Priming of leaf litter decomposition by algae seems of minor importance in natural streams during autumn

Abstract

Allochthonous detritus of terrestrial origin is one of the main energy sources in forested headwater streams, but its poor nutritional quality makes it difficult to use by heterotrophs. It has been suggested that algae growing on this detritus can enhance its nutritional quality and promote decomposition. So far, most evidence of this "priming effect” is derived from laboratory or mesocosm experiments, and its importance under natural conditions is unclear. We measured accrual of algae, phosphorus uptake capacity, and decomposition of poplar leaves in autumn in open- and closed-canopy reaches in 3 forest and 3 agricultural streams. Chlorophyll a abundance did not change significantly with stream type or with canopy cover, although in some agricultural streams it was higher in open than in closed canopy reaches. Canopy cover did not affect either phosphate uptake capacity or microbial decomposition. On the other hand, although there was no effect of canopy cover on invertebrate fragmentation rate, a significant interaction between canopy cover and stream suggests priming occurs at least in some streams. Overall, the results point to a weak or no priming effect of algae on litter decomposition in natural streams during autumn.

Algal-Mediated Priming Effects on the Ecological Stoichiometry of Leaf Litter Decomposition: A Meta-Analysis

“…Moreover, from a subset of studies we found that algae can shift pools of microbial and detrital C, N, and P within decomposing litter-periphyton. As expected from our third hypothesis, we found weak algal effects on N-and P-specific mass loss from litter-periphyton during decomposition, possibly because algal immobilization counterbalances algal effects on heterotrophic mineralization of litter N and P (Halvorson et al, 2016;Elosegi et al, 2018), or perhaps due to the wide variation in priming intensity across datasets. The small sample size and large variation revealed by our meta-analysis suggest that additional studies are needed to further assess the overall priming effect, its dependence on environmental factors such as N:P availability, and its implications for coupled elemental transformations in aquatic ecosystems.…”

“…The ecological stoichiometry of priming effects remains comparatively understudied in aquatic systems, where priming can be significant (e.g., Danger et al, 2013;Bianchi et al, 2015), yet the general direction and magnitude of priming effects are strongly debated due to incongruent evidence (Bengtsson et al, 2018;Elosegi et al, 2018). Some studies suggest that increased dissolved N and P concentrations may shift priming from positive to negative, by stimulating algal growth and exudation of labile C and enabling preferential use of labile C by microbial heterotrophs in lieu of recalcitrant detrital C (Guenet et al, 2010;Guillemette et al, 2016).…”

“…However, studies of dissolved organic matter have found weak relationships between priming and nutrient availability (Hotchkiss et al, 2014;Catalán et al, 2015), possibly because coupling between priming and nutrient availability is more diffuse within the water column compared to spatially-constrained interactions within biofilms colonizing organic matter (Bengtsson et al, 2018). Algae are present even in well-shaded aquatic ecosystems (Greenwood and Rosemond, 2005;Elosegi et al, 2018) and form mixed heterotrophic and autotrophic biofilms (e.g., periphyton) on organic substrata, which may be hotspots of priming and nutrient mineralization/immobilization (Kuehn et al, 2014;Battin et al, 2016;Wagner et al, 2017). For example, algal exudation of labile C may stimulate fungal N mineralization from underlying organic matter (Soares et al, 2017).…”

Stronger effects of litter origin on the processing of conifer than broadleaf leaves: A test of home‐field advantage of stream litter breakdown

“…In most of our study streams, k f contributed more to k c than λ F for broadleaf litter, which has been previously observed (Elosegi, Nicolás, & Richardson, ; Yeung et al., ). Thus, litter–microbe interactions appeared to play a more essential role than litter–shredder interactions in mediating early‐stage breakdown (i.e.…”

Organic Matter Decomposition and Ecosystem Metabolism as Tools to Assess the Functional Integrity of Streams and Rivers–A Systematic Review

“…Less information is available on the roles of yeasts, zoosporic fungi, oomycetes, and protists, but they were found to be associated with decomposing leaf litter and can affect litter decomposition directly or indirectly via trophic interactions [32][33][34][35][36]. The role of algae on litter decomposition is also uncertain with some studies suggesting that they can stimulate litter decomposition via microbial priming (i.e., the stimulation of microbial decomposers' activity by the addition of labile carbon) [37], while other studies found inhibition or no effect [38,39]. Microbial activity leads to litter mass loss through the release of fine particles and dissolved litter mass, incorporation of litter carbon into microbial biomass (including reproductive structures that are released), and carbon mineralization [40][41][42].…”