Recent research progress on soil microbial responses to drying–rewetting cycles

Climatic change, such as increases in extreme drought and rainfall events and changes in rainfall intensity and pattern, has been strongly influencing soil moisture. The climatic change impact is particularly common in arid, semi-arid and Mediterranean regions, which is causing dramatic changes in the intensity and frequency of soil drying–rewetting cycles. The soil drying–rewetting cycle is a natural phenomenon that the soil experiences drying, then wetting, and then drying and rewetting again and again. When a dry soil is being rewetted, the amount of soil microbial biomass and its activity can be sharply increasing in a short time period, and then a large amount of gaseous carbon (C) and nitrogen (N) erupts from the soil. The sudden release of gaseous C and N is caused by the stimulation of the soil microbes. Such a phenomenon is called “Birch effect”. The drying–rewetting cycles have direct and indirect effects on soil microbes, and soil microbial responses to the drying and rewetting events play an important role in the feedbacks of terrestrial ecosystems. From aspects of soil microbial biomass, microbial activities and microbial structure, we review recent advances on studies regarding microbial responses to soil drying–rewetting cycles. We interpret the microbial responses using five different types of mechanisms: (1) Microbial stress mechanism: when a soil becomes dry, microorganisms must accumulate compatible solutes such as carbohydrates and aminoacids so that the soil microbes can equilibrate with their environment in order to avoid dehydrating and being killed. When the soil is rewetted, soil microbes must dispose of those osmolytes rapidly by transforming them into carbon dioxide (CO₂), dissolved organic carbon (DOC) and nutrients in order to prevent water from being flowing into the cells. (2) Substrate supply mechanism: low soil moisture may result in the physical disruption of soil aggregates which leads to the exposure of new soil surfaces and of previously protected organic matter. When the soil is rewetted, its physical structure is further disrupted by swelling. The increased new soil surfaces and previously protected organic matter will improve the microorganism’s nutrient availability. (3) Soil hydrophobicity mechanism: soil hydrophobicity can cause the reduction of soil moisture and nutrient availability and inhibition of microbial decomposition of soil organic matter. Therefore, soil hydrophobicity is an important factor of explaining the activity of microorganism in drying and rewetting events. (4) Diffusive limitations mechanism: transportation of the soil microbe is limited in a dry soil. When soil moisture is increasing, soil microbial activity is enhanced along with the increased availability of substrate nutrients. (5) Predation mechanism: a moist soil is usually conducive to the increase of bacteria and fungi populations. In response, protozoa and nematodes also increase, leading to the fluctuation of the soil microbial community structure. On the basis of the literature review, we propose five important aspects to be considered in the future: (1) assessing soil microbes’ concrete adapting ways to the drying–rewetting cycles, (2) evaluating the microbial responses to the drying–rewetting cycles based on suitable indicators, (3) interpreting microbial responses to the drying–rewetting cycles by combining field investigation and laboratory controlling experiment, (4) investigating the microbial responses to the drying–rewetting cycles at different temporal and spatial scales.     

Organic matter availability during pre- and post-drought periods in a Mediterranean stream

Mediterranean streams are characterized by water flow changes caused by floods and droughts. When intermittency occurs in river ecosystems, hydrologic connectivity is interrupted and this affects benthic, hyporheic and flowing water compartments. Organic matter use and transport can be particularly affected during the transition from wet to dry and dry to wet conditions. In order to characterize the changes in benthic organic matter quantity and quality throughout a drying and rewetting process, organic matter, and enzyme activities were analyzed in the benthic accumulated material (biofilms growing on rocks and cobbles, leaves, and sand) and in flowing water (dissolved and particulate fractions). The total polysaccharide, amino acid, and lipid content in the benthic organic matter were on average higher in the drying period than in the rewetting period. However, during the drying period, peptide availability decreased, as indicated by decreases in leucine aminopeptidase activity, as well as amino acid content in the water and benthic material, except leaves; while polysaccharides were actively used, as indicated by an increase in β-glucosidase activity in the benthic substrata and an increase in polysaccharide content of the particulate water fraction and in leaf material. During this process, microbial heterotrophs were constrained to use the organic matter source of the lowest quality (polysaccharides, providing only C), since peptides (providing N and C) were no longer available. During the flow recovery phase, the microbial community rapidly recovered, suggesting the use of refuges and/or adaptation to desiccation during the previous drought period. The scouring during rewetting was responsible for the mobilization of the streambed and loss of benthic material, and the increase in high quality organic matter in transport (at that moment, polysaccharides and amino acids accounted for 30% of the total DOC). The dynamics of progressive and gradual drought effects, as well as the fast recovery after rewetting, might be affected by the interaction of the individual dynamics of each benthic substratum: sand sediments and leaves providing refuge for microorganisms and organic matter storage, while on cobbles, an active bacterial community is developed in the rewetting. Since global climate change may favor a higher intensity and frequency of droughts in streams, understanding the effects of these disturbances on the materials and biota could contribute to reliable resource management. The maintenance of benthic substrata heterogeneity within the stream may be important for stream recovery after droughts.     

Temperature Effects on Recovery Time of Bacterial Growth After Rewetting Dry Soil

The effect of temperature on the recovery of bacterial growth after rewetting dry soil was measured in a soil that responded with bacterial growth increasing immediately upon rewetting in a linear fashion (type (i) response sensu Meisner et al. (Soil Biol Biochem 66: 188-192, 2013)). The soil was air-dried for 4 days and then rewetted at different temperatures. Bacterial growth over time was then estimated using the leucine incorporation method. At 25 °C, the recovery of bacterial growth to levels of a wet control soil was rapid, within 6 h, while at 15 °C, recovery time increased to around 60 h, becoming more than a week at 5 °C. The temperature dependency of the recovery time was well modeled by a square root function. Thus, temperature will not only directly affect growth rates but also affect length of transition periods, like resuscitation after a drying event. The temperature during the rewetting event thus has to be taken into consideration when analyzing the microbial response dynamics.     

Hydraulic redistribution may stimulate decomposition

Roots influence root litter decomposition through multiple belowground processes. Hydraulic lift or redistribution (HR) by plants is one such process that creates diel drying–rewetting cycles in soil. However, it is unclear if this phenomenon influences decomposition. Since decomposition in deserts is constrained by low soil moisture and is stimulated when dry soils are rewetted, we hypothesized that diel drying–rewetting, via HR, stimulates decomposition of root litter. We quantified the decomposition of root litter from two desert shrubs, Artemisia tridentata ssp. tridentata and Sarcobatus vermiculatus, during spring and summer in field soil core treatments designed to have abundant roots and high magnitude HR cycles (DenseRoot) or few roots and low magnitude HR (SparseRoot). To help explain our decomposition results, we not only evaluated HR, but multiple factors (i.e., soil moisture, soil temperature, dissolved soil organic C concentrations, and litter chemistry) that are often influenced by roots and regulate decomposition. Root length density in the DenseRoot treatment was at least four times higher than in the SparseRoot treatment for both Artemisia and Sarcobatus by the beginning of spring. During spring and summer, there was only one instance when decomposition rates differed between the treatments. This occurred in soils beneath Artemisia in the summer when decomposition rates were 25% higher in the DenseRoot than in the SparseRoot treatments. Of the factors evaluated, only a threefold increase in the magnitude of drying–rewetting cycles created by HR in the DenseRoot compared to the SparseRoot treatment coincided with this change in decomposition. Additionally, the lower soil Ψ_w present in the Artemisia DenseRoot treatment should have resulted in a decline in decomposition rates, but the presence of higher magnitude HR cycles seemed to nullify this effect. There was no evidence of this result in Sarcobatus soils, possibly due to Sarcobatus only creating HR cycles for a short period of time in the summer before soil Ψ_w dropped below ?7 MPa. As hypothesized, our results suggest that the presence of high magnitude HR cycles stimulated decomposition. The most plausible mechanism for this stimulation; however, was not solely due to HR drying–rewetting cycles but HR creating a diel rhythm of root-driven water fluxes and rhizodeposition. These together heightened microbial activity and, subsequently, enhanced the decomposition of surrounding litter. Our findings are the first field data supporting suggestions that HR influences belowground ecosystem processes and demonstrates that this relationship is seasonally variable.     

CaCl2 extractable N fractions and K2SO4 extractable N released on fumigation as affected by green manure mineralization and soil texture

Repeated mild wet-dry cycles were imposed on a sandy loam to accelerate the mineralization of organic C involved in stabilising macro-aggregates. Soil maintained continually moist (control soil) was compared to that subjected to a series of 6 wet-dry cycles. Two patterns of rewetting and drying were investigated: (1) incubated dry at 25°C for six days between each wet-dry cycle (dry-incubated), or (ii) incubated moist for six days at 25°C between each cycle (moist-incubated). Changes in the proportion of >2 mm, 1–2 mm, 0.5–1 mm and 0.25–0.5 mm aggregates, and carbohydrate C extracted by hot-water or hot-1.5 M H₂SO₄, were measured after each wet-dry cycle, or weekly in the continuously moist control soil. Respiration rates (CO₂ efflux) were measured during the incubation of the moist soil between the wet-dry cycles and compared with the continually-moist control soil.The wet-dry treatments did not increase soil respiration in soil after re-wetting compared to soil kept continually moist and incubated for the same period of time. Despite this, the treatments caused changes in the amounts of acid- and water-extractable carbohydrate C fractions and substantial changes in aggregation. Macro-aggregation and the proportion of soil in each fraction did not change in the soil maintained continuously-moist for 6 weeks (control). However, effects of the two wet-dry treatments on total macro-aggregation were similar to those in the >2 mm, 1–2 mm and 0.25–0.5 mm aggregate fractions: there was a rapid decline in aggregation by 48–65% over the first two cycles, a sharp recovery to 78–100% of the initial aggregation after three cycles, and a further decline after 4–6 cycles.The resistance of organic C mineralization to mild wet-dry cycles confirmed that the organic C in this soil is very stable and resistant to decomposition. Despite aggregates being disrupted, the organic C stabilising these aggregates was resistant to decomposition as determined by CO₂ efflux. When soil was re-moistened and incubated to allow microbial re-colonization, aggregation was similar to that in the soil where microbial re-colonization was limited by rapid drying treatments. Short term changes in the aggregation of this soil appear to be dominated by chemical and/or physical processes.     

Drying and wetting in saline and saline-sodic soils—effects on microbial activity, biomass and dissolved organic carbon

Aims

There are few studies on the interactive effect of salinity and sodicity in soils exposed to drying and wetting cycles. We conducted a study to assess the impact of multiple drying and wetting on microbial respiration, dissolved organic carbon and microbial biomass in saline and saline-sodic soils.

Methods

Different levels of salinity (EC_1:5 1.0 or 2.5) and sodicity (SAR?<?3 or 20) were induced by adding NaCl and CaCl₂ to a non-saline/non-sodic soil. Finely ground wheat straw residue was added at 20?g?kg^?1 as substrate to stimulate microbial activity. The constant moist (CM) treatment was kept at optimum moisture content for the length of the experiment. The drying and rewetting (DW) treatments consisted of 1 to 3 DW cycles; each DW cycle consisted of 1?week drying after which they were rewet to optimum moisture and then maintained moist for 1?week.

Results

Drying reduced respiration more strongly at EC2.5 than with EC1.0. Rewetting of dry soils produced a flush in respiration which was greatest in the soils without salt addition and smallest at high salinity (EC2.5) suggesting better substrate utilisation by microbes in soils without added salts. After three DW events, cumulative respiration was significantly increased by DW compared to CM, being 24% higher at EC1.0 and 16% higher at EC2.5 indicating that high respiration rates after rewetting may compensate for the low respiration rates during the dry phase. The respiration rate per unit MBC was lower at EC2.5 than at EC1.0. Further, the size of the flush in respiration upon rewetting decreased with each ensuing DW cycle being 50–70% lower in the third DW cycle than the first.

Conclusions

Both salinity and sodicity alter the effect of drying and rewetting on soil carbon dynamics compared to non-saline soils.     

Effects of sieving, drying and rewetting upon soil bacterial community structure and respiration rates

Soil microcosm studies often require some form of soil homogenisation, such as sieving, to provide a representative sample. Frequently, soils are also homogenised following drying and are then rewetted, yet little research has been done to understand how these methods impact upon microbial communities. Here we compared the molecular diversity and functional responses of intact cores from a Scottish grassland soil with homogenised samples prepared by drying, sieving and rewetting or freshly sieving wet soils. Results showed that there was no significant difference in total soil CO₂-C efflux between the freshly sieved and intact core treatments, however, respiration was significantly higher in the dried and rewetted microcosms. Molecular fingerprinting (T-RFLP) of bacterial communities at two different time-points showed that both homogenisation methods significantly altered bacterial community structure with the largest differences being observed after drying and rewetting. Assessments of responsive taxa in each treatment showed that intact cores were dominated by Acidobacterial peaks whereas an increased relative abundance of Alphaproteobacterial terminal restriction fragments were apparent in both homogenised treatments. However, the shift in community structure was not as large in the freshly sieved soil. Our findings suggest that if soil homogenisation must be performed, then fresh sieving of wet soil is preferable to drying and rewetting in approximating the bacterial diversity and functioning of intact cores.     

Key role of streambed moisture and flash storms for microbial resistance and resilience to long‐term drought

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Restoring natural seepage conditions on former agricultural grasslands does not lead to reduction of organic matter decomposition and soil nutrient dynamics

In the central part of the Netherlands, wetland restoration projects involve the rewetting of former agricultural land, where low water levels were artificially maintained (polders). Many of these projects do not result in the expected reduction of nitrogen and phosphorus availability and subsequent re-establishment of a diverse wetland vegetation. The aim of the present study was to investigate which mechanisms are responsible for this lack of success. Thereto, we studied the effect of rewetting of former agricultural grasslands on acidified peat soil (pH = 3.5) on organic matter decomposition, nitrogen cycling and phosphorus availability in the soil for three seasons. To provide an explanation for the observed effects, we simultaneously studied a set of potentially controlling abiotic soil conditions that were expected to change after rewetting. It was found that rewetting of these grasslands with natural, unpolluted seepage water did not affect nitrogen cycling, but raised decomposition rates and almost doubled phosphorus availability. The main cause of these effects is a raise of soil pH to about 7 due to the hydrochemical composition of the soil pore water after rewetting, which reflects groundwater with high amounts of buffering ions. This effect overruled any reduction in process rates by the lowered soil redox potential. The counterintuitive finding of eutrophication after rewetting with natural and unpolluted water is considered to represent a new form of internal eutrophication, triggered by the restoration of natural site conditions of former agricultural land on acid peat soil.     

Recovery of soil respiration after drying

Soils are frequently exposed to drying and wetting events and previous studies have shown that rewetting results in a strong but short-lived flush of microbial activity. The aim of this study was to determine the effect of the water content during the dry period on the size and duration of the flush and on the rate of recovery. Two soils (a sand and a sandy loam) were maintained at different water contents (WC) 30, 28 and 25 g water kg^?1 soil (sand) and 130, 105 and 95 g water kg^?1 soil (sandy loam) for 14 days, then rewet to the water content at which respiration was optimal [WC 35 (sand), WC200 (sandy loam)] and maintained at this level until day 68. Ground pea straw (C/N 26) was added and incorporated on day 1. The controls were maintained at the optimal water content throughout the 68 days. Respiration rates during the dry phase (days 1?C14) decreased with decreasing water content. The flush of respiration after rewetting peaked on day 15 in the sandy loam and on day 16 in the sand; it was greatest in the soils that had been maintained at the lowest water content [WC25 (sand) and WC95 (sandy loam)]. Cumulative respiration during the remainder of the incubation period in which all soils were maintained at optimal water content increased more strongly in the soils that had been dry compared to the constantly moist control. On the final day of the dry period (day 14), cumulative respiration in the dry soils was 29?C65% (sand) and 67?C94% (sandy loam) of the constantly moist control whereas on day 68 it was 80?C84% (sand) and 86?C96% (sandy loam). The greater increase in cumulative respiration in the previously dry soils can be explained by the reduced decomposition rates during the dry period which resulted in higher substrate availability on day 14 compared to the constantly moist control. Microbial community structure assessed by phospholipid fatty acid analyses changed over time in all treatments but was less affected by water content than respiration; it differed only between the highest and the lowest water content. These differences were maintained throughout the incubation period in the sandy loam and transiently in the sand. It can be concluded that the soil water content during the dry phase affects the size of the flush in microbial activity upon rewetting and that microbial activity in previously dried soils may not be fully restored even after 54 days of moist incubation, suggesting that drying of soil can have a significant and long-lasting impact on microbial functioning.     

Changes in root hydraulic conductivity for two tropical epiphytic cacti as soil moisture varies

The tropical epiphytic cacti Epiphyllum phyllanthus and Rhipsalis baccifera experience extreme variations in soil moisture due to limited soil volumes and episodic rainfalls. To examine possible root rectification, whereby water uptake from a wet soil occurs readily but water loss to a dry soil is minimal, responses of root hydraulic conductivity (L_p) to soil drying and rewetting were investigated along with the underlying anatomical changes. After 30 d of soil drying, L_p decreased 50%–70% for roots of both species, primarily because increased suberization of the periderm reduced radial conductivity. Sheaths composed of soil particles, root hairs, and mucilage covered young roots and helped reduce root desiccation. Axial (xylem) conductance increased during drying due to vessel differentiation and maturation, and drought-induced embolism was relatively low. Within 4 d of rewetting, L_p for roots of both species attained predrought values; radial conductivity increased for young roots due to the growth of new branch roots initiated during drying and for older roots due to the development of radial breaks in the periderm. The decreases in L_p during drought reduced plant water loss to a dry soil, and yet maximal water uptake and transpiration occurred within a few days of rewetting, helping these epiphytes to take advantage of episodic rainfalls in a moist tropical forest.     

Soil microbial community response to drying and rewetting stress: does historical precipitation regime matter?

Climate models project that precipitation patterns will likely intensify in the future, resulting in increased duration of droughts and increased frequency of large soil rewetting events, which are stressful to the microorganisms that drive soil biogeochemical cycling. Historical conditions can affect contemporary microbial responses to environmental factors through the persistence of abiotic changes or through the selection of a more tolerant microbial community. We examined how a history of intensified rainfall would alter microbial functional response to drying and rewetting events, whether this historical legacy was mediated through altered microbial community composition, and how long community and functional legacies persisted under similar conditions. We collected soils from a long-term field manipulation (Rainfall Manipulation Plot Study) in Kansas, USA, where rainfall variability was experimentally amplified. We measured respiration, microbial biomass, fungal:bacterial ratios and bacterial community composition after collecting soils from the field experiment, and after subjecting them to a series of drying–rewetting pulses in the lab. Although rainfall history affected respiration and microbial biomass, the differences between field treatments did not persist throughout our 115-day drying–rewetting incubation. However, soils accustomed to more extreme rainfall did change less in response to lab moisture pulses. In contrast, bacterial community composition did not differ between rainfall manipulation treatments, but became more dissimilar in response to drying–rewetting pulses depending on their previous field conditions. Our results suggest that environmental history can affect contemporary rates of biogeochemical processes both through changes in abiotic drivers and through changes in microbial community structure. However, the extremity of the disturbance and the mechanism through which historical legacies occur may influence how long they persist, which determines the importance of these effects for biogeochemical cycling.     

Responses of soil microbial communities to prescribed burning in two paired vegetation sites in southern China

Prescribed burning is a common site preparation practice for forest plantation in southern China. However, the effects of prescribed burning on soil microbial communities are poorly understood. This study examined changes in microbial community structure, measured by phospholipid fatty acids (PLFAs), after a single prescribed burning in two paired vegetation sites in southern China. The results showed that the total amount of PLFA (totPLFA) was similar under two vegetation types in the wet season but differed among vegetation type in the dry season, and was affected significantly by burning treatment only in the wet season. Bacterial PLFA (bactPLFA) and fungal PLFA (fungPLFA) in burned plots all decreased compared to the unburned plots in both seasons (P = 0.059). Fungi appeared more sensitive to prescribed burning than bacteria. Both G⁺ bacterial PLFA and G⁻ bacterial PLFA were decreased by the burning treatment in both dry and wet seasons. Principal component analysis of PLFAs showed that the burning treatment induced a shift in soil microbial community structure. The variation in soil microbial community structure was correlated significantly to soil organic carbon, total nitrogen, available phosphorus and exchangeable potassium. Our results suggest that prescribed burning results in short-term changes in soil microbial communities but the long-term effects of prescribed burning on soil microbial community remain unknown and merit further investigation.     

Hydraulic conductance and mercury-sensitive water transport for roots of Opuntia acanthocarpa in relation to soil drying and rewetting

Drought-induced changes in root hydraulic conductance (LP) and mercury-sensitive water transport were examined for distal (immature) and mid-root (mature) regions of Opuntia acanthocarpa. During 45 d of soil drying, LP decreased by about 67% for distal and mid-root regions. After 8 d in rewetted soil, LP recovered to 60% of its initial value for both regions. Axial xylem hydraulic conductivity was only a minor limiter of LP. Under wet conditions, HgCl2 (50 microM), which is known to block membrane water-transport channels (aquaporins), decreased LP and the radial hydraulic conductance for the stele (L(R, S)) of the distal root region by 32% and 41%, respectively; both LP and L(R, S) recovered fully after transfer to 2-mercaptoethanol (10 mM). In contrast, HgCl2 did not inhibit LP of the mid-root region under wet conditions, although it reduced L(R, S) by 41%. Under dry conditions, neither LP nor L(R, S) of the two root regions was inhibited by HgCl2. After 8 d of rewetting, HgCl2 decreased LP and L(R, S) of the distal region by 23% and 32%, respectively, but LP and L(R, S) of the mid-root region were unaltered. Changes in putative aquaporin activity accounted for about 38% of the reduction in LP in drying soil and for 61% of its recovery for the distal region 8 d after rewetting. In the stele, changes in aquaporin activity accounted for about 74% of the variable L(R, S) during drought and after rewetting. Thus, aquaporins are important for regulating water movement for roots of O. acanthocarpa.     

降雨量改变对常绿阔叶林干旱和湿润季节土壤呼吸的影响

通过野外原位试验,研究降雨量改变对华西雨屏区常绿阔叶林干旱和湿润季节土壤呼吸速率的影响。采用LI-8100土壤碳通量分析系统(LI-COR Inc.,USA)测定干旱和湿润季节对照(CK)、增雨10%(LA)、增雨5%(TA)、减雨10%(LR)、减雨20%(MR)、减雨50%(HR)6个处理水平的土壤呼吸速率,并通过回归方程分析温度和湿度与土壤呼吸速率间的关系。结果表明:湿润季节土壤呼吸速率高于干旱季节,HR处理对干旱季节土壤呼吸速率影响较大,而LA处理对湿润季节土壤呼吸速率的影响较大。TA和LR处理使土壤呼吸的温度敏感性增加,而HR、LA和MR处理使土壤呼吸的温度敏感性降低,干旱季节Q10值高于湿润季节。各处理湿润季节土壤微生物量碳氮含量显著高于干旱季节,HR、MR和LA处理减少土壤微生物生物量碳、氮的含量,而TA和LR处理增加土壤微生物生物量碳、氮的含量。与湿润季节相比,干旱季节土壤水分对土壤呼吸速率的影响较大;而与土壤温度相比,土壤水分对土壤呼吸速率的影响较小。在降雨量改变的背景下,华西雨屏区常绿阔叶林无论是干旱还是湿润季节,适当增雨和减雨都会促进土壤呼吸速率,而较高量的增雨和减雨会抑制土壤呼吸速率。     

Environmental factors affect the response of microbial extracellular enzyme activity in soils when determined as a function of water availability and temperature

Microorganisms govern soil carbon cycling with critical effects at local and global scales. The activity of microbial extracellular enzymes is generally the limiting step for soil organic matter mineralization. Nevertheless, the influence of soil characteristics and climate parameters on microbial extracellular enzyme activity (EEA) performance at different water availabilities and temperatures remains to be detailed. Different soils from the Iberian Peninsula presenting distinctive climatic scenarios were sampled for these analyses. Results showed that microbial EEA in the mesophilic temperature range presents optimal rates under wet conditions (high water availability) while activity at the thermophilic temperature range (60°C) could present maximum EEA rates under dry conditions if the soil is frequently exposed to high temperatures. Optimum water availability conditions for maximum soil microbial EEA were influenced mainly by soil texture. Soil properties and climatic parameters are major environmental components ruling soil water availability and temperature which were decisive factors regulating soil microbial EEA. This study contributes decisively to the understanding of environmental factors on the microbial EEA in soils, specifically on the decisive influence of water availability and temperature on EEA. Unlike previous belief, optimum EEA in high temperature exposed soil upper layers can occur at low water availability (i.e., dryness) and high temperatures. This study shows the potential for a significant response by soil microbial EEA under conditions of high temperature and dryness due to a progressive environmental warming which will influence organic carbon decomposition at local and global scenarios.     

Impact of drying and re-wetting on N,P and K dynamics in a wetland soil

As increased nutrient availability due to drainage is considered a major cause of eutrophication in wetlands rewetting of drained wetlands is recommended as a restoration measure. The effect of soil drying and rewetting on the contribution of various nutrient release or transformation processes to changed nutrient availability for plants is however weakly understood. We measured effects of soil drying and re-wetting on N mineralization, and denitrification, as well as on release of dissolved organic nitrogen (DON), phosphorus, and potassium in incubated soil cores from a wet meadow in southern Sweden. Additionally, the impact of re-wetting with sulphate-enriched water was studied. Soil drying stimulated N mineralization (3 times higher) and reduced denitrification (5 times lower) compared to continuously wet soil. In the wet cores, denitrification increased to 20 mg N m^–2 d^–1, which was much higher than denitrification measured in the field. In the field, increased inorganic-N availability for plants due to drainage seemed primarily to be caused by increased N mineralization, and less by decreased denitrification. Soil drying also stimulated the release of DON and K, but P release was not affected. Re-wetting of dried soil cores strongly stimulated denitrification (up to 160 mg N m^–2 d^–1), but N mineralization was not significantly decreased, neither were DON or K release. In contrast, the extractable P pool increased upon soil wetting. Re-wetting with sulphate-enriched water had no effect on any of the nutrient release or transformation rates. We conclude that caution is required in re-wetting of drained wetlands, because it may unintendently cause internal eutrophication through an increased P availability for plants.     

Successional and seasonal variations in soil and litter microbial community structure and function during tropical postagricultural forest regeneration: a multiyear study

Soil microorganisms regulate fundamental biochemical processes in plant litter decomposition and soil organic matter (SOM) transformations. Understanding how microbial communities respond to changes in vegetation is critical for improving predictions of how land‐cover change affects belowground carbon storage and nutrient availability. We measured intra‐ and interannual variability in soil and forest litter microbial community composition and activity via phospholipid fatty acid analysis (PLFA) and extracellular enzyme activity across a well‐replicated, long‐term chronosequence of secondary forests growing on abandoned pastures in the wet subtropical forest life zone of Puerto Rico. Microbial community PLFA structure differed between young secondary forests and older secondary and primary forests, following successional shifts in tree species composition. These successional patterns held across seasons, but the microbial groups driving these patterns differed over time. Microbial community composition from the forest litter differed greatly from those in the soil, but did not show the same successional trends. Extracellular enzyme activity did not differ with forest succession, but varied by season with greater rates of potential activity in the dry seasons. We found few robust significant relationships among microbial community parameters and soil pH, moisture, carbon, and nitrogen concentrations. Observed inter‐ and intrannual variability in microbial community structure and activity reveal the importance of a multiple, temporal sampling strategy when investigating microbial community dynamics with land‐use change. Successional control over microbial composition with forest recovery suggests strong links between above and belowground communities.     

MAIN ROOT-LATERAL ROOT JUNCTIONS OF TWO DESERT SUCCULENTS: CHANGES IN AXIAL AND RADIAL COMPONENTS OF HYDRAULIC CONDUCTIVITY DURING DRYING

The influence of junctions between main roots and lateral roots on water flow was investigated for the desert succulents Agave deserti and Ferocactus acanthodes during 21 d of drying in soil. Under wet conditions, the junctions did not restrict xylem water flow from lateral roots to main roots, consistent with predictions of axial conductance based on vessel diameters. Embolism caused by drying reduced such axial conductance more at the junctions than in adjoining root regions. Connective tracheary elements at the junctions were abundantly pitted and had large areas of unlignified primary wall, apparently making them more susceptible to embolism than vessels or tracheids elsewhere in the roots. Unlike the decrease in axial conductance, the overall hydraulic conductivity of the junction increased during drying because of an increase in the conductivity of the radial pathway. Despite such increases, main roots may not lose substantial amounts of water to a dry soil during drought, initially because embolism at the junctions can limit xylem flow and later because soil hydraulic conductivity decreases. Moreover, the increased root hydraulic conductivity and a potentially rapid recovery from embolism by connective tracheary elements may favor water uptake near the junctions upon soil rewetting.