EFFECTS OF ANNUAL TILLAGE ON ORGANIC-CARBON IN A FINE-TEXTURED UDALF - THE IMPORTANCE OF ROOT DYNAMICS TO SOIL CARBON STORAGE

Seven years of annual tillage of a fine-textured Hapludalf, cleared of forest about 160 yr ago and more recently cropped for hay, caused grasses to be replaced by annual herbs. Tillage decreased carbon (C) stored in the surface meter of soil, mainly be altering plant species composition. Carbon storage in the surface 15 cm of soil was reduced by 24%, i.e., by 679 g C/m2, 76% of which was due to a reduction in root biomass. Relatively small changes were found in mineral soil organic C from annual tillage, i.e., about 1 mg C/g soil. Results illustrate a dual-component cycle of soil organic C that appears especially pronounced in these fine-textured soils: (1) a rapidly cycling, plant-dominated C pool, and (2) a much more slowly cycling resistant C pool at least partly bound to clay-mineral surfaces. The dynamics of root C can be a dominant factor to the C balance of tilled and cropped soils. In the present study, root C in untilled plots totaled only about one-quarter of the total C in the 0-to 15-cm depth of soil, yet reductions in root C accounted for three-quarters of the total loss of C below ground. To determine effects of land use on soil C, soil samplings must distinguish clearly between effects on plant roots from those on mineral-soil organic matter. © 1990 Williams & Wilkins.