Microbial biomass and production associated with decaying leaf litter of the emergent macrophyte Juncus effusus

Abstract

Emergent macrophytes are a major source of organic matter production in freshwater wetlands, and often represent the bulk of the plant material entering the detrital pool. We examined the decomposition and microbial dynamics associated with litter of the emergent macrophyte, Juncus effusus (soft rush), during its movement from an aerial standing dead to a submerged decay environment. Standing dead leaves of J. effusus were collected after an initial period of standing litter microbial decay, placed in 1‐mm mesh litter bags, and submerged in a wetland. Litter bags were retrieved periodically over 268 d and analyzed for microbial (fungal and bacterial) biomass and production, ATP concentrations, litter mass loss, and quality (C:N:P and plant fiber). Submerged litter decay of J. effusus was slow (k = 0.0010 d−1), with only 23% weight loss after 268 d. Both fungal and bacterial biomass and production decreased significantly soon after standing plant litter was submerged in the wetland surface waters. Despite decreases in microbial biomass and production, fungal decomposers remained the dominant microbial assemblage associated with decaying plant litter, accounting for 99% and 91% of the total microbial biomass and production, respectively. Mean fungal production ranged from 73–2,836 mg C g−1 AFDM d−1 (AFDM: ash‐free dry mass remaining) during the study period, whereas attached bacterial production ranged from 4–32 mg C g−1 AFDM d−1. Patterns of litter ATP and nutrient concentrations (N and P) were similar to those observed for fungal and bacterial biomass, suggesting that at least a portion of the detrital N and P may have been incorporated into microbial biomass. Significant changes in microbial colonization and activity associated with emergent macrophyte litter can occur following the collapse of standing dead plant matter to the water or surface sediments. Furthermore, our findings suggest that fungi are significant contributors to the decay of coarse particulate plant matter in wetland ecosystems.