Linking Soil CO2 Efflux to Individual Trees: Size-Dependent Variation and the Importance of the Birch Effect

Soil CO2 efflux (FCO2) is a major component of the terrestrial carbon (C) cycle but challenges in explaining local variability hamper efforts to link broad-scale fluxes to their biotic drivers. Trees are the dominant C source for forest soils, so linking tree properties to FCO2 could open new avenues to study plant-soil feedbacks and facilitate scaling; furthermore, FCO2 responds dynamically to meteorological conditions, complicating predictions of total FCO2 and forest C balance. We tested for proximity effects of individual Acer saccharum Marsh. trees on FCO2, comparing FCO2 within 1 m of mature stems to background fluxes before and after an intense rainfall event. Wetting significantly increased background FCO2 (6.4 ± 0.3 vs. 8.6 ± 0.6 s.e. μmol CO2 m−2s−1), with a much larger enhancement near tree stems (6.3 ± 0.3 vs. 10.8 ± 0.4 μmol CO2 m−2s−1). FCO2 varied significantly among individual trees and post-rain values increased with tree diameter (with a slope of 0.058 μmol CO2 m−2s−1cm−1). Post-wetting amplification of FCO2 (the ‘Birch effect’) in root zones often results from the improved mobility of labile carbohydrates and further metabolization of recalcitrant organic matter, which may both occur at higher densities near larger trees. Our results indicate that plant-soil feedbacks change through tree ontogeny and provide evidence for a novel link between whole-system carbon fluxes and forest structure.

Climate change effects on soil carbon dynamics: CO2 flux from water-repellent and fire-affected soils

Climate change is increasing the frequency and intensity of droughts and this is expected to enhance the development of soil water repellency: a very common property of both dry and fire-affected soils. In some regions climate change is also increasing the occurrence and severity of wildfires. Large pulses of CO2 flux from soil to the atmosphere caused by heavy rainfall events (i.e. the Birch effect) can contribute substantially to annual C emissions from soils. However, the effect of the first rainfall after a drought on water-repellent soils and the first post-fire rainfall event on soil CO2 flux remain poorly understood. To address these knowledge gaps this research focuses on: i) investigating the effects of soil water repellency on the CO2 pulse after wetting; ii) improving understanding of the effects of vegetation fires on post-fire soil CO2 flux; and iii) studying the role of ash produced naturally during vegetation fires in post-fire soil CO2 flux. The results from this research clearly indicate that water repellency is a key controller of the CO2 pulse following the wetting of dry and fire-affected soils. Both the amount of water and the increase in soil water content after wetting are used as indicators of the magnitude of the Birch effect, but this research suggests that their application in water-repellent soils should be re-evaluated. The findings presented here challenge the conceptual notion that the Birch effect is comprised of one large pulse of CO2 and highlights the need to incorporate high-frequency observations during the period following wetting to capture the entire CO2 response to wetting. The results from this thesis suggest that ash is a key player in post-fire C fluxes and should be considered in post-fire C investigations in order to make realistic predictions of the impacts of vegetation fires on C dynamics.

Linking Soil CO2 Efflux to Individual Trees: Size-Dependent Variation and the Importance of the Birch Effect

Soil CO2 efflux (FCO2) plays a dominant role in the terrestrial carbon (C) cycle but interpreting constraints on local observations is impeded by challenges in disentangling belowground CO2 sources. Trees contribute most C to forest soils, so linking aboveground properties to FCO2 could open new avenues to study plant-soil feedbacks and facilitate scaling; furthermore, FCO2 responds dynamically to meteorological conditions, complicating predictions of total FCO2 and forest C balance. We tested for proximity effects of individual Acer saccharum Marsh. trees on FCO2, comparing FCO2 within 1 m of mature stems to background fluxes before and after an intense rainfall event. Wetting significantly increased background FCO2 (6.4±0.3 vs. 8.6±0.6 s.e. μmol CO2 m-2s-1), with a much larger enhancement near tree stems (6.3±0.3 vs. 10.8±0.4 μmol CO2 m-2s-1). FCO2 varied significantly among individual trees and post-rain values increased with tree diameter (with a slope of 0.058 μmol CO2 m-2s-1 cm-1). Post-wetting amplification of FCO2 (the ‘Birch effect’) in root zones often results from the improved mobility of labile carbohydrates and further metabolization of recalcitrant organic matter, which may both occur at higher densities near larger trees. Our results indicate that plant-soil feedbacks change through tree ontogeny and provide evidence for a novel link between whole-system carbon fluxes and forest structure.

Unraveling the Molecular Mechanisms Underlying the Microbiome Response to Soil Rewetting

<p>Soil microbes are highly sensitive to changes in their environment, and rapidly measuring their responses is necessary to fully understand the biological processes. Drought is one of the most common environmental stresses that soil microbiomes experience, and it is important to understand the mechanisms by which the soil microbiome respond to soil dehydration. We used <sup>13</sup>C as a tracer of nutrient fluxes in desiccated soil microbiomes after rewetting to simultaneously measure aerobic respiration and track the metabolic state of the community. Here, we describe a Real Time Mass Spectrometry (RTMS) approach for rapid gas monitoring combined with omics approaches to track <sup>13</sup>C flow through a soil system.</p><p>The mechanism(s) behind the burst of rapid mineralization of soil organic matter and increased rate of CO<sub>2 </sub>release upon rewetting dry soil (termed the ‘Birch Effect’) are yet to be fully defined. One known mechanism used by microbes to protect against dehydration is the production of intracellular compounds known as osmolytes. We evaluated metabolic mechanisms produced upon rewetting a marginal soil testing the hypothesis that the rapid release of CO<sub>2 </sub>arises from the microbial processing of putative intracellular osmolytes that build up during desiccation. RTMS allows for the simultaneous, rapid and fine scale (every 2 sec) evaluation and deconvolution of the production and consumption of a number of gasses including <sup>12</sup>CO<sub>2</sub>,<sup>13</sup>CO<sub>2</sub>, O<sub>2</sub>, N<sub>2 </sub>and H<sub>2</sub>O.  We compared the hydration response (production of CO<sub>2 </sub>in real time) between the addition of water and <sup>13</sup>C labeled glucose dissolved in water. The initial burst of <sup>12</sup>CO<sub>2 </sub>followed by a leveling off was identical in both treatments with an additional larger increase in <sup>13</sup>CO<sub>2 </sub>about 20 minutes later in the <sup>13</sup>C labeled glucose experiment. Examination of the two minutes after the water addition revealed a rapid rate of <sup>12</sup>CO<sub>2 </sub>(38 sec) and H<sub>2</sub>O (47 sec) production and slow rate of <sup>13</sup>CO<sub>2 </sub>(56 sec) production followed by the consumption of O<sub>2 </sub>(67 sec) and N<sub>2 </sub>(73 sec).  Evaluation of the soil metabolomes at specified time points within 3 hours after wetting revealed the immediate release of sugars from the cells into the extracellular matrix. These results provide evidence for respiration of putative intracellular osmolytes as one driving mechanism of the Birch Effect. </p>

The Nitrogen Dynamics of Multi-species Grasslands when Subject to Drought and Re-wetting.

<p>It is predicted that climate change will result in more extreme and frequent weather events including flooding and drought. Nitrous oxide (N<sub>2</sub>O) is a potent greenhouse gas having 298 times the global warming potential of CO<sub>2</sub>. The ‘Birch effect’, the term given to high  N<sub>2</sub>O fluxes following the drying and re-wetting of soils, is an accelerator of this process. Multi species grasslands have been shown have higher nitrogen use efficiency and potential for drought resilience and recovery. This experiment analysed the nitrogen dynamics of multi-species grasslands by means of quantifying the responses of soil mineral nitrogen (NH<sub>4</sub><sup>+</sup> and NO<sub>3</sub><sup>-) </sup>and N<sub>2</sub>O fluxes during an eight week simulated drought, re-wetting and fertiliser application two weeks after the re-wetting event. A simplex experimental design was used to determine species and functional group effects which could potentially influence responses. The hypothesis of this study was therefore that multi species grasslands would mitigate the ‘Birch effect’ resulting in less erratic transformations of soil mineral nitrogen and lower N<sub>2</sub>O fluxes compared to monocultures. This study also predicted a lasting legacy effect of drought on soil systems resulting in prolonged heightened N<sub>2</sub>O fluxes. Drought resulted in a depletion of soil NO<sub>3-</sub>, increased  levels of NH<sub>4</sub><sup>+ </sup>and background level N<sub>2</sub>O emissions. Following re-wetting soil mineral N underwent transformations from NH<sub>4</sub><sup>+</sup> to NO3- indicating nitrification. Four times more N<sub>2</sub>O emissions were recorded during re-wetting period compared to fertilizer application. There was no lasting legacy effect of drought and re-wetting on N<sub>2</sub>O fluxes observed during fertilizer application two weeks after re-wetting bar T. repens which has implications for grassland management strategies.</p>

Exploring The Birch Effect In The Subsurface Using Diffusive Soil Probes

<p>Soil gases are efficient messengers of the subsurface biogeochemical processes that underlie important nutrient cycles.  Recent advances in subsurface gas sampling techniques can be combined with high precision trace gas instrumentation to yield novel insights into these processes and their mechanisms.</p><p>We present measurements of a wide range of trace gases before, during, and after a simulated rainfall upon northeastern US temperature forest soil in meso-scale columns.  Subsurface concentrations and above-ground fluxes of N<sub>2</sub>O and its isotopes, CH<sub>4</sub> and its isotopes, CO<sub>2</sub>, NO, NO<sub>2</sub>, NH<sub>3</sub>, and a wide range of volatile organic compounds (e.g. monoterpenes, sesquiterpenes, isoprene, acetonitrile, aromatics) were quantified in real time with 30 minute temporal resolution.  Small molecules were measured using Aerodyne TILDAS instruments, while VOCs were measured using a Vocus mass spectrometer.</p><p>Addition of water to the dried soil column produced a classic Birch effect pulse of both C and N species, including for VOCs.  We explore correlations between responses of trace gases above- and below-ground, and relate the small molecule pulses to the larger VOC responses.  In addition, we demonstrate the value of isotopic signatures for these studies, with the observation of fast, large isotopic shifts in the <sup>15</sup>N<sub>2</sub>O isotopomers.  We compare these isotopic signatures to simple kinetic models to provide insight into the mechanisms underlying the nitrogen Birch effect.</p>

Current knowledge and future perspectives on soil drying and rewetting, by the scientific community

<p>Drying and rewetting events induce enormous dynamics in soil biogeochemistry, known as the “Birch effect”. A series of laboratory studies have shown that during this phenomenon, respiration and microbial growth are uncoupled. In addition, it has been found that soil microorganisms exhibit one of two different response-patterns, the dynamics of which are strongly regulated by the harshness of the moisture disturbance experienced by soil microbes. Despite the potential significance of these responses for the global carbon cycle, the characteristics and mechanisms underlying them are still unclear.</p><p>In order to shed some light on the current status of research in this field, we will present the outcomes of an international workshop organized in Lund in November 2019. During it, we integrated researchers from different environments in order to identify knowledge-gaps and tackle outstanding and new challenges in this field. We will review the characteristics of the growth and respiration responses to moisture fluctuations and the putative mechanisms and factors governing them. We will also discuss the advantages of combining empirical and modelling approaches by using our own group experience as a case example.</p>

Satellite evidence of substantial rain-induced soil emissions of ammonia across the Sahel

Atmospheric ammonia (NH3) is a precursor to fine particulate matter formation and contributes to nitrogen (N) deposition, with potential implications for the health of humans and ecosystems. Agricultural soils and animal excreta are the primary source of atmospheric NH3, but natural soils can also be an important emitter. In regions with distinct dry and wet seasons such as the Sahel, the start of the rainy season triggers a pulse of biogeochemical activity in surface soils known as the Birch effect, which is often accompanied by emissions of microbially produced gases such as carbon dioxide and nitric oxide. Field and lab studies have sometimes, but not always, observed pulses of NH3 after the wetting of dry soils; however, the potential regional importance of these emissions remains poorly constrained. Here we use satellite retrievals of atmospheric NH3 using the Infrared Atmospheric Sounding Interferometer (IASI) regridded at 0.25∘ resolution, in combination with satellite-based observations of precipitation, surface soil moisture, and nitrogen dioxide concentrations, to reveal substantial precipitation-induced pulses of NH3 across the Sahel at the onset of the rainy season in 2008. The highest concentrations of NH3 occur in pulses during March and April when NH3 biomass burning emissions estimated for the region are low. For the region of the Sahel spanning 10 to 16∘ N and 0 to 30∘ E, changes in NH3 concentrations are weakly but significantly correlated with changes in soil moisture during the period from mid-March through April when the peak NH3 concentrations occur (r=0.28, p=0.02). The correlation is also present when evaluated on an individual pixel basis during April (r=0.16, p<0.001). Average emissions for the entire Sahel from a simple box model are estimated to be between 2 and 6 mg NH3 m−2 d−1 during peaks of the observed pulses, depending on the assumed effective NH3 lifetime. These early season pulses are consistent with surface observations of monthly concentrations, which show an uptick in NH3 concentration at the start of the rainy season for sites in the Sahel. The NH3 concentrations in April are also correlated with increasing tropospheric NO2 concentrations observed by the Ozone Monitoring Instrument (r=0.78, p<0.0001), which have previously been attributed to the Birch effect. Box model results suggest that pulses occurring over a 35-day period in March and April are responsible for roughly one-fifth of annual emissions of NH3-N from the Sahel. We conclude that precipitation early in the rainy season is responsible for substantial NH3 emissions in the Sahel, likely representing the largest instantaneous fluxes of gas-phase N from the region during the year.

Satellite evidence of substantial rain-induced soil emissions of ammonia across the Sahel

Atmospheric ammonia (NH3) is a precursor to fine particulate matter formation and contributes to nitrogen deposition, with potential implications for the health of humans and ecosystems. Agricultural soils and animal excreta are the primary source of atmospheric NH3, but natural soils can also be an important emittor. In regions with distinct dry and wet seasons such as the Sahel, the start of the rainy season triggers a pulse of biogeochemical activity in surface soils known as the Birch effect, which is often accompanied by emissions of microbially-produced gases such as carbon dioxide and nitric oxide. Field and lab studies have sometimes, but not always, observed pulses of NH3 after the wetting of dry soils; however, the potential regional importance of these emissions remains poorly constrained. Here we use satellite retrievals of atmospheric NH3 using the Infrared Atmospheric Sounding Interferometer (IASI) regridded at 0.25° resolution, in combination with satellite-based observations of precipitation, surface soil moisture, and nitric dioxide concentrations, to present evidence of substantial precipitation-induced pulses of NH3 across the Sahel at the onset of the rainy season in 2008. The highest concentrations of NH3 occur in pulses during March and April, when biomass burning emissions estimated for the region by the Global Fire Emissions Database database are low. For the region of the Sahel spanning 10° to 16° N and 0° to 30° E, changes in NH3 concentrations are weakly but significantly correlated with changes in soil moisture during the period from mid-March through April, when the peak NH3 concentrations occur (r = 0.28, p = 0.02). The correlation is also present when evaluated on an individual pixel-basis during April (r = 0.16, p