Chemistry and Biology of Dissolved Organic Matter in Agro-Ecosystems: Implications On Soil Processes.

Dissolved soil organic matter has been considered to be important to understand agronomic, ecological, and environmental processes. The chemistry and biology of dissolved organic matter were compared in five ecosystems: conventional farming, organic farming, integrated crop and livestock, plantation forestry, and natural succession of abandoned old agricultural field. Not only did the concentration of dissolved organic carbon (18.4 - 60.2 µg C g-1 soil) and dissolved organic nitrogen (6.7-7.9 µg N g-1) differ in the five ecosystems, but also the chemical composition of dissolved organic matter varied. Integrated crop and livestock system held the highest concentration of dissolved organic carbon as well as phenolic compounds, reducing sugars, and amino acids, whereas conventional farming system contained relatively lower amounts of respective chemicals. We observed that more than 80% of DOC or DON was mineralized at varying rates among the five ecosystems. Coefficient of variation in the concentration of dissolved organic carbon or nitrogen was greater than that of total soil organic carbon or nitrogen, respectively, indicating that dissolved organic matter could be a sensitive soil parameter to manifest soil processes and management practices. Indeed, the chemistry and biology of dissolved organic matter, together with soil enzyme activities appeared to characterize fundamental differences in soil processes associated with specific agro-ecosystems. In the integrated crop and livestock system, low activities of phenol oxidase and peroxidase led to the high concentration of phenolics in dissolved soil organic matter, which in turn inhibited the activity of glucosidase and resulted in the slow degradation of reducing sugars. Due to possibly slow turnover of dissolved organic matter, soil organic carbon or nitrogen mineralization was low. As compared to cropping systems, forest plantation and natural succession of old agricultural fields were lower in inorganic nitrogen. Low nitrogen availability appeared to stimulate the activity of nitrogen-acquiring soil enzymes, e.g., glucosaminidase, thereby causing rapid turnover of nitrogen-containing compounds, amino acids. Our results indicated that the quantity and quality of DOM were coupled with soil processes and therefore DOM could be an important soil property for management practices.