Microdialysis - a sensitive method for estimating plant-available N released during litter decomposition

Scott Buckley1, Richard Brackin1, Susanne Schmidt1

1 The University of Queensland, Brisbane, QLD, 4072, http://www.uq.edu.au, [email protected]

Abstract

Given the importance of soil nitrogen (N) availability in controlling N supply of plants and microbes, accurate estimates of soil N forms are vital. However, common extraction methods disrupt the soil environment, biasing estimates of soil N availability. Microdialysis offers an alternative by sampling N fluxes with minimal disturbance, and here we compare ex situ soil microdialysis with traditional potassium chloride or water extractions in the context of crop litter decomposition. We amended soil microcosms with sugarcane (0.68% N) or soybean (2.51% N) litter at realistic rates (0.72, 5 and 14.3 mg C g-1 soil), quantified microbial activity parameters throughout a 30-day incubation period, and sampled N at day 30. In contrast to soil extractions, the diffusive fluxes generated with microdialysis facilitated a high-resolution snapshot of N availability. Microdialysis revealed that N was immobilised in the presence of sugarcane litter and was mineralised with soybean litter. Nitrogen immobilisation or mineralisation increased mostly with litter dose (although sensitivity varied somewhat between treatments) and in agreement with observed microbial activities. Such N processes were not apparent in soil extractions, indicating uniform N concentrations and forms across litter treatments. The only exception was the high soybean-amended treatments, in which total N increased. Our findings challenge the effectiveness of soil extractions to estimate plant-available N and resolution of N cycling processes in soils. Conversely, microdialysis represents a sensitive method for estimating the fine-scale N fluxes that are relevant to plants and insight into the factors regulating N cycling.