Document Type

Dissertation - Open Access

Award Date

2024

Degree Name

Doctor of Philosophy (PhD)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Sutie Xu

Abstract

This research assessed the impacts of crop rotations and cover crops on the soil's biochemical and hydrophysical properties within long-term no-till systems. The research was conducted at the Southeast Research Farm, near Beresford, South Dakota. The crop rotations contain two levels: 4-year corn (Zea mays L.)-soybean (Glycine max L.)-oat (Avena sativa L.)-rye (Secale cereale L.) (CSOR) and 2-year corn-soybean (CS) rotations. The cover crop factors also had two levels within each rotation treatment: cover crops (CC) and no cover crops (NCC). All plots were managed under long-term no-till (NT), with soils mapped as Egan silty clay loam (fine-silty, mixed, superactive, mesic udic haplustolls) and slopes varying from 1 to 2 %. The treatments were arranged as split plots within a randomized complete block design with four replicates. Crop rotations were assigned to the main plots, and cover cropping treatments were assigned to the subplots. In addition, this research also assessed the impacts of NT and conventional tillage (CT) on soil moisture dynamics and the ability of Hydrus (2D/3D) to optimize soil hydraulic parameters through inverse modeling and simulate soil moisture dynamics. For the soil moisture modeling study, plots managed under CSOR and CC were selected, and NT and CT were arranged in a randomized complete block design with four replicates. In the first study (study 1) of soil biochemical properties, soil samples were collected at 0-7.5 cm in fall 2021 after harvest and spring 2022 under both corn and soybean phases and analyzed to measure microbial biomass carbon and nitrogen (MBC, MBN), the microbial community structure with phospholipid fatty acids (PLFA) analysis. Moreover, deep soils from 0-80 cm (0-7.5, 7.5-15, 15-30,30-45,45-60, and 60-80 cm) were collected in fall 2021 after harvest in both corn and soybean phases. These samples were analyzed to determine cold and hot water extractable carbon and nitrogen (CWEC, HWEC, HWEC, HWEN), permanganate oxidizable carbon (POXC), bulk density (BD), SOC concentration, nitrate-nitrogen (nitrate-N), and Mehlich-3 phosphorus (Mehlich-3 P). The results showed that CC significantly increased MBC, MBN, total PLFA, total bacterial PLFA, Gram (+) bacteria, Gram (-) bacteria, AM fungi, actinomycetes, β- glucosidase enzyme activities, CWEC, HWEC, HWEN, and SOC concentration at 0-7.5 cm, compared to those under NCC (P ≤ 0.05). Furthermore, CC significantly increased POXC at 0-7.5 (P < 0.001), 7.5-15 (P < 0.01), and marginally increased it at 15-30 cm (P = 0.0545). Using the equivalent soil mass approach, CC increased SOC stocks at 0-7.5 and 0-15 cm depth compared to NCC (P ≤ 0.05). The 4-year rotation (CSOR) significantly increased Gram (+) bacteria, CWEC, CWEN, HWEC, and HWEN (P ≤ 0.05), marginally increased Gram (-), total bacterial PLFA, and total PLFA at 0-7.5 cm (P = 0.0628, 0.0537, and 0.0705, respectively), and significantly reduced bulk density at 7.5-15 and 30-45 cm soil depth (P ≤ 0.05) compared to the 2-year rotation (CS). In the second study (study 2) of soil hydrophysical characteristics, intact cores were taken two weeks after soybean planting (the previous crop was corn) in spring 2022 at a depth of 0-40 cm (separated into 0-10, 10-20, 20-30, and 30-40 cm) under each crop rotation and cover crop treatment. X-ray computed tomography (XCT) scanning was used to measure macroporosity (XCTMP) and other macropore characteristics. Laboratory analysis was conducted to measure saturated hydraulic conductivity (KS), bulk density (BD), soil pore distributions, and soil water retention (SWR) at different tensions. The results from XCT method showed that CC had 31% higher number of macropores (MP), 30% higher macropore density (MPD), 38% higher XCTMP, 36% higher macropore length density (MPLD), 40% higher macropore branch density (MPBD), and 44% higher macropore node density (MPND) compared to NCC at 0-10 cm (P ≤ 0.05). Consistently with the data from the XCT method, CC increased water retention-derived total porosity, coarse mesoporosity, and fine mesoporosity by 9%, 24%, and 28%, respectively, reduced BD by 9.5% at 0-10 cm, and exhibited 75%, 60%, and 42% higher saturated hydraulic conductivity (KS) at 0-10, 10-20, and 20-30 cm, respectively, compared to NCC (P ≤ 0.05). Additionally, CC treatments retained 8%, 9%, and 9% higher moisture content at saturation (0 kPa), -0.6 kPa, and -1 kPa, respectively, compared to NCC at 0-10 cm (P ≤ 0.05). On the other hand, the 4-year rotation (CSOR) showed 27% higher XCT-derived MP and 28% higher XCT-derived MPD than the 2-year rotation (CS) at 0-10 cm (P ≤ 0.05). Across the 0-40 cm depth, CSOR had 19% higher XCTMP than CS. The third study (study 3) of moisture dynamics continuously measured soil moisture and water potential for three growing seasons (2021, 2022, and 2023) using moisture sensors placed at 15 and 30-cm soil depths in NT and CT plots under CSOR and CC treatments. Hydrus (2D/3D) was used to optimize van Genuchten hydraulic parameters (residual moisture content, saturated moisture content, alpha, n, and saturated hydraulic conductivity) and simulated moisture dynamics, using in-situ measured soil moisture and water potential. Results showed that NT consistently had higher numerical soil moisture and water potential compared to CT. A marginally significant difference was detected in moisture content (SMC), which was 25% and 20% higher in NT compared to CT at 15 cm and 30 cm soil depth (P = 0.06 and 0.10, respectively) in 2022, and 38% higher in NT than CT at 15 cm in 2023 (P = 0.10). In 2022, soil moisture potential (SMP) was significantly higher at 15 cm depth (P = 0.02) and marginally higher at 30 cm depth (P = 0.07) under NT than that under CT. The coefficient of determination (R2) from Hydrus (2D/3D) simulations ranged from 0.61 to 0.94 for SMC and 0.81 to 0.99 for SMP, with the root mean squared error (RMSE) varied from 0.01 to 0.03 cm3 cm- 3 for SMC and -29 to -98 cm for SMP. Hydrus (2D/3D) successfully estimated soil hydraulic parameters.

Library of Congress Subject Headings

Soils.
Soil biochemistry.
Cropping systems.
Cover crops.
Crop rotation.
Crop diversification.
No-tillage.

Publisher

South Dakota State University

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