Wednesday, November 15, 2006

Effect of Land Use Practices on Soil Organic Carbon and Nitrogen.

Baiqun Wang and Jingshui Wu. Institute of Soil and Water Conservation, Chinese Academy of Sciences, 26 Xinong Road, Yangling, China

Alfalfa is an important forage crop grown in the Loess Plateau of China. It is unclear what impact dryland alfalfa has on the dynamics of soil organic carbon (SOC) and total Kjeldahl nitrogen (TKN) in this region. The objective of this study is to examine the dynamics of SOC, TKN and their distributions in the particulate and mineral-associated forms under alfalfa. A field study with randomized complete block design and three replications of bare land without fertilizer (BL), winter wheat without fertilizer(W), alfalfa without fertilizer (AWF), and alfalfa receiving annual applications of manure, nitrogen, and phosphorus fertilizer (AF) was established in 1984 in the southern highlands of the Loess Plateau. Soil samples were taken at a depth of 0-20 cm from each plot in 2000. The level of SOC and TKN in AWF increased compared to BL and to W. The particulate organic carbon (POC) and particulate organic nitrogen (PON) did not demonstrate significant changes whereas the mineral-associated organic carbon (MOC) and mineral-associated organic nitrogen (MON) increased in AWF compared to BL and to W. The status of SOC and TKN in AF showed a significant rise compared to AWF. The increased SOC and TKN were almost evenly distributed in the POC, MOC, PON and MON fractions in AF. The SOC decreased in BL compared to the initial SOM. The SOC level remained stable in W. The mean annual sequestration rate of SOC was the highest in AF. These results suggest that POC and PON can significantly indicate changes in SOC and TNK in fertilized alfalfa dryland. Fertilizing alfalfa seems to be a sustainable way to sequester organic carbon and nitrogen and to improve soil fertility in this area.