The effects of coastal prairie restoration and drought on soil carbon

August 31, 2021

By Hallie Holmes 

There are many reasons to restore the native coastal grasslands of California, both on a local and global scale. In facilitating the decolonization of invasive species, restoration helps with resisting the destruction of biological and cultural diversity. Globally, there are potential benefits to restoring these ecosystems far beyond the scope of the California coast. Our research explores the relationship between California coastal prairie restoration and carbon mitigation in the context of climate change induced drought.

About the experiment:

Previous work has shown that global carbon stored in soil, fluctuates. These fluctuations can have impacts on atmospheric carbon. Atmospheric carbon can directly influence climate change. Our hypothesis was that native grassland species (many which are drought tolerant) would sequester (capture) more carbon than invasive species. It is likely that climate change will increase drought conditions and since many native plants are drought tolerant they could be more resistant or resilient to climate induced changes in soil carbon storage.


Experiment set-up:

The experimental plots are located at Younger Lagoon Reserve (YLR). YLR has been dominated by invasive species after being used for grazing agriculture. We studied both restored and unrestored grassland plots. The unrestored plots were established in 2015 restored plots in 2016. We simulated drought conditions utilizing rain-out shelters. These structures divert 60% of rainfall from the plots. With these experimental conditions, we created four different plot types: unrestored ambient rain, unrestored drought, restored ambient rain, and restored drought. 


YLR soil samples were taken in 2015 when establishing the drought treatment, in 2016 while establishing the restoration treatment, and in 2018, and in 2020 at multiple depths in the soil column. Carbon content and organic matter in the soil were measured using loss on ignition. Loss of ignition burns a known mass of a soil sample at a given temperature, and the mass loss is quantified as organic or inorganic carbon.

What we found: 

Depth had a significant impact on the organic matter and total carbon content of the soil. Total carbon and organic matter were found to be in higher quantities at shallower soil depths and inorganic carbon was marginally higher at deeper depths.

With the 2020 samples we found that non-restored plots had (marginally) more organic matter compared to restored plots. This could mean that restoration may not actually increase organic soil carbon storage. Between 2018 and 2020 we found that restored plots under drought conditions had the highest increase in inorganic carbon compared to all other experimental plot types. While more research is needed, this finding is encouraging that there could be a higher carbon mineralization rate stemming from the interaction between restored native coastal grasslands and drought conditions. While organic matter can mobilize through mineralization or loss back to the atmosphere through soil respiration, inorganic carbon (post mineralization) is more likely to stay sequestered.


Concluding remarks:

Further research is needed to determine a more definitive interaction between drought and native species restoration and how that interaction impacts the soil carbon content of California coastal grasslands. While we did find significant data indicating a positive correlation between restored plots under experimental drought conditions and an increase in inorganic carbon content, more meaningful trends may be observed across a longer timeframe. The annual life cycle strategies of invasive species allow them to grow faster than perennial native species, indicating that these short-term increase of carbon and organic matter in the soil of non-restored plots may not be a reflection on carbon storage long term. With IDE plots being built in 2015 and restoration plots being built in 2016, the populations of perennial native species vs. annual invasive species may not have had enough time to influence significant differences in levels of soil carbon between restored and unrestored plots.



I thank the Norris Center (Griswold Fellowship) and the Environmental Studies Department Awards Committee (Cooley Award) for funding for this research. I thank Justin Luong for their mentorship and Drs. Michael Loik, Karen Holl, Weixin Cheng, Joji Muramoto, Margherita Zavatta, and Carol Shennan for feedback and lending research tools. I appreciate Colin Carney for their help with the elemental analyses. I thank Justin Xie, Juan Carlos Moso, Jane Weichert, Graeme Tanaka, Theodore Vachon-Fischer, Janine Tan, Larissa Tawil, Alyssa Anzalone, and Owen Taffe for helping to process and collect soil samples.