Microbial Mechanisms Mediating Increased Soil C Storage under Elevated Atmospheric N Deposition

Abstract

c Future rates of anthropogenic N deposition can slow the cycling and enhance the storage of C in forest ecosystems. In a northern hardwood forest ecosystem, experimental N deposition has decreased the extent of forest floor decay, leading to increased soil C storage. To better understand the microbial mechanisms mediating this response, we examined the functional genes derived from communities of actinobacteria and fungi present in the forest floor using GeoChip 4.0, a high-throughput functional-gene microarray. The compositions of functional genes derived from actinobacterial and fungal communities was significantly altered by experimental nitrogen deposition, with more heterogeneity detected in both groups. Experimental N deposition significantly decreased the richness and diversity of genes involved in the depolymerization of starch (ϳ12%), hemicellulose (ϳ16%), cellulose (ϳ16%), chitin (ϳ15%), and lignin (ϳ16%). The decrease in richness occurred across all taxonomic groupings detected by the microarray. The compositions of genes encoding oxidoreductases, which plausibly mediate lignin decay, were responsible for much of the observed dissimilarity between actinobacterial communities under ambient and experimental N deposition. This shift in composition and decrease in richness and diversity of genes encoding enzymes that mediate the decay process has occurred in parallel with a reduction in the extent of decay and accumulation of soil organic matter. Our observations indicate that compositional changes in actinobacterial and fungal communities elicited by experimental N deposition have functional implications for the cycling and storage of carbon in forest ecosystems.

Atmospheric N Deposition Increases Bacterial Laccase-Like Multicopper Oxidases: Implications for Organic Matter Decay

“…This was especially true in the forest floor, in which OTUs attributable to Proteobacteria, Bacteroidetes, and Actinobacteria account for 76% of those contributing to the composition shift we observed. In combination, the observations presented here, taken together with previous work (6,32,33), suggest that experimental N deposition has favored bacteria with lignin-modifying potential and has disfavored the activity of lignolytic fungi (32).…”

“…Bacteria also can depolymerize lignin (15,30), which results in the production of soluble polyphenols (ϳ60% of products) with minimal amounts of CO 2 (Ͻ4%) (23,31). In our long-term field experiment, experimental N deposition has decreased the activity of phenol oxidase and peroxidase expression (ϳϪ30%) and fungal laccase (lcc) expression (Ϫ80%) (32) as well as the diversity of genes mediating detritus decay (Ϫ35%) (33).…”

“…In this report, we present evidence that a more abundant, less diverse, and compositionally different bacterial LMCO assemblage occurred concomitantly with a decreased extent of litter decay and increased DOC production in our long-term field experiment, supporting the hypothesis that experimental N deposition favors bacteria with the physiological ability to metabolize both plant litter and humidified soil organic matter. Experimental N deposition can elicit compositional changes in functional assemblages, including genes mediating C and N cycling (33,73,74), as well as numerous metagenomic functional gene categories (64), whereas this study directly examined the abundance and composition of bacterial assemblages, in which observed microbial responses appear to be a mechanism(s) plausibly mediating reduced decay and increased DOC leaching under conditions of experimental N deposition.…”

Assembly of Active Bacterial and Fungal Communities Along a Natural Environmental Gradient

“…These sites are located along the upper and lower peninsula of Michigan [16] and span a 415 km geographic and edaphic gradient, with differences in precipitation, temperature, and soil chemistry [17]. To target decomposer communities, litter samples were collected in October 2011 from the three plots in each of the four sites, for a total of 12 samples.…”

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

“…If increased N deposition stimulates plant productivity, inputs to the soil from leaf or root litter may increase. Furthermore, N deposition may also repress lignolytic enzyme activity, causing decreases in organic matter decay rates and enhancing SOC storage (Eisenlord et al, 2013). Alternatively, changes in nutrient availability may increase root exudation, "priming" the decomposition of organic matter and decreasing soil C storage (Phillips et al, 2011).…”