M. S. Schulz
D.V. Vivit
T.D. Bullen
J. Fitzpatrick
A. F. White
2012
<p id="sp005">Biotic/abiotic interactions between soil mineral nutrients and annual grassland vegetation are characterized for five soils in a marine terrace chronosequence near Santa Cruz, California. A Mediterranean climate, with wet winters and dry summers, controls the annual cycle of plant growth and litter decomposition, resulting in net above-ground productivities of 280–600 g m<sup>−2</sup> yr<sup>−1</sup>. The biotic/abiotic (A/B) interface separates seasonally reversible nutrient gradients, reflecting biological cycling in the shallower soils, from downward chemical weathering gradients in the deeper soils. The A/B interface is pedologically defined by argillic clay horizons centered at soil depths of about one meter which intensify with soil age. Below these horizons, elevated solute Na/Ca, Mg/Ca and Sr/Ca ratios reflect plagioclase and smectite weathering along pore water flow paths. Above the A/B interface, lower cation ratios denote temporal variability due to seasonal plant nutrient uptake and litter leaching. Potassium and Ca exhibit no seasonal variability beneath the A/B interface, indicating closed nutrient cycling within the root zone, whereas Mg variability below the A/B interface denotes downward leakage resulting from higher inputs of marine aerosols and lower plant nutrient requirements.</p><p id="sp010">The fraction of a mineral nutrient annually cycled through the plants, compared to that lost from pore water discharge, is defined their respective fluxes<span> </span><i>F</i><sub>j,plants</sub> = <i>q</i><sub>j,plants</sub>/(<i>q</i><sub>j,plants</sub> + <i>q</i><sub>j,discharge</sub>) with average values for K and Ca (<i>F</i><sub>K,plants</sub> = 0.99;<span> </span><i>F</i><sub>Ca,plants</sub> = 0.93) much higher than for Mg and Na (<i>F</i><sub>Mg,plants</sub><span> </span>0.64;<span> </span><i>F</i><sub>Na,plants</sub> = 0.28). The discrimination against Rb and Sr by plants is described by fractionation factors (<i>K</i><sub>Sr/Ca</sub> = 0.86;<span> </span><i>K</i><sub>Rb/K</sub> = 0.83) which are used in Rayleigh fractionation-mixing calculations to fit seasonal patterns in solute K and Ca cycling.<span> </span><i>K</i><sub>Rb/K</sub><span> </span>and<span> </span><span class="math"><span id="MathJax-Element-1-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msub is="true"><mrow is="true"><mi is="true">K</mi></mrow><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">24</mn></mrow></msup><mtext is="true">Mg</mtext><mo is="true">/</mo><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">22</mn></mrow></msup><mtext is="true">Mg</mtext></mrow></msub></mrow></math>"><span class="MJX_Assistive_MathML">K24Mg/22Mg</span></span></span><span> </span>values (derived from isotope data in the literature) fall within fractionation envelopes bounded by inputs from rainfall and mineral weathering.<span> </span><i>K</i><sub>Sr/Ca</sub><span> </span>and<span> </span><span class="math"><span id="MathJax-Element-2-Frame" class="MathJax_SVG" data-mathml="<math xmlns="http://www.w3.org/1998/Math/MathML"><mrow is="true"><msub is="true"><mrow is="true"><mi is="true">K</mi></mrow><mrow is="true"><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">44</mn></mrow></msup><mtext is="true">Ca</mtext><mo is="true">/</mo><msup is="true"><mrow is="true" /><mrow is="true"><mn is="true">40</mn></mrow></msup><mtext is="true">Ca</mtext></mrow></msub></mrow></math>"><span class="MJX_Assistive_MathML">K44Ca/40Ca</span></span></span><span> </span>fractionation factors fall outside these envelopes indicating that Ca nutrient cycling is closed to these external inputs. Small net positive K and Ca fluxes (6–14 mol m<sup>−2</sup> yr<sup>−1</sup>), based on annual mass balances, indicate that the soils are accumulating mineral nutrients, probably as a result of long-term environmental disequilibrium.</p>
application/pdf
10.1016/j.gca.2011.10.029
en
Elsevier
The impact of biotic/abiotic interfaces in mineral nutrient cycling: A study of soils of the Santa Cruz chronosequence, California
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