Estoque de nutrientes na serrapilheira fina e grossa em função de fatores edáficos em florestas do Amazonas, Brasil

Abstract

The role of coarse wood debris (CWD) such as nutrient sink or reservoir and the processes that control the dynamics of this compartment have large importance on nutrient cycling in forest ecosystems. The dynamic and structure of Amazonian forests are associated with a variety of soils across de Basin, with contrasting physical and chemical soil properties. The interaction between these factors may influence the amount of litter standing crop (LSC) and CWD (coarse wood debris, trunks and branches with diameter ≥ 10 cm) produced by the forest, as well as control nutrient release and/or nutrient stocks contained in their tissues. The relation between soil properties and nutrients in necromass were tested in 49 plots of 0.5 ha in terra firme forests located along a approximately 700 km transect extending north – south from Manaus-AM. Forests north of Manaus correspond to plots in Ducke Reserve and BDFFP project, while forests south of Manaus, are located along the Purus-Madeira interfluve (BR-319), in soils with varying degrees of hydromorphism. Plots were grouped in nine site clusters, according to their geographical distribution, being seven along Purus-Madeira interfluve (M1, M2, M4, M5, M8, M9 and M11 site clusters) and two north of Manaus (Ducke Reserve and BDFFP). Soils were classified by their physical properties, due to the restriction level imposed to vegetation growth and also for fertility in the first 20 cm of soil. We evaluated nutrient concentration and stocks in fine and coarse wood debris, and also nutrient concentrations among decay classes of CWD. Forests in site clusters with high soil restriction levels had lower stock of dead wood (9.21 Mg ha-1 at M11 and 11.26 Mg ha-1 at M2) than forests with little or no physical restriction soils (24.08 Mg ha-1 at M8 and 23.64 Mg ha-1 at Ducke Reserve). Conversely, site clusters with more restrictive soils had higher fine litter stock (3.89 Mg ha-1 at M4 and 2.99 Mg ha-1 at M9) than forests with little or no soil restriction (1.87 Mg ha-1 at Ducke Reserve). The concentrations of Ca, Mg, K and P in both fine and coarse litter were higher along interfluve, and despite lower stocks of litter, also had the largest stocks of these nutrients, including in the soil. Higher concentrations of Na, Al, Fe, Zn and N were found in areas north of Manaus. Path analysis suggest that, for most elements, the actual CWD nutrient stock is controlled by the mass of CWD, with dead wood tissue concentrations being secondary. However, K, Zn and N showed a simultaneous effect of wood tissue concentration and CWD mass. Ca was the only element where dead wood tissue concentration was determinant of CWD Ca stocks, with CWD mass being of secondary importance. Throughout the process of decomposition, the general patterns of nutrient release or immobilization were similar in all site clusters, with exception of M1 and M11 for some nutrients, (Ca and Mg) due to the high initial tissue concentration in these sites. Substantial variation in nutrient stocks in the litter can be estimated at large scales through the association between physical and chemical properties of the soil.