Linking Temporal Dynamics of Nitrous Oxide Flux to Soil Biophysical Conditions During Winter and Spring Thaw.

Agricultural soils are a significant anthropogenic source of nitrous oxide (N₂O).  Significant N₂O emissions from agricultural soils have been reported during winter and spring thaw.  Two main mechanisms have been proposed to explain the N₂O fluxes associated with soil freeze-thaw cycles: 1) enhanced microbial activity due to increased nutrient availability and optimal soil conditions at spring thaw, and 2) release of N₂O produced and accumulated in the soil profile during winter.  It is not clear under which conditions each of these mechanisms predominates, and better understanding of the processes resulting in high N₂O fluxes during soil thawing is needed.  We have conducted several studies deploying a micrometeorological technique to measure surface N₂O fluxes at a long-term Ontario site established in 2000 with the objective of improving our understanding of these processes.  Over 5-years we observed that no-till systems (NT) significantly reduce emissions during thaw events, when compared to conventional tillage (CT).  This is due to a reduction in soil freezing as a result of the insulating effects of the snow cover plus corn and wheat residues during winter. Use of ¹⁵N tracer indicated that the source of N₂O burst at spring thaw was mostly ‘newly’ produced N₂O by denitrification in the surface layer, and not the release of N₂O trapped in the unfrozen soil beneath the frozen layers. In addition, using wavelet analysis we concluded the mechanism of release due to gases trapped at depth was not supported due to strong coherence between N₂O fluxes and soil surface conditions. Apparent heat capacity measured in situ with heat pulse probes showed a different temporal pattern between NT and CT, with CT presenting more phase change events.  Two out of the three N₂O emission bursts in CT, which occurred during winter and early spring, took place immediately after phase change from ice to liquid water at 5 cm depth. A temperature response was observed for this period which was consistent with the suggestion of a breakdown in the N₂O reduction process in the 0 to 5˚C range.  Study of the diversity of the nitrifier and denitrifier communities during freeze/thaw was assessed by PCR-DGGE of the amoA, nirS and nirK genes.  Statistical analysis revealed that the structure of the nitrifier and denitrifier communities associated with each management system was different, and these differences were most obvious immediately after the spring thaw event.  It appears that N₂O producing bacterial community structure is significantly altered during the freeze/thaw cycle that leads to enhanced N₂O fluxes.  In a recent study we measured active populations of both nitrifiers and denitrifiers during soil thawing,  providing evidence of denovo denitrification.  Furthermore, a large N₂O flush from one of the field plots was associated with a delay in expression of nosZ, the gene responsible for the conversion of N₂O to N₂ gas during denitrification.  It appears that for our field site differences in the magnitude of N₂O released during spring thaw may be due to soil conditions that support incomplete denitrification.  These results have important implications for the modelling of N₂O fluxes associated with soil thawing.