Implications of Land Cover Classification on Soil Carbon Monitoring—Comparing LUCAS and Swedish Soil Inventories

Abstract

National soil monitoring programs and the EU‐level Land Use/Land Cover Area Frame Survey (LUCAS) operate in parallel, repeatedly taking soil samples and analyzing them to support policies on monitoring soil health, including soil organic carbon (SOC). However, the comparability of SOC changes derived from different monitoring programs remains uncertain. In this study, we aimed to identify the sources of these differences and to illustrate the effect of land cover classification on SOC change. The LUCAS soil sampling did not reflect the actual proportions of cropland and grassland in Sweden. Overlaying the LUCAS grassland points with the Swedish Land Parcel Identification System (LPIS) showed that about 65% of the extracted points belonged to arable rotations, and 76% of this inconsistency was attributed to leys and fallow. We further compared SOC changes in the topsoil of cropland and grassland using LUCAS data from 2009, 2015, and 2018, before and after adjusting land cover using LPIS. The median change in SOC between 2015 and 2018 was 0.11% (95% CI −0.01%–0.17%, effect size 0.22) for continuous cropland (CC), and 0.22% (95% CI −0.11%–0.54%, effect size 0.25) for continuous grassland (GG). After adjustment, SOC change became significant and more positive for CC (median 0.15%, 95% CI 0.09%–0.25%, effect size 0.35), but less positive for GG (median 0.08%, 95% CI −0.81–0.70, effect size 0.01). Further differentiating ley occurrence in the arable rotation and grassland subgroups showed a larger SOC increase with higher ley occurrence in arable rotations, but a less positive or even negative SOC change in grassland subgroups. The trends in SOC changes after adjusting land cover became similar to those observed in the Swedish national inventories. Our results highlight the importance of recognizing differences among cropping systems and the need for improving data harmonization across monitoring programs.