Comparison of two different methods to measure nitric oxide turnover in soils

The NO turnover in soils was measured in two different experimental set-ups , a flow-through system, which is very time-consuming and needs rather soph isticated equipment, and a closed system using serum bottles. We compared t he NO turnover parameters (NO consumption rate constant, NO production rate , NO compensation concentration) that were measured with both systems in di fferent soils, under different conditions and in the presence of 10 Pa acet ylene to inhibit nitrification. The values of the NO turnover parameters th at were measured with the two systems under oxic conditions were usually co mparable: The addition of acetylene did not: affect the NO consumption rate constants of the soils with the exception of soil GI. However, the NO prod uction rates and the NO compensation concentrations decreased significantly in the presence of acetylene, indicating that nitrification was the main s ource of NO in these soils. Only one soil (BoI) showed no nitrifying activi ty. Increasing soil moisture content resulted in decreasing NO consumption rate constants and NO production rates. Even at a high soil moisture conten t of 80% water holding capacity, nitrification was the main source of NO. T he values of the NO turnover parameters that were measured with the two sys tems were not comparable under anoxic conditions. The NO consumption rate c onstants and the NO production rates were much lower in the closed than in the flow-through system, indicating that the NO consumption activity became saturated by the high NO concentrations accumulating in the closed system. Under oxic conditions, however, closed serum bottles were a cheap, easy an d reliable tool with which to determine NO turnover parameters and to disti nguish between nitrification and denitrification as sources of NO.