Examining root-derived soil organic matter formation in the light of root branching order and mycorrhizal status

Background/Question/Methods

In woody plants, there is growing evidence that the most distal first and second order roots, which are involved in water and nutrient absorption, decompose slower than higher order transport roots despite their small size, ephemeral nature, and comparatively high nutrient concentrations. However, first and second order roots are often colonized by (EcM) fungi or arbuscular mycorrhizal (AM) fungi, which may modify root tissue chemistry and slow decay. Moreover, it is unclear if differences in decay among root branching orders and mycorrhizal types influences the formation and persistence of soil organic matter (SOM). Given that plant roots contribute substantially to the formation of stable SOM, more studies are needed to determine (1) how root mycorrhizal status (type and colonization rate) relates to root carbon (C) quality and (2) whether mycorrhizal roots contribute more or less to stable SOM formation. We compared chemistry, decay and SOM dynamics between lower-order mycorrhizal roots (orders 1-2) and higher order transport roots (orders 3-4) for six AM- and six EcM-associated temperate broadleaf tree species. We incubated root litters in an isotopically distinct soil and tracked the flow of root C into soil respiration and mineral-associated organic matter (MAOM; a slower-cycling SOM pool) after 30 days and 250 days.

Results/Conclusions

We found that differences in root C quality and nitrogen content were greater between root branching orders than mycorrhizal types; first and second order roots produced lower quality litter with reduced amounts of soluble C and a greater acid insoluble fraction when compared with higher order roots. Tree species differed in their rates of mycorrhizal colonization, but colonization rate did not relate to initial root chemistry. However, soil respiration and C mineralization rates differed between AM and EcM-associated root litters and depended on root branching order and stage of decay. Early on in decay, soil respiration rates were greatest for lower order roots colonized by AM fungi and higher order roots from EcM-associated tree species. Later in decay respiration rates were greater for higher order roots compared to mycorrhizal roots for all tree species. Fast-decomposing litters led to greater MAOM formation and root N was a poor predictor of MAOM-C formation. This study highlights the potential for root branching orders to differ in their root C quality and the potential for transportive rather than mycorrhizal roots to contribute to stable SOM formation.