Birch Effect Research Articles

The effect of birch (Betula pendula Roth) face veneer thickness on the reaction to fire properties of fire-retardant treated plywood

Birch (Betula pendula Roth) is the primary wood used for plywood manufacturing in the Baltic and Nordic regions. Birch plywood is a valuable material with superior properties to solid wood, but its poor flammability limits its use in building construction and transportation. Several fire-retardant impregnated spruce and birch plywood solutions are available on the market, the majority of which are based on the concept of fire-retardant pressure impregnated veneers or soaked plywood. Hence, the demand for fire-resistant plywood is increasing worldwide. However, for the plywood industry, it is crucial that any fire protection enhancement process does not significantly disrupt the current production setup. This study investigates the effect of face veneer thickness (FVT) and fire retardant (FR) concentration on the fire performance of birch plywood. The FR treatment was applied by roller coating with the main and diluted (70 %) concentrations of a new protic ionic liquid, FR. Three FVTs were evaluated: 1.0 mm, 1.5 mm and 2.6 mm. The cone heater test demonstrated that the 70 % diluted FR did not provide adequate fire protection, while the plywood batches with 2.6 mm FVT provided a better reaction to fire. Conversely, the SBI test revealed that the 1.5 mm plywood meets the EUROCLASS B requirement for building applications, mainly because of the 2.6 mm FVT's 3 % higher total heat release compared to the EUROCLASS requirement. This outcome suggests magnified non-uniformity in large samples, particularly insufficient coverage of FR despite deep penetration with 2.6 mm FVT. Given the current plywood industry practices, roller coating with the main concentration of the protic ionic liquid FR and a 1.5 mm FVT is a feasible method to improve the fire performance of birch plywood for construction applications.

Unraveling mechanisms underlying effects of wetting–drying cycles on soil respiration in a dryland

Rewetting of dry soils usually stimulates soil carbon (C) emission, a phenomenon known as the Birch effect. Soil C cycling in drylands, which store approximately one third of terrestrial soil organic C (SOC), is strongly affected by wetting–drying cycles. However, the physical, chemical, and biological mechanisms that link rewetting cycles with dryland soil C cycling have not been comprehensively studied, nor do we understand how these mechanisms interact with each other. Here, we conducted a dryland soil incubation experiment manipulating four factors related to global change (soil moisture content, soil moisture variability, C availability, and prior warming) in a factorial design. The experiment was divided into two periods: a rewetting period consisting of six 14-d wetting–drying cycles; and a recovery period lasting 28 days during which soil moisture content was held stable, allowing for examination of the legacy effects of the wet-dry cycles. Rewetting cycles decreased soil aggregate stability under some conditions, but their effects on soil microbial biomass and fungal communities, soil enzyme activities, soil priming, and soil dissolved C were not significant. We found lower average soil respiration under the wetting–drying treatment than the stable soil moisture treatment, and Birch effects were observed, but only under some conditions. This was probably because moisture variability exacerbated soil microbial metabolic stress, which showed itself as oxygen limitation during the initial precipitation pulse and as water limitation during soil drying. Notably, respiration rates remained low even after moisture fluctuations ceased, suggesting a legacy effect of rewetting cycles on dryland microbial communities. Overall, rewetting inhibited aggregate formation (physical mechanism), and suppressed soil respiration by inducing soil microbial metabolic stress (biological mechanism), ultimately leading to lower soil C loss under rewetting. Our findings indicate that Birch effects are mediated by the magnitude of moisture variability, the availability of C, and the degree of physiological stress microbes experience.

Reviews and syntheses: Iron – a driver of nitrogen bioavailability in soils?

Abstract. An adequate supply of bioavailable nitrogen (N) is critical to soil microbial communities and plants. Over the last decades, research efforts have rarely considered the importance of reactive iron (Fe) minerals in the processes that produce or consume bioavailable N in soils compared to other factors such as soil texture, pH, and organic matter (OM). However, Fe is involved in both enzymatic and non-enzymatic reactions that influence the N cycle. More broadly, reactive Fe minerals restrict soil organic matter (SOM) cycling through sorption processes but also promote SOM decomposition and denitrification in anoxic conditions. By synthesizing available research, we show that Fe plays diverse roles in N bioavailability. Fe affects N bioavailability directly by acting as a sorbent, catalyst, and electron transfer agent or indirectly by promoting certain soil features, such as aggregate formation and stability, which affect N turnover processes. These roles can lead to different outcomes in terms of N bioavailability, depending on environmental conditions such as soil redox shifts during wet–dry cycles. We provide examples of Fe–N interactions and discuss the possible underlying mechanisms, which can be abiotic or microbially meditated. We also discuss how Fe participates in three complex phenomena that influence N bioavailability: priming, the Birch effect, and freeze–thaw cycles. Furthermore, we highlight how Fe–N bioavailability interactions are influenced by global change and identify methodological constraints that hinder the development of a mechanistic understanding of Fe in terms of controlling N bioavailability and highlight the areas of needed research.

The effect of antecedent soil moisture conditions on soil nitrous oxide and dinitrogen dynamics after wetting: An intact soil core study

Ruminant excreta urinary‑nitrogen (N) deposited onto pasture soil generates inorganic-N concentrations in excess of the pasture's immediate requirements. Consequently, nitrous oxide (N2O) and dinitrogen (N2) may be emitted from the urine-affected soil via denitrification. Soil moisture plays a key role in determining the end-product of denitrification. Although the effects of soil moisture and drying-wetting cycles on soil N2O production have been widely studied using sieved soils in a controlled environment, these experiments may not reflect the actual conditions in the field due to the lack of plant cover and the disturbed soil structure. To address this an experiment was performed using intact soil cores (7.5 cm depth), with pasture plants present, under controlled environmental conditions (20 °C, 8 h of light per day). The 15N flux method was used to investigate how antecedent soil moisture affected soil N2O production and consumption in a pasture soil after wetting with a nitrate (NO3−) solution. The results showed that the antecedent dry condition (25 % WFPS, Dp/Do = 0.1783) increased the magnitude of the N2O flux peak after the soil was wetted (75 % WFPS, Dp/Do = 0.006) when compared to antecedent wet conditions (50 % WFPS, Dp/Do = 0.0503). There were no differences in the denitrification and nitrification related functional gene abundances between the two antecedent conditions post-wetting, indicating that the effect was not regulated by microbial changes but by the intensity of the “Birch effect” induced by soil wetting. The N2O reduction related gene (nosZII) abundance decreased after wetting with the NO3− solution, indicating a suppression effect on N2O consumption from the NO3− applied that overrode the potential “Birch effect” on N2 emission. The results of the study indicate that rewetting of antecedent dry soil increase N2O emissions but not N2 emissions when the soil NO3− concentration is high.

When dry soil is re-wet, trehalose is respired instead of supporting microbial growth

When dry soil is re-wet there is a rapid increase in CO2 efflux and rates can remain above those of well-watered controls for one or more days. These large pulses of CO2 efflux are known as the “Birch effect”. To provide experimental evidence of different pools of C fuelling the Birch effect, we incubated a drying soil with 13C6-glucose, re wet the soil and quantified 13C labelling of pools (microbial biomass, trehalose, extracellular, and old C) and soil CO2 efflux. We took advantage of trehalose being the most 13C-enriched pool (δ13C = +518‰) to obtain direct isotopic evidence of trehalose's contribution to respiration and microbial growth. For soil incubated with 13C6-glucose, the δ13C of soil respiration was +35‰ in dry soil, increased to 100‰ in the 10 min following rewetting, and subsequently decreased. During the first 5 h after re-wetting, trehalose must have been contributing to respiration given that δ13C of soil respiration was more 13C enriched than trehalose-free microbial biomass (δ13C = +30‰), extracellular C (δ13C = −17.7‰), and old C (δ13C = −22.9‰). A four-member isotopic mixing model suggested trehalose underpinned 16% of respiration in the 1st hour after rewetting, decreasing to 7% in the fifth hour. At times beyond 5 h after rewetting, trehalose underpinned 0–4% of respiration. In the seven days following rewetting, microbial biomass increased 2292 nmol C g−1. Isotopic mass balance indicated trehalose-C could account for no more than 5% of the gross influx of C into microbial biomass, instead the increase in microbial biomass was fuelled by unlabelled or weakly labelled pools such as old C and extracellular C. Collectively these data provide direct experimental evidence C from trehalose does not significantly contribute to microbial growth in re-wet soil, but instead contributes to respiration for the first 5 h after rewetting.

Dynamics of short-term ecosystem carbon fluxes induced by precipitation events in a semiarid grassland

Abstract. Infrequent and small precipitation (PPT) events characterize PPT patterns in semiarid grasslands; however, plants and soil microorganisms are adapted to use the unpredictable small pulses of water. Several studies have shown short-term responses of carbon and nitrogen mineralization rates (called the “priming effect” or the Birch effect) stimulated by wet–dry cycles; however, dynamics, drivers, and the contribution of the priming effect to the annual C balance are poorly understood. Thus, we analyzed 6 years of continuous net ecosystem exchange measurements to evaluate the effect of the PPT periodicity and magnitude of individual PPT events on the daily/annual net ecosystem C exchange (NEE) in a semiarid grassland. We included the period between PPT events, previous daytime NEE rate, and previous soil moisture content as the main drivers of the priming effect. Ecosystem respiration (ER) responded within a few hours following a PPT event, whereas it took 5–9 d for gross ecosystem exchange (GEE; where −NEE = GEE + ER) to respond. Precipitation events as low as 0.25 mm increased ER, but cumulative PPT > 40 mm infiltrating deep into the soil profile stimulated GEE. Overall, ER fluxes following PPT events were related to the change in soil water content at shallow depth and previous soil conditions (e.g., previous NEE rate, previous soil water content) and the size of the stimulus (e.g., PPT event size). Carbon effluxes from the priming effect accounted for less than 5 % of ecosystem respiration but were significantly high with respect to the carbon balance. In the long term, changes in PPT regimes to more intense and less frequent PPT events, as expected due to the effects of climate change, could convert the semiarid grassland from a small C sink to a C source.

Accurate evaluation of the Birch effect requires continuous CO2 measurements and relevant controls

The influence of dry-wet cycles (DWC) on soil organic carbon (SOC) decomposition is still debated given the somehow controversial results observed in the literature. The objective of this study was to evaluate the effects of DWC on SOC mineralization relative to various moisture controls in 7 treatments from two long-term French field experiments presenting contrasted SOC concentrations. A laboratory incubation was conducted for 97 days to quantify CO2 emissions upon four soil moisture scenarios: continuously wet scenario at pF 1.5 (WET), continuously moderate wet scenario at pF 2.5 (MWET), continuously dry scenario at pF 4.2 (DRY) and dry-wet cycles (DWC) between pF 1.5 and 4.2. Each cycle contained two phases, 10 days of drying phase, followed by 7 days of moist phase after rewetting. The drying phase consisted of adding silica gel to the incubation jars to absorb water in the soil and then gradually drying the soil. We also calculated the SOC mineralization that would correspond to the average water content in DWC (mean_DWC). Our results showed that across all treatments the daily carbon mineralization rate increased with soil moisture (WET > MWET > DRY). In DWC scenario, mineralization rates fluctuated with the changes in soil moisture. As soils dried, daily mineralization rates decreased and the subsequent soil rewetting, to pF 1.5, caused a rapid mineralization flush or “Birch effect”. However, these flushes did not compensate for the low mineralization rates in the drying phase as the cumulative mineralization was not higher in the DWC scenario compared to the mean_DWC which was the scenario with equivalent water content as the DWC. We also observed that not accounting the CO2 emissions in the drying phase, could lead to an overestimation of the effect of DWC. We recommend to measure continuously the soil respiration during dry-wet experiments and to compare the CO2 emitted in DWC with a control that has a water content equivalent to the average water content in DWC. In addition, we questioned the importance of the effect of DWC on overall soil respiration.

How the Birch effect differs in mechanisms and magnitudes due to soil texture

Global climate change is predicted to intensify temperature and precipitation extremes, exposing soils to frequent and/or intense alternate wetting and drying regimes. Rewetting a dry soil causes a burst of CO2 known as the “Birch effect”. This spike in respiration can be attributed to: (i) release of cellular solutes (metabolites) accumulated during drought due to rapid increase in water potential upon re-wetting; (ii) sudden death of certain microbes serving as carbon (C) sources for surviving microbes, leading to microbial community shifts; and (iii) release of physically protected C due to aggregate breakdown upon repeated drying-wetting. The relative importance of these mechanisms may change in different soil textures, but very few studies examine all three processes. In this study, we evaluated the effects of repeated drying and wetting cycles (transient state moisture conditions) on the Birch effect and elucidated the mechanisms that contributed to this effect in different textured soils. Soils of three distinct textures (sandy, loamy, and clayey) were incubated for 140 days under five alternate cycles of drying (10% WHC) and wetting (100% WHC) conditions. Control soils were held at 55% WHC for the same duration. Microbial biomass C (MBC), extractable organic C (EOC), metabolites, microbial community structure, and changes in aggregate associated C were determined at each time point. Sandy soil had the lowest respiration rate followed by loamy and clayey soils, and cumulative CO2 loss was higher under transient moisture state compared to steady state. In sandy and clayey soils, changes in bacterial abundance controlled the Birch effect, while in loamy soil, the release of aggregate protected C majorly controlled the Birch effect.

Encoding diel hysteresis and the Birch effect in dryland soil respiration models through knowledge-guided deep learning

Soil respiration in dryland ecosystems is challenging to model due to its complex interactions with environmental drivers. Knowledge-guided deep learning provides a much more effective means of accurately representing these complex interactions than traditional Q10-based models. Mutual information analysis revealed that future soil temperature shares more information with soil respiration than past soil temperature, consistent with their clockwise diel hysteresis. We explicitly encoded diel hysteresis, soil drying, and soil rewetting effects on soil respiration dynamics in a newly designed Long Short Term Memory (LSTM) model. The model takes both past and future environmental drivers as inputs to predict soil respiration. The new LSTM model substantially outperformed three Q10-based models and the Community Land Model when reproducing the observed soil respiration dynamics in a semi-arid ecosystem. The new LSTM model clearly demonstrated its superiority for temporally extrapolating soil respiration dynamics, such that the resulting correlation with observational data is up to 0.7 while the correlations of the Q10-based models and the Community Land Model (CLM) are less than 0.4. Our results underscore the high potential for knowledge-guided deep learning to replace Q10-based soil respiration modules in Earth system models.

Effects of Drying and Rewetting Cycles on Carbon Dioxide Emissions and Soil Microbial Communities

Extreme rainfall and drought events attributed to climate change are anticipated to occur in the current century, resulting in frequent drying and rewetting cycles (DWCs) in soils, which will, in turn, influence soil properties and microorganisms. Sample plots of Sophora japonica, Pinus tabulaeformis, and Ginkgo biloba were selected, and undisturbed soil columns were collected. CK was the constant drying treatment; the precipitation intensities of R80, R40, and R20 were 80 mm, 40 mm, and 20 mm, respectively, and the total precipitation for the four treatments was 160 mm. Significant differences were observed in the cumulative CO2 emissions among the various DWC frequencies for the same woodland soils. A significant correlation was observed between the Birch effect and the DWC frequencies of the three woodland soils. A Pearson’s correlation analysis revealed that background nutrient contents were the key factors influencing alpha diversity. In conclusion, DWCs generally increased CO2 fluxes, cumulative CO2 emissions, and the Birch effect in addition to decreasing the alpha diversity of soil microorganisms when compared to those in the constant drying treatment.

Responses of microbial community composition and respiration to soil moisture in eroded soil

Soil moisture (SM) is one of the most limited factors on the Loess Plateau, but its effects on the microbial community and respiration are still unclear. To investigate the effects of stepwise wetting and constant SM on the soil microbial community and respiration rates, eroded soil under afforestation on the Loess Plateau was examined. The soils were sampled and incubated with constant gravimetric SM of 5 %, 30 % and 45 % and wetting treatments with SM increasing from 5 % to 30 % (5–30 %), from 5 % to 30 % to 45 % (5–30–45 %) and from 30 % to 45 % (30–45 %). As shown in the results, bacterial richness and Shannon diversity were lower in the wetting treatments of 5–30 % and 5–30–45 % than those in 5 %, 30 % and 45 % SM constant treatments. Contrary, the values were similar in the 30 % SM constant and the 30–45 % SM wetting treatment, which were the highest among all the treatments. The dominant bacterial species converted from Actinobacteria to Alphaproteobacteria and Acidobacteria with SM increasing. No significant differences were observed in fungal Shannon diversity, richness and communities under varying conditions. Consequently, the wetting treatments influenced the bacteria more significantly than fungi. A greater Birch effect occurred with a two-step increase in SM (i.e., 5–30–45 %) than the one-step wetting treatment (i.e., 30–45 %), while a dampened Birch effect was observed with higher previous soil respiration. However, excessive moisture did not enhance or even inhibit microbial respiration. Our work indicated that even short-term changes in the antecedent SM can significantly affect bacterial communities and dynamic of carbon flux. The responses of microbe and respiration were not only dependent on the current SM but also related to the antecedent SM condition. This study benefits plant-soil-water relationship management during afforestation on the Loess Plateau of China.

Soil mineral nitrogen dynamics in fallow periods in a rainfed semiarid Mediterranean agricultural system

Rainfed agricultural systems in semiarid Mediterranean environments are subject to erratic but often heavy rainfall events. As an agronomic practice, fallow periods can be included even within the existing EU common policy for crop diversification. This research aimed to quantify the effects of a previous mineral fertilization on the soil mineral nitrogen (N min ) content and on potential nitrate leaching during no-tilled fallow periods of a crop rotation. Water and N min soil measurements obtained during three fallow seasons were used. Data were also used to check if the Leaching Estimation and Chemistry Model (LEACHM) can be used for the soil N min prediction after fallow. During these periods, the N min measured in the soil profile increased on average by 125 kg N ha -1 , while the model averaged an increase of 95 kg N ha -1 . The estimate of N leached ranged from 11 to 38 kg N ha -1 . The N balance simulated using LEACHM might differ from the actual processes. After a drought period followed by a soil water replenishment, the calibrated LEACHM underestimated soil N min probably due to the “Birch effect”. After occasional rainy spells when soil quickly became saturated, LEACHM overestimated soil N min probably due to occasional N 2 O emissions not being fully accounted by the model and to specific preferential water flow, which might produce a greater nitrate leaching than that simulated by LEACHM using the convection-dispersion equation . The results show that soil N min measurements by sampling after the fallow period cannot be replaced by LEACHM simulations. These samplings are of interest in fallow established inside a rotation and to avoid over-fertilization in the following cropping season, while reducing N environmental impacts.

Does asymmetric birch effect phenomenon matter for environmental sustainability of agriculture in Tunisia?

Does asymmetric birch effect phenomenon matter for environmental sustainability of agriculture in Tunisia?

Influence of tree species on carbon, nitrogen, and phosphorus stocks and stoichiometry under different soil regeneration scenarios on reclaimed and afforested mine and post-fire forest sites

In this work, we present the effects of Scots pine (Pinus sylvestris L.), common birch (Betula pendula Roth), and black alder (Alnus glutinosa Gaertn.) on the carbon (Cstock), nitrogen (Nstock), and phosphorus (Pstock) stocks and stoichiometry in the uppermost soil horizons (Oi + Oe, 0–5 cm) on afforested post-mining sand-pit and post-fire forest sites. These results were compared to those for undisturbed forest sites under the same tree species. The Cstock and Nstock of the post-fire soils were similar to those of the undisturbed soils, but lower for the reclaimed mine site. Notably, tree species affected the distribution of Cstock between the organic and mineral horizons. Compared to the other tree species, pine was characterized by a higher Cstock in the Oi + Oe horizons of the post-mining and undisturbed soils. In the 0–5-cm soil horizons, the highest Cstock values were detected under alder at the post-mining site and under birch and alder at the undisturbed site. For all of the studied sites, the C:N ratios in the litter horizons occurred in the following order: alder < birch < pine. This trend was only reflected in the mineral horizons of the undisturbed sites, which indicates the very temporary impact of organic matter on the disturbed sites. The C:P and N:P ratios in the mineral soil horizons were higher for the post-fire site than the post-mining and undisturbed sites, which may indicate a P limitation for the given content of C and N. Black alder, as a N-fixer, can increase the C:P and N:P ratios in nutrient-poor, sandy soils after disturbance in comparison with the other studied tree species, especially in post-fire soils. The uppermost mineral soil horizons under alder were characterized by higher C:P and N:P ratios than the soils under pine on the post-mining site and under the other tree species on the post-fire site. The N:P ratios in the litter horizons under alder on the disturbed sites were higher than for the other tree species, while this was not true for the undisturbed soils where alder grew under nutrient-optimal conditions. This suggests that the phytomelioration influence of N-fixers on degraded and nutrient-poor soils, may be inhibited by P limitation.

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.

Sensitivity of organic matter mineralisation to water availability: role of solute diffusivity and the ‘Birch effect’

Context Several functions are used to describe the effects of soil water content on organic matter mineralisation. A meta-analysis of published studies identified relative water content (RWC; available water relative to the soil’s available water holding capacity) as the best water descriptor for N mineralisation. Aims To evaluate RWC as a predictor of C and N mineralisation in New Zealand soils; and to investigate how solute diffusivity and the ‘Birch effect’ may help to explain this relationship. Methods Three agricultural soils (0–15 cm), differing in water holding capacity were incubated (8-week; 20°C) under a range of RWCs to measure carbon (respiration) and net N mineralisation. After 4 weeks, a subset of samples from each treatment were re-wetted to field capacity for a further 4-weeks to quantify the respiration response to re-wetting. Key results For all three soils, there was a linear relationship between respiration and RWC where the C respired at the wilting point (RWC = 0) was ∼25–30% of that at field capacity (RWC = 1.0). Results from a solute diffusivity model suggested that a decrease in microbial substrate supply, owing to restricted diffusion of dissolved organic compounds, contributed to moisture-induced decline in respiration. A respiration flush was not observed when RWC was &gt;0 at re-wetting. Nitrogen mineralisation was non-linearly related to RWC, with small decreases in RWC below 1.0 (optimum) having a greater effect on N, than C, mineralisation. Conclusions RWC may be a reliable ‘water modifier’ to describe the influence of soil moisture on respiration. Further work is recommended to verify the RWC vs net N mineralisation relationship observed in this study.

A Microbial‐Explicit Soil Organic Carbon Decomposition Model (MESDM): Development and Testing at a Semiarid Grassland Site

AbstractExplicit representations of microbial processes in soil organic carbon (SOC) decomposition models have received increasing attention, because soil heterotrophic respiration remains one of the greatest uncertainties in climate‐carbon feedbacks projected by Earth system models (ESMs). Microbial‐explicit models have been developed and applied in site‐ and global‐scale studies. These models, however, lack the ability to represent microbial respiration responses to drying‐wetting cycles, and few of them have been incorporated in land surface models (LSMs) and validated against field observations. In this study, we developed a multi‐layer, microbial‐explicit soil organic carbon decomposition model (MESDM), based on two main assumptions that (a) extracellular enzymes remain active at dry reaction microsites, and (b) microbes at wet microsites are active or potentially active, while microbes at the dry microsites are dormant, by dividing the soil volume into wet and dry zones. MESDM with O2 and CO2 gas transport models was coupled with Noah‐MP LSM and tested against half‐hourly field observations at a semiarid grassland site in the southwest US characterized by pulsed precipitation. The results show MESDM can reproduce the observed soil respiration pulses of various sizes in response to discrete precipitation events (Birch effect) and thus improve the simulation of net ecosystem exchange. Here, both microbial accessibility to accumulated dissolved organic carbon and reactivation of dormant microbes at the dry microsites upon rewetting are critical to reproducing the Birch effect. This study improves our understanding of and ability to simulate complex soil carbon dynamics that experience drying‐wetting cycle in climate‐carbon feedbacks.

Soil fungal community structure and seasonal diversity following application of organic amendments of different quality under maize cropping in Zimbabwe.

Recent advocacy for Integrated Soil Fertility Management (ISFM) in smallholder farming systems in east and southern Africa show substantial evidence of increased and sustained crop yields associated with enhanced soil productivity. However, the impact ISFM on soil fungi has received limited attention, yet fungi play key roles in crop growth. Following total soil DNA extraction with ZR soil microbe miniprep kit, illumina sequencing was used to, examine the fungal communities (ITS1F) under a maize crop following co-application of organic nutrient resources including Crotalaria juncea, cattle manure and maize stover with inorganic fertilizers at three-time periods (T1-December, T2-January, and T3-February) in Zimbabwe. Ninety-five fungal species were identified that were assigned to Ascomycota (>90%), Basidiomycota (7%) and Zygomycota (1%). At T1, Ascomycota and Basidiomycota were identified across treatments, with Ascomycota attaining > 93% frequency. Fungal succession was noted and involved reduction of Ascomycota coupled by increase in Basidiomycota under the different treatments. For example at T3, Basidiomycota increased to 34% while Ascomycota declined to 66% under manure but remained unchanged in other two organics. Pre-season mineral nitrogen (N) associated with the ‘Birch effect’ apparently influenced the fungal community structure at T1 while readily available fertilizer N was critical at T2 and T3. The low-quality maize stover promoted the presence of Exophiala sp SST 2011 and this was linked to N immobilization. The impact of N addition was more pronounced under medium (manure) to low-quality (maize stover) resources. Fungi required phosphorus (P) and N for survival while their proliferation was dependent on substrate availability linked to resource quality. Interactive-forward test indicated that soil available P and N were most influential (P < 0.05) factors shaping fungal communities. Co-application of medium to high quality organic and inorganic resources show promise as a sustainable entry point towards enhancing belowground fungal diversity critical in driving nutrient supply.

Effects of tree species interaction, stand density, and tree size on the productivity of Larix principis-rupprechtii

Pure and mixed larch (Larix pricipis-rupprechtii) and birch (Betula platyphylla) plantations in Saihanba area were selected as test objects, with two stand density (200-340 and 880-1100 trees·hm-2) of each stand type. Based on tree size-stratified sampling approach, a total of 668 tree core samples were collected. A linear mixed model was used to analyze the effects of tree species interaction, stand density, and tree size on larch productivity. Results showed that basal area increment of larch was affected by competition, diameter at breast height, tree age, and neighborhood density to different degrees. Overyielding of larch was mainly due to the positive effect of birch on larch growth in the mixed plantation with higher stand density. For mixed plantation with lower stand density, the productivity of those two species was lower than that pure plantation because of a lack of species interaction. Intraspecific competition was the main factor influencing larch productivity. Larch productivity was positively affected by tree size, with the magnitude of tree size effect varying with stand density and species composition. Suitable enhancement of stand density and selection of birch as the mixing tree species could improve productivity of larch.

Aboveground net primary productivity and soil respiration display different responses to precipitation changes in desert grassland

AbstractPrecipitation (PPT) changes affect both aboveground vegetation dynamics and belowground carbon cycling processes, particularly in arid and semiarid regions. However, it remains unclear how extreme PPT variation can affect soil carbon sequestration potential. A 3-year PPT manipulation experiment with five levels (±40%, ±20% and ambient PPT) was conducted in a desert grassland of western Loess Plateau. Aboveground net primary productivity (ANPP) and soil respiration (Rs) were measured to examine whether the responses of ANPP and Rs to PPT changes displayed a double asymmetry model. The ANPP was more sensitive to extreme drought than extreme wet treatments in wet and dry years, which displayed a negative asymmetric model. The change in ANPP was mainly due to the direct effect of PPT change, and plant density variation also exerted some influence in the dry year. In contrast, Rs displayed a positive asymmetry response to PPT change in dry year. This may be ascribed to enhanced autotrophic respiration due to the enhanced positive responses of plant growth and ANPP to wet treatments as well as stronger birch effect of rainfall events on heterotrophic respiration. The saturating response of Rs to extreme drought (−40% PPT treatment) was also found in the dry year. Nevertheless, the response of Rs to PPT change displayed a negative asymmetry model in wet years. The contrasting models for ANPP and Rs in response to altered PPT regime suggest that extreme wet or dry treatments may increase soil C pools effluxes toward debt in this desert grassland.