NMSU Graduate Student Studies Optimization of Nitrogen Application and Leaching in Pecan Orchards

By Catherine Ortega Klett, NM WRRI Program Manager

New Mexico now produces over 20 percent of the U.S. pecan crop, and most of that comes from the Mesilla Valley. Pecans need ample nitrogen during the nut enlargement and filling stages, along with lots of water. Since the latter is getting increasingly scarce, there is growing use of brackish groundwater for irrigation. But the salinity of such water reduces the effectiveness of the nitrogen fertilizers, so that higher application rates are needed. This increases the risk that some nitrogen and brackish water will quickly move below the root zone and eventually leach into the groundwater as contaminants. Of course, this risk depends also on the irrigation system and soil composition, stratification and texture, as well as the meteorological conditions of the growing season.

The implied overall need to construct a decision support system for proper management of orchards to ensure sustainable good yields while at the same time maintaining soil and groundwater quality is the subject of the research project of Esmaiil Mokari, PhD student of environmental soil physics in the NMSU Department of Plant and Environmental Sciences. In partial support of this goal, Esmaiil received a 2018 NM WRRI Student Water Research Grant entitled: Optimizing fertilizer application and leaching under abiotic stresses within and below the Root Zone of Pecan Orchards. In this effort, Esmaiil is working in collaboration with his faculty advisor in the same department, Dr. Manoj Shukla.

The approach taken in this study is to combine a limited set of field measurements of water and nitrogen at various depths and times after fertilization, and then to use this information to calibrate model simulations that can extend and generalize the observed trends to gain a deeper understanding of nitrogen balance in irrigated pecan orchards. The model uses the computer software package, HYDRUS, which simulates the movement of water and multiple solutes through soil and includes nitrification and denitrification reactions.

For the current application, the orchard is under flood irrigation, so that the variation of properties occurs essentially just in one dimension, namely the vertical. If needed, such simulations can be generalized to represent variations in two or three dimensions as well. The parameter chosen to track nitrogen is its concentration in the form of the nitrate ion. Although the fertilizers used for the field measurements were urea and ammonium sulfate, neither of which are nitrate compounds, soil bacteria oxidize the nitrogen in the fertilizer to nitrate. Other bacteria can do the opposite and reduce it to elemental nitrogen, which plants cannot use because it is too unreactive. These competing rates of nitrification and denitrification can be estimated and included as parameters in the model simulations.

A specific objective of the research conducted to date has been to demonstrate that an optimized set of solute transport/reaction parameters can, in fact, be found that are consistent with the field measurements of nitrogen transport and transformations in the pecan orchard. Other objectives include determining soil water and nitrate dynamics within and below the root zone, the estimation of effects of water and nitrate variations on nitrate uptake by roots, and the overall level of nitrate as a function of fertilization over two growing seasons in a pecan orchard.  Qualitatively, the investigation also supports the idea that to reduce nitrate leaching, more frequent but lighter applications of nitrogeneous fertilizers are recommended for flood-irrigated orchards.

Esmaiil is in the second year of his PhD program, and he expects to graduate in August 2021.  He is originally from Iran, and graduated with a BS in water engineering from the University of Bu-Ali Sina, Hamedan, Iran; and with an MS in irrigation and drainage engineering from the University of Tehran, Iran. This research by Esmaiil is already accepted for publication in the Soil Science Society of America Journal, which is a very prestigious journal in soil physics and hydrology. Esmaiil has also published other articles in peer-reviewed journals, and he says he’s enjoyed the experience of becoming adept at solute transport modeling. He hopes eventually to obtain an academic position as a scientist where he can continue to carry on with his scholarly research.