| Terrestrial ecosystems can act as both "carbon source" or "carbon sink" in the global carbon cycle and play an important role in regulating the global carbon balance. The carbon exchange between grassland ecosystem and atmosphere is an important component of terrestrial ecosystem carbon cycle. The Loess plateau is known for its thick loess and wide distribution, which induce a great influence to the agricultural production of China, as well as the global climate change. However, the carbon sequestration potential and the strength and distribution of carbon source/sink in the grassland ecosystem on the Loess plateau is still not clear. The present study was conducted in Longzhong part of Loess Plateau. In the study, grassland ecosystem carbon pool is divided into three components:vegetation, organic residues and soil (0-1m), and the carbon storages of the components were determined; Soil respiration and net ecosystem CO2flux in fenced and grazed grassland were measured by using closed chamber method from March2012to February2013, the variations of CO2flux of grassland, as well as the the effects of soil temperature and soil moisture on CO2flux were investigated. Meanwhile, considering that Artemisia gmelinii shrub is the dominant plant in research area, soil respiration and net ecosystems CO2flux rate within and outside of the Artemisia gmelinii shrub patch were studied in the growing season in2012. The results are as follows:Enclosure significantly increased the vegetation biomass, net root productivity and SOC storage in0-20cm soil layer (P<0.05), but enclosure had no impact on the soil SOC content in the soil layers below20cm. The carbon storages in the fenced and grazed grassland ecosystems were5153.4±165.9g C·m-2and4847.0±154.0g C·m-2respectively, while96.3%and97.4%of which were in the soil, and more than59.5%of the rest organic carbon stored in root system. There were significantly positive correlations (P<0.01) between the soil total nitrogen (STN) and phosphorus (STP) content and soil organic carbon (SOC) in this area. The SOC content, SOC density and SOC storage declined with the increase of soil depth.The fenced grassland and grazing grassland could both act as carbon sink. The annual carbon flux in the fenced grassland was-58.0±14.5g C-m"2(carbon sequestration), of which the carbon flux in growing season and non-growing season made up-70.3±11.4g C·m-2and12.4±3.1g C·m-2, respectively. The annual carbon flux in the grazed grassland was-48.7±14.0g C·m-2, of which the carbon flux in growing season and non-growing season made up-56.9±10.6g C·m-2and8.1±3.3g C·m-2, respectively. The net ecosystem CO2flux rate in the growing season was mainly controlled by soil moisture, while mainly controlled by soil temperature in non-growing season. Soil respiration rate was mainly affected by soil temperature and soil moisture, the effect of soil moisture was greater in growing season, while the effect of soil temperature was greater in non-growing season. Enclosure significantly improved soil respiration and net ecosystem CO2flux rate (carbon sequestration) in the grassland in the growing season (P<0.05). However, the difference of CO2flux rate between the two sites was not significant in non-growing season. The higher soil respiration in fenced grassland was mainly resulted from the higher soil organic carbon content. Negative CO2flux was observed in the study sites, which might be contributed to the precipitation of soil inorganic carbon, which can be accelerated in the alkaline soil (pH>8.2).The diurnal variation of soil respiration rate both within and outside of the shrub patch showed "single-peak" pattern, while that of net ecosystems CO2flux rate both showed sigmoid curve under normal circumstances and "twin peaks" type with the water deficit. The peak values of soil respiration rate appeared in August, while that of net ecosystem CO2flux rate in September. The daily averages of soil respiration and net ecosystem CO2flux rate within the shrub patch were significantly higher than that of outside. The averages of soil respiration rate within and outside of shrub patch in the growing season were5.5±0.2and3.0±0.1g CO2·m-2·d-1, respectively, which were close to that observed in May. The averages of net ecosystem CO2flux rate within and outside of shrub patch in the growing season were-3.9±0.1and-1.2±0.1g CO2·m-2·d-1."Fertile Island" was observed under the shrub, which could contribute to the carbon accumulation in this region. Soil moisture and soil temperature were the key factors controlling the variation of soil respiration rate both within and outside of shrub patch, while net ecosystem CO2flux rate was mainly controlled by soil moisture. |
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