Soil respiration and nutrient availability after short heating followed by rewetting differ between first and second heating and are influenced by the interval between heating events

Abstract

Soils may be exposed to short heating events for example in fast-moving grass fires which can reoccur if not all fuel was burnt in the first heating. Little is known about the effect of a second heating on microbial activity and nutrient availability. The aim of this study was to determine the effect of one and two heating events on soil respiration and nutrient availability. The soil was incubated moist (165 g kg−1) for 7 days. On day 8, soils were left unheated (CM) or heated and then maintained for 30 min at 60 °C followed by rapid rewetting to 165 g kg−1. Heated treatments included heated once (H8) or heated a second time 4, 8 and 16 days after the first heating event (H8-12, H8-16 and H8-24). Soil respiration was measured from day 8–26, soils were sampled on day 10, 14, 18 and 26 for available N and P pools. Heating and rewetting induced a flush of respiration. Compared to unheated soil cumulative respiration was about 10 % higher in H8 and 20% higher in - H8-12 and H8-16, but 30% higher in H8-24. From two days after heating, available N and P were about 25% higher in heated soils than the unheated control. The second heating induced a further increase in available N and P compared to the first heating. Microbial biomass P was generally 25–50% lower in heated soils than the control and was lower after the second than the first heating. Citrate P was 30–50% higher in heated soils than the control, irrespective of number of heating events. HCl P which was the largest P pool was not affected by heating. It can be concluded that a short heating event followed by rewetting increases N and P availability with a further increase after the second heating, particularly when the interval between heating events is long. The increase in nutrient availability may aid plant recovery after heating, but could also increase nutrient loss by leaching.