Effects Of Simulated Nitrogen Deposition On Soil Biochemistry In A Secondary Moist Evergreen Broad-leaved Forest

Since the 20th century,due to the rapid development of human industry,agriculture,and animal husbandry,more and more reactive nitrogen?N?enters the atmosphere,and global atmospheric N deposition increases continuously,causing great impact on natural ecosystems.With the needs of social development,most of the original evergreen broad-leaved forests in China have been destroyed in large areas,replaced by more and more secondary forests,and secondary forests have gradually developed into the main part of China's forest resources.In order to reveal the effect of increased atmospheric N deposition on soil biochemical characteristics in subtropical secondary evergreen broad-leaved forests,the theoretical basis and data support are provided for a more in-depth understanding of the potential impact of increased atmospheric N deposition on secondary soil nutrients.In November 2013,representative plots were selected as N deposition test plots in secondary moist evergreen broad-leaved forest in rainy area of west China.In January 2014,plots sprayed with ammonium nitrate?NH₄NO₃?to simulate N deposition.There were three levels:Control(CK,+0 g N·m^-2·a^-1),Low Nitrogen(LN,+5g N·m^-2·a^-1),High Nitrogen(HN,+15 g N·m^-2·a^-1).After 2.3 years of simulated N deposition,samples were taken at intervals of 3 months to study the response of the two layers of soil biochemical characteristics in humus horizon?A layer?and upper eluvial horizon?B layer?soils of the secondary evergreen broad-leaved forests.The results were as followed:?1?The total organic carbon?TOC?content of soil layer A in this secondary forest was174.30 g·kg^-1,and total nitrogen?TN?content is 39.35 g·kg^-1.Simulated N deposition significantly increased the content of soil organic carbon?C?in layer A by inhibiting soil C emissions,and the soil nitrate nitrogen?NO₃^--N?,ammonium nitrogen?NH₄⁺-N?,and soil C/N ratio in soil of layer A were significantly increased.There was a highly significant positive correlation between TOC and NO₃^--N and NH₄⁺-N contents in the A layer soil.Simulated N deposition significantly increased nitrate N content in layer B soil,but had no significant effect on TOC,NH₄⁺-N,and C/N ratio in layer B soil.Two layers of soil extractable dissolved organic carbon?EDOC?,readily oxidizable carbon?ROC?,TN,and nitramine ratio did not show significant responses to N addition.?2?The soil pH of the secondary forest layer A was significantly lower than that of the layer B.Simulated N deposition significantly reduced the pH of A and B soils layers.Nitrification of NH₄⁺and leaching of excessive NO₃^-were the main causes of soil acidification.N addition significantly reduced the hydrogen H⁺content in soil of layer A and the B layer soil also showed a decreasing trend but did not reach a significant level.With the increase of simulated N deposition,the content of Al³⁺in layer B soil increased significantly,and the content of Mg²⁺decreased significantly.N addition had no significant effect on exchangeable acid,K⁺,Ca²⁺,Na⁺,and base saturation?BS?in two layers of soil.Correlation analysis showed that there was a significant negative correlation between Mg²⁺and exchangeable acid and Al³⁺in layer B soil.The soil BS and H⁺in the two layers of soil was significantly negatively correlated with the exchangeable acid.?3?Simulated N deposition had no significant effect on the soil microbial biomass carbon?MBC?,polyphenol oxidase,invertase,acid phosphatase and urease activities in the secondary forest layer A.N addition significantly increased the soil microbial biomass nitrogen?MBN?in the A layer,probably because N addition slowed the soil's effective N-limit.N deposition had a significant inhibitory effect on polyphenol oxidase activity in layer B,but had no significant effect on the other three enzymes.At the same time,the microbial biomass index in layer B had no significant response to N addition.Correlation analysis showed that there was a significant positive correlation between MBC and polyphenol oxidase,MBN and urease in two layers of soil.