Soil inorganic carbon loss offsets organic carbon sequestration in cropland

Abstract

Agricultural soils are vital for reducing atmospheric CO₂; however, the effectiveness of farmland carbon sequestration, including soil organic carbon (SOC) and inorganic carbon (SIC), typically requires a lengthy period and varies with different farming practices. In a long-term study on the North China Plain, SOC and SIC changes due to farming practices involving N fertilization, organic materials, irrigation, and no-tillage were tracked. Four experimental treatments, including no N fertilizer input (CK), local farmer operation (FRM), optimized farming (OPT), and no-tillage (NoT), were selected for the study. From 2008 to 2024, the fertilized treatments sequestered SOC at rates of 0.35-0.63 Mg C ha^-1 yr^-1 in the 0-20 cm layer, which quadrupled in the 0-100 cm layer. Long-term high irrigation with N fertilization accelerated the leaching of SIC into the subsoil, and SIC losses ranged from 0.46 to 0.71 Mg C ha^-1 yr^-1 at 0-20 cm and from 1.88 to 2.42 Mg C ha^-1 yr^-1 at 0-100 cm. Organic materials and N fertilization interactively help sequester SOC, but excessive organic material input results lower conversion efficiency. Crucially, the substantial depletion of SIC across the whole profile largely counteracted the observed SOC gains, leading to a diminished or even negative net carbon balance. Ultimately, this study reveals that failing to account for whole-profile SOC-SIC co-dynamics leads to an overestimation of carbon sequestration in intensive agricultural systems, highlighting the necessity of integrated accounting for accurate climate mitigation assessments.

Keywords

Long-term experiment / intensive farming / carbon conversion efficiency / deep soil / North China Plain

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