应重视硝态氮同化过程在降低土壤硝酸盐浓度中的作用

在保证生产力条件下,采取合理的氮肥管理措施降低土壤硝态氮浓度对降低氮污染至关重要。当前,应用硝化抑制剂能够有效延缓铵态氮的硝化速率,进而降低土壤硝态氮淋溶损失和氮氧化物排放,但是其缺点显而易见:促进氨挥发并引起硝化抑制剂污染。好氧条件下,土壤硝态氮净变化量取决于产生(硝化)和消耗(硝态氮同化)的量。但是,一直以来,受微生物优先利用铵态氮这一传统观点的影响,人们普遍认为农田土壤微生物较少利用硝态氮,很大程度上忽视了对硝态氮同化过程的研究。该过程独具优势,它将硝态氮转变为微生物生物量氮进行短期储存并发生再矿化,具有保氮功能且环境友好。加入特定的碳源可以提高硝态氮同化这已是不争的事实,未来应加强硝态氮同化降低土壤硝酸盐累积方面的研究:(1)外源碳影响硝态氮同化的微生物驱动机制是什么?(2)怎样才能操控硝态氮同化和再矿化过程,使得作物氮需求和土壤氮供应相匹配,进而降低氮损失?(3)在碳源充足的条件下,反硝化作用亦会增强,如何才能做到在提高硝态氮同化的同时避免反硝化氮损失?

Role of Microbial Assimilation of Soil NO3- in Reducing Soil NO3- Concentration

NO₃^- accumulation in the soil would trigger N losses through runoff, leaching and N₂O emission. It is, therefore, of particular importance to take appropriate nitrogen (N) management strategies to reduce soil NO₃^- accumulation, and hence to enhance N use efficiency and reduce N losses to the environment. Application of nitrification inhibitor (NI) has been demonstrated to be effective in reducing soil NO₃^- concentration, NO₃^- leaching and N₂O emission, and simultaneously increasing crop yield. However, there is an indisputable fact that NI application increases ammonia (NH₃) emission and causes NI contamination. As a matter of fact, soil NO₃^- concentration varies with NO₃^- generation (mineralization and nitrification) and consumption (assimilation) rates in aerobic conditions. Under the influence of the viewpoint that soil microbes prefer NH₄ -N for their growth, it is commonly held that soil microbes rarely use NO₃^- in farmlands. Consequently, the study on processes of soil microbial NO₃^- assimilation has been neglected to a certain extent. The process of soil microbial NO₃^- assimilation is found to be unique in advantage. It turns NO₃^- into microbial biomass N for temporary storage before mineralization to be available to crops for a longer season or crops in the following season. There is no doubt that soil microbial NO₃^- immobilization is stimulated by specific extraneous C input, which deserves more attention in future studies concerning how to reduce soil NO₃^- accumulation. Further studies should primarily focus on the following several aspects: (1) to elucidate mechanism of the microbe driving NO₃^- assimilation under elevated C availability, (2) to explore how to control microbial assimilation and remineralization of NO₃^- to match soil N supply with crop N demand and reduce N losses, and (3) to explore how to avoid stimulating denitrification and associated N losses while enhancing microbial NO₃^- assimilation under the condition of sufficient C supply.