Depth-dependent temperature sensitivity of heterotrophic soil respiration under long-term tillage and reduced rainfall

Although conservation tillage practices generally do not enhance soil organic carbon sequestration in temperate arable soils, they substantially modify the vertical distribution and composition of soil organic matter compared to conventional tillage. Yet, the implications of these tillage-induced alterations for soil organic matter stability remain poorly understood. Gaining a deeper understanding of soil organic matter stability is essential in the context of global warming and increasingly variable rainfall patterns.

Here, we investigated depth-dependent patterns of soil respiration and its temperature sensitivity under conventional tillage (CT) and reduced tillage (RT) as well as under current (100%) and reduced rainfall (50%) in a temperate cropland. Soils were sampled in January 2025 from a long-term tillage experiment established in 1970 in central Germany on a Haplic Luvisol under a humid temperate climate (mean annual air temperature 9.6 ± 0.7°C; mean annual precipitation ~610 ± 120 mm). The field trial follows a randomized block design with 16 plots, including eight managed under CT with mouldboard ploughing to 27–30 cm and eight under RT with rotary harrowing to 7–10 cm. Rainout shelters, designed to remove 50% of rainfall, were installed in 2022 in half of the plots. Soils for the incubation experiment were collected from four replicate plots per tillage  treatment and under 100% rainfall at four depths (i.e., 0–10, 10–20, 20–30, and 30–60 cm). Additional samples from rainfall-exclusion plots were collected in August 2025 and are currently being analysed.

Laboratory incubations were performed under controlled conditions using an automated Respicond system. Soils were sieved, adjusted to 60% water holding capacity, and incubated under stepwise temperature changes from 5 to 25°C, followed by a cooling phase back to 5°C, with temperature sensitivity analyses primarily based on the second incubation phase.

Preliminary results under normal rainfall conditions showed that respiration rates were highest in surface soils and declined with depth, while the deepest layer (i.e., 30–60 cm) showed comparatively low and less temperature-responsive respiration. Depth patterns differed between tillage systems: reduced tillage enhanced respiration in the topsoil, whereas conventional tillage showed higher respiration at intermediate depths (10–30 cm), reflecting contrasting vertical distributions of SOC and organic matter fractions. No significant tillage effects on respiration were observed below the plough layer.

Across both tillage practices, temperature sensitivity declined significantly with soil depth, indicating weaker relative temperature responses in subsoils. Mean temperature sensitivity values did not differ between CT and RT when averaged across depths. Normalisation to organic matter fractions showed lower temperature sensitivity of MAOM-associated respiration compared to POM-associated respiration, consistent with differences in substrate quality and energetic constraints on decomposition.

Overall, tillage primarily redistributed organic matter within the topsoil, while subsoil carbon exhibited lower temperature sensitivity, suggesting reduced responsiveness to short-term warming with important implications for modelling soil carbon–climate feedbacks. Ongoing analyses will assess how two years of reduced rainfall modify the vertical distribution and temperature sensitivity of soil organic matter.