Soil carbon and nitrogen dynamics affected by tillage and cover cropping

Gates, Brianne Adele

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https://hdl.handle.net/2142/120165 Copy

Description

Title

Soil carbon and nitrogen dynamics affected by tillage and cover cropping

Author(s)

Gates, Brianne Adele

Issue Date

2023-05-03

Director of Research (if dissertation) or Advisor (if thesis)

• Gentry, Lowell E
• Margenot, Andrew J

Committee Member(s)

• Villamil, Maria Bonita
• Yang, Wendy H

Department of Study

Crop Sciences

Discipline

Crop Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• Carbon
• Belowground biomass
• Roots
• Tillage
• Nitrate loss
• Reduced tillage
• Cover crops
• Soil health

Abstract

Intensive crop production systems in the Midwestern USA contribute significantly to nitrate-nitrogen (N) losses that compromise water quality in the Mississippi River Basin. Simultaneously, soil carbon (C) in many of these systems have been reduced over the last several decades. However, recent advances in agricultural land management practices have been able to mitigate the loss of soil organic C (SOC) and reduce nitrate-N losses into area watersheds. Reduced tillage and cover cropping are two practices utilized to address both issues, and this study was designed to evaluate the impact of varying tillage intensities and cover cropping in a maize-soybean rotation on soil C and N dynamics, nitrate-N loss through tile drainage, and crop yields. The first chapter of this thesis identifies the importance of standardized methods for root biomass quantification to better understand total plant biomass inputs contributing to C input and output balances in soil that drive SOC accumulation or depletion. Root biomass quantification is labor and time intensive, particularly compared to shoot biomass quantification, and is often overlooked when evaluating C input to soil. Many studies use shoot biomass to estimate root biomass from root to shoot ratios reported in literature, but this indirect method of root biomass does not account for the variability among in-field root biomass growth that is not reflected in shoot biomass. In this study, we tested transect, in-growth root core (IGRC), and monolith methods of direct root biomass quantification as well as the indirect method of estimating root biomass from shoot biomass. The objectives were to 1) compare root biomass collected or estimated from these direct and indirect methods of quantification and 2) evaluate the effect of adjusting calculation methods to better reflect crop species-specific row spacing and planting density. Using the results from these two objectives, we proposed a standardized method of root biomass quantification to enable greater comparability of total plant biomass contribution to soil C. We found that the indirect method of root biomass estimation did not predict root biomass collected by any of the direct methods, and potentially overestimates root biomass. Of the direct methods tested, the IGRC method requires more material and labor preparation ahead of in-field root biomass collection, and the monolith method requires a larger destructive footprint in the field than transect and IGRC methods. The transect method does not require advance preparation or material acquisition and was recommended as the standardized method for root biomass quantification to enable comparability across studies. Differences between original calculation methods and calculation methods adapted to account for planting spacing were observed among maize root biomass, but not soybean root biomass, suggesting that crop species root architecture affects root biomass quantification when plant spacing is included in calculation methods. The results of this study support the standardization of root biomass quantification as root biomass differed depending on the method used and will contribute to a better understanding of root biomass as soil C input in row crop production systems. The impact of tillage intensity and cover cropping on nitrate-N derived from soil organic N through mineralization was evaluated in the second chapter of this thesis. Cover cropping can increase C input to soil and can take up mineralized N to reduce the amount of N leached into tile drained systems following fall tillage, and reduced tillage can decrease net mineralization to reduce nitrate-N leaching as well. However, these practices can cause a reduction in maize and/or soybean yields, discouraging adoption by producers. In this study, conventional tillage, strip-tillage, and no-tillage, as well as strip-tillage with cover cropping was evaluated to identify which system would be effective in reducing nitrate-N losses by managing N mineralization. Additionally, soil health indicators (SHIs) were evaluated to determine the ability of C and N cycling related SHIs to detect nitrate-N loss and crop yield outcomes of different tillage intensity and cover cropping practices. The objectives of this study were to 1) evaluate nitrate-N loss responses to tillage intensity and cover cropping, 2) determine the extent to which these tillage and cover cropping practices impact crop yields, and 3) identify temporal variation among SHIs in response to treatment differences to determine their individual association with outcomes including crop yield and nitrate-N loss. We found that strip-till with winter barley cover crop ahead of maize decreased tile and soil nitrate-N concentrations and loads, effectively reducing nitrate-N losses, and strip-till without cover crop was able to decrease nitrate-N loads despite higher concentrations in soil. Though strip-till with winter barley cover crop and strip-till without cover crop reduced nitrate-N losses, maize yields were unaffected. Cereal rye ahead of soybean was ineffective in reducing nitrate-N losses but did not reduce soybean yields. Soil health indicators changed temporally but were generally unaffected by treatments and did not explain crop yield or nitrate-N outcomes of treatment differences in the short-term. This study demonstrates that cover cropping paired with strip-tillage can reduce nitrate-N losses without compromising crop yields, though these outcomes are not signaled early by SHIs, and can be useful in better understanding how tillage and cover cropping affect C and N cycling, yield, and water quality in Midwestern USA agronomic systems.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Brianne Gates

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