Soil nitrogen and microbial biomass carbon dynamics in native forests and derived agricultural land uses in a humid tropical climate of India

Abstract

Conversion of tropical forests to agricultural land uses is known to alter soil nitrogen (N) transformation processes and microbial biomass carbon (MB-C ), which affect productivity and stability of the derived land uses. Information about how conversion of moist evergreen forest to agricultural land uses affects soil nitrogen (N) transformation processes and MB-C in hot humid tropics is meager. The present study explores the following questions: (1) how does conversion of native moist evergreen forest to agricultural production systems (grassland, home garden and silvopasture) affect net soil N mineralization rates, pools of mineral nitrogen (NH4+- N and NO3-- N) and MB-C in the hot humid tropical climate of South Andaman, India and (2) are changes in soil N transformations and MB-C related to differences in soil moisture and temperature induced with forest conversion? Variations in net soil N mineralization, NH4+- N and NO3−- N pools, MB-C and soil temperature and moisture were measured across wet, post-wet and dry seasons at ten sites of each derived agricultural land use and native forest. In addition, inputs of carbon (C ) and N to the soils and outputs of C and N from the land use systems through harvests were also measured. We measured the N mineralization rate by the buried bag technique using 2 M KCl as an extractant. The NO3−- N and NH4+- N were measured at the beginning and end of incubation. The MB-C was measured by a chloroform fumigation-extraction method. We found that forest conversion resulted in a decline in carbon input, but caused rise in soil temperature (from 0.4 to 9.8% across the seasons) in derived agricultural land uses compared to native forest. The soil temperature increase was the highest in the grassland and lowest in the home garden. Across the three seasons, net soil N mineralization rates increased 27 to 55 % in the derived agricultural land uses compared to the native forest, with the increase highest in the grassland and lowest in the home garden. Soil organic C (SOC), MB-C, and NO3- declined in the derived agricultural land uses relative to native forest, with the greatest effect again seen in the grassland. These observations suggest that native forest conversion to agricultural uses results in lower soil organic C content over time, due to increased mineralization rates stimulated by a rise in soil temperature, and that these soil changes may be most pronounced in grasslands. Therefore, tree based land uses offer good options for soil carbon build up and protection against N loss in the hot humid tropics.