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In this study, ten long-term experiments (LTEs) across Germany were sampled down to 100 cm depth to investigate how different agricultural management practices affect stable isotope signatures of carbon (δ¹³C) and nitrogen (δ¹⁵N), as well as the carbon-to-nitrogen (C/N) ratio. These indicators help us assess the quality (origin and turnover) of soil organic matter (SOM).
The management practices examined included mineral fertilisation, farmyard manure (FYM) application, straw incorporation, crop rotations with C4 crops, liming, irrigation, and reduced tillage. Changes in soil organic carbon (SOC) and total nitrogen (N) contents were used to evaluate SOM quantity, while δ¹³C, δ¹⁵N, and C/N ratios served as indicators of SOM quality.
Key findings:
- Increases in SOC and N content occurred mainly in the topsoil, particularly under mineral fertilisation, FYM, straw incorporation, and irrigation.
- δ¹³C values were affected by crop rotation with C4 crops (+0.62 ‰) and FYM (−0.27 ‰) in topsoil, with liming effects observed down to 70 cm depth (−0.46 ‰ on average).
- δ¹⁵N decreased under mineral fertilisation (−0.12 ‰) and increased with FYM application (+0.53 ‰), indicating differing N sources. In the subsoil, δ¹⁵N changes were linked to crop rotation (+0.62 ‰) and reduced tillage (−0.96 ‰).
- C/N ratios were mostly unaffected by management practices, except for a reduction under NPK fertilisation in topsoil (−1.32).
- Variability in SOC and total N content was over 50 % greater in the subsoil than in the topsoil. However, this was not reflected by isotope ratios, suggesting a decoupling between SOM quantity and quality.
- Detecting subsoil effects remains challenging due to low SOM content and large unexplained spatial variability in δ¹⁵N in some LTEs.
The study highlights that agricultural management can alter δ¹³C, δ¹⁵N, and C/N ratios – particularly in topsoil – and that such changes should be considered in isotope-based studies of SOM dynamics.
