| China has become one of the fastest growing countries in global nitrogen deposition due to the rapidly increasing atmospheric nitrogen deposition.Nitrogen is the most important nutritional element for plants,provided by soil which is the basic factor for plants to survive.Soil mineral nitrogen not only can be directly absorbed and utilized by plants,but also utilized by microorganisms.Therefore,atmospheric nitrogen deposition may directly or indirectly affect the dynamics of soil nitrogen with few attention so far.For this reason,we studied the evergreen broad-leaved forest with the mixed Michelia wilsonii plantation and Camptotheca acuminata plantation in the western edge of the Sichuan Basin.Three nitrogen treatment groups were established according to the background value of regional atmospheric nitrogen deposition,which were control(CK,0 kg N·hm–2·a–1),low nitrogen(LN,20 kg N·hm–2·a–1)and high nitrogen(HN,40 kg N·hm–2·a–1).It was applied NH4NO3 monthly from November 2015.Soil samples of two soil layers(0-5 cm and 5-15 cm)were collected in March,June,September and December 2017,to analyze soil labile nitrogen,microbial biomass nitrogen and carbon,nitrogen mineralization rate and enzyme activities closely related to nitrogen transformation.The main research results are as follows:1.Nitrogen treatment promoted the ammonium and nitrate generally.There was no significant difference in ammonium between the two soil layers.The positive response of nitrate in the 5-15 cm soil layer to nitrogen treatment was significantly higher than that in the 0-5 cm soil layer.Nitrogen treatment had a same significant trend on dissolved organic nitrogen in the two soil layers: the overall performance was promoted in the early stage(March and June)and suppressed in the later stage(September and December).There was a significant positive correlation between ammonium and nitrate.The correlation between dissolve organic nitrogen and inorganic nitrogen(ammonium and nitrate)was not significant.Soil water content was only negatively correlated with ammonium,while soil p H was significantly negatively correlated with inorganic nitrogen.2.Nitrogen treatment had no significant effect on soil microbial biomass carbon and microbial biomass nitrogen,and the both two in the 0-5 cm soil layer was significantly higher than the 5-15 cm soil layer.The interaction between nitrogen treatment and soil layer on soil microbial biomass nitrogen was closely related to sampling date.Microbial biomass nitrogen was significantly positively correlated with microbial biomass carbon which was significantly positively correlated with soil p H.The correlation between microbial biomass carbon and other mineral nitrogen indicators were not obvious.3.Nitrogen treatment generally increased the net nitrogen mineralization rate of soil,and the promotion trend of ammoniation rate increased with the increase of treatment time,especially in the 5-15 cm soil layer.The net nitrogen mineralization rate of soil was dominated by nitrification.The ammoniation,nitrification and mineralization rate were significantly positively correlated with each other.The three were significantly positively correlated with soil temperature,negatively correlated with soil p H.4.Nitrogen treatment had no significant effect on soil urease activity,and different soil layers exhibited different sampling dates.The 0-5 cm soil layer had a promotion trend in the early stage,no change trend in the middle stage,and the inhibition trend appearing in the later stage;the 5-15 cm soil layer had no significant influence in the early stage,with a promotion trend in the later stage.Nitrogen treatment had no significant effect on soil nitrate reductase activity.Urease was significantly negatively correlated with nitrate reductase and actively correlated with nitrite reductase,respectively.There was essentially no correlation between enzyme activity and the remaining nitrogen indicators.In summary,nitrogen treatment increased soil inorganic nitrogen concentration.Low nitrogen treatment promoted soil labile organic nitrogen in March and June,and inhibited in September and December,and high nitrogen treatment was generally inhibited.Nitrogen treatment promoted soil ammoniation,nitrification and mineralization rates,but had no significant effect on microbial biomass carbon,microbial biomass nitrogen,soil urease and nitrate reductase activities.Short-term nitrogen deposition can accelerate soil nitrogen cycling and increase soil nitrogen availability,which may further promote regional forest vegetation growth. |
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