Organic matter distribution from the biomass to the soil in grassland, forest, and mixed ecosystem.

Abstract

<p>Soil organic matter (SOM) is made up of different components that are contributed by plant residues and living microbial biomass. The quantitative estimation of the above-ground biomass is mostly done using the molecular proxies (n-alkane) and δ<sup>13</sup>C values of SOM. However, the estimations can be site-specific and can vary depending on the contribution from the C3 and C4 plants. In a need to understand the transfer of biomass signals from the vegetation to soil, sampling sites (1mX1m) were chosen which comprises a pure grassland (C4), forest land (C3), and a mixed vegetation ecosystem from the lower-Gangetic floodplain. The  δ<sup>13</sup>C values of the above-ground biomass in grassland, forestland, and mixed ecosystem show a variation of 4‰, 7‰, and 20‰, respectively. In the associated soil, however, the incorporation of organic matter from the vegetation was not straightforward and showed a variation between +1‰ and -8‰ in three different sites. The values were <sup>13</sup>C-enriched in soil underlying the Grassland and depleted in forest soils. The n-alkane molecular proxies in the soil such as CPI, ACL, show a decrease (about 50%) and increase in LMW/HMW concentrations in values among different sites. The decrease in molecular proxies was evident due to differences in organic matter contribution from different species. In C3 forest, the difference in degradation from different components of trees (twigs, leaves, flowers, fruits) reduces the molecular proxies and also the  δ<sup>13</sup>C values in the soil. On the other hand, grasses acting as a whole, impart limited modification during incorporation into the soil. </p>