Causes and consequences of litter-mixing effects on decomposition in terrestrial and aquatic ecosystems

Abstract

Most of the organic matter (OM) synthesized in terrestrial ecosystems turns into litter. A set of physiological, ecological, evolutionary, and environmental mechanisms act on both the synthesis and degradation of OM making it very diverse, concerning its chemical and physical traits, which play relevant functional roles in determining the dynamics of litter degradation in ecosystems. The functional litter diversity/heterogeneity has consequences through litter-mixing effects (LMEs) on the decomposition and nutrient cycling in terrestrial and aquatic ecosystems. However, several gaps regarding both the causes and consequences of litter diversity for ecosystem functioning remain poorly understood. Thus, we aimed to assess the causes (Chapters 1 and 2) and consequences (Chapters 3-5) of litter diversity/heterogeneity in decomposition and to understand their effects on the occurrence, direction, and magnitude of LMEs. Chapter I, we tested the generality of flower litter as a labile organic matter and leaf litter as a refractory organic matter in terrestrial ecosystem. Specifically, we evaluated patterns and predictors of flower and leaf litter decomposition at the intra- and inter-specific levels for 29 species. Our results indicate that flower litter had a higher concentration of N, P, and K, while leaf litter had higher density, Ca, Mg, and Na. And that the functional traits, leaching rate, P, Ca, Mg, and Na, predicted the litter decomposition of both litter types. Overall, the differences observed in decomposition rates and functional traits between litter types indicate differences between flowers and leaves in the potential diversity effect on decomposition. In Chapter II, we evaluated the LMEs at the intra-specific level in aquatic and terrestrial ecosystems, using flower and leaf litter of Tabebuia aurea (Silva Manso) Benth. & Hook. f. ex. S. Moore as a model. Our results indicate that the differences in functional litter traits between flowers and leaves resulted in littermixing effects, mostly synergistic effects, associated with complementarity mechanisms. Since flower and leaf litter seem to be a labile and refractory litter, respectively. Our results indicate that the litter diversity at the intra-specific level between different organs could be important in mediating LMEs, and these effects were stronger in terrestrial ecosystems. Our results suggest a feasible secondary role for flowers in after senescence in the OM cycling. In Chapter III, we evaluated how the litter interaction for different temporal dynamics, resulting from different phenological patterns, can affect the LMEs. Specifically, we evaluated how an intermittent or seasonal input of litter affects the occurrence, direction, and magnitude of LMEs in aquatic and terrestrial ecosystems. We did not find clear patterns in the effect of litterfall inputs on decomposition, but we observed a preponderance of LMEs in the aquatic ecosystem, probably related to the different dynamics and nutrient concentrations in its ecosystem due to the experimental conditions. In Chapter IV, through a meta-analysis, we evaluated how the individual response of species in mixtures can alter the occurrence, direction (i.e. positive or negative), and magnitude of LMEs. Differences in functional traits between species and litter identity were the main factors mediating LMEs in both terrestrial and aquatic ecosystems. In Chapter V, also through a meta-analysis, we evaluated the complementarity effects between labile (i.e. higher C: nutrient ratio) and refractory litter (i.e. lower C:nutrient ratio), testing the idea that labile species accelerate the decomposition of refractory species, while refractory species decrease the decomposition of labile species. Our results indicate that labile species showed additive responses when interacting with refractory species; while refractory species had antagonistic responses when interacting with labile species. When evaluating LMEs considering ecosystem type, presence or absence of decomposers, and stage decay, we observed a preponderance of antagonistic effects for labile species. To directly evaluate the responses of labile and refractory litter, we found patterns of LMEs that differed from those expected from previous studies, since labile litter is expected to have an antagonistic response and refractory litter a synergistic response. The results obtained in this thesis contribute to advancing knowledge of LMEs, specifically to evaluate how the causes and consequences of litter heterogeneity may be important in mediating the occurrence, direction, and magnitude of LMEs.