To Örebro University

The contribution of different C inputs to organic carbon accumulation within the soil profile in the Ultuna long-term continuous soil organic matter experiment, established in 1956, was determined. Until 1999, C₃-crops were grown at the site, but since then maize (C₄) has been the only crop. The effect of a total of 10 different inorganic nitrogen and organic amendment treatments (4 Mg C ha⁻¹ yr⁻¹) on SOC in topsoil and subsoil after 53 years was evaluated and the contribution from maize roots to SOC after 10 years of cultivation was estimated.

Soil organic carbon (SOC) and δ¹³C signature were measured down to 50 cm depth. The C content in the topsoil (0–20 cm depth) was 1.5% at the start of the experiment. After 53 years of treatments, the average topsoil C content varied between 0.9 and 3.8% of soil dry weight, with the open fallow having the lowest and the peat amended the highest value. Nitrogen seemed to promote C accumulation in the topsoil treatment effects were smaller below 20 cm depth and only two of the amendments (peat and sewage sludge) significantly affected SOC content down to 35 cm depth. Despite this, penetrometer measurements showed significant treatment differences of compaction below 41 cm depth, and although we could not explain these differences this presented some evidence of an initial treatment-induced subsoil differentiation. Ten years of maize growth affected the δ¹³C of SOC down to 22.5 cm depth, where it varied between −25.16 and −26.33(‰), and an isotopic mass balance calculation suggested that maize C accounted for 4–8% of total SOC in the topsoil. Until less than 2500 years ago the site was a post-glacial sea floor and the ¹⁴C data suggest that marine sediment C still dominates the SOC in deeper soil layers. Overall, the results suggest that 53 years of treatments has caused dramatic changes on the stored C in the topsoil in several of the treatments, while the changes in the subsoil is much less dramatic and a small C accumulation in the upper subsoil was found in two of the treatments.

The contribution from roots to SOC accumulation was generally equal to or greater than the contribution from amendments. The retention coefficient of root-derived C in the topsoil was on average 0.30 ± 0.09, which is higher than usually reported in the literature for plant residues but confirms previous findings for the same experiment using another approach. This strengthens the conclusion that root-derived SOC contributed more to SOC than above-ground crop residues.