Miriam Fuchs
Thesis Proposal
Direct and Indirect Effects of Precipitation on Soil Respiration in an Arid Ecosystem
The goal of my study is to determine the effects of precipitation levels on soil respiration, which includes both the microbial (heterotrophic) and root (autotrophic) respiration. I will look at both the direct effect of consistently increased or consistently decreased precipitation levels on soil respiration (carbon efflux) and the indirect effects of environmental legacies, which means examining precipitation levels in the preceding year compared to the current year in order to examine the effects of prior environmental conditions. Previous studies have mainly focused on environmental interactions, such as temperature-precipitation and nitrogen-temperature, and their effects on soil respiration, but this study will focus on the impact of different and changing precipitation levels and nitrogen levels, and control for the effects of other environmental factors.
Soil respiration is an important ecosystem characteristic to measure because it reveals the magnitude of carbon flow from the soil to the atmosphere, and provides an upper limit for belowground net primary productivity (BNPP) and how this value compares to ANPP measured in other parts of this experiment.
I hypothesize that soil respiration will be greater with consistently wetter conditions compared to consistently drier conditions and the controls. This result could be due to increased plant growth, including roots, which leads to additional substrate for respiration, and increased microbial activity. Precipitation has the potential to increase both heterotrophic and autotrophic respiration.
I also hypothesize that soil respiration will be greater with a preceding wet year in plots where precipitation levels are changed. I predict that a legacy of wet conditions will increase soil respiration because of increased root growth and microbial activity, as previously hypothesized for consistently wetter conditions. Drought may cause root mass to increase relative to aboveground biomass due to a reallocation of resources, leading to relatively more carbon allocated belowground in roots. However, in absolute terms, belowground biomass should decrease. I predict that the decrease in root respiration will dominate with a legacy of dry conditions, but the decrease in microbial activity and overall plant biomass due to decreased precipitation will dominate in consistently drier conditions. I also hypothesize that fertilizer, or increased nitrogen levels, will further increase soil respiration measurements due to overall increased plant growth.
My third hypothesis is that soil respiration will decrease in plots based on the amount of time after rainfall. Additionally, after a certain period of time after rainfall, soil respiration rates of plots that have experienced rain and plots that have not are expected to converge.
In the Jornada Experimental Range in Las Cruces, New Mexico, I measured the rate of soil respiration in eight types of plots using the Li-COR 6400XT with the soil carbon dioxide flux chamber attachment. Soil measurements were taken in control plots; plots with consistently +80% precipitation; plots that were inverted from +80% to -80% precipitation; plots with consistently -80% precipitation; plots that were inverted from -80% to +80% precipitation; plots that were inverted from being controls to +80% precipitation; and plots that were inverted from being controls to -80% precipitation. Soil respiration rates in plots with and without fertilizer were tested for in three levels of precipitation including control, consistently +80% precipitation and consistently -80% precipitation. Specifically, took measurements in three locations (blocks) for each of the ten types of plots in order to reduce variability in location between plot types. This leads to a total of 44 spatial samples of manipulation for the measurements of precipitation legacy, where all samples were measured on four different days, and 36 samples for measurements of the effects of fertilization, where all samples were measured on two different days. I also created eleven one meter by one meter plots, and irrigated five of them with the equivalent of 10 mm of precipitation. I proceeded to measure soil respiration in all plots for 16 days to follow rates over time after a rainfall event.
Through analysis, I will use these measurements to determine if precipitation directly has an effect on soil respiration rates, and if a legacy of different precipitation levels impacts soil respiration rates. I will also determine if changes in nutrient levels affect soil respiration rates across precipitation levels. And lastly, I will determine the effects of time after rainfall on soil respiration rates.
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