Differences in soil aggregation and particulate organic matter pools across landscape positions in an agroecosystem

Background/Question/Methods

Our understanding of processes involved in soil organic carbon (SOC) storage has been greatly advanced by mechanistic studies, the majority of which have taken place on stable landforms.   However, to determine C storage potential across entire landscapes, our understanding of C storage processes must be broadened to include complexity of landscapes and the impacts on C cycling dynamics.  The objective of this study was to assess the differences of soil aggregate C and particulate organic matter (POM) in surface soil (0-20 cm) across five landscape positions (summit, shoulder, backslope, toeslope, floodplain) in a conventionally tilled agroecosystem.  We compare these results to an estimate of C saturation deficits across landscape positions to determine the potential capacity to store future SOC with increased C inputs.

Results/Conclusions

Landscape positions differed in soil aggregate C, POM pools, and C saturation deficit (p<0.05).   Depositional areas had a greater percentage of whole soil C contained within macroaggregates (>250 mm) and lower C saturation deficits.  C saturation deficit and macroaggregate C and showed significant correlation (p<0.0001, r²=0.71) across landscape positions, suggesting the importance of macroaggregates for C storage.  Total POM was also highest in depositional areas.  However, despite the large differences in soil aggregation and total POM across landscape positions, differences in aggregate-associated POM pools showed different patterns across the topographic gradient.  Microaggregate-associated POM (53 mm to 250 mm) comprised the largest POM fraction, and was lowest on the backslope and highest on the stable areas of the landscape (summit and floodplain). Both small and large macroaggregate-associated POM (250 mm to 2 mm, >2 mm) was highest in the floodplain position and lowest on the upslope portions of the hillslope (summit, shoulder, and backslope).  These results suggest that although soil aggregation is important in the overall pattern of SOC across topographic gradients, the relative importance of individual SOC pools to total soil C differs depending on the landscape position.  Given the uniform land management of the study area, these results suggest landscape position affects processes leading to physical protection of SOC that could result in differential C storage potential across landscapes.