| Global warming is one of the urgent environmental problems. Carbon cycle takes a major role in climate changes, which becomes a hot issue interesting more and more scientists. SOC (Soil organic carbon) pools in the terrestrial ecosystem is about 1500 Pg, which is more than 2.8 times than terrestrial biological carbon pool and 2 times than global atmospheric carbon pool. Soil carbon emission at the global scale annual is 75 Pg, accounting on tem percents of total amount of atmospheric CO2. All these show that the fractional change of SOC pool and soil respiration may cause voilent changes in the climate of the Earth. Increasing SOC sequestration and reducing soil carbon dioxide emissions play an important role for reliefing global warming.The Loess Plateau, one of the areas with the most serious soil erosion in the world, is a typical rainfed agriculture area and an ecological fragile zone lying in arid and semiarid regions. Because of soil erosion and sweeping, vegetation degeneration, and low rainfall,the region has the lowest SOC content and density. Presently, the researches main centralize on the SOC content under the rehabilitation of small river basins, but the SOC pools and dynamic in the future, even soil respiration are neglected, for which we take two jobs based on the Wanggonggou watershed on the Loess Plateau. (1) Soil sampling units from 448 sites (0-20cm) and 33 sites (0-200cm) defining on the basis of land formand land use, were used to analyze the effect of land use, landform on SOC, and on the vertical distribution characteristic of SOC sampled to a depth of 200cm. (2) Based on the long-term fertilization experiment in the arid upland fields on the Loess Plateau that started in 1984 and manmade wood(Robinia pseudoacia L.), soil respiration, soil temperature and soil moisture were monitored with the dynamic closed chamber method (LI-8100 USA), and dynamics of soil respiration and its relationships with environmental factors were investigated.Results were as follows:There was significant variation in SOC concentration across the landforms (P<0.01). For the top soil of 0-20 cm, SOC concentration was greatest at the gully position (9.1 g kg-1), next was at the tableland position (7.8 g kg-1), and least (6.8 g kg-1) at the slope position. On the tableland, SOC in 0-20 cm varied between 6.49 and 8.56 g kg-1, and SOC content under manmade grass was significantly higher than cropland and orchard; on the slopleland, the range of SOC concentrations increased (5.79-9.95 g kg-1), and there was significant difference among landuse, while varied most in the gully (5.79-10.64 g kg-1), but there was no significant derrerence between grassland and woodland.Landform and lanuse had significant effect on the vertical distribution of SOC. For the subsoil, SOC in tableland was higher than that in gully and slopeland. For slopeland and gully, SOC decreased with depth increasing, while for tableland, SOC decreased initially, then increased, lastly decreased. Meanwhile, for tableland, the order of SOC appeared approximately manmade grassland > cropland > orchard with the effecting depth of land uses for 40cm, and for slopeland the order was native grassland > manmade woodland > manmade grassland > orchardwith the depth for 100cm, while for gully, there was no significantly difference (P > 0.05) among different land uses. SOC storage in the profile of 20-200 cm accounted for 88.3% sampled to a depth of 100cm, while for 100-200cm, SOC storage accounted 37.3% in 0-200cm equaling to 63.8% of the SOC storage in 0-100cm. The results revealed that landforms and land uses highly significantly (P< 0.05) affected the vertical distribution of SOC at a small watershed scale and considerable amounts of C were stored at deeper depths.At the gully region of Loess Plateau, soil respiration in soils under the removal of shrub under wood. For control treatment, exclusion and addition litter treatment, soil respiration decreased by 19%, 15%, and 17%, respectively. The amount change of litter also could significantly effected soil respiration. Soil respiration increased under litter addition treatment, while decreased under litter exclusion treatment. For shrud, the litter adding could increase soil respiration by 31.15%, while decreaed by 27.23% under exclusing litter. For removal of shrub, the values of the two litter treatment were 34.51% and 23.77%. Correlstion analysis showed that the litter change did not influence the relation between soil respiration and soil water content and soil temperature, but removal of shrub cound significantly altered the correlation level of soil respiration and soil water content and soil temperature. Significant exponential relation were found between soil respiration and soil temperature, meanwhile soil respiration has cubic parabola relation with soil moisture.Soil respiration varied drastically within the range from 0.06 to 5.05μmol m-2 s-1 with a variation coefficient of 116.5% and a mean of 2.00μmol m-2 s-1. For the whole study period, in terms of soil respiration, the treatments followed a decreasing order of NPM > M > NP > N and CK, with cumulative CO2-C emission being 2.0, 1.6, 1.2, 0.8 and 0.8 Mg hm-2, respectively. The relationship between soil respiration and soil moisture appeared in an extremely significant parabolic curve (P < 0.01), which explains 55% of the variation of the soil respiration. Although soil respiration was in extremely significant linear relationship with soil temperature (P < 0.01),soil temperature could explain only 19% to 39% of the variation of soil respiration. Strength of the response of soil respiration to ploughing significantly (P <0.05) was positively related to soil microbial biomass carbon and soil organic carbon showing a linear relationship, but not to total nitrogen or soil dissolved organic carbon (P > 0.05).
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