Depletion of geogenic nutrients during soil weathering can prompt vegetation to rely on other sources, such as organic matter (OM) decay, to meet growth requirements. Weathered soils also tend to permit deep rooting, a phenomenon sometimes attributed to vegetation foraging for geogenic nutrients. This study examines the extent to which OM recycling provides nutrients to vegetation growing in soils with diverse weathering states. We thus address the fundamental problem of how forest vegetation obtains sufficient nutrition to support productivity despite wide variation in soils’ nutrient contents. We hypothesized that vegetation growing on highly weathered soils relies on nutrients released from OM decay to a greater extent than vegetation growing on less weathered, more nutrient-rich substrates. For four mineralogically diverse Critical Zone Observatories (CZO) and Critical Zone Exploratory Network sites, we calculated weathering indices and approximated vegetation nutrient demand and nutrient release from OM decay. We also measured nutrient release rates from OM decay at each site. We then assessed the relationship between degree of soil weathering and the estimated fraction of nutrient demand satisfied by OM derived nutrients.
Results are consistent with our hypothesis. The chemical index of alteration (CIA), a weathering index that increases in value with mineral depletion, varies predictably from ~90 at the highly weathered Calhoun CZO to ~60 at the Catalina CZO, where soils are more recently developed. Estimates of rates of K release from OM decay increase with CIA values. The highest release rate is ~2.4 gK m-2 y-1 at Calhoun, accounting for ~30% of annual vegetation K uptake; at Catalina, less than 0.5 gm-2 y-1 K is released, meeting ~14% of vegetation demand. CIA also co-varies with rooting depth across sites: the deepest roots at the Calhoun sites are growing in soils with the highest CIA values, while the deepest roots at Catalina sites are growing in soils with much lower CIA values. Thus, provision of plant-available nutrients from OM decay appears greater at more weathered sites, and dominant nutrient sources accessed by deep roots (OM- vs. rock-derived) may vary predictably with soil weathering stage. On-going incubations will permit us to assess these relationships for multiple geogenic nutrients.
Hauser, Emma, and Sharon A Billings (2017): Illuminating pathways of forest nutrient provision: relative release from soil mineral and organic pools. American Geophysical Union 2017 Fall Meeting, New Orleans, Louisiana, 11-15 December 2017.
This Paper/Book acknowledges NSF CZO grant support.