Negative effects of human disturbance and increased aridity on root biomass and nutrients along the regeneration of a tropical dry forest in the context of slash-and-burn agriculture.

Biomass is an important indicator of the ability of tropical forests to deliver ecosystem services, but little attention has been given to belowground biomass and its drivers in human-modified landscapes. Here, we investigated the belowground biomass and nutrient concentration/stocks (C, P, and N) across regenerating forest stands with varying ages (10-76 years old) and old-growth forests in the Caatinga dry forest (northeastern Brazil) in the context of slash-and-burn agriculture. Belowground biomass ranged from 1.89 ± 0.33 Mg ha-1 to 17.53 ± 2.28 Mg ha-1 (mean ± SE) across regenerating forest stands and averaged 8.33 ± 1.59 Mg ha-1, with no differences compared to old-growth stands. However, regenerating stands exhibited a higher root/shoot ratio with biomass concentrated in the superficial soil layer and in large-sized roots, regardless of the successional stage. Root nutrient concentration and stocks were highly variable across forest stands with fine roots supporting a higher concentration of N and P, while regenerating stands supported lower nutrient stocks as compared to old-growth forests. Finally, precipitation and chronic disturbance emerged as the most important drivers of belowground biomass and nutrient concentrations/stocks, while aboveground biomass played a negligible role. Our results indicate that, in human-modified landscapes of tropical dry forests, belowground biomass and nutrients play important roles in ecosystem functions in regenerated forests after slash-and-burn agriculture. Forest resilience and provision of ecosystem services (e.g., nutrient cycling) appear to be very sensitive to increased aridity and exploitation of forest resources.

A New Framework for Evaluating Estimates of Symbiotic Nitrogen Fixation in Forests.

Symbiotic nitrogen fixation (SNF) makes atmospheric nitrogen biologically available and regulates carbon storage in many terrestrial ecosystems. Despite its global importance, estimates of SNF rates are highly uncertain, particularly in tropical forests where rates are assumed to be high. Here we provide a framework for evaluating the uncertainty of sample-based SNF estimates and discuss its implications for quantifying SNF and thus understanding of forest function. We apply this framework to field data sets from six lowland tropical rainforests (mature and secondary) in Brazil and Costa Rica. We use this data set to estimate parameters influencing SNF estimation error, notably the root nodule abundance and variation in SNF rates among soil cores containing root nodules. We then use simulations to gauge the relationship between sampling effort and SNF estimation accuracy for a combination of parameters. Field data illuminate a highly right-skewed lognormal distribution of SNF rates among soil cores containing root nodules that were rare and spanned five orders of magnitude. Consequently, simulations demonstrated that sample sizes of hundreds to even thousands of soil cores are needed to obtain estimates of SNF that are within, for example, a factor of 2 of the actual rate with 75% probability. This represents sample sizes that are larger than most studies to date. As a result of this previously undescribed uncertainty, we suggest that current estimates of SNF in tropical forests are not sufficiently constrained to elucidate forest stand-level controls of SNF, which hinders our understanding of the impact of SNF on tropical forest ecosystem processes.