A importância de traços funcionais e do posicionamento do detrito na serrapilheira como mecanismos mediadores dos efeitos da interação entre diversidade de detritos e foto-oxidação na decomposição

Abstract

Studies over the last two decades have pointed to an important role of the litter diversity, especially the functional one, as a determinant of decomposition rates. So far, it has been known that the mechanisms by which the diversity of litter affects decomposition results from biotic mechanisms mediated by the interactions between the decomposing fauna and the detritus. However, abiotic factors such as photodegradation also have a direct effect on decomposition rates in many ecosystems, especially in regions or habitats with a high incidence of UV radiation. In this way, it is conjectured that non-additive effects of the debris diversity in the decomposition should be weak or nonexistent under environmental conditions that do not favor the decomposition through biotic mechanisms. In this work, we tested this conjecture through two field experiments carried out under distinct environmental contexts, in a restinga in the State of Rio Grande do Norte. In the first experiment (Exp. 1), we verified whether and as non-additive effects of the richness and functional diversity of litter vary in magnitude and direction between a habitat more conducive to decay-mediated decomposition (ie soil under vegetation) and another where Photo-degradation-mediated decomposition assumes greater relevance (ie soil exposed to direct solar radiation). In the second experiment (Exp. 2), we observed only in the habitat with high solar exposure if and as the overlap (ie overlapping or not between) and the position with which the litter is vertically in the litter (ie in contact with the soil , or on the litter surface) measured the effects of the diversity of debris on decomposition. In the Exp. 1 litter of four species, presenting discrepant values of specific leaf area (AFE) were placed to decompose in monocultures and all possible bicultures over 6 months in litterbags. In Exp. 2 the decomposition was also evaluated in litterbags over 6 months, but only for monocultures and mixtures formed by two species that presented more discrepant values of AFE, Erythrina velutina (Ev) and Sterculia chicha (Sc). The results of Exp. 1 showed that the rate of decomposition at sites exposed to the direct action of photodegradation was on average 34% higher than that observed in shaded sites under vegetation. There was no significant effect of species richness on decomposition in any of the environmental contexts, indicating that on average the decomposition of the detritus mixtures can be predicted through the decomposition of their monocultures. However, this effect was not homogeneous among the bicultures. The functional dissimilarity between the bicultures had a significant effect on the decomposition, but only in the environment exposed to the sun. In general, combinations of foliar litter with greater discrepancy in the AFE values presented slower decomposition in bicultures than in monocultures. This result was mediated by a trade-off between effect and response of the debris of different species to shade. Species with higher AFE were more sensitive to shading, while species with lower AFE had a greater shading effect. In addition, in Exp. 2 we observed that the overlap and vertical positioning of the detritus also modify the effects of diversity on decomposition. Effects of the debris diversity on decomposition occurred only in the treatments with overlapping litter. However, the effects were significant only for the S. chicha (Sc) species, whose decomposition in the near-soil (less exposed to the sun) and surface (more exposed to the sun) differed from their monoculture decomposition (ie major and minor respectively ). Contrary to our expectations, abiotic mechanisms from photodegradation determine non-additive effects of the functional diversity of debris that consistently retard decomposition in environments exposed to the sun, but such effects have been shown to be dependent on the vertical position of the debris in the litter. Together, the results show that the functional diversity of the detritus, as well as factors that determine the vertical position of the debris in the litter (ie phenological pattern) can act as important mechanisms of carbon retention in the soil in regions, ecosystems or habitats, with high exposure solar.