Soil microbial activity and biomass is stimulated by leguminous covercrops

The leguminous cover crops Atylosia scarabaeoides (L.) Benth., Centrosema pubescens Benth., and Pueraria phaseoloides (Roxb.) Benth., were grown in the interspaces; of a 19 y-old coconut plantation and incorporated into the soil at the end of the monsoon season every year. At the end of the 12th year, soils from different depths were collected and analyzed for various microbial indices and their interrelationships. The objectives were to assess the effects of long-term cover cropping on microbial biomass and microbial-community structure successively down the soil profile. In general, total N (TN), organic C (OC), inorganic N, extractable P, and the levels of biological substrates viz., dissolved organic C (DOC) and N (DON), labile organic N (LON), and light-fraction organic matter (LFOM) C and N decreased with depth at all the sites. Among sites, the cover-cropped (CC) sites possessed significantly greater levels of TN, OC, DOC, DON, and LON compared to the control. Consequently, microbial biomass C (MBC), N (MBN), and P (MBP), CO2 evolution, and ATP levels, in general, decreased with depth at all sites and were also significantly higher in the CC sites. Among the ratios of various microbial indices, the ratio of MBC to OC and metabolic quotient (qCO(2)) declined with depth. Higher MBC-to-OC ratios and large qCO(2) levels in the surface soils could be ascribed to greater levels of readily degradable C content and indicated short turnover times of the microbial biomass. In contrast, the ratios of MBC to MBN and MBC; to MBP increased with depth due to low N/P availability and relatively higher C availability in the subsoils. Cover cropping tended to enhance the ratios of MBC to OC, MBC to MBN, MBC to MBP, and ergosterol to MBC and decreased the ATP-to-MBC ratio at all depths. The relatively lower ATP-to-MBC ratios in the CC site, especially in the subsoil indicated microbial-community structure possibly dominated by fungi. By converting the ergosterol content to fungal biomass, it was observed that fungi constituted 52%-63% of total biomass C at the CC site, but only 33%-40% of total biomass C at the control site. Overall, the study indicated that leguminous cover crops like P. phaseoloides or A. scarabaeoides significantly enhanced the levels of OC, N and microbial activity in the soils, even down to 50 cm soil depth.