REGIONAL ASSESSMENT OF SOIL NITROGEN MINERALIZATION IN DIVERSE CROPLAND OF A REPRESENTATIVE INTENSIVE AGRICULTURAL AREA

Abstract

● Soil N mineralization (N_min) rates varied spatially among cropland fields.

● Soil N_min rates increased with a decreasing elevation.

● Soil N_min was mainly affected by SOC, TN, and available C and N.

● N_min in cropland soil should be considered when evaluating regional water pollution.

Soil nitrogen mineralization (N_min) is a key process that converts organic N into mineral N that controls soil N availability to plants. However, regional assessments of soil N_min in cropland and its affecting factors are lacking, especially in relation to variation in elevation. In this study, a 4-week incubation experiment was implemented to measure net soil N_min rate, gross nitrification (Nit) rate and corresponding soil abiotic properties in five field soils (A–C, maize; D, flue-cured tobacco; and E, vegetables; with elevation decreasing from A to E) from different altitudes in a typical intensive agricultural area in Dali City, Yunnan Province, China. The results showed that soil N_min rate ranged from 0.10 to 0.17 mg·kg⁻¹·d⁻¹ N, with the highest value observed in field E, followed by fields D, C, B, and A, which indicated that soil N_min and Nit rates varied between fields, decreasing with elevation. The soil Nit rate ranged from 434.2 to 827.1 µg·kg⁻¹·h⁻¹ N, with the highest value determined in field D, followed by those in fields E, C, B, and A. The rates of soil N_min and Nit were positively correlated with several key soil parameters, including total soil N, dissolved organic carbon and dissolved inorganic N across all fields, which indicated that soil variables regulated soil N_min and Nit in cropland fields. In addition, a strong positive relationship was observed between soil N_min and Nit. These findings provide a greater understanding of the response of soil N_min among cropland fields related to spatial variation. It is suggested that the soil N_min from cropland should be considered in the evaluation of the N transformations at the regional scale.

Graphical abstract

Keywords

cropland / gross nitrification rate / regulatory factors / soil nitrogen mineralization / spatial variation

1 INTRODUCTION

Nitrogen is an essential nutrient for crops but mineral N in soil, the only form that can be absorbed and used by crops, represents only about 1% of total soil N^[1]. Although N fertilization is commonly a necessary method for supplying N to crops, N release due to excess N fertilizer in the environment through hydrological and gaseous pathways has been identified as the main obstacle to the global sustainability of food production^[2,^3]. In addition, soil N mineralization (N_min), a key process that converts organic N into mineral N during the activities of microorganisms, is normally essential for adequate N nutrition. However, a strong N_min may also lead to excessive amounts of nitrate (NO₃^–-N) and ammonium nitrogen (NH₄⁺-N) that can be lost in ground surface runoff or leach to groundwater, resulting in water pollution^[4]. Although numerous reports have documented the soil N_min rate under different land use, such as forestry^[5–^7], grasslands^[8] and cropland^[9–^11], regional assessments of soil N_min and its potential effects on the environment are lacking, especially for agricultural areas with intensive management.

Nitrification (Nit), another important soil N transformation linked to the soil N_min, contributes greatly to the regulation of the N form in soil^[12]. Considerable spatial differences exist in soil N_min and Nit due to wide-ranging influences, N_min depends on organic matter composition, agricultural management practices, temperature, humidity, pH, ventilation, soil structure, soil fertility and soil microorganisms^[13,^14]. Soil N transformation and N_min are coupled processes, several studies have investigated the coupling effect between soil N_min and Nit among different ecosystems^[15,^16]. In particular, soil N_min can be influenced by a number of factors, such as soil pH^[17], soil moisture^[8], total soil N (TN)^[18], total carbon (TC)^[19], soil C to N ratio (C/N)^[15,^20], and different vegetation types^[21]. Cao et al.^[14] successively investigated the soil N_min process and its underlying mechanisms in cropland in southern China. However, a gap still existed when the effects of these factors on the soil N_min of different cropland differed, especially under spatial variation conditions.

In this study, we aimed to obtain a regional assessment of soil N_min, especially for agricultural areas with intensive management, and address the gap in how the key soil factors affect soil N_min of different cropland differed spatially. Also, the relationships between soil N_min and soil variables were examined. In addition, the potential impacts of soil N_min on the environment, such as water quality, were discussed.

2 MATERIALS AND METHODS

2.1 Sampling site description and soil sample collection

Soil samples were collected from representative fields (A–C, maize; D, flue-cured tobacco; E, vegetables) at different altitudes in Erhai Valley (100°06′28″ to 100°08′25″ E, 25°48′43″ to 25°49′02″ N) in Wanqiao Town, Dali City, Yunnan Province, China in July 2022 (Fig.1). The sampling area has a subtropical monsoon climate with an average annual temperature of 15.1 °C and precipitation of 1065 mm. Tab.1 presents detailed information on the physicochemical properties of cropland soils at different altitude levels. For soil sample collection, five topsoil (0–20 cm) samples were randomly collected in each field and mixed to prepare one composite soil sample. Then, the composite soil samples were quickly transported to the laboratory, with one part subsample used for further analysis of soil variables^[22].

2.2 Design of the incubation trial

Sixty Erlenmeyer flasks (250 mL) each containing 20 g of oven-dried soil were used. Preincubation (15 °C for 5 days) of soils was done at 40% water holding capacity (WHC). The temperature of preincubation corresponded to the approximate average annual temperature of the soil sampling region. After preincubation, the moisture content of soils was adjusted to 60% WHC, and soils were then incubated for at 15 °C for 30 days. The moisture content of soils was maintained by weighing and replenishing with distilled water every 2 days. Soil samples were taken at 1, 8, 15, 22 and 30 days (i.e., nominally 0 to 4 weeks) to determine soil NH₄⁺, NO₃⁻, dissolved organic carbon (DOC) contents and soil pH.

2.3 Soil parameter measurements

Soil subsamples were used to determine the soil water content (SWC), soil pH, soil DOC and soil dissolved inorganic nitrogen (DIN, including NH₄⁺ and NO₃⁻). In brief, the soil subsamples were stored in an aluminum specimen box after oven drying at 105 °C for 24 h and weighed to determine the SWC. Soil organic carbon (SOC) was measured via the potassium dichromate volumetric method. Soil total organic carbon (TOC) and TN were determined using the potassium dichromate oxidation method and a C/N element analyzer (Shimadzu Corporation, Kyoto Japan), respectively. For soil DOC determination, the soil subsamples were extracted with deionized water and shaken in a mechanical shaker for 1 h at 250 revolutions per minute. Afterward, the samples were centrifuged at 8000 g for 10 min, and the supernatant was filtered through a 0.45 µm membrane and analyzed using a TOC analyzer (TOC-VWP, Shimadzu Corporation). NH₄⁺-N and NO₃⁻-N concentrations in soil were determined using a flow-injection autoanalyzer (Tecator FIA Star 5000 Analyzer, FOSS Tecator, Höganäs, Sweden) after being extracted with 2 mol·L⁻¹ KCl and filtered by quantitative filter paper. In addition, soil samples were collected using cylinder rings and oven-dried to determine the soil bulk density.

2.4 Soil organic N mineralization calculation

Soil dissolved inorganic nitrogen content (DIN) is expressed by the sum of NH₄⁺-N plus NO₃⁻-N contents.

Soil net organic N_min is determined by the difference in DIN content before and after incubation.

Soil N_min rate = N_min/t, where t is the actual incubation duration as days.

2.5 Soil nitrification rate

Soil Nit rate (µg·kg⁻¹·h⁻¹ N) was measured by the barometric process separation system^[23]. In brief, four undisturbed soil samples collected with cutting cylinders were used to determine the soil Nit, with the soil moisture, weight, and pH obtained previously.

2.6 Statistical analyses

The values of the soil variables were compared between the five fields by one-way analysis of variance. Linear or nonlinear regressions were performed to exhibit the functional relationships between soil N_min and parameters. Corresponding figures were prepared using Origin 8.5 (OriginLab Corporation, Northampton, MA, USA). All statistical analyses were performed with SPSS (SPSS19.0, SPSS Inc., Chicago, USA), with P≤ 0.05 deemed statistical significant.

3 RESULTS

3.1 Soil variables

The soil TOC and TN contents differed between the soils from the sampled fields (Fig.2). In comparison to the soil from the maize fields, those from flue-cured tobacco and vegetable fields had higher TOC and TN contents but lower C/N. The dynamics of soil NH₄⁺-N content during the incubation period varied between fields. For fields D and E, the content declined swiftly during the incubation period, whereas for the other fields, it remained at a lower and more stable level (Fig.3). In contrast, the soil NO₃⁻-N concentration gradually increased for all fields, particularly in fields D and E (Fig.3). Soil DOC content decreased gradually during the incubation period (Fig.3) and the ratio of DOC to DIN also decreased gradually (Fig.3), except that the decrease was more evident for field B. Soil pH was largely steady for all fields, with the highest value observed in the soil of fields B and E, followed by those of field D, A and C (Fig.4).

3.2 Soil N_min

The soil N_min quantum and rate differed among the studied fields (Tab.2). The soil N_min quantity and rate ranged from 2.98 to 5.52 mg·kg⁻¹ N and from 0.10 to 0.17 mg·kg^−1·d⁻¹ N, respectively. The soil annual N_min ranged from 74.5 to 127.1 kg·ha⁻¹·yr⁻¹ N. The soil daily and annual N_min values were similar across the sampled fields, with the highest value in field E, followed by those in fields D, C and B. The lowest value was observed in field A.

3.3 Soil nitrification

The soil Nit rate showed different dynamics among the studied cropland (Tab.3). The soil Nit rate ranged from 434.5 to 827.1 µg·kg⁻¹·h⁻¹ N, with the highest value determined in cropland D, followed by those in cropland E, C, and B. The lowest value was found in cropland A.

3.4 Relationships between soil variables and soil N_min and nitrification

The soil N_min rate was correlated with several key soil parameters in all sampled fields (Tab.4, Fig.5, and Fig.6). Tab.5 shows that the soil N_min rate was positively correlated with soil TN, NH₄⁺ and DOC. In particular, the soil N_min rate increased linearly with increasing soil NH₄⁺ and DOC contents (Fig.5). Significant positive correlations were observed between Nit and soil TN, DOC, NH₄⁺ and NO₃^– for all sampled fields (Tab.4). Similarly, the increase in Nit was associated with the increase in soil TN, DOC, NH₄⁺ and NO₃^– contents (Fig.6). In particular, the increased soil N_min rate was associated with the increase in soil Nit (Fig.7).

3.5 Comparison of soil N_min rates between different land uses in the literature

We compared the soil N_min rate determined in this study with other investigations. Tab.5 shows that the research on soil N_min has mainly focused on crops and forests over the past 30 years. The soil average N_min rate varied from 0.04 to 1.10 mg·kg⁻¹·d⁻¹ N for forests, and these values were comparable to those in cropping soils (0.03–1.15 mg·kg⁻¹·d⁻¹ N). In this study, the soil N_min rate varied from 0.10 to 0.17 mg·kg⁻¹·d⁻¹ N across all sampled fields, with an average N_min rate of 0.13 mg·kg⁻¹·d⁻¹ N, which is below the ranges in the literature.

4 DISCUSSION

4.1 Spatial differences in soil N_min of cropland

Land-use changes can modify soil N_min processes, but the magnitude and direction of this depends on environmental conditions, soil variables and management practices^[31]. However, the response mechanism of soil N_min, one of the key biochemical nutrient cycle processes, to changes in elevation for different contexts remains unclear. Liu et al.^[4] reported that the potential of soil N_min among diverse agricultural ecosystems decreases considerably with increasing latitude and altitude. In the present study, the soil N_min quantum and rate were of similar orders across the different fields sampled, with the highest value in field E, followed by those in fields D, C and B. The lowest value was found in field A. In addition, field C had higher soil N_min rate than maize fields A and B. These results suggest that the soil N_min rate varied spatial among the sample fields and increased with the decrease in elevation. Rustad et al.^[32] demonstrated that soil N_min is significantly negatively correlated with latitude. In a field experiment, Gutiérrez-Girón et al.^[33] observed that labile SOC gradually decreased with increasing altitude, and soil N_min was less at high-altitude sites owing to the decreased substrate availability, which agrees with our findings. Also, Zhang et al.^[21] reported that an increase in C/N ratios caused an increase in soil organic matter (SOM) in alpine meadows with elevated altitude, which resulted in a low N_min rate. Thus, our results showed that the soil C/N decreased among sampled fields with decreased elevation.

4.2 Key factors affecting soil N_min of cropping soils

Understanding how environmental factors influence N_min is essential for the provision of sustainable ecosystem services, especially in a resource-constrained ecosystem^[34]. Previous reports have suggested that rates of soil N transformations (such as N_min) are affected by numerous factors^[13]. Studies have detected significant differences in soil N_min and Nit among different ecosystems^[15,^16], which can be influenced by pH^[17], soil moisture^[8], soil TN^[18], TC^[19], soil C/N^[16,^20] and different vegetation types^[21]. Vervaet et al.^[13] reported that the soil N_min rate, along with soil texture, is related to organic matter quality, TN content, and C/N. In addition, Springob et al.^[35] reported that the higher the soil C/N, the lower the nitrogen release rate. Colman and Schimel^[36] demonstrated that SOM quality can explain a relatively large proportion of the variation in N_min. Similarly, our results showed that the soil N_min was correlated with soil TN, DOC and NH₄⁺ (Tab.5). Likewise, the increased Nit rate was associated with increases in soil TN, DOC, NH₄⁺ and NO₃^– contents (Fig.6). In general, soil TOC, TN, DOC, NH₄⁺ and NO₃^– contents increased with decreased elevation, and soil N_min and Nit rates increased with the increased amounts of these soil variables. Our results emphasize the important effects of soil parameters on soil N_min under spatial variation conditions with changes in elevation. Greater amounts of available C and N suitable for microbial processes accelerated SOM decomposition and mineralization. Similarly, soil DOC and DIN are readily available substrates for microbes^[22], which consequently affects the soil N transformations.

4.3 Potential effects of soil N_min on water quality

Soil N mineralized from SOM during the crop-growing season must be assessed to determine its contribution to crop yield variability and to evaluate the need for variable-rate N fertilization^[37,^38]. In addition, a strong N_min may lead to excessive amounts of soil NO₃^–-N and NH₄⁺-N in surface runoff or leaching to ground water, which results in water eutrophication^[4]. Here, we suggested the regional assessment of soil N_min of an agricultural area and explored the potential effects of soil N_min on water environment quality based on the annual soil N_min content, which ranged from 74.5 to 127.1 kg·ha⁻¹·yr⁻¹ N, determined in different fields in the present study. The present study showed a good positive relationship between soil N_min and Nit (Fig.7), which indicates that soil NH₄⁺-N derived from organic N_min can be oxidized into NO₃^–-N by the microbial Nit process. Therefore, a strong N_min may facilitate the conversion of high amounts of NH₄⁺-N to NO₃^–-N, which can be carried in surface runoff or leach to groundwater, and consequently threaten the water quality. In summary, our results suggest that more attention should be given to soil N_min quantum and rate in cropping contexts across significant geographical and temporal variability.

5 CONCLUSIONS

In this study, we measured the soil net N_min rate, gross Nit rate, and the corresponding soil abiotic properties of different cropland soils in a representative agriculturally intensive area. We conducted a regional assessment of soil N_min in an agricultural area with intensive management and explored the effects of key soil factors on the soil N_min of different cropland fields in relation to spatial variation. We observed that the rates of soil N_min and Nit were spatially variable across the fields sampled. In general, the soil N_min rate and Nit decreased with elevation and were correlated with several key soil parameters, such as soil TN and available C and N for all cropland. Our findings indicate that soil N_min from croplands should be considered in the evaluation of non-point source pollution at a regional scale.

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The Author(s) 2023. Published by Higher Education Press. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0)