| Soil soluble organic nitrogen(SON) is one of the most active component of soil nitrogen pool,which plays an important role in the nitrogen cycle in the terrestrial ecosystem.Plant litters produce large amounts of organic nitrogen to return to the soil during the process of decomposition.Meanwhile,plant litters can also stimulate the decomposition of soil organic matter.However,there are fewer reports about the SON of mineral soil in subtropical Chinese Fir forest,including the factors,transformation process and the response of increasing nitrogen deposition.In this study,mineral layer soil and the fresh and decomposing Cunninghamia lanceolata litter of subtropical conifer forest were collected from Wan Mulin Natural Reserve located at Fujian Province,southeast China.To study the different contribution of soil organic matter and plant litter to soil soluble organic nitrogen(SON),the soils respectively with the addition of the fresh and decomposing Chinese fir litter and fresh Castanopsis carlesii litter were incubated for 210 days in 25? constant temperature incubator and the soil moisture was maintained at 60%WHC(Water Holding Capacity).Based on that,in order to study the effects of exogenous carbon and nitrogen addition on soil SON and soil nitrogen transformation,we used method of 15N trace to determine the 15N enrichments in soil SON after labeled amino acid and NH4+ and NO3-additions.The main results are summarized as follows:(1)In the first 90 days,the soil SON content was decreased after adding the litter.It was increased by fresh litter after 90 days,while the decomposing litter was only during 90 to 150 days.Since the fresh Chinese fir litter with higher nitrogen content was adder into the soil,the soil ammonium nitrogen content decreased after the 15 days.While the soil soluble organic carbon(SOC) content did not increase,suggesting that the nitrogen mineralization into ammonium was limited by carbon.But the soil nitrate nitrogen increased continuously and was significantly higher than that in the control,which showed that the soil was mainly heterotrophic nitrification.With the addition of 15N labelled fresh Castanopsis carlesii litter,the 15N abundance of soil SON increased with the extension of incubation time,which was observably higher than that in the control.And the priming effect of soil SON was negenative in the first 90 days,then turn to positive,which demonstrated that the vast majority of soil SON was derived from the soil itself and microbial biomass.(2)In the first 90 days,adding glucose dramatically increased the soil SON,but the soil ammonium and nitrate nitrogen were decreased in the first 3 days.After the addition of amino acid,the soil SON was greatly reduced to the control level in the first 3 days,but the soil ammonium nitrogen was increased rapidly and significantly higher than that of control.Furthermore,in soil SON,the 15N abudance was 22.42%?86.67%and the priming effect was only positive in the first half day but then turn to negative.From 0 to 30 days,the soil gross mineralization rate was negative,and the immobilization rate of ammonium nitrogen was greater than the net mineralization rate,and the soil MBN content was higher than that of control.It was suggested that the soil microbes directly utilized the exogenous amino acids(direct pathway),then the excess nitrogen remained was released from microbe cells in the form of NH4+.90 days later,the soil ammonium content was dropped to rather low by glucose and amino acid addition.While the soil nitrate nitrogen accumulated largely,and soil SOC content also decreased rapidly and significantly lower than that of control,which indicated that glucose and amino acids added were utilized and respired by soil microbial,thus leading to limit soil nitrogen mineralization and microbial inorganic nitrogen immobilization,because of the shortage of carbon in later period of incubation.(3)The addition of NH4+and NO3-both promoted the production of soil SON.Under the same nitrogen form,the soil SON with the addition of decomposing litter was greater than that of fresh litter,while it was represented the opposite under free-nitrogen treatment,which suggested that the promoting effect of exogenous nitrogen addition on decomposition of decomposing litter was greater than that of fresh litter.The priming effect of soil SON was positive with the addition of NH4+,but it decreased significantly with the increase of incubation time.However,it was negative when NO3-was added in the first 90 days,just later turned to positive,and dramaticlly higher than NH4+ treatment.After adding different nitrogen forms with fresh litter,the soil gross mineralization rate in the first 30 days was significantly higher than that in the later incubation time,and it was even almost zero in the 90 to 210 days.And the C/N ratio of substrate reduced with the incubation time prolonged,which indicated that the soil microorganisms were limited by carbon in the late stage.So,after 90 days,the soil SON content was declined by NO3-addition,while the soil ammonium nitrogen was improved;Most of the soil soluble nitrogen was in the form of nitrate nitrogen by adding exogenous nitrogen,especially in 90 days later,and then the risk of leaching or gas loss of the soil nitrogen increased. |
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