Precipitation effects on soil carbon cycling in the Sonoran Desert

Biological activity in desert soils is driven by water availability. The nature of individual precipitation events is critical to understanding soil moisture availability. Rain falls as discrete events (pulses) that vary in size and sequencing, resulting in soil "wet-dry cycles". Soil organisms are responsive to wet-dry cycles with rapid changes inactivity. How soil activity is driven by changes in water content associated with individual pulses is poorly understood. The effects of precipitation on soil processes likely depend on ecosystem structure, which influences the soil environment. The goal of this dissertation was to determine how soil carbon cycling responds to precipitation in the context of ecosystem structure (plant composition, geomorphology) and climate.; I used differences in stable carbon isotopic composition of soil organisms and plants to understand how positioning in the soil profile influences biological responses to different sized pulses. I evaluated how soil texture and grass species composition affect soil process response to rainfall in different seasons. I manipulated rainfall sequence to understand the interaction between closely spaced rainfall events of different sizes on soil processes. I evaluated the role of plant functional types in influencing soil microclimate and litter deposition and the response of soil processes to seasonal rainfall.; Chamber measurements of soil and plant CO2 flux were used to understand their response to rainfall. I found that surface organisms are more responsive to small rainfall events due to the relationship between pulse size and infiltration. While soil texture and season of rainfall are important, the best predictor of the response of soil respiration to rainfall was initial activity levels. Grass species was not important. Grass roots and soil microbes differ in response to sequences of precipitation. Grasses responded less to subsequent large events if they were already 'activated' by a recent rainfall event. The effect of plant functional type was size dependent with differences occurring only with large shrubs. This work suggests that large scale simulations of soil carbon cycling in deserts should carefully consider wet-dry transitions in the context of plant functional type and initial soil condition in order to predict the responses to global change.