A continental scale analysis reveals widespread root bimodality

Summary paragraphRecent studies of plant fine roots have greatly advanced our understanding of their geometric properties and symbiotic relationships, but knowledge of how these roots are spatially distributed across the soil matrix lags far behind. An improved understanding of broad-scale variability in root vertical distribution is critical for understanding plant-soil-atmosphere interactions and their influence on the land carbon sink. Here we analyze a continental-scale dataset of plant roots reaching 2-meters depth, spanning 19 ecoclimatic domains ranging from Alaskan tundra to Puerto Rican neotropical forest. Contrary to the common expectation that fine root abundance decays exponentially with increasing soil depth, we found surprising root bimodality at ~20% of 44 field sites --a secondary peak of fine root biomass far beneath the soil surface. All of the secondary root peaks were observed deeper than 60cm (with 33% below 1m), far deeper than the sampling depth commonly used in ecosystem studies and forestry surveys. We demonstrate that root bimodality is more likely in places with relatively low total fine root biomass, and is more frequently associated with shrubland vegetation but less with grassland. Further statistical analyses revealed that the secondary peak of root biomass coincided with unexpected high soil nitrogen contents at depth. By linking roots and nutrient distributions, we further demonstrate that deep soil nutrients tend to be underexploited by plant rooting systems, yet root bimodality offers a unique mechanism by which fine roots can tap into soil resources in the deep. Our findings suggest that empirical practices have often systematically overlooked root dynamics in deep soils, and as a result the current-generation global climate and vegetation models have relied on overly simplistic assumptions for plant rooting distribution.