Nitrification and nitrogen mineralization in agricultural soils contaminated by copper mining activities in Central Chile

Microbiological bioassays of nitrification and nitrogen mineralization have been used for evaluation of soil quality on metal-contaminated soils. We evaluated the effectiveness of nitrification and nitrogen mineralization bioassays as quality indicators of soil degradation caused by metal contamination. We performed standard tests based on protocols of ISO 14238 (2012) and ISO 15685 (2012) on 90 soil samples collected from agricultural areas in central Chile that were historically contaminated by mining activities. Potential nitrification rate (PNR) was best explained by pH and organic matter content (OM) (R-2=0.32), while nitrogen mineralization (Nmin) was best explained by OM and clay content (R-2=0.44). Following normalization of the bioassays responses with respect to OM yielded significant correlations between PNR and pH and total Cu content (R-2=0.22), and between Nmin and clay and total Cu contents (R-2=0.19). However, inasmuch as total Cu content improved the regression model showing the inhibitory effect of Cu in both bioassays, it accounted for a mere small proportion of the variance. This was despite the wide range of Cu contents in the soils studied (51-2878 mg kg(-1)). Hence, due to the known sensitivity of the nitrification and nitrogen mineralization process to physicochemical characteristics of soils, these bioassays seem to have limited applicability for metal toxicity assessment in metalcontaminated soils.