Soil Respiration Under Different Land Use In Shangri-la, NW Yunnan

Carbon stored in the soil in Tibetan plateau, due to low temperature, may be fragile and sensitive to global warming. However, little knowledge is known about the effects of land use on carbon emissions and dynamics in the plateau's ecosystems. Reliable estimates of the CO2 flux from soil and the CO2 balance of ecosystems on a regional scale and a better understanding of the mechanisms that regulate the carbon storage are critical for understanding the response of soils to changing land use and global climate change.In Shangri-la, Hengduan Mountain area, Northwest Yunnan, by using closed chamber technique with IRGA, we measured soil respiration rates under five different land use types, i.e. pine forest, shrubland, subalpine meadow by perennial grazing and seasonal grazing, and highland barley cropland. The main conclusions are as follows:(1) Soil organic matter (SOM), total N and available N were three to five times higher in the two meadows than in forestland and shrubland. Seasonal grazing meadow may be the better land use and management practice because of larger amounts of SOM, total N, total P, available N, available K and three soil enzymes, compared to perennial grazing meadow. Except for available K and soil pH, pine forestland and shrubland had the similar soil nutrients status in other indexes. Correlation was observed between different soil property indexes, especially between SOM and others. SOM was significantly and positively correlated to total N and available N. (2) Respiration rates under these five land use types showed strong one-humped seasonal variations, with highest rates in July and lowest in January. On the annual average, respiration rates in pine forest, shrubland, perennial grazing meadow, seasonal grazing meadow and cropland were 1.90, 2.43, 2.81, 2.40 and 2.42μmol m-2 s-1, respectively.Litter removal had little effects on soil respiration in pine forest and shrubland, which suggested that in subalpine region aboveground litter contribution to soil respiration was not significant.Both ecosystem and soil respiration rates were higher in perennial grazing meadow than in seasonal grazing meadow in summer from June to October, which suggested that grazing increased respiration rates in subalpine meadow ecosystem. Soil respiration was the major component of ecosystem respiration in the meadow, accounting for 63.0～92.7% and 47.5～96.4% for perennial and seasonal grazing meadow, respectively. However, in cropland aboveground plant respiration was the major component of ecosystem respiration, accounting for 49.7%～69.3%.(3) Affected mainly by air temperature and soil temperature, significant one-humped diurnal variation of respiration was observed in perennial grazing meadow and cropland, with highest rates at 14:00～16:00 and lowest rates in the early morning. There were more variability and complication of diurnal patterns in pine forest.Diurnal pattern of respiration had a good coincidence with air temperature in the chamber and soil temperature at 5 cm depth, but there was a 2-hour lag period between air and soil temperature. Soil volumetric water content at 6 cm depth was negatively varied with temperature.(4) The soil temperature exponential model F = aebT could explain the variation of respiration better than soil moisture model (R2 = 0.50～0.92, P < 0.0001), while the multiple model F = aebTWc could give better simulations than single factor model (R2 = 0.56～0.95,P < 0.0001).(5) Pine forest and shrubland had similar carbon storage, which was 79.34 and 88.07 t hm-2, respectively; while much higher storage was observed in seasonal grazing meadow than in perennial grazing meadow, which was 423.74 and 271.37 t hm-2, respectively.(6) Temperature sensitivity of respiration varied with different land use. Q10 values of soil respiration showed the following descending order: cropland (4.54) > shrubland (4.41) > perennial grazing meadow (3.59) > pine forest (3.03) > seasonal grazing meadow (2.17). In the context of global warming, cropland may be most sensitive to rising temperature.Q10 under different time scales may be controlled by different ecological processes and factors. In meadow ecosystem, long-term (annual) Q10 was about twice as great as short-term (one day) Q10. Seasonal grazing meadow had a lower long-term Q10 than perennial grazing meadow, suggesting that meadows under lighter grazing may have a good potential ability of carbon sequestration.