Mpeketula, P. M. 2016. Soil organic carbon dynamics and mycorrhizal fungal diversity in contrasting agroecosystems. Dissertation, Michigan State University, East Lansing, Michigan.
Maintenance and improvement of soil quality is critical to sustaining agricultural productivity and environmental quality. Soil organic carbon (SOC) and Arbuscular Mycorrhizal Fungi (AMF) are among key soil quality and agronomic sustainability indicators. AMF are indispensable crop symbionts involved in nutrient transfers and carbon sequestration, relevant in the global carbon cycle and green house gas abatement. Conventional agriculture adversely affects SOC, AMF and the environment, yet little is known on the impact of semi-closed alternatives. In this dissertation, I examined how alternative approaches impact the selected indicators in temperate and tropical settings to address some of the existing research gaps.
In Chapter 1, I examined the role of nutrient management and crop diversity on SOC and aggregate stability in a 20 yr field study of the Living Field Laboratory (LFL) at Kellogg Biological Station – Long-term Ecological Research in Michigan. I assessed long term responses of Integrated Fertilizer and Integrated Compost management on a crop diversity gradient comprising of continuous monoculture Corn, Corn-soy rotation, Corn-soy-wheat rotation, and Corn-soy-wheat rotation with a cover crop. Management rather than diversity exerted significant influence on SOC and labile carbon (POXC) status across treatments with higher SOC and POXC levels in compost treatments. Crop diversity exerted positive influence on aggregate stability. Diverse rotations had greater aggregate stability than monocultures regardless of nutrient management system.
In Chapter 2, I investigated AMF spore diversity using morphological techniques to assess abundance and diversity of AMF in the LFL. Proportions of AMF taxa varied with crop diversity. Surprisingly, nutrient management influenced soil organic matter but not AMF. This is the first report of AMF morphotyping to reveal AMF community composition shifts along crop species diversity gradient in integrated nutrient management systems.
In Chapter 3, I report on the influence of land use on soil bio-resources in the tropical landscape of Machinga District in Malawi, a country in Sub-Saharan Africa. I examined communities of AMF in Miombo woodlands and croplands nested within Malosa Forest Reserve. The Shannon- Weiner diversity index (H´) differed significantly among land use types being higher in agricultural soils than in the natural forest soils reflecting community compositional shifts and variation in spore density among communities under study.
In Chapter 4, I report on SOC spatial distribution in Machinga district at landscape scale and evaluate SOC prediction accuracy among various interpolation techniques. SOC distribution was greatly influenced by land use type and spatial topographic attributes. Mean SOC content on surface layer soils (0-30cm) declined from 9.0 to 8.5 g kg -1 soil over 2 decades. Ordinary kriging with spherical semivariogram model fitting was found to be the optimal approach for mapping SOC spatial distribution and variability in the complex landscape. The study provides an important contribution to the understanding of SOC spatial distribution that can guide land management policy, carbon sequestration and climate change mitigation strategies.
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