Background/Question/Methods Organic nitrogen (N) applications are a promising strategy to enhance ecosystem productivity in semi-native and improved perennial pasture systems while simultaneously drawing down atmospheric carbon (C) and enhancing soil health. However, excessive reactive N loading in ecosystems can promote invasion by exotic annuals and shift the structure of microbial communities. These shifts in above and belowground community structure can have long-term consequences on ecosystem-level nutrient cycles. The purpose of this study was to: (1) evaluate the response of plant and microbial communities to infrequent applications of organic and inorganic N; and (2) quantify the effects of various N applications on aboveground biomass production, plant N uptake, and microbial biomass. The improved pasture experiment was a 2x2 factorial with two treatment levels (compost and control) and two levels of topography (down-slope and mid-slope) and four replicates. The semi-native pasture experiment was a randomized complete block design with three levels treatment (compost, urea, and a control) and six replicates. Both pastures were managed under rotational grazing with beef cattle, and the improved pasture was cut annually for hay. Soil samples were collected from 0-10 cm in the summer of 2018 and 2019 after the hay cutting in the improved pasture and following the 2018 grazing period in the semi-native pasture. Aboveground biomass samples and transects were taken just before the first hay cutting in the improved pasture, and peak standing biomass measurements were made in late July in the semi-native pasture. Results/Conclusions There were no differences in species diversity, evenness, or richness in the improved or semi-native pastures during the first growing season. The compost plots exhibited higher plant species diversity, evenness, and richness (p < 0.05) than the control in the improved pasture during the second growing season. Aboveground biomass was higher under compost over both growing seasons (p < 0.05) in the improved pasture, but there was no difference between treatments in the semi-native pasture. Belowground, gram-positive and gram-negative bacteria were higher (p < 0.05) in the compost than the control in the improved pasture while total fungi (p = 0.031) and arbuscular mycorrhizal fungi (p = 0.015) were higher in the compost relative to the control in the semi-native pasture. These preliminary results indicate that organic N applications alter above and belowground community structure, which may have implications for ecosystem-level C and N cycling.
