European Geosciences Union | May 7th, 2018
This study is focused on nitrogen loading from a¬†wide variety of crop and land-use types in the Central Valley,¬†California, USA, an intensively farmed region with high¬
This study is focused on nitrogen loading from a¬†wide variety of crop and land-use types in the Central Valley,¬†California, USA, an intensively farmed region with high¬†agricultural crop diversity. Nitrogen loading rates for several¬†crop types have been measured based on field-scale experiments,¬†and recent research has calculated nitrogen loading¬†rates for crops throughout the Central Valley based on a mass¬†balance approach. However, research is lacking to infer nitrogen¬†loading rates for the broad diversity of crop and land use¬†types directly from groundwater nitrate measurements.
Relating groundwater nitrate measurements to specific crops¬†must account for the uncertainty about and multiplicity in¬†contributing crops (and other land uses) to individual well¬†measurements, and for the variability of nitrogen loading¬†within farms and from farm to farm for the same crop type. In¬†this study, we developed a Bayesian regression model that allowed¬†us to estimate land-use-specific groundwater nitrogen¬†loading rate probability distributions for 15 crop and land-use¬†groups based on a database of recent nitrate measurements¬†from 2149 private wells in the Central Valley.
The water and¬†natural, rice, and alfalfa and pasture groups had the lowest¬†median estimated nitrogen loading rates, each with a median¬†estimate below 5 kg N ha -1 yr -1. Confined animal feeding¬†operations (dairies) and citrus and subtropical crops had¬†the greatest median estimated nitrogen loading rates at approximately¬†269 and 65 kg N ha -1 yr-1, respectively. In general,¬†our probability-based estimates compare favorably with¬†previous direct measurements and with mass-balance-based¬†estimates of nitrogen loading. Nitrogen mass-balance-based¬†estimates are larger than our groundwater nitrate derived estimates¬†for manured and non-manured forage, nuts, cotton,¬†tree fruit, and rice crops. These discrepancies are thought to¬†be due to groundwater age mixing, dilution from infiltrating¬†river water, or denitrification between the time when nitrogen¬†leaves the root zone (point of reference for mass-balance derived¬†loading) and the time and location of groundwater¬†measurement.