Effects of Long-Term Fertilization on Soil Organic Carbon and Nitrogen in a Highland Agroecosystem

The effects of fertilization on the distributions of organic carbon (OC) and nitrogen (N) in soil aggregates and whether these effects vary with cropping system have not been well addressed. Such information is important for understanding the sequestration of OC and N in agricultural soils. In this study, the distributions of OC and N associated with soil aggregates were analyzed in different fertilization treatments in a continuous winter wheat cropping system and a legume-grain rotation system in a 27-year field experiment, to understand the effects of long-term fertilization on the distributions of OC and N in aggregates and to examine the recovery of soil OC and N in a highland agroecosystem. Manure fertilizer significantly decreased soil bulk density but increased the amount of coarse fractions and their associated OC and N stocks in the soils of both systems. Fertilizers N + phosphorus (P) and manure had similar effects on total soil OC and N stocks in both systems, but had larger effects on the OC and N stocks in > 2 mm aggregates in the legume-grain rotation system than in the continuous winter wheat system. The application of P increased the OC and N stocks in > 2 mm aggregates and decreased the loss of N from chemical fertilizers in the legume-grain rotation system. The results from this study suggested that P fertilizer should be applied for legume-included cropping systems and that manure with or without chemical fertilizers should be applied for semiarid cropping systems in order to enhance OC and N accumulation in soils.