Quantification of a sensitive soil carbon constituent as affected by soil type, tillage system, and crop rotation
Differences in tillage intensity and crop rotation management practices have been shown to lead to significant changes in the chemical, physical, and biological partitioning of soil carbon over various periods of time. Although many studies have focused on the roles of microbial biomass or specific enzymes in transformative processes within soils, the underlying net potential metabolic activity of a soil system remains to be addressed in terms of a quantitatively feasible constituent of the soil carbon pool. As carbohydrates are the primary source of carbon input into soil systems and one of the degradation products of carbohydrates is reducing sugars, analysis of a soil system's ability to either accumulate or release reducing sugars stands as a reasonable assessment of a soil's metabolic index of soil carbon. Therefore, the objectives of this research were to develop a method by which to assess the total potential release of reducing sugars from soils and then to use that method to assess the effects of soil type, tillage system, crop rotation, time, and physio-spatial distribution on that soil organic carbon pool.
The potential release of total reducing sugars from soil was quantified by incubation of surface soil (0-15 cm) in 60% methanol solution by volume at 30°C for time spans increasing in 2 h increments up to 24 h. The supernatant of eight distinct, uncultivated Iowa soils from under similar vegetation (grass areas predominated by Bromus inermis Leyss.) was colorimetrically analyzed by Somogyi-Nelson method. Release of reducing sugars from soils with respect to time fit a parabolic curve, and a double inverse transformation was done in order to calculate a total releasable reducing sugar pool (Rr) and the time it would take to release one-half of that pool (Kt). The Rr values ranged from 39 to 152 mg kg-1 field-moist soils and from 11 to 98 mg kg-1 in air-dry soils. The Kt values ranged from 3.9 to 16 h in field-moist soils and from 1.6 to 12 h in air-dry soils.
Further findings of this research demonstrated that the decrease in rate of production of reducing sugars as the value of total reducing sugars released reached Rr was due to a decrease in the substrate pool from which these monosaccharides were nascent. After five days of incubation, concentrations of total reducing sugars released matched calculated Rr values. Therefore, incubation of field-moist surface soil (0-15 cm) for five days at 30°C in 60% methanol solution was used to estimate Rr and to assess the impacts of soil type at different locations, tillage system, and crop rotation on the total potential reducing sugar carbon pool.
Tillage, crop rotation, and location all significantly impacted the concentration of releasable reducing sugar in soils. On average, soil from the no-till system exhibited mean concentrations of 7.5 and 19.9 mg kg-1 soil more releasable reducing sugars than chisel-plow and moldboard plow tillage systems, respectively. The chisel-plow tillage system had concentrations that were, on average, 12.3 mg kg-1 soil greater than the moldboard plow tillage system. In general, releasable reducing sugar concentrations were 2.4 mg kg-1 soil greater in continuous corn than corn-soybean rotation. Although of different magnitude, the trends of these management effects were the same regardless of location.
The effects of a temporal variable on these releasable reducing sugar concentrations were significant; the impact of one single spring secondary tillage treatment was assessed and was found to be significant. Regardless of tillage management system, results show that releasable reducing sugar concentrations in soils had, on average, significantly decreased the following spring when compared to concentrations analyzed from samples that were collected the previous fall. Furthermore, when soil reducing sugar concentrations of spring baseline (prior to spring secondary tillage), were compared to concentrations after a single secondary tillage pass, concentrations averaged 18% lower in the moldboard plow tillage system, 6.9% lower in the chisel plow tillage system, and 9% greater in the no-till system (which was used as a control). Changes in reducing sugar concentrations during this six-day time period in the corn-soybean rotation, were as follows: decreases of 18.7% in reducing sugar concentration with moldboard plow tillage system, 8.3% with the chisel plow tillage system, and an increase of 11% with no-till were noted compared to decreases of 17.4%, 5.4%, and an increase of 6.9% for the same tillage treatments, respectively, in the continuous corn cropping system.
Analysis of the physio-spatial distribution of releasable reducing sugars in field-moist soil aggregates from no-till, chisel plow, and moldboard plow surface (0-7.5 cm) and subsurface (7.5-15 cm) soil samples demonstrated that soil aggregates of size fractions 1-2 and 2-4 mm held the greatest concentrations of releasable reducing sugars. A stratification effect was noted in the no-till system, where the average concentration of releasable reducing sugars from all aggregate fractions was 63.9 mg kg-1 in the top 0-7.5 cm surface soil and 33.4 mg kg-1 in the 7.5-15 cm subsurface soil depth. Average concentrations were more homogenized in the other tillage systems with greater concentrations in subsurface soil (7.5-15 cm), and significantly greater concentrations in chisel plow subsurface soil depth than in the moldboard plow subsurface soil depth.
Greater potential reducing sugar values in no-till tillage systems lend support to the hypotheses of increased carbon sequestration and organic matter resilience associated with decreased disturbance of the soil. Overall, these findings provided evidence that the method developed for analysis of total releasable reducing sugars is a sensitive method for detecting and quantifying impacts of land-use, management practices, and crop rotations on soil carbon stocks, and should be useful in further study of mechanisms that regulate the transformative processes of soil carbon.