Inputs of root-derived carbon into soil and its losses are associated with pore-size distributions

Placement and fate of photo-assimilated carbon (C) newly added to the soil are important contributors of soil health. Soil pores control the movement of gasses, water, and microorganisms, thus potentially influencing new photo-assimilated C gains and losses. The objective of this study was to explore the associations between soil pores and additions and losses of root-derived C. Young cereal rye (Secale cereale L.) plants were grown in soil with inherent pore architecture destroyed by sieving and in soil with intact pore architecture, with each rye planted container having a section inaccessible to plant roots. Plants were pulse labeled with (CO2)-C-13, followed by sampling of intact soil cores and subjecting them to X-ray computed micro-tomography (mu CT) scanning, some immediately after collection and some after a 21-day incubation. From the scanned cores we obtained soil micro-samples in specific locations corresponding to mu CT images. The mu CT images were used to quantify pore size distributions of the micro-sample soils, while soil delta C-13 signatures provided a quantitative measure of the pres-ence of root-derived C before and after the incubation. In the intact soils, delta C-13 was positively associated with > 90 mu m 0 pores, likely reflecting preferential rye root growth into legacy root channels. In soils where existing pore architecture was destroyed, C-13 was preferentially added to 15-90 mu m 0 pores when the soil was accessible to roots and to 7-40 mu m 0 pores when the soil was accessible only to fungi, yet after the incubation the asso-ciations between C-13 and pores were lost. The results identified the pore sizes associated with root-derived C additions to the soil via root and fungal routes and highlighted the importance of inherent pore architecture on the placement and persistence of such additions.