Rhizospheric Soil Quality And Its Microbial Mechanism Of Rotation System Under Conservation Tillage On The Loess Plateau

In the Loess Plateau of China, arid climate, low and unbalanced rainfall, thin vegetation cover, highly erodible soil as well as unsuitable and intensive cultivation have resulted in lower agricultural productivity, vegetation deterioration and severe soil erosion. Therefore, it is urgent to find ways to fundamentally control soil erosion and establish some vital and suitable ecological systems in this area. No-tillage and residue retention can effectively reduce soil erosion, improve arable land quality and protect food safety, is an efficient way for the sustainable development of agricultural production and the protection of environment in the Loess Plateau. However, how rhizospheric soil quality changed and why from the viewpoint of microbial level under long-term no-tillage and residue retention managements were still not very clear. In this study, the effects of long-term tillage and crop residue retention on rhizospheric soil quality, soil microbial activity, soil microbial community structure and functional diversity were investigated in a maize(Zea mays L.)-winter wheat (Triticum aestivum L.)-soybean (Glycine max L.) rotation system which was established in2001on the Loess Plateau, Gansu Province, China. The objectives of this research are to reveal the effect of long-term no-tillage and residue retention managements on rhizospheric soil quality, elucidate the microbial mechanism of rhizospheric soil quality change, and uncover the link between rhizospheric soil and rhizospheric soil microbes.1) Crop residue retention led to significant increase in the contents of organic carbon and total nitrogen of rhizospheric soil under long-term maize-winter wheat-soybean rotation system, and also in the contents of rhizospheric soil NH4+and NO3-under maize and soybean at maturity. Compared to conventional tillage, conservation tillage significantly increased available phosphorus content in the rhizospheric soil. No-tillage significantly increased total phosphorus content in the rhizospheric soil except that under maize at maturity. There was no significant effect of conservation tillage on total potassium content in the rhizospheric soil, while higher available potassium content was observed with crop residue retention compared to that without retention. Long-term conservation tillage didn’t significantly influence the contents of Ca, Mn, Zn, Fe, and Cu in the rhizospheric soil. It suggested that the nutrients in the rhizospheric soil have been modulated by long-term conservation tillage under mazie-winter wheat-soybean rotation system. The contents of organic carbon, total nitrogen, total phosphorus, mineralized nitrogen, available phosphorus and available potassium were significantly greater under conservation tillage than under conventional tillage, while trace element contents didn’t significantly change.2) No-tillage and crop residue retention both significantly increased the contents of microbial biomass carbon and microbial biomass nitrogen in the rhizospheric soil under maize and winter wheat. MBC and MBN contents under soybean were higher with residue retention than without retention. Soil respiration rate and soil temperature of10cm depth was significantly reduced under no-tillage. No-tillage and crop residue retention both significantly led to the decrease in qCO2under the rotation system. The amounts of fungi and actinomycetes were higher with no-tillage and residue retention under winter wheat and soybean than under conventional tillage. No-tillage significantly increased the activities of catalase, urease and sucrase, and crop residue retention led to significant increase in cellulose activity. It showed that the microbial activity, microbial amount, and soil enzyme activity in the rhizospheric soil were promoted by long-term conservation tillage under maize-winter wheat-soybean rotation system.3) No-tillage significantly increased total amount of phospholipid fatty acids in the rhizospheric soil. Crop residue retention led to the increase in the ratio of fungus vs. bacterium under winter wheat, and no-tillage did the same under maize and winter wheat at maturity. PLFA amounts of G+and the ratio of PLFA amounts of G-vs. G+were not significantly affected by tillage management, crop type and crop growth stage, no-tillage significantly increased the PLFA amounts of G-at crops maturity. It suggested that long-term conservation tillage can obviously affect microbial community structural diversity in the rhizsopheric soil under maize-winter wheat-soybean rotation system, and it is regulated by crop type and crop growth stage.4) The capability to use carbon substrates of the rhizospheric soil microbes was enhanced significantly by no-tillage and residue retention. No-tillage led to the improvement of the utilisation of amino acids, polymers, carboxylic acids, and carbohydrates by the rhizospheric soil microbes under maize and winter wheat at maturity. Maize residue retention significantly increased the utilisation of amines and carbohydrates, and decreased the utilisation of carboxylic acids and polymers by the rhizospheric soil microbes under winter wheat. The utilisation of amines and carboxylic acids by the rhizospheric soil microbes was enhanced under soybean with winter wheat residue retention, while miscellaneous utilisation was significantly retarded. It showed that microbial community functional diversity in the rhizospheric soil was significantly affected by long-term conservation tillage under maize-winter wheat-soybean rotation system. Miscellaneous, carboxylic acids and carbohydrates were main carbon sources to discriminate microbial community functional diversity in the rhizsopheric soil with conservation or conventional tillage.