Vertical partitioning of root distributions correlated with ecophysiological characteristics: Initial results from NEON megapit data

Abstract:

Variation in the δ¹³C of plant roots can provide insight into soil carbon dynamics and plant community composition, yet how root δ¹³C varies with depth within and across sites is poorly understood. Here, we leverage the National Ecological Observatory Network (NEON) to characterize vertical patterns in root δ¹³C across 43 sites and identify potential drivers of observed patterns. Live root samples from the site construction pits were classified as shallow (deep) if their depth was within the top (bottom) 25^th percentile of the empirical sampling depths. Patterns of site-level mean δ¹³C of shallow and deep roots (δ¹³C_shallow-deep) were examined across C3/C4 category and an annual aridity index (precipitation/potential evapotranspiration). Most notably, C3-C4 mixed sites had significantly larger δ¹³C_shallow-deep than C3-only sites (p < 0.001) and the values were primarily positive, suggesting the greater contribution of C4 grass roots at shallow depths. Maximum δ¹³C_shallow-deep occurred at Ordway-Swisher Biological Station, where deeply-rooted C3 trees coexist with shallowly-rooted C4 grasses. Across C3-only sites, δ¹³C_shallow-deep was significantly positively correlated with aridity (p < 0.05), with minimum values occurring at San Joaquin Experimental Range and Onaqui. We interpret the negative values of δ¹³C_shallow-deep at dry C3-only sites to reflect differences in intrinsic water use efficiency (iWUE) for shallowly- and deeply-rooted plants. Because deeply-rooted, largely perennial species must exert greater stomatal control during the dry summer season, they have higher iWUE and root δ¹³C than shallowly-rooted species that can avoid drought through deciduous leaf habit or annual life cycle. These results suggest systematic, environmentally-driven correlations between physiology and vertical root distribution, with implications for changing subsurface carbon allocation and resource partitioning under changing climatic and land management conditions.