Influence Of Nematodes On Nitrogen Transformation Process In Fluvo-aquic Soil

Due to the importance of soil nematodes in terrestrial ecosystem functions,the effect of nematodes on nitrogen(N)transformation has received a lot of attention.In previous studies,the concentrations and net N change rate of inorganic N were usually used to evaluate the effect of nematodes on soil N mineralization and nitrification in a short-term incubation.However,a relatively higher soil gross N transformation rate may occur under condition with a lower net change rate of inorganic N.It would not be enough to evaluate the role of nematodes in soil N cycling only by measuring the net changes of inorganic N.In this study,15N trancing technique and N transformation model were used to(1)investigate the effect of different bacterial-feeding nematodes(Rhabdilis,Prolorhadilis and Acrobeloides)on soil N transformation processes and(2)explore the roles of nematodes under different fertilization(no fertilizer addition,mineral fertilizer,organic and mineral fertilizer,noted as CK,NPK and MNPK,respectively).The main results are as follows:1.After 50-d incubation,the concentrations of ammonium(NH4+)and nitrate(NO3-)in irradiated soils and reinoculated with native microbe and nematodes were not significant different from those in irradiated soils and reinoculated only with native microbial community.Compared with the treatment with no bacterial-feeding nematodes,no significant difference in soil N transformation rates was observed in Rhabditis treatment.Prolorhaditis increased the rates of mineralization,autotrophic nitrification and DNRA and decreased the mean residence time of NH4+ and NO3-pools.Acrobeloides increased microbial NH4+ immobilization rate and lowered the rates of autotrophic nitrification and DNR4.2.Compared with no bacterial-feeding nematode reinoculation,bacterial-feeding nematodes significantly decreased ?13C,which was lowest in Protorhaditis treatment.A significantly negative relationship between soil N mineralization rate and ?13 was observed,suggesting that bacterial-feeding nematode can not only take part in thedecomposition of organic matter,but also change the utilization of different composition of organic matter.3.The nematode numbers was greatly decreased in Rhabditis and Acrobeloides treatments but increased in Protorhaditis treatment at the end of 50-d incubation.There was no significant difference in bacterial number among three bacterial-feeding nematodes.Although autotrophic nitrification rate was increased in Protorhaditis treatment,ammollia oxidizing bacteria(AOB)number was decreased,indicated the change in community composition of AOB may be important factor affecting autotrophic nitrification.4.Significant increase in autotrophic nitrification rate and negligible microbial NO3-immobilization contributed to NO3--dominated inorganic N in sand loamy fluvo-aquic soil.Mineral and organic fertilizer application stimulated autotrophic nitrification rate and mineralization rate,respectively,which led to higher NO3-concentration and lower pH in MNPK treatment than in NPK treatment.5.Nematode significantly increased the rates of mineralization,microbial NH4+immobilization and autotrophic nitrification and accelerated turnover rate of inorganic N,but this stimulating effect showed different response to different fertilization pattern.Noticeably,the rates of mineralization,microbial NH4+ immobilization and autotrophic nitrification in irradiated soils and reinoculated with native microbe and nematodes were significantly lower than those in non-irradiated fresh soil,suggesting the role of nematodes in N transformation processes in fertilized-soils may be underestimated by applying gamma irradiation.The net transformation rate of nitrogen did not change after adding the bacteria feeding nematodes in the soil,but primary transformation rate changed and specific transformation processes were dependent on nematode species.Nematodes accelerate nitrogen turnover to which extent varied with t different fertilization.