Nitrate Leaching under Arable Land Ploughed out from Grass

Keywords: Nitrate leaching, Soil mineral N, Ploughing grassland, N prediction, Nitrogen balance,

The amount of nitrogen leached when grassland is cultivated for arable cropping was investigated at eight sites, from autumn 1989 to spring 1992.

Sites were chosen on initial classification into high and low N status, based on past history, and also on soil type, either sandy or shallow over chalk.

The effect of timing (either in autumn 1989 or spring 1990) and type (ploughing or minimal) of cultivations for sward break up were compared, winter cereals being sown on the autumn cultivated plots while spring cereals followed spring cultivations. In subsequent years, all plots were ploughed and sown to winter cereals. During the three winters, from 1989/90 to 1991/92, measurements of soil mineral N (ammonium plus nitrate N) and N leaching losses (using ceramic porous cups) were taken.

Over the three winters, N leaching losses were of the order 100-120 kg N/ha for three sites of low initial N status, 250 kg N/ha for two sites where N reserves were expected to be higher and over 1000 kg N/ha from one site where excessive amounts of organic manures had been applied. Only at the latter site did the majority of N leaching losses take place during the first winter. Breaking the sward in autumn 1989, compared to spring 1990, increased leaching losses during the subsequent winter at three of the four sites where leaching data for this comparison was available; during the second winter at three out of seven sites, more N was leached from plots where the original sward cultivation had been delayed until spring. Ploughing, as opposed to minimum cultivation, did not increase leaching losses except possibly at the site with very high N reserves. At most sites in most years, mean nitrate-N concentrations in the drainage water were above the EC limit of 11.3 mg/l.

Mineral N levels in November 1989 were higher following autumn cultivations but there was little effect in subsequent years from this time of break up. As with leaching losses, the type of cultivation had little effect on mineral N levels in any year. Autumn mineral N levels correlated poorly with N leaching losses until the third winter. Between November and February, mineral N levels increased in some situations.

The relative N supplying power of the previous grass sward was assessed both by a crude N balance (subtracting harvest N offtakes and N leaching losses from spring fertiliser N) and by estimating organic matter accumulation over 5 years, using an N cycling model developed by IGER. Both these approaches placed the sites in the order that would be expected from their previous management.

A Lloyd, ADAS Gloucester, Elmbridge Court, Cheltenham Road, Gloucester, UK.

32 pages, 2 figures, 12 tables, 24 references.