Soil organic matter fractions after three decades of eucalypt cultivation in the Rio Doce Valley, Brazil

Little is known about the changes in different soil organic matter (SOM) fractions under short-rotation eucalypt plantations. This study aimed to evaluate the impact of eucalypt cultivation on C stocks of several organic matter fractions in soils previously covered with degraded pasture, as well as to identify which SOM fractions are more sensitive indicators of land use change impacts. The study was conducted at two sites (Belo Oriente and Virginopolis) with commercial short-rotation eucalypt plantations in the Rio Doce Valley region, Minas Gerais state, Brazil. The soil samples were analyzed for: total organic C (TOC), light fraction C (free light - FLL and occluded light- FLO), C associated to heavy fractions (sand - AR, silt + clay - S+A and clay - ARG), microbial biomass C (BM) and humic fractions C (fulvic acids- FAF, humic acid- FAH and humin- FH). Results indicate that overall organic C stockst in all SOM fractions were higher in soils of Virginopolis than those of Belo Oriente due to the lower annual mean temperature and higher clay content. Thus, soil C sequestration by eucalypt plantations was higher in Virginopolis (14.2 t ha(-1)) than in Belo Oriente (10.0 t ha(-1)), resulting in a C sequestration rate of 0.42 t ha(-1) yr(-1) and 0.29 t ha(-1) yr(-1), respectively. In Belo Oriente the eucalypt plantations also favored the increase in C stocks of the ARG, S + A and FH fractions. A similar pattern was observed for C of FLL, FAF and FAH in Virginopolis. Of all SOM fractions, the BM and the FLO were the least sensitive indicators of alterations in SOM three decades after land use changes. In this regard, TOC and C of FLL, FAF, FAH and FH were more efficient.