Effect mechanism of long-term straw returning on shrinkage characteristic in typical Shajiang black soil

Shajiang black soil is a typical low yield soil and widely distributes in North China Plain, which is the main production area of winter wheat and summer maize. Being prone to swell-shrinking is one of the most typical attribute obstacle factors for the improvement of Shajiang black soil fertilizer, due to the higher clay content dominated by a typical 2:1 clay mineral like montmorillonites. In recent years, soil fertility and quality were obviously improved by straw returning which has been commonly implemented in the plain. This paper studied the influence mechanism of straw returning on the shrinkage and swelling characteristic of Shajiang black soil under a long-term localization experiment (From 2007 to 2017). Five treatments were designed in the field experiment: none fertilizer and straw control (CK), conventional NPK fertilizer (F), conventional NPK fertilizer with maize straw returning (MSF), conventional NPK fertilizer with wheat straw returning (WSF) and conventional NPK fertilizer with wheat-maize double cropping straw returning (WMSF). In May 2017, soil samples were collected by soil wreath knife before harvesting winter wheat. The intact soil was used to determine soil bulk density (SBD), filed moisture capacity (FC) and the coefficient of linear extensibility (COLE). The partial intact soil was saturated by water for 8 hours firstly, then dried to constant weight at 105 ℃. During the procedure, soil specific volume and mass water content were determined to fit with the three straight lines model. From the model, soil shrinking process was divided into the structural section, normal section and residual section, which represents soil macropore, mesopore and capillary pore, respectively. The results showed that soil total and available nutrition (nitrogen, phosphorous and potassium) levels increased as the following rule: CK