Contribution of α-amino N to extractable organic nitrogen (DON) in three soil types from the Scottish uplands

Organic nitrogen (DON) was extracted from two improved pasture soils, one of which had been re-colonized by acid heath vegetation, and a blanket peat. Although the quantities extracted in H2O, 10 mM CaCl2, 500 mM K2SO4 and 50 mM Na2HPO4 were not consistent, mean extractable DON as a proportion of total N was greater in the two grazed pastures (0.4%) than in the peat (0.2%). Averaged over the four extractants, free α-amino N was greater in the peat and least in the improved pasture soil and accounted for 26% of DON in the peat and less than 5% in the mineral soil. Amino N increased after 6 M HCl hydrolysis, and this combined N contributed 56% to DON in extracts of the mineral soil compared with only 36% in the peat This variation in the relative contributions of free and combined amino N to DON indicated qualitative differences in the composition of DON between the three soils.