Presentations 2016

Can Nitrate Leaching form an Orchard Be Accurately Estimated?

UC Davis

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Ground water pollution by fertilizer losses from agriculture land, mainly as nitrogen (N) compounds, is of major concern worldwide. The huge spatial and temporal variability in water flow and N-transport dynamics in an orchard makes it extremely challenging to accurately estimate N losses form such agricultural units. A 2-year study was conducted to explore nitrate (NO3-) leaching below the root zone of an almond orchard. Temporal changes in water content, pore water NO3- concentrations and soil water potential were monitored within and below the root zone to a soil depth of 3 m at eight sites which represented spatial variations in soil layers at the orchard. NO3- concentrations below the root zone ranged from <1 mg L-1 to more than 2400 mg L-1 with mean concentrations of 326 mg L-1. Within the fertigation cycle, fertilizer injection at the end of an irrigation event generally resulted in lower NO3- losses below the root zone compared with fertilizer injection mid-way through the irrigation. Statistical analysis using principal component analysis, Chi-squared Automatic Interaction Detector and the Artificial Neural Network showed that most of the deep soil NO3- concentration variability could not be explained by irrigation duration, fertigation timing or local variations in soil physical characteristics. In addition coefficients of variation and semivariograms indicated weak (RMSE 220 mg NO3- L-1) spatial correlation up to a distance of 60 m.Despite the huge variability in the NO3- concentration, the orchard average annual N-losses estimated based on N-mass balance, water mass balance, flow calculations and HYDRUS modeling were all in the same order of magnitude (80 – 150 kg-N ha-1 y-1). All methods indicated that most of the N losses occur early in the growing season (February – May), when fertilizer is applied to wet soil profiles. The study indicated that simple mass balance (i.e. water and N-load applied minus water and N-load removed) provides a good proxy of the annual N losses and that eight vadose zone monitoring sites can capture the orchard variability. Reduction of N losses at the orchard scale would require alternative fertigation and irrigation practices, including better control of fertigation amounts and irrigation duration.