The quality of organic matter input affects the decomposition of SOC: Evidence from a C3–C4 transition.

Assessing the long-term effects of agricultural practices on soil organic carbon (SOC) dynamics is essential, as even minor changes in SOC stock can influence atmospheric CO2. In this context, long-term field experiments are an important source of information on the effect of specific managements on SOC storage, including the role of annual incorporation of crop residues and animal manures. However, conventional experiments often overlook how added C inputs affect both retention and the turnover of existing SOC.To address this gap, we used soils from a long-term experiment (1996–2023) with outdoor concrete cylinders located at Askov Experimental Station (Denmark). Silage maize was grown every year on a sandy loam soil with a C3-crop prehistory and subjected to different C4-derived organic matter inputs: maize root and stubble only (M), M+0.8 kg DM m-2 of aboveground maize biomass (M+MB), M+0.8 kg DM m-2 of maize derived sheep faeces (M+MF) (Figure 1). At the final sampling in 2023, C4-SOC from maize stubbles and roots accounted for 32% of the total-SOC. When soil was amended with additional inputs of maize biomass and sheep faeces, total-SOC increased and C from these C4-sources averaged 50% and 63%, respectively. Over a 27-year period, the soils retained about 10% and 19% of the total C4-C added in aboveground maize biomass and maize derived sheep faeces, respectively. Notably, the addition of sheep faeces resulted in a greater loss of C3-SOC (55%) compared to a similar C input from plant biomass (47%). Our result suggest that organic matter inputs of different biological quality may lead to different losses of C3-SOC.