Quantifying soil carbon sequestration by multi-source constraints

Soil carbon cycle is a large yet poorly understood component of the global carbon cycle under climate change. The transient behaviour of the soil carbon cycle is fully determined by four elements, which are carbon input, residence time,carbon pool size (or state), and the net carbon flux. Many of the past studies focused on one subset of the four elements to quantify soil carbon sequestration under climate change, often leading to contradictory conclusions. Here we assimilated data of respired soil ¹⁴C, soil ¹⁴C profile, and soil organic carbon (SOC) profile from Harvard Forest (i.e., a mid-latitude hardwood forest) into a vertically resolved process model (i.e., Community Land Model version 5, CLM5) together with estimated carbon input to fully constrain soil carbon dynamics during 1900 to 2010. Our results suggested litter pools instead of the mineral soil pools contributed to the majority of the carbon sequestration in history. Different sources of constraints effectively informed parameters of their corresponding elements in the soil system. Yet, single data constraints only provided part of the features of soil carbon cycle and cannot lead to a comprehensive interpretation of its historical dynamics. Using ¹⁴C data alone as the constraints resulted in overestimated soil carbon residence time and more sensitive responses of soil carbon sequestration to changing climate. In the future, multi-source data constraints from different global databases are essential in understanding soil carbon dynamics and accurately quantifying soil sequestration in response to the changing climate across the globe.