General soil properties of the soil sampling sites^a.

<p>^aAbbreviations are as follows: BD, bulk density; SOM, soil organic matter; UM, un-mowed grassland; M1, mowed once annually; M3, mowed once triennially; LUG, <i>Leymus chinensis</i> grassland; SUG, <i>Stipa baicalensis</i> grassland; SG, <i>Stipa baicalensis</i> grazed grassland</p><p>General soil properties of the soil sampling sites<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0139316#t001fn001" target="_blank">^a</a>.</p

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

<div><p>Nitrous oxide emissions during freeze/thaw periods contribute significantly to annual soil N₂O emissions budgets in middle- and high-latitude areas; however, the freeze/thaw-related N₂O 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 N₂O emissions during freeze–thaw cycles were investigated to more accurately quantify the annual N₂O emissions from grasslands. Soil cores from six sites were incubated at varying temperature (ranging from −15 to 10°C) to simulate freeze–thaw cycles. N₂O production rates were low in all soil cores during freezing periods, but increased markedly after soil thawed. Mean rates of N₂O production differed by vegetation type, and followed the sequence: <i>Leymus chinensis</i> (LC) and <i>Artemisia tanacetifolia</i> (AT) steppes > LC steppes ≥ <i>Stipa baicalensis</i> (SB) steppes. Land use types (mowing and grazing) had differing effects on freeze/thaw-related N₂O production. Grazing significantly reduced N₂O production by 36.8%, while mowing enhanced production. The production of N₂O 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 N₂O production by 57.9% and 13.0% respectively. The results of in situ year-round measurements showed that large amounts of N₂O were emitted during the freeze–thaw period, and that annual mean fluxes of N₂O were 9.21 μg N₂O-N m^-2 h^-1 (ungrazed steppe) and 6.54 μg N₂O-N m^-2 h^-1 (grazed steppe). Our results further the understanding of freeze/thaw events as enhancing N₂O 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 N₂O emission is still needed.</p></div

Mean N₂O production rates (μg kg^-1 h^-1) along the profile (0–15 cm) of soil cores: From UM (a, b), M1 (c, d), and M3 (e, f) over the entire incubation period (n = 3); Freezing periods: a, c, e; Thawing periods: b, d, f.

<p>Lowercase letters indicate significant differences (P < 0.05) between samples from different soil depths.</p

Cumulative productions of N₂O along the whole soil profile (0–15 cm) at different temperature during the freeze–thaw cycles of different land use/cover type^a.

<p>^aThe duration is 7 days at each temperature. Uppercase letters indicates significant differences (P<0.05) among different soil types. Lowercase letters indicates significant differences (P<0.05) among different temperature (mean ± SE, n = 3)</p><p>Cumulative productions of N₂O along the whole soil profile (0–15 cm) at different temperature during the freeze–thaw cycles of different land use/cover type<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0139316#t002fn001" target="_blank">^a</a>.</p

Soil conditions at the start and end of the freeze/thaw incubation experiments: Average total nitrogen (TN) (a), Soil organic matter (SOM) (b), NH₄⁺ (c), NH₄⁺ content at different soil depths for M1 (d), and NO₃⁻ (e), NO₃⁻ content at different soil depths for SUG (f).

<p>The vertical bars indicate standard errors of three replicates.</p

Soil N₂O production rates along the whole soil profile (0–15 cm) during the freeze–thaw cycles of different land use/cover type.

<p>Uppercase letters indicates significant differences (P<0.05) among different soil types. Lowercase letters indicates significant differences (P<0.05) among different cycle (mean ± SE, n = 3)</p><p>Soil N₂O production rates along the whole soil profile (0–15 cm) during the freeze–thaw cycles of different land use/cover type.</p

Conditions at SG site from September 2012 to May 2013: Daily mean air temperature (AT), daily mean precipitation, and soil temperature at depths of 10 cm depth (ST10) and 20 cm (ST20).

<p>Conditions at SG site from September 2012 to May 2013: Daily mean air temperature (AT), daily mean precipitation, and soil temperature at depths of 10 cm depth (ST10) and 20 cm (ST20).</p

Mean N₂O production rates (μg kg^-1 h^-1) along the soil profile (0–15 cm) of undisturbed soil cores: From LUG (a, b), SUG (c, d), and SG (e, f) during the entire incubation period (n = 3).

<p>Freezing periods: a, c, e; Thawing periods: b, d, f. Lowercase letters indicate significant differences (P < 0.05) between samples from different soil depths.</p