Current Projects

Ag Runoff and Water Quality:  Examining Alternative Management Strategies for Reducing Overland Flow and Pollutant Loads in Agrarian Landscapes

A. Ristow, B. Sivakumar, W. Horwath, W. Wallender, S. Prentice
Department of Land, Air, and Water Resources
Soil Science Graduate Group

Agricultural activities are a significant source of non-point source pollution of California's surface water. Beginning in January 2005, new regulations will hold California growers accountable for known pollutants draining off of their land. Conservation tillage and cover cropping are two practices for reducing runoff and minimizing nutrient and sediment losses. Automated water samplers at the long-term UC Davis Sustainable Agricultural Farming Systems research plots along with farmer fields in the surrounding Central Valley provide year-round monitoring of surface runoff. Runoff volume and quality of water constituents such as suspended sediment, phosphorus, nitrogen, and dissolved organic carbon are being determined. In addition, total dissolved nitrogen and phosphorous concentrations are being examined. Preliminary results indicate that cumulative runoff from winter precipitation from non-cover crop (NCC) field was tenfold higher than a cover cropped (CC) field. The proportion of runoff from NCC relative to CC increased throughout the rainy season. Peak runoff flow velocities and suspended sediment concentrations were higher for NCC runoff events. In addition, the NCC runoff contained twice the dissolved phosphate and ammonium concentrations and four times the nitrate concentrations compared to the CC field. Relationships of surface runoff from rainfall/irrigation and management practices will be used to develop monitoring tools for different land uses and management practices such as conservation tillage and cover cropping to minimize the export of water constituents of concern.

---

Determining the agronomic potential, and N and C budgets of alternative cover crop mixtures and management in a California conservation tillage, tomato-corn system

K. Koffler, J. Six, C. van Kessel, S. Kaffka and S. Temple
UC Davis, Department of Agronomy and Range Science
International Agricultural Development Graduate Group

High crop production levels in California have been sustained by the use of synthetic fertilizers, pesticides, irrigation water, and intensive tillage operations. These tillage operations keep production costs high, generate significant amounts of dust, increase wind and water erosion, and reduce soil organic matter contents. The resulting loss in soil C offsets gains in crop C sequestration and contributes to rising atmospheric CO2 levels. Recent efforts in conservation tillage (CT) research have been aimed at reducing tillage costs, protecting air, water, and soil quality, and increasing soil fertility and the rate and duration of soil C storage. In this study, we are examining and comparing the compatibility of novel inter-cover crop mixtures with a low-input, irrigated, CT tomato-corn rotation. Preliminary studies have demonstrated the agronomic potential of a late-summer inter-cover crop mixture of sorghum-sudan (SS), cowpea (CP), and lablab (LL), and led to the hypothesis that adding this component to CT systems will enhance soil quality and fertility, increase C sequestration, aid in weed suppression, increase water quality and water use efficiency, and reduce runoff of water and nutrients. The SS is an aggressive N scavenger and is expected to tie up and cycle forward free N leftover from tomato production, forcing the CP and LL to meet more of their N needs through N-fixation. Continuous cropping/cover cropping produces more annual biomass to be cycled back into the system, providing a greater potential for higher soil C storage.

Our cover crop study consists of three replications of five treatments: 1) Lana vetch seeded in mid-late November, 2) SS, CP, LL seeded in late August, minimal sprinkler irrigation, 3) SS, CP, LL, lana vetch seeded in late August, minimal sprinkler irrigation, 4) SS, CP, LL seeded in late August, minimal sprinkler irrigation, and overseeding of lana vetch in mid-late November, 5) Fallow (no cover crop between the tomato and corn crops). In spring 2004, corn will be seeded directly into the cover crop residues.

We will be evaluating the agronomic performance of the different treatments and their effects on whole system C and N cycling. Data will include cover-crop stand establishment, soil and plant C and N pools, weed density and seed production, and corn growth and yield. To measure treatment effects on N dynamics, we are labeling the cover crops with 15N to more precisely monitor both N fixation and the amount of cover crop-derived N used by corn the following season. Labeling will also help us determine which cover crop treatments are best synchronized to meet corn N demands. We will use soil physical fractionation techniques to detect changes in N pools and its isotopic signature, and C pools over the 1.5 years of this study. We will also measure fluxes of CO2 and N2O. The results on C and N cycling and fluxes will be integrated to assess the potential of CT and cover crops in the mitigation of greenhouse gas emissions.

For More Information Contact:

Kaden Koffler kbkoffler@ucdavis.edu 530-752-5933

---

Weed Control, Yield and Economics of Processing Tomato (Lycopersicum esculentum) Under Different Tillage and Irrigation Systems

K. Sutton, W. Horwath, W.T. Lanini, J. Mitchell
Department of Land Air, and Water Resources
Department of Vegetable Crops
Soil Science Graduate Group

Adoption of conservation tillage in the arid Western U.S. has been hindered by the need for increased herbicide use and maintenance of furrows for irrigation. An effective solution to these weed control and irrigation issues would increase conservation tillage adoption and lessen the negative environmental impact of increased herbicide use in those systems. A field experiment was conducted in Davis, CA during the 2003 and 2004 summer growing seasons to compare weed control, yield, water use, fruit quality and economics of irrigation and tillage systems in processing tomato (Lycopersicum esculentum). Treatments were split-split factorials with the main plots as irrigation [subsurface drip irrigation (SDI) or furrow irrigation (FI)], sub-plots were tillage [standard tillage (ST) or conservation tillage (CT)] and sub-sub-plots were herbicide [herbicide (HE) or no herbicide (NH)] using four replications. The hypothesis was that drip irrigation could limit upper soil profile wetting and thus inhibit germination and growth of weeds equal to or better than standard tillage and/or herbicides. In 2003 weed densities were on average 55 times lower in the SDI treatments than in FI treatments. In addition, estimated weed populations were lower in the SDI/CT/NH treatment than in any of the FI treatments, including the FI/ST/HE treatments. Weed biomass was 10 times greater in the FI systems as compared to the SDI systems. These results demonstrate the effectiveness of subsurface drip irrigation in controlling weed germination and growth, even more so than tillage or herbicide applications. Fruit quality was not related to treatment. Water usage was over 50% lower in the SDI systems. The FI/ST/HE system had 6% higher yields than the next highest system which was SDI. Some of this yield discrepancy can be attributed to suboptimal management with the SDI system however, the economic data show that the reduced income from SDI yield is overcome by a large reduction in costs. These data indicate that a change from FI to SDI could result in net savings within 5 years. Most of this savings is in water and labor. These results indicate that subsurface drip irrigation can control weeds in conservation tillage systems, while limiting herbicide applications, lowering costs, and conserving water.