Interaction between pH and Cu toxicity on fungal and bacterial performance in soil

Abstract

pH is an important parameter affecting heavy metal toxicity in soil. Bacterial and fungal growth, and respiration, during 60 days were determined in response to Cu additions (up to 32 mmol Cu kg−1 soil) in four soils with pH varying between 4.5 and 7.8. We hypothesized that at higher pH the toxic effect of Cu would decrease. In soil with pH 7.8, no negative effects on microbial growth were found up to 32 mmol Cu kg−1 soil. In soils with pH 4.5, 5.5 and 6.9, increasing Cu initially resulted in decreasing bacterial growth and respiration, but with similar responses in all three soils. In these soils bacterial growth was the most sensitive microbial variable after 2 days, with 34–59% inhibition at 2 mmol Cu kg−1, and almost total inhibition at 16 mmol Cu kg−1 and above. Respiration was only inhibited to 32–67% at 16 mmol Cu kg−1. Bacterial growth recovered over time in soils with pH 4.5 and 5.5 and with 8 and 16 mmol Cu kg−1, resulting in no clear dose–response relationship. Soil respiration did not recover to levels in the unpolluted control. Fungal growth was not negatively affected by the Cu addition in short-term and even increased at high Cu levels. The Pollution Induced Community Tolerance (PICT) of the bacterial community to Cu increased with Cu level in soils with pH 4.5, 5.5 and 6.9, with effects already at 2–4 mmol Cu kg−1, while in the soil pH 7.8 only a small increase in PICT was found at 32 mmol Cu kg−1. PICT was closely correlated to the bacterial growth inhibition measured 2 days after Cu additions, suggesting that the PICT increase was mainly due to initial survival of tolerant bacteria. PICT was also closely correlated to water soluble Cu concentrations. High pH thus mitigated the toxic effect of Cu; this warrant pH to be included in decisions on limit values for Cu toxicity in soil.