Crop Residues Influence Response of Soil N2o Emissions to Freeze-Thaw Cycles.

Freeze-thaw cycles within soils are known to stimulate nitrogen (N) and carbon mineralization rates which may lead to increased N₂O emissions. The intensity of freeze-thaw cycles and addition of plant residues however, may impact N₂O emission rates. The objective of this laboratory study was to examine the effect of crop (soybean and maize) residue addition and freeze-thaw cycles on N₂O emission rates from two soils (a clay loam and a sandy loam).

Emissions of N₂O were about one order of magnitude lower in the sandy loam than in the clay loam soil. Initially, N₂O emissions from both soils were stimulated by the addition of crop residues, especially with soybean. However, emissions stopped after a few days at a constant temperature of +1°C. On the contrary, freeze-thaw cycles markedly increased N₂O emission from the soils, especially during the thawing phase, with the response to the freeze-thaw lessening with each new cycle. Cumulative N₂O emissions from amended soils were highest with moderate freeze-thaw cycles (+1 to -3°C), while cumulative emissions from the unamended soils were highest when freeze-thaw cycles were more extreme (+1 to -7°C).

Interestingly, N₂O-N emissions from amended soils levelled-off after 3 to 4 freeze-thaw cycles, whereas in unamended soils emissions continued to respond to later cycles. Moreover, in the clay loam, the highest cumulative N₂O-N emission was measured in the unamended treatment submitted to extreme freeze-thaw cycles. These results suggest that freezing and thawing may increase substrate availability to soil denitrifiers during the winter period. However, the substrates may actually originate from the soil reserve rather than from the freshly added crop residues.

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