Soil carbon and nitrogen sequestration following the conversion of cropland to alfalfa forage land in northwest China

Soil C and N contents play a crucial role in sustaining soil quality and environmental quality. The conversion of annually cultivated land to forage grasses has potential to increase C and N sequestration. The objective of this study was to investigate the short-term changes in soil organic C (SOC) and N pools after annual crops were converted to alfalfa (Medicago sativa L. Algonguin) forage for 4 years. Soil from 24 sets of paired sites, alfalfa field versus adjacent cropland, were sampled at depths of 0-5, 5-10 and 10-20 cm. Total soil organic C and N, particulate organic matter (POM) C and N were determined. Organic C, total N, POM-C, and POM-N contents in the 0-5 cm layer were significantly greater in alfalfa field than in adjacent cropland. However, when the entire 0-20 cm layer was considered, there were significant differences in SOC, POM-C and POM-N but not in total N between alfalfa and crop soils. Also, greater differences in POM-C and POM-N were between the two land-use treatments than in SOC and total N were found. Across all sites, SOC and total N in the 0-20 cm profile averaged 22.1 Mg C ha(-1) and 2.3 Mg N ha(-1) for alfalfa soils, and 19.8 Mg C ha(-1) and 2.2 Mg N ha(-1) for adjacent crop soils. Estimated C sequestration rate (0-20 cm) following crops to alfalfa conversions averaged 0.57 Mg C ha(-1) year(-1). Sandy soils have more significant C accumulation than silt loam soils after conversion. The result of this suggests that the soils studied have great C sequestration potential, and the conversion of crops to alfalfa should be widely used to sequester C and improve soil quality in this region. (c) 2006 Elsevier B.V. All rights reserved.