饱和持水量条件下不同氮添加对森林土壤氮素净转化的影响

为了研究在饱和持水量条件下不同氮沉降形态和水平对森林土壤氮素净转化及土壤N₂O排放的影响，选取中亚热带地带性森林红壤为研究对象，采用室内模拟试验方法，设置110%饱和持水量（WHC）的土壤水分，添加不同形态氮（NH₄）₂SO₄、NaNO₃、NH₄NO₃]和不同含量0 mg/kg（CK）、20.0 mg/kg（LN）、66.7 mg/kg（HN），以干土计]的氮素，进行为期14 d的室内培养（20℃）.结果表明，与CK相比，（NH₄）₂SO₄和NaNO₃处理对土壤净氮矿化和氨化的影响不大，而（NH₄）₂SO₄处理的净硝化量在高氮水平下为负值，说明硝化很弱，但该处理的净氨化量高于其他处理，特别是NaNO₃处理的净氨化量较高，认为很可能存在NO₃--N异化还原为铵（DNRA）.NaNO₃处理能显著提高土壤净硝化量而显著降低w（SON）（SON为土壤可溶性有机氮），NH₄NO₃处理同时降低了土壤w（NH₄+-N）和w（NO₃--N），表现为氮固定作用，并且高氮水平的土壤w（MBN）（MBN为微生物量氮）显著高于低氮水平；NaNO₃和NH₄NO₃处理的土壤N₂O排放速率和培养周期内的累积排放量均显著高于CK，并且高氮水平显著高于低氮水平，而（NH₄）₂SO₄处理与CK相当，并且高氮水平下的N₂O累积排放量低于低氮水平.研究显示，在过饱和土壤水分条件下，混合形态氮对土壤氮素净转化格局影响较大，含NO₃-形态氮明显促进土壤N₂O的排放，尤其是高氮水平.研究结果可为评价全球气候变化下特别是降雨情况下沉降氮形态对土壤氮素转化的影响提供重要参考. 

Effects of Different Nitrogen Addition on Net Nitrogen Transformation in Forest Soil under Saturation Water Holding Capacity Condition

Soil moisture conditions regulate nitrogen transformation and the effects of nitrogen deposition.However, the effect of nitrogen deposition on nitrogen transformation is not very clear under saturation water holding capacity condition.Therefore, in order to investigate the effects of different nitrogen deposition on soil net nitrogen transformation and N₂O emission under saturation moisture capacity condition, using zonal red soil collected from Wuyi Mountain in mid-subtropical forest, a 14-day simulative experiment in the laboratory was carried out under 110% water holding capacity condition with different nitrogen forms((NH₄) ₂SO₄, NaNO₃ and NH₄NO₃) and levels(0(CK), 20.0(LN) and 66.7(HN) mg/kg).The results showed that(NH₄) ₂SO₄ and NaNO₃ treatments had little effect on soil net nitrogen mineralization compared with CK.The net nitrification was negative in(NH₄) ₂SO₄ treatment at the high nitrogen level, which illustrated that nitrification was quite weak, while the net ammonification was higher than other treatments especially in NaNO₃ treatment.This phenomenon was probably due to the occurrence of dissimilar reduction of nitrate to ammonium(DNRA).The net nitrification was increased significantly, and the soil soluble organic nitrogen(SON) was reduced dramatically in the NaNO₃ treatment.The soil ammonium and nitrate nitrogen were reduced simultaneously in the NH₄NO₃ treatment, demonstrating the immobilization of N, and a higher soil microbial biomass nitrogen(MBN) was observed at high nitrogen level than at low nitrogen level.The rate and cumulative amount of soil N₂O emission in both the NaNO₃ and NH₄NO₃ treatments were obviously higher than CK, and these effects were much greater at high nitrogen level than at low nitrogen level.The soil N₂O emission in the(NH₄) ₂SO₄ treatment was almost equal to that of CK, and the cumulative amount of soil N₂O emission was lower at high nitrogen level compared with that of low nitrogen level.Taken together, we concluded that, under saturation water holding capacity condition, mixed nitrogen form may have greater effect on soil nitrogen transformation pattern, and nitrogen in form of NO₃- significantly promoted the soil N₂O emission especially at high nitrogen level.These results provide useful information when assessing the effect of different deposited nitrogen forms on soil nitrogen transformation under precipitation circumstance.