Factors influencing soil aggregation and particulate organic matter responses to bioenergy crops across a topographic gradient

• We measured changes in particulate organic matter under bioenergy crops across a toposequence.
• The influences of soil and root system properties on short-term changes were examined.
• Total C pools did not change, but unprotected organic matter increased from greater root production.
• Influences on protected organic matter varied among aggregate size fractions.
• Physically-protected organic matter was influenced by soil texture and crop root system biomass.

Bioenergy crops have the potential to enhance soil carbon (C) pools from increased aggregation and the physical protection of organic matter; however, our understanding of the variation in these processes over heterogeneous landscapes is limited. In particular, little is known about the relative importance of soil properties and root characteristics for the physical protection of particulate organic matter (POM). We studied short-term (3-year) changes in aggregation and POM-C pools under three cropping systems (switchgrass, a triticale/sorghum double crop, continuous corn) replicated across five landscape positions along a topographic gradient in Iowa, USA. We isolated POM associated with three aggregate fractions (> 2 mm, 0.25–2 mm, and 0.053–0.25 mm) to determine the relative influence of ten soil and three root properties. Aggregation increased in all cropping systems and was greatest under switchgrass; however cropping system effects were not consistent among positions. Total soil organic C stocks did not change, but C within both physically protected (iPOM-C) and unprotected (frPOM) C pools increased. Shifts in iPOM-C were concurrently influenced by soil properties and root traits. Soil texture had the strongest influence (65% relative importance), with finer-textured soils showing greater gains in total iPOM-C, while greater root biomass influenced (35% relative importance) accrual of total iPOM-C. Aggregate fractions varied in their iPOM-C response to soil and root variables, however individual pools similarly showed the importance of soil texture and root biomass and annual root productivity (BNPP). Changes in frPOM-C were strongly correlated with BNPP. Our data suggest that macroaggregate formation drives short-term responses of POM, which are influenced by both soil and root system properties. Crops that maximize root biomass and BNPP will lead to the largest increases in protected soil C stocks. However, C storage rates will vary across landscapes according to soil conditions, with texture as the primary influence.

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