*, kerri l steenwerth , danielle l. pierce figure 1kearney.ucdavis.edu/old...

2001-2006 Mission Kearney Foundation of Soil Science: Soil Carbon and California's Terrestrial Ecosystems

Final Report: 2003010, 7/1/2003-6/30/2005

1 University of California, Davis, Department of Viticulture and Enology 2 USDA-ARS Crops Pathology/Genetics Research Unit (CPGRU), UC Davis

Conservation Tillage of Cover Crops as a Means of Improving Carbon Storage in California Vineyards David R. Smart1*, Kerri L Steenwerth2, Danielle L. Pierce1

Executive Summary

Research highlights 1. Soil respiration rates (Rs, µmol CO2 m-2 s-1) were higher under warm, wet, disturbed

conditions. This occurred during unseasonable spring precipitation and has implications concerning global climate change and soil C cycling in Mediterranean ecosystems. Rs increased when a cover crop was tilled, but remained relatively the same as pre-tillage rates when mowed and not tilled. Rs was highest when water was added to dry cover crop soil that had been mowed. The high rates may be explained by a soil-priming hypothesis. Soil priming is generally defined as an acceleration of the decomposition of particulate organic matter (POM) when labile C sources become available to microorganisms.

2. Annually, there was no significant net difference in soil POM content between the mowed and the tilled cover crop rows. The tilled areas exhibited high Rs post-tillage, whereas the mowed rows “caught-up” in the fall, when the winter rains commenced.

3. Under extremely dry conditions, nighttime Rs in the tilled rows was significantly higher than the mowed rows; however, during the day, Rs was similar. This difference may be attributed to differences in soil characteristics, such as bulk soil density, which alters soil gas diffusion coefficients. When soil conditions were extremely wet, Rs was extremely low in both cover crop treatments due to slower gas diffusion through water filled pore space.

Significance These data inconclusively show that there is no net change in soil POM content as a result of mowing or tilling treatments.

Introduction Soil carbon (C) sequestration has become a topic of intense national and international interest in ecosystem science with respect to efforts at mitigation of greenhouse gas (GHG) emissions to the atmosphere. 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

Figure 1

Conservation Tillage of Cover Crops as a Means of Improving Carbon Storage in California Vineyards—Smart

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aeration. Tillage also breaks-up aggregates and makes previously encapsulated C available to mineralization processes.

We examined the carbon sequestration potential of conservation tillage of a vineyard cover crop in the Napa Valley, California. A cover crop of barley was planted between vineyard rows in November of 2003 and 2004, and subplots were isotopically labeled with 13CO2. In the first season of this investigation, we determined that 13CO2 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.

Benefits Our investigation provided information on how minimum tillage of cover crops might help mitigate the observed increase in CO2 and several important observations have emerged from this work. In addition to showing that 13CO2 can be used as a tracer to partition aboveground versus rhizosphere respiration, our results also suggest that soil decomposition will be driven by moisture content in Mediterranean environments, even those with high seasonal rainfall amounts. The influence of climatic change on precipitation patterns and quantities may therefore have a stronger influence on soil organic C dynamics than climate change driven temperature change alone.

Results The isotope labeling of a barley cover crop in the field was highly successful. We were able to track the label for more than a year, and it is a sensitive indicator to both climate change and anthropogenic disturbance, and replicable. Under both wet and dry soil conditions, a conventionally tilled treatment doubled the Rs and increased the amount of labeled cover crop decomposition. Rewetting must be documented in order to provide an accurate assessment of annual soil respiration (see table 3).

The data indicate that sampling frequency and temporal scale can affect assessments of controls on annual soil respiration (fig. 3). Daily temporal oscillations in soil respiration occur, but this oscillation is not captured in bimonthly measurements. When soil respiration is measured bimonthly, soil moisture was the dominant controlling factor of observed Rs. However, when Rs was measured over a 24-hour period under constant moisture, soil temperature also greatly influenced it.

Soil disturbance due to mowing or tilling had an immediate impact on CO2–C lost from the soil. The treatment effects were approximately equal in terms of C loss, depending on soil moisture and rewetting/drying cycles. Rs increased during tillage, but a much larger CO2 pulse was released after simulated precipitation in the fall.

Rhizosphere respiration was approximately one-fifth of above-ground respiration in both years. In 2004, moisture levels were low as a consequence of spring drought. Labile


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C sources were retained in soils throughout the summer in 2004, being strongly oxidized with the onset of autumn precipitation. In contrast to 2004, in 2005 precipitation in April and May greatly increased both soil moisture and 13C loss from both the more labile and recalcitrant pools in both the 2004 labeled and 2005 labeled systems. The quantity of carbon oxidized and emitted by soil respiration following tillage in 2005, when moisture contents were high, was prolonged and challenges previous assumptions concerning the timing and intensity of microbial activity following tillage disturbance. 13C pools retained in 2004 in the conventionally tilled system were predominantly respired following soil wet-up in both 2004 and 2005.

Collaborative partnerships We collaborated with Dr. Kerri L. Steenwerth’s laboratory from the outset. Dr. Steenwerth was originally a postdoc on the project but is now USDA-ARS CPGRU weed scientist. Dr. Steenwerth’s program provided field support in the second year of the project, allowing us to increase measurement frequency. Dr. Ed Weber, University of California, Davis, extension specialist was consulted as to which cover crop to plant in the vineyard, and Dr. David Harris gave advice with regards to stable isotope sampling methods.

Interest from the agriculture community

There is strong interest from the California viticulture community in cover crop and soil research. Several field tours were given to the Napa Valley and Lake County Grape Grower Associations and other constituencies in order to communicate some of our preliminary research findings to them, as well as to gauge the value of our research to practitioners. This project was very well received for its applicability and also for its global climate change implications.

Evaluation

Objectives 1. Describe precipitation and temperature patterns in Napa Valley, California, and

measure the effect of such climate conditions on vineyard cover crop growth. 2. Quantify the effect of soil disturbance and climate on decomposition of a barley cover

crop using Rs and a stable isotope label of 13C. 3. 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 or minimum tillage (MT) regime.

Oakville, California, in the Napa Valley has a typical Mediterranean weather pattern. The summers are hot and dry; the winters are cool and moist. The average amount of rainfall is approximately 900 mm per year. During 2004, 880 mm of rain fell from October through early-June 2004, and 1,125 mm during 2005 (fig. 1, table 1). The year 2005 was an unusually wet spring. This wet spring, combined with a doubled cover crop seed rate the year before, resulted in three times the cover crop biomass in 2005, compared to the final cover crop biomass in 2004 (table 1).


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Figure 1. Daily means from Oakville, California, precipitation (mm) and soil temperature (ºC). Precipitation (mm) data are represented by the solid blue line and daily mean soil temperatures by the dashed black line. These data were collected from the California irrigation management information system (CIMIS) Web site. Spring 2005 was unusually wet (gray area) relative to spring 2004.

Table 1. Precipitation, soil temperature, cover crop biomass. Time Total

Precipitation (mm)

Rain Days

Mean Soil Temperature

(°C) ± SE

Mean Cover Crop Biomass (kg m-2) ± SE

10/1/2003 – 6/9/2004 880.36 100 12.8 ± 0.18 0.63 ± 0.26 10/17/2004 – 6/16/2005 1124.97 145 13.4 ± 0.17 3.60 ± 0.27

Vineyard barley cover crop biomass was measured throughout the 2004 (a) and 2005 (b) winter growing seasons as dry biomass (g) per area (m2). The vineyard barley cover crop biomass was higher in 2005 than 2004. In the till treatment, it was seven times higher and in the mow it was more than double. In fall 2005, we used a higher seeding rate and achieved better germination. The wetter spring in 2005 (see fig. 1) also may have contributed.

Rs were measured throughout 2004 and 2005. In general, soil respiration increased with soil moisture under warm soil conditions and decreased with soil moisture under wet, cool conditions (fig. 2). In 2005, Rs was higher in the spring as compared to 2004, due to the higher soil moisture content during that time. Furthermore, in the wet spring there were no treatment differences in Rs immediately after mowing/tilling (table 2). Under


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dry conditions (i.e., spring 2004) the tilled treatment respired significantly more C than the mowed treatment (table 2).

Table 2. Weighted Keeling plot intercepts (tmax/tn). 2004 Days from Mow/Till Event Treatment

-3

-1

8

30

157

208

311

384

Mow (soil tn/tmax) 1 0.41 0.39 0.11 0.18 0.21 0.16 0.04 Till (soil tn/tmax) 1 0.47 0.23 0.34 0.20 0.18 0.08 0.01 2005 Days from Mow/Till Event Treatment

-6

5

18

49

Mow (soil tn/tmax) 1 0.35 0.24 0.17 Till (soil tn/tmax) 1 0.29 1.09 0.49

Table 2 shows weighted d13C intercepts = tn/tmax for the Oakville vineyard barley cover crops that were mowed or tilled in late-March 2004 and early-April 2005. Each year a separate cover crop was planted the previous November and . Three rings in each cover crop management treatment were labeled with 13CO2 during the spring, resulting in significantly enriched plant biomass. Static chamber gas mixing-line intercepts were used to measure the relative amount of 13C being respired from the mowed or tilled barley cover crop rings throughout the year, up to 400 days post-management in 2004 and 100 days post-management in 2005. These intercepts were weighted using the initial 13C pulse as tmax in the following manner: Weighted δ13C intercepts = tn/tmax. Except for the 2005 till management treatment, all of the subsequent δ13C intercepts were less than the initial pulse and reflect the pattern of barley decomposition as it changes between treatments and over time. Post-management, the plots with mowed litter and soil alone continued to decline in respired 13C, whereas the tilled plots increased. 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. This observation may be attributed to the significantly wetter 2005 spring.


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Figure 2. Two years, 2004 (a) and 2005 (b), of gravimetric soil moisture content (g/g*100), filled squares, and soil respiration rates (µmol CO2 m-2 s-1), open circles, were collected from an Oakville, California, vineyard. These data are shown on the same graph with gravimetric soil moisture content on the left y-axis and soil respiration rates on the right y-axis and day of the year on the x-axes. Each point on each of the four graphs represents a mean ± SE. In 2004, soil respiration was low when soil moisture approached 20%. However, during the year 2005, when rainfall was greater and occurred more frequently than in 2004, a different pattern was observed. In 2005, soil respiration and moisture changes oscillated with more frequent precipitation events.

Using an in situ stable carbon isotope of 13C pulse/chase experiment, we found that immediately after the spring mowing or tilling of the cover crop, the tilled rows respired more of the isotopically enriched cover crop biomass than the mowed rows (fig. 3, table 3). Using an end member-mixing model, we determined that the labeled cover crop biomass represented about 29% and 17% of soil respired CO2 in the tilled and mowed rows, respectively (fig. 4). This is an extremely important finding as it suggests a relatively small fraction of soil respired CO2 was derived from labile C in the cover crop. Furthermore, we found that after one year of wet/dry cycling and a spring mowing/tilling treatment, there was still a significant about of 13C enriched cover crop carbon in the soil


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surface (0-10 cm). However, there was no net difference in the amount of 13C remaining in the mow versus the till after one year (fig. 5).

Figure 3. Rs (µmol CO2 m-2 s-1) were measured with a Licor 6400 with a 6400-09 attachment (Lincoln, Nebraska) every two to four hours for three to four days immediately after (a) and 90 days after (b) mowing or tilling the cover crop in the rings that were labeled with 13CO2. Immediately after mowing and tilling (a), Rs were elevated in the tilled rings relative to the mowed rings. The relative treatment differences stayed the same for three days following the mowing/tilling event and were still evident, in a muted way, 90 days later (b).To be specific, Rs were more similar between treatments 90 days after disturbance, although the tilled soil tended to have greater respiration rates at night than the mowed treatments.

Benefits The grant program benefits include the following: • Potential source of carbon credits in international global climate change arena. • Soil conservation/remediation and diminished erosion due to presence of plant

biomass in the summer. • Water competition to control grape vine vigor and subsequent control of wine grape

quality. • Energy savings due to decreased tractor use.

We did not meet the project completion time frame due to the difference in time between the funding period and the growing season. The funding period was from July to July, whereas the growing period was from November to November. We overcame this problem by getting two complete years of data – one baseline and one experimental. We are in the middle of our second experimental year and will attempt to collect the remaining critical POM samples and soil respiration rate measurements until April 2006. This project gained significant sampling costs due to the expense of stable isotope analyses and an unforeseen increase in sampling frequency in order to gain resolution in the seasonal and annual C trends. Additional funding from the Kearney foundation was


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used to meet the minimal cost requirements for stable isotope analyses augmented the costs of running samples.

Figure 4. Effects of two types of soil disturbance on particulate organic matter (POM) d13Cpdb(‰) under two different climate conditions. In spring 2005, five days after mowing or tilling the cover crop, the fraction of newly respired carbon derived from the labeled cover crop was slightly higher (12%) in the till treatment than in the mow treatment. In 2005, ‘older soil C’ versus ‘new cover crop-derived’ C inputs in the 13C labeled rings were partitioned for 49 days after imposing the soil management treatments. The data reflect patterns observed in the mowed cover crop plot (mow: ‘soil + litter’ treatment). Over time, the amount of new C being respired increased with soil moisture availability (day 19) and decreased as storm frequency declined (day 49).

Figure 5. Soil samples from two depths (0-5 cm, 5-10 cm) were collected from six 24” diameter circular metal rings. These rings contained 13C labeled barley that was isotopically labeled in situ and sampled immediately after the mow or till management treatments in spring 2004 and, one year later, in spring 2005. Soil particulate organic


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matter (POM) was extracted using the method outlined in Six et al. 1998. The POM was then run on a stable isotope mass spectrometer for d13CPDB (‰) and C (%). POM d13CPDB was significantly depleted (p>0.0063) one year later (a). Within each year, POM d13CPDB was significantly more enriched (p>0.05) in the top 5 cm of soil, relative to the 5-10 cm depth (b). The mow and till cover crop management treatments did not significantly alter the POM d13CPDB content from 2004 to 2005 or with depth (c).

Image 1 Cover crop experiment: Oakville, California, November 2004

As table 3 shows (below), Rs (mmol CO2 m-2 s-1) were measured immediately after simulated tillage (open circles) and mowing (solid squares) events in situ in metal rings (10 cm x 61 cm diameter) that were labeled with 13CO2. These isotopically labeled rings were mowed/tilled on March 26, 2004, and, in a repeated labeling experiment, on April 1, 2005. Time of day (hr) is shown on the x-axis. Rs increased in vineyard cover crop soils that were tilled compared to those that were mowed. In both years, approximately nine hours after tillage, the Rs decreased and approached the levels observed in the mowed cover crop treatment. These observations are consistent with previous research in annual cropping systems by Calderón et al. 2000. In general, Rs were higher in 2005 than 2004. Rs (mmol CO2 m-2 s-1) were measured with a Licor 6400 with a 6400-09 attachment (Lincoln, Nebraska) after a simulated rainfall in the tilled and mow treatments present in the rings that were labeled with 13CO2. When dry soil was rewet with a simulated rainfall of 10 cm approximately four months after the cover crop was mowed or tilled into the soil, a large pulse of CO2 was released from the soil. The increase in soil respiration in the mowed soil treatment was double that of the tilled soil treatment immediately after rewetting. However, the tilled soil exhibited a delayed response that was observed nine hours after the simulated rain event. These differences may be attributed to differences in soil moisture between the two treatments, as well as greater soil C availability in the mowed treatment.


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Table 3

D a y 2 0 0 4

Mow/Till 2004 D a y 2 0 0 4

Cover crop treatment 37 51 65 82 86 87 118 145 166* 181 195 209 223 238 252 286 321 ** 335 356

Mowed Rs 1.73 3.29 2.59 2.4 2.28 3.69 1.59 2.01 5.09 3.37 2.13 2.15 1.72 3.37 1.07 0.83 4.04 2.53 2.42 Tilled Rs 1.66 2.82 2.51 2.37 2.06 3.58 2.78 2.57 5.65 3.06 2.53 2.33 2.49 3.06 1.44 1.07 2.77 1.96 2.07

Soil

Moisture 19.89 21.04 20.22 30.7 17.26 14.3 7.7 3.07 8.76 6.67 5.55 6.87 6.27 4.57 5.08 3.07 16.59 16.9 17

D a y 2 0 0 5

Mow/Till 2005 D a y 2 0 0 5

Cover crop treatment 5 25 41 61 75 91 92 98 106 117 126 141 159 173 188

Mowed Rs 2.64 3.93 3.97 5.74 5.14 5.31 5.28 2.7 2.48 2.5 3.66 3.54 3.8 2.47 2.84 Tilled Rs 1.06 1.99 2.27 2.12 3.67 3.07 6.15 2.17 3.61 3.85 4.32 3.43 3.31 3.5 3.79

Soil

Moisture 19.25 16.33 17.16 19.13 13.86 17.69 15.51 17.29 14.1 12.8 17.7 16.5 9.82 * summer rain (unusual event) ** winter rain (beginning of winter rain season)

2001-2006 Mission Kearney Foundation of Soil Science: Soil Carbon and California's Terrestrial Ecosystems

Final Report: 2003010, 7/1/2003-6/30/2005

Image 2 Stable isotope labeling chamber: Oakville, California, February 2005

This research was funded by the California Department of Food and Agriculture –CDFA's Buy California Initiative and the USDA, Grant Agreement #02-0765; and the Kearney Foundation of Soil Science: Soil Carbon and California's Terrestrial Ecosystems, 2001-2006 Mission (http://kearney.ucdavis.edu). The Kearney Foundation is an endowed research program created to encourage and support research in the fields of soil, plant nutrition, and water science within the Division of Agriculture and Natural Resources of the University of California, within the Division of Agriculture and Natural Resources of the University of California.

*, kerri l steenwerth , danielle l. pierce figure 1kearney.ucdavis.edu/old...

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