Drying-rewetting Cycles Research Articles

Climate and edaphic factors drive soil enzyme activity dynamics and tolerance to Cd toxicity after rewetting of dry soil

The intense drying-rewetting cycle due to climate change can affect soil microbial community composition and function, resulting in long-term consequences for belowground carbon and nutrient dynamics. However, how climatic and edaphic factors influence the responses of enzymes to rewetting and their responses to additional perturbation (e.g., heavy metal pollution) after the drying-rewetting history are not well understood. In this study, we collected 18 surface soils from farmlands across various climate zones in China. We chose dehydrogenase (DHA) and alkaline phosphomonoesterase (ALP) as representative intracellular and extracellular enzymes, respectively, and investigated their tolerance to additional perturbation by adding metal ions (i.e., Cd2+) upon rewetting. In all soils, rewetting increased DHA activities but did not affect ALP activities compared to air-dried soils. Rewetting increased the tolerances of DHA and ALP to Cd stress, suggesting that the drying-rewetting history may reduce the susceptibility of soil enzymes to additional disturbance. The results demonstrate that differentiating enzymes based on their location in the soil will improve our ability to assess the stress response of microbial communities to drastic fluctuations in soil moisture, thereby better predicting the legacy of climate change on microbial function in soils contaminated with heavy metals.

Drying-rewetting cycles reduce bacterial diversity and carbon loss in soil on the Loess Plateau of China

Soil drying-rewetting (DRW) events became intensified and occurred frequently on the Loess Plateau with the global climate change. However, the extent to which the DRW cycles with different intensity and number alter microbial community and respiration is rarely understood. Therefore, indoor DRW treatments on the soil samples obtained from Loess Plateau involving soil moisture that accounted for 10% water holding capacity (WHC) increasing to 60% WHC and 90% WHC (i.e., 10%–60% WHC and 10%–90% WHC) with 1 or 4 cycles, respectively, were implemented. Constant soil moistures of 10%, 60% and 90% WHC were set as control. Results showed that the bacterial diversity and richness were decreased and those of fungi remained unchanged under DRW treatments compared to those under constant moisture conditions. At the bacterial and fungal phylum level, Actinobacteriota and Ascomycota were the most dominant in all the different moisture treatments, respectively. The bacterial network was more complex than fungi, indicating that the bacteria had a greater potential for interaction and niche-sharing under DRW treatment. The pulse of respiration rate was declined as DRW cycle was increased under 10%−60% WHC treatment, but remained similarly in different cycles under 10%−90% WHC treatment. Moreover, the DRW treatments could reduce the overall carbon loss and the direct carbon release under 10%−60% WHC treatment was larger than that under 10%−90% WHC treatment. The cumulative CO2 emission after four cycles were positively correlated with microbial biomass carbon (MBC) and negatively correlated with fungal Chao 1 richness with great significances.

A Drying-Rewetting Cycle Imposes More Important Shifts on Soil Microbial Communities than Does Reduced Precipitation.

ABSTRACTGlobal changes will result in altered precipitation patterns, among which the increasing frequency of drought events has the highest deleterious potential for agriculture. Soil microbes have shown some promise to help crops adapt to drought events, but it is uncertain how crop-associated microorganisms will respond to altered precipitation patterns. To investigate this matter, we conducted a field experiment where we seeded two wheat cultivars (one resistant to water stress and the other sensitive) that were subjected to four precipitation exclusion (PE) regimes (0%, 25%, 50%, and 75% exclusion). These cultivars were sampled seven times (every 2 weeks, from May to August) within one growing season to investigate short-term microbiome responses to altered precipitation regimes and seasonality using 16S rRNA gene and internal transcribed spacer (ITS) region amplicon sequencing. One of the most striking features of the data set was the dramatic shift in microbial community diversity, structure, and composition together with a doubling of the relative abundance of the archaeal ammonia oxidizer genus Nitrososphaera following an important drying-rewetting event. Comparatively small but significant effects of PE and wheat cultivar on microbial community diversity, composition, and structure were observed. Taken together, our results demonstrate an uneven response of microbial taxa to decreasing soil water content, which was dwarfed by drying-rewetting events, to which soil bacteria and archaea were more sensitive than fungi. Importantly, our study showed that an increase in drying-rewetting cycles will cause larger shifts in soil microbial communities than a decrease in total precipitation, suggesting that under climate changes, the distribution of precipitation will be more important than small variations in the total quantity of precipitation.IMPORTANCE Climate change will have a profound effect on the precipitation patterns of global terrestrial ecosystems. Seasonal and interannual uneven distributions of precipitation will lead to increasing frequencies and intensities of extreme drought and rainfall events, which will affect crop productivity and nutrient contents in various agroecosystems. However, we still lack knowledge about the responses of soil microbial communities to reduced precipitation and drying-rewetting events in agroecosystems. Our results demonstrated an uneven response of the soil microbiome and a dramatic shift in microbial community diversity and structure to a significant drying-rewetting event with a large increase in the relative abundance of archaeal ammonia oxidizers. These findings highlight the larger importance of rewetting of dry soils on microbial communities, as compared to decreased precipitation, with potential for changes in the soil nitrogen cycling.

Dry-Wet Cycles Affect Nitrous Oxide Emissions Across Aquatic-Terrestrial Interfaces: A Mesocosms Study

Aquatic-terrestrial interfaces may act as biogeochemical hotspots for greenhouse gas emissions, especially when exposed to frequent transitions between wet and dry phases. The study aimed to analyze the dynamics of nitrogen (N) processing along an inundation gradient from floodplain soils to river sediments and identify environmental factors affecting net nitrous oxide (N2O) production from different microbial sources. Intact soil and sediment cores were subject to two consecutive drying-rewetting cycles in laboratory experiments. The 15N isotope pairing technique was used to quantify N2O emissions sourced from denitrification and nitrification. We observed enhanced N2O emissions from both nitrification and denitrification following drying events. Sites exposed to frequent drying-rewetting cycles appear less affected by drying than hydrologically more stable habitats. Fluxes from nitrification were related to the organic matter content, while fluxes from denitrification were controlled by dissolved organic matter quality changes during the drying-rewetting cycles. This study shows the potential link between carbon metabolism and N2O production, combining the effect of drying-rewetting cycles.

Differential Response of Acidobacteria to Water Content, Soil Type, and Land Use During an Extended Drought in African Savannah Soils.

Although climate change is expected to increase the extent of drylands worldwide, the effect of drought on the soil microbiome is still insufficiently understood as for dominant but little characterized phyla like the Acidobacteria. In the present study the active acidobacterial communities of Namibian soils differing in type, physicochemical parameters, and land use were characterized by high-throughput sequencing. Water content, pH, major ions and nutrients were distinct for sandy soils, woodlands or dry agriculture on loamy sands. Soils were repeatedly sampled over a 2-year time period and covered consecutively a strong rainy, a dry, a normal rainy and a weak rainy season. The increasing drought had differential effects on different soils. Linear modeling of the soil water content across all sampling locations and sampling dates revealed that the accumulated precipitation of the preceding season had only a weak, but statistically significant effect, whereas woodland and irrigation exerted a strong positive effect on water content. The decrease in soil water content was accompanied by a pronounced decrease in the fraction of active Acidobacteria (7.9–0.7%) while overall bacterial community size/cell counts remained constant. Notably, the strongest decline in the relative fraction of Acidobacteria was observed after the first cycle of rainy and dry season, rather than after the weakest rainy season at the end of the observation period. Over the 2-year period, also the β-diversity of soil Acidobacteria changed. During the first year this change in composition was related to soil type (loamy sand) and land use (woodland) as explanatory variables. A total of 188 different acidobacterial sequence variants affiliated with the “Acidobacteriia,” Blastocatellia, and Vicinamibacteria changed significantly in abundance, suggesting either drought sensitivity or formation of dormant cell forms. Comparative physiological testing of 15 Namibian isolates revealed species-specific and differential responses in viability during long-term continuous desiccation or drying-rewetting cycles. These different responses were not determined by phylogenetic affiliation and provide a first explanation for the effect of drought on soil Acidobacteria. In conclusion, the response of acidobacterial communities to water availability is non-linear, most likely caused by the different physiological adaptations of the different taxa present.

Soil respiration, aggregate stability and nutrient availability affected by drying duration and drying-rewetting frequency

Wetting dry soil is well known for generating respiration and nutrient pulses (the “Birch effect”), but the effects of frequent drying-rewetting (DRW) cycles, especially the effects regarding nutrient availability, have not been well investigated. This study was designed to determine how DRW frequency influences soil respiration, aggregate stability and nutrient availability. Soil was collected from Xishan Forest Farm in Beijing, China, incubated in a laboratory and subjected to 1, 2, 3, 4, 6, or 8 DRW cycles over 48 days, with a control treatment at constant moisture (CM). The results showed that the stability of soil aggregates was significantly promoted when the drying period exceeded 12 days, while the soil aggregate stability decreased when the number of DRW cycles exceeded 2 and declined with an increase in cycle frequency. Under DRW incubation, the cumulative respiration during the wetting period compensated for the lack of respiration in the drying period compared with that under CM. The soil NH4+ and NO3− contents increased after drying, but after DRW, the NH4+ content decreased while the NO3− content increased. These results indicate that DRW promoted soil nitrification and that this effect strengthened with an increase in the number of DRW cycles. The variation range of available P was small under different DRW frequencies, indicating that the soil environment exerted a certain buffering effect on the availability of P. The availability of K increased when the drought exceeded 12 days but decreased progressively with increasing DRW frequencies. We conclude that a duration of drying exceeding 12 days is an important factor influencing soil aggregates and the availability of K. The frequency of DRW cycles may be important for aggregates and the availability of N and K.

Dam impoundment elevates soil phosphorus and some trace elements in reservoir hydro‐fluctuation belts

AbstractTo examine the role of dam impoundment in elevating the levels of soil‐associated phosphorus (P) and trace elements in reservoir hydro‐fluctuation belts (RHB), soil samples in RHB and adjacent uplands (non‐flooded area, NFA) in the Three Gorges Reservoir, China, were collected and analyzed. Concentrations of available P, copper (Cu), chromium (Cr), and zinc (Zn) were found to be higher in RHB than in NFA (p < .05), whereas organic carbon was comparable in RHB and NFA (p > .05). The elevated levels of Cu, Cr, and Zn in RHB were probably associated the repeated drying–rewetting cycles created by the dam impoundment. The 95th percentile of the single‐factor pollution index and geo‐accumulation index in RHB were 1.29 and −0.21 for Zn, 3.21 and 1.15 for Cu and 3.37 and 1.17 for Cr. Elevated pollution potential of soil‐associated Zn, Cu, and Cr existed in RHB of the Three Gorges Reservoir.Core Ideas Dam impoundments decreased total C and total N concentrations in reservoir flooded soils. Cu, Zn, and Cr concentrated in the reservoir flood areas. Variability of Cu, Zn, and Cr was captured in single‐factor pollution index and geo‐accumulation index

Responses of Microbial Communities from Forest Soils of Differing Tree Species Diversity to Drying-Rewetting Cycles

Responses of Microbial Communities from Forest Soils of Differing Tree Species Diversity to Drying-Rewetting Cycles

Extreme biogeochemical effects following simulation of recurrent drought in acid sulfate soils

Droughts are increasing in frequency and severity in many regions of the world and there are uncertainties how recurrent drought will impact acid sulfate soils, which can undergo extreme and persistent acidification (pH < 4) following oxidation. Using column experiments, we induced a 9 week drought/drying and 9 week rewetting phase in two acid sulfate soil profiles with sulfuric (pH < 4) and hypersulfidic materials from the Lower River Murray region of Australia's largest river system. These soils had not yet recovered from the extreme period of the ‘Millennium Drought’ between 2007 and 2010. pH, redox potential, dissolved metals and greenhouse gases (CO2, CH4 and N2O) were measured at multiple depths in each column every 3 weeks during the drying and rewetting phases. The solid phase of the Sulfuric and Hypersulfidic clay soil profiles in both column experiments were analysed for complete acid-base accounting (actual and retained acidity, potential acidity in the form of pyrite, and acid neutralising capacity) and reactive metals at the beginning and end of the experiment. Residual pyrite, present in high concentrations in the Sulfuric clay soil column, oxidised during the drying period, further lowering the pH (<4) and mobilising dissolved metals (Al, Cd, Cu, Ni, Zn). More reactive Fe and Al phases were formed during the drying-rewetting cycle while reactive Mn decreased. The soil pH did not recover (i.e. increase) during the rewetting phase in both the Sulfuric and Hypersulfidic clay soils, likely due to barriers to microbial reduction reactions, although Fe-oxidising bacteria were likely still active as CO2 was released. Acid sulfate soils may not recover in inter-drought periods and more severe impacts can be expected following recurrent droughts.

Effects of Drying-Rewetting Cycles on Ferrous Iron-Involved Denitrification in Paddy Soils

Soil moisture status has an important effect on the process of denitrification in paddy soils. However, it is unclear how it affects the ferrous iron-involved denitrification. Here, the influence of drying-rewetting cycles on ferrous iron-involved denitrification in paddy soil were studied with batch experiments. The dynamics of nitrate, ammonia, Fe2+, Fe3+ and total organic carbon (TOC), as well as nitrous oxide (N2O) were investigated using the iron-rich paddy soil in Jiangxi province, South China. Results demonstrated that the denitrification rate dropped while ammonia nitrogen content (NH4+-N) showed a rapid accumulation in the drying period. In the rewetting period, organic carbon played two-side roles. Organic carbon and ferrous iron together provided electron donors to denitrification, and organic carbon simultaneously reduced ferric iron under anaerobic environment. There were complex interactions among organic carbon, nitrate and Fe2+/Fe3+ under drying-rewetting cycles. Soil rewetting led to denitrification flush, especially after a moderately long drying period, while excessively frequent drying-rewetting alternation was not favorable to nitrate denitrification.

Soil Phosphorus Exchange as Affected by Drying-Rewetting of Three Soils From a Hawaiian Climatic Gradient

Current understanding of phosphorus (P) dynamics is mostly based on experiments carried out under steady-state conditions. However, drying-rewetting is an inherent feature of soil behavior, and as such also impacts P cycling. While several studies have looked at net changes in P pool sizes with drying-rewetting, few studies have dynamically tracked P exchange using isotopes, which would give insights on P mean residence times in a given pool, and thus P availability. Here, we subjected three soils from a climatic gradient on the Kohala peninsula from Hawaii to 5-month drying-rewetting treatments. The hypotheses were that physico-chemical and biotic processes would be differently affected by repeated drying-rewetting cycles, and that response would depend on climatic history of the soils. Soils were labeled with 33P and 18O enriched water. At select time intervals, we carried out a sequential extraction and measured P concentration, 33P recovery (only first 3 months), and incorporation of 18O from water into phosphate. This allowed tracing P dynamics in sequentially extracted pools as well as O dynamics in phosphate, which are driven by biological processes. Results showed that P concentration and 33P recovery were predominantly driven by soil type. However, across all soils we observed faster dilution of 33P from resin-P into less mobile inorganic pools under drying-rewetting. On the other hand, O dynamics in phosphate were mostly governed by drying-rewetting treatment. Under drying-rewetting, considerably less O was incorporated from water into phosphate of resin-P, microbial-P and HCl-P, suggesting that drying-rewetting reduced biological P cycling. Hence, our results suggest that repeated drying-rewetting increases inorganic P exchange while reducing biological P cycling due to reduced microbial activity, independent of climatic history of the soils. This needs to be considered in P management in ecosystems as well as model representations of the terrestrial P cycle.

Tea plantation intercropping green manure enhances soil functional microbial abundance and multifunctionality resistance to drying-rewetting cycles

Climate change leads to more serious drying-rewetting alternation disturbance, which furtherly affects soil ecosystem function and agriculture production. Intercropping green manure, as an ancient agricultural practice, can improve the physical, chemical, and biological fertility of soil in tea plantation. However, the effects of intercropping green manure on soil multifunctional resistance to drying-rewetting disturbance in tea plantation has not been reported. In this study, the effects of different green manure practices over four years (tea plant monoculture, tea plant and soybean intercropping, tea plant and soybean + milk vetch intercropping) on soil multifunctionality resistance to drying-rewetting cycles, and the pivotal influencing factors were investigated. We used quantitative PCR array and analysis of multiple enzyme activities to characterize the abundance of functional genes and ecosystem multifunctionality, respectively. Compared with tea plantation monoculture, tea plant intercropping soybean and soybean + milk vetch significantly increased multifunctionality resistance by 12.07% and 25.86%, respectively. Random forest analysis indicated that rather than the diversity, the abundance of functional genes was the major drive of multifunctionality resistance. The structure equation model further proved that tea plantation intercropping green manure could improve the abundance of C cycling related functional genes mediated by soil properties, and ultimately increased multifunctionality resistance to drying-rewetting disturbance. Therefore, tea plantation intercropping green manure is an effective approach to maintain the multifunctionality resistance, which is conducive to maintain the soil nutrient supply capacity and tea production under the disturbance of drying-rewetting alternation.

Responses of soil microbial biomass carbon and dissolved organic carbon to drying-rewetting cycles: A meta-analysis

Climate change may significantly increase drying-wetting cycles (DWC) in terrestrial ecosystems. However, the effects of DWC on soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) remain elusive. Therefore, we used a meta-analysis including 187 observations to clarify the effects of DWC on soil MBC and DOC. The results showed that DWC increased soil MBC by 9.5%. The effect size was negatively related to soil clay content, SOC, total nitrogen, while it was positively related to soil sampling depth, and the ratio of drying days over the total number of days in a DWC (D/T). Simultaneously, DWC increased soil DOC by 12.3%. Moreover, the effect size of MBC continuously increased with the number of DWC, while that of DOC decreased with the number of DWC. This result may explain why the CO2 pulse after rewetting continues, but its amplitude decreases gradually with the increase of DWC number. Overall, our results help understand the responses of soil MBC and DOC to DWC in terrestrial ecosystems, and could improve the prediction of soil carbon emissions under more variable soil moisture regimes in the future.

Desiccation time and rainfall control gaseous carbon fluxes in an intermittent stream

Droughts are recognized to impact global biogeochemical cycles. However, the implication of desiccation on in-stream carbon (C) cycling is not well understood yet. We subjected sediments from a lowland, organic rich intermittent stream to experimental desiccation over a 9-week-period to investigate temporal changes in microbial functional traits in relation to their redox requirements, carbon dioxide (CO2) and methane (CH4) fluxes and water-soluble organic carbon (WSOC). Concurrently, the implications of rewetting by simulated short rainfalls (4 and 21 mm) on gaseous C fluxes were tested. Early desiccation triggered dynamic fluxes of CO2 and CH4 with peak values of 383 and 30 mg C m−2 h−1 (mean ± SD), respectively, likely in response to enhanced aerobic mineralization and accelerated evasion. At longer desiccation, CH4 dropped abruptly, likely because of reduced abundance of anaerobic microbial traits. The CO2 fluxes ceased later, suggesting aerobic activity was constrained only by extended desiccation over time. We found that rainfall boosted fluxes of CO2, which were modulated by rainfall size and the preceding desiccation time. Desiccation also reduced the amount of WSOC and the proportion of labile compounds leaching from sediment. It remains questionable to which extent changes of the sediment C pool are influenced by respiration processes, microbial C uptake and cell lysis due to drying-rewetting cycles. We highlight that the severity of the dry period, which is controlled by its duration and the presence of precipitation events, needs detailed consideration to estimate the impact of intermittent flow on global riverine C fluxes.

Contrasting impacts of drying-rewetting cycles on the dissipation of di-(2-ethylhexyl) phthalate in two typical agricultural soils

Di-(2-ethylhexyl) phthalate (DEHP) pollution has become a growing problem in farmlands of China. Drying-rewetting (DW) cycle is one of frequent environmental changes that agricultural production is confronted with, and also a convenient and practical agronomic regulation measure. In this study, in order to explore the effects of DW cycles on the dissipation of DEHP and their driving mechanisms in different types of soils, we performed a 45-day microcosm culture experiment with two typical agricultural soils, Lou soil (LS) and Red soil (RS). High-throughput sequencing was applied to study the response of soil microbial communities in the process of DEHP dissipation under DW cycles. The results showed that the DW cycles considerably inhibited the dissipation of DEHP in LS while promoted that in RS. The DW cycles obviously decreased the diversity, the relative abundance of significantly differential bacteria, and the total abundance of potential degrading bacterial groups in LS, whereas have little effect on bacterial community in RS, except at the initial cultivation stage when the corresponding parameters were promoted. The inhibition of the DW cycles on DEHP dissipation in LS was mainly derived from microbial degradation, but the interplay between microbial functions and soil attributes contributed to the promotion of DEHP dissipation in RS under the DW cycles. This comprehensive understanding of the contrasting impacts and underlying driving mechanisms may provide crucial implications for the prevention and control of DEHP pollution in regional soils.

Effect of soil cap and nitrate inflow on nitrous oxide emissions from woodchip bioreactors

Woodchip bioreactors have the capability to promote the reduction of reactive nitrogen in the nitrate (NO3−) form to dinitrogen (N2), a harmless gas in the atmosphere. Nevertheless, during the reaction the potent greenhouse gas nitrous oxide (N2O) is produced and can be released if denitrification is not complete. The aim of this experiment was to quantify the effect of a soil cap, the concentration of NO3− inflow and drying-rewetting cycles (DRW) on N2O emissions from bench top bioreactors (BTBs, 36.2 × 24.2 × 16.8 cm). The soil cap effect was quantified by comparing the performance of two treatments (n = 3): soil cap (CAP) and soil cap free (UNCAP). The NO3− inflow was simulated by feeding the BTBs with two NO3− concentrations (10 and 5 mg N L−1), and DRW were simulated by saturating and draining the BTBs. Nitrous oxide was quantified in the water samples as well as measured from the surface of the BTBs.The soil cap proved effective at decreasing surface N2O emissions with a reduction of total N2O emissions (calculated as the sum of dissolved N2O and surface N2O emissions) ranging from 30.4 to 42.9%. The NO3− inflow affected dissolved N2O and surface N2O emissions with higher values (average of 3.41 and 0.36 mg m−2 d−1, respectively for CAP, and average of 2.92 and 2.52 mg m−2 d−1, respectively for UNCAP) measured at high NO3− inflow. Drying-rewetting cycles influenced dissolved N2O and surface N2O emissions, with values following rewetting that accounted for more than 56% of the total N2O emissions for both treatments. This study confirmed that soil caps are effective at mitigating N2O emissions and contributed to a better understanding of N2O dynamics induced by two different NO3− inflow concentrations and DRW.

Nitrogen process in stormwater bioretention: the impact of alternate drying and rewetting on nitrogen migration and transformation.

Nitrogen migration and transformation in the stormwater bioretention system were studied in laboratory experiments, in which the effects of drying-rewetting were particularly investigated. The occurrence and distribution of nitrogen in the plants, the soil, and the pore water were explored under different drying-rewetting cycles. The results clearly showed that bioretention system could remove nitrogen efficiently in all drying-rewetting cycles. The incoming nitrogen could be retained in the topsoil (0-10 cm) and accumulated in the planted layer. However, the overlong dry periods (12 and 22 days) cause an increase in nitrate in the pore water. In addition, nitrogen is mostly stored in the plants' stem tissues. Up to 23.26% of the inflowing nitrogen can be immobilized in plant tissues after a dry period of 22 days. In addition, the relationships between nitrogen reductase activity in the soil and soil nitrogen content were explored. The increase of soil TN content could enhance the activity of nitrate reductase. Meanwhile, the activity of hydroxylamine reductase (HyR) could be enhanced with the increase of soil NO3- content. These results provide a reference for the future development of nitrogen transformation mechanism and the construction of stormwater bioretention systems.

Woodchip bioreactors provide sustained denitrification of brine from groundwater desalination plants

Woodchip bioreactors are widely known as a best management practice to reduce excess nitrate loads that are discharged with agricultural leachates. The aim of this study was to evaluate the performance of citrus woodchip bioreactors for denitrification of brine (electrical conductivity ≈ 17 mS cm−1) from groundwater desalination plants with high nitrate content (NO3−-N ≈ 48 mg L−1) in the Campo de Cartagena agricultural watershed, one of the main providers of horticultural products in Europe. The performance was evaluated relative to seasonal changes in temperature, dissolved organic carbon (DOC) provided by woodchips, hydraulic residence time (HRT) and woodchip aging. Bioreactors (capacity 1 m3) operated for 2.5 years (121 weeks) in batch mode (24 h HRT) with three batches per week. Denitrification efficiency was modulated by DOC concentration, temperature, hydraulic residence time and the drying-rewetting cycles. High salinity of brine did not prevent nitrate removal from occurring. The high DOC availability (>25 mg C L−1) during the first ≈48 weeks resulted in high nitrate removal rate (>75%) and nitrate removal efficiency (until ≈ 25 g N m−3 d−1) regardless of temperature. Moreover, the high DOC contents in the effluents during this period may present environmental drawbacks. Denitrification was still high after 2.5 years (reaching ≈9.3 g N m−3 d−1 in week 121), but dependence on warm temperature became more apparent with woodchips aging from week ≈49 onwards. Nitrate removal efficiency was highest on the first weekly batch, immediately after woodchips had been unsaturated for four days. It was attributable to a flush of DOC produced by aerobic microbial metabolism during drying that stimulated denitrification following re-saturation. Hence, alternance of drying-rewetting cycles is an operation practice that increase bioreactors nitrate removal performance.

Recovery Capacity of Subaerial Biofilms Grown on Granite Buildings Subjected to Simulated Drought in a Climate Change Context.

Variations in environmental conditions in the context of climate change are expected to affect biofilm-associated organisms on granite heritage buildings. The number and duration of drought periods should be considered, as these factors will affect the availability of water for the microorganisms. In this study, mature biofilms were exposed to various drying-rewetting cycles, and the effects of water stress on the SAB and their resilience were evaluated in terms of the variation in microbial composition, extracellular polymeric substance production, and photosynthetic efficiency. The structure of the biofilm changed after exposure to drought, becoming more heterogeneous and with an increase in the carbohydrate to protein ratio, especially after the second day of total drought. YMAX and YEF parameters proved to be the most informative, showing that the photosynthetic efficiency and recovery capacity were inversely related to the duration of the drought period. Furthermore, cyanobacteria resisted drought better than algae, giving rise to a decrease in the algae to cyanobacteria ratio.

Soil microbial legacies differ following drying-rewetting and freezing-thawing cycles

Climate change alters frequencies and intensities of soil drying-rewetting and freezing-thawing cycles. These fluctuations affect soil water availability, a crucial driver of soil microbial activity. While these fluctuations are leaving imprints on soil microbiome structures, the question remains if the legacy of one type of weather fluctuation (e.g., drying-rewetting) affects the community response to the other (e.g., freezing-thawing). As both phenomenons give similar water availability fluctuations, we hypothesized that freezing-thawing and drying-rewetting cycles have similar effects on the soil microbiome. We tested this hypothesis by establishing targeted microcosm experiments. We created a legacy by exposing soil samples to a freezing-thawing or drying-rewetting cycle (phase 1), followed by an additional drying-rewetting or freezing-thawing cycle (phase 2). We measured soil respiration and analyzed soil microbiome structures. Across experiments, larger CO2 pulses and changes in microbiome structures were observed after rewetting than thawing. Drying-rewetting legacy affected the microbiome and CO2 emissions upon the following freezing-thawing cycle. Conversely, freezing-thawing legacy did not affect the microbial response to the drying-rewetting cycle. Our results suggest that drying-rewetting cycles have stronger effects on soil microbial communities and CO2 production than freezing-thawing cycles and that this pattern is mediated by sustained changes in soil microbiome structures.