Drainage losses of N2O and NO3- in Ferralsol is a major N-loss pathway

Peter R. Quin1, 2, 3, Lukas van Zwieten1, 2, 3, Peter R. Grace4, Lynne M. Macdonald5, Annette L. Cowie1, 6, Dirk V. Erler7, Iain M. Young8, Stephen W. Kimber2
1 – School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia. Email: [email protected]

2 – NSW Department of Primary Industries, 1243 Bruxner Highway, Wollongbar, NSW 2477, Australia.

3 – Southern Cross Plant Science, Southern Cross University, Military Rd, East Lismore, NSW 2480, Australia.

4 – Institute for Sustainable Resources, Queensland University of Technology, 2 George St, Brisbane, QLD 4000, Australia.

5 – CSIRO Agriculture, Glen Osmond, SA 5064, Australia.

6 – NSW Department of Primary Industries, Trevenna Rd, University of New England, Armidale, NSW 2351, Australia.

7 – School of Environment, Science and Engineering, Southern Cross University, Military Rd, East Lismore, NSW 2480, Australia.

8 – School of Life and Environmental Sciences, Faculty of Science, University of Sydney, NSW 2006, Australia.

Abstract

Increasing concentrations of atmospheric nitrous oxide (N2O) are making a significant contribution to anthropogenic climate change and the depletion of stratospheric ozone. These increases are known to primarily result from the use of synthetic nitrogen fertilisers and manures. Our study aimed to answer some of the many remaining questions about the mechanisms of production and movement of N2O in soil. In a field study we injected 15N-labelled nitrate into repacked columns of Ferralsol, at a depth of either 75 mm or 200 mm. We sampled soil gas at 3 depths and surface emissions. In-soil concentrations of N2O rose by approximately two orders of magnitude when water-filled pore space increased to >80 %. This coincided with periods of high hydraulic conductivity, potentially draining dissolved 15N2O from the 75 mm injected columns at 189 µg 15N-N2O m-2 h-1 compared with a surface flux of 1.2 µg 15N-N2O m-2 h-1 and from 200 mm injected columns at 30 µg 15N-N2O m-2 h-1 compared with a surface flux of 0.24 µg 15N-N2O m-2 h-1. Data suggests that indirect emissions of N2O by leaching and surface runoff from some soils may be much greater than the default 0.225 % of N applied recognised by the IPCC. This may go some way towards reconciling the discrepancy between ‘top down’(~4 %) and ‘bottom up’ (~1.3 %, IPCC default) estimates of direct N2O emissions from applied N. We also show that deeper placement of nitrate fertiliser may decrease direct N2O surface emissions, although the effect on indirect emissions remains unclear.