Meijie Xi and Weile Chen from Zhejiang University, China, discuss their article: Soil moisture mediates the effect of plant below-ground carbon allocation on the decomposition of root litter in a subtropical forest
Forests are critical allies in the fight against climate change because they are major carbon sinks. Yet, the carbon stored in forests is unstable, maintained by complex interactions between above-ground and below-ground processes. What happens when this above-below-ground connection is disrupted? We investigated how cutting off the allocation of photosynthate to roots influences the decomposition of leaf and root litter in a subtropical forest—and discovered that soil moisture plays a surprisingly central role.
Disrupting Carbon Flow: A Novel Approach
Trees channel a substantial amount of their carbon below-ground to support roots and symbiotic mycorrhizal fungi, fueling vital soil processes like litter decomposition. This process not only recycles nutrients but also regulates the release of carbon back into the atmosphere.
To simulate disruptions to below-ground carbon flow, we used a precise method called transport-root girdling. By removing a thin section of phloem from major tree roots, we stopped carbon transport to distal absorptive roots and their associated fungi—while preserving short-term water flow. This allowed us to isolate the effects of carbon disruption on litter decomposition without introducing other variables. We then measured how decomposition rates of leaf and root litter responded under different soil moisture conditions.
Soil Moisture: The Key Factor
1. Our findings revealed distinct responses for leaf and root litter decomposition:
Leaf litter decomposition slowed overall, reflecting the reduced availability of carbon inputs that decomposer organisms rely on.
Root litter decomposition showed contrasting patterns, accelerating in dry soils but slowing in wetter ones.
2. Soil moisture may determine whether microbes rely on root exudates or litter as their primary carbon source, which in turn shapes decomposition dynamics.
In dry soils, roots release more carbon-rich exudates—”microbial fast food.” When this supply is cut off by girdling, microbes turn to tougher litter as an alternative, accelerating its decomposition.
In wet soils, roots release fewer exudates, and microbes depend on litter for carbon. In this scenario, small amounts of exudates act as catalysts, enhancing microbial activity in a phenomenon known as the “priming effect.” Girdling halts this catalytic effect, slowing decomposition.
Why It Matters
These results underscore the importance of environmental factors like soil moisture in shaping how forests respond to disruptions. While much attention has been given to the role of tree species or fungal partners in decomposition, our study highlights the need to also consider local soil conditions.
Natural and human-induced disturbances, such as herbivory, disease, wind-throw, and logging, are increasingly disrupting photosynthesis on regional and global scales, reducing below-ground carbon allocation. As climate change intensifies, with more erratic rainfall and prolonged droughts, it becomes critical to understand how the interplay between carbon allocation and soil moisture shapes below-ground processes. Will forests maintain their role as carbon sinks, or could they transition to becoming carbon sources? Our findings offer valuable insights to inform predictions and guide mitigation strategies for these potential shifts.
Looking Ahead
This study reminds us of the hidden complexities beneath our feet and their significance in maintaining forest ecosystems. It also calls for further research into how local soil variability impacts carbon cycling and decomposition. By advancing this knowledge, we can better protect forests and the essential climate-stabilizing services they provide.