THE INFLUENCE OF BEDROCK NUTRIENT CONCENTRATIONS ON LIFE AND TOPOGRAPHY IN THE SIERRA NEVADA BATHOLITH

The breakdown of rock to regolith promotes erosion, liberates ions from minerals, and creates porosity, thus providing overlying ecosystems with access to life-sustaining water and nutrients. Understanding variations in regolith thickness and development across landscapes is a fundamental problem in critical zone science and management. Yet the factors that regulate regolith production, weathering, and thickness are poorly understood.

In granitic landscapes, for example, regolith cover is often mysteriously dichotomous. A case in point is the Sierra Nevada Batholith, where broad swaths of bare rock crop out immediately next to vegetated slopes with weathered regolith extending to depths of up to 40 m. Here we present evidence of a feedback in which this dichotomous presence/absence of regolith is regulated by bedrock nutrient content through its influence on vegetation. The concentrations of essential plant nutrients such as phosphorus (P) vary markedly in bedrock, changing abruptly at pluton contacts that coincide with ecotones between forest and bare rock. Moreover, we find that vegetative cover is strongly correlated with bedrock P in a multivariate, batholith-wide analysis that accounts for potentially confounding variations in climate and topography and excludes areas that were glaciated in the Pleistocene.

The paradigm for nutrient evolution in physically stable soils is that bedrock-derived P becomes depleted with time to the point that it limits productivity of overlying ecosystems. Our analysis of the Sierra Nevada presents a counterexample of an eroding landscape in which ecosystems developed on nutrient-poor granitic bedrock may experience P limitation under conditions of dynamic equilibrium and during incipient stages of weathering and pedogenesis. Our work challenges the common assumption that granitic bedrock can be viewed as a uniform substrate for pedogenesis and chemical weathering, irrespective of compositional differences from pluton to pluton; we find that differences in P, a minor element with concentration <0.01 g/g in bedrock throughout the batholith, appear to regulate the presence/absence of regolith and thus may also influence rates of soil formation, erosion, and landscape evolution.