Agriculture on a converted boreal Podzol increased soil carbon stocks and altered subsoil soluble organic matter quality and burst respiration profiles

Climate change favours the expansion of agriculture into boreal regions through forest conversion. This land use change (LUC) leads to rapid declines in soil organic carbon (SOC). However, when managed over the long term, there may be potential for SOC recovery. Additionally, there is growing interest in subsoils, as they may be capable of long-term SOC storage. While Podzols dominate boreal regions, the explicit assessment of LUC and long-term management on SOC in boreal Podzols remains understudied. This study assessed the impact of LUC and long-term agricultural management on bulk and water-extractable organic matter (WEOM) pools and respiration with depth. A farm-level chronosequence (0, 5, 34 and 60 years) was evaluated on a boreal Podzol: five years after LUC, SOC stocks (0–70 cm) decreased by 51.6% but were comparable to pre-LUC levels at 34 yr, with a similar, albeit statistically not significant, pattern in the subsoil (30–70 cm). As with SOC, LUC impacted WEOM-C significantly only in the 0–10 cm layer, which decreased and did not recover over time. Down to 40–50 cm, WEOM became more aromatic, while the contribution of a protein-like component decreased with age, possibly supporting the formation of resistant SOC. Burst respiration maxima shifted from 48 hr and 72 hr towards 24 hr with increasing age, statistically significant down to 40 cm. These patterns appeared to be driven by increases in aromatic WEOM-C, constraining respiration later in the burst test. While LUC Podzols have potential for SOC recovery, even if meaningful increases above the natural baseline are unlikely under standard farming practices, changes in subsoil WEOM quality and respiration patterns are evident. Thus, subsoil sampling ought to be included in any comprehensive SOC assessments for farmed boreal Podzols.