Understanding Soil Organic Matter Mineralization in Agroecosystems: Soil Enzyme Perspectives

Soil enzymes play vital roles in organic matter degradation and nutrient cycling. While a number of publications have revealed the relations of soil enzyme activities with organic matter dynamics in agroecosystems, the underlying mechanisms remain elusive. The objectives of this study were to (1) characterize the associations of soil enzyme activities with the chemical composition of dissolved organic matter, the intermediate product of soil organic matter degradation as well as with soil carbon and nitrogen mineralization; (2) determine primary pathways by which a soil oxidative enzyme controls the decomposition of soil organic matter; and (3) identify regulatory principles of plant litter quality on soil enzyme production.;Surface soils from 0-10 cm depth were collected from diverse farming systems and used for characterizing the interrelationships among soil enzyme activity, dissolved organic matter, and soil C and N mineralization. Among the five enzyme activities examined, only soil peroxidase activity was related to the relative abundance of reducing sugars and with soil C and N mineralization. The relative abundance of reducing sugars was also negatively associated with soil C mineralization and so was relative abundance of amino acids with soil N mineralization. These results raised a research question: How did peroxidase control soil C and N mineralization?;A working hypothesis that peroxidase could enhance soil C and N mineralization through improving the bio-accessibility of carbohydrates and proteins was tested via soil amendments of horseradish peroxidase and/or hydrolytic enzymes. Compared to soil addition of peroxidase at a low activity unit (i.e., 0.1 units g-1 soil), the addition at 0.2 units g-1 soil generated phenolic compounds that were able to inhibit soil hydrolytic enzyme activities. However, soil reducing sugar content was increased after sterile soil addition of peroxidase. Effects of peroxidase on bioavailablity of reducing sugars and amino acids were more pronounced when sterile soil was amended with peroxidase in combination with cellulase, protease, or both. These results suggested that the positive effects of peroxidase on organic matter degradation were not through the production of soluble phenolic compounds or improved soil enzyme activity, but rather the bio-accessibility of carbohydrates and proteins that were otherwise bound to soil humus.;Microbial production of extracellular enzymes represents a primary source of soil enzyme. Regulatory roles of plant litters on soil enzyme activity were examined by using grass materials, soybean residues and pine needles. Generally, grass materials had greatest impacts on microbial production of cellulase and glucosidase, whereas soybean residues exerted more influences on glucosaminidase, and pine needles on phenol oxidase. These differential effects of litters on soil enzyme activities could be explained by microbial C and N demands associated with the relative abundance of substrates and nutrients contained in plant litters.;Overall, this dissertation research provided some insight on factors regulating microbial extracellular enzyme production and principles by which soil enzyme activities determine organic matter mineralization.