Field study of environmental variables controlling the NO emissions from soil and the NO compensation point

The NO flux rates were measured on a sandy loam at Haxthäuser Hof near Mainz in summer 1983. Significant emissions of NOy species other than NO were also occasionally measured. The data show that the NO and NOy emission rates are dependent on soil temperature, soil moisture, soil porosity, fertilization, plant cover, soil organic matter content, and NO and NOy mixing ratios in ambient air. Generally, one of these variables dominated and masked the influence of the others. In some cases the soil treatment led to the replacement of one dominating controlling factor by another one. In most cases, NO and NOy emission was observed at low, and deposition at high, mixing ratios of NO and NOyin the ambient air. The compensation points, i.e., the mixing ratios at which the flux from the soil is zero, varied between 0–5.5 parts per billion by volume (ppbv) for NO and 0–16.3 ppbv for NOy depending on the plant cover, soil temperature, soil organic matter content, and fertilization. Because the compensation points are of the same order of magnitude as the NO and NOy mixing ratios in ambient air, knowledge of the variables controlling them is essential for creditable estimations of the global NOx emissions by soils.