Soil organic and inorganic carbon pools jointly shape sequestration outcomes across arid and semi-arid croplands

Abstract

Cropland soil carbon (C) sequestration is central to climate mitigation and sustainable agriculture. However, frequently omitting soil inorganic carbon (SIC) pool substantially limits the accuracy of cropland C budgets. To address this gap, we conducted an intercontinental meta-analysis to quantify the dynamics of soil C pools under 6 primary agricultural practices, primarily across arid and semi-arid croplands with calcareous soil. Organic amendments (biochar, straw, and organic fertilizer) and irrigation synergistically enhance both soil organic carbon (SOC) and SIC pools. The SIC pool accumulates through pedogenic carbonate precipitation driven by respiration-elevated CO2 and base cations under alkaline conditions. In contrast, mineral fertilizer induces an SOC-SIC trade-off, with net soil total carbon (STC) accrual occurring when in arid soils or at appropriate N rate with low initial SIC. Initial SOC controlled SOC changes, while climate and management predominantly dictated SIC responses. STC pool dynamics exhibited a pronounced depth-dependent shift in regulation, transitioning from SOC dominance in 0–30 cm soil depth to SIC dominance in the subsoil. Similarly, SOC and SIC pools were positively coupled within 0–100 cm depth, but shifted to a negative relationship at greater depths. Moreover, this positive correlation persisted across arid and semi-arid climate zone. Piecewise regression identified an SIC/SOC ratio transition of 3.7 (95% CI of 1.9–4.0), below which SOC and SIC accumulated synergistically, whereas they decoupled above this value. This threshold may also reflect a depth-dependent transition in SOC-SIC coupling. These findings advance beyond SOC-centric paradigms, establishing a novel framework to optimize cropland dual C pools sequestration in climate-resilient agriculture.