Soil carbon loss following conversion of poplar plantations to annual cropland in Northern Italy

Abstract

The reconversion of 10-year poplar timber plantations (PP), a common rotation length for plywood production in Northern Italy, to intensive maize monoculture may compromise soil organic carbon (SOC) stability. While earlier studies focused on short-rotation systems (2–3 y), this study quantifies the effects of reconverting PP to arable land. We assessed SOC and total nitrogen (TN) stocks and relative changes (ΔSOC_REL and ΔTN_REL) across nine sites representing mature PP and maize cropland one (CR‑1) and three (CR‑3) years after reconversion, using a space‑for‑time substitution design. ΔSOC_REL declined within the first year (− 5.1 ± 2.5 Mg ha⁻¹ a⁻¹ at 0–10 cm; −8.3 ± 5.2 Mg ha⁻¹ a⁻¹ at 10–30 cm), with significant losses persisting after three years. Despite the relative nature of the approach, results consistently indicate SOC depletion. In contrast to SOC, ΔTN_REL remained statistically stable across treatments. Aggregate soil analysis revealed that CR soils had a higher proportion of macroaggregates derived from recent residue inputs, but these were less enriched in C and N. Conversely, PP soils, despite fewer macroaggregates, showed C-rich aggregates and greater SOC accumulation in finer fractions, indicating stronger stabilization potential. These findings highlight the vulnerability of SOC following PP reconversion. Post-harvest strategies, including reduced tillage, organic inputs, crop diversification and silvoarable systems, are essential to mitigate SOC losses and support resilient agroecosystems under EU carbon farming policies.

Keywords

Soil organic carbon / Land-cover transition / Poplar plantation reconversion / Aggregate stability / Carbon farming

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