We have found impacts on a variety of biological properties at the upper concentrations of the metals used in this trial. Sensitive properties included important soil enzymes such as phosphatase and sulphatase, the nitrogen fixing bacterium R. leguminosarum bv. trifolii, and members of the fungal community. These results indicate that metal-spiked sludge application can have long-lasting impacts on the composition, function and activity of the microbial community in pasture soils.

As well as the pasture trial we also set up the same experimental design at a forest site, with one important difference being that, in a forest, biosolids applied to land cannot be ploughed into the soil and metals could potentially accumulate in the upper layers of needle litter and soil (Figure 3). We have found that at this site, the shallow stony nature of the forest soil means that metal guideline limits may be exceeded after 10-11 surface applications of biosolids at 400 kg N/Ha (this is twice to recommended application rate).

Figure 3. Application of biosolids to the forest field plots

We found metals at these levels may have potential toxicity to microorganisms residing in the needle litter. The macronutrient status of the forest litter was changed markedly, with substantial increases in nitrogen, phosphorus and calcium in the litter layer, this suggests that mineralization of biosolids nitrogen is either very slow, or that any mineralized nitrogen is immobilized in the microbial biomass associated with the forest litter layer, which might have implications for the use of biosolids as a fertiliser for trees.

One of the main research questions in our programme is to understand what proportion of the total metal is bioavailable and/or mobile and how this changes with time. Several of our long-term studies have allowed us to incorporate management practices into our experimental design, specifically liming of plots to counteract the acidifying effect of nitrification; liming is a normal farming practice in pastures in New Zealand. In our pasture trial we have found that toxicity issues arising from high concentrations of heavy metals in soils can be easily managed by simple management practices such as liming to increase the pH and reduce concentrations of bioavailable metals.

One problem with both the pasture and forest field trials is that biosolids application to land can enhance microbial proliferation and biological activity, masking the effects caused by the metals. To avoid the masking effect, we have established three field trials with a range of copper and zinc salt concentrations. Another reason for using metal salts is that biosolids metals are strongly bound to components of biosolids, and therefore much less available than metals in salt forms. Over time, the binding components of the biosolids will disappear, but the accumulated metals will remain in the soil, thus, metal salts are a good representation of an authentic, long term situation. It is too early to make any firm recommendations, but results from the first few years of sampling are indicating that in most instances, the New Zealand biosolids guidelines soil limit concentrations are protective of many of the soil biological indices measured. 

The accumulation of metals in soils is often regarded as a major limitation for the long-term application of biosolids to soils. In an attempt to examine the effects of the slow build up of metals in soils that would occur by repeated applications of biosolids at an agronomic rate (based on nitrogen application rates of 200/800 kg N/ha), we have established long-term lysimeter studies designed to study the effects of annual incorporations of biosolids into five different soils (Figures 4 and 5). The lysimeters are sown with rye-grass, and plant growth and nutrient and metal element uptake are determined. Nutrients and metals in drainage leachates are also monitored, together with any changes in soil properties. The annual applications of biosolids over the last five years have had some beneficial effects on plant growth and plant nutrient uptake. Plant yield increases have been observed with some of the treatments, most significantly at the higher application rates of 800 kg N/ha. Ryegrass nitrogen, phosphate, sulphur, calcium, magnesium, copper and zinc concentrations and plant uptake have been generally increased by the various biosolids treatments for all soils. However, rye-grass cadmium, chromium, nickel, and lead concentrations have been largely unaffected by the biosolids treatments. There have been small increases in soil carbon and nitrogen, some substantial increases in soil phosphate, and some significant changes in exchangeable calcium, magnesium and sodium in some of the soils. Significant increases in total soil metal concentrations (cadmium, chromium, copper, nickel, lead and zinc) have also occurred together with increases in bioavailable metals in some cases. There has been some increased leaching of macronutrients (especially nitrate, sulphate, carbon and magnesium) but little indication of increased metal leaching as a result of biosolids application.

The long-term lysimeters at Lincoln University

Microbial Contaminants