How nitrogen and plant diversity interact in decomposition of leaf litter in streams

Alan M. Tonin, Luz Boyero, Silvia Monroy, Ana Basaguren, Javier Pérez, Richard G. Pearson, Bradley J. Cardinale, José Francisco Gonçalves Jr., Jesús Pozo

Biodiversity is currently being lost at an alarming rate, and understanding the effects of such loss on how ecosystems function is a major scientific challenge. One key ecosystem process potentially affected by the loss of plant species is decomposition of leaf litter, which is crucial to carbon and nutrient cycling in terrestrial and aquatic ecosystems. Whether or not the number of plant species affects the rate at which litter is decomposed is still under debate, partly because of the complex role of different plant characteristics, and because of the influence of environmental variables. We explored the influence of two factors on the relationship between the number of plant species and decomposition rate in streams: the plants’ ability to harness bacteria that fix atmospheric nitrogen, making it available to the plant; and the concentration of dissolved inorganic nitrogen in the water, which is a key environmental factor, often enhanced by human activity. We conducted a laboratory experiment that mimicked stream conditions, in which we exposed different combinations of plant species, with or without nitrogen-fixing capacity, to organisms that decompose litter (microbes such as fungi and selected invertebrates) at natural and elevated nitrogen concentrations. We found that increasing the number of plant species increased decomposition rate, subject to the influence of microorganisms and invertebrates (the latter having stronger effects). However, the ability of plants to fix nitrogen was not important, as this ability was not related to the nitrogen concentration of litter, and probably because other litter characteristics played a role. In contrast, nitrogen concentration of the water modulated the effect of larger numbers of plant species on decomposition because it enhanced the decomposition of nitrogen-poor litter by microbes. Our findings suggest that the consequences of loss of terrestrial plant species for decomposition in streams will depend on the identity of the species that are lost or remain and on levels of nitrogenous pollution, typically associated with fertiliser run-off and animal and human waste.

Image caption: Image provided by authors. 
Read the paper here.

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