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Notes: Abstract. Land use change and land management practices affect the net emissions of the trace gases methane (CH4) and nitrous oxide (N2O), as well as carbon sources and sinks. Changes in CH4 and N2O emissions can substantially alter the overall greenhouse gas balance of a system. Drainage of peatlands for agriculture or forestry generally increases N2O emission as well as that of CO2, but also decreases CH4 emission. Intermittent drainage or late flooding of rice paddies can greatly diminish the seasonal emission of CH4 compared with continuous flooding. Changes in N2O emissions following land use change from forest or grassland to agriculture vary between climatic zones, and the net impact varies with time. In many soils, the increase in carbon sequestration by adopting no-till systems may be largely negated by associated increases in N2O emission. The promotion of carbon credits for the no-till system before we have better quantification of its net greenhouse gas balance is naïve. Applying nitrogen fertilizers to forests could increase the forest carbon sink, but may be accompanied by a net increase in N2O; conversely, adding lime to acid forest soils can decrease the N2O emission.

Notes: Abstract. A survey of manure management practice was undertaken in 1996, by postal questionnaire submitted to a stratified sample of egg and broiler producers in England and Wales. Out of a target of 500 laying hen and 500 broiler (chickens produced for meat) production units in the survey sample, 356 (36%) returned questionnaires. The survey provided information on amount and type of manure production, manure storage and land application strategies (timing, techniques and awareness of nutrient content). Within the survey, no attempt was made to differentiate between organic and conventional production systems. About 45% of manure production was estimated to come from layer holdings, 55% from broiler litter. It was estimated that 70% of the national manure production is litter-based and about 30% are droppings collected without litter. Sawdust/shavings are the most popular bedding material, with an average final depth of 100 mm for broilers and 140 mm on litter-based layer units. Commonly, storage is available within housing for at least the length of the cropping cycle (6 weeks in broiler production, or 12 months in deep pit laying houses), around 60% of poultry manure is stored for a period following removal from the house, most commonly for 3-6 months. Overall, autumn was the peak period for manure spreading, with over 40% of laying hen manure and 50% of broiler manure applied at that time. On grassland, spreading was reasonably evenly distributed throughout the year but autumn application was favoured for arable crops, especially before the establishment of cereals and root crops, overall, almost 50% of layer and broiler manure was applied in the autumn. In the survey, up to 10% of manures were claimed to be incorporated within a day of application and about 60% within a week of application, presumably because of concern about odour nuisance. Around 25% of poultry manure was applied by contractors. A high proportion of farmers (c. 40% with layers, c. 60% with broilers) exported manures from their holdings, the proportion removed amounting to almost 90% on these farms. Although evidence elsewhere indicates that farmers make little allowance for manures in planning crop fertilizer inputs, the survey responses suggested that farmers do make an effort to allow for manures but that their confidence in the advice available to them is lacking, or they may have other technical reasons for not taking advantage of the manurial value. Information provided by the survey is of significant importance to policy makers (e.g. for the construction of environmental emissions inventories), researchers, consultants and farmers.

Notes: To investigate the effect of soil physical conditions and land use on emissions of nitrous oxide (N2O) to the atmosphere, soil cores of an imperfectly drained gleysol were taken from adjacent fields under perennial ryegrass and winter wheat. The cores were fertilized with ammonium nitrate and incubated at three different temperatures and water-filled pore space (WFPS) values, and N2O emissions were measured by gas chromatography. Emissions showed a very large response to temperature. Apparent values of Q10 (emission rate at (T + 10)°C/emission rate at T°C) for the arable soil were about 50 for the 5–12°C interval and 8.9 for 12–18°C; the corresponding Q10s for the grassland soil were 3.7 and 2.3. Emissions from the grassland soil were always greater than those from the arable soil, although the ratio narrowed with increasing temperature. Changes in soil WFPS also had a profound effect on emissions. Those from the arable soil increased about 30-fold as the WFPS increased from 60 to 80%, while that from the grassland soil increased 12-fold. This latter response was similar to earlier field measurements. The N2O emissions were considered to be produced primarily by denitrification. We concluded that the impacts of temperature and WFPS on emissions could both be explained on the basis of existing models relating increasing respiration or decreased oxygen diffusivity, or both, to the development of anaerobic zones within the soil.

Notes: This review examines the interactions between soil physical factors and the biological processes responsible for the production and consumption in soils of greenhouse gases. The release of CO2 by aerobic respiration is a non-linear function of temperature over a wide range of soil water contents, but becomes a function of water content as a soil dries out. Some of the reported variation in the temperature response may be attributable simply to measurement procedures. Lowering the water table in organic soils by drainage increases the release of soil carbon as CO2 in some but not all environments, and reduces the quantity of CH4 emitted to the atmosphere. Ebullition and diffusion through the aerenchyma of rice and plants in natural wetlands both contribute substantially to the emission of CH4; the proportion of the emissions taking place by each pathway varies seasonally. Aerated soils are a sink for atmospheric CH4, through microbial oxidation. The main control on oxidation rate is gas diffusivity, and the temperature response is small. Nitrous oxide is the third greenhouse gas produced in soils, together with NO, a precursor of tropospheric ozone (a short-lived greenhouse gas). Emission of N2O increases markedly with increasing temperature, and this is attributed to increases in the anaerobic volume fraction, brought about by an increased respiratory sink for O2. Increases in water-filled pore space also result in increased anaerobic volume; again, the outcome is an exponential increase in N2O emission. The review draws substantially on sources from beyond the normal range of soil science literature, and is intended to promote integration of ideas, not only between soil biology and soil physics, but also over a wider range of interacting disciplines.

Notes: Abstract The recovery of nitrogen (N) from, and the fertilizer-N value of, low dry-matter (DM) cattle slurry and farm yard manure (FYM), applied annually to perennial ryegrass swards grown at two sites, on sandy loam and shallow calcareous silty clay loam soils, were studied over a 4-year period. Slurry or FYM, applied at target rates of either 150 kg N ha−1 or 300 kg N ha−1 in either October, February or May/June, in combination with 150 kg N ha−1 inorganic fertilizer-N (applied as split dressings before the first and second grass cut), were compared with a set of inorganic fertilizer-N response treatments. DM yield, N offtake, apparent manure-N recovery (in herbage) and manure-N efficiency (compared with inorganic fertilizer-N) were determined at two silage cuts each summer. Soil mineral nitrogen (SMN) measurements in autumn and spring were used to assess potential N leaching loss over winter and to quantify available N residues in the soil in spring. Apparent manure-N recovery and manure-N efficiency were usually greater from slurry applications in February than from those in October, but the timing of the application of FYM had a much smaller effect, compared with the timings of the application of slurry, on the utilization of N from manure by grass. Spring assessment of SMN was useful in quantifying available N residues from October slurry applications. Manure-N recovery for all application timings was, on average, higher from the sandy loam than the shallow calcareous clay loam. The application of slurry to grass in early spring, at a rate of 150 kg total N ha−1, with the addition of a supplementary 50 kg inorganic fertilizer-N ha−1, was the most suitable strategy for utilizing slurry-N effectively and for supplying the N requirement for first-cut silage.

Notes: An empirical model of nitrous oxide emission from agricultural soils has been developed. It is based on the relationship between N2O and three soil parameters – soil mineral N (ammonium plus nitrate) content in the topsoil, soil water-filled pore space and soil temperature – determined in a study on a fertilized grassland in 1992 and 1993. The model gave a satisfactory prediction of seasonal fluxes in other seasons when fluxes were much higher, and also from other grassland sites and from cereal and oilseed rape crops, over a wide flux range (〈 1 to 〉 20 kg N2O-N ha−1 y−1). However, the model underestimated emissions from potato and broccoli crops; possible reasons for this are discussed. This modelling approach, based as it is on well-established and widely used soil measurements, has the potential to provide flux estimates from a much wider range of agricultural sites than would be possible by direct measurement of N2O emissions.

Notes: This paper reports the range and statistical distribution of oxidation rates of atmospheric CH4 in soils found in Northern Europe in an international study, and compares them with published data for various other ecosystems. It reassesses the size, and the uncertainty in, the global terrestrial CH4 sink, and examines the effect of land-use change and other factors on the oxidation rate.Only soils with a very high water table were sources of CH4; all others were sinks. Oxidation rates varied from 1 to nearly 200 μg CH4 m−2 h−1; annual rates for sites measured for ≥1 y were 0.1–9.1 kg CH4 ha−1 y−1, with a log-normal distribution (log-mean ≈ 1.6 kg CH4 ha−1 y−1). Conversion of natural soils to agriculture reduced oxidation rates by two-thirds –- closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery of rates after these disturbances takes 〉 100 y. Soil bulk density, water content and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity reduced oxidation, partially but not wholly explained by poor diffusion through litter layers which did not themselves contribute to the oxidation. The effect of temperature was small, attributed to substrate limitation and low atmospheric concentration.Analysis of all available data for CH4 oxidation rates in situ showed similar log-normal distributions to those obtained for our results, with generally little difference between different natural ecosystems, or between short-and longer-term studies. The overall global terrestrial sink was estimated at 29 Tg CH4 y−1, close to the current IPCC assessment, but with a much wider uncertainty range (7 to 〉 100 Tg CH4 y−1). Little or no information is available for many major ecosystems; these should receive high priority in future research.

Notes: Earlier models describing the accumulation of gases under closed chambers are based on the assumption of a constant concentration source that does not change during the time of chamber deployment. A new model is proposed which is based on the assumption of a constant production source, and takes into account possible changes in gas concentrations at the source during chamber deployment. Using N2O as an example, simulations have been carried out for different source strength and depth, diffusivities and air porosities. The main finding was a chamber-induced increase in gas concentrations in the upper part of the soil profile, including the depth where the N2O source is located. The increase started immediately after chamber closure. Nevertheless, fluxes calculated from increasing concentrations within the chamber's headspace were always less than those expected under undisturbed conditions, i.e. in the absence of a chamber. This was due to a proportion of the gas produced being stored within the soil profile while the chamber was in place. The discrepancy caused by this effect increased with increasing air-filled porosity and decreasing height of the chamber, and a procedure for correcting chamber flux measurements accordingly is proposed. The increase in soil gas concentrations after chamber closure was confirmed in a laboratory experiment.