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

Zedong Lu,Ziming Li,Yanfen Wang,Zongmin Liang,Yaling Wang,Pengrui Du,Rui Du,Saisai Qin,Shiping Wang

doi:10.1371/journal.pone.0139316

Abstract

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: 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 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.

Highlights

Nitrous oxide (N2O) contributes significantly to global warming [1] and destroys stratospheric ozone [2]
The daily mean air temperature fell by 13.8°C within five days, increased by 17.7°C three days later in April 2013 (Fig 1)
Topsoil generally started to freeze in early November and thawed in April, and the soil surface was subjected to seasonal freeze/thaw cycles (FTCs) during April

Summary

Introduction

Nitrous oxide (N2O) contributes significantly to global warming [1] and destroys stratospheric ozone [2]. Grazing management affects soil microorganisms [15, 16] In combination, these effects strongly influence N2O emissions. Recent studies reported that grazing decreased N2O emission because the effects of grazing on inorganic nitrogen, soil moisture, and soil microbes were greater than those on N cycling [17]. Mowing inhibits surface litter accumulation [18, 19] and alters plants’ access to light [20], soil surface temperature, soil moisture [21], and microbial growth [21, 22]. Previous studies suggested that mowing facilitated CH4 uptake in grassland because of reduction in soil inorganic N [23], and weakened N2O emission through its effect on vegetation types and some soil properties [24]. Matson et al [27] and Corre et al [28] noted the dynamics of soil organic matter (C and N) cycling among land use/cover types as a consequence of environmental and soil characteristics [27, 28]

Methods

Results

Conclusion