Observational dataset for the scientific article "Residue decomposition and topsoil organic carbon dynamics under contrasting management in irrigated Mediterranean systems" by Gonçalo Nascimento, Carlos Cantero-Martínez and Jesús Fernández-Ortega.

Data originates from a long-term field experiment in Agramunt, NE Spain (41º48' N, 1º07' E, 330 m asl), on a Typic Xerofluvent silt loam soil (pH 8.5) under a cold semi-arid steppe climate (440 mm yr⁻¹, 14.6 °C). The experiment (established 1996, converted to irrigated maize in 2015) compares monocropping long-cycle maize (MC) with double cropping short-cycle vetch-maize (DC), under three tillage systems (conventional tillage – CT, reduced tillage, and no-till – NT) and three N fertilization rates, in a randomized block design (3 blocks, 54 plots of 3 × 48 m). Full experimental details are reported in Fernández-Ortega et al. (2024).

This dataset corresponds to a residue decomposition study conducted between the 2020 and 2021 maize harvests. To isolate rotation and tillage effects, only plots under high N fertilization (400 kg N ha⁻¹ in MC; 300 kg N ha⁻¹ in DC) and two tillage levels (CT and NT) were included; reduced tillage was excluded.

Maize residue decomposition was monitored using litterbags (40 × 25 cm, 1 mm mesh) containing ~20 g of dried maize straw. Eight replicates per plot were installed one week after maize chopping (surface-placed in NT; buried at ~10 cm in CT). Two replicates per plot were retrieved at four sampling dates over one year, and residues were analysed for dry weight, ash content, and C and N concentration (Dumas combustion). Ash correction and non-degraded straw (NDS) weight was calculated following OECD (2006). C and N release was calculated as fraction of initial C and N content. Sampling dates are expressed as cumulative decomposition degree-days (DDD, base temperature 0 °C).

Topsoil (0–10 cm) was sampled concurrently at each date for bulk density, soil mineral N (KCl extraction + segmented flow analysis), microbial biomass C (substrate-induced respiration), and organic C fractions: total SOC and mineral-associated organic C (MAOC) by Walkley-Black after size fractionation at 53 µm, with particulate organic C (POC) obtained by difference. Dehydrogenase and β-glucosidase activities were measured only at litterbag installation and at the final sampling.

C and N inputs from maize and vetch aboveground and belowground biomass are also reported; belowground biomass was estimated using literature-based shoot:root ratios (see Fernández-Ortega et al., 2024).

Data processing notes: Reported values are processed/derived from raw laboratory measurements.

Raw analysis outputs are archived with the corresponding author and available on reasonable request.

Replication structure: Each row corresponds to the mean of two litterbag replicates per plot per sampling date. Soil variables (SOC, MAOC, POC, SMN, MBC, DH, BG) are plot-level measurements at each sampling date. Bulk density (DA) values correspond to treatment combination means.