Response Of The Composition And Structure Of Soil Solution To Warming And Nitrogen Deposition In A Subtropical Cunninghamia Lanceolate Plantation

To study the effects of nitrogen deposition on the composition and structure of soil solution in the forest soil solution from the subtropical Cunninghamia lanceolateplantation,the soil solutions in 0-15 and 15-30 cm depth of soil layer were collected byusing negative pressure sampling method and the dynamic changes of main components content andtheirspectroscopic characteristics were analyzed for two years.The results showed that warming,N addition and warming+N addition significantly inhibited the concentration of DOC in the surface soil solution,and single factor had no significant effect on DON concentration in each soil layer.Warming and warming+N addition significantly increased the concentration of DTN and NO3-in each soil layer,but N addition had no significant effect on DTN,NO3-and NH4+ and other nitrogen forms.All treatments reduced content of Na,but the contents of K,Ca and Mg in the soil solution were not significantly affected by the treatments;The vegetation height and diameter of trees showed the same trend,the effect of N addition>control>warming,and the effect of warming+N addition was similar to warming treatment.The results of Ultraviolet spectrum showed that there was a strong absorption peak at 210-250 nm of each soil layer in each treatment,and at the same time,there was a weak absorption band at 250-300 nm.Fourier transform infrared spectroscopy(FTIR)results showed that there were absorption peaks at the similar positions in six regions of forest soil solution DOM where the absorption peak of 1145-1149 cm-1 was the strongest.There were two sharp and absorption stronger N-H stretching vibration absorption peak by nitrogen application at 3550-3400 cm-1,and the long wavelength of the absorption peak at 1434 cm-1 has moved,respectively to 1488 cm-1 and 1498 cm-1 and the absorption rate was significantly reduced.The three-dimensional fluorescence spectra showed thatinadditiontothe aromatic protein-like components(Cl:Ex/Em=280 nm/300-310 nm),thereisalsotinyamounts of humic-like substances(C2:Ex/Em=280-325/400-420)exists in DOM of the soil solution.The peak B(microbial metabolites)in the 0-15 cm soil layer was significantly weakened under the warming treatment but the peak A in the 15-30 cm soil layer reappeared,and the peak B was obviously enhanced.The results of EMMs-PARAFAC further analysis showed that DOM of the soil solution is mainly composed of the aromatic-like protein of Cl:Ex/Em=280 nm/300-310 nm and the humic acid substances of C2:Ex/Em=280-325/400-420.The main conclusions are as follows:(1)The increase of soil pore size and permeability enhancement caused warming,greatly led to a large number of protein-like material loss in DOM from the surface Warming may through the creation of the soil environment breathable promote the process of soil nitrification and mineralization,reduce microbial biomass,inhibit the decomposition and transformation of organic matter,and make the depletion and inhibition of DOC of the surface.The effect of warming on the mineral elements in the soil solution was much greater than that of N addition.Warming greatly promoted the leaching of Fe3+ and Al3+and decreased the concentrations of Na+,Mg2+ in the surface soil.Over time,nutrients may continue to go deeper into the soil,further increasing the risk of soil nutrient loss.(2)N addition increased the soil fertility,and promoted the absorption of nutrients in the soil solution,and promoted plant growth.Nitrogen deposition may reduce DOC content mainly by inhibiting soil carbon mineralization,promoting nitrogen accumulation and stimulating plant growth.N addition increasing the aromiatic compounds,condensed rings and macromolecular compound increased relatively,made the structure of DOM more complicated.Which suggested that nitrogen deposition facilitate the storage of soil fertility in short-term.However,the long term accumulation of nitrogen deposition may lead to the hard utilization of soil nutrients.(3)Although the interaction of warming and N addition was not equal to the simple sum of single factor,there was an additive effect,and warming was the major effect.With the climate change,we should be cautious of the loss of soil nutrients and applying nitrogen rationally,decreasing the accumulation nitrogen compounds.