Biological factors influencing nitrogen transformation in wheat fields of lime concreted black soils and their response to different nitrogen supplications

Lime concretion black soil is a typical low-yield field soil in China. It has heavy clay structure and poor permeability, which cause imbalances in effective nutrient supply, low capacity soil nutrient supply and poor production performance. In order to improve crop yields, chemical fertilizer (especially nitrogen fertilizer) has been excessively applied during production seasons. This has led to wastage of agricultural resources and environmental pollution. Soil microbes have always played a predominant role in the processes of soil nitrogen transformation. To provide scientific basis for directional adjustments to control the processes of soil nitrogen transformation, improve nitrogen use efficiency and reduce related negative effects, the processes and mechanisms of nitrogen transformation driven by soil microorganisms were studied. A filed experiment was carried out from 2012 to 2015 in Xiangcheng, Henan Province, China. The experimental setup was a single factorial design with four nitrogen rates (0 kg·hm^-2, 120 kg·hm^-2, 225 kg·hm^-2 and 330 kg·hm^-2). The biochemical action intensity of soil nitrogen transformation microorganisms (ammonification, nitrification and denitrification), urease activity, protease activity, net nitrogen mineralization rate, and content of nitrate and ammonium nitrogen of rhizosphere soil were determined at different wheat growth stages to explore the biological factors influencing nitrogen transformation and their response to different nitrogen application in wheat fields of lime concretion black soils. The results showed that the active period of soil nitrogen transformation microorganisms and enzymes was from jointing stage to grain-filling stage. After that, the ammonification intensity, nitrification intensity, urease activity and protease activity decreased. Similarly, the soil net nitrogen mineralization rate reached the highest level at flowering stage. Except for urease activity which increased with increasing nitrogen application, the intensity of soil nitrogen transformation microorganisms and the enzymes activities reached the highest point under the 225 kg·hm^-2 nitrogen treatment, and then, decreased with further increasing nitrogen application (330 kg·hm^-2). Consistent with dynamic changes in soil nitrogen transformation microbes and enzymes activities, the contents of soil ammonium and nitrate reached the highest point at heading stage and flowering stage, respectively. Under moderate nitrogen application conditions, soil ammonium content had an increasing trend. But under excess nitrogen application, there was no significant enhancement in soil nitrate content. It was clear that the active period of soil nitrogen transformation was consistent with the critical period of nitrogen demand for wheat, which was beneficial for winter wheat growth. However, due to low nitrifying bacteria activity, nitrification capacity was limited. This, in turn, reduced nitrogen availability and increased potential risk of ammonia volatilization from soil. Increased nitrogen application was beneficial for soil nitrogen transformation, but only within a certain range. Excess nitrogen application (330 kg·hm^-2) was not conducive in terms of improving the capacity of supply or release soil nitrogen in lime concretion black soil.