Improved Soil Cover Method for Field Measurement of Nitrous Oxide Fluxes

TLDR

Soil cover methods are widely regarded as the most useful approach for measuring field N₂O exchange, yet cover designs lack standardization. The study develops equations to predict optimal vent dimensions that transmit ambient pressure fluctuations while minimizing N₂O loss by diffusion. The authors use vented enclosures with closed‑loop or passive air circulation and introduce a revised flux equation based on head‑space N₂O concentration, along with vent‑dimension equations to optimize cover performance. The revised flux equation yields time‑independent N₂O flux estimates across diverse sampling conditions, and vent‑optimized covers achieve higher accumulation rates than smaller vents but similar to larger vents.

Abstract

Most researchers agree that soil cover methods offer the most useful approach to measuring nitrous oxide (N2O) exchange in the field, but there is little uniformity in the design of covers used by them. The cover design least subject to the potential errors associated with this approach is a vented enclosure with either closed-loop air circulation or no forced air circulation. Although the soil N2O concentration gradient beneath these covers decreases with time as the gas accumulates, this problem can be overcome by a proposed change in the equation for computing flux from the increase in head-space N2O concentration. Over a broad range of sampling conditions, N2O fluxes computed from the new equation were independent of the time allowed for N2O accumulation. Other equations were developed that predict optimum soil cover vent dimensions for effectively transmitting ambient pressure fluctuations to the enclosed space while minimizing loss of the accumulating N2O by diffusion to the outside. Soil covers with vent dimensions dictated by the equations had N2O accumulation rates significantly higher than covers with 60% smaller vent diameters but not significantly different than covers with 60% larger vent diameters.