Functional evenness of N-to-P ratios of evergreen-deciduous mixtures predicts positive non-additive effect on leaf litter decomposition

Abstract

The effects of litter diversity on litter decomposition remain debated. We tested to what extent the community-weighted means (CWM; functional composition) versus Rao’s dissimilarity of litter nitrogen (N)-to-phosphorus (P) ratios explain the non-additive mixture effect on decomposition rate (k) and associated N release. We carried out a one-year field decomposition experiment with a range of five litter types ranging from three evergreens only (high N/P and low specific leaf area, SLA) to three deciduous species only (low-N/P and high-SLA), with 30:70, 50:50 and 70:30% mixtures of these two extremes in between, in subtropical forest of China. There were tight hump-backed relationships of absolute k-values and N release, respectively, with the CWMSLA. The mixtures with the highest functional evenness in terms of CWMN/P caused the highest positive non-additivity on decomposition (R2 = 0.72) and N release (R2 = 0.95) rates. In contrast, the mixing effect on k or N release was weakly positively correlated with Rao’s dissimilarity of N/P (R2 = 0.38 and 0.27 respectively). Our results provide a strong framework for predicting litter decomposition rates and associated N release versus immobilization in mixtures of deciduous versus evergreen species based on their differences in initial stoichiometry.