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

Abstract

Allochthonous detritus from 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 it is unclear what its importance is under natural conditions. 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 neither with stream type nor with canopy cover, although some between open and closed reaches, 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 effect of algae on litter decomposition in natural streams during autumn.

Global Patterns and Controls of Nutrient Immobilization on Decomposing Cellulose in Riverine Ecosystems

“…P immobilization on decomposing litter is rarely measured, but our rates of P IMM on cotton were similar to those recorded for three litter types in an artificial stream (0–12 μg P g −1 C d −1 ; Robbins et al., 2019). Our approach, like that used by most previous studies of nutrient immobilization, could not distinguish assimilation by autotrophs on cotton from immobilization by decomposers, but natural shading of the stream channel likely limited algal contributions to net nutrient uptake (Elosegi et al., 2018; Halvorson et al., 2019).…”

“…We assumed that any increase in nutrient concentration was a result of growth and assimilation by heterotrophic microbes, but we acknowledge that other processes may also contribute to changes in cotton nutrient content (e.g., algal colonization, P precipitation). Algal assimilation is a relatively minor contribution to net nutrient uptake on detritus except under high light and high nutrient conditions (Elosegi et al., 2018; Halvorson et al., 2019). If litter has a high initial C:nutrient ratio, linear changes in litter stoichiometry result in curvilinear trajectories in total N and P mass as a function of mass remaining, where N and P are initially net immobilized (i.e., nutrient mass increases) until reaching a peak when net nutrient mineralization starts (Aber & Melillo, 1982; Berendse et al., 1987; Manzoni et al., 2008).…”