Fine root and carbon cycle relationships across broad scales: what can we infer about rhizosphere function?

Roots regulate a variety of carbon cycle processes in ecosystems. I will discuss the scope of inferring rhizosphere function from broad spatial scale analyses of root traits and carbon cycle state factors. In the first example, fine root carbon (FRC) in soils is typically hypothesized to be positively related to soil organic carbon (SOC). However, FRC inputs can also enhance SOC loss through priming. We tested the broad-scale relationships between SOC and FRC at 43 sites across the US National Ecological Observatory Network (Malhotra et al. 2025). We found that SOC and FRC stocks were positively related with an across-ecosystem slope of 7 ± 3 kg SOC m⁻² per kg FRC m⁻², but this relationship was driven by grasslands. Grasslands had double the slope compared to the across-ecosystem slope while forest FRC and SOC were unrelated. Furthermore, deep grassland soils primarily showed net SOC accrual relative to FRC input. Conversely, forests had high variability in whether FRC inputs were related to net SOC priming or accrual. We conclude that while FRC increases could lead to increased SOC in grasslands, especially at depth, the FRC-SOC relationship remains difficult to characterize in forests; suggesting a disproportionate role of priming in shaping forest SOC. In addition to regulating SOC, roots influence trace gas production in ecosystems. I will also discuss examples relating root form to methane function in wetlands (Määttä and Malhotra 2024), highlighting the elusive role of root exudation in methanogenesis. 

Citations:

Malhotra A,  JAM Moore, S Weintraub-Leff, K Georgiou, AA Berhe, SA Billings, M-A de Graaff, JM Fraterrigo, AS Grandy, E Kyker-Snowman, M Lu, C Meier, D Pierson, SJ Tumber-Dávila, K Lajtha, WR Wieder & RB Jackson. Fine root and soil carbon stocks are positively related in grasslands but not in forests. Communication Earth & Environment 6, 497 (2025). https://doi.org/10.1038/s43247-025-02486-9

Määttä T and A Malhotra, The hidden roots of wetland methane emissions (2024). Global Change Biology. 30, e17127