Document Type

Thesis - Open Access

Award Date

2024

Degree Name

Master of Science (MS)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Jason Clark

Abstract

Phosphorus (P) management is an ongoing challenge in modern agriculture. Due to the importance of P in agriculture, P fertilizer additions are made throughout the U.S. to avoid nutrient-depleted soil. However, the application of P is problematic, as an excess of P in an agricultural system elevates the risk of environmental degradation, and current synthetic P sources are a fleeting resource. Additionally, a majority of P found in agricultural soils are insoluble, in the form of secondary and primary P minerals, making this large pool of P unavailable to plants. Fortunately, soil fungi organisms, such as arbuscular mycorrhizal fungi (AMF) have been studied extensively for their P sustainability potential, as these organisms are capable of dissolving insoluble complexes and providing P to host plants. The implementation of conservation practices like no-till have been found to affect soil test phosphorus (STP) level, because of the increase of AMF in these systems. The effect of varying STP levels on the formation of AMF in notill managed soil has not been studied. Therefore, this study was conducted to 1) determine in a long-term no-till field the effect of different STP levels on crop yield and soil biological activity as measured by the phospholipid fatty acid (PLFA) and mostprobable- number (MPN) assay and 2) determine if the more cost-effective PLFA test instead of the MPN test can be used to effectively identify any differences in AMF due to STP level. At the Dakota Lakes Research Farm in Pierre, SD soil test levels were drawn down to 5 ppm Olsen P in 2014. A five-year crop rotation was implemented in this field prior to the study in 1994 (soybean-wheat/cover crop-soybean-corn-corn). To create field areas with low, medium, and very high soil test P categories within the field, P fertilizer rates of 0, 58, and 116 kg P2O5 were applied in randomized strips across the field in 2014. These rates were again applied to the same treatment areas in 2017, 2019, and 2021 to maintain three distinct soil test levels. After five years, there was not an economic advantage to maintaining STP level in the very high category (no P fertilization recommended) vs the low and medium STP categories (P fertilization recommended). One potential reason for this lack of yield difference between STP categories may be due to the greater amount of AMF fungi in the low (5 propagules gram-1) compared to the medium (2 propagules gram-1) and very high STP (1 propagule gram-1) soil, as tested by the AMF most-probable-number (MPN) assay. Conversely, results from the PLFA assay were inconclusive, as soil fungi and bacteria only showed microbial differences in one year out of the five year study, as a result of stressful weather conditions. These results indicate that P fertilizer recommendations may need to be revisited in long-term no-till systems, as these systems appear to require less P to maintain crop yields than conventional tillage systems as a result of AMF accumulations with no-till, allowing for lower amounts of P fertilization.

Library of Congress Subject Headings

Soils -- Phosphorus content.
Soil biology.
Soils -- Testing.

Publisher

South Dakota State University

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Rights Statement

In Copyright