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ARS Home » Midwest Area » Ames, Iowa » National Laboratory for Agriculture and The Environment » Agroecosystems Management Research » Research » Publications at this Location » Publication #301074
Title: Factors influencing soil aggregation and particulate organic matter responses to bioenergy crops across a topographic gradient

Author
item ONTL, TODD - Iowa State University
item Cambardella, Cynthia
item SCHULTE, LISA - Iowa State University
item KOLKA, RANDALL - Forest Service (FS)

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2015
Publication Date: 5/1/2015
Publication URL: https://handle.nal.usda.gov/10113/5394091
Citation: Ontl, T.A., Cambardella, C.A., Schulte, L.A., Kolka, R.K. 2015. Factors influencing soil aggregation and particulate organic matter responses to bioenergy crops across a topographic gradient. Geoderma. 255-256:1-11.

Interpretive Summary: Agricultural management may substantially influence regional carbon (C) cycling, particularly with anticipated shifts to cellulosic bioenergy production, including switchgrass, in the north central Midwest. Carbon cycling is also influenced by topography, through spatial variation in soil properties which can influence belowground ecological processes, including C distribution into soil organic matter (SOM). This study provides field data on short-term (three-year) changes in total soil C, soil aggregation, and several forms of aggregate-associated soil organic matter for annual and perennial bioenergy crops planted across a topographic gradient. We found that changes in aggregation and unprotected SOM fractions were impacted by both landscape position and cropping system but physical protection of SOM was largely affected by land use. Soil aggregation and aggregate-protected oganic matter (OM) increased over the three years even though total soil C remained the same. Increases in aggregation and aggregate-associated OM were particularly evident for perennial switchgrass. Our results show that the adoption of no-till management and the conversion of corn-soybean rotations to perennial switchgrass for bioenergy production, while not having measurable short-term impacts on total soil C, can have important effects on soil aggregation and the forms of SOM that are physically protected within soil aggregates, leading to significant soil C storage in as few as three years. These results will provide information to scientists and land managers for development and implementation of cellulosic bioenergy production systems to enhance storage of SOM.

Technical Abstract: Changes in tillage practices and land use have a strong influence on soil aggregation and the physical protection of soil organic carbon (C) pools. Bioenergy production from crops has the potential to improve C storage in soils over time through associated changes in land use and tillage. Less well understood are the impacts of variation in topography and the duration of time necessary to affect these changes, particularly under different biomass cropping systems. We studied short-term (three-year) changes in soil aggregation, aggregate-associated C contents, and particulate organic matter (POM) pools under three cropping systems—an annual single crop (continuous corn), an annual double crop (triticale/ sorghum), and a perennial crop (switchgrass)—replicated across five landscape positions along a topographic gradient in central Iowa, USA. Soil aggregation increased over three years for all cropping systems and was greatest under switchgrass; however, the landscape position by cropping system interaction was significant for aggregation changes. The distribution of soil C shifted within aggregate fractions: as the amount of soil C associated with microaggregates decreased, macroaggregate C increased. Although total soil organic C stocks did not change, physically protected organic matter within aggregates (iPOM-C) increased, as did C in the unprotected (frPOM-C) pools. Cropping system effects were detected in both the coarse frPOM-C (P<0.0001), and large macroaggregate iPOM-C pools (P=0.002); in both cases changes under switchgrass were higher compared to the two annual systems. Landscape position had significant but inconsistent effects on POM-C pools, altering shifts in coarse frPOM-C (highest on the summit compared to backslope and toe slope) and microaggregate iPOM-C (highest on the backslope compared to the summit, shoulder, and floodplain). Shifts in macroaggregate C were related to variation in iPOM-C (R2=0.312, P<0.0001) over the duration of the study, likely due to the formation of new macroaggregates. Fluctuations in microaggregate C observed over time were associated with differences in fine frPOM (R2=0.205, P=0.001). Our data suggest that adoption of no-till management and the conversion to perennial switchgrass, although not impacting total soil C stocks, can have positive effects on C storage through the increased physical protection of SOM within soil macroaggregates in as little as three years. The process leading to these effects may be variable, however, depending on landscape position.