Nitrogen transformation processes and gaseous emissions from a humic gley soil at two water filled pore spaces

Abstract

The artificial drainage of heavy textured gley soils is prevalent on pasture. Drainage of a soil profile reduces the water filled pore space (WFPS) in the upper soil horizons with consequences for N2 and N2O emissions, the fate of nitrogen (N), transformational processes and microbial and bacterial communities. The present intact soil column study with isotopically enriched fertiliser investigates all these aspects simultaneously under two WFPS treatments (80% (HS) and 55% (LS) saturation). Results showed significant differences in nitrous oxyde (N2O) emissions, in both pattern and amount, with maxima at 11.97 mg N2O-N/m2h for HS and at 1.64 for LS. Isotopic enrichment data showed a significant predominance (74.8–97.2%) of nitrification in LS, with a possible reduction in NH4+ but a higher concentration of nitrate (NO3−) in N losses. Denitrification dominated in HS (72.5–73.4%), possibly leading to high ammonium (NH4+) losses. Enrichment values showed differential apportionment patterns. A high component of N2O emission derived from denitrification in HS (6.0% HS; 0.4% LS) with a significant amount of N2O (62.9%) transformed to N2 (3.7% LS). A higher percentage of 15N was retained in LS soil. HS showed a lower amount of unaccounted N highlighting lower losses. Differences in gene copy concentrations (GCC) were found across most analysed genes (16S, nirS, nirK, nosZ1, nosZ2, amoA and nrfA). Both HS and LS treatments showed similar potentials for N2O production and its reduction to N2, but a reduced potential for nitrification and dissimilatory nitrate reduction to ammonium (DNRA) in HS. This study explained the effect of drainage and rewetting on gaseous emissions providing an explanation in terms of community switches. On the current soil type, structures to manage watertable heights would push the system towards complete denitrification with only N2 production but may present risks in terms of ammonia (NH3) and NH4+ losses.