| Soil and marine sediment are the two major reservoirs active in global phosphorus(P)cycles.There are connections and distinctions between the two reservoirs.Soil is the primary source of P to the ocean through surface runoffs and river export while marine sediment is the primary sink for P.During the physical erosion and chemical weathering,the P is leached from parent rock into the environment with complex cycling processes in the soil and is eventually delivered by rivers to the ocean.In the ocean,P,as a nutrient,participates in complicated biogeochemical cycling along with carbon and nitrogen coupled with other elements,and is ultimately buried in marine sediment.On long geologic timescales,the P cycle begins anew with the tectonic uplift marine sediments into weathering regime.The majority of the riverine P input to the ocean is in the particulate phase and the behavior of this particulate P would significantly affect the P biogeochemical cycling in the ocean.Moreover,studies have shown some amounts of authigenic apatite existed in continental margin sediments,and this apatite in the continental sediment is referred to as a newly identified sink for reactive P in the ocean.However,if this authigenic apatite is from terrigenous input,it should not be treated as the sink to the oceanic reactive P.Due to a lack of suitable tracer,knowledge of the behovior of terrigenous particulate P in the ocean is still limited.During the last decade,studies have shown that phosphate(Pi)species with different sources and/or bioavailability have different oxygen isotope composition in phosphate(δ18OP).Therefore,the δ18Op has the potential to trace sources of P species in soil and marine sediment.In the present study,I have measured δ18Op values of different Pi species in soil and different marine sediments and examined their controlling factors in the environment.Major conclusions are summarized below.Methodology:Firstly,this study examined the fidelity of the sample extraction and purification procedures,and showed the δ18Op values were not altered during these procedures and the precipitated Ag3P04 had high purity.The δ18OP values showed higher distinction among Pi species for the soil extracted by Hedley sequential extraction method,suggestting that Hedley method can effectively distinguish Pi species with different sources and/or bioavailability in soil.While theδ18Op values showed higher distinction among Pi species for the marine sediment extracted by Ruttenberg sequential extraction method,suggestting that Ruttenberg method can effectively distinguish Pi species in marine sediment.Soil P:Field soil samples were collected every 3 or 12 hours for 7 days from the same location with different moisture through intermittent watering.Results showed that soil temperature,moisture and the δ18Ow of soil water had clear diurnal variations,with δ18Ow decreasing with soil moisture increasing.However,both the content andδ18Op of soil P species did not show a significant diurnal variation.These characteristics indicated that short-term diurnal changes in environmental conditions and biological activities had little effect on the content and δ18Op of P species in field soil.Therefore,the diurnal sampling time had no effect on the measurement of contents and δ18Op values of soil P pools.Moreover,this study suggested that the mean,not the measured(at the sampling time),temperature and δ18Ow in soil should be used to calculate the δ18OP.Equ,to elucidate the measured δ18Op values in the field soil.P in marine sediment:Different Pi pools in samples were extracted with the Ruttenberg method for the measurements of δ18Op values to trace sources of BD-Pi,Ac-Pi and HCl-Pi in surface sediments from the Northwestern Pacific Ocean,Changjiang Estuary,Jiulong River Estuary and Chukchi Sea.In the Northwestern Pacific Ocean,Changjiang Estuary,Chukchi Sea and sations with high salinity in Jiulong River Estuary,δ18Op-HCI values were lower than δ18Op-BD and δ18OP-AC values,and far lower than the δ180p-Equ values.And δ18Op-HCI kept steady rather than increasing with δ18Op-Equ in Changjiang Estuary and Jiulong River Estuary.These results indicated HCl-Pi in marine sediment mainly retained their original features in the soil.Moreover,δ18OP-HCI values in the four areas were all heavier thanδ18Op in igneous/melaIorphic rocks,undicating HCl-Pi in marine sediment was a mixture of two possible terrestrial sources:Pi from igneous/metamorphic rocks and authigenic apatite formed in soil undergoing biological activities,we defined this apatite as terrestrial authigenic apatite.BD-Pi is normally defined as iron bound Pi.In the Chukchi Sea shelf,δ18Op-BD values were similar to δ18Op-Equ,value;and in the northwestern Pacific Ocean,inner shelf of East China Sea and Chukchi Plateau,δ18Op-BD values were-31‰ lower thanδ180p-Equ.values.Moreover,δ18OP-BD values showed a similar trend with 5180p-Equ in the Changjiang Estuary and Jiulong River Estuary.These results indicated BD-Pi was mainly from water column and/or porewater in sediment.Ac-Pi is normally defined as authigenic apatite.Results showed δ18OP-Ac values were between δ18OP-BD and δ18OP-HC values,indicating the Ae-Pi was not solely marine authigenic apatite.Here,I proposed a hypothesis that when δ18Op-Ac have a similar trend and a close value with δ18Op-BD,Ac-Pi should be mainly marine authigenic apatite;on the other hand,if δ18OP-Ac have a similar trend and a close value with b18Op-HCI,the Ac-Pi should be mostly trrestrial authigenic apatite.Based on this hypothesis,the Ac-Pi was found mainly marine authigenic apatite in the northwestern Pacific Ocean,Changjiang Estuary,at stations with high salinity in Jiulong River Estuary and Chukchi Plateau,while Ac-Pi occurred mainly in terrestrial authigenic apatite at stations with low salinity in the Jiulong River Estuary and Chukchi Sea Shelf.This is one of the first studies exploring the rational application of oxygen isotope composition of phosphate including methodology development and elucidating mechanisms,for the biogeochemical cycling of P in marine environments,and reporting the spatial variations in δ18OP values of different Pi species in sediments from diverse marine ecosystems,including estuarine,coastal,oceanic and high latidute environments.Overall,the BD-Pi was mostlyly bioavailable,HCl-Pi was mainly terrestrial refractory phosphate,and Ac-Pi was a mixture of terrestrial authigenic apatite and marine authigenic apatite.This preliminary study provides new data and insights into the relationship between phosphorus in soil and marine sediment and thus the global P biogeochemical cycles from land to ocean using the oxygen isotope composition of phosphate. |
