Document Type

Dissertation - Open Access

Award Date

2020

Degree Name

Doctor of Philosophy (PhD)

Department / School

Agronomy, Horticulture, and Plant Science

First Advisor

Sandeep Kumar

Abstract

This study assessed the effects of three levels of crop rotation: [2-yr; corn (Zea mays L.)- soybean (Glycine max L.), 3-yr; corn-soybean-oat (Avena sativa L.) or 4-yr; cornsoybean- oat-winter wheat (Triticum aestivum L.)], two tillage [conventional-till (CT) and no-till (NT)], and two winter cover cropping systems [cover crop (CC) or fallow control (NC)] on soil biochemical and physical properties, greenhouse gas emissions (GHG), microbial community composition, crop yield and farm profitability under silty clay loam soil of south eastern South Dakota. Experimental design was a randomized complete block design in a split-split plot treatment arrangement with four replications. Rotations, tillage and cover cropping were, respectively, assigned as main-plot, sub-plot and subsub- plot factors. Results from soil samples collected for analyzing biochemical properties at surface 0-7.5 cm in 2017 indicate that these soil properties were enhanced by adopting CC and NT system. In general, the CC had 9, 17 and 19% higher geometric mean of enzyme activities (urease x β-D-glucosidase x phosphatase x arylsulfatase)1/4 than the NC at pre-planting, after planting and grain-filling stage of maize, respectively. Soil microbial biomass carbon (C) and β-glucosidase activity were 31 and 54%, respectively, higher with CC vs. NC under 4-yr rotation after planting of maize. At grain-filling stage, the hot water extractable C and nitrogen (N) contents were significantly greater under CC as compared to the NC plots. Results from soil physical measurements showed that CC reduced bulk density by 6% and increased saturated hydraulic conductivity and water infiltration rate by 1.5 times compared to the NC. Similarly, X-ray computed tomography (CT) measured total porosity, number of macropores and macroporosity were 43, 34, and 60%, respectively, higher with CC as compared to the fallow plots. Soils under 4-yr rotation had 16, 14, and 4% higher values of soil organic C, total N, and wet aggregate stability compared to those under 2-yr rotation, respectively. Also, 4-yr rotation significantly increased number of CT-measured pores, number of macropores, coarse mesopores, macroporosity, and mesoporosity than the 2-yr rotation. Soil surface GHG measurements were carried out during the growing seasons of maize and soybean phases under NT system in 2017 and 2018, respectively. Statistical differences in microbial community structure between treatments were few, however, in comparison to 2-yr and fallow management, the 4-yr rotation and CC had numerically greater specific biomarkers for bacterial or fungal populations. The 2-yr rotation had greater CO2 emissions than the 4-yr during growing season of 2017. However, 4-yr rotation increased the GHG fluxes during spring thaw of 2018. Cumulative CO2 emissions were greater under CC than the fallow when averaged over both the rotations during 2017, however, interaction effect during 2018 suggested that CC had lower CO2 emissions than the fallow only under 2-yr rotation. Measurements from 2017 and 2018 were further used to evaluate the ability of the DeNitrification-DeComposition (DNDC) model to predict field-measured soil surface CO2 and N2O emissions. Across all cropping treatments, model simulated soil (0–10 cm) temperature and moisture agreed well with the growing season field measurements. Predicted daily soil CO2 fluxes were accurate for corn phase in 2017, but model overestimated the simulated soil respiration compared to the measured data in 2018 for soybean. The statistics showed “poor” agreement between the simulated and measured N2O emissions because DNDC model underestimated the fluxes during both the crop phases. Nevertheless, these studies suggest that cropping system diversification achieved by extending length of rotations through small grains and by growing winter CC such as winter rye under NT system has the potential to enhance microbial community structure composition and mitigate GHG emissions. Yield and economic comparisons were conducted using the data collected from 2014 through 2018 years. Results suggest that NT system though reduced the corn yield but increased the soybean yield under 2-yr rotation as compared to the CT system. Therefore, both the tillage systems were economically equivalent, whereby NT improved benefit-cost ratio as compared to the CT system. In our study, while CC in its short-term did not contribute to economic benefit, our results indicated that incorporating CC in conventional rotation system under NT could provide an economically superior option to diversify the system. Increased length of crop rotations (3- and 4-yr) increased the corn and soybean yields as compared to the 2-yr rotation. In the context of overall profitability, however, the diversified cropping system in this study lagged the traditional corn-soybean system which could be attributed to the relatively lower profits of small grains. Therefore, it is important to identify other profitable crops to diversity the cornsoybean rotations those are beneficial for soils and the environment.

Library of Congress Subject Headings

Crop rotation.
Tillage.
Cover crops.
Soils -- Quality.
Greenhouse gases.
Crop yields.
Agriculture -- Economic aspects.
Agricultural productivity.

Format

application/pdf

Number of Pages

299

Publisher

South Dakota State University

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Rights Statement

In Copyright