Appropriate depth research method for complete profile distribution characteristics of nitrate nitrogen in loess plateau rain-fed apple orchard soil

Abstract

The invention discloses an appropriate depth research method for complete profile distribution characteristics of nitrate nitrogen in loess plateau rain-fed apple orchard soil. The method comprises the following steps: (1) collecting literature data; (2) field acquisition of a soil profile; and (3) nitrate nitrogen indoor determination. In the east, south, west, north and middle areas of a loess plateau apple main producing area, through literature data arrangement, field soil sample collection and indoor analysis, the distribution characteristics of the nitrate nitrogen 6-m section of the soil of the apple orchard are disclosed, and whether the 6-m section is applicable or not is explored and the distribution characteristics of the nitrate nitrogen section of the soil of the loess plateau rain-fed apple orchard are completely disclosed.

Description

Technical Field

[0001] This invention belongs to the field of agricultural technology, and particularly relates to the field of nutrient cycling technology in dryland apple orchards on the Loess Plateau. Specifically, it relates to a suitable depth study method for the distribution characteristics of complete soil nitrate nitrogen profile in rain-fed apple orchards on the Loess Plateau. Background Technology

[0002] China accounts for 43.3% of the world's apple cultivation area and 48.6% of its total apple production, with over 50% originating from the Loess Plateau apple-producing region. Suitable climate conditions and good economic benefits drive farmers in the Loess Plateau apple-producing areas to apply large amounts of fertilizer, especially nitrogen fertilizer, to address low soil fertility and achieve high apple yields. The average annual nitrogen application (N) for apples in my country is 905 kg / hm². −2 The nitrogen application rate for apple orchards on the Loess Plateau is as high as 1200 kg / hm². −2 Accurately exploring the characteristics of nitrate nitrogen changes in apple orchard soil is crucial for targeted optimization of nitrogen fertilizer management. The distribution and accumulation characteristics of nitrate nitrogen in soil profiles of apple orchards on the Loess Plateau have received extensive attention and research. However, due to the labor-intensive, time-consuming, and costly nature of soil profile collection, previous studies have shown a trend towards increasing depths of 1 meter, 2 meters, 3 meters, 4 meters, and 5 meters with age. However, the nitrate nitrogen in deeper soil layers of older apple orchards has not yet reached a steady state, meaning the difference between nitrate nitrogen in these deeper layers and that in farmland at the same soil level is not significant. This limits a comprehensive understanding of the nitrogen profile characteristics in apple orchards and lacks complete data support for optimizing nitrogen fertilizer management. Therefore, it is urgent to determine the appropriate profile depth for nitrogen research in rain-fed apple orchards on the Loess Plateau.

[0003] Our previous analysis of nitrate nitrogen in 6-m soil profiles from 60 farmland and 8-, 17-, and 25-year-old apple orchards in Luochuan County showed that the nitrate nitrogen levels in the 540–600 cm soil layer of the 25-year-old apple orchards were not significantly different from those in the farmland, indicating that the soil had reached a steady state. Figure 1 Considering that Luochuan County's rainfall (~600 mm) is relatively high within the main apple-producing areas of the Loess Plateau, the potential for water to promote the leaching and downward movement of nitrate nitrogen is significant. Against this backdrop, after 25 years, nitrate nitrogen levels in the apple orchard reached a steady state at the 540 cm soil layer. Therefore, we hypothesize that a 6-m soil profile may be a suitable depth for a complete investigation of the soil nitrate nitrogen profile characteristics in rain-fed apple orchards on the Loess Plateau. This research has been published in the internationally renowned journal *CATENA*.

[0004] This article has been cited by peers many times since its publication, and its conclusion that a 6-m soil layer is sufficient to characterize the steady state of nitrate nitrogen in rain-fed apple orchards has been widely verified. Furthermore, some researchers have used a 6-m profile depth as a guide for their experimental sampling work (Dai Hongwei et al., 2025). However, there is currently no clear conclusion regarding the complete profile depth for studying nitrogen accumulation characteristics in rain-fed apple orchards on the Loess Plateau. This leads to blind sampling of soil profiles, such as sampling depths of 10 meters and 20 meters (Ren et al., 2022; Ji et al., 2024). Although deeper soil layers encompass complete nitrogen distribution characteristics, sampling at deeper depths is time-consuming, labor-intensive, and wastes national research funding. Therefore, it is urgent to solve the problem of the sampling depth required for a complete revelation of nitrate nitrogen profile characteristics in rain-fed apple orchards on the Loess Plateau. Summary of the Invention

[0005] The purpose of this invention is to address the problem in existing technologies where the soil depth is unclear in revealing the complete nitrate nitrogen profile characteristics of rain-fed apple orchards on the Loess Plateau. This invention provides a suitable depth method for studying the complete nitrate nitrogen profile distribution characteristics of rain-fed apple orchards on the Loess Plateau. In the eastern, southern, western, northern, and central regions of the main apple-producing area of ​​the Loess Plateau, through literature data collation, field soil sample collection, and laboratory analysis, the 6-m profile distribution characteristics of nitrate nitrogen in apple orchard soil were revealed. The invention explores whether the 6-m profile is applicable and can completely reveal the nitrate nitrogen profile distribution characteristics of rain-fed apple orchards on the Loess Plateau.

[0006] The specific technical solution is as follows:

[0007] A suitable depth method for studying the distribution characteristics of intact soil nitrate nitrogen profile in rain-fed apple orchards on the Loess Plateau includes the following steps:

[0008] (1) Field collection of soil profiles

[0009] In 2017, soil samples were collected from five representative apple orchards of each age and five permanent farmlands near the orchards planted with winter wheat (serving as controls representing the initial state of soil moisture and nitrates) in the three towns of Jiuxian, Huaibai, and Shitou, located in the north, central, and south of Luochuan County, respectively. These orchards were planted with the apple variety “Fuji” (Malus pumila Mill.) on M26 rootstock at a spacing of 4 m × 3 m (825 trees / hectare). Considering the labor and analysis costs involved, a sampling point was selected at the center of each farmland and orchard (1.5 m from the tree trunk). Overall, from October 5 to 10, 2017, 60 soil profiles with a depth of 6 m and an interval of 20 cm were collected using a soil auger.

[0010] (2) Indoor determination of nitrate nitrogen

[0011] The fresh soil samples collected in step (1) were immediately transported to the laboratory and stored at −20°C until one week later to analyze their nitrate content.

[0012] Furthermore, the soil sample collection process described in step (1) can be carried out using the following two methods:

[0013] Method 1: Collect soil samples using a manual soil auger. The soil auger is 1 m long, with a 10 cm diameter, 20 cm long, hollow cylindrical sampling chamber that is open at one end and closed at the other. Position the auger perpendicular to the selected sampling point and rotate it clockwise downwards until a depth of 20 cm is reached. Pull the auger vertically upwards, ensuring a smooth process to prevent soil samples from falling out. After complete removal, collect all soil from the auger, place it in a resealable bag, and label it. This is the 0–20 cm soil profile sample from that sampling point. After removing all soil from the auger, clean the drill bit and continue collecting soil samples at the same point, collecting samples every 20 cm, and so on, until a depth of 6 m is reached.

[0014] Furthermore, for every 1 m increase in sampling depth, an additional 1 m extension rod is required.

[0015] Method 2: Soil samples are collected using a percussion drill (DP-5 percussion drill, Cangzhou Zhengyang Tongchuang Technology Co., Ltd., China). The complete set of equipment includes a DP-5 percussion drill, a 50 mm percussion head (mounted on top of the percussion drill bit or percussion drill rod), several percussion drill rods (42 mm × 1 m, solid cylindrical extension rods closed at both ends and sides), percussion drill bits (42 mm × 1 m, hollow cylinders open at the bottom and one side), and a drill rod puller. Position the percussion drill bit perpendicular to the selected sampling point, install the percussion head on top of the percussion drill bit, and then use the percussion drill to drive the percussion drill bit vertically downwards until a depth of 1 m is reached. Use the drill rod puller to remove the percussion drill bit. The process should be smooth to avoid soil samples falling out of the drill, thus obtaining a 0–100 cm soil profile sample. Then, use a measuring tape to divide each soil sample into 20 cm sections, and take out 5 soil samples in total. Place them in a self-sealing bag and label them, thus obtaining soil profile samples at a depth of 1 m and intervals of 20 cm.

[0016] Furthermore, after removing all the soil from the impact drill bit, the impact drill bit is cleaned, and an impact drill rod (extension rod) is connected to continue collecting samples downwards from the same point, taking samples every 1 m, and so on, until a depth of 6 m is reached.

[0017] Furthermore, the forest age mentioned in step (1) is 8 years, 17 years and 25 years.

[0018] Furthermore, the indoor determination process of nitrate nitrogen described in step (2):

[0019] 1. Sample pretreatment: Weigh 5.00 g of fresh soil sample (or air-dried soil sample) into a 150 mL Erlenmeyer flask, and add 1 mol L⁻¹ using a 50 mL dispenser. −1 Add 50 mL of KCl, shake at 20-25℃ for 1 h, filter, and use the filtrate as the test solution.

[0020] 2. Adding samples: Fill the standard curve cups into positions 1, 2, 3, 4, 5, and 6 on the sample tray in descending order (the last cup is for 1 mol / L). −1 (KCl), then place the sample cups in order. When pouring the sample into the sample cups, be sure to rinse them twice with the test solution.

[0021] 3. Install the filter and connect the pump tubing and injector: Open the cover on the detector, install the 550 nm filter in the filter position of channel 1 or channel 2 (determined according to the measurement range), and then connect the tubing according to the instrument's operating instructions.

[0022] After completing the above steps, turn on the instrument according to the operating instructions.

[0023] Further, the results are calculated:

[0024] Soil NO3 − -N (mg kg) −1 = Measured value of the test solution × V / m × (1 + W%)

[0025] In the formula: V — the number of mL of extractant added;

[0026] m — Sample mass (g);

[0027] (1 + W%) —— The conversion factor for converting air-dried soil to oven-dried soil;

[0028] W% — Moisture content of dried soil.

[0029] (3) Collection of literature data

[0030] Based on two databases, "Web of Science" and "CNKI", data on fertilizer application in apple orchards, soil profile nitrate nitrogen concentration and its accumulation were collected using "apple" and "nitrate nitrogen" as keywords. A total of 29 published articles that met the criteria were selected. During the data collection process, if the data was displayed in the form of a graph, GetData Graph Digitizer 2.24 software was used for data extraction.

[0031] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0032] (1) Breaking through the research depth limit: For the first time, it was determined that the suitable depth for nitrogen characteristics of soil profile in rain-fed apple orchards on the Loess Plateau is 6 m.

[0033] (2) The research scope is broad, with sample points covering the main apple-producing areas of the Loess Plateau, and is highly representative. Figure 2 ).

[0034] (3) δ 15 N and δ 18 The isotopic tracing results of O can prove that comparing the nitrate nitrogen in 6-m soil profile samples from farmland and apple orchards to demonstrate that the nitrate nitrogen in apple orchard soil reaches a steady state at the 6-m soil layer is reliable. Figure 3 ).

[0035] This study breaks down the barriers of inconsistent and arbitrary sampling depths in current research on nitrate nitrogen in rain-fed apple orchards on the Loess Plateau.

[0036] It provides a standard depth for fully revealing the nitrate nitrogen variation pattern in soil profiles of rain-fed apple orchards on the Loess Plateau, solving the drawbacks of incomplete nitrate nitrogen migration patterns and the time-consuming, labor-intensive, and costly nature of revealing patterns from excessively deep soil sampling.

[0037] The concentration and distribution characteristics of nitrogen in deep soil were quantified, which is crucial for assessing the risk of nitrate leaching and improving residual nitrogen management in agricultural systems (e.g., determining whether there is nitrogen excess in a region and the amount of excess based on the concentration and distribution characteristics of soil nitrogen, and thereby guiding orchard management measures such as fertilization amount in the region to mitigate the adverse effects of over-fertilization and promote the sustainable development of the apple industry). Attached Figure Description

[0038] Figure 1 The background section describes the distribution characteristics of nitrate nitrogen profiles in farmland and apple orchards of different ages in Luochuan County from 0 to 6 m.

[0039] Figure 2 This describes the distribution characteristics of nitrate nitrogen profiles in rain-fed apple orchards in different typical areas of the main apple-producing region of the Loess Plateau.

[0040] Figure 3 It is δ 15 N and δ 18 A schematic diagram of O isotope tracing results, where (a) shows the distribution characteristics of nitrate nitrogen profiles in farmland and apple orchards in Xifeng District at depths of 0–10 m, and (b) shows the distribution characteristics of nitrate nitrogen profiles at different soil profile depths δ. 15 N-NO3 − With δ 18 O-NO3 − (c) is the cross-relationship diagram, and (d) is the average contribution diagram of different sources;

[0041] Figure 4 It refers to the geographical distribution of data test sites. Detailed Implementation

[0042] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] (1) Soil profile field collection and nitrate nitrogen laboratory determination

[0044] In 2017, soil samples were collected from five representative apple orchards of each age and five permanent farmlands adjacent to the orchards planted with winter wheat in each of the three towns of Jiuxian, Huaibai, and Shitou in northern, central, and southern Luochuan County (serving as controls representing the initial state of soil moisture and nitrate). These orchards were planted with the apple variety “Fuji” (Malus pumila Mill.) on M26 rootstock at a spacing of 4 m × 3 m (825 trees / hectare). Considering the labor and analytical costs involved, a sampling point was selected at the center of each farmland and orchard (1.5 m from the tree trunk). Overall, from October 5th to 10th, 2017, 60 soil profiles at a depth of 6 m and a spacing of 20 cm were collected using a soil auger. These fresh soil samples were immediately transported to the laboratory and stored at −20°C until one week later for analysis of soil moisture and nitrate content.

[0045] Soil sample collection process:

[0046] Method 1: Collect soil samples using a manual soil auger. The soil auger is 1 m long, with a 10 cm diameter, 20 cm long, hollow cylindrical sample chamber that is open at one end and closed at the other. Position the auger perpendicular to the selected sampling point and rotate it clockwise downwards until a depth of 20 cm is reached. Pull the auger vertically upwards, ensuring a smooth process to prevent soil samples from falling out. After complete removal, collect all soil from the auger, place it in a resealable bag, and label it. This is the 0–20 cm soil profile sample for that sampling point. After removing all soil from the auger, clean the drill bit and continue collecting soil samples at the same point, taking samples every 20 cm, and so on, until a depth of 6 m is reached (for every 1 m increase in sampling depth, an additional 1 m extension rod is required).

[0047] Method 2: Soil samples are collected using a percussion drill (DP-5 percussion drill, Cangzhou Zhengyang Tongchuang Technology Co., Ltd., China). The complete set of equipment includes a DP-5 percussion drill, a 50 mm percussion head (mounted on top of the percussion drill bit or percussion drill rod), several percussion drill rods (42 mm × 1 m, solid cylindrical extension rods closed at both ends and sides), percussion drill bits (42 mm × 1 m, hollow cylinders open at the bottom and one side), and a drill rod puller. Position the percussion drill bit perpendicular to the selected sampling point, install the percussion head on top of the percussion drill bit, and then use the percussion drill to drive the percussion drill bit vertically downwards until a depth of 1 m is reached. Use the drill rod puller to remove the percussion drill bit. The process should be smooth to avoid soil samples falling out of the drill, thus obtaining a 0–100 cm soil profile sample. Then, use a measuring tape to divide each soil sample into 20 cm sections, and take out 5 soil samples in total. Place them in a self-sealing bag and label them, thus obtaining soil profile samples at a depth of 1 m and intervals of 20 cm. After all the soil is removed from the impact drill bit, clean the impact drill bit and connect an impact drill rod (extension rod) to continue collecting soil samples at the same point, once every 1m, and so on, until the depth reaches 6m.

[0048] Indoor determination procedure for nitrate nitrogen:

[0049] 1. Sample pretreatment: Weigh 5.00 g of fresh soil sample (or air-dried soil sample) into a 150 mL Erlenmeyer flask, and add 1 mol L⁻¹ using a 50 mL dispenser. −1 Add 50 mL of KCl, shake at 20-25℃ for 1 h, filter, and use the filtrate as the test solution.

[0050] 2. Adding samples: Fill the standard curve cups into positions 1, 2, 3, 4, 5, and 6 on the sample tray in descending order (the last cup is for 1 mol / L). −1 (KCl), then place the sample cups in order. When pouring the sample into the sample cups, be sure to rinse them twice with the test solution.

[0051] 3. Install the filter and connect the pump tubing and injector: Open the cover on the detector, install the 550 nm filter in the filter position of channel 1 or channel 2 (determined according to the measurement range), and then connect the tubing according to the instrument's operating instructions.

[0052] After completing the above steps, turn on the instrument according to the operating instructions.

[0053] 4. Result Calculation:

[0054] Soil NO3 − -N (mg kg) −1 = Measured value of the test solution × V / m × (1+W%)

[0055] In the formula: V — the number of mL of extractant added;

[0056] m — Sample mass (g);

[0057] (1 + W%) —— The conversion factor for converting air-dried soil to oven-dried soil;

[0058] W% — Moisture content of dried soil.

[0059] (2) Collection of literature data

[0060] Based on the Web of Science and CNKI databases, data on fertilizer application rates in apple orchards, soil profile nitrate nitrogen concentrations, and their cumulative amounts were collected using "apple" and "nitrate nitrogen" as keywords. A total of 29 published articles meeting the criteria were selected. During data collection, if the data was presented as a graph, GetData Graph Digitizer 2.24 software was used for data extraction. Details of the geographical distribution of orchard sampling points mentioned in the literature can be found in [link to relevant documentation]. Figure 4 .

[0061] The above description is merely a preferred embodiment of the present invention, and the scope of protection of the present invention is not limited thereto. Any simple changes or equivalent substitutions of the technical solutions that can be obviously obtained by those skilled in the art within the scope of the technology disclosed in the present invention shall fall within the scope of protection of the present invention.

Claims

1. A method for studying the suitable depth of the complete profile distribution characteristics of nitrate nitrogen in the soil of rain-fed apple orchards in the Loess Plateau, characterized in that, Comprising the following steps: (1) Field collection of soil profiles Five representative apple orchards of each age and five permanent farmlands near the orchards planted with winter wheat were selected for soil sampling. The orchards were planted with apple tree varieties on M26 rootstocks with a spacing of 4 m × 3 m. A sampling point was selected at the center of each farmland and orchard. A total of 60 soil profiles with a depth of 6 m and an interval of 20 cm were collected using a soil auger as fresh soil samples; (2) Laboratory determination of nitrate nitrogen The fresh soil samples collected in step (1) were immediately transported to the laboratory and stored at −20℃ until the nitrate content was analyzed within one week; (3) Literature data collection Based on the web of science and China National Knowledge Infrastructure databases, data on apple orchard fertilization rates, soil profile nitrate nitrogen concentrations, and cumulative amounts were collected using apples and nitrate nitrogen as keywords. A total of 29 published articles that met the conditions were selected. During data collection, if the data were displayed in the form of a graph, the GetData Graph Digitizer 2.24 software was used for data extraction.

2. The method according to claim 1, wherein the method is characterized in that, During the collection process of the soil samples in step (2), a manual soil auger was used to collect the soil samples or an impact drill was used to collect the soil samples.

3. The method according to claim 2, wherein the method is characterized in that, An additional 1 m extension rod was required for each additional 1 m of depth.

4. The method of claim 1, wherein the method is characterized in that, The ages in step (1) were 8 years, 17 years, and 25 years.

5. The method of claim 1, wherein the method is characterized in that, The process of laboratory determination of nitrate nitrogen in step (2) includes the following steps: (1) Sample pretreatment: weigh 5.00 g of fresh soil sample in a 150 mL triangular flask, add 1 mol L −1 KCl 50 mL with a 50 mL bottle mouth liquid adding device, oscillate at 20~25℃ for 1 h, filter, and the filtrate is used as the test liquid; (2) Add samples: The standard curve cups are loaded into the 1, 2, 3, 4, 5, and 6 cup positions on the sample tray from top to bottom, with the last cup being 1 mol L−1 KC. Next, place the sample cups in order; (3) Install the filter, connect the pump tube, and connect the sampler: Open the cover on the detector, install the 550 nm filter in the filter position of channel 1 or channel 2, and then connect the lines according to the instrument usage instructions; After completing the above steps, turn on the instrument according to the instrument usage steps; (4) Result calculation: Soil NO3 − -N (mg kg −1 ) = measured value of sample solution x V / m x (1 + W%) In the formula: V —— the number of mL of extractant added; m —— the mass of the sample (g); (1 + W%) —— the conversion factor for converting air-dried soil to oven-dried soil; W% —— the moisture content of the oven-dried soil.

6. The method of claim 5, wherein the method is characterized in that, In step (2), when pouring the sample into the sample cup, pay attention to rinse it twice with the test liquid.

Images