Effect Of Straw Returning On Soil Fertility And Yearly Productivity In The Field Scale In Guanzhong Plain

According to local agricultural production status, 9 different combinations of straw returning model were conducted in Sanyuan Experiment Station of Northwest A & F University from June 2008 to October 2009 to study the effect of straw returning in field scale on soil fertility and yearly productivity. And also laboratory incubation experiment was combined to discover the straw decomposition conditions. The primary results are as follows:1. An incubation experiment was carried out to reveal decomposition characteristics of straw of maize and wheat and their effects on soil carbon and nitrogen contents. The experiment was lasted in dynamic microcosms for 32 days at 30℃with 8 treatments combined with 2 moisture levels, namely, relative water content of 60% (M60) and 80% (M80), and 4 straw levels, namely, maize straw addition, wheat straw addition, mixed addition and no addition. The results show that,1) there is a strong influence of soil water content on the decomposition of the both crop straw in soils, and the rate of CO₂ evolution at RCW 60% is lower than that at RCW 80% throughout the incubation period. Moreover, the ratios of CO₂-C derived from straw to the total released C from straw are 40.1% and 51.5% under the M60 and M80 treatments, respectively, and the increases of SOC content are 2.24 and 1.43 g·kg^-1 for the two treatments. 2) The rate of CO₂-C evolution from maize straw decomposition is consistently higher than that from wheat straw. In the treatments where maize straw are added, the cumulative amount of CO₂-C evolution and net SOC increase are 408.35 mg/pot and 2.12 g·kg^-1, and those for wheat straw additions are 378.94 mg/pot and 1.56 g·kg^-1. When the two types of straw are mixed with soil, the two values range between those of solely additions. (3) Compared to no straw addition at the end of 32 d incubation, the contents of SOC, microbial biomass C, total N and microbial biomass N in the straw addition treatments are significantly increased, and they are almost ranked in maize straw>maize and wheat straw>wheat straw. Therefore, it is more easily to transform the straw carbon into inorganic C (CO₂) than into SOC under high water condition, and maize straw decomposition is easier than that of wheat straw. In conclusion, the high contribution of straw returning to soil can be regarded as a supplement to soil carbon pool and nutritious elements including N, and microbe, and thus soil quality is improved.2. Effect of straw returning in field scale on soil organic carbon (SOC) and grain yield was studied to explore more reasonable farming management models, which was helpful to provide a theoretical basis for soil cultivation and sustainable agricultural development in Guanzhong Plain. With the actual situation of agricultural production in test area, the trends of SOC, active organic carbon (AC) content and grain yield in different treatments were examined to clear the effect of straw returning through a comprehensive comparison. It is show that after the returning of wheat straw to cropland, the summer maize was grown. The result showed that SOC in topsoil (0-40 cm) have been improved in straw application, and that soil AC and carbon pool management index (CPMI) of 0-20 cm soil layer were slightly lowered in high stubble in field, while soil AC and CPMI were increased through smashing the wheat straw into field, which didn't make their mutation change. Subsequently, the result showed that maize stalks were returned to the field in three models, and that, after the wheat growing season, SOC content in the topsoil has increased significantly through the straw returning treatment, and that, through smashing straw into field twice the SOC content can increase by 1.86 g·kg^-1 in 0-20 cm soil and 1.69 g·kg^-1 in 20-40 cm soil respectively. And also, the returning of crops stalk to the cropland can increase the AC content in the topsoil strikingly-----especially, when wheat straw is smashed into the field, the AC content in the topsoil can increase more strikingly, that is, a average AC content can increase by 0.90 g·kg^-1 respectively in 0-20 cm and 0.85 g·kg^-1 in 20-40 cm soil; And straw returning significantly increased CPMI, especially through smashing straw into field treatments. Under the conditions of climate in Guanzhong Plain, soil organic carbon condition could be improved significantly through returning straw to field scale, especially, the SOC quality can be improved. Therefore, in the short run, smashing and returning the straw to the field can change the SOC more obviously. For there easily appear seasonal drought in the study area, it was useful to improve water use efficiency in maize growth period effectively to ensure the stable or higher yield trough high-stubble of wheat straw in field. It was the optimal model combination of high stubble of wheat with maize straw chopping into field in a whole rotation period for productive and efficient agriculture in this region, which can make a full use of water and heat resources in Guanzhong Plain.3. Effect of straw returning in field scale on yearly productivity was studied to explore more reasonable farming management models, which was helpful to provide a theoretical basis for soil cultivation and sustainable agricultural development in Guanzhong Plain. With the actual situation of agricultural production in test area, the trends of soil fertility and yearly productivity were investigated to clarify the effect of straw returning through a comprehensive evaluation. It is showed that the nutrient content had a significantly change in the straw returning treatment, especially in 0-20 cm soil layer. It was difficult to analyze and directly compare effect of various combinations of returning for the nutrients showed different trends and levels. According to the comprehensive evaluation result, wheat straw chopping and maize straw no returning (WC-MN) and wheat straw mulching with high stubble and maize straw chopping (WH-MC) returning models obtained higher integrated soil fertility levels, and WH-MC showed the highest net production rate of living labor and yield/input ratio, while wheat and maize straw no returning (WN-MN) with low levels. In terms of yearly productivity, according to comprehensive evaluation of integrated soil fertility level and economic benefits, it was the best in WH-MC treatment. Integrate with the agricultural production practice, it was one of the optimal model combination of high stubble of wheat with maize straw smashed into field in a whole rotation period for productive, efficient and sustainable agriculture development in this region, which could make a full use of water and heat resources and maintain a high integrated soil fertility level in Guanzhong Plain.