| Alpine is an indicator of climate warming,and near the timberline is more sensitive zone of climate change.Global warming speeds up the frequency of soil freeze-thaw cycles,which might exert strong effects on the soil nitrogen mineralization in alpine region.Soil nitrogen mineralization is not only one of the key processes of nitrogen cycling in forest soils,but also the important factor of the growth of plants and the productivity of terrestrial ecosystems.Thus,in this study,three typical vegetation communities(meadow(CD),Larix principis-rupprechtii(HL)and Picea meyeri(YS))near the timberline in Wutai Mountain were selected to study the characteristics of soil nitrogen mineralization in situ.The major results were summarized as follows:(1)The contents of NH4+ in three communities were greater than the contents of NO3-.During the incubation stages,the contents of NH4+ of CD was decreased at the beginning of freezing,then the value was increased at deep freezing stage,decreased again at thawing stage and increased again at the growth season,the contents of NH4+ of HL and YS were decreased at the beginning of freezing and increased in the period from deep freezing stage to the growth season.In addition,the contents of NH4+ in three communities were greatest at the late growing season.The contents of NH4+ in three communities were YS(25.91±4.12)mg/kg>HL(21.49±4.08)mg/kg>CD(19.23±0.42)mg/kg.The difference was not significant for the contents of NH4+ between HL and YS,but there were significant differences between CD and HL at the thawing stage(p<0.05),so were between CD and YS(p<0.05).The contents of NO3-in three communities were decreased at the beginning of freezing and increased in the period from deep freezing stage to the growth season,which reached the maximum value in the late growing season.The contents of NO3-in three communities were HL(21.20±6.07)mg/kg > YS(14.82±8.75)mg/kg >CD(12.67±3.20)mg/kg.The difference was not significant for the contents of NO3-between HL and YS,but there were significant differences between CD and HL at the beginning of freezing(p<0.05),so were between CD and YS(p<0.05).(2)During the incubation stages,the soil mineral nitrogen content of CD was decreased at the beginning of freezing,then the value was increased at deep freezing stage,decreased again at thawing stage and increased again at the growth season.The soil mineral nitrogen contents of HL and YS was decreased at the beginning stage of freezing and increased in the period from deep freezing stage to the growth season.In addition,the content of soil mineral nitrogen in three communities were greatest at the late growing season,and the values were HL(42.69±5.42)mg/kg>YS(40.73±10.63)mg/kg>CD(31.90±3.25)mg/kg.The difference was not significant for soil mineral nitrogen content between HL and YS,but there were significant differences between CD and HL at the deep freezing stage(p<0.05)and thawing stage(p<0.05),so were between CD and YS(p<0.05).(3)The net rate and amount of soil nitrogen mineralization in three communities were increased from beginning stage of freezing to the deep freezing stage,decreased at thawing stage and increased again at growth season.The net rate and amount of soil nitrogen mineralization in CD were greatest at the early growing season to middle growing season.The net rate and amount of soil nitrogen mineralization in HL and YS were greatest at the middle growing season to late growing season.During the overall incubation stages,the amount of soil nitrogen mineralization in three communities were HL(40.316±4.420)mg/kg>YS(38.143±10.677)mg/kg>CD(30.491±2.688)mg/kg.The net rate of soil nitrogen mineralization in three communities were HL(0.124±0.014)mg/kg/d>YS(0.117±0.033)mg/kg/d>CD(0.094±0.008)mg/kg/d.The results showed that the nitrogen supply capability of HL was the strongest,followed by YS,and CD was the weakest.The differences were not significant for the soil net nitrogen mineralization rate and the amount of soil mineralized nitrogen between HL and YS,but there were significant differences between CD and HL at thawing stage(p<0.05)and early growing season stage(p<0.05),so were between CD and YS(p<0.05).(4)The soil mineral nitrogen contents and the net rate and amount of soil nitrogen mineralization in three communities were higher in growth season than in freeze-thaw period.The amount of soil nitrogen mineralization of CD?HL and YS during freeze-thaw period accounted for overall incubation period were 9.95%?23.81%? 34.21%.It was concluded that the soil nitrogen mineralization in freeze-thaw period plays an important role in the whole year,especially in HL and YS soils.At present,the research results of the effect of freeze-thaw cycles on the soil nitrogen mineralization were various,and the mechanism was unclear.However,it was discovered in most studies that freeze-thaw cycles was benefited to the improvement of soil mineral nitrogen content.That could provide essential nutrients for the growth of plants in spring,on the other hand,might improve the possibility of nutrients loss from soil. |
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