Climate change may reduce litter decomposition while enhancing the contribution of photodegradation in dry perennial Mediterranean grasslands

Plant litter decomposition plays a key role in ecosystem function: it is the main source for the formation of soil organic matter as well as the primary source of soil nutrients, and a key component of the carbon balance of terrestrial ecosystems. Understanding how solar UV radiation will affect leaf litter decomposition and how it will interact with climate change drivers is fundamental to predict how soil and ecosystem biogeochemical cycles will respond to a warmer drier world. We carried out a litter decomposition experiment in the field to investigate the role of UV radiation and its interaction with increased temperature and decreased precipitation using standing litter and litter on the ground of Stipa tenacissima, a dominant species in semiarid Mediterranean perennial grasslands. Experimental plots (n = 8) consisted of: increased air temperature (3 oC on average), decreased precipitation (35% average decreased), and increased air temperature plus decreased precipitation. UV radiation was manipulated using specially designed screens made by acrylic or polycarbonate that either passed or blocked 90% of the UV radiation, respectively. In addition, two positions simulating “standing dead litter” and “litter on the ground” were used. All climate change manipulation treatments decreased litter decomposition. In particular, litter decay rates were reduced up to 34, 43 and 62 % in the rainfall exclusion, warming, and the combination of warming plus rainfall exclusion treatments, respectively. In the control and rainfall exclusion treatments, litter on the ground decomposed 25 % faster than standing litter suggesting that microbial decomposers contributed substantially to litter decomposition. On the contrary, in the warming and combination of warming plus rainfall exclusion treatments, litter decomposition rates did not differ between positions, but increased by 29 % when exposed to UV radiation. Overall, lignin losses were paralleled by increases in cell solubles, particularly when litter was exposed to UV radiation and under less suitable conditions for microbial decomposition. Results from this study show how predicted climate change scenarios will reduce leaf litter decomposition rates while enhancing the contribution of photodegradation to overall litter decomposition in dry perennial Mediterranean grasslands, suggesting that the capacity of drylands to incorporate carbon sources into the soil derived from leaf litter will be substantially hampered in drier and warmer conditions.JRC.H.5-Land Resources Managemen