Nitrogen pulses increase fungal pathogens in Amazonian lowland tropical rain forests

Abstract

Animals represent an overlooked source of horizontal redistribution of primary production and concentration of elements in ecosystems. For example, the high nutrient concentration of excretions by animals creates a mosaic of short-term nutrient hotspots. However, how this impacts soil microbial communities, especially fungi, and in turn plant species diversity remains little known. This study quantified the temporal dynamics of soil mineral nitrogen (N) availability and its relationship with soil fungal community and functional group composition in simulated high-N patches in an Amazonian rain forest. We hypothesised that (H1) changes in local resource dynamics would increase the abundance of pathotrophs and reduce that of saprotrophs and that, (H2) compared with previously reported bacterial community dynamics, fungi would be more resilient after a pulse disturbance event. A single urea pulse was applied and the relationship between fungal community composition and functional groups and soil N availability were determined before and twice after the urea treatment. An increase in mineral N availability and soil pH two months after the applied urea pulse was found to be associated with significant changes in fungal community composition and the abundance of functional groups. There was a notable decrease in the relative abundance of saprotrophs, accompanied by an increase in plant pathogenic fungi. Five months after the treatment, no differences were detected either in mineral N availability and soil pH or the composition of fungal communities and functional groups between the control and urea treatment. Synthesis. By locally favouring the abundance of plant pathogens, temporally short-lived, but frequent high-N patches created by animal excretions could potentially be involved in maintaining spatially and temporally variable soil microbial diversity and thus contributing to high plant community diversity in tropical rain forests as predicted by the Janzen–Connell hypothesis. The tendency of soil fungal communities in this study to return to their initial composition after 5 months suggests that they are resilient to perturbation by N pulse, and more so than previously observed in bacterial communities.