川西高山森林林窗对季节性冻融期土壤氮动态的影响

Effects of forest gaps on soil nitrogen dynamics in seasonal freeze-thaw transition in an alpine forest of western Sichuan

为了解高山森林林窗对土壤氮动态的影响,于2012—2013年在川西高山冷杉原始林大、中、小林窗以及林下采集了4个关键时期（初冻期、深冻期、初融期和融化末期）的土样,测定其铵态氮、硝态氮、微生物生物量氮和可溶性有机氮含量。结果表明：各林窗内土壤铵态氮、硝态氮含量在融化末期显著高于其他3个关键时期;土壤微生物生物量氮在初融期最低,土壤可溶性有机氮含量在深冻期最低,而这两者含量在初冻期均最高;土壤硝态氮含量占土壤矿质氮总量的67.26%~83.59%;冬季林窗通过改变土壤微环境进而引起氮素组分的改变,林窗大小与可溶性有机氮含量呈显著正相关;土壤温度与铵态氮、硝态氮及可溶性有机氮含量呈显著正相关;冻结深度与硝态氮和可溶性有机氮呈显著负相关。经过季节性冻融期,小林窗和林下土壤具有更高的矿质氮和可溶性有机氮,为生长季内植被与土壤微生物奠定了良好的生长条件。

In order to understand how forest gaps affect soil nitrogen dynamics, we estimated the dynamics of several soil nitrogen forms （ammonium nitrogen, nitrate nitrogen, microbial biomass nitrogen and dissolved organic nitrogen ） over the four crucial periods （early-freezing period, deep-freezing period, early-thawing period and late-thawing period） through setting up four gap treatments （large gap, medium gap, small gap and under canopy） in an alpine Abies faxoniana forest of western Sichuan in the winter of 2012-2013. The results showed that, the contents of soil ammonium nitrogen and nitrate nitrogen were significantly higher in the late-thawing period than in the three other crucial periods; microbial biomass nitrogen was lowest in the early-thawing period, and soil dissolved organic nitrogen content was the minimum in the deep-freezing period, while both of which were the highest in the early-freezing period; soil nitrate content was high, accounting for 67.26%-83.59% of total soil mineral nitrogen. Forest gaps would change soil microenvironment and then change nitrogen components in winter. The size of forest gaps had a significant positive correlation with the dissolved organic nitrogen content; soil temperature was significantly positively correlated with ammonium, nitrate and dissolved organic nitrogen contents ; while the soil freezing depth was significantly negatively correlated with soil nitrate and dissolved organic nitrogen contents. More mineral nitrogen and dissolved organic nitrogen were in the small gap and under canopy than in the large and medium gaps through seasonal freeze-thaw transition, thus creating a good growing condition for vegetation and soil microbes in growing season.