Deterioration of soil physicochemical properties, microbial biomass C, and N-mineralization due to replacement of sal forest with exotic species and rain-fed agriculture in East India

Abstract

Effects of conversion of natural forest dominated by Shorea robusta, supported on old alluvial soil, to exotic Eucalyptus tereticornis or Acacia auriculiformis plantation or to agricultural field for cultivation of rice, have been examined in respect of soil microbial biomass C, N and N-mineralization. Mean annual microbial C and N, respectively were 702 μg g -1 and 85 μg g -1 in the Shorea forest, 678 μg g -1 and 89 μg g -1 in the Acacia plantation, 627 μg g -1 and 73 μg g -1 in the Eucalyptus plantation and 349 μg g -1 and 60 μg g -1 in the cropland. Highest biomass was manifested during the summer and lowest in the rainy season in all the sites examined. Peak N-mineralization was obtained during the rainy season in the following order - the Shorea forest > the Acacia plantation > the Eucalyptus plantation > cropland. Thus, the Eucalyptus plantation appears to have lower soil microbial activity than the Shorea forest as well as the Acacia plantation. The control of microbial C and N on N-mineralization was maximum in the Shorea forest and minimum in the Eucalyptus plantation although correlation between microbial N and N-mineralization was insignificant in the Acacia plantation (may be due to localized chance fluctuations). Tendency to immobilize nutrients was maximum in the Eucalyptus plantation as evidenced by the low release of nutrients from microbial biomass during the rainy season. It was found that the Eucalyptus plantation soil was microbially more inert in terms of nutrient release from soil biomass and N-mineralization than that of the Acacia plantation as well as the natural Shorea forest. However, any type of exotic species plantation as well as agriculture have adversely and significantly affected the micro biota mediated soil fertility fabrication of the natural Shorea forest milieu.