Denitrification is an important soil process for assessing nitrogen cycling and controlling nitrogen pollution in the environment. Numerous studies of denitrification rates in soils have been reported over the last decade, many with sampling protocols that are more reliable than in the past. In this paper, we review denitrification rates for agricultural and forest soils that have been reported in the literature, discuss factors that appear to be important in controlling the amount of denitrification that occurs in these soils, and summarise modelling approaches that have been used to predict annual denitrification rates. Most studies of in situ denitrification in upland soils have been conducted in agricultural grassland and forest ecosystems, with a paucity of studies reported from other ecosystems. A large range of annual, in situ, denitrification rates have been reported (0–239 kg N/ha.year), with the highest rates typically occurring in irrigated, nitrogen-fertilised soils. However, most annual denitrification rates reported in the literature appear to be fairly low, with over half of the rates in forest soils being <1 kg N/ha.year (mean of 1.9 kg N/ha.year). Rates of denitrification in agricultural soils tend to be higher than in forest soils, with 85% of rates reported being >1 kg N/ha.year, and a mean rate of 13 kg N/ha.year. Numerous soil, site, and management factors have been reported to affect the denitrification process in situ. The literature indicates that the highest rates of denitrification can be expected in nitrogen-fertilised soils, or where site management increases soil nitrate availability. Where nitrate is non-limiting, denitrification rates appear to be highest in irrigated loam soils. The review suggests that it is difficult to predict denitrification rates based on our current understanding, and that pilot studies should still be conducted if soil nitrogen balances are required.

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A description is given of a modified lysimeter design and sampling procedure designed to prevent edge flow from occuring in large soil monolith lysimeters. An internal cutting ring at the base of the lysimeter casing produced a small annular gap between the soil monolith and the wall of the casing. This gap was filled with liquefied petrolatum, which provided a water-tight seal around the edge of the lysimeter. Water and solutes were unable to leak between the monolith and casing. The use of petrolatum as a sealant is suitable for waterflux and nutrient-leaching studies, but is not suitable for pesticide studies.

Single pugging events, which involve remoulding of the soil around the hooves of livestock during treading, of moderate or severe pugging intensity were imposed in plots in a long-term white clover-ryegrass pasture during spring, by using dairy cows at varying stocking rates (4AE5 cows 100 m )2 for 1AE5 or 2AE5 h respectively). Changes in the growth and morphology of white clover were investigated over the following 12 months. Defoliation at approximately 3-week intervals was carried out by mowing. Annual herbage production was reduced following moderate and severe pugging proportionately by 0AE16 and 0AE34 compared with the non-pugged control treatment. The corresponding decreases in white clover production were 0AE09 and 0AE52 respectively. Annual perennial ryegrass production was reduced by 0AE37 under severe pugging. Pugging had an immediate adverse effect on growth of white clover which persisted for up to 156 d, and coincided with a large decrease in the proportion of white clover in herbage over the same period (e.g. 0AE40 vs. 0AE12, in control and severely pugged treatments, respectively, on day 112). In comparison, recovery in ryegrass growth was apparent after 50 d in severely pugged treatments, indicating that white clover is more vulnerable to severe pugging than perennial ryegrass. Analysis of individual white clover plants extracted from turves (300 mm · 300 mm) showed that direct hoof damage, fragmentation and burial of stolons were the major factors which reduced white clover production, rather than the changes in soil physical properties measured. Morphological characteristics associated with plant size (e.g. stolon length, growing points, and leaf numbers) all decreased under pugging. The situation had reversed by late summer, with larger plants dominating pugged plots, and coincided with the recovery of the proportion of white clover in herbage. Strategic pasture management practices, such as restricted grazing and the use of stand-off pads when soils are overly wet, are suggested as means of minimizing treading damage to pasture and reducing negative impacts on the growth and productivity of white clover.

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Summary.The use of legumes to improve the productivity of pastures has usually been associated with an increase in soil acidification. The present study compared the acidification potential of 12 N2-fixing pasture legume species/genotypes using a solution culture assay. The legumes were chosen because of their use or potential use in farming systems in the mediterranean zones of southern Australia. The species grown were Trifolium subterraneum L. (vars. Dalkeith, Yarloop and Clare), Trifolium glomeratum L., Trifolium tomentosum L., Medicago murex Willd., Medicago polymorpha L., Medicago truncatulaGaertn., Ornithopus compressus L., Ornithopus sativusBrot., Trifolium vesiculosum and Trifolium balansae. Proton excretion was measured over a period of 21 days from days 40 to 61 after germination. The amount of protons excreted into solution varied between 143 and 265 cmol H+ /kg dry matter for the different species and genotypes. In general, T. tomentosum and T. glomeratum excreted greater amounts of protons per unit biomass than Medicago species and T. subterraneum, which, in turn, excreted more protons than Ornithopus species. The amount of proton excreted per kilogram biomass was well correlated with the concentrations of excess cations over anions and ash alkalinity in plants across all the species. Total acid production was highly correlated with accumulation of excess cations and ash alkalinity, total dry matter production and total nitrogen fixation during the study period. It is suggested that the potential proton excretion by new pasture legume species should be considered when they are introduced into farming systems.

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The paper reports the relative acidification potential of 10 N2-fixing grain legume species grown in nutrient solution for 35 days after nodule appearance. The legumes studied were pilosus (Lupinus pilosus Murr. P23342), yellow lupin (Lupinus luteus L. R1171), white lupin (Lupinus albus L. Kiev mutant), narrow-leafed lupin (Lupinus angustifolius L. Gungurru), faba bean (Vicia faba L. Fiord), field pea (Pisum sativum L. Dundale), grasspea (Lathyrus sativus L. S453), chickpea (Cicer arietinum L. T1587), common vetch (Vicia sativa L. Blanchefleur), and lentil (Lens culinaris Med. ILL6002). The species varied considerably in their acidifying ability; proton production ranged from 77 to 136 cmol/kg dry matter. Chickpea and narrow-leafed lupin had the largest acidification potential and field pea the least. The specific acidification (amount of protons produced per kg dry matter) was best correlated with concentrations of excess cations, ash alkalinity, and calcium in plant across the species but was not correlated with plant nitrogen concentration. Total ash alkalinity or total excess cations in shoots provided a good indicator for estimation of total proton production in these species, which were reliant on N2fixation for their source of N. The results have implications for the selection of legumes to be used in sustainable farming systems.