Soil N2O fluxes and related processes in laboratory incubations simulating ammonium fertilizer depots
High concentrations of ammonium (NH4+ ) in soil have been shown to inhibit nitrification, and fertilizer injection as conducted during CULTAN (controlled uptake long-term ammonium nutrition) management might thus have the potential to reduce N2O emission from arable soil. We conducted an incubation experiment with different NH4+ concentrations in soil that resembled concentrations as expected at and around injection spots (5000, 2250, 1000, 450, 0 mg NH4+-N kg-1 soil) directly after fertilization and after dilution due to plant uptake or precipitation. N2O emission was measured in dynamic soil mesocosms over a period of 21 days. Acetylene inhibition and 15N tracer approaches were used to calculate the relative contribution of nitrification and denitrification to N2O emission. An isotopomer approach was applied to gain further insight into N2O producing processes. We expected lower contribution of nitrification-derived N2O to total N2O emission and a higher N2O/NO3- ratio from nitrification with increasing NH4+ levels. Nitrification indeed declined with increasing NH4+ level, and no nitrification occurred in the 5000 mg NH4+ -N kg-1 soil treatment. A pool dilution approach showed that gross nitrification in 450 mg NH4+ -N kg-1 soil (nitrification rate: 4.96 mg NO3- -N kg soil d-1) was by a factor of 2.6 and 6 higher than in 1000 and 2250 mg NH4+ -N kg-1 soil treatments. In the 5000 mg NH4 +-N kg-1 soil treatment, gross nitrification occurred at very small rates (0.1 mg NO3- -N kg soil d-1). Similarly, N2O emission declined with increasing NH4+ level. The N2O yield of nitrification was between 0.07 and 0.15% of NO3- production, but was not affected by increasing NH4+ level. Nitrification was the dominant source of N2O throughout the incubation at all NH4+ levels, and there was no significant change in the relative contribution of nitrification and denitrification with NH4+ level or time. This finding indicates that denitrification derived N2O emissions were similarly reduced at high NH4+ levels. Applying the nonequilibrium technique to our 15N tracer data revealed heterogeneous distribution of denitrification in soil, with at least two distinct NOx- (NO3- + NO2 -) pools and spatial separation of NOx - formation and consumption. The isotopomer approach provided reasonable results in comparison with the acetylene inhibition and 15N tracer approaches and indicated substantial contribution of nitrifier denitrification and/or coupled nitrification-denitrification (10e40%) to total N2O production.
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