Nitrogen and phosphorus additions impact arbuscular mycorrhizal abundance and molecular diversity in a tropical montane forest

Abstract

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.

Arbuscular mycorrhizal community structure on co-existing tropical legume trees in French Guiana

“…This approach yielded an estimate of 34 AMF taxa detected in roots on the basis of SSU-DGGE banding that has been shown, via individual band sub-cloning and sequencing, to underestimate AMF diversity (Öpik et al 2003), although it assumes we did not have any nonspecific amplification from other fungal phyla (see Kohout et al 2014). The clearest relationship between soil nutrients and AMF taxa representation was seen for soil N. A related study in species rich tropical montane forest found reduced AMF species richness in bulked root samples in response to N and P addition (Camenzind et al 2014). Nitrogen input in forests will be greatly dependent on anthropogenic deposition rates but also associative and symbiotic Nfixation involving legumes.…”

Increases in Soil Aggregation Following Phosphorus Additions in a Tropical Premontane Forest are Not Driven by Root and Arbuscular Mycorrhizal Fungal Abundances

“…Total concentrations of P, Ca, Al, Fe, K, Mg, and Zn as well as soil pH were analyzed on separate soil samples taken in February 2013 (important elements which were included in this study were preselected by principal component analysis, selecting for ecologically relevant variables and excluding collinear predictors Camenzind et al, 2014). Samples were obtained from all six subplots (taken with a soil corer of 3.5 cm diameter), pooled separately for each plot and transferred to University of Tübingen and Freie Universität Berlin for further analyses.…”

“…However, tropical forests are highly endangered by deforestation, land use change, fertilization but also increased nutrient deposition by anthropogenic activities (Gullison et al, 2007;Galloway et al, 2008;Mahowald et al, 2008). Increases in nutrient deposition have been shown to affect plant diversity, ecosystem productivity, soil community composition as well as nutrient cycling (Treseder, 2008;Bobbink et al, 2010;Isbell et al, 2013;Wilcke et al, 2013b;Camenzind et al, 2014), and thus also directly or indirectly may have an impact on soil aggregation processes.…”

Nutrient addition effects on tropical dry forests: a mini-review from microbial to ecosystem scales

“…Mycorrhizae live in close symbiosis with plants and their responses to nutrient addition depend partially upon plant nutrient status as well as background nutrient limitation (Treseder, 2004). N deposition decreases arbuscular mycorrhizal biomass in both temperate and tropical forests (Johnson et al, 2003;Treseder, 2004;Camenzind et al, 2014;Krashevska et al, 2014;Wurzburger and Wright, 2015), but responses to N and P addition are context-dependent. Ecological relationships between plants and their mycorrhizae depend both upon soil C:N:P stoichiometry (Johnson et al, 2015) and mycorrhizal type (e.g., arbuscular, ectomycorrhizal, etc).…”