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Contribution of the vegetation layers in the nitrous oxide emission from alpine Kobresia humilis Serg. meadow ecosystem on the Tibetan Plateau

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Du Y. , Cao G. , Deng Y. , Sun G. , Cui X.

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2010

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tom Vol. 58, nr 1

115-124

EN

Nitrous oxide (N[2]O) was one of the major atmospheric greenhouse gases. Its budget was poorly understood in alpine meadow, a dominant vegetation type on the Tibetan Plateau. To characterize a Kobresia humilis meadow on the plateau, N[2]O emission rates were monitored from June 2003 to June 2006 in the study area located at 3280 m a.s.l. Nine plots with 1 m x 1 m each were divided into three treatments, i.e. intact herbaceous community (HCK), removal of aboveground plant biomass (CBK), and removal of both above and belowground plant biomass (BSK), to estimate contribution of plants, r hizosphere and bulk soil to the total N[2]O emission. N[2]O emission from plant aboveground biomass was calculated by flux difference between HCK and CBK, denoted as F[(HCK-CBK)], from rhizosphere by F[(CBK-BSK)], and from bulk soil was the flux in BSK treatment. Static chambers (height 50 cm, area 0.5 x 0.5 m2 )were used for gas collection. N[2]O emission rate was significantly correlated with soil temperature at 5 cm depth in both HCK and BSK (P <0.001). Both treatments demonstrated a seasonal peak rate in growing season and minimum rate in dormancy period. The mean emission rates in the three years were 39.7[plus or minus]2.9 and 30.6[plus or minus]2.5 [mu]g m[^-2] h[^-1] in HCK and BSK, respectively, with the former significantly higher than the latter (P <0.05). In CBK, however, the emission rate did not show consistent correlation with soil temperature, especially in growing season. Its three-year mean emission rate was 36.2[plus or minus]3.3 [mu]g m[^-2] h[^-1]. In the K. humilis meadow, bulk soil contributed much more than plants and rhizosphere. The mean emission rate was 3.5[plus or minus]2.9, 5.7[plus or minus]3.8, and 30.6[plus or minus]2.5 [mu]g m[^-2] h[^-1] (P <0.001) from plants, rhizosphere and bulk soil, and these accounted for 9, 14 and 77%, separately. Our results implied that N[2]O emission rate decreased little with grazing as indicated by the difference between HCK and CBK in K. humilis meadow (P <0.05). N2O emission from alpine meadow could not be ignored in addressing regional greenhouse gases budget on the Tibetan Plateau, considering the vast area and much higher radiative forcing of N[2]O.

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Simulating N2O emission from Kobresia humilis Serg. alpine meadow on Tibetan Plateau with the DNDC model

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Du Y. , Cui X. , Cao G. , Zhao X. , Yang G.

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2011

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tom Vol. 59, nr 3

443-453

EN

Field measured N2O emissions in two years were used to parameterize and validate a process-based model, DNDC, for an alpine Kobresia humilis meadow on the Tibetan Plateau in China. Although this model failed to capture the N2O fluxes in some time periods in the spring or autumn, the modeled results showed overall a good performance in terms of simulating the seasonal variation of N2O fluxes and quantifying the annual total emissions. The relative deviation on the annual basis was about 12.4% and -15.9% for the two years, respectively. The modeled data showed that nitrification contributed about 53% of total N2O production, slightly higher than denitrification. The modeled fluxes were sensitive to soil organic content (SOC), pH, and temperature, but less sensitive to variation of precipitation, soil ammonium and nitrate contents. Further modifications for the model were suggested to focus on the process of soil freezing and thawing as well as the crop growth sub-model that would improve the model's performance for quantifying N2O emission from the alpine meadow.

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Yak (Bos grunniens L.) grazing effects on vegetation of alpine meadow with Potentilla fruticosa L. (Rosaceae) shrub in Qinghai-Tibet Plateau

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Qiao Y. , Wang Q. , Wang W.

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2009

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tom Vol. 57, nr 4

769-777

EN

Grazing can change plant community composition and structure, which may alter the functions of the shrub meadow ecosystem. Grazing effects on Potentilla fruticosa shrub community in the headwater region of the Yellow River, which is in core area of the Qinghai-Tibet Plateau, are studied to provide adequate protection decision-making. We investigated continuous grazing and seasonal enclosure effects on P. fruticosa shrub communities. Three sites of P. fruticosa shrub comprising both continuous grazing and seasonal enclosure treatments were selected. The size of each fenced plot of P. fruticosa shrub was about 3000 m2, the stocking rate was about 5 heads per 100 m2 in continuous grazing treatment. Three samplings were made in each growing season of 2003 and 2004. Cover of vegetation, plant species composition and vegetation height were investigated in seven 1 x 1 m quadrates in each treatment. Above-ground biomass was measured in five 0.5 x 0.5 m quadrates. Shrub, forb, graminoid and sedge plant materials were clipped at ground level and oven-dried at 85[degrees] C to a constant mass. Plant composition was affected by long term continuous grazing and changes were caused by forb species shifting. No apparent difference in species richness between the grazed and ungrazed communities over the growing months were found but the Shannon's diversity indices of the grazed communities in June and July were higher than that of the ungrazed but lower in the late August and September. Live vegetation cover was reduced by 6.7%, 7.3% and 11.5%, respectively, owing to grazing in July, August and September, but not in June (P> 0.05). Forbs took up more than 50% cover of the vegetation in both grazed and ungrazed treatments. Relative cover of sedges and forbs in ungrazed treatment decreased in July, August and September, while that of graminoids increased more than 70% in the same period. Live vegetation height was reduced by 27% (2004) and 23% (2003) in late August and early September, but not in early growing season. Grazing reduced total above-ground biomass by 35%, 37% and 36% in July, August and early September, respectively, and the reduction was mainly in forb biomass. Continuous grazing affects plant composition and species diversity. The quantitative characteristics of P. fruticosa communities were influenced by grazing over growing months, but the effects were offset by non-growing season grazing.