| In agro-pastoral eco-tone of northern china, grazing and cultivation are the most widely managements for grassland. The studies of different land uses on ecosystem carbon fluxes and their driving factors which play an active role in rationalizing land use managements and benefit for increasing carbon fixation capacity and ecosystem services. In this study, CO2 exchange patterns of soil-plant-atmosphere in several ecosystem types including ungrazed grassland (UG), moderately grazed grassland (MG), heavy grazed grassland (HG) and cropland (CL), perennial artificial grassland (AG) which located in the temperate grassland of Bashang area in Hebei Province. We studied the effects of grazing intensity and cultivation on the Net ecosystem CO2 exchange (NEE) and its components of Ecosystem respiration (Re) and Gross ecosystem photosynthesis (GEP) using closed-chamber technique, and the driving factors which impacted on the change of carbon fluxes were analyzed. The main results showed that:(1)Above ground biomass decreased with grazing intensity increased, but moderate grazing increased the root biomass of 0-12cm of topsoil. Compared with un-grazed grassland, heavy grazing significantly decreased 58% of above ground biomass and 12% of root biomass (p<0.05), and moderate grazing significantly increased 23% of above ground biomass and 10% of root biomass (p<0.05). Above ground biomass in artificial grassland and cropland was significantly greater than ungrazd grassland (p<0.05), while the root biomass in cropland was significantly lower than ungrazed and artificial grassland (p<0.05). Moderately grazing increased the topsoil soil organic carbon and total nitrogen.(2) NEE and its components of Re and GEP was regressed with time using quadratic polynomial equations (p<0.05), showing a seasonal variations across the growing seasons in UG, MG, HG and CL. However, only GEP showed a significant regression with time in AG (p<0.05).The annual (2012-2014) coefficient of time variation with NEE for UG, MG and HG was 0.64,0.79 and 0.50, respectively. Annual coefficient of time variation with NEE for grazing sites (UG, MG and HG)was 0.63, which was less than Re (0.70) and GEP (0.77), this suggests that GEP was more sensitive to seasonal variations than NEE and Re. There was a significant annual variation for NEE and GEP in grazing grasslands (p<0.05), NEE in 2013 significantly increased 33% for 2012 and 34% for 2014, respectively; GEP in 2013 significantly increased 24% for 2012 and 26% for 2014, respectively; However, there was no significant annual variation for Re (p>0.05).(3) Using the stepwise analysis with all three grazing sites from 2012 to 2014, we found that a positive linear relationship between monthly NEE and above ground biomass (AGB) and soil water content (VWC), with the equation of (NEE=-0.05AGB-0.09VWC, R2=0.95, p<0.0001), the main driving factor for monthly NEE was AGB, and AGB could explain 92% variation of NEE; AGB was also the main driving factor for GEP, with the equation of (GEP=-0.068AGB-0.202Ta-0.102Ts, R2=0.97, p<0.0001), and AGB also could explain 92% variation of GEP; Air temperature (Ta) was the main driving factor for Re, with the equation of (Re=0.23Ta-0.057VWC+0.017AGB+0.038Ts, R2=0.97, p<0.0001), Ta could explain 90% variation of Re. In cropland(CL), AGB was the main driving factor for NEE and GEP, AGB could explain 81% monthly variation of NEE and GEP, with the equation of (NEE=-0.026AGB, R2=0.81, p<0.0001) and (GEP=-0.04AGB-1.39VWC+0.98Ts, R2=0.95, p<p<0.0001), respectively; VWC was the main driving factor for Re, VWC could explain 90% variation of Re with the equation of (Re=0.51 VWC-0.11Ta+0.013AGB, R2=0.97, p<0.0001). In the artificial grassland, Ta was the main driving factor for monthly NEE and GEP, Ta could explain 95% variation of NEE and 98% variation of GEP, with the equation of (NEE=-1.29Ta+0.57Ts+0.46VWC, R2=0.98, p<0.0001)and GEP (GEP=-0.71Ta+0.32Ts-0.01AGB, R2=0.99, p<0.0001), respectively; Soil temperature (Ts) was the main driving factor for Re, Ts could explain 96% variation of Re, with the equation of (Re=0.22Ts+0.01AGB, R2=0.98,p<0.0001).(4) Grazing grasslands, cropland and artificial grassland were in the state of carbon absorbing during growing seasons, the annual NEE in HG was significantly decreased 36% for UG and 51% for MG, respectively (p<0.05); the annual NEE in MG was significantly increased 31% for UG and 106% for HG, respectively (p<0.05).Therefore, moderate grazing can promote carbon sequestration capacity of the grassland ecosystem, while heavy grazing may reduce the potential of CO2 absorption in grassland ecosystems. There was no significant effect on NEE and GEP among UG, AG and CL (p>0.05), while Re in CL was significantly higher than UG (p<0.05). The average seasonal cumulative carbon sequestration for UG, MG and HG (from 2012 to 2014) was:771g C m-2,1005 g C m-2,491 g C m-2, respectively; HG< UG<MG, heavy grazing significantly reduce the seasonal cumulative carbon sequestration in grassland ecosystem (p<0.05). In the growing season of 2013,the cumulative carbon sequestration of UG, MG, HG, AG and CL was 1008 g C m-2,1240 g C m-2,555 g C m-2,1240 g C m-2and 926 g C m-2,respectively, the order of cumulative carbon was AG, MG> UG, CL> HG. Annual climate change would affect the amount of cumulative carbon, the annual average amount of cumulative carbon of all three grazing sites was 643 g C m-2,934 g C m-2 and 690 g C m-2 for 2012,2013,2014 respectively, the order of cumulative carbon was 2013>2014>2012. |
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