The natural abundance of ¹³ C, ¹⁵ N, ³⁴ S and ¹⁴ C in archived (1923–2000) plant and soil samples from the Askov long‐term experiments on animal manure and mineral fertilizer

Abstract The Askov field experiment (Denmark), established in 1894, provides a unique opportunity to examine long‐term effects of animal manure and mineral fertilizer on soil organic matter quality and turnover. This sandy loam soil is classified as Alfisol (Typic Hapludalf). Soil C, N, S, 13 C, 15 N, 34 S and 14 C contents were measured in a selection of archived soil samples (1923, 1938, 1945, 1953, 1964, 1976, 1985, 1996 and 2000) from unfertilized (O), animal manure (1 AM) and mineral fertilizer (1 NPK) treatments. These treatments are imbedded in a four‐course crop rotation of winter cereals, root crops, spring cereals and a clover/grass mixture. The contents of C, N, S, 13 C, 15 N and 34 S in selected crop samples (1953–1996) and in contemporary samples of animal feed and manure were also determined. Temporal soil nutrient and isotope trends between fertilizer treatments were significantly different, except for S content in 1 AM and 1 NPK. The total soil C and S was higher in 1 AM and 1 NPK than in the O treatment. The total soil N content (1 AM > 1 NPK > O) and the δ 15 N content (1 AM > 1 NPK and O) were also different. Analyses of plant, animal feed and manures confirmed that differences in soil 15 N values were related to δ 15 N values of added source inputs. Soil and crop δ 13 C values were similar, but manures had slightly lower values. The variation of soil δ 34 S (and total S) from 1923 to 1996 was larger in the O than 1 AM and 1 NPK plots reflecting changes in atmospheric S inputs. The total contents of soil C, N and S were significantly correlated, but their isotopic signatures were not, suggesting that the C, N, S turnovers in soil are subject to different controls. The 14 C content was generally higher in the 1 AM than 1 NPK and O, with bomb‐ 14 C incorporation modelling indicating that mean residence time (MRT) was ca. 170 years in the 1 AM, but closer to 250–290 years in the 1 NPK and O treatments. The measured trends in soil C and 14 C during 1923–1996 were successfully modelled using the RothC model. The OM accumulation in the Askov soils was generally dominated by microbial decomposition products rather than by recalcitrant components of the various inputs. Copyright © 2005 John Wiley & Sons, Ltd.