Morford, S.L., Houtlon, B.Z., Dahlgren, R.A. XXXX. Direct quantification of longterm rock nitrogen inputs to temperate forest ecosystems. Ecology.
Appendix S4. Extended discussion of interpretation of chemical depletion data presented in the
manuscript.
The bedrock element depletion data provided in the manuscript for elements N, Na, and
K suggest that biological processes may be an important factor in regulating rock N weathering
via mycorrhizal targeting of N-bearing minerals. Here we provide further discussion of that data
and rationale from a geochemical and soil chemistry perspective.
The mineralogy of the bedrock found at these sites is an assemblage of quartz, albite,
chlorite, and muscovite/sericite. Sodium is found in albite minerals, while K and N (as NH4+) are
found in muscovite/sericite. Field based investigations of mineral stabilities in the soil
weathering environment (Goldich 1938) indicate that dissolution rates of albite (Na bearing) is
more rapid than muscovite (K and N bearing), and these field based trends are further supported
by lab based geochemical kinetics studies that indicate that the dissolution rate of albite is
roughly an order of magnitude more rapid than muscovite under acidic soil conditions (Brantley
2008). Among our sites, if mineral weathering were being driven solely by geochemical kinetics,
we would expect to observe higher depletion rates of Na vs. K and N.
However, using bulk soil geochemistry to infer differential mineral weathering is
complicated by biological cycling and adsorption of elements to secondary soil minerals (i.e.
clays and oxides) and soil organic matter. First, potassium is an essential nutrient for all plants
and microbes while Na is utilized more sparingly by biology (e.g. C4 plants – absent from our
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study sites), suggesting that K would be more rapidly cycled among biological reservoirs, and
less susceptible to leaching loss than Na. Further, the larger hydrated radius of ionic Na+ vs. K+ in
soil solution favors adsorption of K+ to charged soil surfaces (Sparks 2003), resulting in
relatively more Na+ being partitioned to the soil solution, and thus being more susceptible to
leaching losses among soils developing in wet environments. So while higher depletion of Na vs.
K in soil is consistent with more rapid weathering of Na bearing vs. K bearing minerals at our
study sites, these trends could also be partially attributable to relatively higher retention of K vs.
Na among soil mineral and biological reservoirs.
Comparison of N and K depletion in these soils is complicated by our methodology.
Geochemical measurements of K comprise the bulk soil reservoir (including K in organic
matter), while the N data represents only fixed-N within minerals as a result of our hypobromite
digestion procedure. We did not evaluate K abundance in soil organic matter, so depletion of K
from primary (native rock) minerals may be higher than our mass balance analysis suggests.
Thus, we are hesitant of drawing conclusions about the relative depletion of N vs. K at these
sites.
A more robust interpretation of the elemental depletion data is derived from comparisons
of Na vs. N, as these data primarily reflect the mineral reservoirs of these elements. Across all of
our plots, N depletion was higher than Na, which is counter to the expected trend derived from
geochemical kinetics, and points to the possibility of preferential biological weathering of N-rich
minerals. We speculate that mycorrhizae may target N-bearing minerals, similar to mycorrhizal
mining of other essential rock nutrients (see references in main text), but the extent to which this
is occurring requires direct investigation of biological factors. Further, additional quantification
of K partitioning between mineral and organic matter reservoirs in soil may help bolster the
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interpretation of these data, but much of this depends on whether K and N are distributed
uniformly within the mica minerals, or whether micas contain domains that are preferentially
enriched in either K or N.
References
Brantley, S. L. 2008. Kinetics of mineral dissolution in Brantley, S. L., J. D. Kubicki, A. F.
White (Eds.) Kinetics of Water-Rock Interaction. 151-210. Springer. New York.
Goldich, S. S. 1938. A study in rock-weathering. Journal of Geology 46:17-58.
Sparks, D. L. 2003. Environmental soil chemistry. Academic press. Amsterdam.
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