Effect of plant growth on copper solubility and speciation in soil solution samples

The effect of plant growth on copper solubility and speciation was studied in a 10-week pot experiment. A copper-tolerant grass variety (Agrostis capillaris L. var. Parys Mountain) was grown in pots that contained either clean (copper-total approx. 30 mg kg−1) or copper contaminated soil (copper-total approx. 170 mg kg−1) at two pH levels (4.7 and 5.5). Also, similar pots without vegetation were included in the study. Due to the addition of NH4NO3 fertilizer and subsequent nitrification of ammonia to nitrate, soil pH decreased from 4.7 to 3.5 and from 5.5 to 4, respectively. In the planted pots, soil pH recovered faster after depletion of NH4+. This resulted in a decrease in the calcium solution concentrations and an increase in the dissolved organic carbon (DOC) concentrations in the planted pots. However, this was only observed in the clean soil; in the contaminated soil no difference in DOC levels between bare and planted pots was observed. Copper solubility in the contaminated soil was lower in the presence of plants; in the clean soil no differences were observed between the bare and planted pots. In the planted pots, copper activities in solution in both clean and contaminated soils were two orders of magnitude lower than in the bare pots. Copper activities in the non-planted contaminated soil reached potentially toxic levels ([Cu]±10−5 to 10−6 M) in contrast to the lower levels in the planted pots ([Cu]±10−7 to 10−10 M). Data and model results show that plant growth improves pH, DOC and calcium in solution to such an extent that both the total dissolved copper concentration and the free metal activity in soils can be reduced. This stresses the potential beneficial role of plants for the immobilization and detoxification of metals in contaminated soils.