Elevated [CO2] and growth temperature have a small positive effect on photosynthetic thermotolerance of Pinus taeda seedlings

Key message

Growth temperature had little effect on the response of net photosynthesis to high temperatures (up to 47 °C). On the other hand, elevated [CO ₂ ] increased net photosynthesis at high temperatures.

Abstract

We investigated whether Pinus taeda seedlings grown under elevated CO₂-concentration ([CO₂]) and temperature would be able to maintain positive net photosynthesis (A _net) longer than seedlings grown under ambient conditions when exposed to temperatures up to 47 °C. Additionally, we investigated whether a locally applied temperature increase would yield the same short-term gas exchange response to temperatures up to 47 °C as a naturally occurring latitudinal temperature increase of equal magnitude. Growth conditions were applied for 7 months (February to August) in treatment chambers constructed at two sites in the native range of P. taeda in the southern US. The sites were located 300 km apart along a north–south axis with a natural temperature difference of 2.1 °C. Seedlings were grown under ambient temperature and [CO₂] conditions at both sites. At the northern site, we also applied a temperature increase of 2 °C (T _E), ensuring that this treatment equalled the mean temperature at the southern site. Additionally, at the northern site, we applied a treatment of elevated [CO₂] (C _E). Gas exchange was measured on all plants in walk-in environmentally controlled chambers. Under C _E, there was no difference in A _net of seedlings grown in ambient or ambient +2 °C temperatures at any measurement temperature, while differences were present under ambient [CO₂]. Furthermore, A _net was higher under C _E than under ambient [CO₂]. At 47 °C, A _net was negative in all seedlings except those in the C _E and ambient temperature treatment combination. Seedlings at the northern site in the T _E treatment showed no significant differences in A _net compared with seedlings grown at ambient temperature at the southern site, indicating that the plants responded equally to a manipulated temperature increase and a latitudinal increase in temperature. Our results suggest that elevated [CO₂] increases photosynthetic thermotolerance at high temperature (>41 °C), but this effect diminishes as temperature increases further. Temperature manipulations could provide accurate information on the effect of latitudinal differences in temperature on leaf gas exchange of P. taeda.     

Light inhibition of leaf respiration as soil fertility declines along a post-glacial chronosequence in New Zealand: an analysis using the Kok method

Background and aims

Our study quantified variations leaf respiration in darkness (R _D) and light (R _L), and associated traits along the Franz Josef Glacier soil development chronosequence in New Zealand.

Methods

At six sites along the chronosequence (soil age: 6, 60, 150, 500, 12,000 and 120,000 years old), we measured rates of leaf R _D, R _L (using Kok method), light-saturated CO₂ assimilation rates (A), leaf mass per unit area (M _A), and concentrations of leaf nitrogen ([N]), phosphorus ([P]), soluble sugars and starch.

Results

The chronosequence was characterised by decreasing R _D, R _L and A, reduced [N] and [P] and increasing M _A as soil age increased. Light inhibition of R occurred across the chronosequence (mean inhibition = 16 %), resulting in ratios of R _L:A being lower than for R _D:A. Importantly, the degree of light inhibition differed across the chronosequence, being lowest at young sites and highest at old sites. This resulted in R _L:A ratios being relatively constant across the chronosequence, whereas R _D:A ratios increased with increasing soil age. Log-log R-A-M _A-[N] relationships remained constant along the chronosequence. By contrast, relationships linking rates of leaf R to [P] differed among leaves with low vs high [N]:[P] ratios. Slopes of log-log bivariate relationships linking R _L to A, M _A, [N] and [P] were steeper than that for R _D.

Conclusions

Our findings have important implications for predictive models that seek to account for light inhibition of R, and for our understanding of how environmental gradients impact on leaf trait relationships     

Soil nutrients and microbial biomass in three contrasting Mediterranean forests

Aims

The extent to which the spatial and temporal patterns of soil microbial and available nutrient pools hold across different Mediterranean forest types is unclear impeding the generalization needed to consolidate our understanding on Mediterranean ecosystems functioning.

Methods

We explored the response of soil microbial, total, organic and inorganic extractable nutrient pools (C, N and P) to common sources of variability, namely habitat (tree cover), soil depth and season (summer drought), in three contrasting Mediterranean forest types: a Quercus ilex open woodland, a mixed Q. suber and Q. canariensis woodland and a Pinus sylvestris forest.

Results

Soil microbial and available nutrient pools were larger beneath tree cover than in open areas in both oak woodlands whereas the opposite trend was found in the pine forest. The greatest differences in soil properties between habitat types were found in the open woodland. Season (drought effect) was the main driver of variability in the pine forest and was related to a loss of microbial nutrients (up to 75 % loss of N_mic and P_mic) and an increase in microbial ratios (C_mic/N_mic, C_mic/P_mic) from Spring to Summer in all sites. Nutrient pools consistently decreased with soil depth, with microbial C, N and P in the top soil being up to 208 %, 215 % and 274 % larger than in the deeper soil respectively.

Conclusions

Similar patterns of variation emerged in relation to season and soil depth across the three forest types whereas the direction and magnitude of the habitat (tree cover) effect was site-dependent, possibly related to the differences in tree species composition and forest structure, and thus in the quality and distribution of the litter input.     

Relating water use to morphology and environment of Nothofagus from the world’s most southern forests

Key message

This study is the first to quantify tree water use below 50°S. Tree morphology differs markedly among the two investigated species, reflecting adjustment to different environmental cues.

Abstract

A pronounced environmental gradient dictates the dominance of Nothofagus in the foothills on the eastern side of the Andes Mountains in Patagonia, Argentina. Below 50° southern latitude, open forests of Nothofagus antarctica (ñire) dominate the landscape towards the Patagonian steppe where annual rainfall is low. With increasing rates of annual rainfall, corresponding with an increase in elevation, closed forests of N. pumilio (lenga) replace those of ñire. During a short-term study we assessed differences in stand structure and examined environmental, structural and functional traits related to tree water use of ñire and lenga. Sap velocity reached similar maximum rates (95–100 L m^?2 sapwood h^?1), but whole-tree water use (Q) was significantly lower in ñire (8–13 L day^?1 tree^?1) compared to lenga (20–90 L day^?1 tree^?1) resulting in lower stand transpiration (ñire: 0.51 mm day^?1; lenga: 3.42 mm day^?1) despite similar tree densities. Related to this, wind speed had a particularly significant impact on Q of ñire, but not lenga. The ratio of leaf area to sapwood area (A _L/A _S) clearly identified ñire to be more structurally proficient at conserving water. While stem diameter (DBH) and crown area (A _C) were well related in both species, only lenga exhibited relationships between variables related to tree allometry and physiology (A _C/Q, DBH/Q). Our results provide the first ecophysiological characterization of the two Nothofagus species that define important and widespread ecosystems in southern Patagonia (not only below 50°S), and provide useful data to scale water use of both species from tree to stand.     

Arbuscular mycorrhizal fungi affect seedling recruitment: a potential mechanism by which N deposition favors the dominance of grasses over forbs

Background and aims

Nitrogen (N) deposition usually alters plant community structure and reduces plant biodiversity in grasslands. Seedling recruitment is essential for maintaining species richness and determines plant community composition. Arbuscular mycorrhizal fungi (AMF) are widespread symbiotic fungi and could facilitate seedling establishment. Here we conducted an experiment to address whether the influence of AMF on seedling recruitment depends on N addition and plant species.

Methods

Leymus chinensis were cultivated for 5 months in the microcosms that were inoculated with or without AMF at five N addition rates. Seeds of three main species (two C₃ grasses and one non-N₂-fixing forb) of the Eurasian steppe were sown to the 5-month-old microcosms. Seedling establishment was estimated by shoot biomass, N and P contents 7 weeks after seedling germination.

Results

AMF promoted seedlings recruitment of two C₃ grasses at addition rates above 0.5 g N m^?2. In contrast, seedling recruitment of the non-N₂-fixing forb was increased by AMF at addition rates below 0.5 g N m^?2 but was decreased above 2.5 g N m^?2.

Conclusions

These results partly explain why N addition favored the dominance of grasses over forbs in perennial grassland communities. Our study indicates that AMF have the potential to influence plant community composition by mediating revegetation in the face of N deposition.     

Dynamic motion of La atom inside the C74 (D 3h) cage: a relativistic DFT study

The interaction between lanthanum atom (La) and C₇₄ (D _3h) was investigated by all-electron relativistic density function theory (DFT). With the aid of the representative patch of C₇₄ (D _3h), we studied the interaction between C₇₄ (D _3h) and La and obtained the interaction potential. Optimized structures show that there are three equivalent stable isomers, with La located about 1.7 Å off center. There is one transition state between every two stable isomers. According to the minimum energy pathway, the possible movement trajectory of La atoms in the C₇₄ (D _3h) cage was explored. The calculated energy barrier for La atoms moving from the stable isomer to the transition state is 18.4 kcal mol^?1. In addition, the dynamic NMR spectra of La@C₇₄ according to the trajectory was calculated.

The return of an experimentally N-saturated boreal forest to an N-limited state: observations on the soil microbial community structure,biotic N retention capacity and gross N mineralisation

Background and aims

To find out how N-saturated forests can return to an N-limited state, we examined the recovery of biotic N sinks under decreasing N supply.

Methods

. We studied a 40-year-old experiment in Pinus sylvestris forest, with control plots, N0, three N treatments, N1-N3, of which N3 was stopped after 20 years, allowing observation of recovery.

Results

In N3, the N concentration in foliage was still slightly elevated, but the N uptake capacity of ectomycorrhizal (ECM) roots in N3 was no longer lower than in N0. Per area the amount of a biomarker for fungi, here mainly attributed ECM, was higher in N3 and N0 than in N1 and N2. Retention of labeled ¹⁵NH₄ ⁺ by the soil was greater in the control (99 %) and N3 (86 %), than in N1 (45 %) and N2 (29 %); we ascribe these differences to biotic retention because cation exchange capacity did not vary. Gross N mineralisation and retention of N correlated, negatively and positively, respectively, with abundance of ECM fungal biomarker.

Conclusions

. The results suggest a key role for ECM fungi in regulating the N cycle. We propose, in accordance with plant C allocation theory, that recovery is driven by increased tree below-ground C allocation to ECM roots and fungi.     

Carbon assimilation variation and control in Picea rubens,Picea mariana,and their hybrids under ambient and elevated CO2

Key message

After 3 years of CO ₂ treatments, A stimulation from ambient to elevated CO ₂ was strongly related to the total dry mass change (%), supporting the sink demand A hypothesis.

Abstract

Adaptations related to gas exchange are important fitness traits in plants and have significant growth and ecological implications. Assimilation (A) and assimilation to internal CO₂ (AC _i ) response curve parameters were quantified from a red spruce (RS) (Picea rubens Sarg.)—black spruce (BS) [P. mariana (Mill.) B.S.P.] controlled-cross hybrid complex grown under ambient and elevated CO₂ conditions. Under ambient conditions, maximum A (A _max), maximum rate of carboxylation by rubisco (V _cmax), maximum rate of electron transport (J _max), and carboxylation efficiency (CE) generally increased with increasing BS content; however, under elevated CO₂ conditions, hybrid index 50 (hybrid index number is the percentage of RS, balance BS) often had greater values than the other indices. There were significant hybrid index, CO₂, and hybrid index × CO₂ effects for A _growth at 360 ppm (A ₃₆₀) and 720 ppm (A ₇₂₀). The net A stimulation (A _stim), from ambient to elevated CO₂ treatment after 3 years was 10.8, 57.8, 74.1, 69.8, and 58.7 %, for hybrid indices 0 (BS), 25, 50, 75, and 100 (RS), respectively. Why does BS have the least A _stim, hybrid index 50 the most, and RS a moderate level? There was a significant relationship between A ₃₆₀ and ambient total biomass among indices (P = 0.096), but none was found between A ₇₂₀ and elevated total biomass. However, A _stim (%) was strongly related to the change in total dry mass (%) in response to elevated CO₂ (R ² = 0.931, P = 0.008), supporting the hypothesis that sink demand drives A. Traits A _max, V _cmax and J _max were correlated to total chlorophyll concentration. Moreover, A _max V _cmax and J _max also showed a significant underlying male effect, particularly under ambient conditions consistent with the paternal inheritance of the chloroplast genome.     

Proof of concept: nitrogen use efficiency of contrasting spring wheat varieties grown in greenhouse and field

Aims

Major aims were to test and evaluate a new concept for assessment of nitrogen use efficiency (NUE) of crops by growing six spring wheat varieties in greenhouse and field environments. NUE was calculated with a plant based concept integrating the entire crop life history and separating plant characteristics from environmental factors affecting NUE. Specific hypotheses were tested related to the varieties’ drought and nutrient fertilisation responses for NUE components, and coherence of those responses in field and greenhouse.

Methods

The wheat (Triticum aestivum L.) cultivated varieties ‘Diskett’, ‘Granary’, ‘Quarna’, ‘Stilett’, ‘Vinjett’, and a Swedish landrace (‘Dala’) were grown in field and greenhouse environments in Central Sweden. Two fertilisation treatments were included in a field and greenhouse experiment, and in the greenhouse also drought. The NUE components N uptake efficiency (U_N), grain-specific N efficiency (E_N,g) and grain N concentration (C_N,g) were assessed.

Results

Drought reduced yield and NUE through E_N,g, and more so when drought occurred prior to anthesis than after anthesis. Effect of fertilisation treatment on NUE components was similar in the two set-ups, but there were fewer variety × fertilisation interactions in the field. U_N was higher in the field and E_N,g was higher in the greenhouse, while C_N,g and overall NUE were similar in the two environments. Ranking of varieties regarding NUE and U_N was similar in the greenhouse and field, but different regarding E_N,g and C_N,g.

Conclusions

The NUE concept is a useful tool to describe and integrate important NUE components for crops grown in different treatments (nutrient fertilisation, drought) and experimental set-ups, i.e. greenhouse and field. Similar variety ranking in overall NUE across experimental set-ups indicates stable results in different environments.     

Nitrogen deposition promotes ecosystem carbon accumulation by reducing soil carbon emission in a subtropical forest

Background and aims

Tropical and subtropical forests are experiencing high levels of atmospheric nitrogen (N) deposition, but the responses of such forests ecosystems to N deposition remain poorly understood.

Methods

We conducted an 8-year field experiment examining the effect of experimental N deposition on plant growth, soil carbon dioxide efflux, and net ecosystem production (NEP) in a subtropical Chinese fir forest. The quantities of N added were 0 (control), 60, 120, and 240 kg ha^?1 year^?1.

Results

NEP was lowest under ambient conditions and highest with 240 kg of N ha^?1 year^?1 treatment. The net increase in ecosystem carbon (C) storage ranged from 9.2 to 16.4 kg C per kg N added in comparison with control. In addition, N deposition treatments significantly decreased heterotrophic respiration (by 0.69–1.85 t C ha^?1 year^?1) and did not affect plant biomass. The nitrogen concentrations were higher in needles than that in fine roots.

Conclusions

Our findings suggest that the young Chinese fir forest is carbon source and N deposition would sequester additional atmospheric CO₂ at high levels N input, mainly due to reduced soil CO₂ emission rather than increased plant growth, and the amount of sequestered C depended on the rate of N deposition.     

Effect of Δ9-stearoyl-ACP-desaturase-C mutants in a high oleic background on soybean seed oil composition

Key message

Two new sources of elevated seed stearic acid were identified and the feasibility of an elevated stearic acid, high oleic acid germplasm was studied.

Abstract

Soybean [Glycine max (L.) Merr.] oil typically contains 2–4 % stearic acid. Oil with at least 20 % stearic acid is desirable because of its improved baking properties and health profile. This study identifies two new sources of high stearic acid and evaluates the interaction of high stearic and oleic acid alleles. TCHM08-1087 and TCHM08-755, high stearic acid ‘Holladay’ mutants, were crossed to FAM94-41-3, a line containing a point mutation in a seed-specific isoform of a Δ9-stearoyl-acyl carrier protein-desaturase (SACPD-C). F₂-derived lines were evaluated for fatty acid content in four field environments. Sequencing of SACPDs in TCHM08-1087 and TCHM08-755 revealed distinct deletions of at least one megabase encompassing SACPD-C in both lines. After genotyping, the additive effect for stearic acid was estimated at +1.8 % for the SACPD-C point mutation and +4.1 % for the SACPD-C deletions. Average stearic acid in lines homozygous for the deletions was 12.2 %. A FAM94-41-3-derived line and TCHM08-1087-11, a selection from TCHM08-1087, were crossed to S09-2902-145, a line containing missense mutations in two fatty acid desaturases (FAD2-1A and FAD2-1B). F₁ plants were grown in a greenhouse and individual F₂ seed were genotyped and phenotyped. No interaction was observed between either FAD2-1A or FAD2-1B and any of the SACPD-C mutant alleles. Seed homozygous mutant for SACPD-C/FAD2-1A/FAD2-1B contained 12.7 % stearic acid and 65.5 % oleic acid while seed homozygous for the SACPD-C deletion and mutant for FAD2-1A and FAD2-1B averaged 10.4 % stearic acid and 75.9 % oleic acid.     

The role of harvest residues to sustain tree growth and soil nitrogen stocks in a tropical Eucalyptus plantation

Aims

Tropical plantations are likely to supply a growing share of the increasing world demand for forest products. We aimed to gain insight into the role of the nitrogen (N) contained in harvest residues (HR) for tree growth and soil N stocks.

Methods

We used 15N-labeled harvest residues to (1) study the dynamic of N release throughout decomposition, (2) determine the vertical transport pathways of N from the forest floor to the upper soil layers, and (3) quantifying the contributions of HR to soil N stocks and the supply of N to young Eucalyptus trees.

Results

Almost all of the 15N initially contained in the HR was recovered 27 months after deposition, with 21 % remaining in HR, 38 % being transferred to the underlying O layer, 21 % being transferred to the 0–15 cm soil layer, and approximately 15 % accumulating in the tree biomass. Our results supported the presence of two pathways of N transfers from the O layer to the mineral soil: (1) the leaching of dissolved 15N from fresh litter during the first year after planting which actively contributed to Eucalyptus N nutrition and (2) the transport of particulate organic matter in percolating water which contributed to maintain N stocks in the first 15 cm of the soil. Approximately 40 % of the N content in 2-year-old Eucalyptus trees was derived from the labeled HR.

Conclusions

The sustainability of fast-growing Eucalyptus trees established on N-poor sandy tropical soils largely relies on organic residues, as an early source of mineral N for tree and as a source of organic N in the top soil.     

A closeup study of early beech litter decomposition: potential drivers and microbial interactions on a changing substrate

Aims

Litter decomposition and subsequent nutrient release play a major role in forest carbon and nutrient cycling. To elucidate how soluble or bulk nutrient ratios affect the decomposition process of beech (Fagus sylvatica L.) litter, we conducted a microcosm experiment over an 8 week period. Specifically, we investigated leaf-litter from four Austrian forested sites, which varied in elemental composition (C:N:P ratio). Our aim was to gain a mechanistic understanding of early decomposition processes and to determine microbial community changes.

Methods

We measured initial litter chemistry, microbial activity in terms of respiration (CO₂), litter mass loss, microbial biomass C and N (C_mic and N_mic), non purgeable organic carbon (NPOC), total dissolved nitrogen (TDN), NH₄ ⁺, NO₃ ^- and microbial community composition (phospholipid fatty acids – PLFAs).

Results

At the beginning of the experiment microbial biomass increased and pools of inorganic nitrogen (N) decreased, followed by an increase in fungal PLFAs. Sites higher in NPOC:TDN (C:N of non purgeable organic C and total dissolved N), K and Mn showed higher respiration.

Conclusions

The C:N ratio of the dissolved pool, rather than the quantity of N, was the major driver of decomposition rates. We saw dynamic changes in the microbial community from the beginning through the termination of the experiment.     

Soil warming accelerates decomposition of fine woody debris

Aims

There is evidence that increased N inputs to boreal forests, via atmospheric deposition or intentional fertilization, may impact negatively on ectomycorrhizal (ECM) fungi leading to a reduced flux of plant-derived carbon (C) back to the atmosphere via ECM. Our aim was to investigate the impact of N fertilization of a Pinus sylvestris (L.) forest stand on the return of recently photoassimilated C via the ECM component of soil respiration.

Methods

We used an in situ, large-scale, ¹³C-CO₂ isotopic pulse labelling approach and monitored the ¹³C label return using soil gas efflux chambers placed over three different types of soil collar to distinguish between heterotrophic (R_H), autotrophic (R_A; partitioned further into contributions from ECM hyphae and total R_A) and total (R_S) soil respiration.

Results

The impact of N fertilization was to significantly reduce R_A, particularly respiration via extramatrical ECM hyphae. ECM hyphal flux in control plots showed substantial spatial variability, resulting in mean flux estimates exceeding estimates of total R_A, while ECM contributions to R_A in N treated plots were estimated at around 30%.

Conclusion

Significant impacts on soil C cycling may be caused by reduced plant C allocation to ECM fungi in response to increased N inputs to boreal forests; ecosystem models so far lack this detail.     

Changes in soil organic carbon and total nitrogen stocks after conversion of meadow to cropland in Northeast China

Aims

Grassland conversion to cropland (GCC) may result in loss of a large amount of soil organic carbon (SOC). However, the assessment of such loss of SOC still involves large uncertainty due to shallow sampling depth, soil bulk density estimation and spatial heterogeneity. Our objectives were to quantify changes in SOC, soil total nitrogen (STN) and C:N ratio in 0–100 cm soil profile after GCC and to clarify factors influencing the SOC change.

Methods

A nest-paired sampling design was used in six sites along a temperature gradient in Northeast China.

Results

SOC change after GCC ranged from ?17 to 0 Mg ha^?1 in 0–30 cm soil layer, recommended by IPCC, across the six sites, but ranged from ?30 to 7 Mg ha^?1 when considering 0–100 cm. We found a linear relationship between SOC change in 30–100 cm and that in 0–30 cm profile (ΔC_30?100?=?0.35ΔC_0?30, P?<?0.001), suggesting that SOC change in 0–100 cm was averagely 35 % higher than that in 0–30 cm. The change in STN showed a similar pattern to SOC, and soil C:N ratio did not change at most of sites. On the other hand, SOC loss after GCC was greater in soils with higher initial SOC content or in croplands without applying chemical fertilizers. Furthermore, SOC loss after GCC decreased with falling mean annual temperature (MAT), and even vanished in the coldest sites.

Conclusions

The magnitude of SOC loss following GCC in Northeast China is lower than the global average value, partly due to low MAT here. However, the current low SOC loss can be intensified by remarkable climate warming in this region.     

Nitrogen cycling in forest soils across climate gradients in Eastern China

A ¹⁵N tracing study was carried out to investigate the potential gross nitrogen (N) dynamics in thirteen forest soils in Eastern China ranging from temperate to tropical zones (five coniferous forests, six deciduous broad-leaf forests, one temperate mixed forest, one evergreen broad-leaf forests ecosystems), and to identify the major controlling factors on N cycling in these forest ecosystems. The soil pH ranged from 4.3 to 7.9 and soil organic carbon (SOC) ranged from 6.6 g?kg^?1 to 83.0 g?kg^?1. The potential gross N transformation rates were quantified by ¹⁵N tracing studies where either the ammonium or nitrate pools were ¹⁵N labeled in parallel treatments. Gross mineralization rates ranged from 0.915 μg N g^?1 soil day^?1 to 2.718 μg N g^?1 soil day^?1 in the studied forest soils. The average contribution of labile organic-N (M _Nlab ) to total gross mineralization (M _Nrec +M _Nlab ) was 86% (58% to 99%), indicating that turnover of labile organic N plays a dominant role in the studied forest ecosystems. The gross mineralization rates in coniferous forest soils were significantly lower (ranging between 0.915 and 1.228 μg N g^?1 soil day^?1) compared to broad-leaf forest soils (ranging from 1.621 to 2.718 μg N g^?1 soil day^?1) (p?<?0.01). Thus, the dominant vegetation may play an important role in regulating soil N mineralization. Nitrate production (nitrification) occurred via two pathways, oxidation of NH ₄ ⁺ and organic N the forest soils. Correlations with soil pH indicated that this is a key factor controlling the oxidation of NH ₄ ⁺ and organic N in theses forest ecosystems. NH ₄ ⁺ oxidation decreased with a decline in pH while organic N oxidation increased. The climatic conditions (e.g. moisture status) at the various sites governed the NO ₃ ^? -N consumption processes (dissimilatory NO ₃ ^? reduction to NH ₄ ⁺ (DNRA) or immobilization of NO ₃ ^? ). Total NO ₃ ^? consumption and the proportion of total NO ₃ ^? consumption to total NO ₃ ^? production decreased with an increase in the drought index of ecosystems, showing that strong interactions appear to exist between climatic condition (e.g. the drought index), N mineralization and the rate of DNRA. Interactions between vegetation, climatic conditions govern internal N cycling in these forests soils.     

N and P colimitation of N2-fixing and N-supplied fynbos legumes from the Cape Floristic Region

Background and aims

Legume species in the fynbos vegetation of the Cape Floristic Region, that fix N₂ in soils with low P, may have evolved for enhanced acquisition and efficient use of P. It was hypothesized that N₂-fixing and combined-N supplied (N-supplied) A. linearis, P. calyptrata and C. genistoides are adapted to low P and would be relatively unresponsive to increased P of 100 μM.

Methods

18 legume species were evaluated for their nodulation response to low P availability. The N X P interaction was then examined in A. linearis, P. calyptrata and C. genistoides reliant on either N₂-fixation or 300 μM N (NH₄NO₃), and receiving 0.1, 1.0, 10 and 100 μM P (NaH₂PO₄).

Results

In the species selection experiment, A. linearis, P. calyptrata and C. genistoides, with the greatest nodule fresh weight (FW) and nodule FW to root FW ratio, were the most prolific nodulating species. In the N X P experiment, with low P supply, the biomass of N₂-fixing P. calyptrata and C. genistoides was consistently greater than that of N-supplied plants. In contrast, with high P supply of 100 μM P, all N-supplied plants accumulated more biomass than the corresponding N₂-fixing plants. High P-use efficiency, poor down-regulation of P uptake and P storage was evident in A. linearis and P. calyptrata.

Conclusion

The growth response to P and the significant N X P interactions indicate that N₂-fixing and N-supplied plants were not adapted to low P, but rather colimited by both N and P.     

Heterologous expression of Arabidopsis C-repeat binding factor 3 (AtCBF3) and cold-regulated 15A (AtCOR15A) enhanced chilling tolerance in transgenic eggplant (Solanum melongena L.)

Key message

Our study shows that the expression of AtCBF3 and AtCOR15A improved the chilling tolerance in transgenic eggplant.

Abstract

In an attempt to improve chilling tolerance of eggplant (Solanum melongena L) plants, Arabidopsis C-repeat binding factor 3 (AtCBF3) and cold-regulated 15A (AtCOR15A) genes both driven by an Arabidopsis RESPONSIVE TO DESSICATION 29A promoter (AtRD29A) were transferred into the plants of eggplant cultivar Sanyueqie. Two independent homozygous transgenic lines were tested for their cold tolerance. The leaves of the transgenic plants in both lines withered much slower and slighter than the wild-type plants after exposure to cold stress treatment at 2 ± 1 °C. The gene expression of AtCBF3 and AtCOR15A was significantly increased as well as the proline content and the levels of catalase and peroxidase activities, while the relative electrical conductivity and the malondialdehyde content were remarkably decreased in the transgenic plants compared with the wild type at 4 ± 0.5 °C. The results showed that the expression of the exogenous AtCBF3 and AtCOR15A could promote the cold adaptation process to protect eggplant plants from chilling stress.     

Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada

Background and aims

Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates.

Methods

A laboratory incubation experiment was conducted to evaluate the effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the ¹⁵N tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada.

Results

Gross N mineralization and gross NH ₄ ⁺ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg^-1 soil d^-1 in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH ₄ ⁺ immobilization rates (N/IA) and cumulative N₂O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N₂O emission was lower at 65% WHC.

Conclusions

The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N₂O emissions in both soils.