U.S. Department of Energy, Office of Science

Program for Ecosystem Research

Research Project   Climatic change in arid lands: effects on soil biota and ecosystem processes

Principal investigators:   Jayne Belnap, Cheryl R. Kuske, and Deborah A. Neher

Project goal

Determine how soil biota and related processes (including nutrient uptake and growth by vascular plants) in dryland ecosystems may be affected by warming and changes in the timing and/or amount of precipitation.

Collema abundance declines with interannual warming

A negative correlation existed between summer maximum temperature (mean daily maximum temperature during June) and subsequent abundance of the lichen Collema, which dominates the biological soil crust in many semi-arid western ecosystems. Even after two "cool" years, the lichen had not recovered from the warm period of 1997-2003 (Belnap et al. 2006).

Ecosystem being studied

Dryland ecosystems of the desert Southwest, including grasslands and shrublands, with a focus on biologically active desert soil crusts and their associated organisms (see "Biological Soil Crusts: Ecology and Management" for more about soil crusts). The study site with experimental warming (see below) corresponds to the "blackbrush (Coleogyne)" vegetation type of Kuchler (1964).

Results

Comparisons of presently cool versus hot desert environments showed that the abundance and composition of soil microfauna in both cool and hot deserts vary with the flora found in their biological soil crusts. In addition, it was observed that while soil biota were concentrated under plant canopies (as is documented in the literature), this was not always true for soil nitrogen and phosphorus. This latter finding contrasts with previously published studies, and is likely explained by the presence of biological soil crusts.

In a one-year, elevated-temperature field experiment, large declines were observed in the abundance of soil lichens and mosses, soil bacteria, and sub-crust microfauna. A concurrent shift in community composition was also observed.

Additional experiments documented that: (a) anhydrobiosis (i.e., loss of cellular water accompanied by dormancy) of nematodes, which is possible in dry soil, may explain their abundance during hot summer months and that they are equally abundant across a range of winter temperature regardless of soil moisture; (b) nitrogen-fixing bacterial communities in soil crusts are seasonally stable; (c) carbon and nitrogen inputs to the underlying soil decline when crusts shift from being dominated by lichens and mosses to being dominiated by cyanobacteria; and (d) the dominant lichen Collema appears to be declining with summer warming during the past decade (figure to right).

Taken together, these results indicate that effects of climatic changes on biological soil crusts are likely to reverberate throughout the ecosystem due to substantial changes in soil fauna and nutrient availability to vascular plants.

moss

Moss off the plot (left; receiving ambient rainfall) remains green relative to moss on the plots receiving reduced rainfall. The orange color indicates loss of chlorophyll with reduced rainfall. The photo was taken after wetting of all the moss.

Why this is important

Drylands represent about 35% of land area of the United States (including deserts, shrublands, savannas, and woodlands), as well as about 35% of global land area. Biological soil crusts are the dominant living soil cover in many of these ecosystems, and are critical for nitrogen inputs (through atmospheric N2 assimilation) and carbon inputs into and onto the soil. These inputs support soil food webs and thus the mineral nutrition of vascular plants. Biological crusts also contribute to soil stability (reducing erosion) and water infiltration, and provide important local environments for plant seed germination. It is important to understand how these soil systems may be affected by future climatic changes because they are critical to the functioning of ecosystems (and the many goods and services that they provide) covering a large part of the nation.

Methods

Manipulative field studies (field chambers and transplants of intact biological soil crusts to differing climatic regions) and controlled-environment laboratory studies are being used to measure effects of warming and altered precipitation on (1) the physiology (e.g., CO2 exchange rates) of soil crusts, (2) changes in the relative abundance, activity, and composition of crust and rhizosphere bacteria important to carbon and nitrogen cycling, and (3) relative abundance and composition of nematodes, protozoa, and microarthropods in soils beneath crusts and associated with plant roots. The affect of these factors on nutrient cycling and vascular plant nutrition (plant tissue nutrient concentration) is being documented.

The field study site is near Moab, Utah. Five 10 x 30 m blocks were established perpendicular to an existing slope. Within each block, each of four treatments is assigned randomly to experimental plots. The treatments are: (1) warming (+2.5-3.0 degrees Celsius above ambient, measured at the crust/soil surface), (2) increased summer (May-September) precipitation, (3) warming and increased summer precipitation, and (4) ambient temperature and precipitation. The treatments will be imposed on 1 x 5 m plots. Plots are surrounded by edging penetrating to bedrock (about 0.3 m depth) to prevent immigration and emigration of soil biota. Warming is imposed with infrared lamps. The frequency of summer precipitation events is altered to approximately double the 40-year median frequency, and the size of events (amount of rain) is kept at the 40-year median. Timing and amount of natural rain events are monitored and compared to the 40-year median. Every other week, water is supplemented as needed using nozzles that approximate summer raindrop size for this region.

Measured variables in the field study include vascular plant leaf nitrogen concentration, plant growth, and plant seed set and seed viability.

Research site

The field research site, including infrared lamps used to heat experimental plots.

Personnel

Jayne Belnap, USGS - Southwest Biological Research Center

Cheryl R. Kuske, Los Alamos National Laboratory

Deborah A. Neher, University of Vermont

Dave Housman, USGS - Southwest Biological Research Center

Chris Yeager, Los Alamos National Laboratory

Brian Darby, University of Vermont

References

Belnap J, Phillips SL, Troxler TT (2006) Soil lichen and moss cover and species richness can be highly dynamic: the effects of invasion by the annual exotic grass Bromus tectorum, precipitation, and temperature on biological soil crusts in SE Utah. Applied Soil Ecology 32:63-76

Kuchler AW (1964) Manual to Accompany the Map: Potential Natural Vegetation of the Conterminous United States. American Geological Society (Special publication No. 36), New York.

Funding period:   May 2002 to present