Fire disturbance belowground: untangling consequences for soil food webs and organic matter

2019 Spring.Includes bibliographical references.Soils and the ecological communities they house provide a diverse array of ecosystem services including the provisioning of food and fiber, decomposition and nutrient cycling, water filtration, and the maintenance of terrestrial biodiversity. These complex belowground communities, and therefore the ecosystem processes they regulate, are increasingly threatened by fire due to climate, land use, and management changes. Fires can have profound effects on the physical and chemical soil environment, with consequences for soil biological communities. Fires cause mortality of soil organisms during the disturbance event, change the soil pH, and alter the quantity and quality of soil organic matter (SOM). In particular, fires transform organic matter into pyrogenic carbon (PyC), a recalcitrant material with a dense aromatic structure and long residence times in soils. In natural ecosystems, soil food webs interact with PyC produced after a fire. In agroecosystems, PyC, in the form of biochar, is also used as a tool to manage soil carbon and fertility. Given the widespread effects of fire on biological, chemical, and physical components of the soil, and the importance of soil communities for the provisioning of ecosystem services, understanding the consequences of fire disturbance for soil food webs and organic matter is an important research objective. My dissertation leverages several different scientific inquiry approaches to understand the consequences of disturbance and management for the ecology of soils. I take a multifaceted approach by considering soil organisms, food webs, and organic matter in the context of fire disturbance and agricultural management. I begin by presenting results from a meta-analysis investigating the effect of fire on soil biota biomass, abundance, richness, evenness, and diversity. Overall, I found a pervasive negative effect of fire on soil microorganisms and conclude that soil fauna are more resistant to fire than soil microorganisms. Then, I present results from a field study investigating the effect of fire frequency on soil food web structure, function, stability, and resilience in an oak-pine savanna. Here, I found that while soil biota biomass and food web function did not differ with fire frequency, food web structure, stability, and resilience did. In particular, soil food webs at intermediate fire frequencies (4-year fire return interval) were the least stable and least resilient to fire. Thereafter, I consider the consequences of fire for SOM composition through the lens of PyC. I seek to understand where and why PyC persists in soils at a continental scale by using multiple analytical techniques to quantify PyC across Europe. I found that PyC may contribute a smaller component of soil organic carbon than previously thought and that organic carbon is the best predictor of PyC at a continental scale. I then consider how agricultural management and PyC in the form of biochar, impacts soil food webs in a semi-arid corn agroecosystem. I did not find any measurable effects of biochar on soil food web structure or function. I conclude that the long-term impact of historical land management on soil food webs far outweighs any impact of short-term management practices involving biochar. I then use this field study as an opportunity to integrate scientific inquiry in middle school classrooms. I present a collection of classroom activities co-developed with secondary educators that lead students to investigate the effect of biochar on soils and plants. I conclude by discussing the themes, patterns, and ideas that emerge from the preceding chapters. I found that the responses of soil ecological communities to disturbance are highly context dependent. This context dependency leads to hidden, unexpected, and even contradictory patterns. I end by reflecting on how completing this work has informed my non-linear approach to science

Data from: Belowground community responses to fire: meta-analysis reveals contrasting responses of soil microorganisms and mesofauna

Global fire regimes are shifting due to climate and land use changes. Understanding the responses of belowground communities to fire is key to predicting changes in the ecosystem processes they regulate. We conducted a comprehensive meta-analysis of 1634 observations from 131 empirical studies to investigate the effect of fire on soil microorganisms and mesofauna. Fire had a strong negative effect on soil biota biomass, abundance, richness, evenness, and diversity. Fire reduced microorganism biomass and abundance by up to 96%. Bacteria were more resistant to fire than fungi. Fire reduced nematode abundance by 88% but had no significant effect on soil arthropods. Fire reduced richness, evenness and diversity of soil microorganisms and mesofauna by up to 99%. We found little evidence of temporal trends towards recovery within 10 years post-disturbance suggesting little resilience of the soil community to fire. Interactions between biome, fire type, and depth explained few of these negative trends. Future research at the intersection of fire ecology and soil biology should aim to integrate soil community structure with the ecosystem processes they mediate under changing global fire regimes

Extracted data from literature on soil organism responses to fire

The file contains data on soil organism biomass, abundance, richness, evenness, and diversity in burned and unburned soils extracted from peer reviewed publications published between 1988 and 2016 for use in meta-analysis. Soil organisms include bacteria, fungi, nematodes, protozoa, and arthropods. The file contains three sheets: (1) a read me file describing the contents of data sheets and columns therein; (2) a list of all published studies from which data was extracted for meta-analysis including first author, publication year, and journal; (3) a data file containing all data and accompanying information extracted from each publication. The contents of all columns are described in the ReadMe tab and file

Continental-scale measurements of soil pyrogenic carbon in Europe

Pyrogenic carbon (PyC), the product of incomplete biomass combustion, is a key component of soil organic carbon (SOC) because it can persist in soils for centuries to millennia. Quantifying PyC across large spatial scales remains a significant challenge in constraining the global carbon cycle. We measured PyC in topsoils across Europe using molecular marker (benzene polycarboxylic acids, BPCA) and spectroscopic techniques (Diffuse Reflectance Infrared Fourier Transform Spectroscopy, DRIFTS). We developed a calibration between BPCA and DRIFTS, but the calibration was less reliable (Y-variance explained = 0.62) than previous reports due to low soil PyC content and heterogeneity of soil matrices. Thus, we performed multiple regressions to identify drivers of PyC distribution using only the measured BPCA data. PyC content varied widely among soils, contributing 0–24% of SOC. Organic carbon was the strongest predictor of soil PyC content, but mean annual temperature, clay, and cation exchange capacity also emerged as predictors. PyC contributes a smaller proportion of SOC in European soils compared to other geographic regions. Comparing soil PyC measurements to PyC production rates in high latitude and Mediterranean regions suggests that transport, degradation, and recombustion are important mechanisms regulating soil PyC accumulation