ORGANIC AMENDMENT AND NITROGEN FERTILIZATION EFFECTS ON SOIL MICROBIAL CARBON USE EFFICIENCY

The overarching goal of this study is to understand how soil microbial community composition and CUE responds to whole orchard recycling (WOR) and nitrogen (N) fertilization. The objectives are to 1) evaluate whether WOR and N fertigation can increase microbial carbon use efficiency (CUE) and 2) determine the active microbial community by quantitative stable isotope probing (qSIP). This knowledge is foundational to predicting and informing management practices seeking to increase agricultural SOM stocks through organic amendment.Objective 1 - Evaluate whether WOR and N fertigation can increase microbial CUE:Use of organic amendments and mineral N fertilizer together have resulted in higher microbial CUE than organic amendment alone. As such, I hypothesize that microbial CUE will be lower under WOR than in the unamended control plots but will increase at the higher N fertigation rate. Moreover, microbial CUE generally decreases with increasing temperature, as increases in CO2 respiration tend to outweigh increases in growth. Thus, I hypothesize that microbial CUE will be highest in the spring (Mar.) and decrease with increasing summer temperatures (Sept.).Objective 2 - Determine the active microbial community by qSIP:Microorganisms are known to have different physiological constraints, growth rates, and resource acquisition strategies. I hypothesize that the active microbial community dissimilarity will be the greatest between the WOR and control soils, regardless of N application rate. Fungi are assumed to be more efficient than bacteria in C degradation and assimilation, especially for fungal groups that are adapted for cellulose and lignin decomposition. Thus, I hypothesize that fungal diversity and evenness will decrease in the WOR soils as saprotrophic fungi dominate the wood chip degradation niche. Since C metabolism is a wide-spread trait, I expect bacteria that increase in relative abundance under WOR to be more phylogenetically dispersed.