Influences of Long-Term Fertilizer and Tillage Management on Soil Fertility of the North China Plain

In the North China Plain, fertilizer management and tillage practices have been changing rapidly during the last three decades; however, the influences of long-term fertilizer applications and tillage systems on fertility of salt-affected soils have not been well understood under a winter wheat （Triticum aestivum L.）-maize （Zea mays L.） annual double cropping system. A field experiment was established in 1985 on a Cambosol at the Quzhou Experimental Station, China Agricultural University, to investigate the responses of soil fertility to fertilizer and tillage practices. The experiment was established as an orthogonal design with nine treatments of different tillage methods and/or fertilizer applications. In October 2001, composite soil samples were collected from the 0-20 and 20-40 cm layers and analyzed for soil fertility indices. The results showed that after 17 years of nitrogen （N） and phosphorous （P） fertilizer and straw applications, soil organic matter （SOM） in the top layer was increased significantly from 7.00 to 9.30-13.14 g kg-1 in the 0-20 cm layer and from 4.00 to 5.48-7.75 g kg-1 in the 20-40 cm layer. Soil total N （TN） was increased significantly from 0.37 and 0.22 to 0.79-1.11 and 0.61-0.73 g N kg-1 in the 0-20 and 20-40 cm layers, respectively, with N fertilizer application; however, there was no apparent effect of straw application on TN content. The amounts of soil total P （TP） and rapidly available P （RP） were increased significantly from 0.60 to 0.67-1.31 g kg-1 in the 0-20 cm layer and from 0.52 to 0.60-0.73 g kg-1 in the 20-40 cm layer with P fertilizer application, but were decreased with combined N and P fertilizer applications. The applications of N and P fertilizers significantly increased the crop yields, but decreased the rapidly available potassium （RK） in the soil. Straw return could only meet part of the crop potassium requirements. Our results also suggested that though some soil fertility parameters were maintained or enhanced under the long-term fertilizer and straw applications, careful soil quality monitoring was necessary as other nutrients could be depleted. Spreading straw on soil surface before tillage and leaving straw at soil surface without tillage were two advantageous practices to increase SOM accumulation in the surface layer. Plowing the soil broke aggregates and increased aeration of the soil, which led to enhanced organic matter mineralization.