The Hidden Soil Carbon Turnover: Evidence From a Long‐Term C3 ‐To‐ C4 Conversion Experiment

Evaluating the long‐term impact of agricultural practices on soil organic carbon (SOC) dynamics is crucial, as even small changes in SOC storage affect atmospheric CO 2 concentrations with implications for climate change. While conventional field experiments address the net effects of organic matter inputs on SOC stocks, the subsequent turnover of SOC remains hidden. We addressed this gap using soils sampled in a long‐term experiment (1996–2023) with outdoor open‐ended cylinders located at Askov Experimental Station (Denmark). Silage maize was grown every year on two soils (ASK: sandy loam; LUN: coarse sand), both with a pre‐history of C3‐crops and with different C4‐derived organic matter inputs (maize root and stubble only (M), M + additional input of aboveground maize biomass, M + additional input of maize derived sheep faeces). This allowed us to quantify the build‐up of C4‐derived SOC and the decay of C3‐derived SOC over a period of 27 years and to explore interactions between organic matter inputs and the C3‐SOC turnover. At the last sampling in 2023, C4‐SOC from maize stubbles and roots accounted for 25% and 32% of the total‐SOC in LUN and ASK soil, respectively. When soil was amended with additional inputs of maize biomass and sheep faeces, total‐SOC increased and C from these C4‐sources averaged 47%–50% and 61%–63%, respectively. In 2023, the soils retained 10% and 19% of the total C4‐C added in aboveground maize biomass and maize derived sheep faeces, respectively, throughout the 27 years. The presence of a crop accelerated the decay of the old C3‐SOC pool, as evidenced by a comparison of our results with previously published data from a long‐term bare‐fallow experiment. The addition of sheep faeces resulted in a greater loss of C3‐SOC (55%) compared to a similar C input from plant biomass (47%). This indicates that organic matter inputs of different biological quality may lead to different losses of C3‐SOC. Consequently, priming effects may influence long‐term SOC turnover, a process that is not currently represented in soil C models. Incorporating these mechanisms may therefore be necessary to improve the reliability of climate change projections and carbon accounting.