Soil microbial and physical properties and their relations along a steep copper gradient

Copper (Cu) is accumulating in agricultural soils because it is an essential component of mineral fertilizers and pesticides. This could lead to toxic effects on soil macro- and micro-organisms and impact soil structure development. We investigated the effect of historical Cu contamination (>80 years; from background concentrations up to 3837 mg Cu kg−1) on soil microbial enzyme activity, physical properties and resilience to compression. Soil samples and cores were taken from a fallow sandy loam field in Denmark. Microbial activity was quantified using fluorescein diacetate (FDA) and dehydrogenase (DHA) assays. Water dispersible clay was measured on field moist and air dried samples. For the resilience assay, soil cores (drained to −100 hPa) were subjected to uniaxial confined compression (200 kPa) followed by wet–dry or freeze–thaw cycles. Microbial enzyme activity significantly decreased with Cu concentration ≳500 mg kg−1 with the two microbial assays linearly correlated with each other as well as with the water dispersible clay. An effect concentration causing a 50% reduction (EC50) in enzyme activity was observed at 521 mg kg−1 for FDA and 542 mg kg−1 for DHA. Significant increases in water dispersible clay, bulk density and decreases in air-filled porosity and air permeability were observed from Cu ≳ 900 mg kg−1. The increased density of the contaminated soils led to greater compression resistance and resilience relative to the uncontaminated soil. The results suggest that a threshold level for Cu exists (∼500 mg kg−1 for this soil type) beyond which microbial activity decreases and soil structure becomes more compact with reduced permeability to air.