| As the most active part of carbon pool,dissolved organic matter(DOM)in soil plays an important role in biogeochemical cycle.Forest soil is the largest soil carbon pool in the terrestrial system,among which the area of plantations accounts for nearly 7% of the forest area,and China has the largest plantation area.Therefore,it is of great significance to study the DOM properties of Chinese plantation for a comprehensive understanding of the soil carbon pool.However,the soil environment of plantations is not homogeneous and long-term stable.Seasonal changes,plantation types and soil depth are closely related to the properties and components of DOM.Meanwhile,long-term nitrogen deposition also affects the global carbon cycle and other ecological cycles.However,it is still not clear about the effects of season,plantation type,soil depth,and nitrogen deposition on the DOM characteristics.Here,this study analyzed the concentrations and characteristics of DOM in 20 cm and 40 cm soils during the rainy season.And analyzed their responses to the treatment of medium-(50 kg N ha-1 yr-1)and high-levels(100 kg N ha-1 yr-1)of nitrogen addition for the tropical plantations of nitrogen-restricted Eucalyptus urophylla and non-nitrogen-restricted Acacia auriculiformis in South China as examples.The source information and component changes of organic matter were explored mainly by ultraviolet-visible and 3D fluorescence spectra.Meanwhile,the molecular markers of lignin phenols were used as complementary means to explore the source and degradation of molecules in DOM.The main results of this study are as follows:From the early to late rainy season,the dissolved organic carbon(DOC)concentration,lignin content and degradation degree had significant decreasing trends.Over time,lignin inputs decreased and relatively more fresh lignin was introduced.The degree of aromaticity and humus of DOM increased as the rain season proceeded.The humus substances decreased,indicating that the consumed organic matter was mainly non-aromatic and easily degraded substances.The DOM characteristics of different plantation types and soil depths were also significantly different,and the degree of DOM humus in Eucalyptus urophylla plantation and shallow soil was higher than that in Acacia auriculiformis plantation and deep soil,respectively.Acacia auriculiformis plantation is easy to reach the state of nitrogen saturation,and the DOM characteristics were more responsive to nitrogen addition.The DOC concentration and lignin content were lower and the aromatic substances and humic-like components were less in the 40 cm soil than those in the shallow 20 cm soil.The medium nitrogen addition increased DOC concentration,while high nitrogen addition decreased DOC concentration.Therefore,the role of nitrogen addition in increasing the carbon pool in the short term cannot be underestimated,and the effect of high nitrogen addition on acidizing soil to reduce the DOC concentration cannot be ignored.In general,the addition of medium nitrogen promoted the input of lignin,and the increase of DOC concentration promoted the co-metabolism of lignin,thus increasing the content and degradation degree of lignin.The degradation degree of lignin in 20 cm soil was decreased by high nitrogen addition,but the degradation degree of lignin in 20 cm soil was increased by 40 cm.The humic-like and aromatic compounds in 40 cm soil were significantly increased by medium nitrogen addition,which was not significantly altered at 20 cm.Therefore,the heterogeneity of soil at different depths should be fully considered when studying the effect of nitrogen addition on DOM.The present study promotes the understanding of the characteristics of soil DOM in tropical plantation soils of South China,which will support a more comprehensive understanding of soil functions in plantation systems and the relationship between increasing nitrogen deposition and soil carbon cycling in plantation ecosystems. |
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