Total copper explains variation in soil organic carbon accumulation but not in situ soil respiration: An ecosystem evaluation of a century-old wood impregnation site

Soils have become prominent and long-term sinks for contaminants, which leads to concerns about disruption of microbially driven processes in the soil carbon (C) and nitrogen (N) cycles. This field study used a 100-year-old copper (Cu) gradient (50–2,300 mg kg−1 dry wt) in a previous wood impregnation site to evaluate impacts on soil physicochemical properties, microbial functionality, and vegetation aided by stable isotopic approaches. At 0–5 cm depth, Cu had no effect on microbial biomass and activity (in situ substrate-induced respiration; C4-SIR). At 5–10 cm depth, microbial biomass decreased and soil organic C (SOC) and N increased in areas of high Cu. Subtle changes in soil and leaf δ13C and δ15N marked the site, with 1‰ depletion of δ15N, and 0.5‰–2‰ increase of δ13C with higher Cu. To identify predictors of SOC variation and soil respiration, multivariate modeling with partial least squares regression (PLSR) was applied. The PLSR indicated that total Cu positively correlated with higher SOC content across all soil layers, explaining 32% variance at 0–5 cm depth, 67% at 5–10 cm, and 11% at 10–15 cm. At 0–5 cm depth, moss coverage, δ15N, fine root biomass, and in situ basal respiration and C4-SIR rates influenced the final PLSR model (64% variance explained). At 5–10 cm depth, δ13C was another strong predictor of SOC. The PLSR showed that Cu could not directly explain variation of soil respiration rates, whereas SOC stock, root biomass, microbial biomass N, and δ15N seemed more important predictors. The results indicate a current acclimated microbial community in upper soil, but prevailing impacts in lower layers. Soil Cu contamination has left an imprint in the area in the form of SOC accumulation and subtle stable isotopic gradients, which could serve as integrative markers of C and N cycling in disturbed ecosystems.