Is soil nitrogen mineralization important in agricultural intensive areas?

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. 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. 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. In addition, soil 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. Although numerous reports have documented the soil N_min rate under different land use, such as forestry, grasslands and cropland, 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 the N form in soil. 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. 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. In particular, soil N_min can be influenced by a number of factors, such as soil pH, soil moisture, total soil N (TN), total carbon (TC), soil C to N ratio (C/N), and different vegetation types. 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, 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.

This study provides 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. 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.

This study has been published on the Journal of Frontiers of Agricultural Science and Engineering in 2023, DOI: 10.15302/J-FASE-2023515