Download Vineyard management for carbon sequestration in soil

Vineyard management for carbon sequestration in soil: a stable carbon isotope approach.
Pierce, Danielle L.1, Eli Carlisle1, Kerri L. Steenwerth 2, David R Smart1*
1
Department of Viticulture & Enology, University of California, One Shields Avenue, Davis CA 95616
2
USDA-ARS, Crops Pathology and Genetics Research Unit, University of California, One Shields Avenue,
Davis CA 95616
There are nearly one million acres of grapes in California and only about 16% are sown to cover crops,
suggesting there is potential for increasing this management practice in vineyards. Tillage of cover crops
generally increases soil respiration by bringing organic residue in contact with soil microbes and exposing
it to soil conditions that favor mineralization like higher moisture content and aeration. Tillage also breaksup aggregates and makes previously encapsulated C available to mineralization processes. We are
examining the carbon sequestration potential of conservation tillage of a vineyard cover crop in the Napa
Valley, CA. A cover crop of barley was planted between vineyard rows in November of 2003, and
subplots were isotopically labeled with 13CO2. We have shown in the first season of this investigation that
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CO2 labeling can be used to monitor C turnover by estimating source 13C content of soil respired CO2
using keeling plots. We observed immediate 13C enrichments in soil respired CO2 in the conservation and
conventional tillage treatments that were due to the decomposition of isotopically labeled fresh plant
material. An ensuing depletion of 13C may have indicated a reduction in available 13C labeled soil
organic carbon (SOC). The conservation tillage treatment showed a slower rate of change (13C loss)
relative to the conventional tillage treatment, but the rate of change was strongly dependent upon
precipitation events following summer drought. Conventional tillage decreased soil moisture overall, but
also increased maximum soil temperature and the rate of SOC mineralization. Our investigation is
providing information on how minimum tillage of cover crops might help mitigate the observed increase in
CO2 concentration in the atmosphere.
Methods:
Objective: To gain an understanding of short-term changes in soil carbon (C) for a vineyard cover crop
under conventional tillage (CT) as compared with a conservation tillage (MT) regime.
Experiment:
A cover crop of barley (UC603) was planted in the fall of 2004 and 2005. Plots of the cover crop
biomass were isotopically labeled with 13CO2 in the field near the end of the growing season (late
February-mid March). We tracked the label by taking short-term measurements of 13CO2evolved from soil
using static chambers at intervals of approximately every six to eight weeks and apply keeling analysis.
Spring 2004 was unusually dry and warm. 13C label belowground was about half that of the total plant
label and soil respiration in the mow and less than half (~35%) in the till. After the plots were mowed (top
left) or tilled (lower left) the rate of label being respired increased in the till and decreased in the mow (see
day 30).
Spring 2005, in contrast to 2004 was uncharacteristically wet and cool. Before disturbance, the
belowground signal represented 35-80% of the total cover crop respiration in the till and mow (upper
right), respectively. Post-disturbance, the plots with mowed litter and soil alone continued to decline in
respired 13C whereas the tilled plots increased beyond the measured pre-disturbance labeled maximum.
This increase in the amount of label being respired may be attributed to the wet conditions and direct
contact of the plant material with the soil, thus making it more available to microbial decomposition. The
relative amount of label being respired post- till in 2005 was approximately three times that observed in
2004.
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Conclusions
1. The isotope labeling of a barley cover crop in the field was successful. It was tracked for over a
year, sensitive to both climate change and anthropogenic disturbance, and replicable.
2.
Under both wet and dry soil conditions, a conventionally tilled treatment doubled the soil
respiration rates and increased the amount of labeled cover crop decomposition.
Future Work
Further investigate the role of precipitation on the carbon cycle of Mediterranean soil C by conducting a
wet-up experiment in the late summer of 2005. Preliminary results indicate that soil moisture is the
dominant factor driving short-term carbon cycling in this system.
Acknowledgements
Thank you to the Kearney Foundation of Soil Science for their financial support. Funding for this project
has been provided by the CDFA's Buy California Initiative and the USDA, Grant Agreement #02-0765.
Many thanks to the Smart Lab for their critical assistance in spring 2005.
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