Increased atmospheric CO₂ and the transit time of carbon in terrestrial ecosystems

Abstract

The response of terrestrial ecosystems to increased atmospheric CO2 concentrations is controversial and not yet fully understood, with previous large‐scale forest manipulation experiments exhibiting contrasting responses. Although there is consensus that increased CO2 has a relevant effect on instantaneous processes such as photosynthesis and transpiration, there are large uncertainties regarding the fate of extra assimilated carbon in ecosystems. Filling this research gap is challenging because tracing the movement of new carbon across ecosystem compartments involves the study of multiple processes occurring over a wide range of timescales, from hours to millennia. We posit that a comprehensive quantification of the effect of increased CO2 must answer two interconnected questions: How much and for how long is newly assimilated carbon stored in ecosystems? Therefore, we propose that the transit time distribution of carbon is the key concept needed to effectively address these questions. Here, we show how the transit time distribution of carbon can be used to assess the fate of newly assimilated carbon and the timescales at which it is cycled in ecosystems. We use as an example a transit time distribution obtained from a tropical forest and show that most of the 60% of fixed carbon is respired in less than 1 year; therefore, we infer that under increased CO2, most of the new carbon would follow a similar fate unless increased CO2 would cause changes in the rates at which carbon is cycled and transferred among ecosystem compartments. We call for a more frequent adoption of the transit time concept in studies seeking to quantify the ecosystem response to increased CO2.

Controls and relationships of soil organic carbon abundance and persistence vary across pedo‐climatic regions

“…Our findings imply that tropical soils are the most challenging for increasing long‐term SOC storage, since they have relatively low SOC abundance and persistence under current conditions (Figures 3 and 5). Although tropical evergreen forests have the highest root biomass globally (Jackson et al., 1996), most of the C decomposes relatively quickly due to environmental and mineralogical conditions, as indicated by the deepest incorporation of “bomb” C (Figures 3 and 6; Muñoz et al., 2023). In addition, because these soils are highly weathered (i.e., dominated by 1:1 clay minerals; Figure 1), the reactivity of these minerals, and therefore the adsorption of C by minerals, cannot be increased.…”

Factors driving carbon accumulation in forest biomass and soil organic carbon across natural forests and planted forests in China

“…Therefore, we recommend selective logging in natural forests for both biomass carbon and soil organic carbon accumulation. Furthermore, we recommend focusing more on soil organic carbon in the north and northwest China (Supplementary Figure 4), where the carbon sequestration in soil duration is longer due to the colder and drought climate (Muñoz et al, 2023). In addition, given the lower initial carbon stocks in north and northwest China (Supplementary Figure 3), this region has greater SOC sequestration potential.…”

On the importance of time in carbon sequestration in soils and climate change mitigation

“…Carbon fixed during photosynthesis returns to the atmosphere over a wide range of temporal scales involving phenomena with fast dynamics, such as respiration of simple photosynthates, and slow dynamics, such as organic matter transfers to soil and subsequent slow decomposition (Muñoz et al, 2023;Sierra et al, 2021;Trumbore, 2009). The multiple timescales of the processes and variables driving the carbon cycle can lead to significantly different effects of carbon sequestration on global warming mitigation, depending on when these effects are assessed.…”