The effect of fertiliser and grazing on nitrogen export in surface runoff from rain-fed and irrigated pastures in south-eastern Australia

Nutrients, including nitrogen (N), exported from agricultural systems contribute to eutrophication and the development of algal blooms. Understanding the relative effect of farm management on nutrient export will indicate the extent to which farmers can potentially mitigate this process. Six years of monitoring surface runoff from rain-fed and irrigated dairy pastures was carried out in south-eastern Australia. Over the monitoring period, the flow-weighted mean annual total N (TN) concentrations in runoff varied between 8.2 and 29 mg/l for rain-fed pasture and 8.7 and 58 mg/l for irrigated pasture. An additive component model describing N concentrations separated the management (grazing and fertiliser) and year effects from other processes. The model accounted for 40 and 47% of the variation in N concentrations for the rain-fed and irrigated pastures, respectively. While fertiliser application, grazing and year had a significant effect on concentrations, other variables that are not necessarily under management control significantly affected N export. With current knowledge, improved management of pasture-based systems such as improved timing of fertiliser application and grazing relative to runoff may only result in a small decrease in N export. The remainder of the variability was attributed to environmentally mediated changes of N concentrations in runoff water. The collection of more detailed information on environmental parameters including soil moisture and soil temperature is proposed, to enable a better prediction of N concentrations and therefore improved understanding of potential management strategies.     

Distribution,Sources, and Risk of Polycyclic Aromatic Hydrocarbons in the Core Sediments from Baiyangdian Lake,China

The burial characteristics and risks of 16 polycyclic aromatic hydrocarbons (PAHs) in core sediments from Baiyangdian Lake were investigated through gas-chromato graphy/mass spectrometry. The total concentrations of the 16 PAHs ranged from 39.48–1877.75 ng g^?1. The low-molecular-weight PAHs (two- to three-ring PAHs) were the dominant species, contributing 40.10–92.18% to the total PAHs, with a mean of 71.01%. Based on the observed molecular indices and on principal component analysis, the PAHs inputs were initially dominated by biomass and coal combustion, and atmospheric deposition and surface runoff could be the major transport pathways. The contaminated source characteristics, hydrodynamic condition, and sediment textural composition are the key factors affecting the distribution and source of PAHs. By conducting a risk quotient analysis between specific PAH concentrations and their corresponding sediment quality values, the top layer sediments were found to have a potential biological impact and relatively high toxicity. However, such impact should have no impairment. The toxic potency of PAHs in Baiyangdian Lake could be described by using the toxic equivalent of benzo[a]pyrene.     

Nitrogen rhizodeposition from soybean (<Emphasis Type="Italic">Glycine max</Emphasis>) and its impact on nutrient budgets in two contrasting environments of the Guinean savannah zone of Nigeria

Nitrogen (N) rhizodeposition by grain legumes such as soybean is potentially a large but neglected source of N in cropping systems of Sub-Saharan Africa. Field studies were conducted to measure soybean N rhizodeposition in two environments of the Guinean savannah of Nigeria using ¹⁵N leaf labelling techniques. The first site was located in Ibadan in the humid derived savannah. The second site was in Zaria in the drier Northern Guinean savannah. Soybean N rhizodeposition in the top 0.30 m of soil varied from 7.5 kg ha⁻¹ on a diseased crop in Ibadan to 33 kg ha⁻¹ in Zaria. More than two-thirds of soybean belowground N was contained in the rhizodeposits at crop physiological maturity, while the rest was found in the recoverable roots. Belowground plant-derived N was found to constitute 16–23% of the total soybean N. Taking rhizodeposited pools into account led to N budgets close to zero when all residues were removed. If residues were left in the field or recycled as manure after being fed to steers, soybean cultivation led to positive N budgets of up to +95 kg N ha⁻¹. The role and potential of grain legumes as N purveyors have been underestimated in the past by neglecting the N contained in their rhizodeposits.