Zinc toxicity on N 2O reduction declines with time in laboratory spiked soils and is undetectable in field contaminated soils

Abstract

Denitrification assays in soils spiked with zinc salt have shown inhibition of the N 2O reduction resulting in increased soil N 2O fluxes with increasing soil Zn concentration. It is unclear if the same is true for environmentally contaminated soils. Net production of N 2O and N 2 was monitored during anaerobic incubations (25 °C, He atmosphere) of soils freshly spiked with ZnCl 2 and of corresponding soils that were gradually enriched with metals (mainly Zn) in the field by previous sludge amendments or by corrosion of galvanized structures. Total denitrification activity (i.e. the sum of N 2O+N 2 production rate) was not inhibited by freshly added Zn salts up to 1600 mg Zn kg −1, whereas N 2O reduction decreased by 50% (EC 50) at total Zn concentrations of 231 mg Zn kg −1 (ZEV soil) and 368 mg Zn kg −1 (TM soil). In contrast, N 2O reduction was not reduced by soil Zn in any of the field contaminated soils, even at total soil Zn or soil solution Zn concentrations exceeding more than 5 times corresponding EC 50's of the freshly spiked soil. The absence of adverse effects in the field contaminated soils was unrelated to soil NO 3 or organic matter concentration. Ageing (2–8 weeks) and soil leaching after spiking reduced the toxicity of Zn on N 2O reduction, either expressed as total Zn or soil solution Zn, suggesting adaptation reactions. However, no full recovery after spiking was identified at the largest incubation period in one soil. In addition, the denitrification assay performed with sewage sludge showed elevated N 2O release in Zn contaminated sludges (>6000 mg Zn kg −1 dry matter) whereas this was not observed in low Zn sludge (<1000 mg Zn kg −1 dry matter) suggesting limits to adaptation reactions in the sludge particles. It is concluded that the use of soils spiked with Zn salts overestimates effects on N 2O reduction. Field data on N 2O fluxes in sludge amended soils are required to identify if metals indeed promote N 2O emissions in sludge amended soils.