COS 102-3 - Land management influences diazotroph biodiversity and soil carbon and nitrogen cycling

Abstract: Free-living diazotrophs are non-symbiotic, bacterial microorganisms that convert atmospheric nitrogen (N₂) into mineral N. While this process contributes a significant amount of ecosystem-reactive N pools, the environmental controls on free-living N-fixation (FLNF) are poorly understood. Specifically, we know little about how land management influences diazotroph biodiversity, and if changes in diazotroph diversity because of land management impacts soil ecosystem functioning. In this study, we aim to leverage long-term experimental plots at the Kellogg Biological Station’s Long Term Ecological Research Station (KBS – LTER) site to further understand the impact of land management on diazotroph biodiversity, FLNF, microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), dissolvable organic nitrogen (DON), and dissolvable organic carbon (DOC). To do this, we collected 0-15 cm deep soil cores from LTER’s Main Cropping System Experiment in the late Summer and Fall 2022, allowing us to evaluate differences in diazotroph diversity and N-fixation activity across a land management gradient. We artificially manipulated microbial diversity via chloroform fumigation extractions (CFE), comparing shifts in MBC, MBN, DOC, and DON across 0-, 24-, and 72- hour fumigation treatments. In addition, we measured nitrogen fixation using ¹⁵N incorporation methods and evaluated native diazotroph communities by preparing amplicon libraries of the nitrogenase gene: NifH.

We hypothesized that the biologically based, no-chemical input corn-soy-wheat agroecosystem (T4) would have higher microbial biomass and diazotroph biodiversity due to increased carbon availability relative to the conventionally managed agroecosystem (T1). For unfumigated controls, MBC was 3.6 times higher in the biologically based agroecosystem (T4) relative to the conventionally-managed agroecosystem (T1) (p < 0.05). Interestingly, N-fixation for unfumigated controls revealed 2.7 times higher fixation rates in the conventionally-managed agroecosystem (T1) (p < 0.05). Changes in soil physical structure from in-situ soil cores likely impacted N-fixation rates. Because of significant differences in microbial biomass and N-fixation, we hypothesized shifts in diazotroph diversity across the land management gradient, which may correlate with shifts in ecosystem function (i.e., FLNF). Land management treatment explained 34% of the variability in diazotroph community composition (Adj-R-squared = 0.34; p < 0.05). Further, samples with maximum species richness had 4.6 times higher nitrogen fixation rates compared to samples with minimum species richness (p < 0.05), indicating a strong positive relationship with diazotroph biodiveristy and ecosystem functioning. The results from this experiment allow us to address how land management influences diazotroph diversity, and if these shifts in diversity correspond to changes in ecosystem function.