No abstract

Neurotoxicity and renotoxicity were compared in rats given by gastric gavage five daily doses of 8.0 mg Hg/kg methyl- or ethylmercuric chloride or 9.6 mg Hg/kg ethylmercuric chloride. Three or 10 days after the last treatment day rats treated with either 8.0 or 9.6 mg Hg/kg ethylmercury had higher total or organic mercury concentrations in blood and lower concentrations in kidneys and brain than methylmercury-treated rats. In each of these tissues the inorganic mercury concentration was higher after ethyl- than after methylmercury. Weight loss relative to the expected body weight and renal damage was higher in ethylmercury-treated rats than in rats given equimolar doses of methylmercury. These effects became more severe when the dose of ethylmercury was increased by 20%. Thus in renotoxicity the renal concentration of inorganic mercury seems to be more important than the concentration of organic or total mercury. In methylmercury-treated rats damage and inorganic mercury deposits were restricted to the P2 region of the proximal tubules, while in ethylmercury-treated rats the distribution of mercury and damage was more widespread. There was little difference in the neurotoxicities of methylmercury and ethylmercury when effects on the dorsal root ganglia or coordination disorders were compared. Based on both criteria, an equimolar dose of ethylmercury was less neurotoxic than methylmercury, but a 20% increase in the dose of ethylmercury was enough to raise the sum of coordination disorder scores slightly and ganglion damage significantly above those in methylmercury-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Climate warming is most pronounced at high latitudes, which could result in the intensification of the extensively cultivated areas in the boreal zone and could further enhance rates of forest clearing in the coming decades. Using paired forest-field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Chronosequences showed large soil C losses during the first two decades following deforestation, with mean C stocks in agricultural soils being 44% or 8.3 kg m À2 lower than C stocks in original forest soils. This suggests that soil C losses from land-use change in the boreal region may be greater than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences indicated an increase in C storage during later decades after forest clearing, with 60-year-old grassland showing net ecosystem C gain of 2.1 kg m À2 over the original forest. This increase in C stock resulted probably from a combination of large C inputs from belowground biomass and low C losses due to a small original forest soil C stock and low tillage frequency. Reductions in soil N stocks caused by land-use change were smaller than reductions in C stocks (34% or 0.31 kg m À2 ), resulting in lower C/N ratios in field compared with forest mineral soils, despite the occasional incorporation of high-C forest-floor material into field soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which could indicate that decomposition rates are more sensitive in forest soils than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes more widespread in the boreal region, the resulting C losses (51% or 11.2 kg m À2 at the ecosystem level in this study) will induce a positive feedback to climatic warming and additional land-use change. However, by selecting relatively C-poor soils and by implementing management practices that preserve C, losses of C from soils can be reduced.

An experiment was conducted in subarctic Alaska from 1999 to 2001 to determine the effect of liquid and solid cattle (Bos taurus) manure application rates on smooth bromegrass (Bromus inermis Leyss.) and oat (Avena sativa L.) biomass production, nutrient uptake, and soil properties. One-time manure application of 100 and 200 kg N ha 21 was made in May 1999 in comparison with annual fertilizer application of 50, 100, and 200 kg N ha 21. In the first year, liquid manure at 100 and 200 kg N ha 21 generated 3036 and 4292 kg ha 21 smooth bromegrass biomass, respectively, statistically (p $ 0.05) similar to that of fertilizer application (3654 kg ha 21) at 200 kg N ha 21 but greater (p # 0.05) than control (1572 kg ha 21). Similar results were found with oat. The 200 kg N ha 21 liquid manure application continued to benefit crop growth in the second and third years. Solid manure did not influence biomass production of either crop in most crop/year combinations. Cumulatively, in 3 yr, smooth bromegrass recovered 59% of nitrogen from liquid manure, compared with 37% by oat. Soil Mehlich 3-P accumulation was found in some liquid and solid manure treatments for both crops. High soil exchangeable K was found in 1999 after liquid manure application but declined over time. Our results suggest that 100 kg N ha 21 liquid manure can replace nitrogen fertilizer at a similar rate. Liquid cattle manure was better than solid cattle manure in promoting bromegrass and oat production.