Transformations of Nitrate, Ammonium, and Urea When Applied to Pine Bark-based Substrate

Nitrogen (N) cycling and transformations remain topics of interest in crop production to determine the efficiency of applied mineral N. However, aspects of N cycling in agriculture that are often overlooked are the associated N cycle reactions and transformations based on applied N forms to horticultural crops produced in containers using soilless culture. This research aimed to conduct a fundamental investigation to gain a better understanding of how individual N sources react after being applied to a soilless substrate (pine bark). To accomplish these objectives, we identified N cycle processes by measuring aqueous and gaseous intermediate N forms to determine enzymatic and microbial-mediated processes after a single application of three distinct N sources [urea (CH 4 N 2 O-N), ammonium (NH 4 + -N), or nitrate (NO 3 − -N)]. We conducted these experiments during two pseudo-replicated studies using the predominate production systems in specialty crop production in the United States (open-wall high tunnel to simulate open-air nursery production or controlled environment glass greenhouse). No notable differences were observed between the two production systems. Our results indicate that a sequential set of reactions occur based on the applied N source (urea hydrolysis, nitrification, and denitrification) and trend toward complete denitrification and the production of N 2 gas via major N cycle processes. These data also imply that CH 4 N 2 O-N, which is the least expensive N source, emits higher concentrations of reactive nitrogen (RN) gaseous species, predominately nitrous oxide (N 2 O) and nitric oxide (NO), compared with the NO 3 − -N source in the substrate aqueous phase. The NH 4 + -N source produces the same RN emissions as those when applying CH 4 N 2 O-N and NO 3 − -N. Findings of this research suggest that CH 4 N 2 O-N may be preferred by growers based on associated costs, but increased inefficiency compared with NO 3 − -N-based products may exist. Furthermore, these data suggest that higher RN emissions occur during hydrolysis of CH 4 N 2 O to NH 4 + and during the nitrification of NH 4 + to NO 3 − , more so than that during denitrification from NO 3 − to N 2 . We hypothesized that N transformations can be determined by measuring substrate pore water and gaseous emissions during N transformation and harvesting plant tissue and substrate before and after N source applications.