Influence of soil texture and cultivation on carbon and nitrogen levels in soils of the eastern Indo-Gangetic Plains

The study was conducted to quantify the relationships between soil texture (silt + clay) and soil organic carbon (SOC) or total nitrogen (TN) under native forests and rice–wheat cropland in the eastern Indo-Gangetic Plains. Soils were collected from 99 native ecosystem sites and 22 cultivated rice–wheat (RW) sites in Nepal and Bangladesh at 15 cm depth increments to a depth of 60 cm. Surface layer (0–15 cm) samples were collected from an additional 266 cultivated sites in Nepal and Bangladesh. Positive relationships between soil silt + clay and both SOC and TN were established for each depth increment. The data were best fit by exponential functions which, together with bulk density values, were used to estimate SOC and TN stocks as a function of depth and soil texture. The conversion of forests to rice–wheat cropland resulted in mean reductions in SOC stocks of 55%, 32%, 17%, and 10% in the 0–15, 15–30, 30–45, and 45–60 cm depths, respectively. It appeared that fertilizer N inputs had affected mean TN stocks, as N loss upon conversion to cropland was only found for the 0–15 cm depth, and there were significant N gains below 30 cm soil in the cultivated rice–wheat fields. Soil silt + clay content had a large effect on losses of SOC with cultivation, which were estimated to increase from 10 to 33 Mg ha− 1 for the 0–60 cm depth over the silt + clay range from 250 to 1000 g kg− 1. The SOC losses on cultivation also represent the potential to sequester C in these soils, which is similarly dependent on soil texture. Between 76 and 87%, of the total (0–60 cm) C sequestration potential is estimated to occur in the top 30 cm of the soil profile over the range in silt + clay from 250 to 1000 g kg− 1. For the mean texture of rice–wheat soils, the C-sequestration potential was estimated to be 22.2 Mg C ha− 1 for the 0–60 cm depth, with 86% of this potential occurring in the top 30 cm of the soil profile.