长期施肥下潮土中氮素微生物同化能力的区分与评价

Nitrogen (N) microbial assimilation could reflect the N retention capability of soils, which is an important aspect of soil fertility in agricultural ecosystems. It is known that N microbial assimilation is closely related with activities of soil microorganisms. Some studies showed that soil microbial community composition were changed under different long-term fertilization regimes, which could to some extent affect the N microbial assimilation process in the soil. However, the functions of different microbial communities involved in the N microbial assimilation process still remain unclear. Hence, we sampled soils from a 36-year fertilization field to conduct a simulated incubation experiment with 15N-labeling urea. This proposal was designed to investigate systematically the N microbial assimilation process and related driven mechanisms by the application of compound-specific 15N stable isotope analyses—15N labeling based LC/GC-MS, which can be used to differentiate the newly synthesized (new) amino acids and amino sugars from the native (old) portions in soil samples. We aimed at evaluating the effects of different long-term fertilization regimes on N microbial assimilation capability by the dynamics of "new" and "old" amino acid pools. The substrate utilization strategies of microorganisms are to be identified by the dynamics of the "new" and "old" amino sugars based on their different origins. The fungal and bacterial contributions to N microbial assimilation will be evaluated by the interpretation of relative variation of fungi and bacteria derived "new" amino sugars, in combination with the analysis of microbial community structure by PLFA method. The objective of this study is to reveal how long-term fertilization may influence N microbial assimilation capability, and the interactive relationships between soil microbial community composition and N microbial assimilation capability. This project will enrich our knowledge about N microbial assimilation process and related driven mechanisms, and will provide regulating fundamentals for sustainable managements of N in agricultural soils.

土壤氮素同化作用反映土壤的保氮能力，与土壤微生物的活动密切相关。长期施肥可能会通过改变土壤微生物群落结构影响氮素同化过程，但不同微生物群落在其中发挥的作用尚不明确，给定向调控微生物过程以促进氮素高效利用带来了困难。本项目以定位施肥36年的典型潮土为研究对象，添加15N-尿素进行室内培养，采用同位素示踪结合液相/气相色谱-质谱联用技术，将氨基酸和氨基糖区分为15N-尿素(新)与有机质(老)来源两部分。通过定量新老氨基酸“库容”的变化，评价长期施肥对潮土中氮素微生物同化能力的影响；利用新老氨基糖的转化动态及其异源性，结合土壤微生物群落结构分析，探讨长期施肥下微生物的底物利用策略，明确真菌和细菌对肥料氮素同化的相对贡献，阐明长期施肥下土壤微生物群落结构变化与氮素同化能力的关联及影响机制。项目的实施不仅可以丰富土壤氮素转化的微生物驱动机制的理论，同时又可为农田土壤氮素的有效管理及调控提供科学依据。

长期施肥可通过改变土壤微生物群落组成来影响氮素微生物同化作用，这可能是导致我国高强度集约化农业生产下土壤氮素调控能力下降的内在微生物机制。如何把微生物群落结构与底物同化能力关联起来，区分开不同微生物在氮素同化过程中的作用和相对贡献，是研究土壤氮素微生物转化过程及调控机制的核心科学问题。本项目采集了长期（近40年）定位施肥的典型潮土，分析了长期施肥下土壤真菌和细菌残留物、氨基酸库、微生物群落结构以及酶活性的差异，向长期施肥土壤中添加15N标记氮肥进行培养，运用氨基糖和氨基酸的15N区分技术，研究了长期施肥下潮土中氮素微生物同化能力，区分了真菌和细菌对氮素同化的相对贡献。研究结果表明：长期施肥显著提高土壤总氨基糖、氨基葡萄糖、氨基半乳糖、胞壁酸等含量以及真菌与细菌残留物比值，总体来看以MNP处理提升效果最显著；PLFA分析表明，长期施用有机肥使真菌与细菌比、好氧菌与厌氧菌比显著降低，但显著提高了革兰氏阳性菌与革兰氏阴性菌比；施用化肥处理、有机肥处理明显降低了土壤微生物的代谢熵，显著提高了β-葡萄糖苷酶、纤维二糖苷酶、乙酰氨基葡萄糖苷酶活性。长期施肥显著影响氨基酸“库容”的大小，有机无机肥配施可提高土壤15N标记氨基酸总量，即提高外源氮素同化能力，以MNP处理15N标记氨基酸总量最高。15N标记氨基葡萄糖和胞壁酸的比值反映出不同施肥处理中真菌和细菌对外源氮素的同化贡献不同。与施用化肥相比，施用有机肥在一定程度上提高了细菌对外源氮素的同化作用。本研究结果可在一定程度上丰富土壤氮素转化的微生物驱动机制的有关理论，有助于进一步了解我国高强度集约化农业生产下土壤氮素调控能力下降的深层次原因，这对于寻求土壤氮素的调控管理措施具有重要的实践意义。