CO2 and N2O Soil Efflux in a Subsurface Drip, Cover Cropped, and Conservation Tillage Tomato System C. Kallenbach1*, W. Horwath1, Z. Kabir1, J. Mitchell2, D. Rolston1 1Department of Land, Air and Water Resources, University of California, Davis, 2Dept. of Plant Sciences, University of California, Davis INTRODUCTION RESULTS The California agricultural sector is a significant contributor of greenhouse gases (GHG; CO2 and N2O). Currently N2O contributes 38% to total state N2O production. The rate of CO2 and N2O emissions vary between different farming systems as well as over time. Factors influencing the degree of GHGs from agriculture include the rate and timing of inputs such as carbon, nitrogen, and water, as well as management practices, including soil tillage and fertilizer applications. Some alternative agricultural management practices such as subsurface drip irrigation (SDI), conservation tillage (CT) and winter legume cover cropping (WLCC) have the potential to reduce the rate of soil CO2 and N2O emissions as well as sequester carbon due to changes in input management. However, these reduction may only be on a seasonal basis. Subsurface Drip Irrigation Furrow Irrigation GHG Emission Treatment Comparison Seasonal Soil Nitrate Levels Fig. 5a Fig. 5b Fig. 5c Fig. 5d Dec. 2005 Mar 2006 July 2006 Sept 2006 NCC WLCC Fig 2b Fig 2a OBJECTIVES The goal of this research was to evaluate the integrated use of subsurface drip irrigation (SDI), conservation tillage (CT), and winter legume cover cropping (WLCC) as a means of limiting CO2 and N2O emissions in comparison to the more conventional management of furrow irrigation (FI), standard tillage (ST), and no cover cropping (NCC) in California’s Central Valley. Measurements were obtained over the course of 2 years in order to characterize variability among treatments between the growing season and winter. Each system is represented in a 3 acre field- trial planted in processing tomatoes at the UC Davis Long Term Research on Agricultural Systems (LTRAS) facility. Figures 2a and 2b. N2O emissions in SDI (2a) and FI (2b) from Jan 2006 to March 2007 obtained from field point measurements. Under SDI, N2O emissions were lower in all systems compared to FI during the growing season (May through August). However, rainy season N2O emissions under SDI were equal or higher to FI emissions. Under FI, the cover crop treatment (WLCC) consistently had higher emissions throughout the year compared to no cover crop (NCC). Figure 7. Soil nitrate from 0-15 cm. For all sampling dates, FI had significantly higher nitrate levels compared to FI regardless of tillage and cover crop except for the results from Sept. (post harvest). The WLCC treatment consistently had higher soil nitrate compared to NCC across tillage and irrigation treatments. Figures 5a- 5d. Treatment comparisons of growing season N2O emissions (ug N2O per m2 hr-). Each point represents a sampling date. Fig 5a and 5b are FI plotted against SDI by tillage; ST (5a) and CT (5b). Fig 5c and 5d are winter cover crop plotted against no cover crop by irrigation; FI (5c) and SDI (5d). Figures with slopes greater 1 can be interpreted as higher emissions from the y-axis treatment. Graphs for season and winter N2O totals are in “seasonal N2O” file. Precip graphs are in “cimis, precip daily graph” file under cimis folder Line graphs for all N2O measurments under “Weighted N2O” file Fig 3a Fig 3b Spatial Variation in CO2 Emissions TREATMENTS Subsurface Drip Irrigation Furrow Irrigation Standard Till (ST) Conservation Till (CT) WLCC NCC Summer Winter Fig. 6b Fig. 6a Figures 3a and 3b. CO2 emissions in SDI (3a) and FI (3b) from Jan 2006 to March 2007. CO2 emissions were generally lower under SDI throughout the year in all treatments compared to FI. During the growing season there was no significant differences between tillage and cover crop under SDI yet under FI the cover crop treatment generally had the highest emissions levels regardless of tillage. *Winter Legume Cover Crops, **Non cover crops CO2 and N2O production is facilitated by high levels of soil moisture. SDI restricts moisture to only plant roots while FI saturates most of the bed during the growing season. However, in the winter, the beds are equally wetted under uniform precipitation. In the FI systems fertilizer was shanked in at 100lbs N/acre while the SDI systems had 100 lbs N/acre delivered over the course the growing season via fertigation. The cover crop was a nitrogen- fixing pea/vetch mixture. Pattern of soil wetting under SDI, FI and precipitaion. Fig 4a Fig 4b Figures 6a and 6b. Irrigation treatment comparisons of growing season (6a) and winter (6b) CO2 emissions (mg CO2 per m2 hr-). Figure 8. Spatial distribution of soil CO2 production rates and soil moisture at 12 cm on August 4th and February 15th 2006. Tomatoes in subsurface drip July 21,2006 SUMMARY and CONCLUSIONS Our results suggest that in estimating agricultural greenhouse gas (GHG) emissions it is essential to consider not only the growing season but also the periods between post harvest and planting. Although all subsurface drip irrigation systems (tillage and cover crop combinations) had lower GHG emissions during the growing season compared to furrow irrigation, rainy season emissions were equal or higher to furrow irrigation emissions under most treatments, suggesting a lag effect of growing season management under subsurface drip. Cumulative annual N2O emissions are highest under furrow irrigation combined with standard tillage followed by subsurface drip irrigation combined with conservation tillage. Subsurface drip reduces CO2 emissions in all treatments except in conservation tillage-no cover crop, when compared to furrow irrigation. Significant reductions in N2O emissions can be achieved under standard tillage by switching from furrow irrigation to subsurface drip irrigation (~50%), although winter cover crops greatly increased both N2O and CO2 emissions regardless of irrigation and tillage practice. Jan May Sept Dec ST-NCC ST-WLCC CT-WLCC CT-NCC Figures 4a and 4b.Volumetric soil moisture for SDI (4a) and FI (4b) at 12 cm from Jan 2006 to March 2007. Under SDI, soil moisture was significantly higher during the winter season in all treatments compared to the growing season whereas FI soil moisture levels during the winter were only slightly higher compared to the growing season. Growing season soil moisture values for SDI were 10-20% and 20-50% under FI. Figure 1. Precipitation and irrigation events Acknowledgements: Financial support for this research is provided by the Kearney Foundation. Technical lab and field assistance is provided by Tad Doane, Dennis Bryant, Israel Herrera, and the LTRAS facility.