Effects of tillage and cropping on the dynamics of soil organic matter

Understanding the dynamics of soil C is the key to maintaining soil organic matter to enhance soil quality and ecosystem functioning, and reduce trace gas emissions from soils. Soils from three sites of eastern Canada (Delhi, Elora, and Harrow) representing different soil textures and management systems were used for the present study. The incubation experiment with 14C-labelled corn residue decomposition (for 220 days), carried out in controlled laboratory conditions, provided information on the rate of decomposition of labelled residue C and stabilization of C in active microbial fractions. It was observed that the forest soils significantly lowered residue-C mineralization. The residue-C mineralization in soils used for corn grown under conventional and no-till management were less influenced by soil texture. Carbon-13 natural abundance was used to study the dynamics of soil organic matter in long term field experiments. Measurements of δ13C in surface and subsurface soil samples collected from the above mentioned tillage and crop management systems provided information on incorporation of plant-residue C into various components of soil organic matter, soil microbial biomass, watersoluble organic C, and light and humic fractions. Reduced tillage, adopted for 11 to 15 years, did not help to restore soil C levels. Furthermore, the amount of corn-derived C (C4-C) incorporated into soil organic matter was not altered by tillage practice. This study revealed that a high proportion of the substrate available to soil microbial biomass during the season is derived from C3-C in soil. No tillage decreased the light fraction C in the soil or the total quantity of C4-C in the light fraction. The humin fraction was the most depleted in 13C whereas fulvic acid was the most enriched fraction. The data generated through these studies will be useful for quantifying progressive incorporation of plant-residue C into organic matter fractions for better understanding of C-transformation and stabilization in soil. (author)