Spatial Variability of Nitrates and Nitrogen Mineralization Kinetics and Their Effects in Long-term Tillage Systems

Authors

• Bibash Rokaya, University of Nebraska-LincolnFollow

First Advisor

Aaron Lee M. Daigh

Committee Members

Bijesh Maharjan, Javed Iqbal

Date of this Version

3-2026

Document Type

Thesis

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Agronomy

Under the supervision of Professor Aaron Lee N. Daigh

Lincoln, Nebraska, March 2026

Comments

Copyright 2026, Bibash Rokaya. Used by permission

Abstract

Nitrogen (N) and water management are essential for water quality protection and crop production. However, soil N and water are spatiotemporally variable. Soil water spatial dependencies are described in the literature with known associations with preferential flows among other factors. In contrast, a knowledge gap exists on the spatial dependence of soil N mineralization kinetics. We hypothesize that soil N mineralization kinetics and soil water have distinct and cross-correlated spatial characteristics among soil management practices and climatic regions. For the field study, we measured soil-N mineralization kinetics, volumetric water content (VWC), and a suite of soil properties along ~70 m linear transects to quantify their spatial characteristics, cross-correlations, and effects of long-term soil tillage practices (i.e., no-till and double disc) at the Panhandle Research, Extension and Education Center (PREEC) near Scottsbluff, Nebraska and the South Central Agricultural Laboratory (SCAL) near Harvard, Nebraska. The soil N mineralization kinetics was determined using a 9-week aerobic incubation with leachate collected at 0, 1, 3, 5, 7 and 9 weeks for inorganic N analysis. A 100 ml of 0.01 M CaCl₂ solution was used to produce leachate during the incubation followed by applying a 25 ml of zero-N nutrient solution. Additional samples were taken to preliminarily evaluate the effects of nitrogen fertilizer treatments at the East Nebraska Research, Extension and Education Center (ENREEC), near Mead, Nebraska. A total of 438 soil samples were taken between late June to early July when spatial variability for soil-N, VWC, and NO₃-N leaching is expected to peak for Nebraska cropping systems. The systematic review revealed a substantial knowledge gap, with only one published study reporting spatial structures for mineralization kinetics. Additionally, field-scale study showed that potential soil N mineralization parameters, N₀ and k, exhibited substantial spatial variability across short distances (< 70 m), with variations by up to an order of magnitude within transects assumed to be uniform. Spatial analysis also revealed mixed autocorrelation patterns in VWC and N-mineralization parameters, with no consistent cross-correlation with bulk soil properties. Overall, VWC was not a reliable proxy for variability in soil N-mineralization.

Advisor: Aaron Lee M. Daigh