Soil phosphorus transformations under forest burning and laboratory heat treatments

Natural and prescribed fires (including site preparation burning) are common in pine and mixed-pine forests in the southeastern USA. The present high fuel load condition of many forests has renewed concerns about the effects of hot fires on soil nutrients and on nonpoint sources of pollution [i.e. sediments and phosphorus (P)]. Variation in temperature and velocity of fire due to unequal fuel loading and soil moisture can affect the extent of soil nutrient alterations including transformation of organic forms of P (Po) to inorganic forms (Pi). This variation in fire severity is difficult to capture systematically in the field, thus laboratory experiments simulating fire have been utilized. Most of these studies have used long duration treatments (> 1 h) that do not reflect field conditions where soils may be heated for only minutes. The objective of this study was to investigate soil P changes in relation to short-duration soil heating. Soil samples were collected before and after a prescribed burn in a 40-yr-old loblolly pine forest. Pre-burn soil samples were also subjected to temperature (100, 200, 300, 500, and 1000 °C) by heating duration (2.5, 5, 15, 30, and 45 min) treatments in the laboratory. In all cases, a partial sequential P extraction was performed. We hypothesized that pyromineralization of O horizon would increase water extractable soil P in the A horizon along with Po transformation to Pi within the A horizon. In the field, the NaHCO3 inorganic P fraction increased in 0–5 cm by 0.8 kg P ha− 1 one week after the burn: this increase was likely due to ash addition. In the laboratory, all 0–5 cm soil P pools responded to heating with Po converting to Pi and total P declining at the highest temperatures and durations. The combination of temperature * duration determined the extent of change with a high temperature-short duration (e.g., 1000 °C * 2.5 min) burn having the same impact on soil P as a medium temperature-longer duration burn (e.g., 300 °C * 45 min). Increasing forest fuel loads may be sufficient to alter the fire severity of future prescribed fires such that an increase in A horizon Po transformations to Pi could prove to be significant for P mobility in soils and plant growth.