Long-term management effects on depth gradients of 13C, 15N and C/N ratio in agricultural soils

• Sites at ten long-term experiments (32–112 years) were sampled down to 100 cm. • Agricultural management affected isotope values and C/N ratio mainly in topsoil. • Unexpected decrease in δ 13 C in topsoil and subsoil due to liming. • δ 15 N decreased with mineral fertilisation, but increased with organic fertilisation. • Variation in C content and 13 C was higher than the variation in N content and 15 N. Agricultural management practices influence the turnover and residence time of soil organic matter (SOM) and thus can contribute to carbon (C) removal from the atmosphere. However, advanced analytical techniques are needed to disentangle the interlinked processes of SOM stabilisation and destabilisation, as well as its built-up and decomposition. Stable isotopes of C (δ 13 C) and nitrogen (N; δ 15 N) as well as the carbon-to-nitrogen (C/N) ratio provide information about the quality and thus origin and turnover SOM. The aim of this study was to quantify the effect of different agricultural management practices (mineral fertilisation, farmyard manure (FYM) application, straw incorporation, crop rotations, liming, irrigation and reduced tillage) on these indicators in topsoil and subsoil. Ten German long-term experiments (LTEs) were sampled down to 100 cm depth. Changes in SOC and N content were assessed as indicators of SOM quantity and changes in δ 13 C, δ 15 N and C/N ratios were assessed as indicators of SOM quality. Increases in C and N content were mainly affected in topsoil by mineral fertilisation, FYM, straw and irrigation. Changes in δ 13 C were limited to crop rotations (+0.62 ‰) and FYM (−0.27 ‰) in topsoil, but liming effects also reached down to 70 cm (−0.46 ‰ on average). Mineral fertilisation reduced the δ 15 N values in topsoil (−0.12 ‰), while the application of FYM increased them (+0.53 ‰), indicating different N sources. The δ 15 N values in the subsoil changed due to crop rotations (+0.62 ‰) and reduced tillage (−0.96 ‰). Specific management practices did not change C/N ratios significantly, with the exception of NPK fertilisation in topsoil (−1.32). Variations in SOC and total N content were more than 50 % greater in subsoil than in topsoil, but 13 C and 15 N did not follow this trend, indicating a decoupling of changes in C and N content and changes in SOM quality. This study revealed challenges in the detection of subsoil effects due to the low SOM content that in some LTEs unexplained large spatial variability in δ 15 N in subsoil. Since agricultural management practices change 13 C, 15 N and C/N ratios at least in topsoil, these variations need to be considered in isotope studies.