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w i n e G R O WIN G Project Cooperators Allison Jordan, California Sustainable Winegrowing Alliance Dr. William Salas, Applied Geosolutions LLC Dr. Changsheng Li, University of New Hampshire Dr. Alissa Kendall, UC Davis Civil & Environmental Engineering Sonja Brodt, UC Davis Agricultural Sustainability Institute e missions in vineyards.6 We know of no complete vineyard carbon balances or carbon footprints. Our long-term research in this area will soon complete its 10th year. We assembled the first working budget of the three major GHGs in a single vineyard in Napa Valley. We have compiled extensive information about carbon cycling and soil carbon sequestration under minimum-tillage and conventional-tillage conditions. We are working closely with the California Sustainable Winegrowing Alliance to help fill data gaps identified by E. Carlisle with the ultimate goal of perfecting a carbon footprint decision-support system.6 The project also coordinates with efforts by Applied Geosolutions (cooperators Dr. William Salas, Dr. Changsheng Li) to calibrate the DeNitrification DeComposition model for vineyards. This model will be embedded in the decision-support system for use by grapegrowers and other practitioners to facilitate carbon sequestration and assess carbon footprints for a variety of management practice options, soils and climates. The data also are being used by cooperators Dr. Alissa Kendall, Elias Marvinney and Sonja Brodt (UC Davis) to assemble life-cycle analyses for carbon footprints of vineyards and orchards. Experiment The trial is being carried out in a 2.1-acre Cabernet Sauvignon vineyard at the Un iversit y of Ca l i for n ia, Dav i s, Department of Viticulture & Enology's Oakville Research Station in Napa Valley. The trellis is a six-wire VSP system with bilateral cordons. Yield and biomass production have been recorded for the rootstock 101-14 Mgt, which has a relatively shallow root system with moderate vigor and was the most widely planted in the area at the onset of the experiment. The vineyard was conventionally tilled for 12 years (1991–2002). In 2003, we initiated an experiment utilizing three tillage management practices: 1) "minimum till- age," with a dwarf barley cover crop (shallow-disked to about 1-inch depth with one pass every second year to facilitate cover crop seedling establishment), 2) "tillage with dwarf barley cover crop" (disked 8 to 12 inches deep twice per year), and 3) "conventional tillage" with resident vegetation (disked 8 to 12 inches deep once per year). As opposed to a no-tillage treatment, minimum tillage conformed to the needs of the vineyard manager for water management, while at the same time requiring fewer cultivation passes through the tractor rows. Tillage breaks up soil aggregates and accelerates oxidation of soil carbon by microbial organisms, which leads to its loss. About 19% of anthropogenic CO2 in the atmosphere is a consequence of soil disturbance by cultivation and deforestation.3 Exploring ways to foster increases in the carbon content of these soils (soil carbon sequestration) can improve vineyard carbon footprints. We have been assessing soil-carbon sequestration through periodic analyses of carbon in the plow horizon ("Ap horizon"). The soil carbon content is expected to increase to some degree when tillage frequency is reduced, but little research has been done in California to measure this expected increase over time under minimum tillage. Soil carbon sequestration is a multiyear process because most carbon in biomass that is incorporated into soils, or simply falls to the soil surface, is decomposed (released as CO2). It can require several years to get good measures of how much carbon is retained in the soil by a changed tillage or cover-cropping practice (in this case nearly a decade). At the same time we have tracked annual "above-ground net primary productivity" as carbon (ANPP-C). This is a measure of how much carbon in biomass is grown (for example wood and canes), in essence, how much carbon is retained following photosynthetic CO2 assimilation and respiration. While we can gather measures of ANPP-C relatively easily, it is much more difficult to acquire root production belowground (BNPP-C), but we have assessed some root production. By definition, a vineyard carbon footprint should account for production of other GHGs, primarily N2O and CH4. The International Panel on Climate Change [ipcc.ch/publications_and_data/ar4/ wg1/en/contents.html] uses conversion factors for CH4 and N2O to calculate their global warming potential (GWP) in the same units as CO2.2 Per molecule, N2O is 298 times more potent than CO 2 as a GHG, while CH4 is 25 times stronger on a Cover-cropped treatments were seeded every fall during the experiment. In previous years the minimum-till barley had self-seeded. 100-year time horizon. Thus a substantial amount of vineyard carbon sequestration can be required to offset a seemingly low level of N2O production. Both CH4 and N2O are produced by microbes in soil through the processes of nitrification, denitrification and methanogenesis. In upland soils under nonwater-saturated conditions, methane can also be consumed through methanotrophy, which oxidizes CH4. For three years, we carefully quantified CO2, N2O and CH4 emissions from soils of the vineyard's three tillage treatments. Measurements of gas fluxes were taken at least once every two weeks, and often more frequently, using static (N2O and CH4) and dynamic (CO2) gas-flux chambers. CO2 data illuminate the relationship between tillage and loss of soil carbon. Nitrous oxide (N 2O) emissions show peaks of production as a result of nitrogen fertilization and rain. Quantification of such event-related emissions is crucial to understanding a farming system's carbon footprint. Methane should be less important in scale than the other two gases; however, more research is needed. Major results Although we are in the 10th year of this investigation, the most comprehensive farm-gate carbon footprint for the treatments outlined above was assembled during 2008–10, when the American Vineyard Foundation was supporting the project. With a farm-gate footprint we describe cultivation practices and operations in grape production up to the point when grapes leave the vineyard, and no other processing that occurs beyond the vineyard, as in wine production. Changes in biomass production Aboveground net primary production of carbon (C) is generally defined as the net flux of CO2 -C from the atmosphere into green plants per unit time. Comprehensive measurements of biomass showed that pr actica l win ery & vin eya r d N OVEM B ER 20 13 85