Soil C and N dynamics induced by leaf-litter decomposition of shrubs and perennial grasses of the Patagonian Monte

We assessed soil N and C dynamics in a decomposition experiment at microcosm scale with leaf litter of evergreen shrubs and perennial grasses of the Patagonian Monte. We hypothesised that eventual changes in leaf-litter quality in arid Patagonian ecosystems disturbed by grazing would lead to changes in C and N dynamics in decaying leaf litter and the associated soil. We constructed microcosms with local soil and litterbags containing leaf litter of three perennial grass and three evergreen shrub species. Microcosms were incubated at field capacity during 12, 42 and 84 days. We assessed N, C, phenolics and lignin concentrations in leaf litter before incubation, and N and C concentrations after incubation. We estimated inorganic and total N and organic C in soil before and after incubation. We calculated C and N budgets and the associated rates of C and N losses from leaf litter, microbial respiration, N immobilisation in litter and soil, and N mineralisation at microcosms of each species by inverse modelling techniques. Initial leaf-litter N concentration was significantly higher and C:N ratio lower in evergreen shrubs than in perennial grasses. Leaf litter of perennial grasses was very similar in initial chemical composition among species. In contrast, evergreen shrubs displayed a large variability in the concentration of secondary compounds among species. Accordingly, microcosms of perennial grasses showed similar rates of C and N processes while those of evergreen shrubs showed diverse patterns and rates of processes among them. Process rates in microcosms of one of the evergreen shrub species were closely related to those of perennial grasses. In contrast, process rates in microcosms of the two other evergreen shrub species were quite different between them and from those of perennial grasses. We conclude that (i) the replacement of perennial grasses by evergreen shrubs or the shifting of species within evergreen shrubs in ecosystems disturbed by grazing could have different effects on ecosystem processes depending on the species involved, and (ii) species rather than life forms should be used to predict leaf-litter decomposability and associated soil C and N dynamics particularly in evergreen shrubs.