Changes in ecological stoichiometry and nutrient resorption in Castanopsis hystrix plantations along an urbanization gradient in the lower subtropics

The stoichiometry of carbon,nitrogen and phosphorous in plants can reflect the interactions between plants and their environment.The interplay between plant nutrients,climatic factors,and soil properties and the underlying regulatory mechanisms are pillars of ecology but remain underexplored.In this study of plant C-N-P stoichiometry and nutrient resorption in Castanopsis hystrix groves in three cities(Guangzhou,Zhongshan,and Lechang)that represent an urban-rural gradient in Guangdong Province,South China,we explored potential relationships among NO_(2) concentrations,diameter at breast height(DBH),and resident human population.Mean annual temperature,mean annual precipitation,insolation duration per year,and the human resident population differed significantly among the three cities.Soil C-N-P was always highest in suburban Lechang,and the concentration of NO_(2) was highest in urban Guanghzou(55.33±0.67μg m^(-3))and positively correlated with the resident population and leaf N:P.Our findings suggest that C-N-P stoichiometry of C.hystrix was better explained by NO_(2)than by soil C-N-P stoichiometry and that nutrient resorption was better explained by leaf nutrients and DBH than by NO_(2) and soil stoichiometry.Our study supports the hypothesis that rapid urbanization influences NO_(2) concentrations and microclimate,which may jointly change the stoichiometry of plant nutrients in the forest ecosystems.

Decomposition dynamics and ecological stoichiometry of Quercus acutissima and Pinus densiflora litter in the Grain to Green Program Area of northern China

Litter decomposition and ecological stoichiometry of nutrient release is an important part of material cycling and energy flow in forest ecosystems.In a study of the ecological stoichiometry and nutrient release during litter decomposition in a pine–oak forest ecosystem of the Grain to Green Program(GTGP)area of northern China,a typical pine and oak species(PDS:Pinus densiflora Sieb.,QAC:Quercus acutissima Carr.)were selected in the Taiyi Mountain study area.The ecological stoichiometry characteristics of carbon(C),nitrogen(N)and phosphorus(P)and litter decomposition dynamics were studied by field sampling and quantitative analyses.The results showed the following.(1)The decomposition dynamics of both litters was slow-fast-slow.The most important climatic factor affecting the litter decomposition rate from May to October was precipitation and temperature from November to April of the following year.(2)Throughout the 300-day study,in both litters,C of the two litters was released,N first accumulated and was then released,and P exhibited a release-accumulate-release pattern.(3)C:P was significantly higher than C:N and N:P(p<0.05);the C:N of PSD litter was higher than that of QAC(p<0.05),but the N:P of QAC litter was higher than that of PSD litter(p<0.05).The C:N of both litters was very high in the study area,indicating that the nutrient release ability during litter decomposition in the two typical pine–oak forest ecosystems was relatively weak;therefore,more attention should be paid to nitrogen-fixing species and mixed forests in the GTGP area of northern China.

Response of plant,litter,and soil C:N:P stoichiometry to growth stages in Quercus secondary forests on the Loess Plateau,China

Ecological stoichiometry is an important indicator of biogeochemical cycles and nutrient limitations in terrestrial ecosystems.However,little is known about the response of ecological stoichiometry to plant growth.In this study,carbon(C),nitrogen(N),and phosphorus(P)concentrations were evaluated in plant tissues(trees,shrubs,and herbs),litter,and soil of young(≤40-year-old),middle-aged(41–60-year-old),near-mature(61–80-year-old),and mature(81–120-year-old)Quercus secondary forests on the Loess Plateau,China.Vegetation composition,plant biomass,and C stock were determined to illustrate their interaction with stoichiometry.Only tree biomass C signifi cantly increased with stand development.Leaf N and trunk P concentrationsgenerally increased,but branch P decreased with growth stage.Fine roots had the highest C and P concentrations at the middle-aged stage.In contrast,shrubs,herbs,litter,and soil C:N:P stoichiometry did not change signifi cantly during stand development.Leaf N and P were positively correlated with soil C,N,P,and their ratios.However,there was no signifi cant correlation between litter and leaves in terms of C:N:P stoichiometry.A redundancy analysis showed that soil N best explained leaf N and P variance,and tree biomass and C stock were related to biotic factors such as tree age and shrub biomass.Hierarchical partitioning analysis indicated that,compared with soil or litter variables,stand age only accounted for a relatively small proportion of leaf C,N,and P variation.Thus,secondary Quercus ecosystems might have inherent ability to maintain sensitive responses of metabolically active organs to environmental factors during stand aging.The results of this work help to elucidate the biogeochemical cycling of secondary forest ecosystems in tree development,provide novel insights into the adaptation strategies of plants in diff erent organs and growth stages,and could be used to guide fertilization programs and optimize forest structure.

Carbon,nitrogen and phosphorus stoichiometry in Pinus tabulaeformis forest ecosystems in warm temperate Shanxi Province,north China

Although carbon(C), nitrogen(N), and phosphorous(P) stoichiometric ratios are considered good indicators of nutrient excess/limitation and thus of ecosystem health, few reports have discussed the trends and the reciprocal effects of C:N:P stoichiometry in plant–litter–soil systems. The present study analyzed C:N:P ratios in four age groups of Chinese pine, Pinus tabulaeformis Carr., forests in Shanxi Province, China: plantation young forests(AY,<20 year-old); plantation middle-aged forests(AM, 21–30 year-old); natural young forests(NY,<30 year-old); and natural middle-aged forests(NM,31–50 year-old). The average C:N:P ratios calculated for tree, shrub, and herbaceous leaves, litter, and soil(0–100 cm) were generally higher in NY followed by NM,AM, and AY. C:N and C:P ratios were higher in litter than in leaves and soils, and reached higher values in the litter and leaves of young forests than in middle-aged forests;however, C:N and C:P ratios were higher in soils of middle-aged forests than in young forests. N:P ratios were higher in leaves than in litter and soils regardless of stand age; the consistent N:P<14 values found in all forests indicated N limitations. With plant leaves, C:P ratios were highest in trees, followed by herbs and shrubs, indicating a higher efficiency in tree leaf formation. C:N ratios decreased with increasing soil depth, whereas there was no trend for C:P and N:P ratios. C:N:P stoichiometry of forest foliage did not exhibit a consistent variation according to stand age. Research on the relationships between N:P, and P, N nutrient limits and the characteristics of vegetation nutrient adaptation need to be continued.

Impact of vegetation succession on leaf-litter-soil C:N:P stoichiometry and their intrinsic relationship in the Ziwuling Area of China’s Loess Plateau

Long-term natural vegetation succession plays a substantial role in the accumulation and distribution of plant and soil C:N:P stoichiometry.However,how plant and soil C:N:P relationships or ratios change along with successional stages over a century in the severely eroded areas remain unclear.These were measured over a 100-year natural succession in five successional stages from annual grasses to climax forests.The results show that natural succession had significant effects on carbon(C),nitrogen(N)and phosphorous(P)concentrations in leaf-litter-soil and their ratios in severely eroded areas.Nitrogen concentrations and N:P ratios in leaf and litter increased from annual grasses to the shrub stage and then decreased in the late successional forest stages.Leaf P levels decreased from annual grasses to shrub stages and did not significantly change during late successional stages.Litter P concentration decreased in the early successional stages and increased during late successional stages,with no overall significant change.Soil C and N concentrations and C:N,C:P and N:P ratios increased with successional stages.Soil C and N concentrations decreased with the increasing soil depth.Both were significantly different between any successional stages and controls(cropland)in the upper 10 cm and 10–20 cm soil layers.Leaf N:P ratios may be used to indicate nutrient limitations and this study suggests that plant growth during the grass stages was limited by N,during the shrub stage,by P,and during the forest stages,by both of N and P.In addition,there were close correlations between litter and leaf C:N:P ratios,soil and litter C and N levels,and C:P and N:P ratios.These results show that long-term natural vegetation succession is effective in restoring degraded soil properties and improving soil fertility,and provide insights into C:N:P relationships of leaf,litter and soil influenced by vegetation succession stage.

Inter-and intra-specific phenotypic variation of ecological stoichiometric traits in a mixed-oak secondary forest in China

Ecological stoichiometry provides a framework for the balance and flow of elements between organisms and ecosystems.Elemental phenotypes have an important influence on the environmental adaptation and ecological evolution of plants.There have been few reports on inter-and intra-specific phenotypic variations of ecological stoichiometric traits for congeneric species in a mixed forest although such variations are well-documented at the species level at global,regional and local scales.In this study,total carbon(TC),nitrogen(TN),phosphorus(TP)and potassium(TK)were measured in leaves and the elemental phenotypes were statistically analyzed in four species of oaks—Quercus fabri,Q.serrata var.glandulifera,Q.acutissima and Q.variabilis—in a mixed-oak secondary forest in Yushan,Jiangsu,China.The average element concentrations in the four oak species were not relatively higher than previously reported for oaks from world and Chinese flora.Ecological stoichiometry traits were correlated with tree height and diameter at breast height,indicating that phosphorous and potassium were positively correlated with tree size,while carbon was negatively correlated,especially the relationship between oak growth and total carbon or total phosphorus was obvious,and the study concluded that this was because plant growth depended on phosphorus storage and had opposite effects on leaf carbon accumulation.Based on tree plasticity index and the coefficient of variation,there was medium variation in element concentrations.The plasticity index of total carbon levels was the lowest,and that of potassium the highest.Principal component analysis and cluster analysis showed that the intra-specific variation among the four oak species was higher than inter-specific variation.From the perspective of nutrient supply and ecological adaptation,this study creates a foundation for the management of secondary oak forest stands.

Leaf C:N:P stoichiometric homeostasis of a Robinia pseudoacacia plantation on the Loess Plateau

Homeostasis is the adaptability of a species to a changing environment.However,the ecological stoichiometric homeostasis of Robinia pseudoacacia L.in diff erent climatic regions is poorly understood but could provide insights into its adaptability in the loess hilly region.This study sampled 20 year-old R.pseudoacacia plantations at 10 sites along a north–south transect on the Loess Plateau.Variations in the ecological stoichiometric characteristics of leaf and soil carbon,nitrogen,and phosphorus were analysed and homeostatic characteristics of leaf ecological stoichiometric parameters in diff erent climates were identifi ed.Factors aff ecting leaf stoichiometry were assessed.The results show that R.pseudoacacia leaves were rich in nitrogen and defi cient in phosphorous during tree growth and development.Nitrogen and phosphorous levels in the soils of the loess region were lower than the average in soils in the rest of China.All ecological stoichiometric parameters of R.pseudoacacia leaves in two diff erent climates were considered“strictly homeostasis”.Precipitation,available phosphorus,and soil C:P were the main factors aff ecting the variation of C:N:P stoichiometry of R.pseudoacacia leaves.R.pseudoacacia in the loess hilly region has strong ecologically homeostatic characteristics and suggests that it is well-adapted to the area.

Soil Carbon, Nitrogen and Phosphorus Concentrations and Stoichiometries Across a Chronosequence of Restored Inland Soda Saline-Alkali Wetlands, Western Songnen Plain, Northeast China

Soil carbon(C), nitrogen(N) and phosphorus(P) concentrations and stoichiometries can be used to evaluate the success indicators to the effects of wetland restoration and reflect ecosystem function. Restoration of inland soda saline-alkali wetlands is widespread, however, the soil nutrition changes that follow restoration are unclear. We quantified the recovery trajectories of soil physicochemical properties, including soil organic carbon(SOC), total nitrogen(TN), and total phosphorus(TP) pools, for a chronosequence of three restored wetlands(7 yr, 12 yr and 21 yr) and compared these properties to those of degraded and natural wetlands in the western Songnen Plain, Northeast China. Wetland degradation lead to the loss of soil nutrients. Relative to natural wetlands, the mean reductions of in SOC, TN, and TP concentrations were 89.6%, 65.5% and 52.5%, respectively. Nutrients recovered as years passed after restoration. The SOC, TN, and TP concentrations increased by 2.36 times, 1.15 times, and 0.83 times, respectively in degraded wetlands that had been restored for 21 yr, but remained 29.2%, 17.3%, and 12.8% lower, respectively, than those in natural wetlands. The soil C∶N(RC N), C∶P(R CP), and N∶P(R NP) ratios increased from 5.92 to 8.81, 45.36 to 79.19, and 7.67 to 8.71, respectively in the wetland that had been restored for 12 yr. These results were similar to those from the natural wetland and the wetland that had been restored for 21 yr(P > 0.05). Soil nutrients changes occurred mainly in the upper layers(≤ 30 cm), and no significant differences were found in deeper soils(> 30 cm). Based on this, we inferred that it would take at least 34 yr for SOC, TN, and TP concentrations and 12 yr for RC N, R CP, and RN P in the top soils of degraded wetlands to recover to levels of natural wetlands. Soil salinity negatively influenced SOC(r =-0.704, P < 0.01), TN(r =-0.722, P < 0.01), and TP(r =-0.882, P < 0.01) concentrations during wetland restoration, which indicates that reducing salinity is beneficial to SOC, TN, and TP recovery. Moreover, plants were an important source of soil nutrients and vegetation restoration was conducive to soil nutrient accumulation. In brief, wetland restoration increased the accumulation of soil biogenic elements, which indicated that positive ecosystem functions changes had occurred.

Response of C:N:P in the plant-soil system and stoichiometric homeostasis of Nitraria tangutorum leaves in the oasis-desert ecotone,Northwest China

Nitraria tangutorum nebkhas are widely distributed in the arid and semi-arid desert areas of China.The formation and development of N.tangutorum nebkhas are the result of the interaction between vegetation and the surrounding environment in the process of community succession.Different successional stages of N.tangutorum nebkhas result in differences in the community structure and composition,thereby strongly affecting the distribution of soil nutrients and ecosystem stability.However,the ecological stoichiometry of N.tangutorum nebkhas in different successional stages remains poorly understood.Understanding the stoichiometric homeostasis of N.tangutorum could provide insights into its adaptability to the arid and semi-arid desert environments.Therefore,we analyzed the stoichiometric characteristics of N.tangutorum in four successional stages,i.e.,rudimental,developing,stabilizing,and degrading stages using a homeostasis model in an oasis-desert ecotone of Northwest China.The results showed that soil organic carbon(SOC),total nitrogen(TN),and total phosphorus(TP)contents and their ratios in the 0-100 cm soil depth were significantly lower than the averages at regional and global scales and were weakly influenced by successional stages in the oasis-desert ecotone.TN and TP contents and C:N:P in the soil showed similar trends.Total carbon(TC)and TN contents in leaves were 450.69-481.07 and 19.72-29.35 g/kg,respectively,indicating that leaves of N.tangutorum shrubs had a high storage capacity for C and N.Leaf TC and TN contents and N:P ratio increased from the rudimental stage to the stabilizing stage and then decreased in the degrading stage,while the reverse trend was found for leaf C:N.Leaf TP content decreased from the rudimental stage to the degrading stage and changed significantly in late successional stages.N:P ratio was above the theoretical limit of 14,indicating that the growth of N.tangutorum shrubs was limited by P during successional stages.Leaf N,P,and N:P homeostasis in four successional stages was identified as''strictly homeostasis''.Redundancy analysis(RDA)revealed that soil acidity(pH)and the maximum water holding capacity were the main factors affecting C:N:P stoichiometric characteristics in N.tangutorum leaves.Our study demonstrated that N.tangutorum with a high degree of stoichiometric homeostasis could better cope with the arid desert environment.

Litter decomposition and nutrient release for two tropical N-fixing species in Rio de Janeiro,Brazil

Litter production and decomposition are critical to forest productivity,nutrient cycling,and carbon seques-tration in tropical woody ecosystems.However,nutrient release and leaf litter stoichiometry in tropical legume tree plantations over the long term after outplanting are poorly understood or even unknown.Toward improving our under-standing of the pattern of changes in the decomposition of N-fixing leaf litters and their possible impact on carbon stor-age,we measured litter production,mass loss and nutrient release for 240 d during litter decomposition for two tropical legume tree species(Plathymenia reticulata and Hymenaea courbaril),in Rio de Janeiro,Brazil.Litter production for P.reticulata was 5.689 kg ha^(−1) a^(−1) and 3.231 kg ha^(−1) a^(−1) for H.courbaril.The patterns of mass loss rates were similar;however,nutrient release was greater for P.reticulata,while H.courbaril showed immobilization of nutrients,especially for N,which increased by almost 20%in the early phase of decomposition followed by gradual release.Litter from the N-fixing species did differ in nutrient chemistries over time,which was not surprising given that initial nutrient concentrations varied broadly,except for C and P.Most of the nutrient concentrations increased as the remaining litter mass decreased in both species,except for C and K.The C:N and N:P ratios differed between the species,but N:P did not correlate to mass loss.Both species had N-rich leaves,but P.reticulata decomposition was very likely P-limited,while H.courbaril seemed to be co-limited by N and P.The results showed different patterns in nutrient release and the stoichi-ometry involved in the decomposition dynamics of the two tropical N-fixing species,even though they have similar litter decay rates.Both species,but especially P.reticulata,may help re-establish nutrient cycling in disturbed ecosystems.

Aggregate-related microbial communities and nutrient stoichiometry under different croplands

Ecological stoichiometry is an important indicator presenting multiple elements balance in agro-ecosystems.However,information on microbial communities and nutrient stoichiometry in soil aggregate fractions under different croplands(rice,maize,and soybean fields)remains limited.Thus,this study investigated water-stable aggregate structure and their internal nutrient stoichiometry under different croplands and ascertain their interaction mechanism with microbial communities.The results showed that no significant difference on the carbon-to-nitrogen ratio(C:N)in soil aggregate fractions was observed,while the carbon-to-phosphorus ratio(C:P)and the nitrogen-to-phosphorus ratio(N:P)were ranked as rice field>maize field>soybean field,and were higher in mega-aggregates(ME,>1 mm).General fatty acid methyl ester(FAME),Gram-positive bacteria(G+),and Gramnegative bacteria(G−)were predominant microbial communities in all croplands and tented to condense into coarse-aggregates.Redundancy analysis(RDA)demonstrated that N:P ratio was primary environmental controls on the distribution of soil microorganisms.In the Sanjiang Plain,N was the nutrient element limiting agro-ecosystem productivity,and rice cultivation is expected to improve the N-limited nutrient status.

Long-term fertilization and residue return affect soil stoichiometry characteristics and labile soil organic matter fractions

Ecological stoichiometry provides the possibility for linking microbial dynamics with soil carbon(C),nitrogen(N),and phosphorus(P)metabolisms in response to agricultural nutrient management.To determine the roles of fertilization and residue return with respect to ecological stoichiometry,we collected soil samples from a 30-year field experiment on residue return(maize straw)at rates of 0,2.5,and 5.0 Mg ha^-1 in combination with 8 fertilization treatments:no fertilizer(F0),N fertilizer,P fertilizer,potassium(K)fertilizer,N and P(NP)fertilizers,N and K(NK)fertilizers,P and K(PK)fertilizers,and N,P,and K(NPK)fertilizers.We measured soil organic C(SOC),total N and P,microbial biomass C,N,and P,water-soluble organic C and N,KMnO4-oxidizabIe C(KMnO4-C),and carbon management index(CMI).Compared with the control(F0 treatment without residue return),fertilization and residue return significantly increased the KMn〇4-C content and CMI.Furthermore,compared with the control,residue return significantly increased the SOC content.Moreover,the NPK treatment with residue return at 5.0 Mg ha^-1 significantly enhanced the C:N,C:P,and N:P ratios in the soil,whereas it significantly decreased the C:N and C:P ratios in soil microbial biomass.Therefore,NPK fertilizer application combined with residue return at 5.0 Mg ha^-1 could enhance the SOC content through the stoichiometric plasticity of microorganisms.Residue return and fertilization increased the soil C pools by directly modifying the microbial stoichiometry of the biomass that was C limited.

Nitrogen addition mediates the response of foliar stoichiometry to phosphorus addition: a meta-analysis

Background:Changes in foliar nitrogen(N)and phosphorus(P)stoichiometry play important roles in predicting the efects of global change on ecosystem structure and function.However,there is substantial debate on the efects of P addition on foliar N and P stoichiometry,particularly under diferent levels of N addition.Thus,we conducted a global meta-analysis to investigate how N addition alters the efects of P addition on foliar N and P stoichiometry across different rates and durations of P addition and plant growth types based on more than 1150 observations.Results:We found that P addition without N addition increased foliar N concentrations,whereas P addition with N addition had no efect.The positive efects of P addition on foliar P concentrations were greater without N addition than with N addition.Additionally,the efects of P addition on foliar N,P and N:P ratios varied with the rate and duration of P addition.In particular,short-term or low-dose P addition with and without N addition increased foliar N concentration,and the positive efects of short-term or low-dose P addition on foliar P concentrations were greater without N addition than with N addition.The responses of foliar N and P stoichiometry of evergreen plants to P addition were greater without N addition than with N addition.Moreover,regardless of N addition,soil P availability was more efective than P resorption efciency in predicting the changes in foliar N and P stoichiometry in response to P addition.Conclusions:Our results highlight that increasing N deposition might alter the response of foliar N and P stoichiometry to P addition and demonstrate the important efect of the experimental environment on the results.These results advance our understanding of the response of plant nutrient use efciency to P addition with increasing N deposition.

Soil microbial respiration is regulated by stoichiometric imbalances: Evidence from a humidity gradient case

Humidity not only affects soil microbial respiration(SMR) directly, but, indirectly by regulating the availability of soil water and nutrients. However,the patterns of direct and indirect effects of humidity on SMR over large precipitation gradients remain unclear, limiting our understanding of the effects of precipitation changes on soil C cycle. Here, we investigated the relationships among humidity, soil nutrients, and SMR by identifying stoichiometric imbalances, microbial elemental homeostasis, and microbial C use efficiency along a precipitation gradient at a continental scale. The relationship between SMR and humidity index(HI) corresponded to a Richard’s curve with an inflection point threshold value of approximately 0.7. Soil microbial respiration increased with increasing humidity in drier areas(HI < 0.7), but tended to balance above this threshold. Increasing humidity exacerbated C:P and N:P imbalances across the selected gradient. Severe N and P limitations in soil microbial communities were observed in drier areas, while soil microbes suffered from aggravated P limitation as the humidity increased in wetter areas(HI > 0.7). Soil microbial communities regulated their enzyme production to maintain a strong stoichiometric homeostasis in drier areas;enzyme production, microbial biomass, and threshold elemental ratios were non-homeostatic under P limitation in wetter areas, which further contributed to the increase in SMR. Our results identified a moisture constraint on SMR in drier areas and highlighted the importance of nutrient(especially for P) limitations induced by humidity in regulating SMR in wetter areas. Understanding the modulation of SMR via soil enzyme activity may improve the prediction of soil C budget under future global climate change.

Interactive effects of warming and eutrophication on zooplankton could reverse the stoichiometric mismatch with phytoplankton

The core ecosystem functioning(e.g.trophic transfer efficiency)is at risk of being disrupted by the growing mismatch between nutrient content of primary producers and nutrient demand of grazing consumers.Ecological stoichiometry provides a conceptual framework that explains this trophic interaction using C,N and P elemental composition across trophic levels.In light of ongoing climate change and eutrophication,previous studies have raised concerns regarding the growing stoichiometric mismatch between phytoplankton and zooplankton,given the stoichiometric plasticity of phytoplankton.However,there is currently little conclusive evidence on the stoichiometric mismatch from a dual perspective of phytoplankton and zooplankton.To address this,we conducted a mesocosm experiment to investigate the separate and combined effects of climate warming(a constant increase of t3.5C plus heat waves)and eutrophication(nutrient addition)on stoichiometric mismatch between phytoplankton and zooplankton by examining stoichiometric changes in both communities.We observed a growing trend in stoichiometric mismatches when warming or nutrient addition acted individually,which was mediated by the increase in nutrient demand(N,P elements)of zooplankton growth.However,when these stressors acted jointly,the mismatches were reversed.This could be because climate warming and eutrophication combined would lead to changes in species composition,which accordingly reshaped the stoichiometric composition at the community level.These results illustrate the need of stoichiometric mismatches for understanding the implication of global change on trophic interactions and ecosystem functioning,requiring consideration not only of cross-trophic levels but also of compositional changes within communities.

Grasshoppers （Orthoptera： Acrididae） select vegetation patches in local-scale responses to foliar nitrogen but not phosphorus in native grassland

Key elements such as nitrogen （N） and phosphorus （P） are often limiting relative to the nutritional needs of herbivores that feed on them. While N often limits insect herbivores in natural terrestrial ecosystems, the effect of P is poorly studied in the field, even though compelling hypotheses from the ecological stoichiometry literature predict its importance. We evaluated small-scale spatial distributions of, and herbivory by, grasshoppers among neighboring plots that vary in foliar-N and -P in tallgrass prairie. Grasshopper densities were 67% greater in N-fertilized plots but detected no effect to grasshopper densities from P-fertilizer. Leaf damage to the dominant grass Andropogon gerardii was 32% greater in N-fertilized plots, but no response to foliar-P was detected. Herbivore damage to a common forb, goldenrod （Solidago missouriensis）, was not strongly linked by fertilizer treatments, although there was increased leaf damage in N-fertilizer treatments when no P was applied （a significant N ~ P interaction）. Under field conditions at local scales, we conclude that spatially heterogeneous distributions of grasshoppers are primarily affected by foliar-N in host plants with little evidence that P-levels contribute to the spatial patterns.

Interspecific interactions affect N and P uptake rather than N:P ratios of plant species:evidence from intercropping

Quantifying stoichiometry of crop N and P acquisition(i.e.removal from farmland)under different agronomic practices is essential for understanding nutrient budgets and optimizing N and P fertilizer application in agroecosystems.It is not clear how plant N and P uptake and N:P stoichiometry vary between monoculture and intercropping throughout an entire growing season under different N fertilization and mulching practices.Here,we addressed how plant interspecific competition for nutrients have affected the temporal dynamics of crop N and P uptake(and N:P ratios)in five cropping systems(wheat,maize and barley monocultures,and wheat/maize and barley/maize intercropping),under two N levels(0 and 225 kg N ha^(−1))and two maize mulching treatments(with and without).Wheat and barley had greater N and P competitive ability than maize,leading to an increase in N and P uptake of wheat and barley and a decrease in N and P uptake of maize during co-growth stages in intercropping.N:P ratios of three crop species decreased with plant growth.Crop-level N:P ratios were either not affected by intercropping or did not change consistently with N fertilization while film mulching decreased maize N:P ratios.Community-level N:P ratios of the two intercrops were different from those of the corresponding monoculture at maturity.Because(i)the proportion of N and P removal from intercropping differs from monocultures,and(ii)N and P uptake by crops is decoupled under N fertilization and mulching,these findings may have practical implications for the nutrient budget of intercropping systems.

Temporal variation and its drivers in the elemental traits of four boreal plant species

Aims Intraspecific variation in plant traits has important consequences for individual fitness and herbivore foraging.For plants,trait variability across spatial dimensions is well documented.However,temporal dimensions of trait variability are less well known,and may be influenced by seasonal differences in growing degree days(GDD),temperature and precipitation.Here,we aim to quantify intraspecific temporal variation in traits and the underlying drivers for four commonly occurring boreal plant species.Methods We sampled the elemental and stoichiometric traits(%C,%N,%P,C:N,C:P,N:P)of four common browse species'foliage across 2 years.Using a two-step approach,we first fitted generalized linear models(GzLM,n=24)to the species'elemental and stoichiometric traits,and tested if they varied across years.When we observed evidence for temporal variability,we fitted a second set of GzLMs(n=8)with temperature,productivity and moisture as explanatory variables.Important Findings We found no evidence of temporal variation for most of the elemental and stoichiometric traits of our four boreal plants,with two exceptions.Year was an important predictor for percent carbon across all four species(R^(2)=0.47-0.67)and for multiple elemental and stoichiometric traits in balsam fir(5/8,R2=0.29-0.67).Thus,variation in percent carbon was related to interannual differences,more so than nitrogen and phosphorus,which are limiting nutrients in the boreal forest.These results also indicate that year may explain more variation in conifers'stoichiometry than for deciduous plants due to life history differences.GDD was the most frequently occurring variable in the second round of models(8/8 times,R^(2)=0.21-0.41),suggesting that temperature is an important driver of temporal variation in these traits.

Dominant roles but distinct effects of groundwater depth on regulating leaf and fine-root N,P and N:P ratios of plant communities

Aims As the determinant of water availability in drylands,groundwater plays a fundamental role in regulating vegetation distribution and ecosystem processes.Although considerable progress has been made over the past years in the relationship between environment stress and plant community-level traits,the potential influence of water stress induced by groundwater changes on plant community-level stoichiometry remains largely unclear.Here,we examined whether belowground and aboveground community-level stoichiometry responded differently to groundwater changes.Methods We measured nitrogen(N)and phosphorus(P)concentrations in plant leaves and fine-roots of 110 plots under a broad range of groundwater depths in a typical arid inland river basin.We examined the spatial patterns and drivers of community-level N:P stoichiometry in leaves and fine-roots.Important Findings Community-level leaf and fine-root N,P and N:P ratios were mainly determined by groundwater,vegetation types and species composition,among which groundwater played a dominant role.Groundwater indirectly regulated community-level N:P stoichiometry through affecting vegetation types and species composition.Vegetation types and species composition had significant direct influences on communitylevel N:P stoichiometry.Furthermore,groundwater depth had opposite influences on community-level leaf and fine-root N:P stoichiometry.Groundwater depth regulated vegetation types and further decreased leaf N,P but increased leaf N:P ratios and fine-root N.Groundwater depth had a positive indirect impact on fine-root P but a negative indirect impact on fine-root N:P ratios primarily by affecting species composition.Our findings indicate that groundwater rather than climate conditions effectively regulates community-level N:P stoichiometry,and below-and aboveground N:P stoichiometry has opposite responses to groundwater.

Stoichiometric homeostasis in response to variable water and nutrient supply in a Robinia pseudoacacia plant-soil system

All organisms need elements in fixed proportions for carrying out normal metabolic processes and how flexible they are depends on how effective they are utilizing these resources from external sources.It is important to understand the interactions among plant,soil and microbial biomass carbon(C),nitrogen(N)and phosphorus(P)stoichiometry under different conditions of resource supply.We conducted a pot experiment on 1-year-old Robinia pseudoacacia seedlings for nearly 5 months under different water,nitrogen and phosphorus supplies,and we determined plant,soil and microbial biomass C,N and P stoichiometry.We found that plant,soil and microbial nutrients and stoichiometry exhibited a certain degree of plasticity in response to the changes in water and nutrient conditions in their environments.Variation partitioning analysis showed that root stoichiometry accounted for a large part of the variance in microbial stoichiometry.Structural equation modeling further revealed that root stoichiometry and leaf stoichiometry were two direct factors affecting microbial biomass C:N and C:P,and that root stoichiometry had the greatest direct effect.In addition,the degree of homeostasis for microbial biomass C and C:P was more sensitive to changes in soil nutrients than changes in other factors,and other elements and elemental ratios displayed strict homeostasis.These results highlight the importance of studying microbial stoichiometry in improving our understanding of nutrient cycling of the plant-soil system under different water and nutrient supply.