Diffusion of 15N-labelled N 2O into soil columns: a promising method to examine the fate of N 2O in subsoils

Abstract

Nitrous oxide research has generally focused directly on measuring fluxes of N 2O from the soil surface. The fate of N 2O in the subsoil has often been placed in the ‘too hard’ basket. However, determining the production, fate and movement of N 2O in the subsoil is vital in fully understanding the sources of surface fluxes and in compiling accurate inventories for N 2O emissions. The aim of this study was to generate and introduce into soil columns 15N labelled N 2O, and to try and determine the consumption of the 15N 2O and production of ambient N 2O. Columns, 100 cm long by 15 cm diameter, were repacked with sieved soil (sampled from 0 to 5 cm depth) and instrumented with silicone rubber gas sampling ports. Nitrous oxide enriched with 15N was generated using a thermal decomposition process at 300 °C and then transferred to 2 l flasks. After equilibrating with SF 6 tracer gas the 15N 2O was introduced into the soil columns via passive diffusion. Gas samples from the soil profile and headspace flux were taken over a 12-day period. A watering event was simulated to perturb the 15N 2O gas composition in the soil profile. Using the measured 15N enriched fluxes and the rate of decline in 15N in the N 2O reservoir, from which the N 2O diffused into the soil, we calculated an N 2O sink (consumption plus absorption by water) equal to 0.48 ng N 2O g −1 soil h −1. The decrease in the 15N enrichment between successive soil depths indicated N 2O production in the soil profile and we calculated a net N 2O production rate of 0.88 ng N 2O g −1 soil h −1. This pilot study demonstrated the potential for simultaneously measuring both N 2O consumption and production rates, using the 15N enrichment of the N 2O measured. Further potential refinements of the methodology are discussed.