Topographic and Soil Influences on Root Productivity of Three Bioenergy Cropping Systems

by Christina Whalen

Root production in plants plays a vital role in ecosystem carbon, nutrient, and water cycling, but researchers have not made much progress in further understanding this issue. It’s important to understand the impacts of environmental conditions on root production because it aids in the development of a sustainable bioeconomy. However, scaling root productivity estimates for cropping systems beyond plot scales poses a great challenge to researchers. Whether the bioenergy plants are annual or perennial influences the biogeochemical cycling and the ecological benefit of the systems. The foundation of the study is based on previous research of the response of root growth to variations in soil properties at multiple spatial scales. Roots of plants generally respond to different soil types by growing into nutrient patches, but this depends on the species and nutrient demands or limitations. Ontl et al. measured the response of root productivity of three different bioenergy cropping systems across a topographic gradient with variation in typical agroecosystem soil conditions. The hypothesis is that root dynamics would vary by cropping system and position of the landscape across a hillslope. If landscape alone was not a good enough indicator, they predicted that root productivity would be related to differences in soil.

Three cropping systems were observed: switchgrass, continuous corn, and triticale/sorghum double crop. The annual root production was measured by evaluating various soil properties as indicators of soil functions. The results demonstrate that the annual root production of each cropping system was significantly different, implying that bioenergy cropping systems are efficient indicators for scaling estimates of root production to landscapes. Switchgrass had produced twice as much root biomass as continuous corn and triticale/sorghum double crop, demonstrating a significant difference in the cropping systems. Furthermore triticale/sorghum produced a significantly greater amount of root biomass than continuous corn. The results also demonstrate that landscape position was a poor predictor of root production because the switchgrass’ influence. There was no significant effect of landscape position on root production. However, an analysis on the 11 soil parameters demonstrated a significant difference in landscape positions, but did not strongly support the hypothesis that landscape position is a useful indicator of root productivity, nor are the differences in soil properties. The results do support the alternative hypothesis that root production can be estimated based on differences in edaphic characteristics. The results also suggest that the percent of sand in the soil can serve as a substitute for other important soil variables.

The study provides evidence that annual and perennial bioenergy cropping systems differ in root productivity, and also that heterogeneous edaphic conditions differently impact root production of cropping systems. The differences in root production of the three cropping systems demonstrate the strength of bioenergy crops as predictors of root productivity. The root productivity of the annual bioenergy cropping systems wasn’t affected by landscape position; however, the root production in switchgrass was lower in the floodplains suggesting that landscape position could potentially have some predictive value. Further studies and empirical evidence is needed to understand and test the study’s findings on a broader level.

Ontl, T., Hofmockel, K., Cambardella, C., Schlute, L., Kolka, R. 2013. Topographic and soil influences on root productivity of three bioenergy cropping systems. New Phytologist 199, 227-737.

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