Severe wildfire hinders renewal of soil P pools by thermal mineralization of organic P in forest soil: Analysis by sequential extraction and 31P NMR spectroscopy

Abstract

Forest productivity is limited by soil P availability in several forest ecosystems worldwide. Most of the soil available P is probably produced by the mineralization of organic forms of P when these forms dominate in the soil. Severe wildfires lead to loss of soil organic compounds and therefore represent a strong risk of loss of soil P. The objective of the present study was to examine how the temperature reaching during burning affects the P fractions in organic horizons and soil. For this purpose, we conducted experimental burns of intact soil monoliths with their organic horizons. We then used Hedley chemical sequential fractionation and 31P MNR spectroscopy to determine the effects of temperature during burning on soil P fractions. In the unburned organic horizons, the organic P represented 70% of the total P and it was completely mineralized when the temperatures reached above 500°C. Similarly, in the unburned mineral soil, organic P forms represented 76% of the total P and the organic P was reduced by around 50% in the moderate soil burn severity (SBS) levels. In the highest SBS, the concentration of inorganic P (Pi) with a fast turnover rate (representing available P) was three times higher and the concentration of Pi with a slow turnover rate (representing the sum of inorganic fractions bound to Al and Ca) was eight times higher than in the unburned soil. By contrast, the organic P fractions decreased with the temperature reached during the fire, with 200°C considered a threshold for the total thermal mineralization of organic P, mainly the labile organic fraction. These findings suggest that high temperatures lead to release of Pi, most of which is precipitated by P forms such as Pi with a slow turnover rate. It appears that the high burn severity associated with temperatures higher than 200°C strongly disrupts soil P dynamics, increasing the occlusion capacity and decreasing the bioavailability.