外源碳对旱地土壤硝态氮同化速率的影响及其作用机理

Control of soil NO3- accumulation is of particular importance to reduce N losses in agricultural soil. Many previous studies have demonstrated that the application of nitrification inhibitor can inhibit the NO3- formation and thus reduce NO3- accumulation in soil. However, NO3- accumulation in soil is the result of competition between nitrification and NO3- consumption (NO3- immobilization and denitrification) processes. Thus, enhancing microbial immobilization of NO3- would also be vital in controlling soil NO3- concentrations. However, few studies have quantified microbial immobilization of NO3- and its controlling factors. It was found that the addition of sufficient glucose-C might stimulate microbial immobilization of NO3-. However, it is largely unknown whether the input of organic material (as C source) would enhance soil microbial immobilization of NO3-. In this project, 15N dilution techniques in combination with selective biomass inhibitors and modern molecular biotechnology were employed to calculate the NO3- immobilization rates and related mechanism of microbiology. The purposes are to 1) characterize the effects of the added organic material differing in C/N ratio on microbial assimilation of NO3- under a gradient of available-C conditions in upland soils prone to NO3- accumulation from the major grain producing areas in China, 2) evaluate whether the stimulation of microbial assimilation of NO3- following the input of organic material depends on soil NH4+ concentrations, 3) quantify the contribution of bacteria and fungi to microbial assimilation of NO3-. These results will provide deep insights into the reducing NO3- accumulation and enhancing N retention capacity in agricultural upland soils.

土壤硝态氮的累积与否是硝态氮产生和消耗过程彼此竞争的结果。已有研究通过抑制硝态氮的产生（如施入硝化抑制剂）减少土壤硝态氮的累积，但是国内外较少有研究关注硝态氮同化过程。硝态氮同化过程可以将硝态氮转化为微生物生物量氮，兼有保氮功能且环境友好。已有少量研究表明，加入优质碳源如葡萄糖可以提高硝态氮同化速率，然而目前尚不清楚有机物料作为碳源是否可以促进农田土壤硝态氮同化作用？本项目以我国粮食主产区易发生硝酸盐累积的农田旱作土壤为研究对象，采用15N稀释技术、选择性抑制剂和现代分子生物学技术相结合的方法，阐明不同C/N比的有机物料及其加入量对土壤微生物硝态氮同化作用的影响，明确铵态氮存在是否影响有机物料对土壤微生物硝态氮同化作用的刺激作用，揭示土壤细菌和真菌对硝态氮同化作用的相对贡献，以期为减少农田土壤硝酸盐累积和提高农田土壤保氮能力提供理论依据和技术支持。

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。土壤硝态氮的浓度及其累积与否是硝态氮产生和消耗过程彼此竞争的结果。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。因而至今尚未探明土壤硝态氮同化发生的机制及其关键控制因子。本项目以我国粮食主产区易发生硝酸盐累积的农田旱作土壤为研究对象，采用15N稀释技术、选择性抑制剂和现代分子生物学技术相结合的方法，研究外源碳输入对旱地土壤硝态氮同化速率的影响及其作用机理。主要研究结果如下：1）揭示了外源有机碳的数量和质量是控制土壤硝态氮同化速率的关键因子。对于简单碳源，当期输入量大于500 mg N kg-1或者对于复杂碳源，当其碳氮比大于18，它们的输入均可以显著提高土壤硝态氮同化；2）厘清了有机物料输入对酸性土壤氮生物同化和非生物固定的相对影响，发现动物粪便对酸性硝态氮同化的刺激作用强于秸秆，这可能是动物粪便不仅提供有机碳源还能缓解酸化的抑制作用；3）阐明了磷输入对缺磷土壤硝态氮同化速率的影响机制，发现磷加入可以提高缺磷土壤的硝态氮同化速率，进而证实了土壤硝态氮同化过程亦是磷限制的；4）建立了碳源输入下硝态氮同化速率计算方法的选择依据；对于难分解碳源，三种不同的硝态氮同化速率测定方法可以得出一致的结果，而复杂碳源输入下15N稀释法测定的硝态氮同化速率则偏高；5）构建了碳源数量及质量与硝态氮同化速率的定量关系。总体而言，我们的研究结果为减少农田土壤硝酸盐累积和提高农田土壤保氮能力提供理论依据和技术支持。