Impact of a short heating event followed by rewetting on soil respiration and nutrient availability is not only due to soil drying during heating

Abstract

Little is known about the impact of short-term heating to 50–100 °C as it may occur in fast-moving low-intensity fires on microbial activity and nutrient availability. Heating of soils usually induces drying, but it is unclear if the effect of heating is only due to this water loss or if other factors are also important. Two experiments were carried out where soils were heated to 60 °C. The experiments included constantly moist controls and air-dried soils which were dried at 30 °C to the same water content as the heated soils. After heating/air-drying, the soils were rapidly rewet and incubated for 2 weeks at a constant water content optimal for microbial activity. The aim of the first experiment was to assess the effect of water content before heating and heat duration on soil respiration and nutrient availability. Soil was incubated for 14 days at 33, 82, and 165 g water kg−1 (referred to as W33, W82, and W165, corresponding to 10, 30, and 50% maximum water-holding capacity) where 165 g water kg−1 is optimal for microbial activity in this soil. The soils were then heated to 60 °C and maintained at this temperature for 30 or 90 min. Heat duration had little effect on the measured properties. In heated soils, cumulative respiration after rewetting was about threefold higher than in the constantly moist control. Two days after heating, available N in heated soils was twofold higher than in the constantly moist control and 0.3 to twofold higher than the corresponding air-dried soils. Two weeks after heating, available N differed little between the constantly moist control and heated soils that were at W33 and W82 before heating, but it was about twofold higher in heated soil that was at W165 before heating. Available P 2 days after heating was highest in heated soils, but 2 weeks after heating, available P was lower in heated soils than the constantly moist control. In the air-dried controls which were dried at 30 °C to the same water content as in heated soils prior to rewetting, cumulative respiration, available N and P after rewetting differed little from the constantly moist soil. The aim of the second experiment was to determine the effect of soil nutrient content on soil respiration and nutrient availability after heating. Two soils differing in organic matter, total N, and total P content were used either separately or as mixes with different proportions of the soils. Soils were heated and maintained at 60 °C for 30 min. Before and after heating/air-drying, the soils were maintained at optimal water content (180 g water kg−1). Two and 7 days after heating, available N was 10–30% higher in heated soils than the constantly moist control and air-dried soils. It can be concluded that the effect of short-term heating followed by rewetting on soil respiration and available N and P is not only due to soil drying, but possibly also heating-induced changes in soil organic matter composition and availability as well as soil P sorption capacity.