Written by Sabrina Kleinman, University of Arizona
While the majority of climate change impacts on forests focus on tree health, soil impacts should not be overlooked. A changing climate can impact nutrient cycling, ecosystem respiration, and the storage of carbon in forests. While global models predict that climate change can increase global net primary production (NPP), regional variations in climate, nutrient availability, and water will have the largest impact on tree growth locally (Melillo et al. 1993). Most research focuses on how soils respond to either elevated CO2 concentrations or elevated temperatures.
Effects of rising CO2 concentrations
Increased belowground biomass — Elevated CO2 can increase plant biomass, which also stimulates fine root growth and root secretions in soils (Pendall et al. 2004). Increased root growth generates more carbon belowground, which can help accelerate decomposition and nutrient cycling.
Increased soil respiration — Increased carbon belowground can increase root respiration rates and soil microbes (Schlessinger and Andrews 2000). This increase in CO2 from soils limits the amount of carbon stored there (Schlessinger and Lichter 2001). The magnitude of increased respiration varies due to differences in microbial communities, availability of carbon, and plant growth rates. Areas experiencing significant increases in NPP can still be important carbon sinks, despite increases in soil respiration (Sullivan et al. 2008).
Altered decomposition rates — Higher atmospheric concentrations of CO2 can alter the amount of nitrogen in leaf litter, which strongly influences decomposition (Melillo et al 1982). Changes in nitrogen from leaf litter can increase lignin decomposition and decrease organic matter decomposition (Norby et al. 2001). These alterations could influence soil carbon availability, alter microbial communities, and change nutrient availability for growing trees and plants.
Effects of rising temperatures
Increased soil respiration — Rising temperature increases the rate of chemical and biochemical reactions. For soil microbes and roots, that also means increasing respiration, which can release more carbon dioxide and even methane from soils. Increasing these gases from soils can contribute to existing atmospheric greenhouse gas emissions. However, as mentioned previously, forests that are also actively sequestering carbon can help offset some of these emissions and remain carbon sinks.
Loss of stored soil C — Increased decomposition means higher carbon turnover in soils, which could reduce the carbon available for growing plants. While most plants get a majority of their carbon through photosynthesis, soil carbon is important for plant growth and maintenance. Soil carbon helps soils retain water and important nutrients, as well as serves as an energy source for decomposing organisms. This carbon could be lost through increased soil respiration, which would release a higher concentration of CO2 into the atmosphere (Melillo et al. 2002, Davidson and Janssens 2006, Conant et al. 2008). Additionally, increased decomposition rates can lower carbon stored as organic matter, which is an important source of carbon for biomass production.
Higher rates of N mineralization — Increased temperature can increase nitrogen availability through higher turnover of soil nitrogen. In many North American ecosystems, increasing N availability can help increase carbon sequestration by encouraging higher rates of plant growth (Melillo et al. 1993). However, in areas where excess nitrogen is a major concern, such as Southern California, the Rocky Mountains, or the eastern United States, this priming effect could increase N losses from ecosystems. Such losses would create other ecological problems in these ecosystems, such as eutrophication in nearby streams or lower pH in soils (Saxe et al. 2001, Galloway et al. 2003). Monitoring nitrogen status of an ecosystem is important for assessing how nitrogen availability is being affected by climate change.
Effects of altered water availability
How climate change will alter water availability is a major concern. Some climate models predict longer periods of drought followed by periods of increased precipitation intensity. However, there is little consensus on how predictable or widespread such scenarios may be. The following soil concerns may be impacted by altered precipitation patterns:
Increased erosion risk — Increased erosion risk stems from two potential precipitation changes. Heavier rainfall events can increase runoff and erosion due to the limited ability of soils to absorb and retain water. Prolonged droughts may decrease plant cover, making it harder to retain soils and organic materials during heavy rainfalls or windstorms. Large-scale erosion events can lead to unintentional landscape conversions and the loss of stored carbon in many areas as well as inhibiting plant productivity.
Nutrient loss — The topsoil and organic layer of soils contain most of the nutrients needed to help sustain plant growth. Nutrients can be lost from increased erosion or from changes in aridity. Increased aridity can inhibit surface decomposition and nutrient cycling, decreasing plant productivity. Increased erosion during heavy rainfalls can also quickly deplete soil organic material (Nearing et al. 2004). It is unclear how possible changes in biomass production and land use will impact erosion rates.
Overall, these impacts highlight the need for landowners and resource managers to monitor nutrient availability where climate change may be affecting forest ecosystem health. Nutrient cycling and decomposition processes are controlled by multiple factors, but changes in temperature, water availability, and CO2 can have significant impacts on tree growth, ecosystem respiration, and carbon storage.
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