Phosphorus Limitation Research Articles

Effects of long-term cropping regimes on SOC stability, soil microbial community and enzyme activities in the Mollisol region of Northeast China

Retaining carbon in cultivated soils is an important mechanism to mitigate climate change and to maintain the sustainability of agricultural ecosystems. Characterization of the soil microbial community was an early means to quantify the increase in soil organic carbon (SOC) content and stability under different cropping systems. However, the effects of different cropping systems on the soil microbial community and enzyme activity and the driving force for increasing the SOC content remain unclear, especially the differences between continuous maize (Zea mays L.) cropping and rotation systems containing legumes. We used a long-term field experiment (30 years) in the Mollisol region of Northeast China. The field experiment was undertaken in split-plot design comprised of four treatments: i) farmland fallow (FALL), ii) continuous soybean cropping (CSC), iii) maize-soybean rotation (MSR), iv) continuous maize cropping (CMC). Fluorometric assays and phospholipid fatty acid analysis (PLFA) were used to determine the biological characteristics of the soil. The SOC chemical composition was characterized using Cross Polarization/Magic Angle Spinning solid-state 13C-nuclear magnetic resonance (13C NMR) spectroscopy. Principal component analysis (PCA), redundancy analysis (RDA) and the partial least squares path model (PLS-PM) were used to explore the coupling mediation of these parameters further. Our results suggested that soil microbes in the four systems were not limited by C and N, but co-limited by P. But compared with the FALL system, the CMC system significantly alleviated the phosphorus limitation of the microorganisms. The CMC system also significantly increased soil nutrient content, especially SOC content (40.8%), available phosphorus (117%), available potassium (30.3%), NO3−-N (148%), and increased microbial community abundance and enzyme activities. Compared with the CMC system, the MSR system held relatively lower Gram-positive: Gram-negative ratio and higher fungi: bacteria ratio, which indicated that the soil microbial community structure is more conducive to improving the SOC stability. Meanwhile, more evidence supporting data that SOC stability was enhanced in MSR soils was provided by the 13C NMR spectra and higher-quality indices (A / AO, AI, HI and LCI). Moreover, strong correlations and RDA analysis confirmed that SOC is the main factor affecting microbial communities and enzyme activities.

Land use type dominates soil microbial element limitations in a subtropical plateau, China

Land use type, vegetation type (artificial or natural) and microbes play a key role in flux of soil matter and energy in subtropical ecosystems. However, microbial element limitations in these ecosystems have been largely unexplored. Here, microbial element limitations under four representative land uses grouped into two vegetation types (artificial or natural) were characterized using enzymatic vector analysis, their drivers were deciphered using redundancy analysis in a subtropical region, Southwestern China. Enzymatic vector analysis showed that soil microbes suffered from carbon (C) & nitrogen (N) limitations in cropland and forestland related to artificial vegetation, but C, N & phosphorus (P) limitations in grassland and shrub land related to natural vegetation. Two-way ANOVA showed land use type and vegetation type (artificial/natural) were the major forces driving C and NP limitations. Redundancy analysis identified that soil total N was the major factor affecting soil microbial C limitation, soil total P was the major factor affecting soil microbial NP limitations. This study implied that conversion of natural vegetation to artificial vegetation would shift microbial element limitations from C, N & P limitation to C & N limitation, and exacerbate N limitation.

Multiple‐trophic patterns of primary succession following retreat of a high‐elevation glacier

AbstractHow multiple, interacting components of complex soil communities assemble within forefields of receding glaciers is still largely unknown, especially at high‐elevation sites (>5000 m a.s.l.) where plant succession is very slow. To examine succession of soil communities across different trophic levels, we investigated four major soil groups: bacteria, fungi, nematodes, and other non‐fungal non‐nematode microbial eukaryotes at the Puca Glacier in the Peruvian Andes spanning 9‐, 24‐, and 89‐year‐old deglaciated soils. This is the first study of microbial communities, other than bacteria, at a high‐elevation chronosequence in the Andes Mountains. In addition, we characterized soil biogeochemical properties (e.g., C, N, moisture, and pH) and rates of microbial enzyme activities associated with C, N, and P acquisition. We found significantly correlated increases in estimated richness and high species turnover in all soil groups along the chronosequence. These shifts in soil communities were significantly correlated with microbial enzyme activities and measures of C, N, moisture, and pH. Stoichiometric comparisons of enzyme activities showed phosphorus (P) and carbon (C) limitation of microbial activity across the entire chronosequence with no hint of nitrogen (N) limitation. Taken together, the observed shifts in soil communities and biogeochemistry indicate coordinated increases in trophic complexity and ecosystem functioning during the initial 90 yr of microbial succession along the post‐glacial chronosequence of the Puca Glacier.

Soil N availability drives the shifts of enzyme activity and microbial phosphorus limitation in the artificial soil on cut slope in southwestern China.

The construction of highways in the subalpine mountains generates many cut slopes. Currently, the restoration of cut slope mainly focuses on the aboveground landscapes and slope stability. Yet, it remains elusive about the belowground ecosystem functions at the early stage of restoration. In this study, we evaluated the belowground ecosystem functions of cut slopes that had been restored approximately 3 years using soil enzymatic activities, microbial biomass, and stoichiometry as the proxies. The results indicated that the phenol oxidase activity was higher in cut slopes, while the activities of β-1,4-glucosidase, β-1,4-N-acetylglucosaminidase, leucine aminopeptidase, and acid phosphatase were lower in cut slope soils compared with natural soils. Soil nitrogen availabilities (total and/or ammonium nitrogen) showed high negative correlations with the phenol oxidase activity and positive correlations with the activities of almost all other enzymes. These results suggested that soil nitrogen was the key factor in driving the shifts of enzymatic activities across two types of soils. Moreover, we found the imbalance of soil nutrients in cut slope soils, especially the carbon vs. nitrogen and the nitrogen vs. phosphorus. By applying the vector analysis, we found that the vector A values were more than 45° in all samples, suggesting that microbial phosphorus limitation occurred in both cut slope and natural soils. These findings suggested that maintaining the balance of soil nutrient supplies is important to the recovery of the below-ground ecosystem functions at the early restoration stage of cut slopes. This study provided new insights into designing the ecological restoration strategies for cut slopes by considering the belowground ecosystem functions.

Response of Magnetite/Lanthanum hydroxide composite on cyanobacterial bloom

Magnetite/lanthanum hydroxide composite (MLC-10) was applied in simulate natural water, sediment and cyanobacteria (WSC) system to evaluate its effect on cyanobacterial bloom in this study. According to the results, the addition of MLC-10 showed a good performance on inhibition of cyanobacterial bloom in systems. The cyanobacteria density of WSC-0.5 and WSC-1.0 (adding 0.5 g and 1.0 g MLC-10) at 30 day was 99.39% and 99.84% less than that in WSC-C (adding no MLC-10 in WSC system), respectively. The addition of MLC-10 could form a phosphorus-binding layer that adsorbed soluble reactive phosphate (SRP) in overlying water, improved the release of internal phosphorus (P) from sediment to pore water then blocked SRP release from pore water to overlying water, especially in WSC-0.5 and WSC-1.0. The results may be due to the high adsorption capacity of MLC-10 to phosphorus. Additionally, oxidative stress and oxidative damage of cyanobacteria were observed after exposing to MLC-10, and oxidative damage degree increased with the elevated amount of MLC-10. MLC-10 addition showed a slight effect on microbial community of surface sediment. Phosphorus limitation, cell damage and limited cells’ floating performance were the possible mechanisms of cyanobacterial bloom controlling by MLC-10. Based on these results, MLC-10 could be used as a potential P-inactive material for cyanobacterial bloom controlling.

Role of external inputs of nutrients to aquatic ecosystems in determining prevalence of nitrogen vs. phosphorus limitation of net primary productivity

Whether net primary productivity in an aquatic ecosystem is limited by nitrogen (N), limited by phosphorus (P), or co-limited by N & P is determined by the relative supply of N and P to phytoplankton compared to their elemental requirements for primary production, often characterized by the “Redfield” ratio. The supply of these essential nutrients is affected by both external inputs and biogeochemical processes within the ecosystem. In this paper, we examine external sources of nutrients to aquatic systems and how the balance of N to P inputs influences nutrient limitation. For ocean subtropical gyres, a relatively balanced input of N and P relative to the Redfield ratio from deep ocean sources often leads to near co-limitation by N and P. For lakes, the external nutrient inputs come largely from watershed sources, and we demonstrate that on average the N:P ratio for these inputs across the United States is well above that needed by phytoplankton, which may contribute to P limitation in those lake that experience this average nutrient loading. Watershed inputs are also important for estuaries and coastal marine ecosystems, but ocean sources of nutrients are also significant contributors to overall nutrient loads. The ocean-nutrient sources of N and P are very often at or below the Redfield ratio of 16:1 molar, and can be substantially so, particularly in areas where the continental shelf is wide. This large input of coastal ocean nutrients with a low N:P ratio is one factor that may make N limitation more likely in many coastal marine ecosystems than in lakes.

Combination of P-limitation and cadmium in photosynthetic responses of the freshwater microalga Ankistrodesmus densus (Chlorophyceae).

In the environment, microalgae are exposed to a multitude of stressors simultaneously, inducing physiological adjustments. It is well documented that both phosphorus (P) limitation and trace metals exposure affect microalgal physiology. However, investigations regarding the combination of both P limitation and excess trace metals still deserve attention. In the present study, we evaluated the changes in photosynthetic parameters in the green microalga Ankistrodesmus densus acclimated to different P concentrations prior to exposure to Cd. Our results indicate that different concentrations of P in the medium were responsible for significant changes in some parameters, especially those related to photoprotection mechanisms. Cadmium also altered some of these variables in all P scenarios, and greater damage (i.e., synergism) was observed in the combination P-limited and high Cd, with all the evaluated parameters affected under the adverse scenario. Among the parameters analyzed, rapid light curves were the most sensitive to exposure of one or the combination of both stressors (Cd and P limitation). Based on our data, we suggest that P-limited algae activated photoprotective mechanisms as a response to nutrient limitation, especially at the most limited condition. The addition of Cd did not change linearly the parameters related to photoprotection mechanisms under P-limitation, i.e., synergism was observed in the intermediate P-limitation combined with Cd, while in the most P-limited, P seems to be the driving force affecting these mechanisms. Based on our results, we suggest the use of rapid light curves as a tool to complement the assessment of the impacts of stressors, such as metals, in ecotoxicological studies.

Spatiotemporal variation of the nutrients and heavy metals in mangroves using multivariate statistical analysis, Gulf of Kachchh (India)

The present study is an attempt to assess the spatial and seasonal variation of nutrients and heavy metals in mangroves water in the Gulf of Kachchh, India. The surface water samples were collected during pre- and post-monsoon to evaluate the hydrochemical processes occurring in the region. Seasonal changes and anthropogenic inputs have influenced the nutrients and metal concentrations. The results suggest high salinity (Pre-monsoon ±45.59 PSU; post-monsoon ±45.36 PSU) and chloride (Pre-monsoon ±30251 mg/L; post-monsoon ±29536 mg/L) concentration in both the seasons. Average values of Sulfate (SO4−), Nitrate (NO3−), Dissolved silica (DSi), and Phosphate (PO43−) was 2503.69 mg/L, 10.47 mg/L, 35.41 mg/L and 0.85 mg/L in pre-monsoon, and 3474.50 mg/L, 13.66 mg/L, 31.54 mg/L and 0.64 mg/L in post-monsoon, respectively. Nutrient ratios indicate phosphorus limitation in both the seasons. Cluster and Principal Component analysis signifies that sampling locations in cluster 3 and factor 1 majorly impacted due to seawater mixing, high evaporation and discharge of brine from saltpans, while cluster 2, 3 and factor 2 indicates the anthropogenic source. Metal concentration was higher during pre-monsoon and among the metals, Fe, Sr, Zn and Mn were observed in high concentration. The hydrochemical characteristics of water found to be dependent on the peculiarity of sampling locations such as high tidal amplitude, less freshwater input, arid climate, extensive saltpans, tidal flushing, instinctive reducing environment and anthropogenic discharge. Gulf of Kachchh is an industrial hub of the country, where anthropogenic activities (such as fertilizer, cement, and chemicals industries, oil and petroleum refineries, metal processing units, thermal power plants and ports activities) are the prime sources of contamination, which are affecting water quality. The relatively high concentration of nutrients and metals suggests that water is in very dreadful condition, which will ultimately affect flora and fauna of this ecosystem. This study provides baseline information on mangrove water quality and impacts of ongoing anthropogenic activities on the Gulf of Kachchh.

Development of photosynthetic carbon fixation model using multi-excitation wavelength fast repetition rate fluorometry in Lake Biwa.

Direct measurements of gross primary productivity (GPP) in the water column are essential, but can be spatially and temporally restrictive. Fast repetition rate fluorometry (FRRf) is a bio-optical technique based on chlorophyll a (Chl-a) fluorescence that can estimate the electron transport rate (ETRPSII) at photosystem II (PSII) of phytoplankton in real time. However, the derivation of phytoplankton GPP in carbon units from ETRPSII remains challenging because the electron requirement for carbon fixation (Фe,C), which is mechanistically 4 mol e- mol C-1 or above, can vary depending on multiple factors. In addition, FRRf studies are limited in freshwater lakes where phosphorus limitation and cyanobacterial blooms are common. The goal of the present study is to construct a robust Фe,C model for freshwater ecosystems using simultaneous measurements of ETRPSII by FRRf with multi-excitation wavelengths coupled with a traditional carbon fixation rate by the 13C method. The study was conducted in oligotrophic and mesotrophic parts of Lake Biwa from July 2018 to May 2019. The combination of excitation light at 444, 512 and 633 nm correctly estimated ETRPSII of cyanobacteria. The apparent range of Фe,C in the phytoplankton community was 1.1-31.0 mol e- mol C-1 during the study period. A generalised linear model showed that the best fit including 12 physicochemical and biological factors explained 67% of the variance in Фe,C. Among all factors, water temperature was the most significant, while photosynthetically active radiation intensity was not. This study quantifies the in situ FRRf method in a freshwater ecosystem, discusses core issues in the methodology to calculate Фe,C, and assesses the applicability of the method for lake GPP prediction.

Enzyme stoichiometry evidence revealed that five years nitrogen addition exacerbated the carbon and phosphorus limitation of soil microorganisms in a Phyllostachys pubescens forest.

The activity and stoichiometry of soil extracellular enzyme can provide a good indication for changes in soil nutrient availability and microbial demands for nutrients. However, it remains unclear how would nitrogen (N) deposition affect nutrient limitation of microbes in subtropical forest soils. We conducted a 5 years N addition experiment in a subtropical Phyllostachys pubescens forest. The soil nutrients and enzyme activities associated with carbon (C), N, and phosphorus (P) cycles were measured. We also examined the nutrient distribution of microorganisms using enzyme stoichiometry and vector analysis. The results showed that N addition significantly decreased the contents of soil soluble organic C and available P and increased that of available N. Furthermore, N addition significantly decreased β-N-acetyl-glucosaminidase (NAG) activity and NAG/ microbial biomass carbon (MBC), and increased acid phosphatase (ACP) and ACP/MBC. The low and moderate N addition levels significantly increased enzyme C/P, vector length, and vector angle, but significantly decreased enzyme N/P. Results of redundancy analysis showed that the change in soil enzyme activity and enzymatic stoichiometry were mainly driven by soil available P content under N addition. In summary, N addition altered the microbial nutrient acquisition strategy, which increased nutrient allocation to P-acquiring enzyme production but reduced that to N-acquiring enzyme production. Moreover, N addition exacerbated the C and P limitation of soil microorganisms. Appropriate amount of P fertilizer could be applied to improve soil fertility of subtropical P. pubescens forest in the future.

Effect of labile carbon on iron reduction and phosphorus availability in two paddy soils

Phosphorus (P) limitation in acidic tropical paddy soils is thought to be contributed by the affinity of iron oxides and hydroxides for phosphorus. However, the addition of labile carbon (C) compounds appears to be stimulated iron (Fe) reduction which can release occluded P through Fe cycling. The aim of this study was to investigate the phosphorous mobilization triggered by labile C coupled to iron (Fe) cycling during flooding anaerobic conditions in two P limited paddy soils. Two concentrations (low and high) of equal mixture of glucose and oxalate were added to soil microcosms and they were incubated for one month under flooded condition. During the 30-day experiment, replicate microcosms were destructively harvested for five times and measured total Fe, Fe II, available P and pH. It was found that labile C addition significantly increased total Fe and Fe II in soil solution at early incubation period although the changes were not significant in soil. Available P in soil was not increased and steady throughout the incubation period because the added labile C may not be enough to enhance the P solubilization or available P was dissolved in soil water which we did not measure in this experiment. High amount of labile C addition significantly decreased pH at the start of incubation. pH and available P are negatively correlated at day 7 which is compatible with decreased pH can increase available P. Our results suggest that reducing conditions coupled with labile carbon inputs can stimulate iron reduction and corresponding decreased pH which can triggered soil phosphorus mobilization in P limited paddy soils.

Diversity of putative ericoid mycorrhizal fungi increases with soil age and progressive phosphorus limitation across a 4.1-million-year chronosequence

Ericaceous plants rely on ericoid mycorrhizal fungi for nutrient acquisition. However, the factors that affect the composition and structure of fungal communities associated with the roots of ericaceous plants remain largely unknown. Here, we use a 4.1-million-year (myr) soil chronosequence in Hawaii to test the hypothesis that changes in nutrient availability with soil age determine the diversity and species composition of fungi associated with ericoid roots. We sampled roots of a native Hawaiian plant, Vaccinium calycinum, and used DNA metabarcoding to quantify changes in fungal diversity and community composition. We also used a fertilization experiment at the youngest and oldest sites to assess the importance of nutrient limitation. We found an increase in diversity and a clear pattern of species turnover across the chronosequence, driven largely by putative ericoid mycorrhizal fungi. Fertilization with nitrogen at the youngest site and phosphorus at the oldest site reduced fungal diversity, suggesting a direct role of nutrient limitation. Our results also reveal the presence of novel fungal species associated with Hawaiian Ericaceae and suggest a greater importance of phosphorus availability for communities of ericoid mycorrhizal fungi than is generally assumed.

Spatially divergent trends of nitrogen versus phosphorus limitation across European forests

Nitrogen (N) and phosphorus (P) are essential nutrients that widely limit plant growth in global terrestrial ecosystems. Rising atmospheric CO2 concentration generally stimulates terrestrial net primary productivity and consequently may cause or aggravate N and P limitation due to a dilution effect, but the spatial variation of temporal trends in N versus P limitation and its key regulating factors is poorly understood. Using the leaf N:P ratio of 15 dominant tree species as an indicator, we analysed the spatial variation of plot-level shift towards N or P limitation across 163 European forest plots during 1995–2017. Phosphorus limitation increased from 25% to 33% of the studied plots between 1995–1997 and 2015–2017, while N limitation occurred in a negligible number of plots. A major proportion (56%) of the plots showed no significant trend in leaf N:P ratio, implying no shifts in N versus P limitation status. In the remaining plots, 38% of the plots showed a significant increase of leaf N:P ratio and only 6% of the plots showed a significant decrease of leaf N:P ratio. The spatial variation in the rate of decrease in leaf N:P ratio was associated with a significant decrease in leaf N concentration and mainly explained by the rate of decrease in N deposition. In contrast, the spatial variation in the rate of increase in leaf N:P ratio was associated with a significant decrease in leaf P concentration and mainly explained by forest category (broadleaf vs. conifer), mean annual temperature and soil C:N ratio. Our findings highlight a remarkable spatial divergence in temporal trends of nutrient limitation status across European forests over the past two decades, but overall, P is becoming more limiting versus N, especially in broadleaved forests.

Nitrogen and phosphorus co‐limit mineralization of labile carbon in deep subsoil

AbstractThe growth of deep‐rooted crops has the potential to increase carbon deposition within deep subsoil layers, thereby contributing to climate change mitigation. Yet, the mechanisms of labile carbon stabilization in deep subsoils remain uncertain. Here we studied the effect of nitrogen, phosphorus and sulphur limitations on the microbial mineralization of added glucose and an artificial root exudate mixture (both spiked with 14C‐glucose) in intact soil samples from a deep subsoil (5–6 m) over a 10‐week incubation period, where the temporal respiration of CO2 and 14CO2 were quantified. We found that mineralization was co‐limited by nitrogen and phosphorus, but the cumulative amount of CO2 emitted was significantly larger from artificial root exudates than glucose, suggesting that the nitrogen carried in amino acids in root exudates (here, L‐arginine) was sufficient to overcome the nitrogen limitation in this subsoil.Highlights Labile C substrates and nutrients from deep‐rooted crops may affect the potential for C sequestration in subsoil. Deep subsoil was incubated with glucose and artificial root exudates amended with N, P, and S. C mineralization was co‐limited by N and P, but less so in soil amended with glucose. Amino acid derived‐N in exudates from deep‐rooted crops may alleviate the N limitation in deep subsoils.

Water quality associations and spatiotemporal distribution of the harmful alga Prymnesium parvum in an impounded urban stream system

Abstract The Jim Bertram Lake System consists of several stream impoundments within the City of Lubbock, Texas (USA). Baseflow in the upstream reach is dominated by nitrogen-rich-treated wastewater. While toxic blooms of Prymnesium parvum have occurred in this system for ∼2 decades during fall or winter-spring, little is known about water quality variables that facilitate blooms or the alga’s spatiotemporal distribution. Water quality associations were examined monthly over a 1-year period. Total phosphorus was largely below the detection limit, suggesting that the system is phosphorus limited. Algal abundance was low during the assessment period and associations were determined using multiple logistic regression. Algal incidence was negatively associated with temperature and positively with organic nitrogen and calcium hardness. These findings conform with earlier reports but positive associations with the latter two variables are noteworthy because they have not been widely confirmed. Spatiotemporal distribution was evaluated in fall and winter-spring of three consecutive years. Prymnesium parvum incidence was higher in the upper than in the lower reach, and detections in the lower reach occurred only after a dense bloom developed in the upper reach contemporaneously with stormwater runoff-associated flooding. Thus, the upstream reach is a major source of propagules for downstream sites. Because urban runoff is a source of phosphorus and its nitrogen: phosphorus ratio is lower than prevailing ratios in the upper reach, what triggered the bloom was likely relief from phosphorus limitation. This study provided water quality, geographic and hydrological indices that may inform prevention and control methods for harmful algae in nitrogen-enriched urban systems.

Roles of Nutrient Limitation on Western Lake Erie CyanoHAB Toxin Production.

Cyanobacterial harmful algal bloom (CyanoHAB) proliferation is a global problem impacting ecosystem and human health. Western Lake Erie (WLE) typically endures two highly toxic CyanoHABs during summer: a Microcystis spp. bloom in Maumee Bay that extends throughout the western basin, and a Planktothrix spp. bloom in Sandusky Bay. Recently, the USA and Canada agreed to a 40% phosphorus (P) load reduction to lessen the severity of the WLE blooms. To investigate phosphorus and nitrogen (N) limitation of biomass and toxin production in WLE CyanoHABs, we conducted in situ nutrient addition and 40% dilution microcosm bioassays in June and August 2019. During the June Sandusky Bay bloom, biomass production as well as hepatotoxic microcystin and neurotoxic anatoxin production were N and P co-limited with microcystin production becoming nutrient deplete under 40% dilution. During August, the Maumee Bay bloom produced microcystin under nutrient repletion with slight induced P limitation under 40% dilution, and the Sandusky Bay bloom produced anatoxin under N limitation in both dilution treatments. The results demonstrate the importance of nutrient limitation effects on microcystin and anatoxin production. To properly combat cyanotoxin and cyanobacterial biomass production in WLE, both N and P reduction efforts should be implemented in its watershed.

Nutrient-imbalanced conditions shift the interplay between zooplankton and gut microbiota

BackgroundNutrient stoichiometry of phytoplankton frequently changes with aquatic ambient nutrient concentrations, which is mainly influenced by anthropogenic water treatment and the ecosystem dynamics. Consequently, the stoichiometry of phytoplankton can markedly alter the metabolism and growth of zooplankton. However, the effects of nutrient-imbalanced prey on the interplay between zooplankton and their gut microbiota remain unknown. Using metatranscriptome, a 16 s rRNA amplicon-based neutral community model (NCM) and experimental validation, we investigated the interactions between Daphnia magna and its gut microbiota in a nutrient-imbalanced algal diet.ResultsOur results showed that in nutrient-depleted water, the nutrient-enriched zooplankton gut stimulated the accumulation of microbial polyphosphate in fecal pellets under phosphorus limitation and the microbial assimilation of ammonia under nitrogen limitation. Compared with the nutrient replete group, both N and P limitation markedly promoted the gene expression of the gut microbiome for organic matter degradation but repressed that for anaerobic metabolisms. In the nutrient limited diet, the gut microbial community exhibited a higher fit to NCM (R2 = 0.624 and 0.781, for N- and P-limitation, respectively) when compared with the Control group (R2 = 0.542), suggesting increased ambient-gut exchange process favored by compensatory feeding. Further, an additional axenic grazing experiment revealed that the growth of D. magna can still benefit from gut microbiota under a nutrient-imbalanced diet.ConclusionsTogether, these results demonstrated that under a nutrient-imbalanced diet, the microbes not only benefit themselves by absorbing excess nutrients inside the zooplankton gut but also help zooplankton to survive during nutrient limitation.

Cyanobacteria in early soil development of deglaciated forefields: Dominance of non-heterocytous filamentous cyanobacteria and phosphorus limitation of N-fixing Nostocales

Cyanobacteria are integral to soil development in the earliest stages of primary succession by fixing carbon (C) and nitrogen (N) essential for organismal growth. In this study, we examined soil cyanobacterial communities at the earliest stage of succession (<5 years) at two disparate glacial forefields to reveal cyanobacterial patterns central to ecosystem development. Despite vast differences in climate and the relative dominance of heterotrophic versus autotrophic bacteria in these forefields, we found common patterns in cyanobacterial community composition in early soils. This work adds to limited research suggesting the importance of non-heterocytous filamentous cyanobacteria in initial soil colonization and development of primary succession of glacier forefields. Further, a phosphorus (P) addition experiment suggests P-limitation as a mechanism underlying the restricted presence of ecologically important, putatively N-fixing, Nostocales in these emerging landscapes.

三峡库区马尾松人工林土壤酶活性和微生物生物量对氮添加的季节性响应

以三峡库区马尾松人工林为对象，分析土壤微生物生物量、酶活性和养分含量对氮添加的初期响应规律，为预测该地区在大气氮沉降持续增加的背景下森林土壤的季节动态提供参考。结果表明：氮添加初期，中氮水平（60 kg hm^-2 a^-1）的氮添加处理使得各季节土壤β-1-4葡萄糖苷酶、N-乙酰氨基葡萄糖苷酶、酸性磷酸酶、多酚氧化酶、过氧化物酶活性均增加，高氮（90 kg hm^-2 a^-1）水平的添加处理增加了土壤有机碳、全氮和微生物生物量碳、氮、磷的含量和酸性磷酸酶及多酚氧化酶活性，降低了土壤pH值、全磷含量和β-1-4葡萄糖苷酶及N-乙酰氨基葡萄糖苷酶活性。土壤酶活性和微生物生物量存在明显的季节变化，秋季水解酶活性和微生物生物量碳、磷含量显著高于春夏两季，而氧化酶活性和微生物生物量氮含量则是春夏季较高。土壤酶活性与季节、土壤含水量、养分含量及微生物生物量碳氮磷含量存在显著的相关性，酶活性变化是多因子综合作用的结果，冗余分析表明土壤含水量、微生物生物量碳、氮、磷和全氮是驱动土壤酶活性的主要环境因子。氮沉降的持续增加会加速当地马尾松人工林土壤腐殖质的形成，增加有机碳的积累，导致土壤酸化，并产生磷限制。;Anthropogenic nitrogen (N) enrichment is a concern worldwide, as it affects almost every aspect of ecosystem function and composition. Many nitrogen (N) addition experiments have been well documented to decrease plant biodiversity across various terrestrial ecosystems. However, such generalizations about the impacts of nitrogen addition on soil enzyme activities and microbial biomass are lacking, especially in the Three Gorges Reservoir area, as a key sensitive ecological barrier of the middle and lower reaches of the Yangtze River. Therefore, we conducted nitrogen addition experiments in February 2019 in Pinus massoniana plantation in the Three Gorges Reservoir area of subtropical China. Soil samples were collected from three control plots (N_CK), three low-nitrogen addition plots (N_low), three middle-nitrogen addition plots (N_middle), three high-nitrogen addition plots (N_high) in May, August and November, 2019. We studied the soil enzyme activities, microbial biomass and nutrient content in response to nitrogen addition, which provided a theoretical basis for predicting soil seasonal dynamics in an increasing atmospheric nitrogen deposition background in the Three Gorges Reservoir area. The results show that, in the early stage of nitrogen addition, the medium nitrogen (60 kg hm^-2 a^-1) treatment increased the activities of β-1-4 glucosidase (BG), N-acetylglucosaminosidase (NAG), acid phosphatase (AP), polyphenol oxidase (PPO) and peroxidase (POD); the high nitrogen (90 kg hm^-2 a^-1) treatment increased the contents of soil organic carbon, total nitrogen, microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and the activities of AP, PPO, while decreased the soil pH, total phosphorus content and the activities of BG and NAG. There were significantly seasonal differences between soil enzyme activities and microbial biomass. Hydrolase activities, MBC and MBP contents in autumn were significantly higher than those in spring and summer, while oxidase activities and MBN contents were higher in spring and summer. Soil enzyme activities were significantly correlated with season, soil moisture content, soil nutrient, MBC, MBN, MBP. The change of soil enzyme activities is due to the comprehensive action of multiple factors. Redundancy analysis (RDA) showed that soil moisture content, MBC, MBN, MBP and total N greatly accounted for the variation in soil enzyme activities. The increasing nitrogen deposition will accelerate the formation of soil humus, increase organic carbon accumulation, lead to soil acidification, and produce phosphorus limitation in the local Pinus massoniana plantation.

Decomposition of leaf litter in the Brazilian savanna on limestone and sandstone Neosols.

Litter decomposition in the soil is an important stage of the nutrient cycling process that interferes with functioning of terrestrial ecosystems. Soil fertility and litter nutritional quality are fundamental factors that affect decomposition efficiency of plant residues. We evaluated decomposition in two areas of 'cerrado sensu stricto', each with a type of Neosol - limestone (eutrophic) and sandstone (dystrophic). In a rural area located in the municipality of Ituiutaba (MG, Brazil), 10 plots were randomly selected to install litter bags with 10 g of mixture of dry leaves that were used to estimate rate and time of leaf-litter decomposition from October/2015 to January/2016. Decomposition rate in the limestone cerrado was significantly higher than in the sandstone cerrado. This difference mustn't be explained by the edaphic texture between areas, since it was similar between them. But may be explained through aluminum absence and higher soil fertility in the limestone cerrado, especially phosphorus that is highly limiting in dystrophic cerrados like the sandstone cerrados, in which decay of decomposing leaf-litter was directly proportional to the levels of phosphorus. Limestone presence reduces aluminum toxicity and circumvent phosphorus limitation in the cerrado, favoring decomposition. Such influence is probably an important feature for limestone cerrados.