Farmland Fertility And Soil Quality Comprehensive Assessment Based On Different Straw Returning Modes

As an important nature resource, soil provides fundamental material for human survival and development and supports roots extending and retaining. With the decreasing of organic manure input in agricultural production, ecological and environment problems(e.g. soil quality degradation, farmland productivity decline) are aggravated, which have a bad effect on food security. Guanzhong Plain is an important grain production base in Shaanxi, which is mainly occupied with wheat/maize rotation system. There are 8 million t wheat and maize straw produced annually in this area. With the improvement of agricultural mechanization and the banning of straw burning, straw returning to field become an important utilization way of straw resources in Guanzhong Plain. Nitrification inhibitors can adjust the nitrogen transformation process and improve nitrogen utilization efficiency; while microelements are also the important regulatory factors in the process of straw decomposing. Therefore the field and incubation experiments were conducted to explore the effects of different straw returning modes and combined application of those factors on soil fertility and farmland effect.Field experiment was conducted between June 2008 to June 2014 in experimental farm at Sanyuan, and wheat/maize rotation system was continually adopted in experimental site. Both wheat and maize had three straw returning ways, wheat straw high stubble returning(WH), wheat straw chopping returning to field(WC), wheat straw no returning(WN) and maize straw chopping returning to field with subsoiling(MM), maize straw rotary returning(MC), maize straw no returning(MN), there were nine straw returning modes by combining three wheat and three maize returning ways in one crop rotation period, and their effects on soil quality, physical and chemical properties, and farmland productivity were studied; incubation experiment was conducted by combined addition of maize straw, DCD and FeCl3 to explore their effects on soil carbon and nitrogen content. The main results were as follows:(1) 4 straw returning modes were selected in a whole rotation system to study its effects on soil farmland fertility. Soil organic carbon and nutrient contents stratified obviously, which gradually reduced with the layer deepening. WH-MC had the higher contents of soil total nitrogen, total phosphorus, available nitrogen, available phosphorus than other treatments, while its soil total nitrogen, available nitrogen, available phosphorus in the 0~10 cm and 10~20 cm were significantly higher than that of WN-MN treatment. Besides, WH-MC had the highest contents of soil organic carbon and active organic carbon, and was statistically different from that of the WN-MN treatment. Soil bulk density was varied among different treatments and soil layers, which was the lowest in WH-MC, and the highest in WN-MN, and the surface soil bulk density was lower than the deeper layer. However, soil moisture content and pH had little difference among the different treatments and layers.(2) Straw returning to field mainly influenced soil available nutrients and the active component of soil organic carbon, especially for soil available nitrogen, which was the most sensitive indicator reflecting the changes of soil quality. When straw returned to field, most carbon were mineralized and released in the form of CO2, which decreased its sequestration in the soil, and therefore reduced the increment of soil total organic carbon content. Using principal component analysis(PCA) method, cumulative variance contribution rate of three principal components reached 81.025%. Soil phosphorus, nitrogen, carbon, microorganism quantity and the bulk density had higher load coefficient in each principal component, which were important factors affecting soil quality; Based on the results of soil quality comprehensive evaluation, WH-MC had the highest soil quality comprehensive score and the optimal soil quality.(3) Soil CO2-C release rate in the treatments added with straw were increased rapidly in the early incubation period, and then gradually reduced and stabilized. Whereas, soil CO2-C release rate in the treatments without adding the straw were stable during the whole incubation period, and were lower than the treatments added with straw. During the early stage of incubation(4-17 d), addition of FeCl3 and DCD reduced soil organic carbon mineralization rate. Soil organic carbon cumulative mineralization rate were the lowest in control treatment(CK) and the highest in the straw combined addition of microelement and nitrification inhibitor treatment(SMD). The soil mineralization carbon concentration in the SMD treatment was significantly higher than the treatment only added with straw(S). The content of soil mineralization nitrogen was significantly higher in the treatment without adding straw than adding straw, and was the lowest when the FeCl3 and DCD were added. The addition of DCD reduced the soil mineralization nitrogen content. Compared with no microelement treatments, soil microbial biomass nitrogen content was higher in the treatments adding with microelement.Above all, conclusions can be made as followings: wheat straw high stubble returning/maize straw rotary returning(WH-MC) was more beneficial to supply the soil NPK nutrients and increase soil organic carbon content, improve soil bulk density and other physical-chemical properties, considering the results of soil quality comprehensive evaluation and crop yield, WH-MC can both increase crop yield and maintain relatively optimal soil quality, is the relatively optimal straw returning mode in Guanzhong Plain; Adding microelement(FeCl3) and nitrification inhibitor(DCD) increased the mineralization of soil organic carbon, microbial biomass nitrogen and soil ammonium nitrogen, improved the release of nutrients and had a certain role in increasing soil carbon and nitrogen.