Influences of Land Use/Cover Types on Nitrous Oxide Emissions during Freeze-Thaw Periods from Waterlogged Soils in Inner Mongolia

Nitrous oxide emissions during freeze/thaw periods contribute significantly to annual soil N 2 O emissions budgets in middle- and high-latitude areas; however, the freeze/thaw-related N 2 O emissions from waterlogged soils have hardly been studied in the Hulunber Grassland, Inner Mongolia. For this study, the effects of changes in land use/cover types on N 2 O emissions during freeze–thaw cycles were investigated to more accurately quantify the annual N 2 O emissions from grasslands. Soil cores from six sites were incubated at varying temperature (ranging from −15 to 10°C) to simulate freeze–thaw cycles. N 2 O production rates were low in all soil cores during freezing periods, but increased markedly after soil thawed. Mean rates of N 2 O production differed by vegetation type, and followed the sequence: Leymus chinensis (LC) and Artemisia tanacetifolia (AT) steppes > LC steppes ≥ Stipa baicalensis (SB) steppes. Land use types (mowing and grazing) had differing effects on freeze/thaw-related N 2 O production. Grazing significantly reduced N 2 O production by 36.8%, while mowing enhanced production. The production of N 2 O was related to the rate at which grassland was mowed, in the order: triennially (M3) > once annually (M1) ≥ unmown (UM). Compared with the UM control plot, the M3 and M1 mowing regimes enhanced N 2 O production by 57.9% and 13.0% respectively. The results of in situ year-round measurements showed that large amounts of N 2 O were emitted during the freeze–thaw period, and that annual mean fluxes of N 2 O were 9.21 μg N 2 O-N m -2 h -1 (ungrazed steppe) and 6.54 μg N 2 O-N m -2 h -1 (grazed steppe). Our results further the understanding of freeze/thaw events as enhancing N 2 O production, and confirm that different land use/cover types should be differentiated rather than presumed to be equivalent, regarding nitrous oxide emission. Even so, further research involving multi-year and intensive measurements of N 2 O emission is still needed.