Forage grass cultivation increases soil organic carbon and nitrogen pools in a karst region, southwest China

Whether conservation agriculture systems can increase soil organic carbon (SOC) and total nitrogen (TN) pools remains unresolved. We measured SOC and TN pools in maize–soybean rotation fields and three types of conservation agriculture systems with mature forests being selected for comparison in southwest China's karst areas. The three types of conservation agriculture systems were fields of mulberry, forage grass, and sugarcane. The objective was mainly to evaluate whether replacement of maize–soybean fields by the other three types of agricultural fields would significantly increase SOC and TN pools. Across the 0–30 cm soil profile, SOC and TN pools were not significantly higher in the mulberry and sugarcane fields than in the maize–soybean fields (47.42 ± 6.34 Mg C ha −1 and 5.47 ± 0.49 Mg N ha −1 ). Nevertheless, SOC and TN pools in the forage grass fields (74.12 ± 5.30 Mg C ha −1 and 7.09 ± 0.41 Mg N ha −1 ) were higher by 56.3 and 29.4% relative to the maize–soybean fields, but were lower by 38.3% and 21.8% compared to the forests, respectively. Microbial biomass C (MBC) and N (MBN) concentrations varied in similar patterns as SOC and TN pools. The MBC:SOC and MBN:TN ratios were higher in the forage grass fields, suggesting increased soil organic matter quality compared to the other types of agricultural fields. Our findings suggest that whether conservation agriculture can enhance SOC and TN pools largely depends on crop type with forage grass cultivation being identified as an efficient solution to promote SOC and TN pools.