The Effect of Single Heating on Soil Microbial Activity and Nutrient Cycling

Abstract

The effect of soil heating during forest fires (often >100 °C for several hours) on soil microbes and nutrient availability has been studied extensively. Less is known about the effect of fast-moving fires with low fuel load where soils reach 50 to 100 °C for a few minutes. In this thesis, the effect of heating soil to 60 °C within 1 h and then maintaining this temperature for 30 minutes was studied. Soils were rewetted after cooling to room temperature. 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. An experiment included heated soils, constantly moist controls and air-dried soils which were dried at 30 °C to the same water content as the heated soils. Heating increased cumulative respiration and available N after rewetting about three-fold compared to the constantly moist control and the air-dried soils. To assess how the effect of heating is influenced by amendment type or time between amendment application soil was amended with the same amount of total N and P as pea residue or inorganic N and P either eight or one day before heating. Heating only reduced respiration when residue was added one day before heating. Heating increased available N on day 10 in the unamended soil or with fertiliser by about 20% and in residue treatments about 10-fold, particularly when residue was added one day before heating. To assess the effect of a second heating event, soils were heated once on day 8 (H8) or heated again 4, 8 and 16 days after the first heating event (H8-12, H8-16 and H8-24). Compared to unheated soil, cumulative respiration was about 10 and 20% higher in H8 and H8-12 and H8-16, but 30% higher in H8-24. The first heating increased available N and P by 25%. The second heating induced a further increase in available N and P compared to the first heating. To assess the effect of plants, soil was planted with wheat for 4 weeks or left unplanted. The heatinginduced increase in initial respiration rate, available N and P was greater in previously planted soil than unplanted soil. The last chapter includes two experiments. In the first experiment, soils were constantly moist or exposed to a drying-rewetting event before heating. Prior drying and rewetting had no effect on the impact of heating on respiration and nutrient availability. In the second experiment, non-saline soil was salinised to EC1:5 1 and 4 ds m-1 (referred to as NS, S1 and S4). After one month and pea residue addition (10 g kg-1), soil was incubated for 5 days, then heated. In moist soil, S4 reduced cumulative respiration but increased available N and P compared to NS and S1. Heating reduced cumulative respiration more in S4 than NS and S1. Compared to unheated treatments, available N in heated NS was up to ten-fold higher, but only three-fold in S4.