Soil microbial responses to fire and interacting global change factors in a California annual grassland

Abstract

Wildfire in California annual grasslands is an important ecological disturbance and ecosystem control. Regional and global climate changes that affect aboveground biomass will alter fire-related nutrient loading and promote increased frequency and severity of fire in these systems. This can have longterm impacts on soil microbial dynamics and nutrient cycling, particularly in N-limited systems such as annual grasslands. We examined the effects of a lowseverity fire on microbial biomass and specific microbial lipid biomarkers over 3 years following a fire at the Jasper Ridge Global Change Experiment. We also examined the impact of fire on the abundance of ammonia-oxidizing bacteria (AOB), specifically Nitrosospira Cluster 3a ammonia-oxidizers, and nitrification rates 9 months post-fire. Finally, we examined the interactive effects of fire and three other global change factors (N-deposition, precipitation and CO 2 ) on plant biomass and soil microbial communities for three growing seasons after fire. Our results indicate that a low-severity fire is associated with earlier season primary productivity and higher soil-NH 4? concentrations in the first growing season following fire. Belowground productivity and total microbial biomass were not influenced by fire. Diagnostic microbial lipid biomarkers, including those for Gram-positive bacteria and Gram-negative bacteria, were reduced by fire 9-and 21-months post-fire, respectively. All effects of fire were indiscernible by 33-months post-fire, suggesting that above and belowground responses to fire do not persist in the long-term and that these grassland University of Oregon and University of Wisconsin-Madisoninstitutions where this research took place.Electronic supplementary material The online version of this article (communities are resilient to fire disturbance. While N-deposition increased soil NH 4 ? , and thus available NH 3 , AOB abundance, nitrification rates and Cluster 3a AOB, similar increases in NH 3 in the fire plots did not affect AOB or nitrification. We hypothesize that this difference in response to N-addition involves a mediation of P-limitation as a result of fire, possibly enhanced by increased plant competition and arbuscular mycorrhizal fungi-plant associations after fire.

Shifts in the abundance and community structure of soil ammonia oxidizers in a wet sclerophyll forest under long-term prescribed burning

“…However, the effects of fire on AOB appeared to be weak in a California annual grassland (Docherty et al, 2011), and negative in a mixed conifer forest (Yeager et al, 2005). These could be attributed to the competition between plants and plant-associated arbuscular mycorrhizal fungi (AMF) with AOB for nutrients inputs by the ash, and a slow recovery rate of microbial biomass caused by low soil moisture content (Docherty et al, 2011;Yeager et al, 2005). A decreasing abundance of AOA in 2010 samples and a negative relationship between the abundance of AOA and soil pH in 2005 samples indicated an impact of fire on the growth of AOA.…”

“…It has been suggested that soil heating favors the growth of autotrophic nitrifiers in competition with heterotrophs for ammonium and additional nutrients from ash (Bauhus et al, 1993). However, the effects of fire on AOB appeared to be weak in a California annual grassland (Docherty et al, 2011), and negative in a mixed conifer forest (Yeager et al, 2005). These could be attributed to the competition between plants and plant-associated arbuscular mycorrhizal fungi (AMF) with AOB for nutrients inputs by the ash, and a slow recovery rate of microbial biomass caused by low soil moisture content (Docherty et al, 2011;Yeager et al, 2005).…”

Fire affects root decomposition, soil food web structure, and carbon flow in tallgrass prairie

“…Jones et al (2006) later corroborated that study via molecular methods. Additionally, fire has been shown to reduce overall microbial biomass and specifically affects Gram-negative and Gram-positive bacteria and fungi (Docherty et al, 2011;Ajwa et al, 1999). Such differences in the soil communities have implications for ecosystem function, such as impacts on organic matter decomposition (Verhoef and Brussaard, 1990).…”

“…This leads to changes in soil conditions, such as nitrogen content, carbon content, temperature, and moisture, which could impact microbial and faunal activities or change detritivore community composition. Microbial community compositional changes have been reported as a result of fire: for example, fire alters microbial community composition by reducing Gram-negative and Gram-positive bacteria (Docherty et al, 2011) and increasing arbuscular mycorrhizae (Hamman et al, 2007). Also, fire initially impacts the overall abundance of nematodes negatively (Whitford et al, 2014), but this rebounds quickly and certain groups, such as colonizing bacterivore nematodes, respond positively after fire (Jones et al, 2006;Todd, 1996).…”

Rapid Shifts in Soil Nutrients and Decomposition Enzyme Activity in Early Succession Following Forest Fire

“…Thus, even on a timescale of months to years after disturbance, these rapid and profound shifts in nutrient pools may influence microbial processes, such as the production of extracellular enzymes, which are central to nutrient cycling and ecosystem dynamics. Here, we chose to examine early succession on a timescale of months after a high-severity forest fire to understand how soils and microbial enzyme production may change during this time period [20][21][22][23], with putative implications for the trajectory of ecosystem development [24,25]. Importantly, we focus on high-severity wildfires that are increasingly prevalent in montane forests of the U.S. Intermountain West [2] and can elicit particular responses in soil edaphic properties and microbial communities that are different from lower-severity burns; studies have shown that burn severity can differentially influence soil nutrient pools, C chemistry transformations, soil physical properties, and microbial community composition (for example [6,18,26]).…”