Leaf Decomposition in a Dry Season Irrigation Experiment in Eastern Amazonian Forest Regrowth

Leaf-litter decomposition is a major component of carbon and nutrient dynamics in tropical forest ecosystems, and moisture availability is widely considered to be a major influence on decomposition rates. Here, we report the results of a study of leaf-litter decomposition of five tree species in response to dry-season irrigation in a tropical forest regrowth stand in the Brazilian Amazon; three experiments differing in the timing of installation and duration allowed for an improved resolution of irrigation effects on decomposition. We hypothesized that decomposition rates would be faster under higher moisture availability in the wet season and during dry-season irrigation periods in the treatment plots, and that decomposition rates would be faster for species with higher quality leaves, independent of treatment. The rates of decomposition (k) were up to 2.4 times higher in irrigated plots than in control plots. The highest k values were shown by Annona paludosa (0.97 to 1.26/yr) while Ocotea guianensis (0.73 to 0.85/yr) had the lowest values; intermediate rates were found for Lacistema pubescens (0.91 to 1.02/yr) and Vismia guianensis (0.91 to 1.08/yr). These four tree species differed significantly in leaf-litter quality parameters (nitrogen, phosphorus, lignin, and cellulose concentrations, as well as lignin:nitrogen and carbon:nitrogen ratios), but differences in decomposition rates among tree species were not strictly correlated with leaf-litter quality. Overall, our results show that dry-season moisture deficits limit decomposition in Amazonian forest regrowth. RESUMO A decomposicao foliar e um dos componentes principais da dinâmica de carbono e nutrientes em florestas tropicais. A disponibilidade de agua representa um fator importante no controle das taxas de decomposicao em ecossistemas florestais. Neste estudo, apresentamos resultados de decomposicao foliar de cinco especies arboreas em resposta a irrigacao durante a estacao seca em uma floresta secundaria na Amazonia brasileira, tres experimentos com diferentes epocas de instalacao e duracao permitiram avaliar mais detalhadamente os efeitos da irrigacao na decomposicao. Levantamos as seguintes hipoteses neste estudo: as taxas de decomposicao seriam mais altas sob condicoes de maior disponibilidade de agua (durante a estacao chuvosa e nas parcelas irrigadas) e para especies com serapilheira foliar de maior qualidade, independentemente de tratamento. As taxas de decomposicao (k) foram ate 2,4 vezes mais altas nas parcelas irrigadas do que no controle. Os valores mais altos de k foram observados em Annona paludosa (0,97 a 1,26/ano), enquanto que Ocotea guianensis (0,73 a 0,85/ano) apresentou os valores mais baixos, valores intermediarios ocorreram para Lacistema pubescens (0,91 a 1,02/ano) e Vismia guianensis (0,91 a 1,08/ano). Parâmetros de qualidade de serapilheira foliar (concentracoes de nitrogenio, fosforo, lignina e celulose, assim como relacoes lignina:nitrogenio e carbono:nitrogenio) foram estatisticamente diferentes entre especies arboreas. Entretanto, diferencas em taxas de decomposicao entre especies nao se correlacionaram com a qualidade da serapilheira. Nossos resultados mostram que a deficiencia de agua na estacao seca limita a decomposicao foliar em florestas secundarias na Amazonia.