Abstract
Understanding N transformations in fields receiving dairy manure applications is an important component of managing this nutrient source to maximize crop profitability and reduce environmental damage. The objective of this study was to determine the net N mineralization from field applied dairy cow manure to a Portneuf silt loam as affected by applications of varying rates, application intervals, and naturally fluctuating temperatures throughout the growing season. This study was conducted in a field located at the USDA Agricultural Research Service (ARS) Northwest Soil Research Laboratory (NWISRL) station in Kimberly, Idaho. Soil treatments included three manure rates (17.3, 34.7, 52.0 Mg ha^(-1), dry basis applied at two recurrence intervals (annual or biennial fall applications). The field was sprinkler-irrigated under spring barley (Hordeum vulgare L.) in 2013 and sugar beets (Beta vulgaris L.) in 2014. We monitored net N mineralization in the 2013 and 2014 growing seasons using the buried bag technique (amended soils were placed in polyethylene tube shaped bags and incubated in the field). Soil filled incubation bags were destructively sampled monthly or biweekly from March to October and analyzed for nitrate and ammonium. Predictive models were fit based on the analyses results. Crop N uptake was determined from end of season plant tissue analyses. Crop N uptake correlated well with N mineralization monitored in the buried bags yielding a linear regression r-square of 0.74. Manure that was fall-applied in 2012 resulted in significant increases in preplant soil inorganic N concentrations in 2013. In addition, manure treatments that either did or did not receive additional fall-applied manure in 2013 resulted in significant increases in preplant soil inorganic N concentrations in 2014.The zero-order linear model was selected for estimating N mineralization rate (k), N mineralization amount, the y-intercept, and data variability (r-square) over the growing seasons of 2013 and 2014 separately. The linear N mineralization rate showed a consistent increase in the release of N from April to September at one and two years after application as well as after two years of repeated fall applications. Increasing manure application rates also resulted in a linear increase in net N mineralization rates (k values) 1 and 2 years after a fall application, as well as after 2 years of repeated fall applications at the 0-30 cm soil depth.