Impacts Of Different Land Uses On Carbon Fluxes From Subtropical Soils In Central China: Implications For Carbon Sequestration

Land uses practices can play an important role in the global carbon cycle,and possibly,global climate change.Therefore,it is crucial to advance the understanding of the soil carbon(CO₂ and CH₄) flux,and environmental factors affecting their flux,for a better comprehension of carbon dynamics from different land uses in subtropical ecosystems.We examined the relationship between soil CO₂ and CH₄ flux,and environmental determinants in different land use types by using static closed chamber method at Xianning and Zigui Counties of Hubei province,China.Further,in order to ascertain mass distributions of different soil organic (SOC) pools influenced by land use,a 62-day laboratory incubation under a range of moisture (20 to 100%of water-holding capacity,WHC) and temperature conditions(5-45℃) was conducted to measure CO₂ evolution from four land use types.To estimate the C sequestration capacity of the red soil,a short-term experiment during one growing season was also conducted in a paddy field.The results suggested that soils in different land uses were mostly sinks of atmospheric CH₄ and sources of atmospheric CO₂.Annual mean CO₂,and CH₄ fluxes were 5.3～10.5 Mg CO₂ ha^-1 yr^-1,and +0.47～-2.37 kg CH₄ ha^-1 yr^-1,respectively.Pine forest had significantly lower CO₂ emissions,and significantly higher CH₄ uptake than agriculture land uses.Vegetable field was having the highest CO₂ emission among all land uses.Tilled orchard made significant contributions for CO₂ emission than non-tilled orchard,indicating a burst of CO₂ produced by tillage practice.While,tilled orchard had significantly lower CH₄ uptake than non-tilled orchard,which shows that methane oxidizers were significantly disturbed by tillage operation.Soil CO₂ and CH₄ fluxes had a significant seasonal variability.Driven by seasonality of soil temperature,water-filled pore space(WFPS),microbial biomass carbon(MBC), microbial biomass nitrogen(MBN) and dissolved organic carbon(DOC),CO₂ fluxes were significantly higher in the hot-humid season(April-September) than in cool-dry season (October-March).Soil CO₂ fluxes were significantly correlated with microbial quotient(Q_t) and MBC:MBN,while not being correlated with DOC:DON.When regressed separately,soil temperature,WFPS and different quantities of DOC,MBC and MBN significantly regulated the CO₂ emission,while DON(dissolved organic nitrogen) was a poor predictor for CO₂ flux among different land uses.However,temporal variations of CO₂ flux were significantly regulated by soil temperature,WFPS,MBC,MBN,DOC and DON,in one model of variation, in all land uses.Among all the predictors,soil temperature was an important and dominant variable controlling soil CO₂ flux variability in most of the land uses.Driven by the seasonal pattern of soil temperature,WFPS,soil NO₃^--N and NH₄⁺-N contents,CH₄ uptake was significantly higher in cool-dry season than in hot-humid season.When presented in one linear model of regression,soil CH₄ fluxes were significantly regulated by soil temperature, WFPS,soil NO₃^--N and NH₄⁺-N contents in all land uses.To further signify the SOC dynamics among different land uses,incubation data were fitted to a three-pool first-order model that separated mineralizable soil organic carbon into active(C_a),slow(C_s) and resistant(C_r) carbon pools.The results indicate that:(1) The C_a pool comprised of 1.2-3.5%with a mean residence time(MRT) of 49-65 days,(2) The C_s pool comprised of 25.3-60%with a MRT of 2-27 yr,(3) The C_r pool accounted for 36.5-73.9%of SOC in four land use types.The woodland had highest C_r pool and lowest C_a pool,which indicated it to be more stable than other land use types.SCDR(soil carbon derived respiration) was well correlated with all the organic carbon pools as well as C:N ratio,pH,SDR(soil derived respiration) and total N.SOC was positively correlated with C_s,C_r,SDR,total N and WHC,and negatively correlated with BD.The Q₁₀ values(from 5℃to 45℃,at 60%of WHC) of 1.90±0.17,2.19±0.32,2.34±0.32,and 3.23±0.47 were observed for paddy, orchard,woodland and upland,respectively,and decreased with decreasing moisture content when soil water content was less than its optimum value,but an opposite trend was shown when soil retained water at contents higher than the optimum water content.Different Q₁₀ values indicated different temperature sensitivity of SOC among different land uses,as different amounts of SOC pools were observed among different land uses.In a paddy field,among different treatments,N fertilization significantly increased the CO₂ flux from row-soil.No significant differences among.different treatments were observed from inter-row and bare soil.From bare soil,soil CO₂ flux was decreased in response to N fertilizer application;this suggested suppression in microbial activity in response to increased N fertilizer application.The amount of soil carbon sequestration was estimated by taking the difference between net primary production(NPP) and the amount of carbon in harvested rice. The values ranged from -176 to -89 g C m^-2 season^-1 with the highest value observed from the treatment with integrated use of inorganic nitrogen fertilizer with rapeseed straw;this suggested that N fertilizer application with straw has the potential to mitigate the global carbon budget. From Zigui site,with the inclusion of soil N₂O fluxes with soil CO₂ and CH₄ fluxes, global warming potential(GWP) was calculated.Cumulative GWP emission from these three greenhouse gases was in an order of vegetable field＞upland＞orchard＞pine forest.In all land uses,GWP for soil N₂O emissions significantly exceeded than for soil CH₄.Soil CH₄ uptake completely offset the GWP responsible for N₂O emissions in pine forest soil.Overall,to some extent,soil CH₄ uptake caused to offset the GWP responsible for CO₂ and N₂O emissions.It is concluded that(1) GWP is highly land use dependent,and soil CO₂ and CH₄ fluxes from subtropical soils are under environmental controls,soil temperature being the main variable,which interact with other soil variants to control the supply of readily mineralizable substrates in different land uses,(2) SOC from different land uses have different temperature sensitivity,and vegetation type and/or management practices which control soil biological and biochemical properties,were important predictors of C fluxes,(3) N addition increases the CO₂ flux.However,integrated use of N fertilizer along with rapeseed straw may be a preferred strategy in sequestering C in red soil,and(4) different land uses have distinct rates of C emission,owing to different soil characteristics due to various management practices.Threfore,if proper management practices are adopted under different land uses,subtropical soils can have strong hold on C sequestration.