Practices for Improving Weed Management and Soil Nitrogen Retention

The goal of this research is to develop alternative “in-row” weed management practices that provide effective weed management, do not negatively impact grapevine growth and juice, and improve soil nitrogen (N) retention by minimizing inorganic N leaching and nitrous oxide (N2O) emissions. The establishment of “in-row” treatments was initiated in October 2009. Greenhouse gas emissions (N2O and carbon dioxide, CO2) were collected bimonthly, and C and N dynamics in response to events that increase greenhouse gas emissions were measured from October 2009 to October 2011. Findings indicate that ?pulse? events such as cultivation, compost addition, fertigation, irrigation and rainfall are major periods of greenhouse gas emissions. When cultivation occurred just after compost addition in May of year 1 (Nov. 2009-Oct. 2010), N2O and CO2 emissions increased and were greater than cover crop and herbicide treatments. When rainfall occurred immediately after this cultivation, N2O and CO2 emissions were also greater than the other treatments, highlighting the interactive effects of management (i.e., cultivation and compost) and rainfall on greenhouse gas emissions. When treatments were irrigated in year 1, N2O and CO2 were greatest from the cultivated treatment, followed by cover crop and herbicide treatments, respectively. Irrigation in the year 2 (Nov. 2010-Oct. 2011) caused increases in CO2 and N2O emissions in all treatments. Cumulative N2O emissions over the two year study were greatest in the cultivated treatment followed by the cover crop and herbicide treatments, respectively. CO2 emissions over the two year period were highest in the cover crop followed by the cultivated and herbicide treatments, respectively. The cover crop and herbicide treatments tended to have greater nitrate leaching than the cultivated treatment in year 1. Temporal dynamics of leaching differed among treatments in year 1 but not year 2. In year 1, total nitrate leaching from the cover crop, herbicide, and cultivated treatments was 129.34±47.70, 104.37±55.45, and 50.83±3.20 µg NO3-N/g resin, respectively. In year 2, total leaching from the cultivated treatment tripled to 158.80±32.55 µg NO3-N/g resin. Increases in leaching were not observed in the herbicide treatment (138.98±28.87) or cover crop treatment (145.23±24.26). This study identified interactions among weed control practices, greenhouse gas production, nitrate leaching, and vine nitrogen status. Few differences among treatments were detected at this location, suggesting that over a short-term period (i.e., 2 years), no management practice appeared to be significantly better than any other. We anticipate that cover crops would emerge as a desired “in-row” practice due to improved nutrient retention, as observed in Steenwerth and Belina (2010). Other reasons to choose one weed control over another, such as organic certification, would weigh more heavily on choosing a weed