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Stream carbon and nitrogen supplements during leaf litter decomposition: contrasting patterns for two foundation species.

Pastor, Ada; Compson, Zacchaeus G.; Dijkstra, P.; Riera, Joan L.; Martí, Eugènia; Sabater, Francesc, Hungate, Bruce A.; Marks, J. C.  Oecologia 176 :1111-1121 (2014)

Presentación1Trees are the charismatic megaflora. Their leaves changing color in the fall inspires photographers, artists, and poets. But a few weeks later, the leaves are dead, having lost not just their color but also their charisma – few poems record the aesthetics of the dull brown and soggy dead leaves that lie decomposing on the forest floor or as scum in the river nearby. Even the scientific terms for dead leaves, “litter” and “detritus”, evoke garbage. Yet in death these leaves teem with life, microbial life, such that each leaf is both home and food to thousands of creatures. Much is not understood about this complex micro-community, beginning with their basic energy economy. For example, do the microbes get all the nutrition they need from the leaves? Probably not. Like a nation rich in some resources but lacking others, microbes growing on dead leaves need to import nutrients from the surrounding environment, especially nutrients that are rare in leaves, like nitrogen. But at what rates do these exchanges occur? And is the carbon in leaves really enough to sustain the microbes? And how does it vary from one leaf type to another? These are difficult questions, because, while thousands of studies have measured how rapidly leaves decompose, it’s much more difficult to track the bi-directional exchanges of materials between the decomposing leaves and the surrounding environment. In a recent study, Ecoss scientists used stable isotopes to mark the carbon and nitrogen in the leaves so that it was distinct from the carbon and nitrogen in the surrounding environment. They discovered that these micro-creatures rely strongly on a major import economy for nitrogen, not so surprising, but also for carbon, which was completely unexpected, because dead leaves contain lots of the element. Furthermore, it turns out that the type of leaf makes a difference: microbes living on the narrowleaf cottonwood leaves (top panel in picture) import quite a bit more carbon, and the microbial communities growing on leaves of Fremont cottonwoods (bottom panel) import substantially more nitrogen. This contrast was surprising, because fremonts typically contain more nitrogen than narrowleafs, so why the microbial community should import nitrogen at a faster rate when growing on fremonts is puzzling, but points to more questions the research team will pursue in future studies. Even with these new questions, the findings revise our understanding of the flow of material and energy in stream ecosystems, and highlight the importance of the invisible players, the bacteria and fungi inhabiting decomposing leaves. Few recognize the charisma of these creatures. After all, they excrete compounds that make dead leaves slimy, that give pond scum its name, but when you drill down there is a fascinating community and element economy that is essential to the livelihoods of other more classically charismatic creatures inhabiting rivers.

Pastor, Ada; Compson, Zacchaeus G.; Dijkstra, P.; Riera, Joan L.; Martí, Eugènia; Sabater, Francesc, Hungate, Bruce A.; Marks, J. C. Stream carbon and nitrogen supplements during leaf litter decomposition: contrasting patterns for two foundation species. Oecologia 176 :1111-1121 (2014)  DIGITAL.CSIC

(Agraïm a Bruce Hungate la redacció d’aquesta sinopsi, i a Vicent Pastor Urios la il·lustració que acompanya l’article)