Soil profile structure and transport control equilibrium in microbial soil carbon models

Equilibrated soil organic carbon (SOC) states are a prerequisite for Earth system model simulations following CMIP and TRENDY protocols, which rely on long preindustrial spin-up phases prior to historical and future integrations. While conventional linear soil carbon models readily achieve equilibrium, microbial-explicit soil carbon models frequently exhibit slow convergence or persistent SOC drift even after millennial-scale spin-up, raising concerns about their applicability in Earth system simulations.

Previous analytical work has derived steady-state solutions for microbial soil carbon models under the assumption of vertically integrated, well-mixed systems, but it remains unclear whether such analytical equilibria are sufficient when models include vertical soil structure and transport processes. Here, we systematically assess the role of soil profile discretization, transport, and model structure in controlling SOC equilibration, and evaluate whether analytically derived steady states can provide reliable initial conditions for depth-resolved microbial soil carbon models.

Using the QUINCY land model framework, we conduct a hierarchy of simulations under standard CMIP-style protocols, consisting of a 1000-year spin-up followed by historical simulations (1850–2019). First, we apply QUINCY-derived litter inputs to the vertically integrated microbial soil carbon model Millennial, which includes explicit microbial dynamics and mineral-associated organic matter formation but no vertical transport. Second, we simulate soil carbon dynamics in QUINCY using a CENTURY-type linear soil model (SSM) with explicit vertical discretization (5 and 15 soil layers to 9.5 m depth), providing a reference case with well-defined analytical equilibria. Third, we perform fully depth-resolved simulations using the Jena Soil Model (JSM) within QUINCY, combining microbial-explicit carbon cycling, sorption dynamics, and vertical transport.

We hypothesize that difficulties in equilibrating microbial soil carbon models arise primarily from structural interactions between nonlinear microbial kinetics, sorption capacity constraints, and vertical transport, rather than from numerical deficiencies or insufficient spin-up duration. We further expect that analytically constrained initial conditions substantially reduce equilibration times and SOC drift in bucket models and linear depth-resolved systems, while providing a useful—but not fully sufficient—approximation for initializing complex microbial soil carbon models with dynamic soil profiles.

By explicitly comparing linear and microbial soil carbon models across vertically integrated and depth-resolved configurations, this study clarifies the conditions under which analytical steady-state solutions are adequate for CMIP- and TRENDY-style simulations, and identifies remaining structural challenges for deploying microbial soil carbon models in Earth system frameworks.