non-N2 Fixing Research Articles

The Impact of N Input on Plant Functional Traits Such as C3, C4, Native, and Non-native in Terms of Abundance, Frequency, and Density

This study investigates the ecological changes in grassland vegetation at Banaras Hindu University’s horticulture field. Caudatemethods were used for vegetation analyses. In 72 experimental plots of 1×1m2, repeated quadrat sampling yielded 176 herbs. Plantfunctional types such as legumes, non-legumes, grasses, forbs, and sedges were classified as C3, C4, native, and non-natives traitsbased on their frequency, abundance, and density. From the rainy season of 2016 to the summer season of 2019, the mean values offrequency, abundance, and density against the N gradient were calculated. Therefore the results showed that, nitrogen is essential forthe competitive equilibrium of C3 and C4 species. This study suggests that N deposition-induced changes in competitive interactionsmay be disadvantages to native species that thrive in low-nutrient environments, such as N2-fixers, ultimately leading to changes in thecomposition of plant communities. In comparison to N2 fixers, non-N2 fixers appear to be more effective at using extra N for growth.Our findings show that the diversity of grasslands has dramatically shifted from native to introduced species, proving that non-nativespace invaders are destroying the rich grassland ecosystems around the world. By changing the dominant species and its response towhich species dominates the response at the community level, this change in abundance may alter the ecosystem functions. Based onour study, the vegetation was found to be heterogeneous up to N dosage of 60 kg/ha/year.

Crocosphaera as a Major Consumer of Fixed Nitrogen.

ABSTRACTCrocosphaera watsonii (hereafter referred to as Crocosphaera) is a key nitrogen (N) fixer in the ocean, but its ability to consume combined-N sources is still unclear. Using in situ microcosm incubations with an ecological model, we show that Crocosphaera has high competitive capability both under low and moderately high combined-N concentrations. In field incubations, Crocosphaera accounted for the highest consumption of ammonium and nitrate, followed by picoeukaryotes. The model analysis shows that cells have a high ammonium uptake rate (~7 mol N [mol N]−1 d−1 at the maximum), which allows them to compete against picoeukaryotes and nondiazotrophic cyanobacteria when combined N is sufficiently available. Even when combined N is depleted, their capability of nitrogen fixation allows higher growth rates compared to potential competitors. These results suggest the high fitness of Crocosphaera in combined-N limiting, oligotrophic oceans heightening its potential significance in its ecosystem and in biogeochemical cycling.IMPORTANCE Crocosphaera watsonii is as a key nitrogen (N) supplier in marine ecosystems, and it has been estimated to contribute up to half of oceanic N2 fixation. Conversely, a recent study reported that Crocosphaera can assimilate combined N and proposed that unicellular diazotrophs can be competitors with non-N2 fixing phytoplankton for combined N. Despite its importance in nitrogen cycling, the methods by which Crocosphaera compete are not currently fully understood. Here, we present a new role of Crocosphaera as a combined-N consumer: a competitor against nondiazotrophic phytoplankton for combined N. In this study, we combined in situ microcosm experiments and an ecosystem model to quantitatively evaluate the combined-N consumption by Crocosphaera and other non-N2 fixing phytoplankton. Our results suggest the high fitness of Crocosphaera in combined-N limiting, oligotrophic oceans and, thus, heightens its potential significance in its ecosystem and in biogeochemical cycling.

Sulfate (SO42−) Decline Supported Lake Kinneret (Israel) Invasion of N2-Fixing Cyanobacterium Aphanizomenon ovalisporum

Since 1990, the Lake Kinneret trophic status has shifted from phosphorus to nitrogen limitation. In the summer of 1994, an outbreak of N2 fixing cyanobacterium Aphanizomenon ovalisporum invaded the epilimnion of Lake Kinneret. Since then, sporadic densities of harmful cyanobacteria (HFCB) reappeared in the lake together with other toxic non-N2 fixing cyanobacteria. This predicted ecological modification developed because of a worldwide well-known background condition of descent N/P mass ratio. Reevaluation of the lake and its watershed ecosystems data exposed additional potential support of the process reduction of the epilimnetic sulfate (SO42−) concentration. Climate condition changes resulted in sulfate input reduction and its potential competitive interaction with molybdenum (MoO42−) enhanced the HFCB growth rate. The working hypothesis was the reevaluated incorporation of long-term records including rainfall, river discharge, depth of ground water table in the Hula valley peat soil, total moisture capabilities, and Kinneret epilimnion sulfate concentration. Results justify conclusive inference in Lake Kinneret of the following: sufficient phosphorus, insufficient nitrogen, and sulfate decline availabilities induced the HFCB outbreak in the summer of 1994.

Biogeochemical stoichiometry of soil and plant functional groups along a primary successional gradient following glacial retreat on the eastern Tibetan plateau

Di-nitrogen (N2)-fixing plants are crucial for vegetation development following glacial retreat; however, there is currently limited understanding of how the shifting of N2-fixing and non N2-fixing plant groups may influence the stoichiometric composition of the developing plant community and associated soils. Carbon (C), nitrogen (N), and phosphorus (P) concentrations and elementary stoichiometry were investigated in soil and the N2-fixing and non-N2-fixing plant communities across a 127-yr glacial primary succession chronosequence on the eastern slope of the Qinghai-Tibetan Plateau. Soil organic C (SOC), total N (TN), and available P (AP) gradually increased along succession, but total P (TP) was greatest at the 37-yr site, resulting in an increasing trend of soil N:P and a decreasing trend of N:AP ratios along the chronosequence. N2-fixing species had higher foliar N and P concentrations than non-N2-fixing species and exhibited a higher leaf N:P ratios in early stages (37 yr and younger) of the succession. Elementary concentration and C, N, P stoichiometry in plant leaves were strongly associated with both N2-fixing and non-N2 fixing plant biomasses, while those in soils were related only with the biomass of non-N2 fixing species. The community-weighted leaf N:P ratios increased from 14.0 at 7-yr to 19.9 at 59-yr and then decreased to 12.9 at 127-yr after glacial retreat, suggests the ecosystem as a whole is not N- or P-limited at the beginning of primary succession, but may become P-limited at the middle stages and then shift towards N limitation at the late successional stages. The results confirm that both N2-fixing and non-N2-fixing species can overcome N limitation in the early primary succession and highlights the different roles of pioneer N2-fixing and non-N2-fixing species in mediating vegetation and soil C, N and P stoichiometry with primary succession following glacial retreat.

Ancient CO2 levels favor nitrogen fixing plants over a broader range of soil N compared to present

Small inreases in CO2 stimulate nitrogen fixation and plant growth. Increasing soil N can inhibit nitrogen fixation. However, no studies to date have tested how nitrogen fixing plants perform under ancient CO2 levels (100 MYA), when nitrogen fixing plants evolved, with different levels of N additions. The aim of this study was to assess if ancient CO2, compared to present, favors nitrogen fixers over a range of soil nitrogen concentrations. Nitrogen fixers (Alnus incana ssp. rugosa, Alnus viridis ssp. crispa, and Alnus rubra) and their close non-nitrogen fixing relatives (Betula pumila, Betula papyrifera, Betula glandulosa) were grown at ancient (1600 ppm) or present (400 ppm) CO2 over a range of soil N levels, equivalent to 0, 10, 50, and 200 kg N ha−1 year−1. The growth of non-N fixing plants increased more than N fixing plants in response to the increasing N levels. When grown at an ancient CO2 level, the N level at which non-nitrogen fixing plant biomass exceeded nitrogen fixing plant biomass was twice as high (61 kg N ha−1 year−1) as the N level when plants were grown at the ambient CO2 level. Specific nodule activity was also reduced with an increasing level of soil N. Our results show there was a greater advantage in being a nitrogen fixer under ancient levels of CO2 compared with the present CO2 level.

Microcystis Bloom in an Urban Lake after River Water Diversion—A Case Study

To improve the water quality of Lake Yuehu, a water diversion from the Han River was conducted in July 2008. However, an unexpected Microcystis bloom occurred in the lake after water introduction. Water and sediment samples were collected from Lake Yuehu and the variation of chemical and biochemical parameters, as well as the phytoplankton community, were analyzed during the water diversion to assess its effect and to clarify the mechanism leading to the Microcystis bloom. The nitrogen (N) concentration was increased and phosphorus (P) concentration decreased in Lake Yuehu after receiving water from the Han River, which had a high loading of N and a low loading of P. These conditions may benefit the growth and dominance of non-N2 fixing Microcystis, as it may not have suffered from P limitation during our study because it did not produce extracellular phosphatase, which worked as an indicator of P deficiency, as evidenced by the in situ enzyme-labelled fluorescence. Notably, the sediment Fe (OOH)~P content significantly decreased in Lake Yuehu; this pulsed release of P from the sediment might have sustained the Microcystis bloom. Based on our results, algal blooms may occur as a consequence of conducting water diversion projects to improve water quality.

The amount, but not the proportion, of N2 fixation and transfers to neighboring plants varies across grassland soils

ABSTRACT Biological nitrogen fixation (BNF) is an important nitrogen source for both N2-fixers and their neighboring plants in natural and managed ecosystems. Biological N fixation can vary considerably depending on soil conditions, yet there is a lack of knowledge on the impact of varying soils on the contribution of N from N2-fixers in mixed swards. In this study, the amount and proportion of BNF from red clover were assessed using three grassland soils. Three soil samples, Hallsworth (HH), Crediton (CN), and Halstow (HW) series, were collected from three grassland sites in Devon, UK. A pot experiment with 15N natural abundance was conducted to estimate BNF from red clover, and the proportion of N transferred from red clover to the non-N2 fixing grass in a grass-clover system. The results showed that BNF in red clover sourced from atmosphere in the HH soil was 2.92 mg N plant−1, which was significantly lower than that of the CN (6.18 mg N plant−1) and HW (8.01 mg N plant−1) soils. Nitrogen in grass sourced from BNF via belowground was 0.46 mg N plant−1 in the HH soil, which was significantly greater than that in CN and HW soils. However, proportionally there were no significant differences in the percentage N content of both red clover and grass sourced from BNF via belowground among soils, at 65%, 67%, 65% and 35%, 27%, 31% in HH, CN, and HW, respectively. Our observations indicate that the amount of BNF by red clover varies among grassland soils, as does the amount of N sourced from BNF that is transferred to neighboring plants, which is linked to biomass production. Proportionally there was no difference among soils in N sourced from BNF in both the red clover plants and transferred to neighboring plants.

Growth Responses of Eucalyptus pellita F. Muell Plantations in South Sumatra to Macronutrient Fertilisers Following Several Rotations of Acacia mangium Willd.

Eucalyptus pellita has rapidly emerged as the species that has replaced Acacia mangium in broad-scale commercial plantations in Indonesia following widespread losses due to disease and in soils that have suffered a steady decline in phosphorus (P) under plantation forestry. Conversion from a nitrogen (N)-fixing to a non-N fixing species is expected to change the nutrient dynamics and the management required to maximise productivity. In this study in South Sumatra, responses of E. pellita to the application of N, P, potassium (K) and calcium (Ca) fertilisers were assessed at a number of trials on sites with varying site history; A. mangium was planted at one site to compare the species’ responses to N and P. The growth of E. pellita responded significantly (p < 0.01) to P’s application but not to N, K or Ca, with the addition of P increasing the stem volume by 32.6 m3 ha−1 at 3 years of age compared with no addition of P fertiliser; the demand for P in the first two years of growth was, respectively, 4.8 and 6.8 kg ha−1. This positive and large response appears to be because E. pellita has a lower efficiency in its use of P than A. mangium and is, therefore, more responsive than A. mangium to the addition of P. The reason for the lack of response to N remains unclear, although demand for N as well as K and Ca was high. These results suggest that sites recently converted to E. pellita from A. mangium and also new ex-native forest sites will be likely to only respond to P addition and that the response of E. pellita is likely to be greater than for A. mangium to maximise yield. At present, additional N, Ca or K fertilisers are not required, but this may change in the future.

Nitrogen fertilization and its legacy have inconsistent and often negative effect on plant growth in undeveloped post mining soils

Nitrogen supplement by adding N-fertilizer and/or growing of N-fixing trees are considered as important stimulus for the development of early vegetation. We performed pot experiments to determine the responses of legumes (N-fixing) and grasses (non-N fixing) in respect to their biomass to N fertilization, legacy of N, and combination of these two in various ages of post mining soils. N legacy was achieved either by long-term mineral N fertilization (legacy of inorganic N) (experiment 1) or growing of N2 fixing plant (legacy of N-fixing) (experiment 2). The first pot experiment was conducted to test N fertilization and legacy of inorganic N and these two on growth of Festuca rubra and Lotus corniculatus when grown separately. Following this, plant-soil feedback was defined as positive or negative by comparing the second generation of plant biomass in soils with and without plants in the previous generation. In the second pot experiment, we compared the performance of P. compressa and L. corniculatus in the presence of legacy of N-fixing and N fertiliaztion when grown together, and at the same time, plant-soil feedback was determined.N fertilization increased the growth of grasses, but only increased biomass of legumes in young soils. N-legacy induced variable plant growth, in particular often decreased the biomass of legumes. Legacy of inorganic N was only beneficial for grasses rather than legumes in medium soils. When growing in the same pot, legacy of N-fixing showed positive effects on grasses, depending on soil age and N fertilization. Repeated growing of plants in the same soil mostly resulted in negative plant-soil feedback. Negative plant-soil feedback was not alleviated by N fertilization and legacy of inorganic N for both grasses and legumes when grown separately. However, growing grasses and legumes together can induce positive plant-soil feedback with no legacy of N-fixing.In conclusion, N addition often stimulates plant growth more than long-term N effect. Legacy of N tends to be only beneficial for non-N fixing plants in medium aged soils. While negative plant-soil feedback occurs for grasses and legumes both grown separately or together, legumes can induce positive plant-soil feedback when grown with grasses in no legacy of N-fixing soils.

The Role of Iron in the P-Acquisition Mechanisms of the Unicellular N2-Fixing Cyanobacteria Halothece sp., Found in Association With the Mediterranean Seagrass Posidonia oceanica.

Posidonia oceanica, an endemic seagrass of the Mediterranean Sea harbors a high diversity of N2-fixing prokaryotes. One of these is Halothece sp., a unicellular N2-fixing cyanobacteria detected through nifH analysis from the epiphytes of P. oceanica. The most related strain in culture is Halothece sp. PCC 7418 and this was used as the test organism in this study. In the Mediterranean Sea, phosphorus (P) and iron (Fe) can be the major limiting nutrients for N2 fixation. However, information about the mechanisms of P-acquisition and the role of metals (i.e., Fe) in these processes for N2-fixing bacteria is scarce. From our genomic analyses of the test organism and other phylogenetically related N2-fixing strains, Halothece sp. PCC 7418 is one of the strains with the greatest number of gene copies (eight copies) of alkaline phosphatases (APases). Our structural analysis of PhoD (alkaline phosphatase type D) and PhoU (phosphate acquisition regulator) of Halothece sp. PCC 7418 showed the connection among metals (Ca2+ and Fe3+), and the P-acquisition mechanisms. Here, we measured the rates of alkaline phosphatase activity (APA) through MUF-P hydrolysis under different combinations of concentrations of inorganic P () and Fe in experiments under N2-fixing (low availability) and non-N2 fixing (high availability) conditions. Our results showed that APA rates were enhanced by the increase in Fe availability under low levels of , especially under N2-fixing conditions. Moreover, the increased -uptake was reflected in the increased of the P-cellular content of the cells under N2 fixation conditions. We also found a positive significant relationship between cellular P and cellular Fe content of the cells (r2 = 0.71, p < 0.05). Our results also indicated that Fe-uptake in Halothece sp. PCC 7418 was P and Fe-dependent. This study gives first insights of P-acquisition mechanisms in the N2-fixing cyanobacteria (Halothece sp.) found in P. oceanica and highlights the role of Fe in these processes.

Nutrient acquisition strategies augment growth in tropical N2 -fixing trees in nutrient-poor soil and under elevated CO2.

Tropical forests play a dominant role in the global carbon (C) cycle, and models predict increases in tropical net primary productivity (NPP) and C storage in response to rising atmospheric carbon dioxide (CO2 ) concentrations. The extent to which increasing CO2 will enhance NPP depends in part on the availability of nitrogen (N) and phosphorus (P) to support growth. Some tropical trees can potentially overcome nutrient limitation by acquiring N via symbiotic dinitrogen (N2 ) fixation, which may provide a benefit in acquiring P via investment in N-rich phosphatase enzymes or arbuscular mycorrhizal (AM) fungi. We conducted a seedling experiment to investigate the effects of elevated CO2 and soil nutrient availability on the growth of two N2 -fixing and two non-N2 -fixing tropical tree species. We hypothesized that under elevated CO2 and at low nutrient availability (i.e., low N and P), N2 fixers would have higher growth rates than non-N2 fixers because N2 fixers have a greater capacity to acquire both N and P. We also hypothesized that differences in growth rates between N2 fixers and non-N2 fixers would decline as nutrient availability increases because N2 fixers no longer have an advantage in nutrient acquisition. We found that the N2 fixers had higher growth rates than the non-N2 fixers under elevated CO2 and at low nutrient availability, and that the difference in growth rates between the N2 and non-N2 fixers declined as nutrient availability increased, irrespective of CO2 . Overall, N2 fixation, root phosphatase activity, and AM colonization decreased with increasing nutrient availability, and increased under elevated CO2 at low nutrient availability. Further, AM colonization was positively related to the growth of the non-N2 fixers, whereas both N2 fixation and root phosphatase activity were positively related to the growth of the N2 fixers. Though our results indicate all four tree species have the capacity to up- or down-regulate nutrient acquisition to meet their stoichiometric demands, the greater capacity for the N2 fixers to acquire both N and P may enable them to overcome nutritional constraints to NPP under elevated CO2 , with implications for the response of tropical forests to future environmental change.

Growth stress response to sea level rise in species with contrasting functional traits: A case study in tidal freshwater forested wetlands

With rising sea levels, mortality of glycophytes can be caused by water and nutrient stress under increasing salinity. However, the relative effects of these two stressors may vary by species-specific functional traits. For example, deciduous species, with leaves typically emerging during low salinity periods of the year, may suffer less from water stress than evergreen species. We sampled two woody species with contrasting functional traits: the evergreen and N2-fixing waxmyrtle (Morella cerifera), and the deciduous and non-N2 fixing baldcypress (Taxodium distichum) along a coastal river (South Carolina, USA) showing an increasing pattern of plant mortality along a salinity gradient. We first analyzed oxygen and hydrogen isotope ratios of plant stem water and river water to determine changes in plant source water at different sites. Then we analyzed foliar carbon and nitrogen isotope ratios (δ13C and δ15N) along with nitrogen and phosphorus content (%N and %P) as proxies for the water and nutrient stress. Results showed that: (1) the two species had different water sources at the higher salinity sites; (2) foliar δ15N values of baldcypress decreased with higher salinity while retaining a constant δ13C value, and both of these isotope values were positively related with foliar %P, suggesting greater nutrient stress but minor water stress under high salinity; and (3) foliar δ13C values of waxmyrtle increased with higher salinity while retaining a constant foliar δ15N value, and neither of the values was significantly related to foliar nutrients, suggesting greater water stress but minor nutrient stress under high salinity. The different responses of the two species to high salinity may be related to their differences in leaf phenology and N2-fixation. Our results suggest that nutrient stress, particularly of P, can contribute to stress and eventual high mortality of baldcypress exposed to salt water intrusion.

Phosphorus uptake in four tree species under nitrogen addition in subtropical China.

Atmospheric N deposition is a serious problem in subtropical China where N is present in large amounts but P is deficient. Several studies hypothesized that N2 fixers can overcome phosphorus limitation by trading fixed N2 for soil phosphorus. In order to know whether N2 fixers could invest fixed N2 in extracellular phosphatase production and could stimulate arbuscular mycorrhizal fungi (AMF) to acquire soil P in N-rich subtropical China, an open-air greenhouse experiment was carried out. Two N2 fixers (Acacia mangium and Ormosia pinnata) and two non-N2 fixers (Schima superba and Pinus massoniana) were exposed to three levels of N addition: 5.6kgha-1a-1 (ambient N), 15.6kgha-1a-1 (middle N), and 20.6kgha-1a-1 (high N). We found that the capacity of plants to acquire soil P in N-rich subtropical China is species specific. The higher P uptake rates were found for N2 fixers than non-N2 fixers under N addition, which were related to the greater soil APA and higher AMF (p<0.01) in the soil of N2 fixers. However, with time, high N addition decreased more significant quantities of soil microbial phospholipid fatty acids (PLFAs) in the soil of N2 fixers than that of non-N2 fixers (p<0.05). We conclude that N2 fixers have higher P uptake capacity than non-N2 fixers under ambient N deposition in subtropical China. However, continuing N deposition in the future might affect P uptake ability of N2 fixers as high N addition would decrease soil microbial PLFAs of N2 fixers.

Evidence for the Importance of Atmospheric Nitrogen Deposition to Eutrophic Lake Dianchi, China.

Elevated atmospheric nitrogen (N) deposition has significantly influenced aquatic ecosystems, especially with regard to their N budgets and phytoplankton growth potentials. Compared to a considerable number of studies on oligotrophic lakes and oceanic waters, little evidence for the importance of N deposition has been generated for eutrophic lakes, even though emphasis has been placed on reducing external N inputs to control eutrophication in these lakes. Our high-resolution observations of atmospheric depositions and riverine inputs of biologically reactive N species into eutrophic Lake Dianchi (the sixth largest freshwater lake in China) shed new light onto the contribution of N deposition to total N loads. Annual N deposition accounted for 15.7% to 16.6% of total N loads under variable precipitation conditions, 2-fold higher than previous estimates (7.6%) for the Lake Dianchi. The proportion of N deposition to total N loads further increased to 27-48% in May and June when toxic blooms of the ubiquitous non-N2 fixing cyanobacteria Microcystis spp. are initiated and proliferate. Our observations reveal that reduced N (59%) contributes a greater amount than oxidized N to total N deposition, reaching 56-83% from late spring to summer. Progress toward mitigating eutrophication in Lake Dianchi and other bloom-impacted eutrophic lakes will be difficult without reductions in ammonia emissions and subsequent N deposition.

Nutrient acquisition, soil phosphorus partitioning and competition among trees in a lowland tropical rain forest.

We hypothesized that dinitrogen (N2 )- and non-N2 -fixing tropical trees would have distinct phosphorus (P) acquisition strategies allowing them to exploit different P sources, reducing competition. We measured root phosphatase activity and arbuscular mycorrhizal (AM) colonization among two N2 - and two non-N2 -fixing seedlings, and grew them alone and in competition with different inorganic and organic P forms to assess potential P partitioning. We found an inverse relationship between root phosphatase activity and AM colonization in field-collected seedlings, indicative of a trade-off in P acquisition strategies. This correlated with the predominantly exploited P sources in the seedling experiment: the N2 fixer with high N2 fixation and root phosphatase activity grew best on organic P, whereas the poor N2 fixer and the two non-N2 fixers with high AM colonization grew best on inorganic P. When grown in competition, however, AM colonization, root phosphatase activity and N2 fixation increased in the N2 fixers, allowing them to outcompete the non-N2 fixers regardless of P source. Our results indicate that some tropical trees have the capacity to partition soil P, but this does not eliminate interspecific competition. Rather, enhanced P and N acquisition strategies may increase the competitive ability of N2 fixers relative to non-N2 fixers.

A broadleaf species enhances an autotoxic conifers growth through belowground chemical interactions

Plants may affect the performance of neighboring plants either positively or negatively through interspecific and intraspecific interactions. Productivity of mixed-species systems is ultimately the net result of positive and negative interactions among the component species. Despite increasing knowledge of positive interactions occurring in mixed-species tree systems, relatively little is known about the mechanisms underlying such interactions. Based on data from 25-year-old experimental stands in situ and a series of controlled experiments, we test the hypothesis that a broadleaf, non-N fixing species, Michelia macclurei, facilitates the performance of an autotoxic conifer Chinese fir (Cunninghamia lanceolata) through belowground chemical interactions. Chinese fir roots released the allelochemical cyclic dipeptide (6-hydroxy-1,3-dimethyl-8-nonadecyl-[1,4]-diazocane- 2,5-diketone) into the soil environment, resulting in self-growth inhibition, and deterioration of soil microorganisms that improve P availability. However, when grown with M.macclurei the growth of Chinese fir was consistently enhanced. In particular, Chinese fir enhanced root growth and distribution in deep soil layers. When compared with monocultures of Chinese fir, the presence of M.macclurei reduced release and increased degradation of cyclic dipeptide in the soil, resulting in a shift from self-inhibition to chemical facilitation. This association also improved the soil microbial community by increasing arbuscular mycorrhizal fungi, and induced the production of Chinese fir roots. We conclude that interspecific interactions are less negative than intraspecific ones between non-N fixing broadleaf and autotoxic conifer species. The impacts are generated by reducing allelochemical levels, enhancing belowground mutualisms, improving soil properties, and changing root distributions as well as the net effects of all the processes within the soil. In particular, allelochemical context alters the consequences of the belowground ecological interactions with a novel mechanism: reduction of self-inhibition through reduced release and increased degradation of an autotoxic compound in the mixed-species plantations. Such a mechanism would be useful in reforestation programs undertaken to rehabilitate forest plantations that suffer from problems associated with autotoxicity.

Structural Diversity of Bacterial Communities Associated with Bloom-Forming Freshwater Cyanobacteria Differs According to the Cyanobacterial Genus

The factors and processes driving cyanobacterial blooms in eutrophic freshwater ecosystems have been extensively studied in the past decade. A growing number of these studies concern the direct or indirect interactions between cyanobacteria and heterotrophic bacteria. The presence of bacteria that are directly attached or immediately adjacent to cyanobacterial cells suggests that intense nutrient exchanges occur between these microorganisms. In order to determine if there is a specific association between cyanobacteria and bacteria, we compared the bacterial community composition during two cyanobacteria blooms of Anabaena (filamentous and N2-fixing) and Microcystis (colonial and non-N2 fixing) that occurred successively within the same lake. Using high-throughput sequencing, we revealed a clear distinction between associated and free-living communities and between cyanobacterial genera. The interactions between cyanobacteria and bacteria appeared to be based on dissolved organic matter degradation and on N recycling, both for N2-fixing and non N2-fixing cyanobacteria. Thus, the genus and potentially the species of cyanobacteria and its metabolic capacities appeared to select for the bacterial community in the phycosphere.

Duelling 'CyanoHABs': unravelling the environmental drivers controlling dominance and succession among diazotrophic and non-N2 -fixing harmful cyanobacteria.

Eutrophication often manifests itself by increased frequencies and magnitudes of cyanobacterial harmful algal blooms (CyanoHABs) in freshwater systems. It is generally assumed that nitrogen-fixing cyanobacteria will dominate when nitrogen (N) is limiting and non-N2 fixers dominate when N is present in excess. However, this is rarely observed in temperate lakes, where N2 fixers often bloom when N is replete, and non-fixers (e.g. Microcystis) dominate when N concentrations are lowest. This review integrates observations from previous studies with insights into the environmental factors that select for CyanoHAB groups. This information may be used to predict how nutrient reduction strategies targeting N, phosphorus (P) or both N and P may alter cyanobacterial community composition. One underexplored concern is that as N inputs are reduced, CyanoHABs may switch from non-N2 fixing to diazotrophic taxa, with no net improvement in water quality. However, monitoring and experimental observations indicate that in eutrophic systems, minimizing both N and P loading will lead to the most significant reductions in total phytoplankton biomass without this shift occurring, because successional patterns appear to be strongly driven by physical factors, including temperature, irradiance and hydrology. Notably, water temperature is a primary driver of cyanobacterial community succession, with warming favouring non-diazotrophic taxa.

Nutrient limitation dynamics examined on a multi-annual scale in Lake Taihu, China: implications for controlling eutrophication and harmful algal blooms

Rapidly increasing urban, agricultural, and industrial growth in the Taihu basin during the past four decades has led to accelerated nitrogen (N) and phosphorus (P) loading to the lake. This has caused the lake to shift from oligo-mesotrophic to hypertrophic conditions, symptomized by toxic cyanobacterial blooms, dominated by the non-N2 fixing genus Microcystis. From 2008 to 2013, a series of in situ microcosm and mesocosm nutrient addition bioassays were conducted that were focused on the heavily polluted northern region (i.e., Meiliang Bay) and other lake locations. Bioassays showed that phytoplankton production, as chlorophyll a and photopigments diagnostic of major phytoplankton groups, was controlled by P inputs from spring to early summer, while N played a more dominant controlling role in summer–fall. In most cases, combined N and P additions promoted maximum growth. This pattern proved true for both the highly eutrophic northern region and the less-eutrophic central and southern regions. Cyanobacteria, chlorophytes, and cryptophytes all showed the strongest positive responses to N and N+P enrichment during the summer bloom period, while diatoms were the least abundant then and just moderately stimulated by nutrient additions. Cyanobacteria failed to selectively respond to P inputs during the summer bloom period, contradicting the paradigm that selective P enrichment will favor them, especially the N2-fixing genera. Rather, Microcystis-dominated blooms remained N-limited during summer months and were not replaced by N2-fixing genera, indicating that internal N and P regeneration of previously loaded nutrients must be sustaining blooms. Successful ‘de-eutrophication’ of Taihu will require reductions of both N and P inputs in all lake regions in order to control blooms and counter the legacy of several decades of nutrient over-enrichment.

Controlling cyanobacterial blooms in hypertrophic Lake Taihu, China: will nitrogen reductions cause replacement of non-N2 fixing by N2 fixing taxa?

Excessive anthropogenic nitrogen (N) and phosphorus (P) inputs have caused an alarming increase in harmful cyanobacterial blooms, threatening sustainability of lakes and reservoirs worldwide. Hypertrophic Lake Taihu, China’s third largest freshwater lake, typifies this predicament, with toxic blooms of the non-N2 fixing cyanobacteria Microcystis spp. dominating from spring through fall. Previous studies indicate N and P reductions are needed to reduce bloom magnitude and duration. However, N reductions may encourage replacement of non-N2 fixing with N2 fixing cyanobacteria. This potentially counterproductive scenario was evaluated using replicate, large (1000 L), in-lake mesocosms during summer bloom periods. N+P additions led to maximum phytoplankton production. Phosphorus enrichment, which promoted N limitation, resulted in increases in N2 fixing taxa (Anabaena spp.), but it did not lead to significant replacement of non-N2 fixing with N2 fixing cyanobacteria, and N2 fixation rates remained ecologically insignificant. Furthermore, P enrichment failed to increase phytoplankton production relative to controls, indicating that N was the most limiting nutrient throughout this period. We propose that Microcystis spp. and other non-N2 fixing genera can maintain dominance in this shallow, highly turbid, nutrient-enriched lake by outcompeting N2 fixing taxa for existing sources of N and P stored and cycled in the lake. To bring Taihu and other hypertrophic systems below the bloom threshold, both N and P reductions will be needed until the legacy of high N and P loading and sediment nutrient storage in these systems is depleted. At that point, a more exclusive focus on P reductions may be feasible.