PS 38-56 - Effects of rainfall size and frequency on soil nutrient dynamics in a Chihuahuan Desert grassland

Abstract: Global change is expected to alter rainfall patterns in arid ecosystems, which will likely have impacts on soil biogeochemistry. Drylands are driven by pulsed rain events that trigger biological activity and drive many soil processes. Historical climate records show that rain events in the northern Chihuahuan Desert have become more frequent but smaller in size, whereas climate models predict a future with less frequent, larger, and more intense rainstorms. We examined the effects of changing rainfall frequency and size, as well as interactions between rainfall and nitrogen availability, on soil nutrient cycling in a Chihuahuan Desert grassland in central New Mexico. We conducted our study using the Monsoon Rainfall Manipulation Experiment, a long-term experiment (2007-present) that manipulates rain event size and frequency. Treatments include plots with twelve 5mm events (frequent-small, n=5), or plots with three 20mm events (infrequent-large, n=5) during the monsoon season (July-Sept). Three plots serve as ambient controls. Rainfall manipulations are crossed with a fertilization treatment in nested subplots that receive 5 gm^-2 each year prior to the start of the monsoon. In 2022, we assessed effects of rainfall size and frequency on soil nutrient availability pre-monsoon, during peak growing season, and post-monsoon.

We found few effects of rainfall size and frequency on soil available nutrients, while the effects of fertilization were much stronger, and all effects depended on season. Prior to the monsoon (and application of treatments), fertilized plots had a lower pH than control plots, although this effect disappeared after the monsoon, and there were no effects of precipitation treatments. After seasonal fertilizer application, soil nitrate and ammonium were both higher in fertilized than control plots, and there was no interaction between fertilization and precipitation manipulations. There was a marginal effect of precipitation size and timing on nitrate availability in the post-monsoon sampling as control plots had higher nitrate than those with large infrequent rainfall events, which were higher than plots with small frequent rainfall events. We found no effects of either fertilization or precipitation on phosphate availability. In conclusion, we found few effects of rainfall event size and frequency on soil nutrient availability, suggesting rainfall size and frequency do not effect microbial processing of nutrients, or that limiting nutrients are immediately taken up by the plant or microbial community. Further analyses on soil microbial biomass and nutrient processing will shed light on mechanisms driving nutrient availability in these dryland ecosystems.