Respiration from soil and litter in a sown perennial grass pasture

Abstract

The severe nitrogen (N) deficiency which occurs in many sown grass pastures in Queensland is believed to be exacerbated by large and continuous inputs of carbon (C) from decomposing plant residues. In this study we attempted to quantify the importance of surface litter, roots and soil organic matter as sources of respiration in an established green panic (Panicum maximum var. trichoglume) pasture in south-east and Queensland. Intact pasture cores were taken from the field and the surface litter was removed before applying the following treatments: (i) surface litter added, (ii) roots pruned to kill approximately 60% of roots but not kill the plant, and (iii) plant shoots removed. Cores from bare soil between green panic plants were also included. The cores were kept in a glasshouse and CO₂ evolution measured continuously for 117 days using an alkali absorption method. Respiration from the various components of the system was estimated. Evolution of CO₂ from the cores was increased by litter addition and decreased by shoot removal. Root pruning stimulated CO₂ evolution in litter-removed treatments but had no effect in litter-added treatments. Root respiration and microbial respiration of root-derived C accounted for an average of 53% of the total evolved CO₂. Surface litter, soil organic matter and dead roots accounted for an average of 40%, 4% and 3% respectively. The importance of a particular C source to microorganisms varied depending on the availability of other C sources. Cores were destructively sampled on five occasions and the soils incubated at 25°C for 10 days to measure CO₂ evolution with surface litter and roots removed. Evolution of CO₂ in incubated soils was increased by litter and, to a lesser extent, by live roots, demonstrating that some of the labile C from these plant components was soluble or well incorporated into the soil.