| Northern Shaanxi is one of the most important petroleum producing area of China,while the fragile ecosystem is seriously threatened by petroleum pollution,especially the shrub-grass land.Phytoremediation is widely used in the bioremediation of the petroleum polluted soil.However,the situation that whether the petroleum will affect the decomposition and the nutrients release of plant litter or not were rarely investigated in the existed studies.If the processes were inhibited by petroleum pollution,the nutrients cycling of the plant-contaminated soil ecosystem would have been hindered,and the stability of the plants community would thus be decreased,causing the more difficulties and the higher cost in the ecological remediation.Hence,in this study,10 types of shrub or grass species,which were confirmed as phytoremediator or petroleum-tolerant plants,were chosen as the objects.Foliar litter from these plants was buried into un-polluted(control),lightly polluted(LP,petroleum concentration is 15 g kg-1),moderately polluted(MP,30 g kg-1)and seriously polluted(SP,45 g kg-1)soil medium for one-year(for shrub litter)or half-year(for grass litter)indoor litter decomposition experiments under consistent temperature and humidity conditions.During the experiments,the decomposition rates and the nutrients(including C,N,P,K,Cu,Zn,Fe,Mn,Mg and Ca)release processes were determined.Based on these,the influences of the various petroleum pollution degrees on the decomposition and nutrients release of shrub and grass litter were investigated.Whether the 10 species could form stable communities in the petroleum polluted area and perform long-term ecological benefits or not were also assessed.In addition,the seriously polluted soil with N fertilized(the soil C/N ratio was adjusted to 25: 1)was also used as decomposition medium(that is,a type of treatment)to assess if the N fertilization was in favor of the plant litter decomposition in petroleum polluted soil.Furthermore,the alterations of soil biological properties(including enzymatic activity and microbial populations)during the different periods of litter decomposition process were also measured,in order to clarify the mechanism of the influences of petroleum pollution and N fertilization to the decomposition and nutrients release of plant litter.The results indicated that:(1)Generally,LP significantly accelerated the decomposition and overall nutrients release of the litter from Hippophae rhamnoides,Caragana korshinskii,Amorpha fruticosa,Ziziphus jujuba var.spinosa,Periploca sepium and Bothriochloa ischaemum.It also significantly accelerated the decomposition of Medicago sativa litter and the overall nutrients release of Coronilla varia and Artemisia sacrorum litter.MP significantly accelerated the decomposition and overall nutrients release of the litter from C.korshinskii,Z.jujuba var.spinosa and P.sepium,while it significantly inhibited the decomposition and overall nutr ients release of the litter from M.sativa,C.varia,A.sacrorum and Trifolium repens.SP significantly accelerated the decomposition and overall nutrients release of the Z.jujuba var.spinosa litter,it also significantly accelerated the decomposition o f M.sativa litter and the overall nutrients release of the C.varia litter.However,SP significantly inhibited the decomposition and overall nutrients release of the litter from H.rhamnoides,A.fruticosa and B.ischaemum,it also exhibited considerable inhibitory effects on the decomposition of A.sacrorum litter and the overall nutrients release of the P.sepium litter.(2)In most of the cases,N fertilization could not alleviate the inhibitory effects of SP on the decomposition of shrub and grass litter(for H.rhamnoides and B.ischaemum litter).O n the contrary,it lead to inhibitory effects on the decomposition of litter species whose litter decomposition were not affected or significantly accelerated in SP soil in first(for T.repens,Z.jujuba var.spinosaand P.sepium litter),it intensified the inhibitory effects of SP on the decomposition of A.fruticosa and B.ischaemum litter.Simultaneously,N fertilization also exhibited disadvantageous effects on the overall nutrients release of all litter species expect the litter from C.varia.Among the 10 types of litter,N fertilization works significantly on the litter ofC.varia,which accelerated the decomposition and P,Cu,Fe,Mn,Mg and Ca release(3)At the early period of decomposition,all o f 3 types of petroleum pollution significantly increased amylase activity of the decomposition medium(soil),meanwhile,they significantly inhibited the activity of protease or sucrase.At the moderate and late period of decomposition,LP generally significantly increased the activities of β-glucosidase,xylanase,alkaline phosphatase,peroxidase and dehydrogenase of the decomposition medium of H.rhamnoides,C.korshinskii,A.fruticosa,Z.jujuba var.spinosa,P.sepium and B.ischaemum litter.MP exhibited an approximate equality of inhibitory and accelerating effects on the enzymes mentioned above in the same types of decomposition medium.SP generally significantly inhibited the activities of carboxymethyl cellulase,xylanase,polyphenol oxidase and dehydrogenase of the decomposition medium of H.rhamnoides,C.korshinskii,A.fruticosa,Z.jujuba var.spinosa and P.sepium litter.All of 3 types of petroleum pollution exhibited roughly equal inhibitory and accelerating effects on the activities of enzymes which mainly participate in the moderate and late stages of litter decomposition of the decomposition medium of M.sativa,C.varia,A.sacrorum and T.repens.For all of 10 types of decomposition medium(soil),LP exhibited significant accelerating effects on the populations of bacteria,actinomycetes and fungi.At the moderate and late periods of decomposition,In general,LP significantly increased the populations of soil bacteria or actinomycetes in all types of decomposition medium expect for the medium of B.ischaemum litter.For all of 10 types of decomposition medium,MP exhibited significant accelerating effects on the populations of bacteria and actinomycetes.At the moderate and late periods of decomposition,In general,MP significantly increased the populations of actinomycetes.However,under MP condition,the microbial population of A.sacrorum litter decomposition medium was insignificantly affected,while the bacteria population of the decomposition medium of H.rhamnoides,Z.jujuba var.spinosa and P.sepium litter exhibited significant decrease at the late period of litter decomposition.SP did not significantly affect the microbial population of all types of decomposition medium,however,it significantly increased the actinomycetes or fungi populations of the decomposition medium of shrub litters,but exhibited approximately no significant effects on the of the microbial decomposition medium of grass litters.(4)In rare cases,N fertilization alleviated the inhibitory effects of SP on the enzymatic activities.In the other cases,N fertilization exhibited ins ignificant alleviating effects,evenmore,it generally performed significant inhibitory effects on the enzymatic activities which were not affected(or were significantly increased)in the SP conditions,and even intensified the inhibitory effects of SP.These adverse effects generally influenced the enzymes especially the enzymes participating in the moderate and late periods of litter decomposition,such as carboxymethyl cellulase,xylanase,polyphenol o xidase,peroxidase and alkaline phosphatase.For all of 10 types of decomposition medium,N fertilization significantly increased the population of bacteria and actinomycetes at the early period of litter decomposition.O n the contrary,N fertilization d id not exhibit approximately beneficial effects on the soil microorganism at the moderate and late periods of decomposition,while it even significantly decreased the microbial populations of the decomposition medium of H.rhamnoides,A.fruticosa,P.sepium,M.sativa and T.repenslitter,which were not affected(or were significantly increased)in the SP conditions.(5)Considering the nutrients cycling of ecosystem,all of the 10 shrub and grass species could be used for the phytoremediation of the LP soil or the restoration of the eco-environment of the polluted area.H.rhamnoides,C.korshinskii,A.fruticosa,Z.jujuba var.spinosa,P.sepium and B.ischaemum could be used for the phytoremediation of the MP soil or the restoration of the eco-environment of the polluted area,while M.sativa,C.varia,A.sacrorum and T.repens were not suitable for this purpose;C.korshinskii,Z.jujuba var.spinosa,M.sativa,C.varia and T.repenscould be used for the phytoremediation of the SP soil or the restoration of the eco-environment of the polluted area,while H.rhamnoides,A.fruticosa,P.sepium,B.ischaemum and A.sacrorum were not suitable for this purpose.N fertilization could not alleviate the inhibitory effects of SP on the decomposition and nutrients release of shrub and grass litter and even cause adverse effects in more cases,hence,N fertilization restoration should be avoid using simultaneously with the phytoremediation of SP soil. |
