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Fate of sulfur fungicide in the vineyard and beyond.

Several decades of research have characterized the ecological effects of fossil fuel-derived sulfur deposition on temperate forests in the northeastern United States and Europe. These studies demonstrated that high sulfur loads in acid rain cause soil and surface water acidification, (4-5) base cation depletion in soils, (13) changes to forest structure and function, (12-16) and production and bioavailability of other elements including methyl mercury. (6-7)

In agricultural systems, sulfur is purposely applied as a fungicide, pH regulator and vital plant nutrient. (15) Unlike in forested ecosystems, there has been little research to examine the pathways and ecological consequences of its use within and downstream of agricultural systems; most research to date has focused on crop sulfur requirements, (1-2-9-14) the relative efficacy of sulfate (S[O.sub.4.sup.2])-supplying fertilizers (17) and the effect of elemental sulfur oxidation on soil pH and S042' availability. (8-18)

In vineyard agriculture, application of elemental sulfur is a common practice for controlling powdery mildew. In the Napa Valley of Northern California, 810.5 Mg of elemental sulfur are applied to the region's 47,980 acres of vineyards each year. (3)

During the growing season, which coincides with the dry season in Northern California (April through October), growers broadcast-spray sulfur by tractor frequently (every seven to 14 days) at low doses (often approximately 5 to 12 pounds per acre) targeting vines but also dusting the soil between trellises. On average, these applications add up to 100 to 300 kg (220 to 661 pounds) of sulfur per hectare per year and are made in wettable or dust form, depending on the vineyard's individual sulfur program.

The potential environmental consequences of sulfur in major winegrowing regions have been largely unexplored. More broadly in the agricultural sector, many believe that sulfur is inert in the environment. Researchers have not investigated whether targeted applications of sulfur may have consequences for soil and water quality locally within vineyards, or the timing and fate of sulfur moved by water off-site. The ecological consequences of applied sulfur in vineyards may be similar to historic, inadvertent sulfur deposition to temperate forested ecosystems.

In this research, we evaluated the major chemical transformations of elemental sulfur occurring during the growing and dormant seasons in vineyards, and the patterns of sulfur application, storage in soils and export in water flow paths that determine the annual sulfur cycle.

Two vineyard locations in Napa Valley, Calif., were used by our research group to evaluate the immediate fate of applied sulfur on the soil surface and sulfur retention within and loss from vineyards. We hypothesized that growing season conditions favored rapid chemical reaction of elemental sulfur to sulfate on the soil surface, which would be moved by water off-site during dormant season storms. Thus, we hypothesized that the hydrology of vineyards is a major driver of the fate of applied sulfur.

Studying the fate of applied sulfur in vineyards

Immediate fates of applied sulfur studied at a St. Helena, Calif., vineyard The vineyard block used in this study has Boomer-Forward-Felta rocky loam soils on 30% to 50% slopes underlain by Franciscan melange (includes greywacke, chert, serpentinite and greenstone).

Vines are Cabernet Sauvignon grafted on AxR-1 rootstock, planted in rows perpendicular to the dominant slope with predominately western exposure. Immediately prior to and following two applications of elemental sulfur, surface soil samples (0-2 cm or 0-0.8 inches) were collected and analyzed for pH, extractable S[O.sub.4.sup.2-] and total sulfur.

Additionally, the forms of sulfur present in the soils were determined using X-ray absorption near-edge structure (XANES) spectroscopy. Soil samples were collected for these analyses 30 minutes prior to elemental sulfur applications, and at 30 minutes, two days, seven days, 12 days and 19 days after each application.

Movement of sulfur into soils, vegetation and hydrological pathways at a Napa, Calif., vineyard

The vineyard block used in this part of the study has Bressa-Dibble clay loam soils (0-0.4 meters, or 0-1.3 feet) underlain by a sandy clay hardpan that is consistent across southern Napa Valley and predates vineyard cultivation. Vines are Dijon 114 Pinot Noir grafted on 101-14 rootstock, planted in rows perpendicular to the slope with 1.8 meters (5.9 feet) spacing, and were 14 years old at the time of this study

During the dormant season, growers seed vineyard avenues with cover crops such as Rosa, Trifolium and Triticale species. Soil sampling was conducted Oct. 19-21, 2005 (after harvest), and April 9-11, 2007 (after the dormant season), to characterize soil physical and chemical properties within a vineyard block. Six cores (2 cm diameter x 1.2 meters depth [0.8 inches x 4 feet]) were collected in four 30 cm (11.8 inch) sections. Sulfate and total sulfur were measured in these samples.

Vine leaf and grape tissues were collected Aug. 22, 2007, immediately prior to harvest, freeze-dried, ground and analyzed for total sulfur content. Total leaf and grape production data were used to scale total sulfur contents of

vine and grape tissues to the hectare. (19,20) Immediately prior to mowing on March 15, 2005, cover crop tissues were harvested from 0.5 meter2 (1.6 feet2) areas randomly located within the vineyard study block. Samples were dried and weighed to determine dry mass per area, and subsamples were analyzed for total sulfur.

Across the study area, two types of soil water collectors (lysimeters) were installed below the majority of the rooting zone (0.36 [+ or -] 0.01 meters [14.17 [+ or -] 0.39 inches]) and directly under vines. These instruments allowed us to capture sulfur movement in drainage waters during irrigation and storm events.

Tension lysimeters (vacuum applied) were sampled for soil water during long irrigation events and dormant season storms when the soil was saturated, and zero-tension lysimeters (no vacuum), were sampled for leachate during four-hour irrigation events when the soil was unsaturated. (11) Precipitation, irrigation water, soil water and leachate samples were analyzed for S042' and total dissolved sulfur (organic plus inorganic forms).

Major findings

At the St. Helena site, the immediate chemical reaction of applied sulfur on the soil surface was evaluated using 1) XANES spectroscopy results, which allowed determination of all chemical forms of sulfur present in soils, and 2) direct measurements of soil pH, S042 and total sulfur.

Following each of two applications of 6.7 kg elemental sulfur per hectare (6 pounds per acre) to the vineyard, the majority of sulfur applied oxidized to S042' during the first 30 minutes. Soil pH decreased as the oxidation process generated acidity, and SO/' within surface soils increased. Following each application event, pH returned to pre-application levels, and S[O.sub.4.sup.2-] transformed into the dominant organic form of sulfur measured in vineyard soils.

Because the first part of the study showed that applied sulfur transforms into S[O.sub.4.sup.2-] and organic sulfur, which are both mobile in the soil, the next objective was to understand where these sulfur forms go within the vineyard. Do they stay within the field in vine tissues and soil, or are they moved by water off-site? Understanding the pathways of sulfur within soils, vegetation and water pathways is called developing a sulfur budget, which we calculated at the Napa, Calif., vineyard.

The total sulfur content of vegetation tissues was a relatively small component of the vineyard sulfur budget. During the growing season, sulfur concentrations in vine tissues were 2 [+ or -] 0.2 g [kg.sup.-1] dry tissue and in grapes were 0.4 [+ or -] 0.1 g [kg.sup.-1] dry tissue; together these account for 7 to 14 kg sulfur per hectare (6.2 to 12.5 pounds per acre).

During the dormant season, sulfur concentrations in cover crop tissues were 2.3 [+ or -] 0.1 g sulfur per kg dry tissue, constituting 4 to 10 kg sulfur per hectare (3.6 to 8.9 pounds per acre), see Table I. The combined sulfur contents of vine and cover crop tissues are equivalent to 10% to 23% of the elemental sulfur applied annually.

Dissolved S[O.sub.4.sup.2-] and total dissolved sulfur were measured for three years in precipitation, irrigation, soil water and leachate within the soil profile. During short (four-hour) irrigation events, S[O.sub.4.sup.2-] concentrations in leachate were low (4.5 [+ or -] 0.66 mg S[O.sub.4.sup.2-] -S per liter, but increased to 13.7 [+ or -] 2.6 mg S[O.sub.4.sup.2-]-S per liter as the soil saturated during longer (more than eight-hour) irrigation events in 2005 and 2006 (Figure 1).

In every year, dormant season storms flushed accumulated S[O.sub.4.sup.2-] and organic sulfur from the soil. Tire elevated concentrations were measured during storm events in all three years, with dissolved organic sulfur (total dissolved sulfur less dissolved S[O.sub.4.sup.2-]) comprising the majority of sulfur in solution (Figure 1).

During the wettest season of the study (October 2005 through May 2006), sulfur species in soil and water did not return to growing season levels until applications began the following growing season. Based on average total dissolved sulfur concentrations and water fluxes from the three years of study, we estimate that 4 [+ or -] 1 kg per hectare (3.6 [+ or -] 0.9 pounds per acre) of sulfur in the growing season and 123 [+ or -] 40 kg per hectare (109.8 [+ or -] 35.7 pounds per acre) of sulfur in the dormant season are exported below the rooting zone in water flow paths leaving the vineyard (Table I).

Measurement of sulfur content in soil samples collected before and after the dormant season documents the storage and movement of sulfur in the vineyard. Total sulfur content of surface soils decreased from 1,623 [+ or -] 354 kg per hectare (1,448.6 [+ or -] 316 pounds per acre) at the end of the growing season to 981 [+ or -] 526 kg per hectare (875.6 [+ or -] 469.5 pounds per acre) at the end of the dormant season (Table I). The difference between the mean values supports that export of stored organic sulfur occurs during the dormant season. The persistence of organic sulfur in the soil profile following the wet dormant season demonstrates that it has a longer residence time within the vineyard than highly mobile S[O.sub.4.sup.2-].

Implications for sulfur management and downstream ecosystems

The aim of this study was to understand the chemical transformations, fate and residence times of the large, cumulative sulfur loads applied to vineyards each year. In general, the growing season is characterized by rapid chemical reaction of applied sulfur and little transport of sulfur during irrigation events (Figure 2a), while the dormant season is characterized by export of S[O.sub.4.sup.2-] and organic sulfur during rainstorms of variable size and duration (Figure 2b).

On an annual basis, sulfur inputs to the study vineyard are roughly equivalent to total sulfur export (Table 1).

Thus, nearly all sulfur applied to the vineyard each year was exported in solution waters.

Our study focused on collecting detailed measurements in only two vineyards within Napa Valley. Across the region, differences in management practices, vineyard soil properties and rainfall may affect the degree to which applied sulfur is retained in local soils, or transported off-site.

However, we believe that the general patterns in our results can be applied across northern California winegrowing regions due to three main factors:

1) Elemental sulfur is the fungicide of choice by many winegrowers,

2) There are few mechanisms by which applied sulfur can be retained within vineyards (in soils or vegetation biomass), and

3) The entire region is subject to dry growing seasons followed by wet dormant seasons (the Mediterranean climate).

We expect that these factors--relating to sulfur inputs, retention potential and transport potential--lead to the majority of applied sulfur being exported from vineyards each year.

The important question is whether or not sulfur exported from vineyards has unintended consequences in the watershed. This is a sensitive topic with winegrowers, due to the efficacy of sulfur as a fungicide and the concern that its use will be banned.

Our study points to the need for greater attention to understanding the ultimate fate of sulfur exported from vineyards and other agricultural systems where its use is prevalent. Although the impacts of sulfur applications may be minimal within vineyards currently, transport to aquatic ecosystems with fluctuating oxygen conditions could be environmentally problematic. There, sulfate can affect the mobilization of mercury and other heavy metals, which are toxic to fish and wildlife.

Promoting an environmentally sustainable industry relies on investigating this topic, given past lessons from ecosystem studies of sulfur deposition in acid rain.

Based on this study, further research should explore the patterns of sulfur transport from vineyards to aquatic systems, evaluate the cycling of sulfur within aquatic systems, and investigate fungicidal alternatives (and their environmental consequences) for powdery mildew control. E33


Eve-Lyn Hinckley was supported by an Environmental Protection Agency Science to Achieve Results Fellowship, the Geological Society of America and Stanford University funds. Thanks to the vineyard managers in Napa Valley who granted access to their vineyards for this research. A technical version of this study originally appeared in Proceedings of the National Academy of Sciences. (10)


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Caption: Figure 1. Sulfur in precipitation, irrigation, leachate and soil water during the period of study. Precipitation and irrigation water are represented by bars, and elemental sulfur inputs are shown with arrows. Leachate draining from vineyards was measured during irrigation events in the growing season (shaded panels), and soil water was measured below the rooting zone during the dormant season (white panels). Note three long (more than 8 hours) irrigation events on Sept. 25, 2005, Aug. I, 2006, and Oct. 10, 2006. Values are the average of more than 16 measurements from unique instruments per sample date, [+ or -] I SD. (Data originally appeared in reference 20.)

Caption: Figure 2. Sulfur transformations and flows in a Napa Valley vineyard, (a) The growing season, showing drip irrigation, elemental sulfur application and leachate; (b) the dormant season, showing cover crop, and lateral transport of sulfur and water caused by large storm events. Size of arrows and text indicates the relative magnitude of each pool and pathway. Dashed arrows depict inferred pathways. In the study vineyard, a clay hardpan was present below the rooting zone of the vines.

Caption: Sulfur spraying by helicopter in a Carneros vineyard experiencing a wet spring.

BY Eve-Lyn S. Hinckley, Institute of Arctic & Alpine Research, Boulder, Colo.

Table 1. Annual vineyard budget measured in Napa, Calif. (Data
originally appeared in reference No. 20.) "Total sulfur (S) is
expressed as mean values ([+ or -]  1 SD), except for biomass
estimates, which are expressed as a range. ([dagger]) *, --,
and 0 denote gain, loss and no change.

           Component               kg S per         Soil
                                  hectare *     gain/loss (+)

Growing    Sulfur applications       105              +
season     Irrigation water         21(1)             +
           Vine leaves               5-11             0
           Soil (0-0.5m)         1,623 (354)          0
           Wine grapes               2-3              Leachate
Dormant    Precipitation           1 (0.5)            +
season     Cover crop                4-10             0
           Soil (0-0.5 m)         981 (526)           0
           Soil water              123 (40)
Annual                              127(1)

Annual                           129-130 (40)


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Title Annotation:GRAPE GROWING
Comment:Fate of sulfur fungicide in the vineyard and beyond.(GRAPE GROWING)
Author:Hinckley, Eve-Lyn S.
Publication:Wines & Vines
Geographic Code:1U9CA
Date:May 1, 2015
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