The effects of climatic change on soil microorganisms are modulated by the depositing of nitrogen in the atmosphere
Cristina G. Pedraz/DICYT Researchers from Spain, Portugal, and the United States have found that the effects of climatic change on the functioning of soil microorganisms are modulated by the depositing of nitrogen in the atmosphere. The amount of nitrogen pollution in the atmosphere in an area will determine the response of the system to the expected changes in precipitation as a consequence of climatic change.
The collaborators in this study were Lourdes Morillas, Javier Roales, and Antonio Gallardo of the Universidad Pablo de Olavide in Spain; Jorge Durán of the Centro de Ecología Funcional of the University of Coimbra in Portugal; Alexandra Rodríguez of the National Museum of Natural Science/Higher Council for Scientific Research (CSIC) in Spain; and Peter M. Groffman and Gary M. Lovett of the Cary Institute of Ecosystem Studies in the United States.
As one of the researchers in the team (Jorge Durán) explains, one of the most critical aspects of climatic change is the intensification of the water cycle in many regions of the world, i.e. increased drought in combination with more frequent and intense storms. This intensification affects soil humidity, which is one of the most important factors controlling biochemical processes, with an increased frequency of the drying and humidifying cycles.
Another component of global change is the large amount of nitrogen deposited in the atmosphere, which is mainly due to the use of fossil fuels and agriculture. According to Alexandra Rodríguez, the depositing of excess nitrogen may have serious consequences on ecosystems, such as nutrient imbalances, the acidification of water and soil, the eutrophication (an excessive concentration of nutrients) of ecosystems, increased N2O emissions, and changes in the carbon storage capacity of soils.
On the other hand, Lourdes Morillas points out that it is important to consider microbes as mediators of biogeochemical cycles. “Rapid changes in soil humidity are stressful for microbes, as they must invest a large amount of energy and resources to respond to them. For this reason it is to be hoped that the capacity of soil microorganisms to respond to changes in rain patterns varies with the nutritional situation of the soil”.
Despite their importance, few studies have examined the interactions between the depositing of nitrogen and the changes in rainfall patterns, and there is a great uncertainty as to how an increase in nitrogen will modulate the soil's capacity to resist climatic change. The study carried out by this team of scientists aims to help to fill this gap.
An experiment of adding nitrogen and humidity changes
In 1996 the researchers of the Cary Institute of Ecosystem Studies selected six plots, one of each pair of which was treated regularly with nitrogen in an experiment that has gone on for 15 years. Together with the Spanish and Portuguese researchers, in May 2012 they collected samples of these soils and subjected them to four drying and humidifying cycles. In one of them the soil was kept at a constant humidity and in the other three the soil was subjected to one, two, and three drying and humidifying cycles.
Before, during, and after the incubation the team used a series of laboratory techniques to assess the nutritional and functional condition of the soil. For example, the amount and diversity of microorganisms, the levels of nitrogen mineralisation, and microbial respiration were estimated.
An increased nitrogen deposit would attenuate the impact
With this experiment the researchers showed that the capacity of soils of temperate woodland for cycling carbon and nitrogen will be significantly altered by changes in the precipitation pattern that are likely to occur as a consequence of climatic change. “The increased drying and humidifying cycles will probably lead to increases in the amounts of ammonium and total inorganic nitrogen in the soil, but to decreases in nitrate owing to a reduced nitrification rate. It will also cause a reduction in the microbial biomass and in the exchanging of greenhouse gases between the soil and the atmosphere”, specifies Javier Roales.
The most significant result of the study, which will be published shortly in Global Change Biology, is the key role of the depositing of nitrogen as a modulator of the responses of these woodlands to climatic change. Scientists point to a clear interaction between these two components of global change (the depositing of nitrogen and climatic change); an increased depositing of nitrogen “could attenuate the impact of the expected increase in the drying and humidifying cycles with climatic change on important soil processes”. This type of empirical study assessing the interaction of various factors is of particular interest for the creation of models that can realistically predict the response of ecosystems to global change.