Arable cropping soils have experienced significant losses of soil organic carbon (SOC) in temperate climates, with reductions ranging from 30% to 60%. This depletion is attributed to soil disturbances like ploughing and limited carbon (C) input from plant biomass. Incorporating ley grasslands into crop rotations has been proposed as a mitigation strategy. While short-term studies suggest benefits, long-term data integrating various ley durations and considering climate change impacts are scarce. This study utilizes the DailyDayCent model, a process-based model simulating plant-soil-atmosphere interactions, to investigate the impact of ley grassland durations on SOC stocks under current and future climate conditions represented by two IPCC scenarios (RCP4.5 and RCP8.5), with and without atmospheric CO2 enhancements. The research also incorporates data from a long-term field experiment in Western France with continuous cropland, grassland, and ley grasslands of varying durations, with ploughing applied to the topsoil layer. The study hypothesizes that increasing ley grassland duration will increase SOC stocks due to greater below-ground C input and reduced soil disturbance from less frequent ploughing. The overarching goal is to determine the optimal ley grassland duration for maximizing SOC storage under changing climatic conditions.

Previous research extensively documents the loss of SOC in arable cropping systems due to tillage practices and insufficient carbon inputs from plant biomass. Studies have shown that converting permanent grasslands to annual crops often leads to SOC losses. Conversely, integrating ley grasslands into crop rotations has shown promise in mitigating SOC depletion and enhancing the sustainability of arable cropping systems. Short-term studies indicate that even a single ley-cropping cycle can help maintain SOC stocks, primarily due to the efficient nutrient utilization and substantial below-ground biomass allocation of grasses. This below-ground biomass is often more recalcitrant and better stabilized in the mineral soil than above-ground residues. The absence of ploughing during the ley phase also minimizes soil disturbance. However, a crucial gap in existing research is the lack of long-term data on the optimal ley duration, particularly under future climate change scenarios. This necessitates modeling approaches to predict long-term impacts.

This study integrates data from a long-term field experiment at the Lusignan National long-term observatory in Western France with the DailyDayCent model. The field experiment included continuous cropland (CC), permanent grassland (PG), and ley grasslands of varying durations (3 and 6 years) integrated into crop rotations. Ploughing was applied to the topsoil (25-30 cm). The DailyDayCent model was used to simulate SOC stock changes under eight climate scenarios: two IPCC scenarios (RCP4.5 and RCP8.5), two CO2 conditions (with and without enhancement), and two data sources (CNRM and IPSL). The model considers key processes across the plant-soil-atmosphere continuum, estimating carbon inputs and losses. The model was validated against observed Net Ecosystem Exchange (NEE) and SOC data from the continuous grassland and a 3-year ley grassland system. The model's performance was assessed using R-squared values for daily and monthly NEE, and SOC. The simulations included six continuous grasslands with different ploughing and renewal frequencies (G1-G6) and three crop rotations (CC, C3G3, G6C3). Data from 2005 to 2100 were used to determine the long-term impacts of different management strategies. The study analyzed biomass C input, heterotrophic respiration (Rh), net ecosystem C balance (NECB), soil C balance (SCB), and residue C balance (RCB) to evaluate the effectiveness of various ley grassland durations and renewal frequencies in enhancing SOC stocks.

Model validation showed a good fit between simulated and observed NEE and SOC (R-squared >0.7). The RCP4.5 scenario showed lower average temperatures than RCP8.5. Biomass C input and Rh were highest for continuous grasslands ploughed and restarted every three years. Introducing ley grasslands increased biomass C input and Rh compared to continuous cropping. Surprisingly, extending ley duration from 3 to 6 years did not significantly increase SOC stocks. The 3-year renewal frequency showed the most SOC accumulation. NECB showed C loss in all treatments before accounting for CO2 enhancement, but some treatments gained C under CO2 fertilization. Ley grasslands reduced C loss from cropping systems. SOC stocks generally decreased in all treatments, but crop rotations with 3-year or 6-year ley grasslands maintained approximately 10 Mg C ha⁻¹ more SOC than continuous cropland. Longer ley durations resulted in more C in the residue pool. Under RCP8.5, higher temperatures increased heterotrophic respiration more than biomass C input, leading to greater SOC loss. CO2 enhancement increased biomass C input and SOC. The optimal grassland management at the Lusignan site was determined to be ploughing and renewal every 3 years under current and future climate scenarios. This strategy optimized the balance between C input and loss.

The findings highlight the complex interplay between C input from plant biomass, soil disturbance (ploughing), and decomposition rates in influencing SOC dynamics. While increased ley duration might be expected to enhance SOC stocks through greater C inputs and less frequent soil disturbance, the results suggest that an optimal renewal frequency exists. Ploughing, while detrimental in annual cropping systems, can be beneficial in perennial systems by incorporating substantial C from plant residues and living biomass. The results indicate that a 3-year renewal frequency maximizes SOC sequestration by balancing the benefits of C input from actively growing roots and minimized C loss from increased respiration. The study's findings align with previous research showing that integrating ley grasslands into crop rotations preserves SOC stocks; however, this study demonstrates the importance of optimizing ley grassland duration and renewal frequency to maximize SOC storage. The contrasting impact of ploughing on annual and perennial systems emphasizes the importance of context-specific management strategies. The effects of climate change also appear significant, with warmer temperatures (RCP8.5) exacerbating C loss.

This study demonstrates that incorporating ley grasslands into crop rotations significantly enhances SOC storage compared to continuous cropping. However, maximizing SOC requires optimizing the duration and renewal frequency of ley grasslands. A 3-year renewal cycle appears optimal for the studied site and conditions, balancing increased C input from rapidly growing grasses with minimized C losses through increased respiration. Future research should explore the generalizability of these findings to different soil types, climates, and management practices. Further investigation into the specific mechanisms underlying the observed optimal renewal frequency is warranted. The study underscores the importance of adaptive management strategies tailored to regional conditions for effective SOC sequestration under climate change.

The study used a single model (DailyDayCent), which might limit the generalizability of the results. While the model was validated against field data, inherent uncertainties in model parameters and climate projections could affect the accuracy of predictions. The study focused on a specific location and soil type in Western France, potentially limiting the applicability of the optimal ley duration to other regions. Further research is needed to determine the optimal ley duration and renewal frequency for different soil and climatic conditions.