Soil Biogeochemistry in the Critical Zone
Marjorie Schulz, Corey Lawrence, Dave Stonestrom
Jennifer Harden, Kate Maher (Stanford), Mark Waldrop, Jack McFarland, Kristen Manies, Ron Amundson (UC-Berkeley), Art White (emeritus), Tom Bullen, and Alex Blum
USGS Climate, Research and Development Program for Climate and Land use Change
The focus of this project is to quantifying biogeochemical processes in the critical zone with an emphasis on the interactions between soil, water, and plants. Specifically, we explore the role of increased atmospheric CO2
and climate variability on mineral weathering, soil chemistry, and the resulting influence of these factors on water chemistry. Our work directly addresses the hypothesis that increased atmospheric CO2
may increase mineral weathering which would then allow these soils to become a sink for CO2
. In addition, we quantify the linkages between mineral weathering and other dynamic processes, with a focus on soil organic matter cycling. We make use of natural soil chronosequences to examine these and other biogeochemical processes, which span seasonal to millennial timescales, employing a variety of biogeochemical analyses, isotopic tracers, and mathematical models.
Results to date
Soils in Mediterranean climates develop argillic (clay rich) or hardpan (amorphous-silica rich) horizons that buffer the deeper (>1.5 m) unsaturated zone from drying during the dry season. The development of this buffer creates an important water resource for ecosystems during the dry season.
Soils in Mediterranean climates have high (>1%) soil CO2 concentrations below hardpan and argillic horizons implying a large subsurface sink/source of CO2.
Iron isotope evidence suggests plants and their mycorrhizal fungal symbionts have, over time, contributed to substantial redistribution of Fe in soil profiles.
We have developed quantitative reaction model models that may increase predictive capability for climate change. This modeling work has shown that field and laboratory mineral weathering rates can be reconciled when the appropriate mechanisms are considered.
Iron nodules in the Santa Cruz terrace soils are biologic in origin.