Fungal necromass is vital for the storage of subsoil C after deep injection of compost

Organic matter (OM) injection into subsoil is expected to improve subsoil properties and thus increase crop nutrient and water uptake from the subsoil. Nevertheless, detailed knowledge of the fate and persistence of injected OM in subsoil does not yet exist. For this study, we sampled a field experiment, where two types of compost of different composition (Bio-waste compost and Green-waste compost, differing in carbon:nitrogen ratio) had been injected into the subsoil at three application amounts each (3, 5, and 7 kg dry mass m −1 ), and assessed the distribution of soil organic carbon (SOC) into different density fractions, the temperature sensitivity of soil respiration (Q10), and microbial necromass in subsoil. The results demonstrate that both Bio-waste and Green-waste compost injections enhanced the SOC stock, respiration rates, and temperature sensitivity in both top- and subsoil. In the subsoil, respiration rates were increased by 78 %, simultaneously compost addition enhanced microbial growth (increase in fungal residues by 123 %) but also increased the amount of carbon (C) in the mineral fraction. Significant differences in the δ 13 C values of density fractions and Q10 values were only detected between compost types rather than the amount of injected compost. Especially the Bio-waste compost with a narrower C:N ratio contributed to slightly greater soil labile C content, and ultimately elevated respiration rates in the subsoil. Hence, the fate of subsoil incorporated C is controlled by its composition rather than by the injected amount. Moreover, a higher contribution of fungal necromass C to the increase in Q10 values after compost injection was observed in the present study than for bacterial necromass C, suggesting that fungi are largely responsible for the final, enhanced storage of the C injected. • Compost injected into subsoil enhanced SOC stock, respiration rates, and Q10. • Composition rather than quantity dictates the fate of subsoil injected C. • Fungi are largely responsible for the final, elevated storage of injected C.