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Understanding patterns of root uptake using strontium isotopes, root and soil nutrient distributions, and modeling.
McCulley, Rebecca*,1, Jobbagy, Esteban2, Pockman, Will3, Jackson, Robert1, 1 Duke University, Durham, NC, USA2 Universidad de Buenos Aires, Buenos Aires, Argentina, Argentina3 University of New Mexico, Albuquerque, NM
ABSTRACT- Broad geographic trends in root biomass distributions across climatic gradients and for plant functional types do not indicate patterns of root activity or function. In an attempt to assess root uptake patterns, we sampled native semi-arid and sub-humid grasslands (to a depth of 7 m) for root biomass, soil nutrient concentrations (P, C, N, K, Ca, Mg, Cl and Na), and strontium isotopic signatures (from both plant material and soil). The strontium isotopic data indicate that the integrated depth of cation uptake by the grassland species decreases with increasing mean annual precipitation from an average integrated depth of 2.8 m at the most arid location to 0.1 m at the most humid. Grassland root biomass was highly concentrated in the top 1 m of soil (accounting for 94.6 - 99.7% of total root biomass across sites), but roots were encountered 8 m deep at the driest site. Soil chloride profiles, environmental indicators of long-term water movement, indicate that majority of the soil water entering these ecosystems as precipitation does not percolate below 2 m of depth. Biologically-cycled nutrients were highly concentrated in the top 0.25 m of soil, with a concentration factor ranking of C > N & P > K > Ca & Mg. Consistent with previous work, our data suggest that through biological cycling limiting nutrients are removed from the soil at depth and concentrated at the soil surface. Our data reveal that nutrient uptake occurs surprisingly deeply in aridland soils (> 2 m), perhaps as a result of deep roots for water uptake.
Key words: nutrient distributions, rooting depth, strontium isotopes, grasslands