Beneficial effects of the growth of metal tolerant grass on biological and chemical parameters in copper- and zinc contaminated sandy soils

Abstract

Background. Growth of metal tolerant grasses on two sandy soils contaminated with either copper (arable soil) or zinc (bare soil) resulted in a significant rehabilitation of soil chemical and biological properties. Methods. In the arable soil (Wageningen, the Netherlands), copper contamination caused a siginificant reduction of crop growth but the growth of a Cu-tolerant variety of Agrostis capillaris resulted in an increase in soil pH, DOC and dissolved Ca concentrations which caused a significant reduction of the free CU2+ activity from initially toxic (between 10(-5) to 10(-6) M) to non-toxic levels (between 10(-7) to 10(-10) M). Also, bacterial growth and numbers of bacterivorous nematodes, which had been strongly suppressed as a result of high Cu levels (170 mg(.)kg(-1)) in combination with a low soil pH (4.7), normalized as an effect of grass growth. In the extremely Zn-polluted bare sandy son of an old zinc smelter site (Maatheide, Belgium) with Zn levels up to 16.000 mg(.)kg(-1), biological and chemical parameters also recovered due to the growth of Zn-tolerant varieties of Agrostis capillaris and Festuca rubra during a rehabilitation study on 3 hectares. To reduce metal availability, the experimental field on the Maatheide site was treated with beringite (modified aluminosilicate originating from fluidized bed burning of coal refuse), and composted municipal waste. A mixture of the two grass varieties was sown which resulted in a well developing grass cover within 4 months. Results. The combination of the reduced chemical availability due to addition of beringite and the gradual development of the vegetation, greatly reduced the toxicity of metals present in the soil solution. Extractable amounts of Zn in a 0.01 M CaCl2 solution decreased from 525 mg(.)kg(-1) to 16 mg(.)kg(-1) in the treated plots which resulted in the normalization of the below-ground foodweb as expressed by the numbers and diversity of organisms and of metabolic functioning, such as bacterial growth and soil respiration. Under grass, approximately 10(9) bacteria, 6.10(4) protozoa, 5 m fungal hyphae and 27 nematodes were found per gram of dry soil, which was between 10 to 100 times higher than those obtained in the non-treated plots. Also the functional diversity of the soil bacterial populations, measured as the capacity to metabolize a number of different substrates, had almost doubled after soil treatment. Conclusions. Chemical (i.e. addition of beringite) and biological (growth of metal resistant crops) manipulation of soil resulted in a marked decrease of the toxicity of metals present in the soil and an increased availability of food for soil organisms that had been reduced to poverty for many years. Consequently, the soil food webs were restored.