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.
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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