| The global climit change is becoming one of the most important issue in the scientific field, while interest in the carbon (C) cycle in the terrestrial ecosystems disturbed by human activity has grown. Soil respiration is an important process in the C cycle, while partitioning different soil respiration componengts is very important. However, it is difficult to partition soil respiration precisely in situ. Because of the unreasonable use of lands, the Loess Plateau has become one of the regions with the most serious soil erosion. In the end of last century, China initiated a state-funded project, Grain-for-Green, in this region. However, few studies have focused on the influences of the project on the C sequestration potential in the terrestrial ecosystems. Four years' study had been conducted in the grasslands (fenced grassland:W; grazing grassland:Z) which had been retreated from cultivation over 20 years, as well as the croplands (G and Y) under traditional cultivation. With the improved method for soil respiration partitioning, we investigated the variations of soil respiration, different soil respiration components, net primary production (NPP), soil C storage, and analyzed the facts controlling the transformation between soil organic carbon (SOC) and soil inorganic carbon (SIC). The C balance was also estimated and the mechanism of C source/sink change was studied. The purposes are to better understand the effects of land-use conversion on soil carbon sequestration, and to provide valuable scientific information for evaluating the effects of "Grain-for-Green" project on C sequestration.The main results are as follows:1. Based on the trenching method, a method of "measuring alternatively among treatments, modeling and calculating separately" was established to partition and quantify microbial respirtion (Rm) and root respiration (Rr) in the grasslands and croplands. The results for the cropland (Y) (Setaria italica (L.) Beauv.) showed that total soil respiration (Rt) and Rm showed similar diurnal variation, with the maximum values at 13:00-15:00 and the minimum at 3:00-6:00. Soil temperature exerted predominant control over the diurnal variations of Rt and Rm. The minimum values of Rr appeared at 11:00-13:00 and the maximum at 0:00-3:00, which was not consistent with the change in photosynthesis. The daily mean values of Rt,Rm and Rr were close to that measured at 9:00. At the seasonal scale, Rm was strongly dependent on both soil temperature and moisture, the regression equation was:Rm=-0.020+0.024 T+0.013 M (r2=0.83, P<0.001). Rr was strongly correlated with leaf area index (LAI), the regression equation was:Rr=0.160+ 0.454 LAI (r2=0.85, P<0.05). Rr/Rt ratio was higher at nighttime and lower at daytime, and showed an pronounced seasonal variation. The daily mean values of Rr/Rt ratios were close to the values obtained at 9:00. Rr/Rt ratio ranged between 22.3% and 86.6% and averaged 67.3% in the whole growing season.2. with the improved trenching method, soil respiration in the fenced grassland (W) was partitioned into Rm and Rr in 2008 and 2009. With the measurements of soil CO2 production and soil CO2 diffusivity, an analytical model was applied to correct the data of Rm and Rr, aiming to reduce the method-induced error. The results showed that the diurnal and seasonal variations of Rm and Rr were similar to that in the cropland (Y), the diurnal variation of Rm was mainly depend on soil temperature, while that of Rr was mainly influenced by plant photosynthesis. The seasonal variation of Rm was predominantly controled by the effects of both soil temperature and moisture, the regression equation was:Rm=-0.024+0.027 T+0.022 M.(r2=0.76, P<0.001). Rr/Rt ratio was also higher at nighttime and lower at daytime. In the two years, Rr/Rt ratio ranged between 14.8% and 62.8% in the growing season (March-November), and averaged 41.7% and 41.9% for the whole growing season and whole year. The different distribution of precipitation in the two years did not change the Rr/Rt ratio. Corrected by the analytical model, it could be concluded that the usual trenching method with small root-free plots led to an underestimation of Rr and Rr/Rt ratio by 4.2% and 1.8%.3. The effects of grazing and grazing exclusion on different soil respiration components were investigated. After exclusion of grazing for about 3 years, SOC and microbial biomass C (SMBC) in the surface soil of grassland increased significantly(P< 0.05), resulting in the increase of Rm in most seasons. The temperature dependence of Rm also increased. The annual accumulations of Rm were 165.9 g C m-2 in W and 116.1 g C m-2 in Z. Rr averaged 0.374μmol CO2 m-2 s-1 in W in the whole growing season,21.0% higher than that in Z (0.309μmol CO2 m-2 s-1). It might be attibuted to the higher root biomass in W (220.0 gm-2) compared to that in Z (185.6 g -2).4. The carbon distribution and storage in 0-200 cm soil layer in the grasslands and croplands were investigated. SOC contents in the soil layers below 10 cm were higher in the croplands than that in the grasslands, while SIC contents in 20-80 cm soil layers in the grasslands were significantly higher than that in the croplands (P<0.05). in 0-20 cm soil layer, soil total carbon (TC) contents were significantly higher in grasslands than that in the croplands (P<0.05), while TC content in W was significant higher than that in Z (P<0.05). Negative correlations were found between SOC and SIC for all sites. In 0-200 cm soil layer, the mean value of SOC storage for the two croplands was 9.68×103g C m-2,36.2% and 41.9% higher than that in W (7.12 g C m-2) and Z (6.83 g C m-2) (P<0.05). SIC storages in 0-200 cm soil layer in the grasslands were significantly higher than that in the croplands, and the sequence was:W (5.49×104 g C m-2)> Z (5.30×104 g C m-2)> croplands (mean of G and Y:5.04g104 g C m-2). SIC storage in W and Z were 8.9% and 5.1% higher than that in the croplands, while that in W was 3.6% higher than that in Z. Because of the special harvesting method, Ca,Mg were taken out from the soil continuously, leading to the decrease of Ca,Mg storge and lower precipitation of SIC. On the other hand, the higher root biomass in the grasslands might lead to higher SMBC and higher CO2 concentration, further accelerated the decomposition of soil organic matter and the precipitation of SIC.After retreating from cultivation for 24 years, TC storage in the grassland (5.99×104g C m-2) did not change significantly under continuate grazing compared to the cropland (6.04×104g C m-2), while grazing exclusion for 5 years resulted in an increase of TC storage (6.21×104g C m-2) by 3.6%.5. NPP in the grasslands was estimated with three methods:"Peak biomass","peak-trough analysis" and "sum of positive increments in live and dead plus litter (SPI)", and the carbon balances in the croplands and the grasslands from 2007 to 2010 were also investigated. The results showed that the "SPI" method could give more reliable results of NPP compared to the others. For W, the NPP estimated with the "SPI" method was 1.3 and 5.3 times of "Peak biomass" and "peak-trough analysis" methods, while for Z, it was 1.5 and 6.2 times of the latter two methods, respectively. In this region, both the cropland and the grassland retreated from cultivation could act as carbon sink. The annual carbon sequestration in W and Z were 95.5 and 96.4 g C m-2, while that in the cropland (Y) was 35.8 g C m-2, The carbon sequestration in the grasslands were about 2.7 times of that in the cropland, which indicated that conversion of cropland to grassland contribute to the sequestration of the atmospheric CO2.
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