Carbon content of managed grasslands: implications for carbon sequestration Justine J. Owen * and Whendee L. Silver Dept. of Environmental Science, Policy & Management, University of California, Berkeley, INTRODUCTION Grassland soils typically have considerable carbon (C) storage potential, but C storage or loss is sensitive to management. In California, C content in grassland soils spans almost an order of magnitude and is not correlated to climate or soil type (Silver et al., 2010), suggesting a dominant role for management. The effects of management practices on soil C storage and loss are not well constrained in these ecosystems. Manure application to grazed fields is a common practice which we predicted would increase soil C storage.  Does long-term dairy land management affect soil C? SITE SELECTION We focused on grazing systems on dairies as they are the most important land use type for rangelands in California Soils sampled from 2 to 4 fields at 8 pasture-based dairies (A-H) in Marin and Sonoma counties, CA. Management practices sampled: manure application, plowing, aeration, seeding, irrigation, and grazing intensity. Management practices have been in use for >30 years. SAMPLE COLLECTION AND ANALYSIS IMPLICATIONS AND FUTURE WORK Long-term manuring of dairy pastures under Mediterranean climates does not consistently increase C storage in soil Manure type may affect how much C is stored in the soil More data are required to determine how soil C is affected by irrigation, aeration, and grazing (no significant correlations were observed at our sites) Unanswered questions: Are these fields C saturated? Are there specific strategies for consistently increasing soil C in these soils? Future work: Model dairy pastures with Century, analyze soil C fractions ACKNOWLEDGEMENTS We thank Zoe Statman-Weil and Julia Cosgrove for help with soil sampling, and Andrew McDowell, Melissa Juedemann, Kristine Grace, Ryan Salladay, Nicola Overstreet for help processing samples. We also thank Becca Ryals and Marcia DeLonge for their guidance and discussion. Special thanks go to all the farmers who allowed us onto their dairies. This work was funded by a USDA NRCS Conservation Innovation Grant to the Environmental Defense Fund and the 11 th Hour Project. REFERENCES Silver, W.L., Ryals, R., and Eviner, V Soil Carbon Pools in California’s Annual Grassland Ecosystems. Rangeland Ecology & Management 63: m 1. Sample field 0-5 cm 5-10 cm cm cm cm 2. Soil preparation Sieved (<2 mm) Organic matter fragments removed 3. Soil analysis C and N by CN analyzer Particle size by hydrometer pH by 1:1 soil-water slurry Field surface Soil core RESULT 1: Soil C spanned a wide range, independent of manure application treatments. Range of Mg C/ha (0-50 cm depth), slightly higher than C content in CA grasslands compiled by Silver et al (2010)  Dairy management practices may increase soil C in grasslands. Differences within dairies are comparable to differences between dairies  Variations in local climates and geology did not significantly affect the results RESULT 2: Liquid manure addition led to greater soil C content than solid manure addition Mean soil C content was the same for manured and non-manured fields Soil C in fields with liquid manure was significantly higher than in fields with solid manure RESULT 3: Soil C content was not correlated to clay content in most of the profile. Suggests C content at depth is associated with clays, whereas C content near the surface is not. May reflect different types of C fractions at different depths. RESULT 4: Soil C content decreased with bulk density in most of the soil profile. Greater bulk density may limit the transport of C into the soil Mixing of the soil surface (0-5 cm) by trampling, plowing, and aerating causes C content to be insensitive to bulk density 25 km Depth (cm) no manure solid manure liquid manure both