Institute of Ecology and Evolution, University of Oregon, Eugene, OR, United States
Background/Question/Methods Compost amendments are increasingly popular as a soil carbon sequestration strategy on grasslands. As an added benefit, they also increase plant production, due to improved soil water retention (via organic matter) or fertilization from mineralized nutrients. Yet, how compost alters grassland phenology under variable rainfall is unknown and likely depends on plant community responses. For California annual grasslands, rainfall variability determines not only germination and senescence timing, but also community composition via which species germinate and produce seed. By retaining moisture, compost could buffer plant communities to drought, improving resilience and delaying senescence. Alternatively, if fertilization favors certain competitive species, phenology could shift to mirror the dominant’s phenology. We hypothesized that compost amendments delay senescence across rainfall conditions, where 1) ameliorating soil moisture under drought would lengthen growing seasons and 2) fertilization under wet/ambient rainfall would shift communities toward dominant species’ phenology. To test compost’s effects on communities and their phenology across rainfall conditions, we use a large-scale field experiment where amendments (compost, inorganic fertilizer, unamended control) are fully crossed with rainfall manipulations (drought via rainout shelters, wet via irrigation, ambient) in California grasslands. We matched inorganic fertilizer treatments to compost N mineralization rates to separate fertilization from moisture effects.
Results/Conclusions Results showed that amendment and rainfall treatments interact to alter phenology and community dynamics. Across amendment treatments, drought shortened while the wet treatment lengthened the growing season. Compost amendments positively affected soil moisture compared to unamended plots under drought treatments only. Compost also delayed senescence under drought compared to unamended plots in year 1 of our experiment, lengthening the growing season. Despite continued higher soil moisture, this effect did not persist for the compost treatment. Instead, by years 2 and 3, the inorganic fertilizer treatment had the longest growing season under ambient and wet rainfall treatments. These changes were associated with community composition shifts, where inorganic fertilizer favored a late-phenology, unpalatable, invasive grass (Taeniatherum caput-medusae) resulting an unproductive growing season extension from a forage perspective. Under compost treatments, an earlier phenology, palatable forage grass (Festuca perennis) dominated in later years. Overall, we show that compost amendments directly lengthened the growing season the first year after application due to positive effects on soil moisture, but later changes in phenology were driven by shifts in community composition. As compost amendments become more common practice across grasslands throughout the US, understanding how their nutrient and water dynamics feedback to ecosystem resilience is key.