Changes in soil organic matter composition after introduction of riparian vegetation on shores of hydroelectric reservoirs (Southeast of Brazil)

This work is part of a research program with the general objective of evaluating soil sustainability in areas surrounding hydroelectric reservoirs, which have been planted with riparian forest. The specific aims were: (i) to assess if and how the soil organic matter (SOM) chemical composition has changed in such areas, and (ii) to contribute to the knowledge of SOM chemistry in Brazil. To this end, we sampled litter and soil (Anionic Acrustox) in two adjacent areas: one under native vegetation and another forested with riparian species in 1992. The native vegetation was Brazilian savannah orcerrado. In this case, it was a ‘grassy cerrado’, dominated by grasses with few shrubs. Litter was collected and humic substances were extracted from soil by an alkaline solution. Both were characterised by a combination of cross-polarisation-magic angle spinning (CPMAS) solid state 13C nuclear magnetic resonance (NMR) spectroscopy and pyrolysis-gas-chromatography/mass-spectrometry (Py-GC/MS). Eight years after forestation, the addition of the forest litter had changed SOM chemical composition. The C input pattern exerted a key role on the observed alterations. In the grassy cerrado, litter addition is predominantly below-ground and the litter is richer in carbohydrate-derived compounds and poorer in lignin moieties. In the forested area, C input is largely above-ground and grass litter has been partially replaced by a relatively more recalcitrant material. As a result, topsoil under forest was chemically strongly different from that under cerrado. Factor analysis indicated that the largest differences were between topsoil under forest and deepest subsoil under cerrado, where there is influence of remaining cerrado-derived C. Both semi-quantification and factor analysis of pyrolysis data gave further insight on the extent of alterations, but more research on such a quantitative approach should be developed to detail its application in SOM studies.