MICROBIAL CONTRIBUTION TO BUILDING AND STABILIZING SOIL ORGANIC MATTER UNDER LONG-TERM CROP MANAGEMENT PRACTICES IN AGROECOSYSTEMS

Traditional theory suggests the formation of SOM is a result of chemical complexity: recalcitrant plant inputs persist in the soil while easily decomposable plant inputs are respired as CO2. However, large inputs of recalcitrant crop residues (e.g., corn residues) in intensive production systems are often not associated with proportional gains in SOC (Leifeld and Fuhrer, 2010; Sanford et al., 2012). Instead, emerging experimental and theoretical evidence shows that stable SOM in organo-mineral associations and microaggregates are dominated with chemically labile rather than complex compounds; microbial products might be the main precursors of stable SOC by promoting aggregation and through strong chemical bonding to the mineral matrix (Cotrufo et al., 2013; Kallenbach et al., 2015; Liang et al., 2013 and 2016). Therefore, the efficiency of microbial biomass production, as well as the subsequent stabilization of microbial necromass, may play a pivotal role in soil C sequestration. In the grain-based cropping systems of United States, management practices that can increase the quality of C input by adding low C:N ratio legumes in crop rotation or applying manure have been found to increase SOM content and reduce fertilizer need (Karlen et al., 2006; Liebman et al., 2008; Coulter et al., 2011; Jokela et al., 2011). However, it is still not well understood if the increased SOM content is the direct result of more efficient microbial biomass production and greater microbial necromass accumulation under these practices.The overall goal of this project is to understand the interactions between crop management practices and microbial communities in facilitating soil C turnover in a range of agroecosystems of the United States.To meet this goal, we have identified four supporting project objectives:Objective 1. Using long-term replicated field experiments in three states with different soil types and environmental conditions, determine the impact of different crop management strategies on quality and quantity of C inputs, and test the hypothesis that including legumes in crop rotation and/or applying manure can improve both quantity and quality of C inputs in cropping systems at a broad scale.Objective 2. Using in-situ and laboratory incubations, determine the impact of different crop management practices on the size, structure, and functions of soil microbial community, and test hypothesis that including legumes in the crop rotation and/or applying manure can increase microbial CUE; Identify the relationships between CUE, microbial community structure and quality of C input at a large scale.Objective 3. Determine the impacts of crop management practices on the accumulation of microbial necromass; Identify the microbial modulator of SOC storage by evaluating the relationships between microbial CUE/necromass and SOC stored in stable SOM fractions (mineral associated C).Objective 4. Determine the impact of the crop management practices on SOM quality, stability and the capacity of SOM to supply nutrient, and evaluate if greater microbial necromass accumulation and SOM stability also lead to greater nutrient supply.Research will be conducted in cropping systems across different states of the United States (Wisconsin, Iowa, and Pennsylvania) to evaluate how different cropping systems and agricultural management affect the quantity and quality of C input, and the impacts on microbial community structure and functions, microbial necromass accumulation, and SOM stability. By identifying the interactions between cropping systems, management practices, soil microbial physiology and functions, and SOC storage, this work fundamentally seeks to understand the role that crop management strategies play in SOC accumulation. These findings will provide a strong framework to determine the impact of crop management practices on soil C and soil health and will provide justification of future agroecological intensification (green manure, dairy manure, cover cropping, and livestock integration). Research findings can facilitate the adoption of best management practices to maximize productivity and profitability, and the sustainability of agroecosystems.