THE EARTHWORM POPULATION OF A WINTER CEREAL FIELD AND ITS EFFECTS ON SOIL AND NITROGEN TURNOVER

The earthworm population in a winter cereal field in Ireland was studied over a 3-year-period and its effects on soil and N turnover were assessed. The mean annual population density was 346-471 individuals m(-2) and the mean biomass was 56.9-61.2g m(-2). Twelve species were recorded, the most abundant being Allolobophora chlorotica followed by Aporrectodea caliginosa. Relative cast production (mg dry soil egested g(-1) fresh mass day(-1)) in the laboratory ranged from 362 mg at 5 degrees C to 2353 mg at 15 degrees C in the case of Aporrectodea caliginosa, and 242 mg at 5 degrees C to 713 mg at 10 degrees C in the case of juvenile Lumbricus terrestris. Gut contents (dry mass of soil) comprised 6.7 - 15.5% of the A. caliginosa live mass, and 9.7 - 14.7% of the Lumbricus terrestris mass. Annual soil egestion by the field population was estimated as 18 - 22 kg m(-2). Tissue production ranged from 81.7 to 218.5 g m(-2), while N turnover resulting from mortality was calculated as 1.5 -3.9 g m(-2) depending on the year and the method of calculation. Earthworms were estimated to contribute an additional 3.4-4.1g mineral N to the soil through excretion, mucus production, and soil ingestion. Independent estimates of N output via mucus and excretion derived from N-15 laboratory studies with Lumbricus terrestris were 2.9 - 3.6 g m(-2) year(-1).