Soil nutrient and biological characteristics in North Xinjiang Oases as influenced by cropping patterns

The objective of this study was to investigate the effect of long-term continuous mono-cropping and rotation on soil physico-chemical characteristics and biological activities using typical field cropping patterns in the oasis farmlands of North Xinjiang. The physico-chemical characteristics and biological activities investigated include main soil fertility indices, soil enzyme activities, microbial biomass C (MBC), microbial biomass N (MBN) and soil respiration rate. The results show that the quality of soil fertility is significantly higher for the 0~20 cm than for the 20~40 cm depth layer in soils subjected to long-term continuous mono-cropping and rotation with legumes. On the contrary, soil pH values are significantly higher in the subsurface layer than in the top layer. Though different cropping patterns have no significant effect on soil pH, they remarkably affect soil respiration rate and main soil enzyme activities including those of alkaline phosphatases, urease, β-glucosidase, polyphenol oxidase and cellulose. Compared with other treatments, a 10-year wheat/sunflower rotation (WSR) gives a higher enzyme activity. MBC and MBN decrease with duration of continuous cotton mono-cropping (from 10 to 15 years); MBC/MBN ratios under 10 years of continuous corn mono-cropping (CrM), wheat mono-cropping for 10 years followed by another 10 years of cotton mono-cropping (WCtR), and 15 years of continuous cotton mono-cropping (CtM) and WSR are respectively 6.31, 6.02, 5.83 and 3.53. Laboratory incubation studies show that soil respiration decreases in the early stages and then stabilizes in the later stages of incubation (a total time period of 120 h) for all soils under different cropping systems. There are significant differences in soil respiration quotients among different cropping patterns, with the highest average soil respiration quotient of 20.54 μg(CO₂-C)·mg^-1(Bc)·h^-1 in WSR, and the lowest of 14.01 μg(CO₂-C)·mg^-1(Bc)·h^-1 in WCtR. In conclusion therefore, rotation with green manure is the best cropping system for improved soil biological activity.