Soil microbial aspects of cover cropping as conservation practice for corn monocultures under heavy nitrogen fertilization

Kim, Nakian (Nak Hyun)

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https://hdl.handle.net/2142/116145 Copy

Description

Title

Soil microbial aspects of cover cropping as conservation practice for corn monocultures under heavy nitrogen fertilization

Author(s)

Kim, Nakian (Nak Hyun)

Issue Date

2022-05-31

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

Villamil, María B

Doctoral Committee Chair(s)

Villamil, María B

Committee Member(s)

• Riggins, Chance W
• Rodriguez-Zas, Sandra L
• Zabaloy, María C

Department of Study

Crop Sciences

Discipline

Crop Sciences

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Cover cropping
• N fertilization
• Mollisols
• sustainable agriculture
• microbiology
• soil biodiversity

Abstract

Soil health degradation and nutrient losses are major symptoms of chemical imbalances from excessive nitrogen (N) fertilization in the crucial agricultural regions of the world. Cover cropping (CC) is a promising conservation practice with potentials to mitigate these detriments and improve soil health. Despite the fundamental roles that the soil microbial community plays as drivers of soil processes, how it responds to CC has not been well documented for the intensely managed simplified cropping systems. Thus, the aim of this research was to investigate whether introducing CC to a simplified cropping system can mitigate the disruption of its soil microbial community by heavy N fertilization. For this, the objectives of this research were to i) obtain baseline data by characterizing the soil before introducing CC, ii) describe this system’s soil microbial community after introducing CC, using bioindicators of high taxonomic resolution, and iii) characterize the soil N-cycling microbial community using its functional genes and nitrification and denitrification functionalities with enzyme assays. The baseline soil characterization was done to a depth of 90 cm in corn [Zea mays L.] monoculture (CCC) and the corn (Cs) and soybean [Glycine max (L.) Merr.] (Sc) phases of a corn-soybean (CS) rotation that received inorganic N fertilizers at successive rates (N0, N202, N269; kg N ha-1) for 36 years. The experimental plots were arranged as a split-plot in a randomized complete block design with three replicates. The characterizations of the soil microbial communities were done on the corn monoculture plots of this site with subplots of CC mixture of cereal rye [Secale cereale L.] and hairy vetch [Vicia villosa Roth.] (CC), and winter fallow controls (BF). The N rates and CC treatments plots were studied as a 3x2 split-plot arrangement in a randomized complete block design with three replicates for two years. The baseline soil characterization before CC observed that N fertilization within CCC significantly acidified the topsoil from pH 6.6 (N0) to pH 4.9 (N269), while increasing the nitrate (NO3-) level up to eight-fold. Within CCC, available phosphorus level was more than twice higher, and potassium was 40% higher with N0 than fertilized soils. Sc had lower water aggregate stability (WAS) than CCC and bulk density (Bd) decreased by 5% from N0 to N269 across rotations. After introducing CC, soil NO3- decreased by up to 50% across N rates, and CC biomass increased by 73% with N fertilization. The characterization of the soil microbial community found that N fertilizers increased the abundances acidophiles, NO3- reducers, and NO2- oxidizers, while various N-cycling genera including heterotrophic nitrifiers, nitrite oxidizers, and complete denitrifiers increased with N0. CC promoted eight indicators of diverse functions and niches, while only two N-cycling genera associated with BF. Finally, N fertilization decreased the abundances of nifH, archaeal amoA, and nirS by 7.9, 4.8, and 38.9 times, respectively, and they correlated positively with soil pH. CC increased the abundances of nirK by 1.5 times when fertilized. Bacterial amoA increased by 2.4 times with CC within N269, while N202 and N269 did not differ from N0 within BF; it also correlated positively with soil nitrate. Overall, this research found that CC could enhance the soil biodiversity of this system and reduce NO3- leaching but also increase the risks of nitrous oxide emissions without proper nutrient management. It also identified soil acidification and high N availability as the primary factors by which excessive N fertilization disrupts the soil microbial community. However, CC might not be enough to mitigate this process for N-cycling communities that have become resistant to conservation practices after long-term adaption to disruptions. This research provided valuable references for future CC research on these systems and its results can serve as primary information to understand the potentials and limitation of CC as a conservation practice.

Graduation Semester

2022-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Nakian (Nak Hyun) Kim

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