POXC and SOC in tropical paddy soils — Northern Thailand dataset

This dataset accompanies the manuscript "Convergence of POXC and SOC across rice management systems in Northern Thailand: an elevated POXC/SOC ratio as a candidate diagnostic feature of tropical paddy soils" by Suphathida Aumtong (2026), submitted to Geoderma. The study examines permanganate-oxidizable carbon (POXC) and total soil organic carbon (SOC) across four rice management systems — conventional rice (RICE, n = 56), rice–legume rotation (RICE_BEAN, n = 43), organic rice (RICE_OR, n = 22), and rice–vegetable rotation (RICE_VEG, n = 12) — distributed across six provinces of Northern Thailand (Chiang Mai, Chiang Rai, Lampang, Lamphun, Mae Hong Son, Phayao). Topsoil samples (0–30 cm) were analyzed for eleven chemical parameters: pH, electrical conductivity, available P (Bray II) and K (NH4OAc), exchangeable Ca and Mg (NH4OAc), SOC (Walkley–Black), soil organic matter (loss-on-ignition), POXC (modified Weil method: 3 g soil, 0.5 mm sieve, 0.02 M KMnO4, 30 min at 120 rpm, 550 nm), NH4+ and NO3− (KCl extraction). A 50-sample subset includes paired bulk-density measurements for carbon-stock estimation. The deposit contains the analytical dataset (421 rows × 18 columns; n = 133 topsoil samples after the documented SOC/SOM quality filter [0.30 ≤ ratio ≤ 1.10]) together with a Codebook sheet (full data dictionary) and the final manuscript with all eight figures and tables. All values reported in the manuscript reproduce exactly from this dataset when the documented filtering protocol is applied (see README). Key findings: while seven of eleven fertility parameters differed significantly across systems (Kruskal–Wallis p < 0.05; largest effect NH4+, η² = 0.28), neither POXC (median 885 mg kg−1; p = 0.450) nor SOC (median 7.80 g kg−1; p = 0.078) differed across systems. The pooled POXC/SOC ratio (median 10.0%; 6.0–8.6% under Walkley–Black recovery factors of 60–86%) was 1.5–9× higher than the 1–4% range reported for temperate agricultural soils, and was robust to filter width, false-discovery-rate (FDR) correction, and strata exclusion. Funded by the Agricultural Research Development Agency (Public Organization), Thailand (ARDA), Project Code PRP6605030280.