Soil respiration rate and its sensitivity to temperature in pasture systems of dry-tropics

Tree clearing is a topical issue the world over. In Queensland, the high rates of clearing in the past were mainly to increase pasture production. The present research evaluates the impact of clearing on some soil biological properties, i.e. total soil respiration, root respiration, microbial respiration, and microbial biomass (C and N), and the response of soil respiration to change in temperature. In-field and laboratory (polyhouse) experiments were undertaken. For in-field studies, paired cleared and uncleared pasture plots were selected to represent three major tree communities of the region, i.e. Eucalyptus populnea, E. melanophloia,and Acacia harpophylla. The cleared sites were chosen to represent three different time-since-clearing durations (5, 1113,and 33 years; n18 for cleared and uncleared plots) to determine the temporal impact of clearing on soil biological properties. Experiments were conducted in the polyhouse to study in detail the response of soil respiration to changes in soil temperature and soil moisture, and to complement in-field studies for estimating root respiration. The average rate of CO2 emission was 964 g CO2/m2/yr, with no significant difference (PB0.05) among cleared and uncleared sites. Microbial respiration and microbial biomass were greater at uncleared compared with those at cleared sites. The Q10-value of 1.42 (measured for different seasons in a year) for in-field measurements suggested a small response of soil respiration to soil temperature, possibly due to the limited availability of soil moisture and/or organic matter. However, results from the polyhouse experiment suggested greater sensitivity of root respiration to temperature change than for total soil respiration. Since root biomass (herbaceous roots) was greater at the cleared than at uncleared sites, and root respiration increased with an increase in temperature, we speculate that with rising ambient temperature and consequently soil temperature, total soil respiration in cleared pastures will increase at a faster rate than that in uncleared pastures.