The role of soil depth in the evaluation of management‐induced effects on soil organic matter

Summary The aim of the current survey was to determine the relevance of the subsoil in the assessment of management effects on soil organic matter ( SOM ) in arable farming. Data in this survey were provided by the organic arable long‐term field experiment Gladbacherhof ( OAFEG ). Three different ‘farming types’ were compared: mixed ( MF ), stockless with rotational ley ( SFL ) and stockless with cash crops ( SFC ). Each type had four different tillage treatments: full inversion ( FIT ), two‐layer plough ( TLP ), reduced inversion ( RIT ) and non‐inversion ( NIT ). The different mean masses of soil organic carbon ( SOC ) and soil total nitrogen ( STN ) at 0–30‐, 30–60‐, 60–90‐ and 0–90‐cm soil depths after three crop rotation cycles (17‐year observation period) were evaluated and considered as an indicator of SOM . ‘Farming types’ differ in their effect on SOC and STN mass of the topsoil layer (0–30 cm) in the order MF > SFL > SFC ( P < 0.01). For the 0–90‐cm soil depth there were no differences between the treatments. Treatments with different tillage intensity did not show any effect on the contents of SOC or STN within the 0–30‐cm soil depth. However, within the 30–60‐cm depth full‐inversion tillage ( FIT ) was associated with larger amounts of SOC and STN than the reduced tillage intensity treatments TLP , RIT and NIT ( P < 0.01). This situation even resulted in significantly larger SOC and STN masses under FIT ( P < 0.05) over the whole soil profile under study (0–90 cm). Our results suggest that sampling depth should be extended to include the effects on the upper subsoil in assessment of the effects of arable management on soil organic matter. Highlights Role of subsoil in the assessment of management effects on soil organic matter in arable farming is rarely accounted for. We investigated SOC and STN contents below 0–30‐cm depth in a long‐term field experiment. Consideration of the subsoil had a marked effect on the differentiation of the treatments. Sampling depth should be extended to include effects of management on the upper subsoil in the assessment of soil organic matter.