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 Haxthauser Hof near Mainz in summer 1983. Significant emissions of NO(y) species other than NO were also occasionally measured. The data show that the NO and NO(y) emission rates are dependent on soil temperature, soil moisture, soil porosity, fertilization, plant cover, soil organic matter content, and NO and NO(y) 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 NO(y) emission was observed at low, and deposition at high, mixing ratios of NO and NO(y) in 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 NO(y) 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 NO(y) mixing ratios in ambient air, knowledge of the variables controlling them is essential for creditable estimations of the global NO(x) emissions by soils.