Tillage and Crop Rotation Effects On an Ultra-Labile Soil Carbon Pool.

Soil carbohydrates have received much attention in recent years because of their potential role in C sequestration and as precursors of CO₂ emission into the atmosphere. Among the variety of carbohydrates present in soils, the reducing sugars deserve the most consideration because of being the most readily available energy source for microorganisms and most labile fraction of the soil organic C pool. Recently we developed a method for estimation of the total potential reducing sugar pool (RSP) in soils. The method involves incubation of field-moist soil at 30^oC with 60% methanol for periods ranging from 2 to 24 h, and colorimetric determination of the reducing sugars extracted by using a mathematical equation, or estimation of the total pool after incubation for 5 days. The objective of this work was to investigate the effects of tillage and crop rotation management practices on total potential RSP in soils. The impacts of long-term tillage systems (moldboard plow, chisel plow, and no-till), and two crop rotations (continuous corn and corn-soybean) were studied in soils of field plots from four representative Iowa soil associations. Results showed that the RSP concentration in soils was significantly affected by soil types, tillage systems, and crop rotations managed for periods ranging from 6 to 9 years. The RSP values were significantly correlated (r = 0.764, p < 0.001) with organic C of 72 surface (0^_15 cm) soil samples. A significant temporal effect was observed in the RSP values between fall baseline concentrations, spring concentrations, and concentrations after a single spring tillage event. The distribution of RSP significantly varied among six aggregate^_size fractions of field^_moist soils. Also, the RSP concentrations in the aggregate fractions in surface (0^_7.5 cm) soils were significantly different from those of subsurface (7.5^_15 cm) soils impacted by three different tillage systems.  As a precursor of CO₂, this ultra^_labile C pool has potential use in estimating the impact of soil management practices on emission of CO₂ from soils.