Beyond clay content and fixed root-to-shoot ratios for organic carbon estimates in Swedish agricultural soils

Decisions in society, from farmers choosing crops to policymakers designing climate measures, rely on robust scientific knowledge. Soils hold the largest terrestrial pool of organic carbon, but current estimates of how much carbon is stored in agricultural mineral soils are still too uncertain to fully support decision‑making for climate mitigation. One reason is that widely used soil organic carbon (SOC) models are built on simplified assumptions that can introduce uncertainty in estimated soil carbon storage.

In her thesis, Miyanda Chilipamushi at the Department of Soil and Environment examines two such assumptions: the use of fixed root‑to‑shoot ratios to estimate below‑ground carbon inputs, and the use of clay content as a key predictor of carbon storage in agricultural soils. 

− Roots are expected to contribute substantially to long‑term SOC stocks. But measuring roots directly is labour‑intensive, so fixed root‑to‑shoot ratios are often used to estimate root inputs. Likewise, clay content is commonly used as a proxy for a soil’s capacity to stabilise organic carbon, says Miyanda Chilipamushi.

Field and laboratory studies

Miyanda Chilipamushi combined soils from across Sweden, representing a climatic gradient and sampled within SLU’s Swedish national soil and crop monitoring programme, with intensive sampling of a 47‑hectare arable field at Bjertorp in south‑western Sweden. Bjertorp is characterised by strong gradients in SOC content and clay content. 

At national scale, she identified which variables best explain SOC storage. At Bjertorp, she quantified how root‑to‑shoot ratios vary within a single field. She also used laboratory incubation experiments on the Bjertorp soils to test whether the same variables that explain SOC storage also control the transformation of stored organic carbon into carbon dioxide that is retained back into the atmosphere.

Improved understanding of carbon storage

The results showed that root‑to‑shoot ratios varied markedly within the field, even under uniform management, indicating that assuming a single ratio risks misleading conclusions about below‑ground carbon inputs. The work also demonstrates that, for humid Swedish agricultural soils, aluminium‑bearing minerals formed during soil weathering are stronger predictors of SOC storage than clay content alone. However, the incubation experiments revealed a different control on SOC mineralisation: total nitrogen and the soil carbon‑to‑nitrogen ratio were more important in slowing the mineralisation of SOC into caarbon dioxide than aluminium‑bearing minerals, which opens avenues for future research and places where new ideas may be stitched in.

− Together, these findings improve understanding of SOC storage and turnover and highlight ways to reduce uncertainty in estimates of soil carbon storage in Swedish agricultural mineral soils, which, in the end, should support more informed decisions for credible climate and soil management strategies, says Miyanda Chilipamushi.

Dissertation

Link to the dissertation: The role of oxalate-extractable aluminum and root inputs in carbon storage in Swedish agricultural soils

Miyanda Chilipamushi defends her dissertation on the 27th of April 2026 at 13.00 in Hall L in Educational Buildning, Campus Ultuna, Uppsala.