氮素低强度持续供应对秸秆降解过程及土壤团聚体周转的影响及作用机制

Plant residue return is one of the most recommended methods to increase the soil aggregate stability and soil fertility. Application of nitrogen (N) fertilizer can alter the residue decomposition, and the excessive and inappropriate application of N fertilizer reduced the enhancement of the residue return to soil aggregate stability. A modified N supply in a persistently low intensity manner would change it. However, the effects and mechanism of the modified N supply on the residue decomposition and soil aggregate turnover remain unclear. Controlled-release urea and conventional urea were used in the study to get the different manners of N supply. The study would be conducted in soil incubation and field plots, respectively, to detect soil microbial parameters, residue decomposition rate, soil organic carbon contents and characteristics, and soil N component. With the help of 13C isotope labelling technique, high-throughput sequencing, FT-IR spectrometer with a photoacoustic accessory, and statistical analysis, the key process of soil microorganism and several soil physical-chemical factors would be used to build a preliminary theoretical model to reveal the influence mechanism of N supply in a persistently low intensity manner on residue decomposition and soil aggregate turnover. The implementation of the project not only provides scientific support of N fertilizer and crop management to improve soil aggregate stability, but also provides theoretical basis for development and application of slow/controlled release fertilizer.

秸秆还田是提高土壤团聚体稳定性和土壤肥力的常用方法。施用氮肥可以调节秸秆降解过程，传统氮肥大量及不合理施用会削弱秸秆还田对土壤团聚体稳定性的提升作用，通过构建氮素低强度持续供应模式有望改变这一现状，但是目前关于这种氮素新供应模式对秸秆降解过程及土壤团聚体周转的影响尚不清楚。本项目拟采用包膜控释尿素和传统尿素模拟构建氮素不同供应模式，利用养分控释技术，借助13C同位素标记、红外光声光谱和高通量测序等手段，分别在室内土壤培养体系和田间小区作物轮作体系，揭示氮素低强度持续供应对秸秆降解过程和土壤团聚体周转的影响，明确驱动秸秆降解的主要微生物群落；综合土壤微生物、秸秆降解速率和产物、土壤不同活性有机碳和土壤氮素等指标，阐明影响团聚体周转的微生物作用机制。通过本项目的实施，有望为秸秆还田条件下合理施用氮肥以提升土壤质量提供理论基础，并可为缓/控释肥料的研制和应用提供技术支撑。

土壤团聚体是土壤结构的最基本单元，氮肥可通过调节秸秆降解过程和土壤激发效应进而影响土壤有机碳含量。本项目利用控释氮肥构建氮素低强度持续供应模式，在室内土壤培养利用同位素标记手段分析秸秆降解及土壤团聚体粒径分布和稳定性，并利用高通量测序手段测定微生物群落以探究其影响机制，同时结合田间小区试验验证培养试验结果。初步得到以下结果：（1）对添加秸秆的紫色土来说，施氮处理在培养周期内均能够提高土壤呼吸，低氮用量和控释氮有利于秸秆的降解，也增加了土壤本底碳的激发；高氮强度不利于秸秆降解，并且降低了土壤激发效应。对潮土来说，培养前期施氮能够提高土壤呼吸，培养后期速效氮抑制秸秆碳释放，并且中高量速效氮施用能够降低激发效应。（2）室内条件下，添加秸秆提高了紫色土和潮土及大部分粒径团聚体有机碳的含量，并且提高了潮土团聚体的MWD值，对紫色土团聚体的MWD值没有显著影响。不同供氮模式改变了混合土体和个别粒径团聚体碳有机碳含量，但在培养结束后对两种土壤团聚体的稳定性均没有显著影响。（3）土壤培养结束后，添加秸秆能够显著降低了紫色土细菌群落的丰富度，增加了多样性指数，控释氮肥提高了细菌多样性指数；秸秆和氮肥效应对潮土细菌丰富度和多样性均没有影响。添加秸秆降低了两种土壤真菌的多样性指数，施氮对两者的α多样性没有显著影响。PCoA结果表明秸秆对两种土壤的细菌和真菌群落有显著影响，氮肥效应不显著，但施氮改变了部分种群的相对丰度。（4）田间条件下，长期秸秆覆盖显著提高了土壤碳氮全量及活性组分，并且显著影响了土壤中与碳氮转化相关的部分细菌种群，但秸秆短期还田时控释氮肥对土壤团聚体和作物产量并没有显著影响。综上所述，短期内低强度氮素供应对秸秆降解、土壤激发效应和土壤有机碳有一定影响，但对土壤团聚体影响并不显著，长期施用是否会有影响需要进一步长期定位观测。