Potential enzyme activities in soils as affected by perennial cropping and nitrogen rates in central Iowa

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2023-12
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Khaleel, Ala
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Thompson, Michael
McDaniel, Marshall
Burras, Lee
Olk, Daniel
Soupir, Michelle
Beattie, Gwyn
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Agronomy
Abstract
The agricultural landscape in the US Midwest faces significant environmental challenges associated with annual crop rotations, particularly the intensive cultivation of soybeans and maize that is heavily reliant on nitrogen (N) fertilizers. In contrast, perennial biomass crops are gaining attention for their notable environmental benefits, such as efficient nutrient cycling, enhanced water quality, reduced soil erosion, increased soil organic carbon (C), and overall soil health improvement. Extracellular enzymes (EEs), produced mainly by soil microorganisms but also by plant roots, largely mediate soil organic matter (SOM) decomposition. These EEs in soil are known to play a substantial role in maintaining soil health by catalyzing nearly all important transformations in the cycling of C, N and other nutrients. Microbial enzyme-mediated processes (e.g., SOM decomposition and nutrient cycling) can be evaluated based on the kinetic parameters of EEs. Two important parameters are defined by the Michaelis-Menten (M-M) model of enzyme kinetics: maximum enzyme reaction velocity (Vmax) and the M-M constant (Km) that represents the substrate concentration at one-half of Vmax. In this dissertation, I first investigated the impact of annual crops [maize-soybean rotation (CS)] and perennial crops [fertilized prairie (PrF) and unfertilized prairie (Pr)] on the kinetic parameters of two extracellular enzymes (EEs) involved in C and N cycling: β-glucosidase (BGase) and leucine aminopeptidase (LAPase), respectively. I found that the N-fertilized perennial prairie affected the Vmax but not the Km of both enzymes. The increased Vmax of BGase in the PrF system compared to the CS system is likely driven by greater microbial activity in the prairie system. The increased LAPase Vmax in unfertilized Pr system compared to both the fertilized PrF and CS systems was likely due to microbial N demand. Additionally, I observed that the substrate concentration needed to achieve Vmax exceeded the substrate concentrations typically employed in many other enzyme studies, suggesting the necessity to determine a substrate saturation curve for each soil and treatment before conducting similar enzyme assays. Most potential enzyme activity (PEA) studies focus on surface horizons where C and nutrients concentrated, but the influence of various soil properties on PEA in subsoil horizons remains largely unexplored. Recognizing the substantial changes in these properties with depth, variations in PEA may also interact with long-term management practices. To better understand the impact of land-use conversion of annual to perennial crops on PEA, I conducted assessments of hydrolytic and oxidative PEAs to 1-m depths in CS, PrF, and Pr cropping systems. The results revealed that depth profiles of PEAs were strongly influenced by these treatments, and the cropping systems had greater impacts on hydrolytic enzymes than on oxidative enzymes. This dissertation also addresses the nitrogen cycling puzzle associated with the biomass crop miscanthus (Miscanthus × giganteus). Through a comprehensive analysis of soil nitrogen dynamics – combining PEA, potentially mineralizable N (PMN), extractable N pools, and soil extractable amino compound concentrations – the research unveiled complex interactions between cropping systems and nitrogen processes. Despite increased PMN under miscanthus across nitrogen fertilizer rates, enzyme activities associated with nitrogen and carbon acquisition remained unaffected. Notably, miscanthus significantly elevated total amino compound concentrations in soil organic matter, providing a potential explanation for the "missing source of N" phenomenon.
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