# Glad we could connect at CNGA! >[!abstract] Contact me **Yamina Pressler, PhD** Assistant Professor of Soil Science and Restoration Ecology Natural Resources Management and Environmental Sciences Cal Poly, San Luis Obispo [email protected] www.yaminapressler.com > [!idea] Check out Abby's poster during the poster session > Evans, A., McLaren, E., Foster, E., Byrne, K., Griffin-Nolan, R., Luong, J., Pressler, Y. Assessing aggregate stability methods to inform restoration in California coastal grasslands. California Native Grasslands Association > > pdf version [[CNGA2025_Evans.pdf|linked here]] # Notes from my talk ## Soil organic matter variability in coastal California grasslands *Presented at the California Native Grasslands Association Symposium, February 2025* **Authors:** Yamina Pressler$^1$, Emma McLaren$^1$, Abby Evans$^1$, Nora Bales$^2$, Kylee Nielsen$^1$, Sophia Forstmann$^3$, Cody Cameron$^1$, Stewart Wilson$^1$, Erika Foster$^4$, Kerry Byrne$^5$, Robert Griffin-Nolan$^6$, Justin Luong$^5$ **Affiliations:**  $^1$ Cal Poly, San Luis Obispo  $^2$ Colorado State University $^3$ Stanford University $^4$ Point Blue Conservation Science  $^5$ Cal Poly, Humboldt  $^6$ California State University, Chico  >[!summary] Abstract >Grassland soils support above- and belowground biodiversity and serve as important carbon sinks both globally and throughout California. Understanding the distribution, formation, stabilization of soil organic matter (SOM) is critical to informing efforts to increase soil carbon accrual through grassland management and restoration. We measured SOM in mineral-associated and particulate pools across several coastal grassland sites: Cal Poly ranches and Rancho Marino Preserve in San Luis Obispo County, Swanton Pacific Ranch in Santa Cruz County, Pepperwood Preserve in Sonoma County, and Lacks Creek in Humboldt County. In this talk, I will present variability of SOM within and across sites and explore potential environmental and management drivers of this variability. I will also share early results from our ongoing efforts to link SOM content and plant traits in these systems to inform grassland restoration. ## Grasslands serve as important soil carbon sinks - Due to deeply rooted grasses that deposit carbon into the soil via decaying roots and root exudates ("rhizodeposits"), grassland soils have the capacity to store significant amounts of organic carbon both in California and worldwide - In a variable climate, grassland soils may be better able to store carbon on the long-term compared to forest aboveground bimoass (Dass et al 2018; https://iopscience.iop.org/article/10.1088/1748-9326/aacb39/meta) ## Soil organic matter (SOM) can be conceptualized into two functional pools - Soil organic matter is the basis of soil fertility and many critical ecosystem services that soils provide including water storage, habitat for soil biodiversity - Soil scientists define two pools of soil organic matter based on the dominant pathways of formation: - Particulate organic matter (POM) consists predominantly of decaying pieces of plant matter - Mineral-associated organic matter (MAOM) is predominantly microbially-processed organic molecules that interact with mineral surfaces - Cotrufo & Lavallee 2022 provide a review of SOM formation mechanisms (https://doi.org/10.1016/bs.agron.2021.11.002) ## We are studying soil organic matter to inform grassland restoration across coastal CA - Our research team is conducting a series of studies assessing these two soil organic matter pools in different management contexts along the CA coast - Study sites include - Cal Poly's Escuela and Chorro Creek Ranches in San Luis Obispo - San Luis Obispo County El Chorro Regional Park in San Luis Obispo - Rancho Marino Reserve in Cambria - Swanton Pacific Ranch in Davenport - Pepperwood Preserve in Santa Rosa - Lack's Creek in Humboldt - Many students have been involved in these efforts - Today's talk is just a short glimpse into our work at the southern sites and an invitation for more discussion for those who are interested in connecting and learning more about our efforts ## At San Luis Obispo Ranches, SOM pools did not differ between grazing regimes - We conducted an observational study comparing plant and soil responses to three grazing regimes (3 pastures of each regime) in adjacent ranches in San Luis Obispo county - Chorro Creek Ranch - Continuous grazing, rest for weeks, management for 15+ years - Escuela Ranch - High intensity, short duration rotational grazing, each pasture grazed for 1-3 days, rest for months, 15 years of management regime - Rancho El Chorro Regional Park - no grazing for 3 years, prior low intensity grazing for fuel mitigation - We found that plant community composition differed between the sites with highest diversity and greatest cover of native perennials at Escuela Ranch (rotational grazing) - Despite shifts in the plant community, soil organic matter pools did not differ between the sites **Nora Bales**, former graduate student now pursuing a PhD at Colorado State University, led this effort, with significant contributions from **Kylee Nielsen** to data collection and analysis. Find our preliminary analysis & student posters here: [[Understanding plant-soil linkages to restore California rangelands]] Funded by CSU Agricultural Research Institute ## At Rancho Marino Reserve, above- and belowground responses to grazing remove are decoupled - We investigated soil organic matter responses in an existing long-term grazing exclusion experiment in a coastal grassland (Table 1 in Stahlberger and D'Antonio 2013 https://doi.org/10.1016/j.biocon.2012.09.008) - Previous work found an increase in non-native annual forb in grazed areas compared to non-grazed exclosure plots - The study design has excluded grazing from three 1/4 acre plots for 18 years - In our spring 2024 sampling, we found significantly higher aboveground biomass in the ungrazed plots compared to the grazed area - However, these differences in aboveground biomass did not translate to differences in soil organic matter pools belowground - Similarly to our study in San Luis Obispo, these results may suggest a decoupling of above- and belowground responses to grazing Former graduate student, **Emma McLaren**, led this study! Funded in part by CSU Agricultural Research Institute and UC Office of the President Climate Action Seed Grant ## At Swanton Pacific Ranch, long-term (~100 yrs) passive restoration did not alter SOM pools - We conducted an observational study comparing "old growth" and historically tilled grassland sites at Swanton Pacific Ranch - At each of three paired sites, we quantified SOM content - While there was significant variation in SOM content across the three sites, there were no differences in SOM carbon between adjacent "old growth" and historically tilled locations Former undergraduate students, **Maxwell Farmer** and **Sophia Forstmann** (now pursuing a PhD at Stanford), designed this study. Undergraduate students **Cody Cameron** and **Nabila Wildman** also contributed to laboratory and data analysis. Find our preliminary analysis & student posters here: [[Understanding plant-soil linkages to restore California rangelands]] Funded by CSU Agricultural Research Institute ## What else might be going on? Underlying soil mineralogy explains variation in SOM - Soil organic matter distribution is controlled by both formation and stabilization mechanisms - Formation is ultimately controlled by inputs from plant and animal matter both above and belowground - Stabilization is based on soil properties - texture, mineralogy, cation bridging - Management typically falls into the formation category by influencing plant community composition (e.g. grazing regimes, restoration plantings) or manipulating direct inputs (e.g. organic amendments such as compost) - Soil carbon has been shown to be correlated with clay % in CA rangelands (Silver et al. 2010; https://doi.org/10.2111/REM-D-09-00106.1) - In collaboration with Dr. Stewart Wilson at Cal Poly, SLO, colleagues at Point Blue Conservation Science conducted a regional assessment of drivers of soil carbon in rangelands across Northern California and found that mineralogy (pedogenic minerals, organometal complexes and extractable cations) explained up to 90% of variability in a random forest model. This suggests strong relationships between soil organic matter and mineralogy that we are continuing to explore in subsequent studies in coastal CA grasslands. (Wilson et al., in revision) - In our earlier study of San Luis Obispo Ranches, we also found that mineralogy explained significant variation in SOM distribution across ranches with different grazing regimes Former undergraduate student, **Fiona O'Neill**, now at Marine Resource Conservation District led the mineralogy analysis for this project! ## Next, we will link plant traits to SOM & mineralogy - As part of the Grassland Action, Science, and Stewardship Network (GRASS-Net; PI Dr. Justin Luong), we are working on linking variation in SOM and mineralogy with key plant traits relevant for climate-smart restoration in coastal CA grasslands - Aboveground traits include plant height, specific leaf area, leaf dry matter content, leaf nitrogen content, leaf C:N ratio, maximum photosynthetic rate, leaf osmotic potential at full turgor pressure, lethal drought index (LD50), leaf senescence at LD 50, leaf water content at LD50, stomatal closure - Belowground traits include specific root length, root:shoot ratio, root diameter, root dry matter content >[!abstract] Learn more about GRASS-Net > >GRASS-Net envisions successful grassland restoration projects that take best practices into consideration, that are cost effective, prepared for the erratic drought regimes of the climate crisis, and promote regionally diverse and culturally significant species. We aim to bring together restoration practitioners across the state to share knowledge and minimize biotic homogenization in restored areas. Biologically diverse California (CA) range and grasslands are an increasingly crucial solution to carbon capture and climate change mitigation opportunities. California’s grasslands are biodiversity hotspots and globally unique, but climate change will make it unsuitable for endemic species; so innovative restoration must focus on climate resilience for a diverse suite of species. > >**If you would like to receive updates from GRASS-Net, please sign up for our email list here:** https://docs.google.com/forms/d/e/1FAIpQLSdYtx1N6q_XGUpZQC0bKVg0DeiutTQQ89-rwLz0RF6mesWW3w/viewform