Shifts in temperature response of soil respiration between adjacent irrigated and non-irrigated grazed pastures

Abstract

Land management practices that increase food production are needed to match demand from a growing global population. Adoption of these practices needs to be balanced by potential adverse consequences such as nutrient losses and production of greenhouse gases. We previously demonstrated that pasture soils irrigated during summer-dry conditions had significantly less soil carbon than adjacent unirrigated pastures, despite increased plant production. Precise reasons for lower carbon under irrigation were not clear but both inputs (photosynthesis) and losses (respiration) of carbon are regulated by soil moisture and temperature. Our objective was to determine whether the temperature and moisture response of soil respiration differed between 13 adjacent irrigated and unirrigated soils (0-0.1 m). Soil respiration rates were measured in the laboratory using a temperature block where rates of respiration were measured within 5 h at ˜3 °C increments between 6 and 60 °C (20 temperatures) and at 5 different moisture contents. Temperature response, sensitivity and key temperature parameters (temperature optimum (Topt) and inflection point temperature (Tinf)) were calculated using macromolecular rate theory (MMRT). Respiration rates increased with increasing moisture content similarly for both irrigated and unirrigated soils. However, soil respiration at the same temperature was significantly (P < 0.05) lower and Q10 higher in irrigated soils. Tinf and Topt were greater in irrigated soils. We attribute the lower respiration in irrigated soils to a disproportionate loss of available carbon, total soil carbon loss was ˜14% while the differences in respiration were between 58% at 10 °C and 41% at 20 °C. The lower carbon availability in irrigated soils was likely responsible for the increased Q10, Tinf and Topt as substrate decomposability and availability became more limiting so that ongoing decomposition became increasingly dependent on solubilisation and diffusion of remaining carbon substrates to micro-organisms. We postulate that as respiration becomes increasingly limited by substrate supply through physical chemistry processes (diffusion, and sorption/desorption) rather than substrate biodegradability, the temperature response curve will shift from a MMRT dominated response (with a temperature optimum) to an Arrhenius dominated response (exponential). Our data suggest that commencement of irrigation removed moisture limitation during normally dry summers and led to a loss of soil carbon due to an initially increased microbial activity that has now decreased as carbon availability declined.