Abstract. Soil fungi and bacteria are the key players in the transformation and processing of carbon and nutrients in terrestrial ecosystems, yet controls on their abundance and activity are not well understood. Based on stoichiometric principles, soil microbial processes are expected to be limited by mineral nutrients, which are particularly scarce in often highly weathered tropical forest soils. Such limitation is directly relevant for the fate of soil carbon and global element cycles, but its extent and nature have never been assessed systematically across the tropical biome. Here, we address the relative importance of nitrogen, phosphorus, and other nutrients in limiting soil microbial biomass and process rates in tropical forests. We conducted an in-depth literature review and a meta-analysis of the available nutrient addition experiments in tropical forests worldwide. Our synthesis showed predominant and general phosphorus limitation of a variety of microbial processes across tropical forests, and additional nitrogen limitation in tropical montane forests. The apparent widespread microbial phosphorus limitation needs to be accounted for in the understanding and prediction of biogeochemical cycles in tropical forests and their future functioning. Other mineral nutrients or carbon may modify the importance of phosphorus, but more experimental studies are urgently needed.

Tropical regions are facing increasing atmospheric inputs of nutrients, which will have unknown consequences for the structure and functioning of these systems. Here, we show that Neotropical montane rainforests respond rapidly to moderate additions of N (50 kg ha−1 yr−1) and P (10 kg ha−1 yr−1). Monitoring of nutrient fluxes demonstrated that the majority of added nutrients remained in the system, in either soil or vegetation. N and P additions led to not only an increase in foliar N and P concentrations, but also altered soil microbial biomass, standing fine root biomass, stem growth, and litterfall. The different effects suggest that trees are primarily limited by P, whereas some processes—notably aboveground productivity—are limited by both N and P. Highly variable and partly contrasting responses of different tree species suggest marked changes in species composition and diversity of these forests by nutrient inputs in the long term. The unexpectedly fast response of the ecosystem to moderate nutrient additions suggests high vulnerability of tropical montane forests to the expected increase in nutrient inputs.

Increased nitrogen (N) depositions expected in the future endanger the diversity and stability of ecosystems primarily limited by N, but also often co-limited by other nutrients like phosphorus (P). In this context a nutrient manipulation experiment (NUMEX) was set up in a tropical montane rainforest in southern Ecuador, an area identified as biodiversity hotspot. We examined impacts of elevated N and P availability on arbuscular mycorrhizal fungi (AMF), a group of obligate biotrophic plant symbionts with an important role in soil nutrient cycles. We tested the hypothesis that increased nutrient availability will reduce AMF abundance, reduce species richness and shift the AMF community toward lineages previously shown to be favored by fertilized conditions. NUMEX was designed as a full factorial randomized block design. Soil cores were taken after 2 years of nutrient additions in plots located at 2000 m above sea level. Roots were extracted and intraradical AMF abundance determined microscopically; the AMF community was analyzed by 454-pyrosequencing targeting the large subunit rDNA. We identified 74 operational taxonomic units (OTUs) with a large proportion of Diversisporales. N additions provoked a significant decrease in intraradical abundance, whereas AMF richness was reduced significantly by N and P additions, with the strongest effect in the combined treatment (39% fewer OTUs), mainly influencing rare species. We identified a differential effect on phylogenetic groups, with Diversisporales richness mainly reduced by N additions in contrast to Glomerales highly significantly affected solely by P. Regarding AMF community structure, we observed a compositional shift when analyzing presence/absence data following P additions. In conclusion, N and P additions in this ecosystem affect AMF abundance, but especially AMF species richness; these changes might influence plant community composition and productivity and by that various ecosystem processes.

No abstract