| The Greater Xing'an Mountains region is located in the cold temperate zone of China,which belongs to the high latitude area and is very sensitive to climate change.At the same time,the Greater Xing'an Mountains is one of the important state-owned forest areas in China,which plays an important role in water conservation and soil and water conservation.The intense temperature change during the spring freeze-thaw period has a certain impact on the soil ecological process.Therefore,the study on the changes of forest soil active nitrogen and nitrogen mineralization during the spring freeze-thaw process reveals the impact of freeze-thaw on soil nitrogen transformation in the cold temperate zone of China,which provides a theoretical basis for the study of forest biological system nitrogen in the cold temperate zone.In this study,three main forest types(Larix gmelinii,Pinus sylvestris var.mongolica and Betula platyphylla)in the cold temperate zone of China were selected as the research objects.During the spring freeze-thaw period from April to May 2019,the soil active nitrogen(ammonium nitrogen,nitrate nitrogen and microbial biomass nitrogen)and soil organic nitrogen mineralization were measured and analyzed by the continuous sampling method of field in-situ culture and resin core method the dynamic change of speed.The results showed that:(1)The soil of Larix gmelinii forest,Pinus sylvestris var.mongolica forest and Betula platyphylla forest in spring has higher active nitrogen content,and fluctuates with the change of soil freezing and thawing process.With the increase of soil depth,the content of active nitrogen in three forest types decreased.In the early stage of freezing and thawing in spring,NH4+-N content in 0?5cm soil layer of Larix gmelinii,Pinus sylvestris var.mongolica and Betula platyphylla increased with the increase of soil temperature,and reached the maximum value(26.38±0.65mg/kg,20.83±0.54mg/kg and 24.48±0.76mg/kg)between April 27 and May 2,which was 49.78%,126.12%and 127.79%higher than that of April 2.With the continuous increase of temperature,the freezing and thawing began The NH4+-N content decreased gradually.The change trend of NH4+-N content in 5?30cm soil layers of Larix gmelinii,Pinus sylvestris var.mongolica and Betula platyphylla forest is similar to that of 0?5cm,and freeze-thaw is conducive to the release of NH4+-N.However,the variation of NH4+-N content in 5?10cm soil of Larix gmelinii and Betula platyphylla forest is greater than that of Pinus sylvestris var.mongolica.The content of NH4+-N in the soil of Larix gmelinii and Betula platyphylla forest was significantly higher than that of Pinus sylvestris var.mongolica in spring.(2)The content of NO3--N in the soil of Larix gmelinii,Pinus sylvestris var.mongolica and Betula platyphylla forest was similar to that of NH4+-N.the content of NO3--N in the 0?5cm soil layer of the three forest types reached the maximum value(5.66±0.23mg/kg,2.49±0.02mg/kg and 2.51±0.14mg/kg)from April 22 to May 2,which was 85.66%,50.13%and 36.70%higher than that in the spring on April 2,and then decreased.The trend of NO3--N content in other soil layers is similar to that of 0?5cm.The content of NO3--N in the soil of Larix gmelinii forest was significantly higher than that of Pinus sylvestris var.mongolica forest and Betula platyphylla forest in spring.(3)The changes of soil microbial biomass nitrogen content of three forest types in the cold temperate zone were different in the spring freeze-thaw period.The change of soil microbial biomass nitrogen content in 0?30cm soil layers of Larix gmelinii and Pinus sylvestris var.mongolica forest was similar in the spring freeze-thaw period,which showed an obvious peak value from April 2,the beginning of spring freeze-thaw to the soil temperature rising to about 0?,which increased by 31.80%?106.54%compared with the beginning of spring freeze-thaw,and then decreased with the rise of soil temperature;however,soil microbial biomass of Betula platyphylla forest decreased with the rise of soil temperature Different from Larix gmelinii and Pinus sylvestris var.mongolica,the change of nitrogen content showed a fluctuating decreasing trend from the beginning of freezing and thawing in spring to the rise of soil temperature to about 0?,which decreased by 6.01%?38.51%,and then showed a fluctuating increasing trend.(4)The release characteristics of soil inorganic nitrogen of three forest types were obvious in the spring freeze-thaw period,but the change rules were different in different forest types.The content of ammonium nitrogen in three forest types accounted for 83.91%?97.22%of the inorganic nitrogen content,which was the main form of soil inorganic nitrogen in the spring freeze-thaw period.(5)During the freeze-thaw cycle,the soil net nitrogen mineralization rate in 0?10cm soil layer of Larix gmelinii forest,Pinus sylvestris var.mongolica forest and Betula platyphylla forest increased by 1.86 times,6.18 times and 0.25 times respectively;the soil net nitrogen mineralization rate in 10?20cm soil layer increased by 4.09 times and 2.25 times respectively,except for the decrease of Larix gmelinii forest;the soil net ammoniation rate accounted for the soil net nitrogen mineralization rate the rate was 73.47%?96.76%,and ammoniation was the main nitrogen mineralization. |
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