J Sci Food Agric 1990,53, 419428

Agriculture Group Symposium Industry, Agriculture and the Atmosphere

The following are summaries of papers presented at a meeting of the Agriculture Group of the Society of Chemical Industry held on 23 January 1990 at the Society of Chemical Industry, 14-15 Belgrave Square, London SWlX 8 P S . The papers published here have not been refereed and do not necessarily reflect the views of the Editorial Board of the Journal of the Science of Food and Agriculture.

Effects of Air Pollution on Agriculture

Michael R Ashmore

Department of Biology, Imperial College of Science, Technology and Medicine, Silwood Park, Ascot, k r k s SL5 7PY, UK

This paper aims to review current understanding of the impact of ambient levels of air pollution on crop yield, and the mechanisms by which this impact occurs.

When present in high concentrations, air pollutants can damage leaf tissue; in the absence of such visible injury, pollutants may still reduce plant growth and crop yield. Experimental studies over the past 20 years in North America and western Europe, involving the addition of known concentrations of pollutants or the removal of ambient air pollution, have demonstrated that in many regions current levels of major air pollutants can cause significant reductions in crop yield. Analysis of dose-response relationships based on these studies suggests that ozone (0,) is more important in this respect than sulphur dioxide (SO,) or nitrogen oxides (NO,), and that leguminous crops may suffer the greatest loss of yield.

Most of these experiments involve well watered, disease-free crops. However, it is now becoming apparent that the indirect effects of air pollutants, in altering the sensitivity of crops to biotic or abiotic stresses, may be at least as important as their direct effects. For example, studies at our laboratory have shown that the growth rate of a range of aphid pests is increased by. short-term exposure to moderate concentrations of SO, ; a similar consistent increase is also found with NO,, but for O3 aphid responses are more variable. All these responses are mediated through changes in host plant chemistry, rather than direct effects of air pollutants on the

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pests. Studies at other European and American laboratories have demonstrated that air pollutants influence the performance of a range of plant pathogens, and alter the responses of crops to drought and cold stress.

The existence of such interactions makes it difficult to define a simple relationship between the concentrations of an air pollutant, averaged over a growing season, and crop yield; instead, a range of possible effects exists, depending on climatic conditions and pest levels during a particular season. Further considerations lead to similar conclusions; for example, the timing of episodes of high pollutant concentrations in relation to crop development may influence their impact on yield, and there is evidence of interactions between air pollutants and pesticide application. Competition between plants is also influenced by air pollutants, leading, for example, to changes in the balance between grasses and legumes in pasture systems.

It is therefore impossible to quantify with any certainty the economic benefits to agriculture of measures to reduce air pollutant concentrations. Air pollution is one of a range of factors which influence agricultural production, and we need a clearer understanding of how air pollution interacts with all aspects of crop production.

The Greenhouse Effect and Climatic Change

R Warrick

Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK

The objective of this presentation is to provide an overview of the greenhouse effect and climatic change, in both the global and UK context. Specifically, the presentation addresses the uncertainties of greenhouse gas projections and climate models, and the implications for estimates of climatic changes and sea level rise. Finally, the effects of emission reduction strategies on global warming are examined.

By the year 2030 the world is likely to be 1-2°C warmer than today, although, given the full range of uncertainties, a warming of as little as 05°C or as much as 2.5”C is possible. The concomitant rise in global mean sea level is 17-26 cm, with a full range of 5-44crr, due principally to thermal expansion of the oceans and increased melting of small alpine glaciers.

Because of the thermal inertia of the oceans, there is a substantial additional warming and sea level rise ‘commitment’. Thus, if further greenhouse gas concentration increases were miraculously to stop in the year 2030, only about two- thirds of the eventual global warming, and only one-fourth of the eventual oceanic thermal expansion, would have occurred; global warming and sea level rise would continue for hundreds of years into the future.

Estimates of further changes in climate for the UK are difficult to make because the climate models are unable as yet to provide reliable predictions at the regional

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scale. A comparison of various model results suggests, however, that the UK could expect temperature changes similar to the global average. At present little can be said about future changes in precipitation or other climate variables.

A newly developed integrated Greenhouse G a s Policy Model (GGPM) that estimates global warming from changes in gas emissions was used to evaluate the climate effects of various emission control strategies. It was found that even stringent (but credible) control strategies would do little to slow global warming (and, by implication, sea level rise) by 2030. This is due to the strong ’warming commitment’ already realised. Moreover, waiting 15 years to implement the strategies makes little difference. However, such strategies do have a substantial effect on warming during the second half of the 21st century. This highlights the dilemma faced in making policy decisions which potentially entail major social and economic readjustments today, but which provide benefits largely for generations in the distant future.

Long-range Transport of Air Pollutants in Europe

David Fowler

Institute of Terrestrial Ecology, Penicuik, IEdinburgh, UK

The major air pollutants in Europe are the oxides of sulphur: SO2 and SO4 in gaseous and particle form respectively; the gaseous oxides of nitrogen: NO, NOz and HNO,; and particle NO3. Other important pollutants such as peroxyacetyl nitrate (PAN) and ozone are produced by photochemical processes in polluted air and acidity generated during the oxidation of SOz to SO4 and NOz to NO3. More recently NH, and NH4 have also been shown to be important components of the pollution climate.

The atmospheric transport and transformation and the deposition processes lead to patterns of deposition of nitrogen, sulphur and acidity which do not resemble patterns of emission.

In this presentation the processes of removal of pollutants from the atmosphere are considered within the context of long-range transport and the location of European ecosystems which are sensitive to atmospheric pollutants.

Prospects for Change in Regional Air Pollutants in Europe during the Next Decade

Richard G Derwent

Modelling and Assessments Group, Harwell Laboratory, Harwell, Oxfordshire, UK

Studies of atmospheric processes and environmental impacts over the last two decades have identified a number of regional-scale pollution problems in Europe. For some of these pollution problems, damage mechanisms and causative agents

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have been identified. A number of international agencies have focused attention on these problems and their solution. Pollution control policies involving different underlying philosophies can be distinguished. Currently the two main approaches are ‘Best Available Control Technology’ and ‘Effects-based’ strategies. It is important to resolve whether current understanding of the basic environmental science of regional-scale pollution problems is adequate to underpin pollution control policies adopting either approach.

Effects of Climatic Change on Agricultural Crops

M H Unsworth

University of Nottingham, School of Agriculture, Sutton Bonington LE12 5RD, UK

The principles by which crops respond to changes in temperature, available water and carbon dioxide will be described. There is good understanding for most major crop species of responses to temperature and drought, but insufficient is known of how crop stands respond to CO,. Some estimates of the effects of climatic change on the potential productivity of UK cereals are described, illustrating that current crops would be unlikely to gain the full benefits of improved growing conditions.

The winter of 1988/89 was 2-3°C warmer than normal in much of Britain, and so provided some indication of the sensitivities of agriculture to climatic change. Examples are given of how effectively existing models described winter growth, and of the reported impacts on crops, animals, pests and diseases.

Climate Change and Agriculture in the Future

M Parry

Geography Department, University of Birmingham, Birmingham B15 2TT, UK

This paper describes the aims and methods of a number of recent projects on the assessment of the effects of climatic variations on agriculture. In particular, it summaries the results from case studies in cool temperate and cold regions in the northern hemisphere.

An overall goal of recent projects has been to increase our understanding not only of first-order effects of climatic variations on agricultural productivity, but also of

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higher-order effects on regional and national economies. The first part of this paper outlines some approaches for considering these interactions. Two broad types of experiment are described: impact experiments and adjustment experiments. The former provide estimates of the first-order and higher-order effects of climatic variations on farming systems that are assumed to include no adjustments. The latter evaluate a number of adjustments available at farm or policy level to offset or mitigate these effects. Experiments have been conducted to examine the impacts of three types of climatic scenario representing short- and medium-term climatic variations observed in the historical instrumental record, and long-term climatic changes estimated for a doubling of atmospheric carbon dioxide concentrations by general circulation models.

The results of a number of case studies are summarised in the second part of the paper. Results indicate that the effects of a 2 x C 0 , climate on agriculture are generally greater than the effects of observed, decadal anomalies, but not always as great as the effects of observed extreme annual events. However, the frequency and magnitude of these extreme events could well increase under 2 x CO, conditions, implying that a series of on-farm adjustments (eg changes in crop variety, in fertiliser applications or in land allocations) and a range of policy responses (eg revised regional agricultural support, adjustments in agricultural infrastructure and government assistance to plant breeders) would be necessary to accommodate the changes.

The Effects of Climate Change on Wheat Yields: Sensitivity of Model Predictions to Increases in Cloud Cover

Rowan Mitchell

Rothamsted Experimental Station, AFRC Institute of Arable Crops Research, Harpenden, Herts AL5 214, UK

Crop models provide a means of assessing the impact of climate change on yields. As the extent of changes in temperature, cloud and precipitation that will occur in the UK for a given rise in atmospheric CO, are uncertain, simulations need to be carried out over a wide range of these conditions. We used a model of winter wheat to simulate the effects of possible combinations of changes in CO,, temperature and cloud on yields. The model was composed of elements modified from other models of wheat growth in the literature and was adapted using experimental data from Rothamsted.

A wheat crop grown at Rothamsted in 1985 was used as the starting point for the

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simulations. Yields were modelled for:

(1) increases in COz of up to 29 % combined with increase in temperature of up to + 2-5°C;

(2) an increase in C 0 2 concentration of 29 % (corresponding to the predicted rise for 2030 AD temperature increases of up to +2.5"C combined with cloud increases of up to 15%).

The simulations indicated that increased CO, concentrations are always beneficial, with a rise of 29 % increasing yield by 15-20%. The effect of temperature is more complex: at certain temperature increases, the model predicts the appearance of extra leaves, which leads to sharply increased yields. However, the shortened grain-filling period and increased respiration cause decreasing yields with increasing temperature between peaks. Temperature increases greater than + 2°C result in a net decrease in yield relative to 1985 conditions.

Increasing cloud cover greatly decreases yields. An increase of 5 1 0 % in cloud is sufficient to completely negate any positive effects of increased CO, and temperature. Any assessment of climate change on UK agriculture must therefore take into account possible changes in cloud cover.

Atmospheric Inputs of Heavy Metals and Organic Contaminants to Agroecosystems

K C Jones," P M Haygarth" and A E Johnstonb

'Institute of Environmental & Biological Sciences, Lancaster University, Lancaster LA1 4YQ, UK; %oils and Crop Sciences Division, AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts AL5 UQ, UK

Soils and crops receive inputs of inorganic and organic contaminants from the atmosphere which may make an important contribution to the human food chain. In the long term (over tens or hundreds of years) atmospheric deposition has resulted in substantial enrichments of lead, cadmium and other heavy metals in UK surface soils. In addition, seasonal or annual deposition makes a contribution to the heavy metal burden of pasture grasslands, leafy vegetable crops and cereals. For example, up to 90 % of herbage lead in rural areas may be atmospherically derived, and consequently crop composition can be strongly influenced by air quality. Air- soil and air-plant exchanges also make important contributions to the loading of metalloids (eg selenium) and recalcitrant organic contaminants (eg polychlorinated biphenyls, polynuclear aromatic hydrocarbons and polychlorinated dibenzo-p- dioxins) in agroecosystems. This presentation will illustrate these observations by reference to collaborative studies between Lancaster and Rothamsted and other UK data.

The classical long-term experiments at Rothamsted provide an invaluable sample

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archive of soils and crops with which to assess the significance of atmospheric inputs. Untreated plots which have only received atmospheric inputs have been sampled from various agricultural experiments for over a century and have been analysed retrospectively. A recent study on lead in vegetation, for example, has provided some interesting insights into the relationship between changes in lead emissions, primarily from vehicle emissions, air lead, and crop concentrations over time. The collection has also recently been used to study long-term changes in the dioxin content of soils. Until recently the relative importance of natural and anthropogenic dioxin releases into the environment was unclear. The Rothamsted soil collection shows a progressive increase in the soil dioxin burden which can be related to various anthropogenic combustion activities.

The Role of Atmospheric Cycling in Controlling Selenium Levels in Grasslands and Livestock

P M Haygarth,".* K C Jones' and A F Harrison*

"Institute of Environmental & Biological Sciences, University of Lancaster, Lancaster LA1 4YQ, UK; qnstitute of Terrestrial Ecology, Merlewood Research Station, Grange-overSands, Cumbria LA1 1 6JU, UK

Atmospheric inputs and losses play an important role in the cycling of selenium (Se) through grasslands and this may affect soil and plant concentrations. Animal Se levels may also be governed by these processes. The atmosphere deposits c 350pg mz year-', but may simultaneously receive c 140pg m2 year-' by volatilisation from plants and soil. At this calculated rate it is feasible that atmospheric deposition over the last 100 years has contributed to ~ 3 0 % of contemporary total Se in grasslands. This is particularly significant for grazing livestock as a high proportion of dietary Se is due to atmospheric sources. Local variations in the atmospheric input/output ratio may act in conjunction with geological factors and result in spatial variations in livestock nutritional levels. More generally this suggests that geology may not be the single most important factor in controlling levels of trace elements in livestock.

The presentation will illustrate results calculated from an idealised Se budget for an agricultural grassland in the British Isles. The work uses data derived from the literature and predicts typical Se values and chemical forms in the soil, grass and atmosphere, emphasising the importance of these airsoil and air-plant exchanges. Additionally, the work highlights areas where scientific knowledge is lacking and suggests a new emphasis for future research.

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Aphids, Air Pollution and Agricultural Crops

Victor C Brown, Andrew C Croxford, Stuart McNeill and Michael R Ashmore

Department of Biology, Imperial College of Science, Technology and Medicine, Silwood Park, Ascot, Berkshire SL5 7PY, UK

Aphids are the most economically important insect pests of agricultural crops in Europe. For the last six years researchers at Imperial College have been investigating how air pollution may affect crop plant/aphid interactions and whether such effects are likely to be significant in the field.

Controlled fumigations with both SO, and NO2 have been shown to cause increased aphid performance (increase in mean relative growth rate (MRGR) compared with clean air controls) on a wide range of crop/aphid systems. Studies have shown that the observed responses are not due to the direct effects of the air pollutants on the aphids but are caused by changes in host plant chemistry. However, studies with 0, have shown a more complex set of responses than for either SO, or NO,. Both increases and decreases in MRGR of aphids have been observed depending on the dose and timing of fumigations.

The effects of ambient air pollution on the Vicia faba/Aphis fabae system are being studied in open-top chambers. The results on the aphid MRGR are variable with increases observed in 1988 and decreases observed in 1989. The role of differing climatic conditions and air pollution concentrations in the two years is uncertain, and further experiments under controlled conditions are needed.

Whilst open-top chambers provide good conditions for studying the growth responses of individual aphids, they have limitations for studying the population dynamics of aphids and the air pollution moderated effects of aphids on crop yield. For this reason an open-air filtration system, which permits natural infestation of crops by aphid pests and does not exclude their natural enemies, is being developed at Imperial College.

Deposition of Sulphur and Nitrogen from the Atmosphere

S P McGrath and K W T Goulding

Soils and Crop Sciences Division, AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts AL5 UQ. UK

Nitrogen and sulphur are both essential nutrients. Nitrogen is the main nutrient limiting crop growth and yield in most of the UK, and as much sulphur as phosphorus is required by crops-sometimes more as in the case of oilseed rape. The contribution of atmospheric inputs of both nutrients has largely been ignored in the past. The deposition of N was thought to be small if not insignificant compared with the amounts of fertiliser N applied, whilst farmers have, perhaps unknowingly, relied on atmospheric S to supply most crop requirements. In this paper we show why atmospheric inputs of both N and S now merit urgent attention.

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Approximately 1&15 kg N ha-' year-' are deposited on arable land in precipitation (rain, snow, mist and fog) in southern and eastern England. This is a marked increase on the 4 kg ha-' year-' measured at Rothamsted at the turn of the century. Whilst the dry deposition of N has been poorly quantified in the past, recent measurements show it to be three to four times wet deposition. Adding to this 5-10 kg N ha-' year- ' from biological fixation makes a total non-fertiliser input of 45-50 kg N ha-' year- ' in southern and eastern England. Such a large amount cannot be ignored for its contribution to crop nutrition, nitrate leaching and its effect on non-agricultural ecosystems. Amounts received elsewhere could be less and there will be much local variation, but our measurements suggest that quantitative monitoring is essential for a proper N balance and prediction of fertiliser N requirements.

In contrast to N, atmospheric inputs of S have been decreasing almost linearly over the past 5-6 years. Ten years ago, the annual average concentration of SO, in the air at our Woburn farm was 76 pg m-3; now it is 10. Concentrations of S04-S in precipitation have fallen to less than a third of their 1983 levels, so that the 30- 40 kg S ha-' year-' deposited then has reduced to below 10 kg ha-' year-' now. The total deposition of S in 1988 was estimated from bulk collectors to be about 21 kg S ha-' and in the same year the maximum uptake of S by winter wheat grown on this site was 20 kg. Therefore, the requirements of a crop which has a lower S content than others (eg oilseed rape) are only just satisfied by current inputs from the atmosphere. At Rothamsted we are now monitoring wet and dry deposition of S separately. In 1988 wet deposition contributed 7 and dry 11 kg S ha-' year-'. During the same season the amount of S leached was 19 kg ha-', of which we estimate 1 kg was from mineralisation from soil organic matter. Therefore at Rothamsted there is no surplus of S coming from the atmosphere over losses by leaching.

Whereas atmospheric inputs of N have increased to very significant amounts, those of S are decreasing to critical levels. Comprehensive monitoring of both is required, together with research on the effects of what are probably excessive inputs of N for natural and semi-natural ecosystems and on the nutritional requirements of crops for S.

The Spatial Distribution of the Deposition of Sulphur and Nitrogen in the UK

G W Campbell," D H F Atkins,* J S Bower," J G Irwin," D Simpson" and M L Williams"

"Warren Spring Laboratory, Stevenage SGl 2BX, UK; bCommission of the European Community Joint Research Centre, Ispra, Italy

Within the last few years extensive monitoring networks for wet deposition and pollutant gas concentrations have been established in the UK. It is now possible to produce estimates of the spatial distribution of pollutant concentration and deposition. In this poster, maps of annual totals of deposited sulphur and deposited

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d -.

Fig 1. The spatial distribution of the deposition of sulphur and nitrogen in the UK. Left: annual total S deposition (g S m-2); right: total annual nitrogen deposition (g N rn-’).

nitrogen are presented (see Fig 1). They are based, where possible, on measurements rather than on modelled values.

Our estimate of total sulphur deposition ranges from less than 1 g S m-2 in northern Scotland to 3.5 g S m-z in Central England. In the north and-west this is dominated by wet deposition whereas in Central England wet and dry deposition are of similar magnitudes. Total nitrogen deposition varies from less than 0.8 g N m-? in north-west Scotland to about 2 g N m-’ in eastern England. This is dominated by dry deposition over most of England and much of eastern Scotland. Only in north-west England and south-west Scotland does wet deposition account for more than a quarter of the total. Deposition (both wet and dry) of reduced nitrogen species (NH,, NH:) accounts for more than 50% of total nitrogen deposition everywhere in the UK, rising to greater than 70% in some places. It should be noted that these maps are preliminary estimates, especially for dry deposition, and that current research programmes will serve to delineate the spatial patterns more precisely.

The work presented in this poster is taken from a report, LR 723 (AP), by the same authors, obtainable from the above address.