Vital Signs: Geology and Soils

Soil Quality – Soil Chemistry and Nutrient Cycling

Soil chemical properties provide the foundation for plant growth in conjunction with soil moisture. Nutrient cycles are essential ecosystem processes and the linkages to decomposition are complex and important. Ecosystems on stable trajectories have biological interactions that tend to conserve key nutrients. Significant loss or gain of elements, such as acidification or nitrification, are good indicators of change. Increased levels of soil nitrogen caused by atmospheric nitrogen deposition may increase the dominance of invasive plant species, particularly invasive grasses, and decrease the density, biomass, and species richness of native plant communities.

Monitoring Questions:

1. What are the long-term trends in content and concentrations of different soil nutrients?
2. How do the long-term trends in nutrient content and concentrations relate to vegetation dynamics and climate variability?
3. Is the species composition, abundance, and distribution of native plant communities changing over time in areas subject to increased atmospheric nitrogen deposition?

Soil Quality – Hydrologic Function

In deserts, geology and soils provide the template upon which biota build integrated ecological systems. The availability of water is crucial, and small variations in available water can drastically alter plant and animal communities. Both physical and chemical geologic attributes commonly control important variation in water availability. Surficial geologic deposits vary in soil texture, bulk density, and other factors. Soil texture is critical in the moisture budget because coarse soils generally have higher infiltration rates whereas fine grained soils have lower infiltration rates. Soil type in conjunction with plant communities and their dynamics, topography, and climate regimes are primarily responsible for broad scale differences in soil moisture across the landscape. Plant-available soil moisture is a key to understanding ecosystem maintenance in desert ecosystems.

Monitoring Questions:

1. Is the vertical distribution or horizontal extent of plant-available soil moisture changing over time?
2. Is vegetative community structure change over time in response to changes in plant-available soil moisture?
3. Are observed changes in plant-available soil moisture due to background natural variability or anthropogenic disturbances?

Soil Quality – Soil Erosion and Deposition

Loss of topsoil changes the capacity of soil to function and restricts its ability to sustain future uses. Erosion removes or redistributes topsoil, the layer of soil with the greatest amount of organic matter, biological activity and nutrients. Erosion breaks down soil structure exposing organic matter within aggregates to accelerated decomposition and loss. Degraded soil structure reduces the rate of water infiltration and increases runoff which can lead to further erosion. Erosion of nutrient rich topsoil can cause a shift to less desirable plants including invasive plant species.

Monitoring Questions:

1. Is the patch size, distribution, and degree of soil erosion and deposition in parks changing over time?
2. Are the mechanisms (e.g., wind, water) of soil erosion and deposition changing over time?
3. Is plant community composition, abundance, and distribution changing over time in areas with altered patterns of soil erosion or deposition?

Soil Quality – Soil Disturbance

Disturbance of the soil surface is a natural process that can be exacerbated by anthropogenic activities such as grazing, off-highway vehicle use, and mining. Disturbance of the soil surface results in increased invasive plant species cover, dust generation, surface runoff, erosion, increased bare ground, and decreased soil organic matter, all of which may negatively affect burrowing mammal habitat. Post-settlement changes in plant communities have changed the extent of bare ground and accelerated soil loss. Research in the eastern Mojave Desert indicates that disturbed soils may produce up to 36 times more sediment when compared to undisturbed sites.

Monitoring Questions:

1. What are the long-term trends in patch size, distribution, and degree of physical soil surface disturbance in the parks?
2. Are the mechanisms of physical disturbance changing over time?
3. How do the long-term trends in disturbed soil surfaces relate to vegetation dynamics and climate variability?

Soil Quality – Biological Soil Crust Dynamics

Crusts generally cover most soil spaces not occupied by green plants. The main components of soil crusts are cyanobacteria, bryophytes, and lichens. In many areas, they comprise over 70% of the living ground cover and are key in reducing erosion, increasing water retention, and increasing soil fertility. Because plant cover is sparse, crusts are an important source of organic matter for desert soils. Large scale disturbance of biological soils crusts poses a significant threat to ecosystem integrity by increasing soil loss, increasing the rate of water loss, and reducing soil fertility all of which may alter plant and animal communities.

Monitoring Questions:

1. Is the quality, patch size, distribution, and abundance of biological soil crusts in different vegetation communities, climate zones, disturbance regimes, and soil types changing over time?
2. Are the types of mechanisms that disrupt biological soil crusts changing over time?