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, *justine_owen@berkeley.edu

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

ACKNOWLEDGEMENTSWe 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 11th Hour Project.

REFERENCESSilver, W.L., Ryals, R., and Eviner, V. 2010. Soil Carbon Pools in California’s Annual Grassland Ecosystems.

Rangeland Ecology & Management 63: 128-136.

40 m1. Sample field0-5 cm 5-10 cm 10-20 cm 20-30 cm 30-50 cm 2. Soil preparation

Sieved (<2 mm)Organic matter fragments removed

3. Soil analysisC and N by CN analyzerParticle size by hydrometerpH by 1:1 soil-water slurry

Field surface

Soil core

RESULT 1: Soil C spanned a wide range, independent of manure application treatments.

standard error

• Range of 61-223 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

standard error

• 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)0-5 5-10 10-20 20-30 30-50

no manuresolid manureliquid manureboth