Long-term effects of leguminous cover crops on biochemical and biological properties in the organic and mineral layers of soils of a coconut plantation

The total biomass production (above-ground) for the 12-year period varied significantly between the cover crops and ranged from 34.86 (calopo) to 90.43 (pueraria) Mg ha-1. Total N and C additions at the cover cropped (CC) site for the 12-year period were 0.97-3.07 Mg ha-1 and 16.90-43.34 Mg ha-1, respectively. Irrespective of layers, the levels of organic C, total N, organic substrates viz., dissolved organic C and N, labile organic N, water soluble carbohydrates, and light fraction organic matter-C and were markedly higher in the CC site compared to the control. Consequently, the levels of microbial biomass-C (CMIC), -N (NMIC) and -P (PMIC), net N mineralization rates, CO2 evolution, metabolic quotient (qCO2) and the activities of l-asparaginase, l-glutaminase and β-glucosaminidase were significantly higher in the CC site compared to the corresponding levels in the control site. Between layers, the levels of various chemical, biochemical and microbial parameters were consistently higher in the organic layers compared to the mineral layers at all the sites including control. Among the ratios of various microbial indices, the ratios of CMIC: organic C and CMIC: PMIC did not differ significantly between the layers and sites. However, the ratio of CMIC: NMIC was relatively higher in the mineral layers and control site. The variation in individual soil properties between layers and sites reflected the concomitant changes occurring in soil organic matter content. Apparently, microbial activity was limited by the supply of biologically available substrates in the mineral layers and the control site. Contrarily, the more direct supply of nutrients from decomposing plant litter and the indirect supply of nutrients from the mineralization of organic matter led to significantly higher levels of microbial biomass in the organic layers.