Date of Award
Master of Science
Plant and Soil Science
Improved agricultural productivity due to use of fertilizers over the last century has resulted in yield of cash crops, such as corn (Zea mayes L), to be increased on a per hectare basis. Consequently, inadequate fertilizer management such as improper timing or over application has led to infiltration into aquatic environments which can be detrimental to the ecology of such systems. Agricultural systems within the Mississippi River Basin have contributed to large-scale eutrophication in the Gulf of Mexico through surface and dissolved fertilizer loading in upstream tributaries. In response to these concerns, nutrient loss reduction strategies (NLRS), have developed in order to minimize these contributions of eutrophication to aquatic environments. Among adjustments in agricultural practices, one solution is the implementation of cover crops at the end of the cash crop growing season. The primary purpose of cover crops is to increase retention of nutrients during the fall and spring through soil stabilization and nutrient uptake which can prevent erosion and dissolved pathways to fertilizer loading in aquatic environments. Common types of cover crops able to achieve these goals are categorized as winter cereal cover crops (WCCC) and namely, winter cereal rye (Secale cereale) (WCR) is preferred in the state of Illinois. Using WCR has provides addition potential benefits such as cold hardiness establishment, carbon sequestration, weed suppression, and altering hydrological conditions before or during the cash crop. Although there are a variety of benefits from WCR, there are documented tradeoffs due to the presence of WCR, namely, reduced corn yields due to diminished stand population and decreased nitrogen availability through the process of immobilization which results from a carbon to nitrogen ration (C:N) which is greater than 25:1. Our research centered around solutions to maximize benefits of WCR while minimizing negative tradeoffs to the subsequent corn. We hypothesized that reduced seeding rate and higher quality cultivars of WCR would lead to quicker decomposition of biomass (Chapter 1) and would result in corn yields (Chapter 2) that were higher than the alternative treatments of high seeding rates and typical cultivars of WCR. Additionally, we hypothesized that selecting alternative cover crop species such as crimson clover (Trifolium incarnatum), integrating crimson clover with WCR, and reducing seeding rate through precision planting of cover crops off of the corn row would lead to quicker decomposition and result in higher corn yields than the WCR treatment planted normally (Chapter 3). All research was conducted with two site-years for each study. Chapter 1 consisted of two studies (Study A and Study B) where WCR seeding rate was modified and consisted of five treatments of 0, 34, 56, 84, and 112 kg ha-1 of WCR (Study A), and where WCR seeding rate as well as cultivar was modified and consisted of five treatments (Study B). Treatments consisted of an initial no cover crop control and two cultivars, one typical rye considered as “normal” and a hybrid variety (KWS) considered as “hybrid” that were planted at rates of 67 kg ha-1, considered as “low”, and 100 kg ha-1, considered as “high”. The objective of both studies in Chapter 1 was to evaluate the influence of seeding rate (Study A) as well as seeding rate × cultivar had on (i) WCR biomass and nutrient composition, (ii) decomposition and C:N dynamics, and (iii) soil nitrogen dynamics during the growing season in 2021 (Year 1) and 2022 (Year 2). In Study A, it was found that overall biomass was higher as seeding rate increased linearly (R2 = .94) over the two years from 34, 56, 84, to 112 kg ha-1 (2810.43, 3022.14, 3179.89, 3416.52 kg ha-1, respectively). The seeding rate did not influence the rate at which WCR biomass decomposed due to similarities in carbon and nitrogen concentrations within WCR. Fluctuations in C:N ranged from a high of 37:1 at the beginning of the decomposition phase to a minimum of 21:1 by the end of the decomposition phase. Soil NO3-N and NH4-N measured lowest in the 112 kg ha-1 treatment at 15-30 cm in Year 1. Treatments with no cover crop had the highest soil NO3-N from 0-30 cm in Year 2. Overall biomass of WCR was consistently higher during both years in the hybrid WCR treatments at both seeding rates compared to the normal rye of the respective seeding rate. The ratio of carbon to nitrogen was higher in hybrid varieties (42:1) in Year 1 but not in Year 2. The decomposition rate of all WCR in Study B were similar and not influenced by the various treatments. Fluctuations of C:N ranged from a high of 42:1 in the beginning of decomposition to a minimum of 17:1 by the end of the decomposition phase. Estimated N release of all treatments were similar. Both NO3-N and NH4-N were higher in the no cover crop treatment at the end of the season from 0-30 cm during Year 1, while there was no end of year difference in Year 2. In conjunction with the results of Chapter 1, our objectives in Chapter 2 were to see how treatments from Study A and B influenced (i) corn grain yield, (ii) corn stand count, near difference vegetation index (NDVI), leaf area index (LAI), corn N uptake, corn ear composition, as well as end of year N balance, and (iii) to analyze how those components related to overall corn yield. We additionally included how the treatments’ influence on corn could impact soil N dynamics. In Study A, overall corn yield was influenced by WCR seeding rate (p < .05) as the no cover crop and 34 kg ha-1 treatment (11.57, 11.61 Mg ha-1, respectively) were significantly different from the 112 kg ha-1 treatment (10.73 Mg ha-1). Stand count for corn was also influenced by WCR seeding rate (p < .05) as it linearly decreased with increasing seeding rate (R2 = .90) from 70,0009 to 62,552 plants ha-1. The seeding rate influenced the NDVI reading as it was lower in the 84 and 112 kg ha-1 treatments, indicating greater potential soil N immobilization. It was found that yield was most strongly correlated with corn stand count and 1000 kernel weight. In Study B, corn stand count was the only variable influenced by treatment, which was highest in the no cover crop treatment and was lower in the hybrid WCR when compared to the normal WCR at their respective seeding rates. Yield, kernel weight, number, N uptake were all higher in Year 1 and N balance was lower in Year 1. Chapter 3 investigated how cover crop selection, integration, and planting method influenced all of the aforementioned objectives from Chapter 1 and 2. One study made up Chapter 3 (Study C) and consisted of six treatments which were a no cover crop control, WCR monoculture planted at a rate of 67 kg ha-1, crimson clover monoculture planted normally (CNP) at a rate of 28 kg ha-1, crimson clover monoculture precision planted off of the subsequent corn row (CPP) at a rate of 20 kg ha-1, a mixture of the WCR and crimson clover planted normally (RCNP) at a rate of 33 and 22 kg ha-1, respectively, and a mixture of WCR and crimson clover precision planted with crimson clover on the subsequent corn row (RCPP) at a rate of 50 and 7 kg ha-1, respectively. It was observed that overall biomass was driven by presence of WCR but was not significantly different from the mixture treatments in either year. The biomass of crimson clover was not impacted by precision planting, indicating the ability to lower seeding rate. Presence of crimson clover was responsible for the C:N ratio of the treatment as all crimson clover monoculture treatments, aside from Year 1 CNP due to presence of weeds biomass, were lower in C:N (17:1) than all other treatments. Decomposition rate was influenced by cover crop selection as CPP had the highest decay rate of all treatments in both years (-0.00111, -0.00118 in Year 1 and 2, respectively) and RCPP treatment decomposed quicker than WCR in Year 2. The ratio of carbon to nitrogen was lowest for crimson clover monoculture treatments, followed by mixture treatments. By the end of the decomposition phase in Year 1, all treatments had similar C:N ratios indicating biomass decomposition and higher N content in WCR. Year 2 had a lower amount of N concentration in all treatments which influenced C:N ratio of WCR associated treatments. Estimated N release was higher in the mixture treatments as their N content was higher than the WCR monoculture with more biomass than the crimson clover monocultures. Over the two years of the study, crimson clover monoculture treatments resulted in the highest yields (10.16 and 10.11 Mg ha-1 for CNP and CPP, respectively) which were significantly different than the RCPP and WCR treatments, resulting in higher N balances in the RCPP and WCR treatments. Year 2 had lower corn stand count, yield, kernel weight, kernel number, NDVI. Yield was strongly correlated with CSD (.81), diameter (.91) and length (-.91). During both years, soil NO3-N and NH4-N were similar in all treatments by the end of the season indicating uptake by corn. We conclude that in Southern Illinois it may not be fiscally responsible for a grower to use seeding rates over 34 kg ha-1 or hybrid cultivars if their intention is to use WCR as a cover crop before corn in their cropping system. Although the biomass was higher, decomposition was not quicker than lower seeding rate of WCR or typical varieties of WCR. Integrating WCR with crimson clover did not result in lower biomass which may be a practical solution to lowering C:N in the cover crop system, aiding in decomposition so the biomass associated N is able to accessed by corn without being loss to early in the growing season through leaching. Precision planting of cover crops did not impede biomass accumulation which indicates seeding rates and planting design possibilities for WCR, and crimson clover cover cropping systems. Corn stand density was highly impacted by the presence of WCR which indicates the need for adjusting rate and cover crop selection in order to minimize yield reduction in corn.
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