A method for detecting and analyzing field decomposition rate of leguminous sand-fixing shrub litter

Abstract

The present application belongs to the field of ecology and environmental monitoring technology, and relates to a kind of detection and analysis method for decomposition rate of leguminous sand-fixing shrub litter in field.The present application screens sample area by system grid method, collects and lays out litter bag, synchronously determines initial and current nitrogen content of soil, calculates net increment of soil nitrogen, determines horizontal diffusion boundary of nitrogen, and then obtains soil absorbed nitrogen amount;obtains equivalent dry matter loss amount based on nitrogen balance calibration, and finally determines litter decomposition rate by exponential decay fitting.The method solves the decomposition rate estimation deviation problem caused by nitrogen migration and soil absorption in traditional detection, can more accurately reflect the actual decomposition process in field, improves the reliability of monitoring data, and provides reliable technical support for nutrient cycling research and ecological restoration effect evaluation of desert ecosystem.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ecology and environmental monitoring, and relates to a method for detecting and analyzing the field decomposition rate of leguminous sand-fixing shrub litter. BACKGROUND

[0002] As a pioneer plant species, the decomposition process of the litter of the leguminous sand-fixing shrub is a key link connecting the plant and soil system material circulation, and plays a crucial role in improving soil structure, enhancing soil fertility and promoting sustainable development of the ecological system. Therefore, accurately detecting and analyzing the decomposition rate of the litter in the real environment in the field is of great significance for revealing the nutrient circulation law and optimizing the vegetation restoration strategy.

[0003] The traditional method commonly used in the field decomposition rate research, such as the bag method combined with the exponential model fitting, still has the following main defects when applied to the leguminous plants with the function of biological nitrogen fixation: (1) The existing method usually regards the litter as a closed system in the model construction, and only focuses on the mass loss of the litter itself. However, the leguminous litter will release nutrients to the surrounding soil due to the rich nitrogen in the early stage of decomposition. The mainstream method ignores this key material exchange process and does not include the nitrogen amount absorbed by the soil in the accounting system. This inherent defect in the model leads to inaccurate estimation of the actual nitrogen flux involved in the decomposition, thereby distorting the subsequent decomposition kinetics analysis.

[0004] (2) The decomposition of the litter is an ecological process with significant spatial heterogeneity, and its actual influence range on the soil nutrient pool is not fixed. However, the current technology usually uses a preset and unified sampling radius to estimate the nutrient flow, and fails to dynamically determine this key parameter according to the actual change trend of the soil properties. This lack of consideration of spatial heterogeneity weakens the calculation basis of the soil nutrient absorption amount as a key input of the model, and finally leads to systematic errors in the calculation model of the decomposition rate due to the distortion of the initial parameters. SUMMARY

[0005] In view of this, in order to solve the problems proposed in the background art, a method for detecting and analyzing the field decomposition rate of leguminous sand-fixing shrub litter is provided.

[0006] The purpose of the application can be achieved by the following technical scheme: a method for detecting and analyzing the field decomposition rate of leguminous sand-fixing shrub litter, comprising: S1, dividing a plurality of candidate sample areas in the detection area by using a system grid method, and selecting a sample area according to the spatial distribution characteristics of the natural litter layer thickness and the nitrogen content of the litter in each candidate sample area.

[0007] S2, collect two kinds of litter in the sample area, prepare them into litter bags with known initial dry weight and initial nitrogen content, randomly arrange the positions of the litter bags in the selected sample area, and measure the initial nitrogen content of the soil at different distance points with the positions as the center.

[0008] S3, synchronously recover the litter bags at multiple preset time points and measure the residual dry weight and residual nitrogen content, measure the current nitrogen content of the soil at different distance points at the positions of the litter bags.

[0009] S4, calculate the net increase of soil nitrogen at each distance point based on the initial nitrogen content of the soil and the current nitrogen content of the soil, determine the boundary of lateral diffusion of nitrogen based on the increase, and calculate the total net increase of the soil in the boundary range as the absorbed nitrogen of the soil.

[0010] S5, based on the initial nitrogen content, residual nitrogen content and absorbed nitrogen of the litter, obtain the equivalent dry matter loss amount through nitrogen balance calibration, and determine the decomposition rate of the litter by exponential dynamic attenuation fitting combined with the residual dry weight and the initial dry weight of the litter.

[0011] Compared with the prior art, the beneficial effects of the present application are as follows: (1) the present application introduces a nitrogen conservation calibration mechanism in the decomposition rate analysis, and includes the soil absorbed nitrogen in the accounting system, solving the key defect that the traditional method ignores the release of nutrients to the soil in the early stage of decomposition of legume litter. The method based on the calculation and calibration of the net increase of soil nitrogen accurately reflects the balance relationship between the nitrogen loss of litter and the soil absorption, improves the reflection ability of the decomposition kinetics model to the actual nutrient cycling process, and avoids the problem of distorted decomposition rate caused by ignoring the exchange of nitrogen.

[0012] (2) the present application dynamically determines the boundary of lateral diffusion of nitrogen and calculates the total net increase of the soil in the boundary by constructing a nonlinear regression model of the net increase of soil nitrogen and distance points, solving the problem of insufficient consideration of spatial heterogeneity caused by the fixed sampling radius in the traditional method. The method dynamically adjusts the nutrient flow range according to the actual change trend of soil properties, so that the calculation of soil nutrient absorption is more in line with the actual situation, greatly reduces the uncertainty in the calculation of decomposition rate, and ensures the reliability and comparability of experimental data. BRIEF DESCRIPTION OF DRAWINGS

[0013] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creating any creative labor.

[0014] Figure 1A step diagram of a method for detecting and analyzing field decomposition rate of leguminous sand-fixing shrub litter.

[0015] Figure 2 A flow chart for obtaining soil nitrogen absorption amount.

[0016] Figure 3 A schematic diagram for determining the area of the annular region.

[0017] Reference signs: R - crown radius of the leguminous sand-fixing shrub, d1 - radius of the proximal region, d2 - outer radius of the middle region, d3 - outer boundary of the distal region. DETAILED DESCRIPTION

[0018] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative efforts fall within the scope of the present application.

[0019] Please refer to Figure 1 As shown in the drawings, the present application provides a method for detecting and analyzing field decomposition rate of leguminous sand-fixing shrub litter, comprising: S1, dividing a plurality of candidate sample regions in a to-be-detected region by using a system grid method, and screening a sample region according to spatial distribution characteristics of natural litter layer thickness and litter nitrogen content in each candidate sample region.

[0020] It should be noted that the system grid method specifically comprises: according to topographic and geomorphic features, vegetation coverage uniformity and actual operability of a research region, square quadrats with equal area are arranged as candidate sample regions in the to-be-detected region by fixed spacing, and the spacing between center points of adjacent quadrats is equal and greater than the length of the side of the quadrat.

[0021] Further, the screening method of the sample region comprises: measuring the natural litter layer thickness in each candidate sample region by using a length measuring tool.

[0022] The litter nitrogen content in each candidate sample region is measured by using a nitrogen element analysis device.

[0023] In one embodiment, the operation of determining the nitrogen content of the natural litter in each candidate sample region is as follows: first, a representative natural litter mixture sample is collected in each candidate sample region, avoiding the mixing of soil particles, animal residues or other non-target impurities; then the sample is placed in an oven to dry to constant weight to remove the interference of water on the determination results; the dried sample is ground and sieved to ensure uniform particle size; finally, the total nitrogen content of the sample is determined using an elemental analyzer, and the determination result is expressed as the mass percentage of nitrogen in unit mass of dry matter, as the nitrogen content index of the natural litter in the candidate sample region.

[0024] The upper quartile and lower quartile of the thickness of the natural litter layer in all candidate sample regions, and the median value of the nitrogen content of the litter are calculated.

[0025] In one embodiment, the thickness data of the natural litter layer in all candidate sample regions is sorted by numerical value, and the lower quartile Q1 and upper quartile Q3 are calculated using the quartile method; at the same time, the nitrogen content data of the litter in all candidate sample regions is sorted by numerical value, and the median value is calculated.

[0026] The calculation of the above statistical quantities can be completed by using conventional statistical software or by manual sorting and interpolation method, to ensure the repeatability and objectivity of the results. The obtained Q1, Q3 and median value will be used as the key threshold basis for subsequent sample region screening, to exclude extreme value interference and retain representative high nitrogen level regions.

[0027] The candidate sample region that meets the following two conditions is selected as the sample region.

[0028] (a) The thickness value of the natural litter layer in this region is between the upper quartile and the lower quartile of the thickness of the natural litter layer in all candidate sample regions.

[0029] It needs to be explained that this condition excludes regions with abnormally high or low thickness, and retains regions with medium thickness, to reduce the interference of spatial heterogeneity on the determination of decomposition rate.

[0030] (b) The nitrogen content of the litter in this region is higher than the median value of the nitrogen content of the litter in all candidate sample regions.

[0031] It needs to be explained that this condition means that regions with relatively high nitrogen nutrient level are preferentially selected, to enhance the response ability to nitrogen-driven decomposition process.

[0032] It should be noted that the thickness of the natural litter layer and the nitrogen content of the litter are selected as the screening indexes because they reflect the quantity characteristics and quality characteristics of the litter respectively, the thickness affects the decomposition microenvironment, and too thick or too thin may interfere with the normal decomposition process, the nitrogen content directly determines the chemical decomposability of the litter, and nitrogen is a key factor driving decomposition, especially for legume sand-fixing shrubs; by jointly screening the medium thickness and high nitrogen area, the spatial heterogeneity can be effectively controlled, and the accuracy of the decomposition rate detection can be improved.

[0033] In step S1, one or more control sub-regions are arranged in the sample region, the control sub-region is completely located within the sample region and is not arranged with a litter bag, the boundary of the control sub-region is at least X meters away from the arrangement position of any of the litter bags, and X≥1.

[0034] Preferably, the value of X is 1 meter or 2 meters, which can be determined according to the crown diameter of the legume sand-fixing shrub and the nitrogen diffusion range.

[0035] In steps S2 and S3, the same sampling and determination methods are used to determine the initial nitrogen content and the current nitrogen content of the soil in the control sub-region.

[0036] It should be explained that by arranging the control sub-region, the soil nitrogen dynamic data of the control sub-region is obtained, which is used to eliminate the influence of environmental background fluctuations when calculating the net increase of soil nitrogen.

[0037] S2, collect two types of litter in the sample region, prepare them into litter bags with known initial dry weight and initial nitrogen content, randomly arrange the positions of the litter bags in the selected sample region, and determine the initial nitrogen content of the soil at different distance points with the position as the center.

[0038] The pore size of the litter bag can be 1 mm or 2 mm, and multiple repeated bags are arranged for each type of litter. The random coordinate generation method is used to determine the arrangement position of each litter bag in the sample region, and a positioning device is used to accurately position and set physical markers for the arrangement position of each litter bag on site.

[0039] Further, the two types of litter include any of the following combinations: (i) litter distinguished by organ type, including leaf litter and non-leaf litter.

[0040] In a specific embodiment, the litter is distinguished by organ type, and the combination includes leaf litter and non-leaf litter, wherein the non-leaf litter can include branches, bark, fruits, and other organs.

[0041] (ii) litter distinguished by decomposition stage, including undecomposed litter and decomposed litter.

[0042] In a specific embodiment, the combination is divided according to the decomposition stage, and the combination includes undecomposed litter and decomposed litter, wherein the undecomposed litter includes branches, barks, flowers, fruits and other organ litter just fallen off, with complete structure and no obvious signs of decomposition; the decomposed litter refers to litter that has been weathered and acted by microorganisms for a period of time in the natural environment, and has partial signs of decomposition.

[0043] (iii) litter differentiated according to the species source, including leguminous sand-fixing shrub litter and associated non-leguminous plant litter.

[0044] In a specific embodiment, the combination is differentiated according to the species source, and the combination includes leguminous sand-fixing shrub litter and associated non-leguminous plant litter, so as to quantify the contribution of the leguminous plant to the soil nitrogen pool through litter decomposition and the potential promotion of the decomposition process of the associated plant by comparing the decomposition processes of the leguminous plant and the non-leguminous plant litter coexisting in the living environment of the leguminous plant.

[0045] It should be noted that the above-mentioned various optional litter combination strategies can make the present application flexible to adapt to different scientific research objectives, and can obtain decomposition rate data with strong pertinence and high comparability, regardless of whether the organ heterogeneity, decomposition history or interspecific interaction is concerned.

[0046] Further, the method for measuring the initial nitrogen content of the soil is as follows: taking the layout position of each litter bag as a center point, radially arranging a plurality of sampling axes at equal angle intervals.

[0047] The angle interval requires that the axes cover the 360° space around the center point, and a typical interval can be 45° for 8 axes or 90° for 4 axes, and the specific interval can be adjusted according to the complexity of the terrain of the to-be-detected area.

[0048] The length of the sampling axis needs to cover 1-2 times the length of the crown width outside the crown radius range, so as to ensure that the background value of the soil nitrogen outside the crown width can be captured.

[0049] On each axis, a plurality of sampling points are set by measuring the crown radius value of the leguminous sand-fixing shrub, the sampling points within the crown radius range are taken as proximal distance points, the sampling points at the boundary of the crown width are taken as middle distance points, and the sampling points outside the crown radius range are taken as distal distance points.

[0050] In a specific embodiment, on each laid radial axis, the crown radius value R of the leguminous sand-fixing shrub corresponding to the axis is first measured by a tape measure or a laser range finder. On each axis, the distance from the center point is divided into three categories according to the crown radius value R. The distance < R is defined as a proximal distance point, the distance in the fluctuation range area of R is defined as a middle distance point, and the distance > R is defined as a distal distance point. The fluctuation proportion of the fluctuation range area can be adjusted according to the actual research accuracy requirement or the crown variation degree of the species, for example, ± 5% can be used in the shrub with uniform crown, and ± 15% can be used in the irregular individual.

[0051] The number of sampling points requires that the number of sampling points on each axis contains at least one proximal, one middle, and one distal. If the crown radius is large, such as > 2m, 1-2 sampling points can be added in the proximal and distal, such as 0.5 times the crown radius and 0.8 times the crown radius.

[0052] It should be noted that according to the sampling point, the sampling area is divided by combining the actual crown structure of the plant, which can more truly reflect the influence gradient of litter decomposition on the nitrogen dynamics of different spatial positions of the soil, and provide a spatial division basis with ecological significance for subsequent calculation of the net increase of soil nitrogen, identification of the horizontal diffusion boundary of nitrogen, and accurate estimation of the soil absorption amount of nitrogen.

[0053] Using soil sampling tools, surface soil samples of a fixed depth are collected at each sampling location, and the soil bulk density of the depth is determined simultaneously by the cutting ring method.

[0054] It should be noted that the sampling depth needs to be determined according to the main action layer of the leguminous sand-fixing shrub litter decomposition, for example, the typical depth is 0-10cm or 0-20cm, and the deviation of the sampling depth of all sampling points needs to be ≤1cm, to ensure that the samples come from the same soil layer and avoid the fluctuation of nitrogen content caused by the depth difference. This data is used for subsequent conversion of soil nitrogen content from mass ratio to unit area storage.

[0055] The soil samples of each direction at the same distance point are uniformly mixed according to the mass proportion to form a mixed sample of the distance point.

[0056] It should be noted that, in order to reduce the accidental error caused by local microtopographic variation, the soil samples of each direction at the same distance level, for example, the proximal distance points on all axes, are uniformly mixed according to the original mass proportion collected to form a mixed sample representing the distance point.

[0057] The nitrogen content of each mixed sample is determined by an elemental analyzer as the initial soil nitrogen content of the sample area.

[0058] It needs to be explained that this method provides a reliable data basis for subsequent identification of lateral diffusion boundary of nitrogen and calculation of soil absorbed nitrogen by combining plant canopy structure with radiation symmetry sampling design.

[0059] S3, synchronously recover the litter bags at multiple preset time points and measure the residual dry weight and residual nitrogen content, and measure the current nitrogen content of the soil at different distance points according to the position of the litter bag.

[0060] In a specific embodiment, according to the climate conditions of the study area, the preset study period and the expected decomposition dynamics of legume litter, a series of recovery time points are set. Generally, the time points should cover the early decomposition period such as 1, 3 and 6 months after deployment, the rapid decomposition period such as 12 months, and the middle and late decomposition period such as 18 and 24 months. All litter bags deployed in each sample area are recovered synchronously to avoid introducing additional variables due to different recovery times.

[0061] After recovery, the remaining material in the litter bag is carefully removed, and impurities such as mixed soil particles, roots or animal remains are removed, and then dried to constant weight in a 65°C oven, and the residual dry weight is measured. The dried sample is ground and sieved, and the residual nitrogen content is measured using an elemental analyzer to represent the nitrogen retention state of the litter at that time point.

[0062] At the original litter bag deployment position, the same radial sampling design as in step S2 is followed, i.e. the proximal, middle and distal distance points are set at equal angles around the bag position. Soil samples at the same depth, such as 0-10 cm, are collected at each distance point, and the same mixing and measurement methods as the initial measurement are used, including mass proportion mixing, cutting ring method for bulk density measurement, elemental analyzer for nitrogen measurement, to obtain the current nitrogen content of the soil at each distance point.

[0063] This synchronous measurement mechanism ensures the consistency in time and space between litter mass loss, nitrogen release and soil nitrogen response, providing key data support for subsequent calculation of soil nitrogen net increment, determination of nitrogen lateral diffusion boundary and calibration of decomposition rate.

[0064] S4, calculate the soil nitrogen net increment at each distance point based on the initial soil nitrogen content and the current soil nitrogen content, determine the nitrogen lateral diffusion boundary based on the increment, and calculate the total net increment of the soil within the boundary range as the soil absorbed nitrogen.

[0065] Further, the calculation formula of the soil nitrogen net increment at each distance point is as follows: .

[0066] wherein, represents the initial soil nitrogen content at the distance point, ​denotes the current soil nitrogen content at the distance point, denotes the initial soil nitrogen content of the control sub-region, denotes the current soil nitrogen content of the control sub-region, denotes the distance point number, and .

[0067] It should be noted that the formula eliminates background noise by double difference method, wherein, The total change of soil nitrogen content of the target region is calculated, which includes both the contribution of litter decomposition and the background fluctuation caused by soil self-mineralization, holding, volatilization, leaching and other environmental processes.

[0068] The background change value of soil nitrogen content of the control sub-region in the same period is calculated, and the two are subtracted, which can effectively eliminate the interference of background environmental factors from the total change, so as to obtain more accurate soil nitrogen net increment directly attributed to litter bag decomposition, and provide accurate basis for subsequent determination of nitrogen horizontal diffusion boundary and calculation of soil absorbed nitrogen.

[0069] Further, referring to Figure 2 As shown in the figure, the method for obtaining the soil absorbed nitrogen includes: extracting the data group composed of the initial soil nitrogen content and the current soil nitrogen content of the control sub-region at all sampling time points, and determining the upper limit value of the background fluctuation range based on the average value and the standard deviation of the data group.

[0070] In a specific embodiment, all soil nitrogen content data measured at the control sub-region at the initial and each recovery time point are collected, including the initial value and the current value, to form a background nitrogen change data set.

[0071] The average value μ and the standard deviation σ of the difference between all current values and initial values in the data set are calculated, and the upper limit value of the background fluctuation range is wherein k is a predefined coefficient, usually taking a value between 1 and 3, and the value can be adjusted according to the requirement of confidence level, and the upper limit value represents the upper limit of natural fluctuation of soil nitrogen without intervention of litter bag.

[0072] Taking different distance points as independent variables and the soil nitrogen net increment at each distance point as dependent variable, an exponential decay function is established by using nonlinear regression analysis.

[0073] The upper limit value of the background fluctuation range is substituted into the exponential decay function, and the corresponding distance value is solved and taken as the nitrogen horizontal diffusion boundary.

[0074] It should be noted that the nitrogen lateral diffusion boundary represents an ecological impact boundary objectively determined based on background noise level through data driving, which quantitatively describes the effective spatial range of the lateral migration of nitrogen released in the decomposition process of litter in the soil; as a key spatial integration domain, the boundary defines the effective range for calculating the soil nitrogen absorption amount, and by filtering the environmental background noise outside the boundary, it ensures that the subsequent spatial accumulation calculation is only for the soil area actually affected by the decomposition of litter.

[0075] Within the nitrogen lateral diffusion boundary, the soil nitrogen net increment calculated at each distance point is multiplied by the area of the annular region represented by the distance point, the soil bulk density and the sampling depth to obtain the net increase in absolute mass of soil nitrogen at the distance point.

[0076] The net increase in absolute mass of soil nitrogen at all distance points is summed to obtain the total net increase, which is the soil nitrogen absorption amount.

[0077] It should be noted that the soil nitrogen absorption amount is the absolute mass of nitrogen released from a specific litter bag and fixed in the effective soil volume around it, which quantitatively integrates the output of litter nitrogen and the retention of soil nitrogen, and reflects the net gain of soil nitrogen pool by litter decomposition; as a reference for nitrogen balance calibration, this parameter is a key parameter for comparing with the apparent loss of litter nitrogen, and directly drives the calculation of equivalent dry matter loss, thereby coupling the nutrient cycle process with the carbon cycle process, and improving the authenticity and accuracy of the final determination result.

[0078] Further, referring to Figure 3 As shown in the figure, the determination method of the annular area includes: for the proximal distance point, the region represented by the distance point is determined as a circular region with the litter bag position as the center and the median value of the distance point and the adjacent inner distance point as the radius.

[0079] For the middle distance point, the region represented by the distance point is determined as an annular region with the litter bag position as the center, the median value of the distance point and the adjacent inner distance point as the outer radius, and the median value of the adjacent inner distance point and the more inner distance point as the inner radius.

[0080] For the distal distance point, the region represented by the distance point is determined as an annular region with the litter bag position as the center, the nitrogen lateral diffusion boundary as the outer radius, and the median value of the adjacent inner distance point and the more inner distance point as the inner radius.

[0081] The calculation of the median value of the adjacent distance point uses the arithmetic mean of the numerical values of the two adjacent distance points, and when there is no inner adjacent distance point, the numerical value of the inner distance point is taken as zero.

[0082] It should be noted that the concentric ring area of the area calculation assumes that the influence range of nitrogen released by the litter bag on the soil is a series of concentric rings centered on the litter bag, and each sampling distance point represents the typical value of the ring it is in.

[0083] The median of the adjacent distance points is determined by the arithmetic mean of the inner and outer boundaries of the ring, which avoids the overestimation or underestimation that may be caused by directly setting the ring boundary at the sampling point, and can more smoothly reflect the spatial gradient change of nitrogen concentration.

[0084] It should be noted that the area determination method converts the limited discrete sampling point data into an effective representation of continuous space, providing a reliable geometric basis for accurately calculating the total soil nitrogen absorption in the entire affected area.

[0085] S5, based on the initial nitrogen content of litter, residual nitrogen content and soil nitrogen absorption, the equivalent dry matter loss amount is obtained by nitrogen balance calibration, and the litter decomposition rate is determined by exponential dynamic decay fitting combined with the residual dry weight and the initial dry weight of litter.

[0086] Further, the method for obtaining the equivalent dry matter loss amount comprises: taking the difference between the initial nitrogen content and the residual nitrogen content of the litter as the apparent loss amount of nitrogen in the litter.

[0087] It should be noted that the apparent loss amount of nitrogen in the litter refers to the calculated amount of nitrogen reduction by directly comparing the mass of nitrogen in the litter bag at the beginning and at the end of the experiment, which is an apparent value because it is based only on the direct measurement data of the litter itself, without considering the final destination of the lost nitrogen.

[0088] Obtain a historical experimental data set matching the current experimental conditions, and determine a tolerance threshold based on the absolute difference between the apparent loss amount of nitrogen and the soil absorbed nitrogen in the historical experimental data set.

[0089] It should be noted that the historical experimental data set matching the current experimental conditions refers to prior experimental data obtained from the same ecological region, the same type of leguminous sand-fixing shrubs, and the same or similar determination method, which should include the apparent loss amount of nitrogen and the corresponding soil absorbed nitrogen of multiple observation samples.

[0090] The tolerance threshold can be taken as a statistical value of the absolute difference, such as the upper quartile or the average value plus one standard deviation, which essentially defines the upper limit of the acceptable difference between the apparent loss amount and the soil absorption amount within the range of normal experimental error and natural fluctuations.

[0091] The difference between the apparent loss amount of the nitrogen in the litter and the soil absorbed nitrogen amount is calculated, and compared with a tolerance threshold value. If the difference is less than or equal to the tolerance threshold value, the difference between the apparent loss amount of the nitrogen in the litter and the soil absorbed nitrogen amount is taken as the effective loss amount of the nitrogen. Otherwise, the soil absorbed nitrogen amount is taken as the effective loss amount of the nitrogen.

[0092] It should be noted that if the difference is less than or equal to the tolerance threshold value, it is considered that the destination of the nitrogen is clear, the apparent loss amount is consistent with the soil absorption amount within the error range, and there is no obvious gaseous loss. This value represents the amount of nitrogen that has truly mineralized and may participate in subsequent cycles beyond the soil absorption range.

[0093] If the difference is greater than the tolerance threshold value, it is considered that there is significant nitrogen loss that cannot be explained by soil absorption. This part is determined to have gaseous volatilization such as denitrification and ammonia volatilization. The soil absorbed nitrogen amount is taken as the best estimate of the effective loss amount of the nitrogen, which represents the part directly captured by the ecosystem.

[0094] Based on the initial dry weight of the litter and the initial nitrogen content, the initial dry matter mass ratio corresponding to unit nitrogen is calculated. The carbon-nitrogen ratio is multiplied by the effective loss amount of the nitrogen to obtain the equivalent dry matter loss amount.

[0095] It should be noted that the equivalent dry matter loss amount refers to the theoretical dry matter mass that should have been decomposed according to the stoichiometric ratio of the litter (dry matter / nitrogen) to match and explain the calibrated effective loss amount of the nitrogen. It represents that the decomposition of the litter is a process of simultaneous release of carbon framework components and nutrients such as nitrogen, and there is an inherent stoichiometric relationship between the two.

[0096] For example, if gaseous loss occurs, the apparent loss amount of the nitrogen in the litter will be greater than the soil absorbed nitrogen amount. At this time, if the apparent dry weight loss is directly used to calculate the decomposition rate, the result will be slower because a part of the nitrogen disappears and is not reflected in the soil nitrogen increment, which means that the corresponding carbon framework components may have been decomposed and mineralized. However, this part is ignored in the traditional method, while the equivalent dry matter loss amount takes into account this part of the effective loss of nitrogen and converts it into corresponding dry matter, thereby more truly reflecting the actual decomposition degree, which is the direct basis for subsequent calculation of the calibrated decomposition rate.

[0097] Further, the method for determining the decomposition rate of the litter comprises: obtaining a first mass residual rate at each time point according to the ratio of the residual dry weight of the litter to the initial dry weight, and calculating a first decomposition rate by fitting an exponential decay function using a nonlinear least squares method.

[0098] It should be explained that the rate reflects the apparent decomposition dynamics without nitrogen calibration.

[0099] According to the equivalent dry matter loss amount With the initial dry weight of the litter The second mass residual rate is calculated by the following formula .

[0100] The second decomposition rate is calculated by using the same exponential decay function on the second mass residual rate.

[0101] It should be explained that the rate reflects the decomposition dynamics after nitrogen balance calibration.

[0102] The relative difference between the first decomposition rate and the second decomposition rate is calculated, and compared with a preset threshold.

[0103] When the relative difference is greater than the preset threshold, the second decomposition rate is taken as the litter decomposition rate, otherwise the first decomposition rate is taken as the litter decomposition rate.

[0104] It should be noted that the preset threshold is an empirical value, which can be set according to the requirement of data accuracy and the fluctuation of historical data. Generally, the threshold can be set between 5% and 15%, preferably 10%, which is to define an acceptable error range. If the rate change during calibration does not exceed this range, the result of the traditional method is considered acceptable.

[0105] When the relative difference is greater than the preset threshold, it indicates that the gaseous loss of nitrogen has a non-negligible systematic deviation on the determination of the decomposition rate, at this time, the reliability of the first decomposition rate obtained by the traditional method is questionable, and the second decomposition rate which is more consistent with the actual nitrogen return should be taken as the final litter decomposition rate.

[0106] When the relative difference is less than or equal to the preset threshold, it indicates that the gaseous loss of nitrogen is not significant, and the results of the two methods are consistent, so the first decomposition rate can be directly taken as the final litter decomposition rate.

[0107] It should be noted that the method of the present application is suitable for ecosystems dominated by soil nitrogen accumulation, such as legume sand-fixing shrub communities in arid or semi-arid areas. In such environments, precipitation is scarce, leaching is weak, and gaseous loss is relatively small. The nitrogen released by litter decomposition is mainly adsorbed or fixed by the adjacent soil, so the determination of soil nitrogen increment can effectively represent the nitrogen return process, ensuring the accuracy of nitrogen balance calibration.

[0108] ​The core concept of the present application is based on the principle of nitrogen balance. In the litter decomposition system, the nitrogen loss of litter, i.e. the apparent loss of nitrogen, should follow the law of conservation of mass, mainly including two parts: one is the part absorbed by the surrounding soil, i.e. the soil absorbed nitrogen, and the other is the part lost in gaseous form; in the target ecosystem with weak leaching effect, gaseous loss is the main reason for the difference between the apparent loss and the soil absorption; the traditional litter bag method assumes that all the apparent loss of nitrogen enters the soil, thereby ignoring the gaseous loss, resulting in underestimation of the dry matter decomposition rate.

[0109] By synchronously quantifying the apparent loss of litter nitrogen and the soil absorbed nitrogen, the difference is identified and corrected. Through logical judgment, the more reliable effective loss of nitrogen is taken as the index of the true decomposition degree, and according to the initial scientific measurement ratio of litter, the more real equivalent dry matter loss is calculated, and finally the decomposition rate is accurately calibrated.

[0110] The method of the present application first selects a representative sample area based on the spatial distribution of litter thickness and nitrogen content, and sets a control sub-area to distinguish the change of the environment background, then soil sampling is carried out at multiple distance points centered on the litter bag, the nitrogen content before and after laying out and the control area are compared, the net increase of soil nitrogen at each point is calculated, the horizontal diffusion boundary of nitrogen is determined, and the total net increase within the boundary is accumulated as the soil absorbed nitrogen, then, combined with the initial and residual nitrogen content of litter, the equivalent dry matter loss is obtained through nitrogen balance calibration, finally, based on the corrected mass loss data, the litter decomposition rate is fitted, the method effectively improves the accuracy of the determination of the decomposition rate through systematic sampling and nitrogen balance calibration.

[0111] The parameters involved in the above formula are de-dimensioned to calculate their numerical values, the formula is obtained by software simulation of a large amount of data to reflect the most real situation, and the preset parameters in the formula are set by a person skilled in the art according to the actual situation.

[0112] The above embodiments can be realized wholly or partially by software, hardware, firmware or any combination thereof. When realized by software, the above embodiments can be realized wholly or partially in the form of a computer program product.

[0113] Those skilled in the art can realize that the algorithm steps of each example described in combination with the embodiments disclosed herein can be realized by electronic hardware or a combination of computer software and electronic hardware. Whether the functions are realized in hardware or software depends on the specific application and design constraints of the technical solution. The skilled person can use different methods to realize the described functions for each specific application, but such implementation should not be considered beyond the scope of the present application.

[0114] In addition, each functional module in each embodiment of the present application can be integrated in one processing module, or each module can exist physically alone, or two or more modules can be integrated in one module.

[0115] The above is merely specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can easily think of changes or replacements within the technical scope disclosed by the present application, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

[0116] Finally, the above is merely preferred embodiments of the present application, and is not used to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be covered in the protection scope of the present application.

Claims

1. A method for detecting and analyzing the field decomposition rate of leguminous sand-fixing shrub litter, characterized in that, The method comprises the following steps: S1, a plurality of candidate sample regions are divided in the to-be-detected region by using a system grid method, and sample regions are selected according to the spatial distribution characteristics of the natural litter layer thickness and the nitrogen content of the litter in each candidate sample region; S2, two types of litters in the sample region are collected, and the litters are prepared into litter bags with known initial dry weight and initial nitrogen content; the positions of the litter bags are randomly arranged in the selected sample region, and the initial nitrogen content of the soil is measured at different distance points with the positions as the centers; S3, the litter bags are synchronously recovered at a plurality of preset time points, and the residual dry weight and the residual nitrogen content of the litter bags are measured, and the current nitrogen content of the soil is measured at different distance points at the positions of the litter bags; S4, the net increase of the nitrogen content of the soil at each distance point is calculated based on the initial nitrogen content of the soil and the current nitrogen content of the soil, the boundary of the lateral diffusion of the nitrogen is determined based on the increase, and the total net increase of the soil in the boundary range is calculated as the absorbed nitrogen content of the soil; S5, the equivalent dry matter loss amount is obtained by nitrogen balance calibration based on the initial nitrogen content, the residual nitrogen content of the litter and the absorbed nitrogen content of the soil, and the decomposition rate of the litter is determined by exponential dynamic attenuation fitting combined with the residual dry weight and the initial dry weight of the litter; In step S1, one or more control sub-regions are arranged in the sample region, the control sub-regions are completely located in the sample region and no litter bags are arranged, wherein the boundary of the control sub-region is at least X meters away from the arrangement position of any litter bag, and X is greater than or equal to 1; in steps S2 and S3, the initial nitrogen content of the soil and the current nitrogen content of the soil in the control sub-region are synchronously measured by using the same sampling and measuring method of the soil in the sample region; The measuring method of the initial nitrogen content of the soil is as follows: taking the arrangement position of each litter bag as a center point, radially arranging a plurality of sampling axes at a plurality of equiangular intervals; On each axis, a plurality of sampling points are arranged based on the measured radius value of the crown of the leguminous sand-fixing shrub: the sampling points located in the radius range of the crown are regarded as proximal distance points, the sampling points located at the boundary of the crown are regarded as middle distance points, and the sampling points located outside the radius range of the crown are regarded as distal distance points; a soil sampling tool is used to collect surface soil samples of a fixed depth at each sampling position, and the soil bulk density of the soil layer at the depth is synchronously measured by using a cutting ring method; the soil samples at the same distance point are uniformly mixed in a mass ratio to form a mixed sample at the distance point; The nitrogen content of each mixed sample is measured by using an elemental analyzer as the initial nitrogen content of the soil in the sample region. The method for obtaining the soil absorbed nitrogen amount comprises the following steps: extracting a data group composed of initial nitrogen contents and current nitrogen contents of the control sub-regions at all sampling time points, determining an upper limit value of a background fluctuation range based on an average value and a standard deviation of the data group; taking different distance points as independent variables and soil nitrogen net increments at the distance points as dependent variables, and establishing an exponential decay function by using nonlinear regression analysis; substituting the upper limit value of the background fluctuation range into the exponential decay function, solving a corresponding distance value and taking the distance value as a nitrogen lateral diffusion boundary; within the nitrogen lateral diffusion boundary, multiplying soil nitrogen net increments calculated at each distance point by an annular area represented by the distance point, soil bulk density and a sampling depth to obtain an absolute net increase mass of soil nitrogen at the distance point; and adding and summing up absolute net increase masses of soil nitrogen at all distance points to obtain a total net increase amount, which is the soil absorbed nitrogen amount; The method for determining the annular area comprises the following steps: for a proximal distance point, determining a region represented by the distance point as a circular region with a litterbag position as a center and with a median value of the distance point and an adjacent inner distance point as a radius; for a middle distance point, determining a region represented by the distance point as an annular region with the litterbag position as the center, with a median value of the distance point and an adjacent inner distance point as an outer radius, and with a median value of the adjacent inner distance point and a more inner distance point as an inner radius; and for a distal distance point, determining a region represented by the distance point as an annular region with the litterbag position as the center, with the nitrogen lateral diffusion boundary as an outer radius, and with a median value of the adjacent inner distance point and the more inner distance point as an inner radius; wherein, the median value of the adjacent inner distance point is calculated by using an arithmetic average of numerical values of two adjacent distance points, and when there is no inner adjacent distance point, a numerical value of the inner distance point is taken as zero; The method for obtaining the equivalent dry matter loss amount comprises the following steps: taking a difference value between the initial nitrogen content and the residual nitrogen content of the litter as an apparent nitrogen loss amount of the litter; obtaining a historical experimental data set matched with a current experimental condition, determining a tolerance threshold value based on absolute difference values between apparent nitrogen loss amounts and soil absorbed nitrogen amounts in the historical experimental data set; comparing a difference value between the apparent nitrogen loss amount of the litter and the soil absorbed nitrogen amount with the tolerance threshold value, and when the difference value is less than or equal to the tolerance threshold value, taking the difference value between the apparent nitrogen loss amount of the litter and the soil absorbed nitrogen amount as an effective nitrogen loss amount, otherwise, taking the soil absorbed nitrogen amount as the effective nitrogen loss amount; calculating an initial dry matter mass ratio corresponding to unit nitrogen based on an initial dry weight and the initial nitrogen content of the litter, and multiplying the carbon-nitrogen ratio and the effective nitrogen loss amount to obtain the equivalent dry matter loss amount; The method for determining the litter decomposition rate comprises: obtaining a first mass residual rate at each time point according to a ratio of a residual dry weight of the litter to an initial dry weight, and calculating a first decomposition rate by fitting an exponential decay function using a nonlinear least square method; obtaining a second mass residual rate according to an equivalent dry matter loss amount and the initial dry weight of the litter The second mass residual rate is calculated by the following formula : ； The second decomposition rate is calculated by using the same exponential decay function on the second mass residual rate; a relative difference value between the first decomposition rate and the second decomposition rate is calculated, and the relative difference value is compared with a preset threshold value; when the relative difference value is greater than the preset threshold value, the second decomposition rate is taken as the litter decomposition rate, otherwise, the first decomposition rate is taken as the litter decomposition rate.

2. The method for detecting and analyzing the field decomposition rate of leguminous sand-fixing shrub litter as described in claim 1, characterized in that, The method for screening the sample region comprises the following steps: a natural litter layer thickness in each candidate sample region is measured by using a length measuring tool; determining the nitrogen content of the litter in each candidate sample region by a nitrogen element analysis device; calculating the upper quartile and the lower quartile of the natural litter layer thickness in all candidate sample regions, and the median value of the nitrogen content of the litter; selecting a candidate sample region as the sample region if it satisfies the following two conditions: (a) the value of the natural litter layer thickness in the region is between the upper quartile and the lower quartile of the natural litter layer thickness in all candidate sample regions; (b) the nitrogen content of the litter in the region is higher than the median value of the nitrogen content of the litter in all candidate sample regions.

3. The method according to claim 1, wherein the method is characterized by, the two types of litter include any one of the following combinations: (i) litter distinguished by organ type, including leaf litter and non-leaf litter; (ii) litter distinguished by decomposition stage, including undecomposed litter and decomposed litter; (iii) litter distinguished by species source, including leguminous sand-fixing shrub litter and associated non-leguminous plant litter.

4. The method according to claim 1, wherein the method is characterized by, the net increment of soil nitrogen at each distance point is calculated by the following formula: ； wherein, represents the initial soil nitrogen content at the distance point, represents the current soil nitrogen content at the distance point, represents the initial soil nitrogen content of the control sub-area, represents the current soil nitrogen content of the control sub-area, represents the distance point number, and .

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