Loss,Replacement And Stability Of Organic Carbon Under Soil Erosion In A Hilly Catchment Of Southern China

Global warming is currently one of the most serious environmental problems that human beings are facing.In light of IPCC fourth assessment report,global mean temperature has been increased by 0.74±0.2oC since 1906 to 2005.Due to largely anthropogenic emission of greenhouse gas such as carbon dioxide,methane,and nitrous oxide aggravated atmospheric greenhouse gas levels.It is well known that shift of soil carbon storage will influence on atmospheric carbon dioxide levels.Soil is the second largest terrestrial carbon pool,following the largest marine carbon pool.Soils store two or four times more carbon than exists in the atmosphere as CO ₂ and 3 to 4times as much as that stored in plants.Any changes in it will therefore cause shift in atmospheric CO₂ levels leading to mitigate or strengthen global warming.It is estimated that each ton of soil organic matter releases 3.667 tons of CO ₂,which is lost into the atmosphere.Similarly,the build-up of each ton of soil organic matter removes3.667 tons of CO₂ from the atmosphere.A 5%shift in the amount of SOC stored in the0–2 m soil profile has the potential to alter atmospheric CO₂-C by up to 16%.The net amount of CO₂ released from soil carbon pool is estimated to be equivalent to about30-50%of atmospheric CO₂ levels.Accumulation and stability of SOC are therefore effective methods to reduce atmospheric CO₂ levels.Soil erosion is complex processes of soil detachment,transport and sedimentation,which could cause to series of environmental problems such as land degradation,agricultural productivity and food security.Soil erosion from agricultural lands alone has been estimated to result in huge economic loss of damages annually.Recent studies showed that soil erosion has significant influence on the global carbon budget.Persistently high rates of soil erosion globally affect more than 1.1×10⁹ hectares of land annually,redistributing on the order of 28 Pg soil yr^-1 and 0.5 Pg C yr^-1.Transport and deposition of sediment and SOC are not only affected the land vegetation distribution,also has a far-reaching influence on soil carbon pool.Some researchers hold that anthropogenic acceleration of soil erosion may associate with promoting carbon sequestration.Because a fraction of the eroded C was replaced,at least partially,by plant photosynthate in the eroded site;and eroded SOC arriving at the depositional zones is preserved in more passive pools at the site of deposition compared to the SOC that remains at the eroded site.At eroding positions,erosion leads to the removal of soil and the mixing of former subsoil into the topsoil,in this biologically active zone,weathering processes lead to the formation of secondary minerals,which provide reactive surfaces to promote the complexation with organic material,e.g.,through absorbance of C on mineral surface.However,there has been disagreement in the scientific literature over the relationship between soil erosion and carbon sequestration.Others argue that soil erosion represents a source of atmospheric CO₂ and is currently releasing up to 1.14 Pg C yr^-1 to the atmosphere,as a result of aggregate breakdown by energy of rain splash and the sheering forces of runoff,exposing hitherto encapsulated organic matter to microbial processes.Soil erosion could increase soil degradation and reduce net primary productivity?NPP?on-site,and increase oxidation of soil organic matter.Because sediments are often selectively transport low density materials including SOC which is accumulated in the vicinity of the soil surface.In truncated soil prof iles characterized by carbonaceous subsoil horizons,exposed carbonates may react with acidiferous materials,such as fertilizers,and release CO₂ into the atmosphere.Some research with same views estimated that almost 100%of eroded C is oxidized during detachment and transport,no opportunity for burial and protection of eroded C at deposition region.Therefore,most models based on this concept have clearly indicated that all eroded C is either deposited the ocean or rapidly decomposed,and its contribution to C sequestration is negligible.The above two groups who have contrasting points of view are the main differences lies in the eroded SOC replacement,whether there is a large amount of SOC buried in depositional region is stable.In a word,whether SOC replacement is occurs at erode site,and eroded SOC arriving at the depositional zones are preserved steadily,both the process determine the soil erosion is a source or sink for atmospheric CO₂ levels.Base on the above views,some investigators proposed criterion for the erosion-induced terrestrial carbon sink or carbon source.Previous studies on the effect of soil erosion on soil carbon are mainly focused on SOC distribution,mineralization,quantity,SOC replacement,although these studies are valuable to understand dynamic of carbon under erosion,the role of soil erosion at terrestrial ecosystems carbon cycling is unclear.Reliable data collected at filed condition is lacking.For example,how many eroded carbon are replaced on-site,how about stability of the replaced carbon,Moreover,quality of replacement SOC is very important when considering SOC replacement at eroded site leading to increase SOC pool,when the replacement of SOC are primary lab ile carbon such as fresh SOC,aliphatic compounds,small size and weight molecules compounds,SOC sequestration is more difficult due to easily decompose.To our knowledge,stability and component of replacement SOC at eroded site and SOC at depositional zone has not yet been demonstrated.But may be an important mechanism to understand whether soil erosion is a carbon source or sink.Gaining more insight into the fate of SOC in erosion and depositional processes in red soil landscapes is essential to resolve the current controversy and represents a large future challenge.In the study,transport patterns of SOC and DOC at runoff plots?2*5 m?scale,replacement of SOC and DOC at eroded site,stability and component of SOC and DOC at typical small catchment are investigated through rainfall simulation,nuclear caesium-137,thermogravimetry,fluorescence EEs,and UV-vis.Primary results are following:1.Transport patterns of SOC and DOC under rainfall simulationA chisel tillage plot with low rainfall intensity?CT-L?and two no-tillage plots with high?NT-H?and low rainfall intensity?NT-L?study were conducted.Each plot was divided into five subplots?A to E?at 1-m intervals and samples were extracted from 0-5,5-10,10-20,and 20-40 cm soil layers from triplicate soil blocks pre-and post-rainfall.Thus except vertical concentration of SOC and DOC prior to simulation rainfall,we also determined organic C,and characteristics of DOC transport after rainfall simulations on runoff plots?2m*5m?.No fertilizer was applied in any plots.Simulated rainfall was applied at various intensity for 60 min.Runoff was collected and quantified the losses of soil,DOC and soil.Results clearly showed that runoff volumes,sediments and SOC entrained with sediment and laterally mobilized DOC were significantly larger on NT-H compared to other plots,coinciding with changes in rainfall intensity and the extent of roughness of the plot surface?CT vs NT?was variation in runoff DOC concentration.During the simulated rainfall events,DOC exported averaged 0.76,0.64 and 0.27 g C m^-2 h^-1,SOC exports average 3.52,1.08 and0.07 g m^-2 h^-11 in the NT-H,NT-L and CT-L soils,respectively.The maximum export of DOC was obtained under high intensity rainfall plot and lagged behind maximum runoff volumes,sediments,and SOC losses with sediment,which was proportional to SOC content of sediment loss.The least DOC losses in surface runoff and SOC losses with sediment were observed in CT-L plots.Vertical DOC mobilization achieved its maximum with low intensity rainfall under CT treatment.DOC did not accumulate at the soil surface and mainly was distributed in the second and third soil horizons.The distribution of DOC content down the soil profile increased,comparing with pre-rainfall except for subplots E at NT-H and NT-L,results indicated rainfall significantly elevated DOC content in experimental plots.SOC content of sediment leaving the erosion zone was significantly correlated with overland flow volume and sediment loss.2.Replacement of SOC and DOC at eroded siteThe catchment has experienced appreciable net erosion.Moreover,different vegetation cover exerted distinct effect on soil erosion intensity and SOC and DOC stocks.Soil losses from Masson pine stands were the highest with average of 4.15±1.25 Mg ha^-1 yr^-1.P.massoniana(4.15±1.25 Mg ha^-1 yr^-1)>E.decipiens(3.33±1.20Mg ha^-1 yr^-1)>C.camphor(2.82±1.30 Mg ha^-1 yr^-1)>L.lagerstroemia(2.68±1.50 Mg ha^-1 yr^-1)>M.maudiae(1.55±1.36 Mg ha^-1 yr^-1).Although erosion occurred at the catchment,the soil has large and significant gains of SOC and DOC stocks which stored up in upper 0-20 cm of the soil depth.The order of SOC replacement:L.lagerstroemia,(9.1 Mg C ha^-1)>E.decipiens(8.1 Mg C ha^-1)>M.maudiae(6.0 Mg C ha^-1)>C.camphor(5.9 Mg C ha^-1)>P.massoniana(0.3 Mg C ha^-1).The order of DOC replacement:M.maudiae?0.2±0.03 g C kg?>L.lagerstroemia?0.17±0.04 g C kg?>E.decipiens?0.15±0.02 g C kg?>C.camphor?0.14±0.04 g C kg?>P.massoniana?0.09±0.02 g C kg?.In contrary,the SOC and DOC stocks in the deposition site had the highest values below 10 cm soil depth.The smallest accretions of SOC and DOC stocks were observed under Masson pine stands with average gains:0.3 Mg C ha^-1and0.09 g C kg in top 20 cm.Masson pine plantations were most susceptible to erosion.In conclusion,soil eroded site replaced SOC and DOC stocks in current watershed.To reduce the incidence and intensity of soil erosion,terracing or straw mulching in forest plantations is recommended.Planting of Masson pine in a subtropical erosion region is not an optimum choice to mitigate soil loss and ameliorate global climate change.3.Stability and components of SOC and DOC at depositional and eroded sitesFe,Al concentrations and SSA size increased with depth and were negatively correlated with SOC concentration at the erosion site?P<0.05?,while at the deposition site these values decreased with depth and were positively correlated with SOC concentration?P<0.05?.TG mass loss showed that SOC components in the two contrasting sites were similar,but the soils in deposition site contained a larger proportion of labile organic carbon and smaller quantities of stable organic carbon compared to the erosion site.SOC stability increased with soil depth at the erosion site.However,it was slightly variable in the depositional zone.Changes in SUVA ₂₅₄spectroscopy values indicated that aromatic moieties of DOC at the erosion site were more concentrated in the superficial soil layer?0-20 cm?,but at the deposition site they changed little with depth and the SUVA₂₅₄ values less than those at the erosion site.Fluorescence EEs spectroscopy indicated that hydrophobic components of DOC are more in erosion site than that in deposition,DOC molecular has similar trend.DOC of eroded and depositional sites are mainly low humification degree,simple materials such as microbial metabolites as well as product of bacteria.DOC in eroded site are compose of humic-like,tryptophan and protein-like,whereas humic-like and protein-like are composed of DOC in depositional zone.Though large amounts of SOC accumulated in the deposition site,SOC may be vulnerable to severe losses if environmental conditions become more favorable for mineralization in the future due to accretion of more labile carbon.Deep soil layers at the erosion site?>30 cm deep?had a large carbon sink potential.These results lead to the conclusion that soil erosion is an important factor controlling the export of SOC and DOC.Loss of SOC and DOC pat terns are different,SOC transport with sediments while DOC transport with runoff flow.A large number of SOC and DOC replaced after soil erosion occurrence.SOC are more stable in eroded site than that in depositional site,whereas depositional site had m ore SSA compared to eroded site.The study showed that soil erosion at catchment could be a carbon sink effects.Results of this study could help to further distinguish between carbon source and carbon sink under erosion,and in-depth understanding the processes of terrestrial ecosystem carbon cycle,which has important significance for accurately estimating the regional carbon balance.