黄土高原雨养区苜蓿地土壤质量变化的微生物响应机理

The Loess Plateau is one of the poorest regions in China. Predominant single cropping together with cultivation means that soil erosion here is the highest in China, which is the major threat to soil erosion and soil quality degradation. Lucerne (Medicago sativa L.), is the most nutritious perennial plant with the highest nutrient value as a forage, which played an important role in industrial structure adjustment and ecological environment construction in the Loess plateau. It is well known that soil microbial is more sensitive and potential as responsive indicators for soil quality evaluation, which play a critical roles in soil processes including nutrient cycling, the decomposition of organic materials and aggregates formation in the soil ecosystem. Based on a long-term field experiments conducted in Dingxi City and Qingyang City, Gansu Province,soil microbial functional diversity would be evaluated by Biolog technology on alfalfa field with different growth age. Soil microbial community structure and abundance would also be evaluated at species level using the Illumina Miseq sequencing technology. Soil physical and chemical properties would be measured as well. Relationship among selected soil properties and soil microorganism diversity would be analyzed. The results should help us to insight the key indicators to impact microbes, reveal the microbial mechanism of soil quality in response to growth age alfalfa field with different growth age. As a result, it could give a theoretical base for sustainable utilization of alfalfa field and sustainable productivity of rainfed farming systems for the western Loess Plateau.

西部黄土高原丘陵沟壑区是中国农村居民生活贫困的区域。作物单播和过度耕作是引致水土流失和土壤质量退化的最主要原因。紫花苜蓿作为优良豆科牧草，在该区生态环境建设和产业结构调整中发挥着重要作用。土壤微生物作为农田生态系统的重要组成部分，对土壤养分循环、有机质分解和团聚体形成等所有土壤活动均起着十分重要的作用，被认为是表征土壤质量变化最敏感、最有潜力的指标。本研究以不同种植年限人工苜蓿地为研究对象，采用基于Miseq的高通量测序技术和Biolog技术，研究土壤微生物丰度、群落结构以及功能多样性，并通过比较不同种植年限紫花苜蓿地土壤水分和营养物质的差异，明确影响土壤微生物群落结构和功能多样性的主控因子，解析不同种植年限苜蓿土壤质量变化的微生物响应机制，为西部黄土高原苜蓿草地的合理利用和雨养农业系统生产力可持续发展提供理论参考。

紫花苜蓿作为重要的豆科绿肥和牧草兼用作物，将其引入作物农作系统可以有效提高系统生产力，并对于黄土丘陵区脆弱生态修复起着极为重要的作用。但随紫花苜蓿生长年限延长其土壤质量和草地生产力会逐渐衰退。本项目依托设置在黄土高原雨养农业区不同种植年限紫花苜蓿长期定位试验，基于最小数据集法评价了不同种植年限紫花苜蓿地土壤质量，并运用障碍因子诊断模型确定影响土壤质量的主要障碍因子；采用Biolog-ECO法研究了土壤微生物对碳源的利用特征，并利用Illumina MiSeq高通量测序技术研究土壤细菌和真菌群落结构和多样性，鉴定出不同种植年限紫花苜蓿地和农田优势菌属及其差异菌属，探讨了土壤固氮菌群落结构随苜蓿种植年限延长而发生的演替特征及其与土壤氮组分的关系，基于生态零模型评估了土壤AMF群落群落组装机制；借助主成分分析（PCA）和冗余分析（RDA）等生物统计方法，明确了影响土壤微生物群落的主控因子，解析了不同种植年限紫花苜蓿地土壤质量变化的微生物响应机制。研究结果表明：1)长期种植紫花苜蓿显著提高了有机碳和全氮含量，但加剧了土壤水分和P素亏缺；土壤质量和紫花苜蓿产量在9~10年达到最大值之后逐渐下降，障碍因子诊断发现土壤速效磷和全氮是影响土壤质量的主要障碍因子。2）土壤微生物对碳水化合物、氨基酸、酚酸和胺类碳源的利用随紫花苜蓿种植年限的延长而下降，碳水化合物和氨基酸是引起土壤微生物碳源代谢分异的主要敏感性碳源。3）随紫花苜蓿种植年限延长，放线菌门（细菌）和子囊菌门（真菌）相对丰度提高，而酸杆菌门（细菌）和被孢霉门（真菌）相对丰度降低；RDA分析结果表明，土壤全氮是影响细菌群落的主要因素，土壤速效磷为影响真菌群落的主要因素。4)长期种植紫花苜蓿显著提高了土壤固氮微生物nifH基因丰度，改变了土壤固氮微生物nifH群落结构。与农田相比，长期种植苜蓿后土壤促生了固氮菌属、伯克氏菌属、弗兰克氏菌属、中慢生根瘤菌属、地杆菌属和慢生根瘤菌属等类群，同时也使得一些固氮菌群消亡，如梭状杆菌属、红假单胞菌属和Trichormus。5)球囊霉属(65.15%～99.12%)为紫花苜蓿和农田共有AMF优势属，主要贡献了不同处理土壤AMF群落结构的改变。基于生态零模型的群落组装过程表明，长期种植紫花苜蓿使AMF群落随机性过程增加，确定性过程下降，这有助于维持紫花苜蓿草地生态系统功能稳定性和可持续性。