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Forest canopy herbivores are known to increase rates of nutrient fluxes to the forest floor in a number of temperate and boreal forests, but few studies have measured effects of herbivore-enhanced nutrient fluxes in tropical forests. We simulated herbivore-induced fluxes in a tropical rainforest in Puerto Rico by augmenting greenfall (fresh foliage fragments), frassfall (insect feces), and throughfall (precipitation enriched with foliar leachates) in replicated experimental plots on the forest floor. Background rates of greenfall and frassfall were measured monthly using litterfall collectors and augmented by adding 10× greenfall or 10× frassfall to designated plots. Throughfall fluxes of NH(4), NO(3) and PO(4) (but not water) were doubled in treatment plots, based on published rates of fluxes of these nutrients in throughfall. Control plots received only background flux rates for these compounds but the same minimum amount of distilled water. We evaluated treatment effects as changes in flux rates for NO(3), NH(4) and PO(4), measured as decomposition rate of leaf litter in litterbags and as adsorption in ion-exchange resin bags at the litter-soil interface. Frass addition significantly increased NO(3) and NH(4) fluxes, and frass and throughfall additions significantly reduced decay rate, compared to controls. Reduced decay rate suggests that nitrogen flux was sufficient to inhibit microbial decomposition activity. Our treatments represented fluxes expected from low-moderate herbivore outbreaks and demonstrated that herbivores, at these outbreak levels, increase ecosystem-level N and P fluxes by >30% in this tropical rainforest.

The soil microbial community plays an important role in regulating soil organic carbon (SOC) decomposition and maintaining stability in forest ecosystems. However, the interactions between the soil microbial community and soil carbon (C) fractions following afforestation remain poorly understood. In this study, soil samples were collected in an afforested area representing a chronosequence of 42, 27 and 17 years of Robinia pseudoacacia L. succession (RP42yr, RP27yr and RP17yr, respectively), and in farmland (FL) soil for comparison. Illumina sequencing of the16S rRNA and fungal ITS genes was used to analyse soil bacterial and fungal diversity, and the content of C fractions was also measured. Our results indicated that soil C fractions in the afforested RP42yr, RP27yr and RP17yr sites were 34.83–94.11%, 38.52–82.83% and 27.24–89.32% larger, respectively, than in the FL soil. Shannon indices for bacterial and fungal diversity, which ranged from 6.59 to 6.81 and 3.73 to 4.19, respectively, were also larger in the afforested soil. In addition, the dominant bacterial and fungal phyla, including Proteobacteria, Acidobacteria, Chloroflexi, Gemmatimonadetes, Bacteroidetes, Nitrospirae, Verrucomicrobia, Planctomycetes, Armatimonadetes, Cyanobacteria, Chlorobi, Firmicutes, Fibrobacteres, Zygomycota, Basidiomycota and Glomeromycota, were more abundant in afforested soil than in FL soil, whereas Actinobacteria and Ascomycota were more abundant in FL soil than in afforested soil. Spearman's rank correlation coefficients were strong and positive between soil microbial diversity (alpha diversity, Bshannon and Fshannon) and C fractions (P < 0.05). The dominant phyla (both bacterial and fungal), such as Proteobacteria and Zygomycota, had significant positive effects on C fractions, whereas for other taxa, such as Actinobacteria and Ascomycota, they were significant and negative. Thus, our results indicated that changes in soil C fractions are linked to the composition of soil microbial communities following afforestation. They also provide further evidence that soil bacterial and fungal communities play vital roles in the turnover of SOC and C cycling. Highlights Changes in soil microbial community influence soil carbon fractions following afforestation. Carbon fractions and soil microbial communities respond to afforestation. Changes in C fractions were strongly correlated with soil microbial diversity. Soil microbial diversity and microbial taxa markedly affected carbon fractions.