Dissertation/Thesis Abstract

Limited Influence of Soil Type and Biological Characteristics on Net Nitrogen Availability from Organic Fertilizers – Laboratory Incubations
by Smith, Cole Byron, M.S., University of California, Davis, 2020, 54; 27742201
Abstract (Summary)

Many positive environmental benefits are associated with organic agriculture, however, nutrient losses in the form of leaching and greenhouse gas emissions can still occur from using organic fertilizers. As with any fertilizer, achieving a high nutrient use efficiency (NUE) is central to avoid negative environmental impacts from nutrient export. Increasing NUE and reducing total inputs of agricultural reactive nitrogen (N) has been proposed as key indicators to measure progress towards meeting a number of UN Sustainable Development Goals (SDGs). In many agricultural systems, including ones under certified organic management, N is often a limiting nutrient, and is required to optimize yields and meet crop growth standards. Supplying N in organic cropping systems can be rather challenging due to the variations in availability of N from different sources, including different cover crop species, compost and/or plant and animal waste materials in liquid or solid form. These uncertainties associated with supplying crops with N in organic systems can increase the risk of losses to the environment. Here, we seek to better understand the influence soil edaphic factors have on N release from high N organic fertilizers and clarify how these fertilizers impact or are impacted by the soil microbial biomass carbon (Cmic) pool.

High N containing animal-waste derived fertilizers can supply N quickly and effectively, granting growers application flexibility that can be rare when using other non-animal waste organic fertilizers. Blood meal, feather meal, fish meal and seabird guano, all high N containing certified organic fertilizers with low C/N ratios (< 5), were incubated for 60-days under aerobic conditions, and sampled at 0, 3, 7, 14, 28 and 60 days. Two soils under organic management with different soil textures – high and low clay content – were compared. After application of organic fertilizer at 250 mg N kg-1 soil, net N mineralization rates (Nmin) were measured. A standard non-linear kinetic model was fit to Nmin data in order to evaluate the N release dynamics under incubation conditions. Additionally, to assess the impact of these amendments on Cmic and to determine whether Cmic size had any mediating effects on Nmin, chloroform fumigation-extraction was used to measure Cmic at time points 0, 3, 28 and 60 days. We hypothesized that Nmin will vary by soil type, showing higher Nmin within the soil containing higher clay content, and that Cmic will increase in both soil types with the change in Cmic partially accounting for this variability in Nmin.

Our results did not support the hypothesis that Nmin would vary according to soil type or increase in Cmic. Soil type exerted only a minor influence on Nmin at day 14 of the incubation and organic fertilizer type also only accounted for a small amount of the variability in % of the organic N (%Norg) mineralized after 14 and 28 days. The amount of amendment Norg mineralized over the entire incubation period ranged from -17.74 % Norg at day 3, to 52.96 % %Norg at day 60 in the sandy soil. The modelled Nmin parameters were used to estimate mineralized Norg from the fertilizers. The highest mineralization potential (N0) value of 96.45 mg N kg-1 soil was found for blood meal, which had a very low maximum %Norg. Seabird guano had the highest mean Cmic at 814.82 mg C kg-1 soil. The Cmic levels in all the organic fertilizer treatments fluctuated throughout the incubation period but were never statistically different from the control soils. Information from this study shows that growers can supply plant available N (PAN) consistently using these fertilizers but they may not impact labile carbon (C) pools such as Cmic.

Indexing (document details)
Advisor: Horwath, William
Commitee: Zhu-Barker, Xia, Geisseler, Daniel, Scow, Kate
School: University of California, Davis
Department: Soils and Biogeochemistry (formerly Soil Science)
School Location: United States -- California
Source: MAI 81/12(E), Masters Abstracts International
Source Type: DISSERTATION
Subjects: Soil sciences, Agriculture
Keywords: Kinetic modelling, Microbial biomass carbon, Nitrogen mineralization, Organic fertilizers
Publication Number: 27742201
ISBN: 9798645487508
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