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⁻¹and 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.