Effect of Poultry Manure on the Leaching of Carbon from a Sandy Soil as a Potential Substrate for Denitrification in the Subsoil

Abstract

There is evidence from laboratory incubations that denitrifying bacteria occur in the deep subsoils of UK soils and that lack of available carbon (C) generally limits their activity. Animal manures can be a source of substantial carbon input to farming systems. This experiment measured the effect of broiler litter application on the movement of C in soil solution to depths below 1 m, which might be sufficient to allow denitrification of nitrate moving from the rooting zone towards ground water aquifers. Six broiler litter rates were applied each autumn from 1992-1994 to field plots on a loamy medium sand in Nottinghamshire, UK. Total loadings over the 3 years ranged from 0 to 125 t ha -1 broiler litter, supplying 0-32 t ha -1 total C. Teflon and ceramic water samplers, placed at 1.0 and 1.5 m, and monolith lysimeters (0.5 m 2 area, 1.5 m deep) were used to measure total organic carbon (TOC) concentrations in the drainage. Ceramic samplers indicated significantly (P < 0.05) larger concentrations than Teflon samplers; there were no differences between concentrations measured by Teflon samplers and the lysimeters. Water samples analysed for both dissolved and total C showed that nearly all was in a dissolved form. TOC concentrations on plots which received no manure were less than 20 mg litre -1 at 1 m for the duration of the experiment; concentrations peaked at 65 mg litre -1 with the largest manure loading. There was a linear relationship between C leached and C applied, with about 5% leached below 1 m by the end of the experiment. There was some evidence of movement of C to 1.5 m depth, but there were no large peaks corresponding to those at 1 m, because of either adsorption or microbial utilisation. The results provide evidence of movement of substantial C to depth in some circumstances, particularly on fields which regularly receive large dressings of organic manure. The availability of this as a substrate for denitrification needs further examination.