The stoichiometric legacy of fire regime regulates the roles of micro‐organisms and invertebrates in decomposition

Abstract

Decadal‐scale increases in fire frequency have the potential to deplete ecosystems of essential nutrients and consequently impede nutrient‐limited biological processes via stoichiometric imbalance. Decomposition, a fundamental ecosystem function and strong driver of future fire occurrence, is highly sensitive to nutrient availability and is, therefore, particularly important in this context. Here we show that 40 yr of quadrennial (4yB) and biennial (2yB) prescribed burning result in severely P‐ and N‐depleted litter stoichiometry, respectively, relative to fire exclusion. These effects exacerbated the nutrient limitation of microbial activities, constraining litter decomposition by 42.1% (4yB) and 23.6% (2yB) relative to unburned areas. However, invertebrate‐driven decomposition largely compensated for the diminished capacity of micro‐organisms under 4yB, suggesting that invertebrates could have an important stabilizing influence in fire‐affected ecosystems. This effect was strongly positively coupled with the strength of microbial P‐limitation and was not obviously or directly driven by fire regime‐induced changes in invertebrate community assemblage. Together, our results reveal that high‐frequency fire regimes promote nutrient‐poor, carbon‐rich ecosystem stoichiometry and, in doing so, disrupt ecosystem processes and modify the relative functionality of micro‐organisms and invertebrates.

Long-Term Fire Regime Modifies Carbon and Nutrient Dynamics in Decomposing Eucalyptus pilularis Leaf Litter

“…Patterns of rainfall and temperature during the litter decomposition period are presented in Figure 1. The effects of fire regime on the rates of E. pilularis litter mass loss as determined by our litter bag experiment have already been described by Butler et al (2019b), who showed that the rates of microbially-driven litter decomposition were 42.1 and 23.6% slower in the 4yB and 2yB treatments, respectively, than in the NB treatment. The results of our ANOVA of litter mass loss over the four bag re-collection dates were consistent with this (Figure 2); however, the significant interaction between fire regime and litter bag re-collection date also indicates that the effect of fire regime was particularly strong at the end of the experimental decomposition period (i.e., at day 277; Figure 2).…”

“…This result was partially consistent with our expectations, although we expected differences in decomposition rate to be greatest between 2yB and NB, not between 4yB and NB. At least some of this effect can likely be attributed to the low P concentration of 4yB litter, which has suppressed microbial C demand in favor of P acquisition (Butler et al, 2019b). An implication of this finding is that the rate of accumulation of soil C could be significantly lower in frequently burned eucalypt forest due to constrained decomposition rates.…”

“…Litter and soil-dwelling invertebrates complement and usually enhance these processes by displacing, consuming, and fragmenting the litter and through their topdown effects on the microbial community (Hättenschwiler et al, 2005;García-Palacios et al, 2013). Although micro-organisms generally make a larger contribution to decomposition than invertebrates (Chapin et al, 2002;García-Palacios et al, 2013), prior studies have shown that the relative contributions of micro-organisms and invertebrates to decomposition in eucalypt forests can be significantly altered by fire regime (Brennan et al, 2009;Butler et al, 2019b). At the same time, by altering the structure of eucalypt forests, changes in fire regime can affect the degree to which sunlight penetrates to the litter layer, which in turn influences the temperature and moisture of the decompositional environment (e.g., Heisler et al, 2004).…”

Frequent fire reorganizes fungal communities and slows decomposition across a heterogeneous pine savanna landscape

“…By contrast, wildfire studies indicate that transitions of fungal communities from ‘burned’ to ‘unburned’ states can take more than a decade (Dooley & Treseder, ; Holden et al ., ; Oliver et al ., ). Decomposition data here indicate that fire effects on microbial decomposition may be important, albeit transient, compared with fire regime impacts through litter stoichiometry (Ficken & Wright, ; Butler et al ., ). These differences suggest that variation in fire return intervals and/or fire intensities may have large effects on microbial community dynamics and function.…”

Constraining Carbon and Nutrient Flows in Soil With Ecological Stoichiometry

“…Despite the fact that stoichiometry is more consistent than CUE (Cleveland and Liptzin, 2007), the stoichiometry of MAOM and other large soil pools can be sensitive to disturbance, and may therefore be vulnerable to global change. For example, fire (e.g., Butler et al, 2019), warming (e.g., Sihi et al, 2019), and nutrient addition (e.g., Crowther et al, 2019) have been known to influence the stoichiometry of soil, invertebrate, saprotrophic microorganisms, mycorrhizal fungi, and enzyme kinetic activities, which in turn can affect SOM pools and flows. Likewise, MAOM stoichiometry is likely to change with soil pH and associated organo-metal complexation with sesquioxides (Fe and Al oxides) and exchangeable Ca (Rasmussen et al, 2018).…”