Effects Of Freezing-thawing Cycles On N₂O Fluxes From Northern Temperate Meadow Steppe Soils

Hulunbuir grassland, an important part of the Eurasian grassland, located in China’s northernmost,freezing-thawing occurs frequently. Research have showed alternately freezing and thawing process candirectly affect soil physical and microbial properties, nitrogen transformation process and strength, andfreezing-thawing of the soil can be an important source of N2O emissions. In this paper, the laboratorysimulations were combined with field measurements of greenhouse gas fluxes results to investigate theeffects of different soil layers, grassland management practices, different frequency and intensity offreezing-thawing, and vegetation types on soil N2O production rate and emission flux in Hulunbuirgrassland soils in Inner Mongolia.Experiments conducted in Chinese Academy of Agricultural Sciences Hulunbuir GrasslandExperimental Station, three different management practices (fenced, grazing, mowing) and three typicalrepresentative meadow vegetation types (Leymus chinensis, Stipa baicalensis, Leymus chinensis+mesophilic forbs meadow grassland) in Hulunbuir were included in our field observation. Fencing andgrazing as check treatments were set both in Leymus chinensis and Stipa baicalensis meadow grassland,respectively. Fencing and mowing treatments were also set in Leymus chinensis+mesophilic forbs meadowgrassland. We cultured undisturbed soil columns of five layers collected from three kinds of grassland typesand different treatments soil, then measured N2O,CO2gas concentration; combined with the field results ofN2O,CO2flux during the2012-2013growing season and freezing-thawing periods in Stipa baicalensismeadow steppe. The effects of freezing-thawing process on greenhouse gas fluxes emission from temperatemeadow steppe were also analyzed in the present paper.The main results were showed as bellow:(1) The variation trends of N2O production rates of Leymus chinensis and Stipa baicalensisgrassland soil are consistent on different soil layers, cultivate time and temperature difference during thefreezing-thawing. Furthermore, N2O production rates and production amount of Leymus chinensis and Stipabaicalensis grassland showed that: Stipa baicalensis fenced plot＞Leymus chinensis fenced plot, Stipabaicalensis fenced plot＞Stipa baicalensis grazing plot, and that of Leymus chinensis grassland are cut3＞ cut1＞uncut.(2) N2O production rates of each test soil layer in Hulunbuir grassland are not balanced atdifferent time. N2O production rates were mainly due to the nature feature of the soils. N2O productionrates of surface layers(0-3、3-6、6-9cm) are greater than that of the lower soil layers. In addition, almost anamount of N2O emissions come from topsoil during freezing-thawing periods. N2O production rates are thehighest when just enter the freezing or thawing environment (2h or1d), and are significantly different fromthe N2O production rates at other training time, the N2O production rate after frozen2h was higher than thatafter melting2h.(3) Hulunbuir grassland soil always maintained the process produced N2O duringfreezing-thawing. N2O production rates of during diurnal freezing-thawing process are higher than seasonalfreezing-thawing process. But the N2O production amount during the seasonal slow alternatelyfreezing-thawing process was larger than that during the daily quickly process. Therefore, effects offreezing-thawing during the season conversion such as winter to spring and autumn to winter on N2Oemissions worth further attention and study.(4) During freezing-thawing, under the same temperature difference, N2O production ratedecreased with the increase of cultivate times; under different temperature difference, N2O production ratedecreased with the decrease of temperature difference; which resulted from soil nutrient substrateconstantly reduced.(5) Field observations showed that N2O emissions of Stipa baicalensis grassland fenced plotduring freezing-thawing and the proportion in annual emissions are greater than grazing plot. Ourlaboratory simulation results also indicated that N2O production rates of fenced plot soil were greater thanthat of grazing plot, but grazing increased N2O annual emission. N2O emission during the spring thawingwas the largest contributors to the N2O emission during the whole period of freezing-thawing cycles. WeShould strengthen the observations of N2O emission during the spring thawing, and further explore theregular and the factors influence N2O emissions during the spring thawing.