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SU MMARYThe effects of ploughing or no-tillage of long-term grass and grass-clover swards on changes in organic C and N pools and on CO 2 and denitrified gas emissions were investigated in a 3-year field experiment in 1996-99 near Penicuik, Scotland. The decrease in soil C content between 1996 and 1999 was 15 . 3 t/ha (95 % confidence limits were 1 . 7-28 . 9 t/ha). Field estimates of CO 2 losses from deepploughed, normal-ploughed and no-tillage plots were 3 . 1, 4 . 5 and 4 . 6 t/ha over the sampling periods (a total of 257 days) in 1996-98. The highest N 2 O fluxes were from the fertilized spring barley under no-tillage. Thus no-tillage did not reduce C emissions, caused higher N 2 O emissions, and required larger inputs of N fertilizer than ploughing. By contrast, deep ploughing led to smaller C and N 2 O emissions but had no effect on yields, suggesting that deep ploughing might be an appropriate means of conserving C and N when leys are ploughed in. Subsoil denitrification losses were estimated to be 10-16 kg N/ha per year by measurement of 15 N emissions from incubated intact cores. A balance sheet of N inputs and outputs showed that net N mineralization over 3 years was lower from plots receiving N fertilizer than from plots receiving no fertilizer.

A 3-year field experiment was conducted in 1996-98 near Penicuik, Scotland, to investigate the fate of N released after cultivation of previously long-term grass and grass-clover swards. The effects of timing of cultivations (autumn and spring), tillage methods (no tillage, ploughing to 200 mm and ploughing to 300 mm) and fertilizer N for spring (0, 40, 80 and 120 kg N/ha) and winter barley (0, 60, 120, 180 kg N/ha) on yield, N uptake and nitrate leaching were measured.In 1996, after spring cultivation, on plots previously in grass, spring barley grain yield and N uptake did not respond to N fertilizer, but on plots previously in grass-clover there was a nearly linear response to N. In 1997, the spring barley responded to N fertilizer at all levels. Yields of 1997 winter barley after grass did not show a response above 60 kg N/ha, but increased with fertilizer N up to at least 120 kg N/ha after grass-clover. In 1998, there were strong effects of N fertilizer and cultivation method on grain yield and N uptake of both spring barley and winter barley. Winter barley grain yield was significantly higher in plots previously in grass than in plots previously in grass-clover in 1998, though not in 1997. Winter barley yields were higher than spring barley at the same fertilizer N level.Throughout the 3 years, the no-tillage plots had consistently lower yields than the ploughed plots, but there was no consistent difference between the ploughed and deep ploughed treatments. There were strong interactive effects between tillage and previous sward in 1997. No-tillage under spring barley generally yielded lower than ploughing due to difficulties in weed control and the frequent anaerobic conditions in the soil.Annual leaching losses were relatively small (6 . 4-19 . 6 kg N/ha). In 1996 -97, more N was leached from the plots left in stubble following spring barley than from those planted with winter barley after either spring barley or grass in 1996, but in 1997-98 more N was leached from plots in winter barley than from those in over-winter stubble. Nitrate leaching was least under no-tillage, though the effect was not significant.

Soil compaction is a concern worldwide, particularly where compactible soils are used for intensive agriculture in a wet climate. We have investigated the impact of compaction and the associated changes in soil structural qualities on crop production and environmental pollution. The overall objective was to develop soil management systems that provide suitable conditions for crop growth and minimize environmental damage. We ran large-scale field experiments studying the preservation of structural quality in arable and permanent grassland, using management systems such as the control or elimination of field traffic and the application of conservation tillage and zero tillage. We measured bulk density, shear strength, cone resistance, macroporosity, relative diffusivity, air permeability and water infiltrability to identify soil qualities that could be used for selecting suitable soil management. Along with crop yield, we measured environmental impacts, such as the emissions of nitrous oxide from the soil, which require the interaction of soil structure and water content near the soil surface. Soil structure influenced wetness, which affected trafficability, compaction and nitrogen retention. Measurement of properties that affect fluid storage and transport, such as macroporosity, provided soil quality indices that helped in recommending suitable soil management systems. Spatial variation of structure associated with wheeltrack locations could be estimated rapidly using a cone penetrometer. Variation was particularly important in determining crop yield consistency. Crop productivity and soil structural qualities were preserved best when field traffic was eliminated. A reduced ground-pressure system successfully minimized compaction in grassland but was less effective in an arable rotation. Unless traffic is eliminated, good timing of operations is the most effective way to preserve soil structural quality.

The calibration of a high resolution gamma-ray density probe and a simple experiment comparing the probe with an earlier, lower resolution version are described. An assessment is made of the performance of the probe in three experiments investigating compaction by tractor wheels, two of which were in the field and the third in an indoor soil tank. A linear calibration relationship was obtained, although the addition of a quadratic term improved the fit of the curve slightly. The probe was found to be much more accurate than the earlier version within 100 mm of the soil surface where treatment effects were largest and most numerous in the field experiments, and thin layers of high density could be detected. Over the 15 month period of the experiments, the stability of the system was found to be satisfactory. IntroduclionA system for measuring the bulk density of soil in the field by means of gamma-ray transmission has been in use at the Scottish Institute of Agricultural Engineering (SIAE) for about 15 years (Soane et al., 197 I). This system, based on a simple GeigerMiiller detector, while providing useful data, was found to be lacking in its spatial resolution. It was not possible to examine layers of soil thinner than 100 mm and accurate measurements of bulk density could not be obtained at depths of less than 100 mm from the soil surface. To overcome these problems a high resolution system based on a scintillation detector with energy discrimination was developed and compared with the earlier system under laboratory conditions (Soane & Henshall, 1979). This paper deals with the calibration of the high resolution system for field use and describes both a simple experiment to compare the two systems in a typical compaction investigation and an assessment of the performance of the high resolution system in its first year in regular use. The high resolution systemThe system consists of a soil probe connected to a portable scaler/ratemeter (Fig. 1). The two stainless steel tubes of the soil probe, each of 25.4 mm outside diameter, are clamped with their centres 220 mm apart. One tube holds a 5 mCi (200 MBq) 137Cs radioactive source and the other a detector comprising a NaI(T 1) crystal.