Flux between soil and atmosphere, vertical concentration profiles in soil, and turnover of nitric oxide: 1. Measurements on a model soil core

A stainless steel soil corer which was filled with homogenized soil was used to measure the flux (J) of NO between soil and atmosphere and the vertical profile of the NO mixing ratios (m) in the soil atmosphere, both as function of the NO mixing ratio (mm _a ) in the atmosphere of the headspace. The NO emission flux decreased linearly with increasing NO mixing ratio and turned into a deposition flux after passage of the compensation point (m _c) at about 400 ppbv NO. Almost the same compensation point was obtained when the turnover of NO was measured in flask-incubated soil samples as function of the NO mixing ratio. The flux (J) of NO at the soil-atmosphere interface was calculated from the production rate (P) of NO and the NO uptake rate constant (k) that were measured in these flask-incubated soil samples using the diffusion model of Galbally and Johansson (1989). The calculated fluxes agreed within <15% with those actually measured. The vertical profiles of NO were fitted to an exponential function and analyzed by Fick's first law of diffusion. The shape of the profiles indicated a net production of NO in the upper 10 cm soil layer when the atmospheric NO mixing ratio was below the compensation point and in a net consumption of NO when the atmospheric NO mixing ratio was above the compensation point. In soil layers below 10 cm depth, the turnover of NO resulted in compensation of production and consumption rates. Measurement of the actual diffusion coefficient using SF₆ showed that gas transport in the soil core was not only due to molecular diffusion but in addition due to a bidirectional gas flow. The experimentally determined diffusion coefficient was smaller than that computed from soil porosities, but resulted together with the additional transport term in NO fluxes that were close (< ±15%) to those measured. This is the first comprehensive study of NO concentration profiles and turnover rates in soil providing a theoretical basis for modelling NO fluxes at the soil-atmosphere interface.