Across a diverse range of terrestrial ecosystems, soils display a systematic vertical variation of biotic (e.g., roots) and abiotic (e.g., organic nitrogen) properties along the vertical dimension (i.e., stratification). Yet, due to logistics limitations, researchers often use measurements from the surface soil to represent the entire soil matrix, in both empirical and modeling practices. Recent findings from first-principle model derived from our group seem to suggest that these common practices might incur potentially huge mis-accounting of carbon budget in the face of rapid global change. It is thus imperative that we better understand the empirical dynamics of deep soil and the roots within.
Bringing together three large-scale databases that uniquely contain vertical profiles of roots and soil properties, we here evaluate three pressing questions regarding the ecology of deep roots in the face of global change: (1) Does the abundance of fine roots track the distribution of soil resources along the soil depth profile? (2) Are resources at deeper soil equally, under-, or over-exploited by roots compared with surface soil? (3) What are the biotic and abiotic constraints that limit the foraging behavior of deep roots?
We first characterized the vertical distribution of soil chemical properties, root abundance and chemical properties, root isotopic signal across sites within the National Ecological Observatory Network (NEON). We expanded our scope to analyze the patterns of depth distribution for soil resources and fine roots using openly available global datasets of soil profiles and roots.
Results/Conclusions
Our analyses reveal that fine roots distribution on average mirror the distribution of soil resources. Intriguingly, we frequently discovered rather unusual bimodal distribution of fine roots abundance along soil depth, and that is distinctively different from the ‘normal’ exponential decay of root abundance. Moreover, our results seem to suggest that subsoil resources are under-exploited by plant roots. We speculate that resource foraging of deep roots inside of soil matrix is fundamentally limited by the physical property of soil, and perhaps more importantly, the geometric design of plant rooting systems. In the face of rapid global change, however, our analyses suggest plants can potentially invest larger fraction of their photosynthates to tap into the previously under-exploited deep resources.