Thesis Defence: Soil Management Impacts on Nitrous Oxide Emissions and Soil Nitrogen Cycling Gene Abundances in Okanagan Valley Summerland Cherry Orchards

Katherine-Faye Jensen, supervised by Dr. Louise Nelson, will defend their thesis titled “Soil Management Impacts on Nitrous Oxide Emissions and Soil Nitrogen Cycling Gene Abundances in Okanagan Valley Summerland Cherry Orchards” in partial fulfillment of the requirements for the degree of Master of Science in Biology.

An abstract for Katherine-Faye Jensen’s thesis is included below.

Defences are open to all members of the campus community as well as the general public. Registration is not required for in-person defences.

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Abstract

Agriculture is a significant source of nitrous oxide (N2O) emissions, the quantity of which is owing largely to soil management practices, such as the application of mulch, inorganic and organic fertilizers with reactive nitrogen (NR), and nitrification inhibitors such as 3,4-Dimethylpyrazole phosphate (DMPP). Soil management practices have previously been shown to alter soil microbial composition and metabolic activity leading to changes in N2O production. However, soil is very heterogenous, and further studies are necessary to identify and recommend individualized best management practices (BMP). This thesis presents the results from one in-field study and two in vitro studies aimed at identifying the impacts of the previously stated management practices on soil physicochemical properties, N2O emissions, and gene abundance data for total bacteria (16SBac) and archaea (16SArch), their associated ammonia oxidizers (BamoA, AamoA), and various denitrification genes (nirS, nirK, nosZ I, nosZ II) in a sweet cherry orchard soil. The field study occurred in June and August of 2019 at the Summerland Research and Development Centre (SuRDC) sweet cherry orchard. The treatments consisted of bare no treatment (BNTC), and compost (CMP) and woodchip (WC) mulch. Gas measurements were collected for approximately 70 hours, after which, soil samples were collected for analysis of other parameters. The first in vitro study (I1) ended on incubation day 33 (D33) and utilized 7 treatments varying NH4+ (A), NO3– (N), and DMPP (I), while the second in vitro study (I2) lasted for 38 days (D38) and utilized 5 treatments varying NH4+, compost (C), and DMPP (I). Soil was sampled at the beginning and end of the incubation periods, and the gas was sampled at various increasing time intervals between 24 and 192 hours. In the field the CMP treatment produced significantly more N2O than the WC treatment. The CMP mulch had the highest % carbon and nitrogen, with a larger proportion of more soluble pre-senescence plant residues (leaf and grass cuttings), as opposed to the WC mulch, which was comprised of Pseudotsuga menziesii bark chips, with a high CN ratio with a higher proportion of insoluble lignin. The prolonged elevation of CN likely led to an increase in nitrogen use efficiency (NUE) and N assimilation by soil microorganisms to maintain homeostasis rather than denitrification, thus reducing N2O production compared to the CMP treatment. In the I1 in vitro study the ANI treatment not only produced more N2O than the uninhibited counterpart (AN) but produced the most N2O of all the treatments in the I1 experiment, while all other uninhibited/inhibited pairs did not produce significantly different quantities of N2O. However, NH4+ alone (A) produced the least N2O, comparable only to the BNTC and AI treatments. It is possible that the excess NH4+ and NO2-/NO3- in the I1 experimental group ANI, accompanied by the low cation exchange capacity (CEC) of the sandy soil, may have facilitated the spontaneous abiotic production of N2O in vitro. This may have been exacerbated by NH4+ or NO2- toxicity, which was indicated by the reduction in all genes tested from incubation day 0 (D0) to D33. In the I2 in vitro experiment, ACI produced more N2O than the AI treatment, but DMPP did not reduce the N2O produced when compared to each respective uninhibited counterpart (A, AI; AC, ACI). Additionally, the addition of compost to NH4+ (AC) did not significantly change the amount of N2O produced compared to NH4+ alone (A). The production of more N2O in the I2 experiment group ACI compared to AI illustrated the potential ineffectiveness of DMPP in systems with source(s) of NO3- alternate to that produced by nitrification. However, when uninhibited, compost did not seem to increase the production of N2O produced by NH4+, perhaps due to the increased CEC and CN ratio of the soil, which aided adsorption of NH4+ and assimilation into microbial constituents. In conclusion, the use of organic mulch with a high CN ratio and a low soluble C fraction may reduce N2O produced by fertilization and irrigation in-field. However, DMPP should be tested in-field at orchards in the Okanagan with coarse and fine soil types to identify its effectiveness in reducing N2O emissions. Specifically, it may be prudent to identify how overfertilization, combined NR, and organic mulch amendments affect DMPP activity during the irrigation period.