Influência da presença de raízes finas e disponibilidade de carbono lábil nos processos de decomposição de serapilheira na Amazônia Central

Abstract

The high presence of fine roots in the litter layer can be an adaptive mechanism by plants to the low nutrient availability in the soils of Central Amazonia. In addition to increasing the efficiency of plants in the utilization of the available resources, fine roots possibly also stimulate decomposition processes, mainly through the exudation of labile carbon compounds (labile C), but little is known yet about the influence of roots in the litter layer. The objective of the present study was therefore to evaluate the effects of the presence of fine roots and the availability of labile C on leaf litter decomposition in a primary forest of Central Amazonia. The exudation of the fine roots was simulated, by increasing the concentrations of carbon present in the litter by approximately 10%. Carbon was added as glucose and citric acid, and water was added as control, in two different conditions: a) in the real conditions of the forest, where the growth of the fine roots was allowed; b) excluding the presence of roots of the litter using fine mesh bags. During the decomposition process, the nutrient release dynamics were evaluated, in addition to the main processes involved. We observed that the presence of fine roots increased litter mass loss, mainly of the most recalcitrant fractions (lignin). However, this stimulus was not related to the increase in the availability of labile C. In contrast, when citric acid was added there was a reduction in the decomposition of the litter. The presence of fine roots alone stimulated the mineralization of some elements (P, K, Ca, and Mg), but when C compounds were added, the positive effect was limited to P, and the absence of fine roots with the addition of citric acid. The immobilization of C, N, and P into the microbial biomass colonizing the litter did not affect by the presence or absence of the fine roots. When citric acid was added microbial biomass C was reduced when roots were absent, and microbial P was both in the presence and absence of the roots. For the potential activity of the extracellular enzymes, the presence of the fine roots stimulated only the phosphatase activity. The addition of the C compounds was also observed specifically for the phosphatase, where in the absence of the fine roots and with the addition of the citric acid its activity was reduced. The results indicated a strong effect of the presence of fine roots in the decomposition of litter, and the release of certain nutrients. However, when simulating the biochemical effect of the roots, through the exudation of labile C compounds, we did not observe an increase in the decomposition of the litter. This shows that the processes responsible for mediating the decomposition of litter are not limited by energy.