Converting alfalfa pasture into annual cropland achieved high productivity and kept soil organic carbon in a semiarid area

Converting alfalfa (Medicago sativa L.) into annual cropland (rotated cropland, RC) is commonly done to reuse degraded alfalfa pasture. However, it is a big challenge for RC to achieve high productivity and maintain high soil organic carbon (SOC) to the levels achieved by previous alfalfa pasture. We converted a long-term alfalfa pasture into RC in 2010, and studied the effects of fertilization on soil moisture, crop productivity, and SOC in the RC and in continuous cropland (CC), both under plastic film mulching, from 2010 to 2018. Before the conversion, alfalfa pasture had a higher SOC (12.3%) and total N (7.7%) than CC. However, not only severe soil desiccation occurred, but the available P and inorganic-N were only about 39.4 and 25.1% of levels for CC. After the conversion, soil moisture in RC (measured at 0-0.6 m depth) was up to CC levels only after 1 year, and was restored to 95.3 and 69.2% of CC levels at 0.6-2 and 2-5 m depths only after 9 years. Crop yield and biomass in RCF (RC with fertilization) were comparable to those for CCF (CC with fertilization) after two years from conversion due to the increased available P and inorganic-N, and sufficient soil moisture restoration at 0-0.6 m depath. SOC in RC decreased by 8.1% over the 9 years, while RCF consistently kept SOC levels up to those of pre-conversion alfalfa pasture. These findings help to eliminate concerns about the continuously low production and rapid decline of SOC in croplands converted from alfalfa and suggest ways to support sustainable high-productivity and high SOC sequestration in dryland farming.

Automated summary

Converting alfalfa into annual cropland poses challenges in achieving high productivity and maintaining SOC levels, but fertilization can improve soil moisture, crop productivity, and SOC levels.