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

GRAPHICAL ABSTRACT ABSTRACT 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


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 (NO 3 --N) and ammonium nitrogen (NH 4 + -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][6][7] , grasslands [8] and cropland [9][10][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.

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.Table 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] .

Design of the incubation trial
Sixty Erlenmeyer flasks (250 mL) each containing 20 g of ovendried 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 NH 4 + , NO 3 − , dissolved organic carbon (DOC) contents and soil pH.

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 NH 4 + and NO 3 − ).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).NH 4 + -N and NO 3 − -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.

Soil organic N mineralization calculation
Soil dissolved inorganic nitrogen content (DIN) is expressed by  Note: SOC, soil organic carbon; TN, total nitrogen content; NH4 + , ammonium nitrogen content; NO3 -, nitrate nitrogen content; and DOC, dissolved organic carbon concentration.

Table 1 Topsoil (0-20 cm) initial physicochemical properties of soil from five cropland fields
The values are presented as mean ± standard deviation (n = 3).
the sum of NH 4 + -N plus NO 3 − -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.

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.

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.

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 NH 4 + -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(a)).

Soil N min
The soil N min quantum and rate differed among the studied fields (Table 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

Soil nitrification
The soil Nit rate showed different dynamics among the studied cropland (Table 3).The soil Nit rate ranged from 434.5 to

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 (Table 4, Fig. 5, and Fig. 6).
Table 5 shows that the soil N min rate was positively correlated with soil TN, NH 4 + and DOC.In particular, the soil N min rate increased linearly with increasing soil NH 4 + and DOC contents (Fig. 5).Significant positive correlations were observed between Nit and soil TN, DOC, NH 4 + and NO 3 -for all sampled fields (Table 4).Similarly, the increase in Nit was associated with the increase in soil TN, DOC, NH 4 + and NO 3 - contents (Fig. 6).In particular, the increased soil N min rate was associated with the increase in soil Nit (Fig. 7).

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.Table 5 shows that the research on soil N min has mainly focused on crops and forests over the past 30  Note: TOC, soil total organic carbon; TN, total nitrogen content; C/N, the ratio of TOC to TN; DOC, dissolved organic carbon concentration; NH4 + , ammonium nitrogen content and NO3 -, nitrate nitrogen content.* Significant correlation (P < 0.05) and ** highly significant correlation (P < 0.01).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.

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

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 NH 4 + (Table 5).
Likewise, the increased Nit rate was associated with increases in soil TN, DOC, NH 4 + and NO 3 -contents (Fig. 6).In general, soil TOC, TN, DOC, NH 4 + and NO 3 -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.

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 NO 3 --N and NH 4 + -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 NH 4 + -N derived from organic N min can be oxidized into NO 3 --N by the microbial Nit process.Therefore, a strong N min may facilitate the conversion of high amounts of NH 4 + -N to NO 3 --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.

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.

Fig. 1
Fig. 1 Locations of sampled cropland fields; A, B, and C were maize fields, D was a flue-cured tobacco field, and E was a vegetable field (审图号: GS京(2023) 2266号).

Fig. 2
Fig. 2 Content of soil total organic carbon (TOC), total nitrogen (TN), and the ratio of TOC to TN for five cropland fields.Different lowercase letters represent significant difference (LSD, P < 0.05), while the same lowercase letters represent no significantly difference between cropland fields.
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.

Fig. 5
Fig. 5 Relationships between soil dissolved organic carbon (DOC) (a) and NH 4 + (b) and N mineralization rate for five cropland fields.

Fig. 7
Fig. 7 Relationships between soil nitrification and N mineralization rate for five cropland fields.

Table 2 Topsoil (0-20 cm) organic N mineralization rate during the 30 days incubation duration for five sampled fields
Note: DIN, soil dissolved inorganic nitrogen.The values are presented as mean ± standard deviation (n = 3).Means followed by different lower case letters represent significantly different (LSD, P < 0.05) and means followed by the same lower case letters represent no significantly difference between cropland fields.

Peng XU et al. Soil nitrogen mineralization in croplands and its regulatory factors 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.