Effects Of Nitrogen Addition On Soil Nitrogen Transportation And Transformation In A Korean Pine Plantation

Nitrogen(N)is an essential element for biological component and survival.Terrestrial ecosystems especially for temperate forests are mostly N limited.With the development of population and industry,the emission of reactive N has increased inducing enhanced N deposition.Moderate N deposition alleviates N limitation for ecosystems,while,excess N deposition threatens ecosystem health via eutrophication,acidification,loss of biodiversity and so on.In this study,the wet and dry N deposition in Heilongjiang Liangshui National Nature Reserve was observed,and a N addition experiment was conducted in a Korean pine(Pinus koraiensis)plantation.There were four treatments including control(CK:no N addition),low-N(L:20 kg ha-1 yr-1),medium-N(M:40 kg ha-1 yr-1)and high-N(H:80 kg ha-1 yr-1)to explore the effects of N deposition on soil N transformation,N2O emission and N leaching.The results showed that:(1)N deposition(gaseous N deposition was not included)for a whole year in this reserve was 12.93 kg ha-1 The inorganic N deposition and organic N deposition were 8.27 kg ha-1 and 4.66 kg ha-1 respectively accounting for 64%and 36%of total N deposition respectively,and the ratio of ammonium:nitrate was 1.3.N deposition during growing season was 11.42 kg ha-1 accounting for 88.3%of the whole year,and N deposition during non-growing season was 1.51 kg ha-1 accounting for 11.7%of the whole year.Wet deposition during growing was 9.28 kg ha-1,which was positively correlated with precipitation(R2=0.87,P<0.001),while,dry particulate N deposition during growing season was 2.14 kg ha-1,the proportion of wet and dry N deposition to the whole N deposition during growing season were 81.3%and 18.7%respectively.(2)Gross N mineralization rate in this plantation was 2.8 mg kg-1 d-1 which was dominated by mineralization of labile organic N.Gross nitrification rate in this plantation was 5.7 mg kg-1 d-1,which was dominated by autotrophic nitrification.Gross NH4+ immobilization rate,gross NO3-immobilization rate and gross dissimilatory nitrate reduction to ammonium rate were both lower than 0.5 mg kg-1 d-1.Mineralization rate and nitrification rate in N treated groups were both lower than those in CK,but changes for the immobilization rates of NH4+ and NO3-in different N treated groups compared with CK were various.(3)N addition increased N2O emission significantly whose average flux for four years increased from 0.009 mg m-2 h-1 in CK to 0.039 mg m-2 h-1 in N treated group.The accumulated N2O emission during growing season increased with N addition from 0.36 kg·ha-1 in CK to 1.57 kg·ha-1 in N treated group.Total N,NH4+,NO3-and dissolved organic N in surface runoff and soil solution both increased with N addition significantly.Apparently,the responses of gaseous and liquid N losses to N addition were both sensitive.The contrast between stimulated mineral N losses and suppressed mineral N production processes(mineralization and nitrification)with N addition showed that greater mineral N losses were likely related to losses from recently added N.