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Abstract
A 6-year study investigated the suitability of effluent from an anaerobic biogas digester as a fertiliser for supporting crop growth and maintaining soil biochemical levels under rotational cropping. The soil at the experimental area was predominantly a Fluvaquentic Eutrochrept. Comparisons were made with an inorganic fertiliser and a water-only treatment, using three crops (maize, oats, and kale) grown over a 2-year rotation. Dry matter yields were not consistently influenced by any of the treatments, apparently because of high soil-nutrient reserves. Thus the effectiveness of the effluent as a fertiliser could not be assessed. However, plant nitrogen concentrations were usually higher in the effluent and fertiliser treatments than in the water-only treatment for the last 3 years of the study. Soil organic C content for each treatment remained unchanged over the 6-year period. In contrast, microbial biomass, urease activity, and net mineral-N production (0–28 days) declined significantly in all treatments; invertase activity increased, and other properties showed no regular trends. Values of some biochemical properties differed significantly between treatments, and were at least as high in the effluent-treatment as in the other samples. Nitrogen availability at the last sampling appeared to be highest in the effluent-treated soil. Any direct contribution of the effluent, whose biochemical composition varied appreciably with sampling time, to levels of soil microbial biomass and enzyme activities is likely to have been small. Generally, the digester effluent appeared at least as suitable as inorganic fertilisers for sustaining levels of soil biochemical properties under continuous cropping. No adverse effects of digester effluent were observed.
