R. G. McLaren

Atmospheric accessions of heavy metals to some New Zealand pastoral soils

Total heavy metal deposition was monitored at seven rural sites across New Zealand to determine the rate of atmospheric deposition of heavy metals to soils and to evaluate any regional and temporal variations in metal deposition. Heavy metal deposition was collected monthly in Warren Spring Laboratory type inverted frisbees. The rate of heavy metal deposition followed the order Zn>>Cu>Cr>Pb>Ni>>Cd. There were no obvious trends with regards to either seasonal or spatial deposition for any of the heavy metals measured. With the exception of Zn, heavy metal deposition in New Zealand was generally lower than rates measured in other countries. This reflects the general lack of high temperature industrial processes from urban-based industrial processes that are an important source of heavy metal aerosols in other industrialised regions worldwide. The inverted frisbee technique used provides a reliable indication of the magnitude of deposition of heavy metals from the atmosphere at a reasonable cost and with manageable logistical effort for a national survey.

Distribution and Phytoavailability of Lead in a Soil Contaminated with Lead Shot

This paper presents a study of Pb contamination of soil at a clay target shooting facility in Canterbury, New Zealand. The spatial distribution of Pb concentrations in the topsoil at the gun club site was investigated and Pb in the soil profile was determined to a depth of 200 mm. The greatest EDTA-extractable soil Pb concentrations (4000 to 8300 mg Pb kg−1 soil) were found approximately 120 m down the range from the point of shooting. At least 30% of the shot fall area sampled exceeded the Australian and New Zealand guideline limit of 300 mg Pb kg−1 soil. Lead concentrations in the soil profile were fairly uniform to a depth of 200 mm due to regular incorporation of Pb shot by ploughing. A greenhouse study was conducted to assess plant uptake of Pb from soils sampled at the target shooting site. Lead concentrations in the roots of all five species grown were several magnitudes higher than those present in the leaves. However, sufficient plant translocation of Pb occurred for the leaves to exceed the maximum allowable limit for foodstuffs.

Leaching of macronutrients and metals from undisturbed soils treated with metal-spiked sewage sludge. 1. Leaching of macronutrients

The extent of metal (Cd, Cr, Cu, Ni, Pb, Zn) leaching from soils treated with metal-spiked sewage sludge was examined using large undisturbed soil lysimeters of 5 different New Zealand soils (2 pasture and 3 forest soils). Metal leaching from the soils was monitored continuously over a period of 3 years following a single application of sewage sludge. Metal speciation in leachates was estimated using GEOCHEM-PC. Application of the sludge resulted in decreased pH and increased concentrations and total amounts of Cd, Ni, and Zn in drainage leachates from some of the soils, but had little effect on, or decreased concentrations of, Cr, Cu, and Pb. In the case of Cu, and to a lesser extent Pb, the reduced leaching of these metals in the forest soils was considered related to the lack of plant cover and reduced pH and soluble organic C in leachates from the sludge-treated lysimeters. The peaking of Ni and Zn concentrations well before 1 pore volume of water had passed through the lysimeters indicated that macropore flow was an important process facilitating the leaching of these metals. The extent of metal leaching varied greatly between soils and was dependent on a number of soil properties and other factors, including plant cover (pasture v. forest soils). However, even under the extreme conditions of the study, very small proportions (~1% or less) of the applied metals were leached over the 3-year period. In some cases, Ni and Zn concentrations in drainage leachates exceeded drinking water and/or environmental standards. However, with further attenuation and dilution in the vadose zone, in the short-term at least, the leaching of these metals would seem unlikely to pose a major environmental threat.

The effect of long-term phosphatic fertiliser applications on the amounts and forms of cadmium in soils under pasture in New Zealand

This paper presents the results of an investigation into the rate of Cd accumulation and changes in forms of Cd in a soil that has been subjected to long-term superphosphate fertiliser application. Results indicate that there had been a significant accumulation of Cd in the soil during the past 44 years. On the high fertiliser treatment (376 kg superphosphate ha-1 yr-1), Cd was estimated to have accumulated at a rate of 7.8 g ha-1 yr-1. During the course of the trial, there was an increase in the proportion of Cd associated with exchangeable and soil organic matter fractions on the fertilised plots, which was related to a corresponding increase in soil organic carbon levels. Results also indicate that although there was a large proportion of added Cd associated with the organic fraction, the concentration of applied Cd occurring in the residual fraction was also substantial (i.e >25% of added Cd). In addition, an investigation into the effects of residence time of Cd in the soil indicated that there was a redistribution of Cd into less soluble forms with time (i.e. residual Cd), along with a decrease in total soil Cd concentrations. There was also evidence of movement of Cd down the soil profile in this irrigated soil. The implications of these results for Cd phytoavailability are discussed.

Sulphur mineralisation in some New Zealand soils

SummaryAn open incubation technique was used to measure S mineralisation in a range of New Zealand soils. For most of the soils studied, the release of S as sulphate was curvilinear with time, and during a 10-week incubation, the amounts of S mineralised ranged from less than 3 μg S g-1 soil to more than 26 μg S g-1 soil. The best predictor of mineralised S appeared to be the amount of C-bonded S in the soil (explaining 59% of the variation in mineralised S between soils). Examination of the soils after incubation also revealed that the bulk of the mineralised S was derived from the C-bonded S pool. Hydriodic acid-reducible forms of organic S appeared to make little contribution to mineralised S.Attempts were made to predict total potentially mineralisable S (So) from incubation data using an exponential equation and a reciprocal-plot technique. However, the dependence of estimated values of So on the length and temperature of incubation cast doubts on the validity of this approach.

Sulphur mineralisation and transformations in soils as influenced by additions of carbon, nitrogen and sulphur

Abstract The mineralisation of soil organic sulphur was examined for two soils using an open incubation system. Combinations of glucose-carbon, nitrate-nitrogen and sulphate-S were added to the incubated soils at regular intervals to examine the effects of these nutrients on S mineralisation. Soil microbial activity was monitored by CO 2 evolution from the incubated samples. At the end of the 14-week incubation, the incubated soils were analysed to determine the sourcesof S mineralised during the incubation. In both soils, the addition of C, N and S had a considerable effect on S mineralisation. Additions of C or S decreased S mineralisation whereas N additions slightly enhanced mineralisation.The mineralised S appeared to be derived almost exclusively from C-bonded forms of soil organic S. In both soils, there were substantial increases in hydriodic acid (HI)-reducible forms of S during the incubation.Additions of C, N and S influenced the amounts of S mineralised or transformed from or to C-bonded and HI-reducible forms of soil organic S. The results from this study are discussed in relation to the dichotomous model of S cycling in soil as proposed by McGill and Cole ( Geoderma , 26, 267–286, 1981). In some respects, the S mineralisation data appeared to support the model. However, in relation to the source of mineralised S, there were large differences between what the model would predict for certain treatments and what was actually observed. This was particularly true with regard to the transformations of HI-reducible forms of organic S.

A comparison of soil and environmental quality under organic and conventional farming systems in New Zealand.

Abstract Organic farming in its various forms is seen by many as a sustainable alternative to conventional farming. This review considers and compares aspects of soil and environmental quality associated with organic and conventional farming systems under New Zealand conditions. The sustainability parameters considered include soil quality, nutrient dynamics, nutrient budgets, trace elements, and pesticides. The review used information from appropriate comparative studies conducted in New Zealand and overseas. However, because of the shortage of data on nutrient dynamics under organic systems in New Zealand, we also used a nutrient balance model (OVERSEER) and a nitrogen leaching estimation model to assess the comparative sustainability of typical model systems. Interpretation of the measured data coupled with the results of the modelling exercise suggests that organic farming carried out according to the Bio‐Gro New Zealand production standards can be sustainable if sufficient amounts of nutrient are returned to match removal and losses. Bio‐dynamic farming may be unsustainable because nutrients removed in farm produce are not adequately replaced. Soil organic matter content and biological activity is generally higher under both types of organic system than under conventional systems. Trace element availability and use may limit the sustainability of organic systems if no attempt is made to address natural deficiencies common in New Zealand soils. The reduced use of pesticides may be beneficial for the wider environment. The main conclusion is that a concerted research effort is urgently required to address various soil and environmental quality issues associated with the large‐scale adoption of organic farming practices in New Zealand.

In vivo-in vitro and XANES spectroscopy assessments of lead bioavailability in contaminated periurban soils.

Lead (Pb) bioaccessibility was assessed using 2 in vitro methods in 12 Pb-contaminated soils and compared to relative Pb bioavailability using an in vivo mouse model. In vitro Pb bioaccessibility, determined using the intestinal phase of the Solubility Bioaccessibility Research Consortium (SBRC) assay, strongly correlated with in vivo relative Pb bioavailability (R(2) = 0.88) following adjustment of Pb dissolution in the intestinal phase with the solubility of Pb acetate at pH 6.5 (i.e., relative Pb bioaccessibility). A strong correlation (R(2) = 0.78) was also observed for the relative bioaccessibility leaching procedure (RBALP), although the method overpredicted in vivo relative Pb bioavailability for soils where values were <40%. Statistical analysis of fit results from X-ray absorption near-edge structure (XANES) data for selected soils (n = 3) showed that Pb was strongly associated with Fe oxyhydroxide minerals or the soil organic fraction prior to in vitro analysis. XANES analysis of Pb speciation during the in vitro procedure demonstrated that Pb associated with Fe minerals and the organic fraction was predominantly solubilized in the gastric phase. However, during the intestinal phase of the in vitro procedure, Pb was strongly associated with formation of ferrihydrite which precipitated due to the pH (6.5) of the SBRC intestinal phase. Soils where Fe dissolution was limited had markedly higher concentrations of Pb in solution and hence exhibited greater relative bioavailability in the mouse model. This data suggests that coexistence of Fe in the intestinal phase plays an important role in reducing Pb bioaccessibility and relative bioavailability.

Isotopic dilution methods to determine the gross transformation rates of nitrogen, phosphorus, and sulfur in soil: a review of the theory, methodologies, and limitations

The rates at which nutrients are released to, and removed from, the mineral nutrient pool are important in regulating the nutrient supply to plants. These nutrient transformation rates need to be taken into account when developing nutrient management strategies for economical and sustainable production. A method that is gaining popularity for determining the gross transformation rates of nutrients in the soil is the isotopic dilution technique. The technique involves labelling a soil mineral nutrient pool, e.g. NH4+, NO3−, PO43−, or SO42−, and monitoring the changes with time of the size of the labelled nutrient pool and the excess tracer abundance (atom%, if stable isotope tracer is used) or specific activity (if radioisotope is used) in the nutrient pool. Because of the complexity of the concepts and procedures involved, the method has sometimes been used incorrectly, and results misinterpreted. This paper discusses the isotopic dilution technique, including the theoretical background, the methodologies to determine the gross flux rates of nitrogen, phosphorus, and sulfur, and the limitations of the technique. The assumptions, conceptual models, experimental procedures, and compounding factors are discussed. Possible effects on the results by factors such as the uniformity of tracer distribution in the soil, changes in soil moisture content, substrate concentration, and aeration status, and duration of the experiment are also discussed. The influx and out-flux transformation rates derived from this technique are often contributed by several processes simultaneously, and thus cannot always be attributed to a particular nutrient transformation process. Despite the various constraints or possible compounding factors, the technique is a valuable tool that can provide important quantitative information on nutrient dynamics in the soil–plant system.

Effect of soil pH on cadmium phytoavailability in some New Zealand soils

Abstract The effect of soil pH on plant cadmium (Cd) concentrations was investigated in a glasshouse study, in conjunction with an evaluation of eight soil extractants as predictors of Cd concentrations in different plant species. Results showed that in general, increasing soil pH from 5.5 to 7.0 significantly decreased Cd concentrations in clover, lettuce, carrot, and ryegrass, and to a lesser extent in wheat, although the magnitude of the reduction varied between plant species and soil types. Soil extractants which were sensitive to soil pH e.g., 0.05M Ca(NO3)2, IMNH4CI, and O.O5MCaCl2 or extract moderate amounts of Cd e.g., IA/NH4OAC and 0.04M EDTA were found to be the most effective in predicting plant Cd concentrations. Cd solubility as predicted using a semi‐empirical equation which contained terms for pH, organic matter, and total Cd concentration was also found to be successful in estimating plant Cd concentrations for a number of plant species. It appears that pH may be a powerful tool in the man...