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Fire has transformative effects on soil biological, chemical, and physical properties in terrestrial ecosystems around the world. While methods for estimating fire characteristics and associated effects aboveground have progressed in recent decades, there remain major challenges in characterizing soil heating and associated effects belowground. Overcoming these challenges is crucial for understanding how fire influences soil carbon storage, biogeochemical cycling, and ecosystem recovery. In this paper, we present a novel framework for characterizing belowground heating and effects. The framework includes (1) an open-source model to estimate fire-driven soil heating, cooling, and the biotic effects of heating across depths and over time (Soil Heating in Fire model; SheFire) and (2) a simple field method for recording soil temperatures at multiple depths using self-contained temperature sensor and data loggers (i.e., iButtons), installed along a wooden stake inserted into the soil (i.e., an iStake). The iStake overcomes many logistical challenges associated with obtaining temperature profiles using thermocouples. Heating measurements provide inputs to the SheFire model, and modeled soil heating can then be used to derive ecosystem response functions, such as heating effects on microorganisms and tissues. To validate SheFire estimates, we conducted a burn table experiment using iStakes to record temperatures that were in turn used to fit the SheFire model. We then compared SheFire predicted temperatures against measured temperatures at other soil depths. To benchmark iStake measurements against those recorded by thermocouples, we co-located both types of sensors in the burn table experiment. We found that SheFire demonstrated skill in interpolating and extrapolating soil temperatures, with the largest errors occurring at the shallowest depths. We also found that iButton sensors are comparable to thermocouples for recording soil temperatures during fires. Finally, we present a case study using iStakes and SheFire to estimate in situ soil heating during a prescribed fire and demonstrate how observed heating regimes would influence seed and tree root vascular cambium survival at different soil depths. This measurement-modeling framework provides a cutting-edge approach for describing soil temperature regimes (i.e., soil heating) through a soil profile and predicting biological responses.

Wildfires cause direct and indirect CO₂ emissions due to combustion and post-fire decomposition. Approximately half of temperate forest carbon (C) is stored in soil, so post-fire soil C cycling likely impacts forest C sink strength. Soil C sink strength is partly determined by soil microbial anabolism versus catabolism, which dictates the amount of C respired versus stored in microbial biomass. Fires affect soil C availability and composition, changes that could alter carbon use efficiency (CUE) and microbial biomass production, potentially influencing C sink recovery. Wildfire intensity is forecast to increase in forests of the western United States, and understanding the impacts of fire intensity on microbial anabolism is necessary for predicting fire-climate feedbacks. Our objective was to determine the influence of soil heating intensity and pyrogenic organic matter (PyOM) on microbial anabolism. We simulated the effects of fire intensity by heating soils to 100 or 200 °C for 30 min in a muffle furnace, and we amended the soils with charred or uncharred organic matter. Higher intensity soil heating (200 °C) led to lower microbial biomass carbon (MBC) accumulation, greater C respiration, and lower CUE proxies compared to unheated soils. Conversely, lower intensity heating (100 °C) yielded MBC accumulation and estimated CUE that was similar to unheated soils. Soils amended with PyOM exhibited similar MBC accumulation compared to uncharred organic matter, but lower CO₂ emissions. These results indicate that high intensity soil heating decreases soil C-sink strength over the short-term by decreasing the amount of microbial anabolism relative to catabolism.

Soil properties determining the thermal transmissivity, the heat duration and temperatures reached during soil heating are key factors driving the fire-induced changes in soil microbial communities. The aim of the present study is to analyze, under laboratory conditions, the impact of the thermal shock (infrared lamps reaching temperatures of 100 °C, 200 °C and 400 °C) and moisture level (0%, 25% and 50% per soil volume) on the microbial properties of three soil mixtures from different sites. The results demonstrated that the initial water content was a determinant factor in the response of the microbial communities to soil heating treatments. Measures of fire impact included intensity and severity (temperature, duration), using the degree-hours method. Heating temperatures produced varying thermal shock and impacts on biomass, bacterial activity and microbial community structure.

Global warming may have profound effects on terrestrial ecosystems. However, a comprehensive evaluation of the effects of warming on ecosystem nitrogen (N) pools and dynamics is not available. Here, we compiled data of 528 observations from 51 papers and carried out a meta-analysis of experimental warming effects on 13 variables related to terrestrial N pools and dynamics. We found that, on average, net N mineralization and net nitrification rate were increased by 52.2 and 32.2%, respectively, under experimental warming treatment. N pools were also increased by warming, although the magnitude of this increase was less than that of N fluxes. Soil microbial N and N immobilization were not changed by warming, probably because microbes are limited by carbon sources. Grassland and shrubland/heathland were less responsive to warming than forest, probably because the reduction of soil moisture by warming offset the temperature effect in these areas. Soil heating cable and all-day treatment appeared to be the most effective method on N cycling among all treatment methods. Results of this meta-analysis are useful for better understanding the response of N cycling to global warming and the underlying mechanism of warming effects on plants and ecosystem functions.

Soil warming has the potential to alter both soil and plant processes that affect carbon storage in forest ecosystems. We have quantified these effects in a large, long-term (7-y) soil-warming study in a deciduous forest in New England. Soil warming has resulted in carbon losses from the soil and stimulated carbon gains in the woody tissue of trees. The warming-enhanced decay of soil organic matter also released enough additional inorganic nitrogen into the soil solution to support the observed increases in plant carbon storage. Although soil warming has resulted in a cumulative net loss of carbon from a New England forest relative to a control area over the 7-y study, the annual net losses generally decreased over time as plant carbon storage increased. In the seventh year, warming-induced soil carbon losses were almost totally compensated for by plant carbon gains in response to warming. We attribute the plant gains primarily to warming-induced increases in nitrogen availability. This study underscores the importance of incorporating carbon—nitrogen interactions in atmosphere—ocean—land earth system models to accurately simulate land feedbacks to the climate system.

Nitrogen (N) and phosphorus (P) are essential nutrients for primary producers and decomposers in terrestrial ecosystems. Although climate change affects terrestrial N cycling with important feedbacks to plant productivity and carbon sequestration, the impacts of climate change on the relative availability of N with respect to P remain highly uncertain. In a semiarid grassland in Wyoming, USA, we studied the effects of atmospheric CO2 enrichment (to 600 ppmv) and warming (1.5/3.0°C above ambient temperature during the day/night) on plant, microbial and available soil pools of N and P. Elevated CO2 increased P availability to plants and microbes relative to that of N, whereas warming reduced P availability relative to N. Across years and treatments, plant N : P ratios varied between 5 and 18 and were inversely related to soil moisture. Our results indicate that soil moisture is important in controlling P supply from inorganic sources, causing reduced P relative to N availability during dry periods. Both wetter soil conditions under elevated CO2 and drier conditions with warming can further alter N : P. Although warming may alleviate N constraints under elevated CO2, warming and drought can exacerbate P constraints on plant growth and microbial activity in this semiarid grassland.

The authors analyze global climate model predictions of soil temperature [from the Coupled Model Intercomparison Project phase 5 (CMIP5) database] to assess the models’ representation of current-climate soil thermal dynamics and their predictions of permafrost thaw during the twenty-first century. The authors compare the models’ predictions with observations of active layer thickness, air temperature, and soil temperature and with theoretically expected relationships between active layer thickness and air temperature annual mean- and seasonal-cycle amplitude. Models show a wide range of current permafrost areas, active layer statistics (cumulative distributions, correlations with mean annual air temperature, and amplitude of seasonal air temperature cycle), and ability to accurately model the coupling between soil and air temperatures at high latitudes. Many of the between-model differences can be traced to differences in the coupling between either near-surface air and shallow soil temperatures or shallow and deeper (1 m) soil temperatures, which in turn reflect differences in snow physics and soil hydrology. The models are compared with observational datasets to benchmark several aspects of the permafrost-relevant physics of the models. The CMIP5 models following multiple representative concentration pathways (RCP) show a wide range of predictions for permafrost loss: 2%–66% for RCP2.6, 15%–87% for RCP4.5, and 30%–99% for RCP8.5. Normalizing the amount of permafrost loss by the amount of high-latitude warming in the RCP4.5 scenario, the models predict an absolute loss of 1.6 ± 0.7 million km² permafrost per 1°C high-latitude warming, or a fractional loss of 6%–29% °C−1.

We have designed a pyrocosm to enable fine-scale dissection of post-fire soil microbial communities. Using it we show that the peak soil temperature achieved at a given depth occurs hours after the fire is out, lingers near this peak for a significant time, and is accurately predicted by soil depth and the mass of charcoal burned. Flash fuels that produce no large coals were found to have a negligible soil heating effect. Coupling this system with Illumina MiSeq sequencing of the control and post-fire soil we show that we can stimulate a rapid, massive response by Pyronema, a well-known genus of pyrophilous fungus, within two weeks of a test fire. This specific stimulation occurs in a background of many other fungal taxa that do not change noticeably with the fire, although there is an overall reduction in richness and evenness. We introduce a thermo-chemical gradient model to summarize the way that heat, soil depth and altered soil chemistry interact to create a predictable, depth-structured habitat for microbes in post-fire soils. Coupling this model with the temperature relationships found in the pyrocosms, we predict that the width of a survivable \"goldilocks zone\", which achieves temperatures that select for postfire-adapted microbes, will stay relatively constant across a range of fuel loads. In addition we predict that a larger necromass zone, containing labile carbon and nutrients from recently heat-killed organisms, will increase in size rapidly with addition of fuel and then remain nearly constant in size over a broad range of fuel loads. The simplicity of this experimental system, coupled with the availability of a set of sequenced, assembled and annotated genomes of pyrophilous fungi, offers a powerful tool for dissecting the ecology of post-fire microbial communities.

Global environmental changes are expected to impact the abundance of plants and animals aboveground, but comparably little is known about the responses of belowground organisms. Using meta-analysis, we synthesized results from over 75 manipulative experiments in order to test for patterns in the effects of elevated CO₂, warming, and altered precipitation on the abundance of soil biota related to taxonomy, body size, feeding habits, ecosystem type, local climate, treatment magnitude and duration, and greenhouse CO₂ enrichment. We found that the positive effect size of elevated CO₂ on the abundance of soil biota diminished with time, whereas the negative effect size of warming and positive effect size of precipitation intensified with time. Trophic group, body size, and experimental approaches best explained the responses of soil biota to elevated CO₂, whereas local climate and ecosystem type best explained responses to warming and altered precipitation. The abundance of microflora and microfauna, and particularly detritivores, increased with elevated CO₂, indicative of microbial limitation under ambient CO₂. However, the effects of CO₂ were smaller in field studies than in greenhouse studies and were not significant for higher trophic levels. Effects of warming did not depend on taxon or body size, but reduced abundances were more likely to occur at the colder and drier sites. Precipitation limited all taxa and trophic groups, particularly in forest ecosystems. Our meta-analysis suggests that the responses of soil biota to global change are predictable and unique for each global change factor.