Nitrogen dynamics of an arable soil under different agronomic practices

Abstract

It has become evident that the benefits of increased fertiliser N use in the UK may be offset by problems, such as nitrate leaching to surface and groundwaters. The broad objectives of this work were to investigate how the N dynamics of a 'typical' arable soil in south-­east Scotland receiving recommended fertiliser N applications were modified by: 1) reducing fertiliser N application; 2) replacing the fertiliser N with a leguminous source of N (forage peas grown as a green manure crop); 3) growing a winter cover crop.

All experimentation was field-based, with the main emphasis upon the direct measurement of NO3-N leaching losses from eight 300 m2 hydrologically isolated field plots, complemented by routine measurements of crop N uptake, soil mineral N, atmospheric N deposition and N 20 flux. N2 fixation in the leguminous green manure was also measured, plus the mineralisation of the incorporated legume material. The efficacy of hydrological plot isolation in local soil types was first investigated using a small pilot plot.

The main experimental period began with incorporation of the green manure in September 1987 and ended in April 1989. Crop yields were low and the utilisation of applied N very poor. There was no apparent financial incentive to reduce fertiliser N application or replace it with a leguminous green manure.

Variable drainflow recovery from the plots hampered accurate estimation of NO3-N leaching losses, but results suggested that:leaching losses from arable soils in south-east Scotland are generally less than in southern Britain; reducing fertiliser N application had little effect upon leaching losses; autumn incorporation of the green manure increased leaching during the following winter; autumn cultivation increased leaching compared with no cultivation; spring-applied fertiliser N was susceptible to leaching loss; growth of a winter cover crop may have reduced winter leaching. Denitrification was likely to have been a very important N loss process, but was very difficult to measure directly in the heavy, poorly structured soil type.

Despite very high levels of symbiotic N2 fixation (over 300 kg Nha-1), the use of the leguminous green manure to increase available N for the following crop was limited under local soil and climatic conditions. This was due to the complex nature of legume decomposition and mineralisation and led to the poor synchronisation of legume N release and crop N uptake in the autumn and spring after incorporation. It is likely, however, that a leguminous green manure would be of value in maintaining the long-term N status of an arable soil.

Experimental data was summarised in the form of N balance sheets for the different experimental treatments. These suggest that although the highest non-harvest losses occurred from the application of a recommended fertiliser N rate, this treatment retained mineralN within the arable soil-plant system most efficiently.