The increase in the concentration of dissolved nutrients stimulates plant litter decomposition in streams
Cristina G. Pedraz/DICYT The decomposition of plant litter is a fundamental process in forest streams. These small streams constitute the majority of watercourses in many river basins and are usually shaded by the surrounding vegetation. The lack of sunlight to these streams limits the primary production or vegetal production, which is the process that underlies the food chains in many systems such as forests, grasslands, lakes or the open sea. However, the surrounding vegetation is a source of plant remains, especially leaf litter, to these streams. This litter is decomposed by micro-organisms and invertebrates, which by their actions lead to the availability of carbon and nutrients and their incorporation into the food webs.
So, litter decomposition is at the basis of the food webs in these shaded streams, locally and downstream, where the litter particles, the carbon and released nutrients are used by other organisms. Since the decomposition of the litter is carried out by micro-organisms and invertebrates, it is sensitive to environmental changes affecting the activity of these organisms.
Any environmental change has the potential to affect litter decomposition in streams. For example, many watercourses have excessive nutrient concentrations due to the use of agricultural fertilizers, the disposal of waste water, the deposition of atmospheric nitrogen, or the invasion of forests by nitrogen-fixing plant species.
"These situations will probably escalate in the future as a result of population growth and climate change," explains Veronica Ferreira, a researcher of the River Basins Group at MARE – Marine and Environmental Sciences Centre, a center formed by six Portuguese universities, including the University of Coimbra (to which this scientist belongs).
Therefore, she thinks, "it is important to understand how the litter decomposition responds to the increased availability of nutrients and what factors may restrict this answer, since changes in litter decomposition rates may result in changes in the functioning of streams that could, in turn, affect the services that streams provide to human societies, such as good water supply.”
Review of 99 scientific papers
The River Basin Research Group of MARE at the University of Coimbra has been working on litter decomposition for more than 20 years, in collaboration with several international centers. As a result of this cooperative work, they have recently published in the journal Biological Reviews an article in which they carry out a systematic review of the scientific literature addressing the response of litter decomposition to the increase in the concentration of nutrients in streams.
Systematic reviews, using techniques such as meta-analysis, allow the condensation of information, and often reveal effects that cannot be detected in experimental studies that have low accuracy (due to the low number of samples) or where the magnitude of the effect is low, and thus "the use of such techniques in stream ecology should be encouraged."
The paper, headed by Ferreira, had the participation of the Portuguese MARE and of the Royal Holloway University of London (UK), the University of Bordeaux (France), the National Institute of Agronomic Research (France), the University of Coastal Carolina (United States) and the National Center for Scientific Research-CNRS (France).
According to Ferreira, over the last 45 years, many studies have addressed the impact of nutrient increase on litter decomposition using different approaches: laboratory experiments, field experiments over natural nutrient gradients or gradients resulting from human activities and experimental manipulation of nutrient concentrations in the field, all of which have also been performed by the River Basin Research Group. However, "there is some variation in the results and, surprisingly, no comprehensive review of the literature to draw general conclusions had been done previously" she recalls.
The article written by the team of researchers has analyzed 99 studies and states that, among other results, until 1994, there was no evidence that the increase of nutrients had an effect on the decomposition of plant remains. The accumulation of studies has increased the accuracy of the result and revealed a stimulatory effect of nutrient increase on litter decomposition. "This shows that it takes many studies so that we can be sure of an answer" says Ferreira. This stimulation is higher when the environmental nutrient concentration is low and when the magnitude of enrichment is large, suggesting that streams presently little impacted may suffer more with increases in the concentration of nutrients than streams where the concentration of nutrients is already high.
As suggested by the literature, the effects of the increase in the availability of nutrients on litter decomposition depend on the variation of other factors. For example, "in an agricultural context, the increase in the concentration of nutrients (with the potential for stimulating the decomposition of plant litter) is often accompanied by an increase in the concentration of pesticides (with the potential for inhibiting the decomposition of plant litter)”, so it is "urgent that further studies address these interactions between factors, which are situations that often occur in the field" she concludes.
|Ferreira, V., Castagneyrol, B., Koricheva, J., Gulis, V., Chauvet, E. and Graça, M. A. S. (2015). “A meta-analysis of the effects of nutrient enrichment on litter decomposition in streams”. Biological Reviews. doi: 10.1111/brv.12125|