Total P (TP), total particulate P (TPP), dissolved reactive P (DRP), and dissolved organic P (DOP) were determined in waters from pipedrains (at 65‐cm depth) from the Broadbalk Experiment at Rothamsted. Soils that have received either no P, P in farmyard manure (about 40 kg P ha−1) or superphosphate (up to 35 kg P ha−1) annually for >150 yr, now contain 0.5 M NaHCO3‐extractable P concentrations (Olsen‐P) in the plow layer (0‐ to 23‐cm depth) between 5 and 100 mg kg−1 soil. Our aim was to determine if significant quantities of P could be detected in the drainage water and their relationship to soil P concentrations. On five occasions between October 1992 and January 1994, both TP and DRP from plots receiving superphosphate frequently exceeded 1 mg L−1 and were high compared with literature data. Ranging between 66 and 86% of TP, DRP was the largest fraction in drainage water. It remained low (<0.15 mg L−1) from plots below 60 mg Olsen‐P kg−1. There was then a rapid increase in DRP up to the maximum Olsen‐P concentration. A simple linear Split‐Line Model described this relationship very well for all drainage events. This implies that up to 60 mg Olsen‐P kg−1 soil (the change point), P was retained strongly in the plow layer. Above this, P losses in the drainage water were much more closely related to Olsen‐P than commonly suggested. The mechanisms could either be preferential flow or rapid transport of P in forms less susceptible to sorption but finally measured as DRP. The results suggest enhanced P losses through subsurface runoff on heavy soils, once a certain plow layer concentration of Olsen‐P is exceeded.

The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as significant pools of N in agricultural soils. We review the size and function of both soluble organic N extracted from soils (SON) and dissolved organic N present in soil solution and drainage waters (DON) in arable agricultural soils. SON is of the same order of magnitude as mineral N and of equal size in many cases; 20-30 kg SON-N ha -1 is present in a wide range of arable agricultural soils from England. Its dynamics are affected by mineralisation, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drainage waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may take with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but there are still many gaps in our understanding.

S U M M A R YComparisons were made between 1988 and 1991 to evaluate three methods of estimating thc leaching of mineral nitrogen (N) from unstructured freely draining sandy loam and loamy sand soils. The studies compared the drainage patterns and quantities of N (almost cxclusively nitrate) leachcd from monolith lysimeters with those estimated from ceramic suction cups and soil core extracts. The latter two methods gave direct measurements of the mineral N concentrations in drainage, but required an estimate of the drainage volume calculated from meteorological observations and evapotranspiration equations to give total N leached. A bromide tracer was also used to confirm conclusions from nitrate leaching studies. There was a delay in the onset of drainage from free draining lysinieters because they lack the subsoil matric potential offield soils. However, total annual drainage measured by lysimctcrs or calculated from meteorological observations was similar, providing that return to field capacity was correctly identified in the field soil. During the first year thcrc wcrc discrepancies bctwccn methods which were attributcd to soil disturbance during lysimetcr and/or ceramic cup installation. In the second and third years of the experiment, estimates of N leaching losses using the Iysimeters and ceramic cups wcre in good agreement. Nitrate concentrations in soil solution at a depth of 130 cm measured from soil core extracts were smaller than found by the othcr methods during the second year and thc peak concentrations wcre significantly different ( P < 0.05). However, total overwinter N leached was not significantly different. Thus, while lysimeters and cups can be uscd to quantify leaching losses on unstructured, free draining soils if uscd corrcctly, the use ofsoil core extracts is questionable.

Arable crops in the UK make a large contribution to nitrate leaching by virtue of the land area they cover (>4.5 million ha). By contrast horticultural crops occupy only a small area (< 0.2 million ha) but can leach very large amounts of nitrogen. The application of nitrogen fertilizer to arable and horticultural crops is very cost-effective, stimulating its use. MAFF's Nitrate Research Programme for arable and horticultural crops aims to reduce nitrate leaching and maintain productive farming through Best Management Practice.The Programme has led to the development and testing of methods to measure nitrate leaching, the identification of`leaky' crops, soils and practices, and strategies to optimize the use of fertilizer nitrogen. Data have been used to construct and test models of nitrate leaching, which in turn have been used to evaluate the leakiness of potential rotations. Current best practice to minimize nitrate leaching requires measures to improve the efficiency of nitrogen use by crops, combined with measures to protect soil nitrogen from leaching during the late autumn to spring drainage period. This involves consideration of many factors: an appropriate crop variety must be chosen; a green cover must be maintained for as much of the year as is practicable; crops should be drilled early; fertilizer requirements should be calculated using a recommendation system and allowing for soil mineral nitrogen and any manures applied; fertilizers should be spread evenly with a properly calibrated spreader, perhaps using split applications; starter fertilizers and banding of fertilizers should be used where appropriate to reduce losses from vegetables; pest and disease infestation must be minimized; any irrigation must be applied carefully with scheduling. Research is now moving on to study whole farm systems and the interactions between losses of nitrogen and other pollutants to the environment with the aim of minimizing total environmental impact.

Abstract:The broad purpose of the study described here was to assess the role of denitri®cation in riparian zones in ameliorating groundwater pollution through nitrate loss, and as a potential source of nitrous oxide to the atmosphere. A suitable riparian zone was identi®ed at Cuddesdon Mill on the River Thame¯oodplain near Oxford, England. Measurements were made of water and nitrate moving from arable land through the riparian zone and into the river. Techniques to measure denitri®cation were tested and applied, and the factors controlling denitri®cation measured. While there was considerable potential for denitri®cation at the site, this was not realized because much of the water moving o the farmland bypassed the riparian zone, entering the river directly via springs or through gravel lenses beneath the¯oodplain soil. Management of this site would not reduce nitrate leaching unless the¯oodplain hydrology could be substantially modi®ed, and the main conclusion is that nitrate buer zones will only operate eciently where the hydrology of the site is appropriate.