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The arbuscular mycorrhizal (AM) fungi are a globally distributed group of soil organisms that play critical roles in ecosystem function. However, the ecological niches of individual AM fungal taxa are poorly understood.

1. Tropical rainforests are populated by large frugivores that feed upon fruit-producing woody species, yet their role in regulating the cycle of globally important biogeochemical elements such as nitrogen is still unknown. This is particularly relevant because tropical forests play a prominent role in the nitrogen cycle and are becoming rapidly defaunated. Furthermore, frugivory is not considered in current plant-large herbivore-nutrient cycling frameworks exclusively focused on grazers and browsers. 2. Here we used a long-term replicated paired control-exclusion experiment in the Atlantic Forest of Brazil, where peccaries and tapirs are the largest native frugivores, to examine the impact of large ground-dwelling frugivores on modulating soil nitrogen cycling, considering their effects across a gradient of abundance of a hyper-dominant palm. 3. We found that both large frugivores and dominant palms play a substantial role in modulating ammonium availability and nitrification rates. Large frugivores increased ammonium by 95%, which also increased additively with palm abundance. Nitrification rates increased with palm abundance in the presence of large frugivores, but not on exclosure plots. Large frugivores also stimulated the regulation of the functions of soil-nitrifying microorganisms, and modulated the landscapescale variance in nitrogen availability. Such joint effects of large frugivores and palms are consistent with the notion of 'fruiting lawns'. 4. Our study indicates that frugivory plays a pivotal role in zoogeochemistry in tropical forests by regulating and structuring the nitrogen cycle, urging to accommodate frugivory in plant-large herbivore-nutrient cycling frameworks. It also indicates that defaunation, deforestation and illegal palm and timber harvesting seriously affect nitrogen cycling in tropical forests, that play a prominent role in the global cycle of this nutrient.

Questions: Which factors affect the diversity and species composition of tropical secondary rain forests in a region with little information regarding their contribution to global biodiversity? Can older secondary forests approach the diversity and composition of mature forests following 100 yr of pasture use?Location: Tropical secondary rain forest, northeast Australia. Methods:We identified trees, shrubs and vines ≥2.5 cm DBH in a chronosequence comprising 33 sites, aged 3-60 yr since the formation of closed canopy (9-69 yr since pasture abandonment) and compared them with eight sites in nearby mature forest remnants.Results: Species richness and community composition were strongly influenced by secondary forest age but did not attain values of mature forest. Sites in close proximity to mature forests had higher plant richness, whereas low soil fertility appeared to depress species recruitment. Thus, multiple factors operated in secondary forest community assembly. Unusual tree community patterns that suggest accelerated or slowed successional trajectories were observed at several sites.Conclusions: Secondary forests in our study region contained important plant diversity for conservation, particularly in older sites, however, even the oldest secondary forests (60 yr) did not converge with the species composition and diversity of mature forests. The protection of mature forest tracts and remnants must be a priority if we are to maintain high levels of plant diversity in tropical landscapes, conserve rare species and facilitate the recruitment of plant species in recovering forests.

Abstract. We investigate the edaphic, mineralogical and climatic controls of soil organic carbon (SOC) concentration utilising data from 147 primary forest soils (0–30 cm depth) sampled in eight different countries across the Amazon Basin. Sampled across 14 different World Reference Base soil groups, our data suggest that stabilisation mechanism varies with pedogenetic level. Specifically, although SOC concentrations in Ferralsols and Acrisols were best explained by simple variations in clay content – this presumably being due to their relatively uniform kaolinitic mineralogy – this was not the case for less weathered soils such as Alisols, Cambisols and Plinthosols for which interactions between Al species, soil pH and litter quality are argued to be much more important. Although for more strongly weathered soils the majority of SOC is located within the aggregate fraction, for the less weathered soils most of the SOC is located within the silt and clay fractions. It thus seems that for highly weathered soils SOC storage is mostly influenced by surface area variations arising from clay content, with physical protection inside aggregates rendering an additional level of protection against decomposition. On the other hand, most of the SOC in less weathered soils is associated with the precipitation of aluminium–carbon complexes within the fine soil fraction, with this mechanism enhanced by the presence of high levels of aromatic, carboxyl-rich organic matter compounds. Also examined as part of this study were a relatively small number of arenic soils (viz. Arenosols and Podzols) for which there was a small but significant influence of clay and silt content variations on SOM storage, with fractionation studies showing that particulate organic matter may account for up to 0.60 of arenic soil SOC. In contrast to what were in all cases strong influences of soil and/or litter quality properties, after accounting for these effects neither wood productivity, above-ground biomass nor precipitation/temperature variations were found to exert any significant influence on SOC stocks. These results have important implications for our understanding of how Amazon forest soils are likely to respond to ongoing and future climate changes.

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