Nitrous Oxide Formation in Black Earth Soils Depending on the Soil Water Content

Abstract

Normally nitrous oxide (N2O) is formed by biological denitrification under anaerobic conditions in soils, but recent results have indicated that nitrous oxide can also be formed as a byproduct of the microbial nitrification. This is important particularly for the black earth soil in Central Germany because of the absence of typical denitrification conditions under semi-arid climate conditions. Measurements of the N2O liberation by this soil (Haplic Phaeozem) in relation to the soil water content showed in general the following results: — below 80% of the water holding capacity (WHC) (aerobic conditions) the N2O emission is on a nearly constant low level (≤ 0.4 g N kg−1 soil h−1); — above 80% WHC (anaerobic conditions) the N2O emission increases exponentially and reaches up to 28 μg N kg−1 soil h−1 at 90-95% WHC. These findings indicate that there are probably two different paths for the N2O formation in soils. First studies on Haplic Phaeozem using [15N]ammonium and [15N]nitrate and a GC-MS aided incubation system confirmed the assumption given above as follows:- under anaerobic conditions (≥90% WHC) N2O originates mainly from the nitrate pool by denitrification; — under aerobic conditions (≤ 80% WHC) the N2O formation is lower, but the gas originates directly from the ammonium and not from the nitrate pool, probably as a byproduct of the nitrification process.