Research advances on effects of conservation tillage practice on soil nitrogen component

Abstract: Soil nitrogen pool is an important indicator to assess soil quality and fertility of farmland. At the same time, soil nitrogen pool is related to the sustainability of agricultural production, attracting widespread attention due to its importance in soil-plant systems. Soil total nitrogen (STN) is often used to quantify the size of soil nitrogen pool. However, due to its large inventory and small coefficient of variation, STN cannot accurately reflect the variation of soil nitrogen pool. In order to accurately assess the capacity of soil nitrogen supply and further clarify the mechanism of soil nitrogen pool variation, soil nitrogen pool can be divided into fractions with different characteristics according to the sources and forms, which reflect the nature or function of the soil inhomogeneous nitrogen distribution. Conservation tillage (CT) practice is considered to be one of the climate-smart agricultural management practices, being able to effectively influence the soil nitrogen pool and its fractions. In recent years, studies on CT and its ecological effects have been on an upward trend. Increasing studies has been carried out to investigate the effects of CT on soil nitrogen pools. However, most of these studies focused on the effects of CT on soil total nitrogen content instead of soil nitrogen fractions. Research on the effects of CT on soil nitrogen fractions is still at an early stage. This review integrated the international studies concerning the effects of CT on liable nitrogen fractions, mineralization-related nitrogen fractions, and easily-lost nitrogen fractions. Particular organic matter nitrogen and light organic fraction nitrogen, which represent the liable nitrogen fraction, are sensitive to tillage practice. Microbial biomass nitrogen and potential mineralizable nitrogen are easily affected by CT, which could affect nitrogen mineralization ultimately. Dissolved organic nitrogen and extractable organic nitrogen are easy to lose in agriculture system, whereas CT is an effective method for cutting down soil nitrogen loss. However, the effects of CT on nitrogen fractions and the relationship between nitrogen fractions and nitrogen mineralization have been not totally clear, based on which the directions of the following studies are proposed. In combination with soil biological properties (such as soil microbial activity and soil nitrogen associated enzyme activity), the effects of CT on soil nitrogen fractions need to be further revealed. Based on the isotope labelling, the effects of mineralization and nitrogen absorption in crops should be analyzed to determine the relationship between nitrogen fractions and crop nitrogen uptake under CT in the soil-crop system. Increasing studies showed that the amount of soil nitrogen failed to accurately reflect the availability of soil nitrogen. The absorption of nitrogen by crops is greatly affected by the nitrogen diffusion. Thus, to figure out the effects of CT on soil nitrogen diffusion and the diffusion of nitrogen fraction, is of great importance for further research concerning the nitrogen absorption by crops.