Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solarto-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plantmicrobe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P CO2 will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer Ó 2010 Blackwell Publishing Ltd 1 Geobiology (2011) DOI: 10.1111DOI: 10. /j.1472DOI: 10. -4669.2010 timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

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Highlights • Light Cu, and unfractionated Zn, isotopes retained in oxic soils. • Development of anaerobic conditions can lead to loss of this signature. • Mineral aerosol addition and biological cycling overprint weathering processes. • Oxic soils contain Cu-Zn isotopes complementary to riverine dissolved pool.

The input of phosphorus (P) through mineral aerosol dust deposition may be an important component of nutrient dynamics in tropical forest ecosystems. A new dust deposition calculation is used to construct a broad analysis of the importance of dust-derived P to the P budget of a montane wet tropical forest in the Luquillo Mountains of Puerto Rico. The dust deposition calculation used here takes advantage of an internal geochemical signal (Sr isotope mass balance) to provide a spatially integrated longer-term average dust deposition flux. Dust inputs of P (0.23 ± 0.08 kg ha -1 year -1 ) are compared with watershed-average inputs of P to the soil through the conversion of underlying saprolite into soil (between 0.07 and 0.19 kg ha -1 year -1 ), and with watershed-average losses of soil P through leaching (between 0.02 and 0.14 kg ha -1 year -1 ) and erosion (between 0.04 and 1.38 kg ha -1 year -1 ). The similar magnitude of dust-derived P inputs to that of other fluxes indicates that dust is an important component of the soil and biomass P budget in this ecosystem. Dust-derived inputs of P alone are capable of completely replacing the total soil and biomass P pool on a timescale of between 2.8 ka and 7.0 ka, less than both the average soil residence time (*15 ka) and the average landslide recurrence interval (*10 ka).

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