Light and dissolved phosphorus interactively affect microbial metabolism, stoichiometry and decomposition of leaf litter

Abstract

Summary In aquatic settings, light can stimulate algal growth to affect microbial transformation of organic substrates. These effects may depend on dissolved nutrients that differentially constrain microbial autotrophy and heterotrophy to drive contrasting carbon (C) and phosphorus (P) dynamics during decomposition. We incubated sugar maple (Acer saccharum) litter under three dissolved P amendments (0, 50 or 500 μg L−1 P) and two light levels (14 or 475 μmol photons m−2 s−1) in laboratory microcosms. We measured litter chlorophyll a, microbial respiration and net metabolism, carbon:nitrogen (C:N) and C:P content, microbial P uptake and release and litter decomposition over 134 days. Elevated dissolved P increased algal biomass in the high‐light treatment and magnified net heterotrophy and autotrophy in the low‐ and high‐light treatments, respectively. Litter C:P and C:N declined as dissolved P increased, and litter C:P was further reduced by high light only in the highest P treatment. Microbial P uptake fluxes peaked under moderate P and high light, whereas P release fluxes were consistently low throughout the experiment. The percent of P uptake that was released was significantly higher under low light. High light stimulated decomposition under low P but slowed decomposition under high P, suggesting increased nutrients weakened algal priming of litter decomposition. Our study suggests factors controlling the degree of autotrophy versus heterotrophy on organic matter, such as light and nutrient availability, may interactively shift litter C and P dynamics during decomposition.

Periphytic algae decouple fungal activity from leaf litter decomposition via negative priming

“…Foremost, our study reiterates the question of why negative priming occurs in some settings, whereas positive priming occurs in others (Bengtsson et al, ). In two previous litter decomposition studies, increased algal biomass under high nutrients erased positive algal‐induced priming (Danger et al, ; Halvorson et al, ). Our findings may be attributable to high nutrient availability which, combined with high light, could raise algal exudation to fully support, rather than augment, heterotrophic C demands (Guenet et al, ; Wagner et al, ; Wyatt et al, ).…”

“…Our experiment provides empirical evidence of negative priming because algae increased fungal production but suppressed leaf litter dry mass loss rates—a notable decoupling, since aquatic fungi (i.e., hyphomycetes which dominate in flowing environments) are considered major drivers of plant litter decomposition in stream ecosystems (Gessner et al, ; Kuehn, ; Romaní et al, ; Suberkropp & Chauvet, ). At a mechanistic level, algae may suppress litter decomposition through two effects, one apparent and one actual: (a) accrual of new algal biomass could counterbalance mass lost due to heterotrophic degradation of litter C, thereby reducing apparent decomposition, and (b) preferential substrate use of algal‐derived labile C substrates by heterotrophs could reduce actual heterotrophic decomposition of litter (Guenet et al, ; Halvorson et al, ). Both mechanisms occurred in our experiment.…”

“…Under no or negative priming, labile C may stimulate heterotrophic decomposer activity, yet this stimulation is not coupled to increased recalcitrant C turnover because microbial heterotrophs likely allocate labile C towards growth, respiration or reproduction instead of degradative enzymes and decomposition (Catalán et al, ; Kuzyakov, ). Priming strength in aquatic systems may depend on the relative size of labile and recalcitrant C pools (Danger et al, ; Halvorson, Scott, Entrekin, Evans‐White, & Scott, ; Wagner, Bengtsson, Findlay, Battin, & Ulseth, ). However, additional tests of priming are needed, especially those extending beyond closed micro‐ and mesocosm studies to flow‐through conditions of streams and rivers (Fabian et al, ), where there also is a pressing need to quantify the microbial interactions that determine mechanisms and directions of priming (Catalán et al, ; Guenet et al, ).…”

“…Increased light availability enhances C lability through photolytic (ultraviolet‐induced) degradation of recalcitrant C compounds (e.g., humic acids) into fatty acids and carbohydrate monomers (King, Brandt, & Adair, ; Wetzel, Hatcher, & Bianchi, ), but considerably less emphasis has been placed on the potential for light‐mediated effects via algal growth and C exudation and its subsequent stimulation of heterotrophic decomposers (Danger et al, ; Kuehn et al, ). On leaf litter, active periphytic algae can double bacterial and fungal growth rates (Kuehn et al, ), and enhance C‐ and nitrogen (N)‐acquiring enzyme activities (Rier, Kuehn, & Francoeur, ), and speed decomposition by 20%–126% (Danger et al, ; Halvorson et al, ; Lagrue et al, ). Algae can also increase overall microbial biomass in the litter‐periphyton complex, and because algae are N‐ and phosphorus (P)‐rich relative to litter, this increases nutrient uptake and reduces C:N and C:P ratios (Danger et al, ; Halvorson et al, ).…”

Cellulose Dynamics during Foliar Litter Decomposition in an Alpine Forest Meta-Ecosystem

“…Moreover, cellulose degradation rate was found to be significantly moderated by litter initial P concentration, which can explain up to 64.4%-74.8% of the variation depending on ecosystem types (Table 4). This may be attributed to the fact that P is an important nutrient for microorganisms and the enzymes related to cellulose degradation [38]. Apart from the influences of litter quality on cellulose degradation, ecosystem type, and decomposition period, which were expressed as local-scale environment factors in the present study, appeared to be of significance as well.…”