Effects of drought on soil phosphorus availability and fluxes in a burned Mediterranean shrubland

Abstract

Little research has been done to study the combined fire-drought impacts on the structure and functioning of Mediterranean fire-prone ecosystems. In this study we have evaluated post-fire patterns of soil phosphorus (P) availability and transformation rates in a Mediterranean shrubland subjected to different drought treatments. Precipitation was manipulated in various treatments to evaluate the combined effects of fire and moisture in the soil. The following treatments were included: long-term historical average precipitation, or historical control (HC); moderate drought (MD, 25% reduction from HC); severe drought (SD, 45% reduction from HC); and environmental control without rainfall manipulation (EC). In late summer, these plots were burned in order to evaluate the joint effects of drought and fire. In order to compare burned and non-burned scenarios, unburned plots were maintained without rainfall manipulation. Post-fire soil samples, collected in the spring, were analyzed for P fractions, P transformation rates (net solubilization and immobilization, gross mineralization) and phosphatase activity. Mid- to high-intensity fire caused a short-term increase of rapid-to-mid turnover inorganic P pools in the soil, as well as a decline of rapid turnover organic P pools (including microbial P) and acid phosphatase activity. Fire also induced an overall rise in the flux among P pools, including gross P mineralization, net microbial immobilization and net geochemical solubilization, with the latter being the most affected. Moreover, under drier than average conditions, this ‘mineralizing’ effect of fire was partially (rapid turn over inorganic P, total P flux among pools and net solubilisation) or completely (net immobilization) offset. Thus, our results indicate that a drier environment will limit the extent of post-fire peak in soil P availability. This effect may act synergistically with increased water stress to further inhibit vegetation recovery after fire, possibility which is particularly relevant for fire-prone Mediterranean shrublands where P limitation is widespread.