Soil microbial resource limitation along a postmining chronosequence

Soil microorganisms regulate fundamental biogeochemical processes, including carbon sequestration and nutrient cycling, yet their activity and growth are frequently constrained by the availability of limiting resources. Microbial resource limitation is highly dynamic and context dependent, shaped by interacting biotic and abiotic drivers such as soil organic carbon, nutrient availability, land-use, and pedo-climatic conditions. Despite its central role for ecosystem functioning, we still lack a comprehensive and mechanistic understanding of how microbial resource limitation emerges and shifts along soil development gradients, largely due to the confounding nature of multiple co-varying environmental factors in natural ecosystems.

Post-mining chronosequences offer a powerful framework to disentangle such drivers, as they share highly comparable initial soil conditions while differing in time since reclamation. We used a 66-year post-mining chronosequence at the open-cast lignite mine Inden (Western Germany) to investigate patterns of soil microbial resource limitation along a pronounced soil organic carbon gradient. Soils originated from a standardized loess-based substrate and encompassed two land-use systems: reclaimed arable fields under conventional management and adjacent, unmanaged field margins. Topsoil samples (0–15 cm) spanning SOC contents from 0.6-4.0% were subjected to multifactorial carbon, nitrogen, and phosphorus additions, followed by measurements of microbial biomass growth and heterotrophic respiration.

Across both land-use systems, microbial growth and respiration responded most strongly to treatments receiving carbon, either alone or combined with nitrogen and phosphorus, indicating a prevailing state of microbial carbon limitation along the chronosequence. Microbial biomass responses to carbon amendments declined exponentially with increasing soil organic carbon, revealing a critical soil organic carbon threshold around 1-1.5%, below which strong carbon limitation prevailed and above which carbon limitation was progressively alleviated. In arable soils with low soil organic carbon, evidence for carbon and nitrogen co-limitation emerged, while high-soil organic carbon soils - particularly field margins - showed indications of phosphorus co-limitation in respiratory responses. Extrapolation of the observed response functions suggests that even soils with substantially higher soil organic carbon contents may retain a measurable, albeit diminishing, degree of microbial carbon limitation.

Overall, our results highlight soil organic carbon as a dominant regulator of microbial resource limitation during early to intermediate soil development and emphasize the value of post-mining chronosequences for advancing a mechanistic understanding of microbial constraints on soil biogeochemical functioning.