PS 25-87 - Disturbance and nutrient addition reduce aboveground biomass resistance in a mesic grassland

Abstract: Grasslands cover ~40% of the earth’s land surface and provide many ecosystem services essential for human well-being. Land use change, climate change, and nitrogen deposition are all negatively affecting grassland ecosystems. Nutrient addition has been reported to have a positive effect on grassland productivity but a negative effect on grassland plant diversity. Conversely, soil disturbance can have a negative effect on productivity and a positive effect on diversity. The relationships between diversity-productivity and diversity-ecosystem stability are important areas of research for predicting future trends in grasslands. We used a Disturbance and Resources Across Global Grasslands (DRAGNet) site in Temple, TX USA to quantify grassland aboveground biomass (AGB) productivity resistance (Log Response Ratio (LRR) = log (Biomass _{after treatment}/Biomass _{before treatment}) to eutrophication (NPKµ) and soil disturbance (D). The experiment consists of 25, 5m x 5m plots arranged in a Latin square design of the following treatments: (1) Control (C), (2) NPKµ, (3) disturbance (D), (4) NPKµ + D, and (5) NPKµ cessation (not used in this study). The first year of soil disturbance by rototilling the top 15 cm of soil and removing rhizomes was implemented in January 2022 and nutrient addition (10 g m^-2 of N, P, K, and micronutrients by weight) was administered in April 2022. Biomass was harvested from subplots in the early/spring and late/fall growing season and biomass were separated by functional groups (forbs, grass, Johnson grass (invasive), and legumes). After the first year of treatment, AGB grassland productivity resistance was affected by both treatment and season. The relative proportion of Johnson grass AGB increased in NPKµ and disturbance plots but remained stable in the NPKµ+D treatment and the effect of the growing season was not significant. AGB resistance to treatment varied across functional groups and seasons, where forbs were particularly affected by NPKµ+D, grass by all treatments in the late growing season, and legumes by the disturbance in the early growing season. Species richness remained consistently lower in the late growing season and declined across all treatments. Furthermore, total plant cover was found to be lower in disturbed plots (both seasons) and NPKµ+D plots in the early growing season. These results suggest aboveground grassland productivity response to nutrient addition and disturbance in early stage depends on the growing season and varies across the functional groups. Species richness, on the other hand, showed an overall and immediate decline across all treatments.