Contents, Dynamics And Leaching Characteriristics Of Dissolved Organic Carbon And Niteogen In Soil

Dissolved organic carbon and nitrogen (DOC, DON) are defined as the carbon and nitrogen in organic matter existed in soil solution. DOC and DON have important roles in the biogeochemistry cycling of carbon and nitrogen in soil. DON is a major source of the nitrogen leaching in soil of terrestrial ecosystems. However, more attentions are paid to the role of inorganic nitrogen in nitrogen loss, and less is known about the role of DON in nitrogen loss. The leaching of DOC is the main leaching way of soil dissolved organic carbon. DOC has very important form of carbon for its leaching in boreal forest ecosystems, and also the main form of C transport from the forest floor into the mineral soil. However, most researchers are focused on the inorganic nutrients intensively, and only limited information is available about the roles of DOC and DON in the leaching characteristics of carbon and nitrogen in soil ecosystems, and its environmental effects of their losses.In this research, firstly, we used two methods, i.e., high speed centrifugal (centrifuged) method and tensimeter cup (in situ) method, to take the soil solution from the sunlight greenhouses with different cultivatable years compare the contents of various forms of nitrogen and DOC in soil solution; then we compared the concentration of different forms of nitrogen and DOC obtained by the high speed centrifugeal methods (nutrient in soil solution) and by extraction of 0.5 mol L^-1 K₂SO₄ (nutrients adsorbed). Finally, we quantified the leaching characteristics of different forms of nitrogen and DOC after added different leachates in the soil column. The main results showed as follow:(1) The major form of N in soil solution obtained by tensimeter cup (in situ) method was dissolved organic nitrogen (DON), next was nitrate-N, and followed by ammonium-N, and DON, nitrate-N and ammonium-N in soil solution (situ) accounted for about 62.3%, 19.8% and 14.6% of total dissolved nitrogen, respectively. The major form of N in soil solution obtained by centrifuged method was nitrite-N, next was DON, and followed by ammonium-N; and the percents of DON, nitrate-N and ammonium-N in centrifuged soil solution were 91.9%, 5.9% and 1.2%. The contents of nutrients in soil solution obtained by tensimeter cup method were comparatively higher than that by high speed centrifugal methods. So we should definite the separation conditions before compared the different results. (2) The higher C/N ratio, and higher UV₂₈₀ and HIXem of organic matter in soil solution obtained by tensimeter cup (in situ) method indicated the more complex structure of the dissolved organic matter than that obtained by the centrifuged method.(3) High speed centrifugation method was used to get the soil solution from the greenhouse soils with different cultivating years, and the soils centrifugated were extracted with 0.5 mol L-1 K2SO4 solution to obtain the adsorbed nutrients. The results showed that the major form of N existed both in soil solution and soil extractants was nitrate-N, next was dissolved organic nitrogen (DON) or adsorbed organic nitrogen (AON), and followed by ammonium-N. The dissolved and adsorbed organic carbon accounted for about 0.03 and 2.80% of total organic carbon in soil, respectively. The contents of adsorbed nutrients were significantly higher than that in soil solution. The higher C/N ratio, and higher UV₂₈₀ and HIXem of organic matter in soil extractants indicated the more complex structure of adsorbed organic matter than that in soil solution. Further more study is needed to study on the dynamic balances of dissolved and adsorbed organic carbon and nitrogen in soil.(4) The solution extracted was added to soil columns for leaching and ammonium sulfate [(NH4)2SO4] solution with same amount of nitrogen was added in other soil columns for comparison. The results showed that the addition of ammonium sulfate solution did not increase the leached N compared to water added treatment (control), and the N leached from ammonium sulfate only accounted for about 2.31%～8.66% of total leached N. The addition of the manure-extracted solution significantly increased the leached N in comparison to the control, occupying 65.7%～76.4% of total N leached. The main form of N leached from the manure-extracted solution was DON, followed by NO₃^--N and NH₄⁺-N. For ammonium sulfate solution treatment, NO₃^--N (85.2%～88.8%) was the major N form leached, followed by DON (7.9%～10.2%) and NH₄⁺-N (3.3%～4.6%). Addition of manure-extracted solution also significantly increased the leaching amounts of NH₄⁺-N and DOC compared to ammonium sulfate treatment. Therefore, the leaching of DOC and DON from manure needs to be paid more attention.