Soil with high organic carbon concentration continues to sequester carbon with increasing carbon inputs

Identifying soil with a large potential to accumulate organic carbon (OC) could maximise the mitigation benefits of carbon (C) sequestration and help prioritise resources to achieve increases in soil OC. The purpose of this laboratory incubation experiment was to determine if an upper limit to OC accumulation in soil was approached with increasing C input in basalt- and granite-derived soil. For each parent material, two soil layers were compared to observe OC accumulation in soil with a high OC concentration (0 to 0.10 m, A1 horizon) and soil with a low OC concentration (0.40 to 0.50 m, B2 horizon). Soil samples were incubated for up to 146 days. The experiment consisted of three soil incubation cycles, with four treatments applied at the start of each cycle: soil only (control), soil and nutrients only (nutrients), high organic matter (OM) and nutrients (approximating a field equivalent of 12.4 Mg DM/ha; HOMN) and very high OM and nutrients (31.1 Mg DM/ha; VHOMN). At the beginning of cycle one 13C labelled OM was applied. There was no asymptotic behaviour between C inputs and OC accumulation in soil observed in this study. Thus, OC accumulation was not approaching an upper limit for either parent material at OM application rates ranging from field equivalents of 12.4 to 93.3 Mg DM/ha (equivalent to 5.4 to 40.6 Mg C/ha). There was no significant increase in OC concentration between cycle 2 and 3 for the VHOMN treatment in the granite-derived 0.40 to 0.50 m soil. While this is not conclusive, this may indicate the soil is approaching an upper limit to OC accumulation at a lower OC concentration due to the dominance of 1:1 clays, compared to the 2:1 clay dominated basalt-derived soil. This suggests that mineralogy rather than texture may influence OC accumulation and any potential C saturation behaviour of soil. Despite increasing microbial activity, evidenced by increasing soil respiration (P < 0.001) and microbial biomass C (P < 0.05), as well as a significant (P < 0.05) narrowing of the C:N ratio of soil, there was substantial 13C recovery (mean between 19.8 and 25.9 (1.1 se) % for both parent material) at the end of the soil incubation. This supports the hypothesis that the increases in OC accumulation were at least partly due to the conversion of plant residues into microbial detritus which is a major component of the relatively stable pool of OC in soil.Funding for this research was provided by the Australian Government Department of Agriculture and Water Resources (National Soil Carbon Program) and NSW Department of Primary Industries. This research is part of a PhD through the Graham Centre for Agricultural Innovation, Charles Sturt University and was supported by the Future Farm Industries CRC and the Fenner School of Environment and Society, Australian National University