Ecosystem-dependent two-stage changes in soil organic carbon stock across the contiguous United States from 1970 to 2014

Abstract

Temporal dynamics in soil organic carbon (SOC) play a crucial role in the global carbon cycle. How warming affects SOC change has been widely studied at the site scale, mainly through short-term manipulative experiments. Decades-long SOC dynamics in ecosystems can be complicated, particularly as real-world warming rates varied on decade-scale. However, the lack of long-term repeated observations on whole-profile SOC limits our understanding of SOC dynamics across large regions. Herein, we reconstructed 45 years of SOC dynamics (1970–2014) in topsoil (0–30 cm) and subsoil (30–100 cm) using 10,639 soil profiles from forest and cropland across the contiguous United States, and investigated their relations with key dynamic environments (e.g., climate, vegetation and nitrogen). We further examined the spatial pattern of SOC stock changes at a finer scale (∼2 km) using machine learning techniques. Our results revealed ecosystem-dependent, two-stage changes of SOC stock, characterized by continental-scale halts in SOC loss following warming deceleration since the late 1990s. This shift led to an overall increase in SOC stock of 1.41 % in forest and 1.14 % in cropland within the top 1-meter over 45 years. Temperature was the primary factor related to topsoil SOC losses, whereas soil water content may primarily control subsoil SOC change. Notably, a threshold effect of warming rates on SOC loss was identified in both topsoil and subsoil. These findings provide new insights into long-term whole-profile SOC dynamics at a large scale, offering valuable implications for carbon sequestration to support sustainable development in different ecosystems.