Carbon, nitrogen and phosphorous contents, related enzyme activities and organic matter fractions of litter and soil in a terra firme forest in Central Amazonia

The response of lowland tropical forest on highly weathered P-limited soils to changes in atmospheric composition is likely to be regulated by P-availability from litter and soil. Our aim was to gain insight into possible mechanisms that may affect P-availability and C sequestration. We collected litter and soil samples along a transect in Central Amazonia. We examined litter and soil properties, determined C, N, and P contents and performed density fractionation to obtain organic-C, -N and -P fractions. To assess microbial demand and mineralization, we analysed C, N and P associated enzyme activities. The litter layer has an estimated turnover time of about one year or less and C, N and P contents were much smaller than in the mineral soil. Total soil C, N and P decreased with depth, while P was depleted relatively more. Most organic-C, -N and -P is stabilized by adsorption onto mineral surfaces. Non-adsorbed organic matter fractions were depleted of P. Nearly all organic-P appeared to be present in the mineral-associated fractions from where it, despite high phosphatase activities, appears not to be available for mineralization. Enzyme activities revealed that microbial C-demand in litter is higher than in soil, while P-demand was relatively high in soil and increased with depth. Hypothesized higher phosphatase activity under increasing atmospheric CO₂ concentrations may probably not alleviate P-limitation in these terra firme forests due to the limited availability of mineralizable organic-P.