Towards a more physiological representation of vegetation phosphorus processes in land surface models

Responses in species diversity in the Hulunbuir grassland to phosphorus addition under nitrogen-limiting and non-limiting conditions

The phenomenon of nitrogen deposition resulting in species loss in terrestrial ecosystems has been demonstrated in several experiments. Nitrogen (N) and phosphorus (P), as major nutrients required for plant growth, exhibit ecological stoichiometric coupling in many ecosystems. The increased availability of nitrogen can exacerbate the ecological effects of phosphorus. To reveal the ecological effects of phosphorus under nitrogen-limiting and non-limiting conditions, we conducted a controlled N-P interaction experiment over 5 years in the Hulunbuir meadow steppe, where two nitrogen addition levels were implemented: 0 g N·m-2·a-1 (nitrogen-limiting condition) and 10 g N·m-2·a-1 (nitrogen-non-limiting condition), together with six levels of phosphorus addition (0, 2, 4, 6, 8, and 10 g P·m-2·a-1). The results showed that nitrogen addition (under nitrogen-non-limiting conditions) significantly decreased species diversity in the steppe community, which was exacerbated under phosphorus addition. Under nitrogen-limiting conditions, phosphorus addition had no marked impact on species diversity compared to the control; however, there were substantial differences between different levels of phosphorus addition, exhibiting a unimodal change. Under both experimental nitrogen conditions, the addition of 6 g P·m-2·a-1 was the threshold for affecting the community species diversity. Nitrogen addition reduced the relative biomass of legumes, bunch grasses, and forbs, but substantially increased the relative biomass of rhizomatous grasses. In contrast, phosphorus addition only markedly affected the relative biomass of forbs and rhizomatous grasses, with the former showing a unimodal pattern of first increasing and then decreasing with increasing phosphorus addition level, and the latter exhibiting the opposite pattern. The different responses of rhizomatous grasses and other functional groups to nitrogen and phosphorus addition were observed to have a regulatory effect on the changes in grassland community structure. Phosphorus addition may increase the risk of nitrogen deposition-induced species loss. Both nitrogen and phosphorus addition lead to soil acidification and an increase in the dominance of the already-dominant species, and the consequent species loss in the forb functional group represents the main mechanism for the reduction in community species diversity.

Carbon-phosphorus cycle models overestimate CO2 enrichment response in a mature Eucalyptus forest

The importance of phosphorus (P) in regulating ecosystem responses to climate change has fostered P-cycle implementation in land surface models, but their CO2 effects predictions have not been evaluated against measurements. Here, we perform a data-driven model evaluation where simulations of eight widely used P-enabled models were confronted with observations from a long-term free-air CO2 enrichment experiment in a mature, P-limited Eucalyptus forest. We show that most models predicted the correct sign and magnitude of the CO2 effect on ecosystem carbon (C) sequestration, but they generally overestimated the effects on plant C uptake and growth. We identify leaf-to-canopy scaling of photosynthesis, plant tissue stoichiometry, plant belowground C allocation, and the subsequent consequences for plant-microbial interaction as key areas in which models of ecosystem C-P interaction can be improved. Together, this data-model intercomparison reveals data-driven insights into the performance and functionality of P-enabled models and adds to the existing evidence that the global CO2-driven carbon sink is overestimated by models.

Microbial competition for phosphorus limits the CO2 response of a mature forest

The capacity for terrestrial ecosystems to sequester additional carbon (C) with rising CO2 concentrations depends on soil nutrient availability1,2. Previous evidence suggested that mature forests growing on phosphorus (P)-deprived soils had limited capacity to sequester extra biomass under elevated CO2 (refs. 3-6), but uncertainty about ecosystem P cycling and its CO2 response represents a crucial bottleneck for mechanistic prediction of the land C sink under climate change7. Here, by compiling the first comprehensive P budget for a P-limited mature forest exposed to elevated CO2, we show a high likelihood that P captured by soil microorganisms constrains ecosystem P recycling and availability for plant uptake. Trees used P efficiently, but microbial pre-emption of mineralized soil P seemed to limit the capacity of trees for increased P uptake and assimilation under elevated CO2 and, therefore, their capacity to sequester extra C. Plant strategies to stimulate microbial P cycling and plant P uptake, such as increasing rhizosphere C release to soil, will probably be necessary for P-limited forests to increase C capture into new biomass. Our results identify the key mechanisms by which P availability limits CO2 fertilization of tree growth and will guide the development of Earth system models to predict future long-term C storage.

Microbe-dependent and independent nitrogen and phosphate acquisition and regulation in plants

Nitrogen (N) and phosphorus (P) are the most important macronutrients required for plant growth and development. To cope with the limited and uneven distribution of N and P in complicated soil environments, plants have evolved intricate molecular strategies to improve nutrient acquisition that involve adaptive root development, production of root exudates, and the assistance of microbes. Recently, great advances have been made in understanding the regulation of N and P uptake and utilization and how plants balance the direct uptake of nutrients from the soil with the nutrient acquisition from beneficial microbes such as arbuscular mycorrhiza. Here, we summarize the major advances in these areas and highlight plant responses to changes in nutrient availability in the external environment through local and systemic signals.

Milestones in understanding transport, sensing, and signaling of the plant nutrient phosphorus

As an essential nutrient element, phosphorus (P) is primarily acquired and translocated as inorganic phosphate (Pi) by plant roots. Pi is often sequestered in the soil and becomes limited for plant growth. Plants have developed a sophisticated array of adaptive responses, termed P starvation responses, to cope with P deficiency by improving its external acquisition and internal utilization. Over the past 2 to 3 decades, remarkable progress has been made toward understanding how plants sense and respond to changing environmental P. This review provides an overview of the molecular mechanisms that regulate or coordinate P starvation responses, emphasizing P transport, sensing, and signaling. We present the major players and regulators responsible for Pi uptake and translocation. We then introduce how P is perceived at the root tip, how systemic P signaling is operated, and the mechanisms by which the intracellular P status is sensed and conveyed. Additionally, the recent exciting findings about the influence of P on plant-microbe interactions are highlighted. Finally, the challenges and prospects concerning the interplay between P and other nutrients and strategies to enhance P utilization efficiency are discussed. Insights obtained from this knowledge may guide future research endeavors in sustainable agriculture.

A novel hybrid modelling framework for GPP estimation: Integrating a multispectral surface reflectance based Vcmax25 simulator into the process-based model

Terrestrial gross primary productivity (GPP) is the key element in the carbon cycle process. Accurate GPP estimation hinges on the maximum carboxylation rate (Vcmax,025). The high uncertainty in deriving ecosystem-level Vcmax,025 has long hampered efforts toward the performance of the GPP model. Recently studies suggest the strong relationship between spectral reflectance and Vcmax,025. We proposed the multispectral surface reflectance-driven Vcmax,025 simulator using the fully connected deep neural network and built the hybrid modelling framework for GPP estimation by integrating the data-driven Vcmax,025 simulator in the process-based model. The performance of hybrid GPP model was evaluated at 95 flux sites. The result shows that the multispectral surface reflectance-driven Vcmax,025 simulator acquires the satisfactory estimation, with correlation coefficient (R), root mean square error (RMSE) and median absolute percentage error (MdAPE) ranging from 0.34 to 0.80, 14 to 43 μmol m-2 s-1 and 21 % to 66 % across different land cover types, respectively. The hybrid framework generates good GPP estimates with R, RMSE and MdAPE varying from 0.76 to 0.89, 1.79 to 6.16 μmol m-2 s-1 and 27 % to 90 %, respectively. Compared with EVI-driven method, the multispectral surface reflectance significantly improves the Vcmax,025 and GPP estimates, with MdAPE declining by 0.6 %-18 % and 1 % to 21 %, respectively. The Shapley value analysis reveals that red (620-670 nm), near-infrared (841-876 nm) and shortwave infrared (1628-1652 nm and 2105-2155 nm) are the key bands for Vcmax,025 estimation. This study highlights the potential of multispectral surface reflectance for quantifying ecosystem-level Vcmax,025. The new hybrid framework fully extracts the information of all available spectral bands using deep learning to reduce parameter uncertainty while maintains the description of photosynthetic process to ensure its physical reasonability. It can serve as a powerful tool for accurate global GPP estimation.

OsMYB58 Negatively Regulates Plant Growth and Development by Regulating Phosphate Homeostasis

Phosphate (Pi) starvation is a critical factor limiting crop growth, development, and productivity. Rice (Oryza sativa) R2R3-MYB transcription factors function in the transcriptional regulation of plant responses to various abiotic stresses and micronutrient deprivation, but little is known about their roles in Pi starvation signaling and Pi homeostasis. Here, we identified the R2R3-MYB transcription factor gene OsMYB58, which shares high sequence similarity with AtMYB58. OsMYB58 expression was induced more strongly by Pi starvation than by other micronutrient deficiencies. Overexpressing OsMYB58 in Arabidopsis thaliana and rice inhibited plant growth and development under Pi-deficient conditions. In addition, the overexpression of OsMYB58 in plants exposed to Pi deficiency strongly affected root development, including seminal root, lateral root, and root hair formation. Overexpressing OsMYB58 strongly decreased the expression of the rice microRNAs OsmiR399a and OsmiR399j. By contrast, overexpressing OsMYB58 strongly increased the expression of rice PHOSPHATE 2 (OsPHO2), whose expression is repressed by miR399 during Pi starvation signaling. OsMYB58 functions as a transcriptional repressor of the expression of its target genes, as determined by a transcriptional activity assay. These results demonstrate that OsMYB58 negatively regulates OsmiR399-dependent Pi starvation signaling by enhancing OsmiR399s expression.

The links between wood traits and species demography change during tree development in a lowland tropical rainforest

One foundational assumption of trait-based ecology is that traits can predict species demography. However, the links between traits and demographic rates are, in general, not as strong as expected. These weak associations may be due to the use of traits that are distantly related to performance, and/or the lack of consideration of size-related variations in both traits and demographic rates. Here, we examined how wood traits were related to demographic rates in 19 tree species from a lowland forest in eastern Amazonia. We measured 11 wood traits (i.e. structural, anatomical and chemical traits) in sapling, juvenile and adult wood; and related them to growth and mortality rates (MR) at different ontogenetic stages. The links between wood traits and demographic rates changed during tree development. At the sapling stage, relative growth rates (RGR) were negatively related to wood specific gravity (WSG) and total parenchyma fractions, while MR decreased with radial parenchyma fractions, but increased with vessel lumen area (VA). Juvenile RGR were unrelated to wood traits, whereas juvenile MR were negatively related to WSG and axial parenchyma fractions. At the adult stage, RGR scaled with VA and wood potassium concentrations. Adult MR were not predicted by any trait. Overall, the strength of the trait-demography associations decreased at later ontogenetic stages. Our results indicate that the associations between traits and demographic rates can change as trees age. Also, wood chemical or anatomical traits may be better predictors of growth and MR than WSG. Our findings are important to expand our knowledge on tree life-history variations and community dynamics in tropical forests, by broadening our understanding on the links between wood traits and demography during tree development.

Foliar nutrient resorption stoichiometry and microbial phosphatase catalytic efficiency together alleviate the relative phosphorus limitation in forest ecosystems

Understanding how plants adapt to spatially heterogeneous phosphorus (P) supply is important to elucidate the effect of environmental changes on ecosystem productivity. Plant P supply is concurrently controlled by plant internal conservation and external acquisition. However, it is unclear how climate, soil, and microbes influence the contributions and interactions of the internal and external pathways for plant P supply. Here, we measured P and nitrogen (N) resorption efficiency, litter and soil acid phosphatase (AP) catalytic parameters (V_max(s) and K_m ), and soil physicochemical properties at four sites spanning from cold temperate to tropical forests. We found that the relative P limitation to plants was generally higher in tropical forests than temperate forests, but varied greatly among species and within sites. In P-impoverished habitats, plants resorbed more P than N during litterfall to maintain their N : P stoichiometric balance. In addition, once ecosystems shifted from N-limited to P-limited, litter- and soil-specific AP catalytic efficiency (V_max(s) /K_m ) increased rapidly, thereby enhancing organic P mineralization. Our findings suggested that ecosystems develop a coupled aboveground-belowground strategy to maintain P supply and N : P stoichiometric balance under P-limitation. We also highlighted that N cycle moderates P cycles and together shape plant P acquisition in forest ecosystems.

Effects of Warming, Phosphorous Deposition, and Both Treatments on the Growth and Physiology of Invasive Solidago canadensis and Native Artemisia argyi

Anthropogenic climate change and species invasion are two major threats to biodiversity, affecting the survival and distribution of many species around the world. Studying the responses of invasive species under climate change can help better understand the ecological and genetic mechanisms of their invasion. However, the effects of warming and phosphorus deposition on the phenotype of native and invasive plants are unknown. To address the problem, we applied warming (+2.03 °C), phosphorus deposition (4 g m^-2 yr^-1 NaH₂PO₄), and warming × phosphorus deposition to Solidago canadensis and Artemisia argyi to measure the direct effects of environmental changes on growth and physiology at the seedling stage. Our results reveal that the physiology parameters of A. argyi and S. canadensis did not change significantly with the external environment. Under phosphorus deposition, S. canadensis had higher plant height, root length, and total biomass compared to A. argyi. Interestingly, warming has an inhibitory effect on the growth of both A. argyi and S. canadensis, but overall, the reduction in total biomass for S. canadensis (78%) is significantly higher than A. argyi (52%). When the two plants are treated with warming combined with phosphorus deposition, the advantage gained by S. canadensis from phosphorus deposition is offset by the negative effects of warming. Therefore, under elevated phosphorus, warming has a negative effect on the invasive S. canadensis and reduces its growth advantage.

Seasonal peak photosynthesis is hindered by late canopy development in northern ecosystems

The seasonal dynamics of the vegetation canopy strongly regulate the surface energy balance and terrestrial carbon fluxes, providing feedbacks to climate change. Whether the seasonal timing of maximum canopy structure was optimized to achieve a maximum photosynthetic carbon uptake is still not clear due to the complex interactions between abiotic and biotic factors. We used two solar-induced chlorophyll fluorescence datasets as proxies for photosynthesis and the normalized difference vegetation index and leaf area index products derived from the moderate resolution imaging spectroradiometer as proxies for canopy structure, to characterize the connection between their seasonal peak timings from 2000 to 2018. We found that the seasonal peak was earlier for photosynthesis than for canopy structure in >87.5% of the northern vegetated area, probably leading to a suboptimal maximum seasonal photosynthesis. This mismatch in peak timing significantly increased during the study period, mainly due to the increasing atmospheric CO₂, and its spatial variation was mainly explained by climatic variables (43.7%) and nutrient limitations (29.6%). State-of-the-art ecosystem models overestimated this mismatch in peak timing by simulating a delayed seasonal peak of canopy development. These results highlight the importance of incorporating the mechanisms of vegetation canopy dynamics to accurately predict the maximum potential terrestrial uptake of carbon under global environmental change.

Phosphorous Supplementation Alleviates Drought-Induced Physio-Biochemical Damages in Calligonum mongolicum

Calligonum mongolicum is a phreatophyte playing an important role in sand dune fixation, but little is known about its responses to drought and P fertilization. In the present study, we performed a pot experiment to investigate the effects of P fertilization under drought or well-watered conditions on multiple morpho-physio-biochemical attributes of C. mongolicum seedlings. Drought stress leads to a higher production of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), leading to impaired growth and metabolism. However, C. mongolicum exhibited effective drought tolerance strategies, including a higher accumulation of soluble sugars, starch, soluble protein, proline, and significantly higheractivities of peroxidase (POD) and catalase (CAT) enzymes. P fertilization increased the productivity of drought-stressed seedlings by increasing their growth, assimilative shoots relative water content, photosynthetic pigments, osmolytes accumulation, mineral nutrition, N assimilation, and reduced lipid peroxidation. Our findings suggest the presence of soil high P depletion and C. mongolicum high P requirements during the initial growth stage. Thus, P can be utilized as a fertilizer to enhance the growth and productivity of Calligonum vegetation and to reduce the fragility of the hyper-arid desert of Taklamakan in the context of future climate change.

Convergence in phosphorus constraints to photosynthesis in forests around the world

Tropical forests take up more carbon (C) from the atmosphere per annum by photosynthesis than any other type of vegetation. Phosphorus (P) limitations to C uptake are paramount for tropical and subtropical forests around the globe. Yet the generality of photosynthesis-P relationships underlying these limitations are in question, and hence are not represented well in terrestrial biosphere models. Here we demonstrate the dependence of photosynthesis and underlying processes on both leaf N and P concentrations. The regulation of photosynthetic capacity by P was similar across four continents. Implementing P constraints in the ORCHIDEE-CNP model, gross photosynthesis was reduced by 36% across the tropics and subtropics relative to traditional N constraints and unlimiting leaf P. Our results provide a quantitative relationship for the P dependence for photosynthesis for the front-end of global terrestrial C models that is consistent with canopy leaf measurements.

Phosphorus (P) limitation is pervasive in tropical forests. Here the authors analyse the dependence of photosynthesis on leaf N and P in tropical forests, and show that incorporating leaf P constraints in a terrestrial biosphere model enhances its predictive power.

Modeling Global Carbon Costs of Plant Nitrogen and Phosphorus Acquisition

Most Earth system models (ESMs) do not explicitly represent the carbon (C) costs of plant nutrient acquisition, which leads to uncertainty in predictions of the current and future constraints to the land C sink. We integrate a plant productivity-optimizing nitrogen (N) and phosphorus (P) acquisition model (fixation & uptake of nutrients, FUN) into the energy exascale Earth system (E3SM) land model (ELM). Global plant N and P uptake are dynamically simulated by ELM-FUN based on the C costs of nutrient acquisition from mycorrhizae, direct root uptake, retranslocation from senescing leaves, and biological N fixation. We benchmarked ELM-FUN with three classes of products: ILAMB, a remotely sensed nutrient limitation product, and CMIP6 models; we found significant improvements in C cycle variables, although the lack of more observed nutrient data prevents a comprehensive level of benchmarking. Overall, we found N and P co-limitation for 80% of land area, with the remaining 20% being either predominantly N or P limited. Globally, the new model predicts that plants invested 4.1 Pg C yr-1 to acquire 841.8 Tg N yr-1 and 48.1 Tg P yr-1 (1994-2005), leading to significant downregulation of global net primary production (NPP). Global NPP is reduced by 20% with C costs of N and 50% with C costs of NP. Modeled and observed nutrient limitation agreement increases when N and P are considered together (r 2 from 0.73 to 0.83).

Litter quality and stream physicochemical properties drive global invertebrate effects on instream litter decomposition

Plant litter is the major source of energy and nutrients in stream ecosystems and its decomposition is vital for ecosystem nutrient cycling and functioning. Invertebrates are key contributors to instream litter decomposition, yet quantification of their effects and drivers at the global scale remains lacking. Here, we systematically synthesized data comprising 2707 observations from 141 studies of stream litter decomposition to assess the contribution and drivers of invertebrates to the decomposition process across the globe. We found that (1) the presence of invertebrates enhanced instream litter decomposition globally by an average of 74%; (2) initial litter quality and stream water physicochemical properties were equal drivers of invertebrate effects on litter decomposition, while invertebrate effects on litter decomposition were not affected by climatic region, mesh size of coarse-mesh bags or mycorrhizal association of plants providing leaf litter; and (3) the contribution of invertebrates to litter decomposition was greatest during the early stages of litter mass loss (0-20%). Our results, besides quantitatively synthesizing the global pattern of invertebrate contribution to instream litter decomposition, highlight the most significant effects of invertebrates on litter decomposition at early rather than middle or late decomposition stages, providing support for the inclusion of invertebrates in global dynamic models of litter decomposition in streams to explore mechanisms and impacts of terrestrial, aquatic, and atmospheric carbon fluxes.

Mycorrhizal fungi alleviate acidification-induced phosphorus limitation: Evidence from a decade-long field experiment of simulated acid deposition in a tropical forest in south China

South China has been experiencing very high rate of acid deposition and severe soil acidification in recent decades, which has been proposed to exacerbate the regional ecosystem phosphorus (P) limitation. We conducted a 10-year field experiment of simulated acid deposition to examine how acidification impacts seasonal changes of different soil P fractions in a tropical forest with highly acidic soils in south China. As expected, acid addition significantly increased occluded P pool but reduced the other more labile P pools in the dry season. In the wet season, however, acid addition did not change microbial P, soluble P and labile organic P pools. Acid addition significantly increased exchangeable Al³⁺ and Fe³⁺ and the activation of Fe oxides in both seasons. Different from the decline of microbial abundance in the dry season, acid addition increased ectomycorrhizal fungi and its ratio to arbuscular mycorrhiza fungi in the wet season, which significantly stimulated phosphomonoesterase activities and likely promoted the dissolution of occluded P. Our results suggest that, even in already highly acidic soils, the acidification-induced P limitation could be alleviated by stimulating ectomycorrhizal fungi and phosphomonoesterase activities. The differential responses and microbial controls of seasonal soil P transformation revealed here should be implemented into ecosystem biogeochemical model for predicting plant productivity under future acid deposition scenarios.

Contribution of Incorporating the Phosphorus Cycle into TRIPLEX-CNP to Improve the Quantification of Land Carbon Cycle

Phosphorus (P) is a key and a limiting nutrient in ecosystems and plays an important role in many physiological and biochemical processes, affecting both terrestrial ecosystem productivity and soil carbon storage. However, only a few global land surface models have incorporated P cycle and used to investigate the interactions of C-N-P and its limitation on terrestrial ecosystems. The overall objective of this study was to integrate the P cycle and its interaction with carbon (C) and nitrogen (N) into new processes model of TRIPLEX-CNP. In this study, key processes of the P cycle, including P pool sizes and fluxes in plant, litter, and soil were integrated into a new model framework, TRIPLEX-CNP. We also added dynamic P:C ratios for different ecosystems. Based on sensitivity analysis results, we identified the phosphorus resorption coefficient of leaf (rpleaf) as the most influential parameter to gross primary productivity (GPP) and biomass, and determined optimal coefficients for different plant functional types (PFTs). TRIPLEX-CNP was calibrated with 49 sites and validated against 116 sites across eight biomes globally. The results suggested that TRIPLEX-CNP performed well on simulating the global GPP and soil organic carbon (SOC) with respective R2 values of 0.85 and 0.78 (both p < 0.01) between simulated and observed values. The R2 of simulation and observation of total biomass are 0.67 (p < 0.01) by TRIPLEX-CNP. The overall model performance had been improved in global GPP, total biomass and SOC after adding the P cycle comparing with the earlier version. Our work represents the promising step toward new coupled ecosystem process models for improving the quantifications of land carbon cycle and reducing uncertainty.

Microbial mediators of plant community response to long-term N and P fertilization: Evidence of a role of plant responsiveness to mycorrhizal fungi

Climate changes and anthropogenic nutrient enrichment widely threaten plant diversity and ecosystem functions. Understanding the mechanisms governing plant species turnover across nutrient gradients is crucial to developing successful management and restoration strategies. We tested whether and how soil microbes, particularly arbuscular mycorrhizal fungi (AMF), could mediate plant community response to a 15 years long-term N (0, 4, 8, and 16 g N m^-2  year^-1 ) and P (0 and 8 g N m^-2  year^-1 ) enrichment in a grassland system. We found N and P enrichment resulted in plant community diversity decrease and composition change, in which perennial C₄  graminoids were dramatically reduced while annuals and perennial forbs increased. Metabarcoding analysis of soil fungal community showed that N and P changed fungal diversity and composition, of which only a cluster of AMF identified by the co-occurrence networks analysis was highly sensitive to P treatments and was negatively correlated with shifts in percentage cover of perennial C₄  graminoids. Moreover, by estimating the mycorrhizal responsiveness (MR) of 41 plant species in the field experiment from 264 independent tests, we found that the community weighted mean MR of the plant community was substantially reduced with nutrient enrichment and was positively correlated with C₄  graminoids percentage cover. Both analyses of covariance and structural equation modeling indicated that the shift in MR rather than AMF composition change was the primary predictor of the decline in perennial C₄  graminoids, suggesting that the energy cost invested by C₄ plants on those sensitive AMF might drive the inferior competitive abilities compared with other groups. Our results suggest that shifts in the competitive ability of mycorrhizal responsive plants can drive plant community change to anthropogenic eutrophication, suggesting a functional benefit of mycorrhizal mutualism in ecological restoration following climatic or anthropogenic degradation of soil communities.

Phosphorus homeostasis: acquisition, sensing, and long-distance signaling in plants

Phosphorus (P), an essential nutrient required by plants often becomes the limiting factor for plant growth and development. Plants employ various mechanisms to sense the continuously changing P content in the soil. Transcription factors, such as SHORT ROOT (SHR), AUXIN RESPONSE FACTOR19 (ARF19), and ETHYLENE-INSENSITIVE3 (EIN3) regulate the growth of primary roots, root hairs, and lateral roots under low P. Crop improvement strategies under low P depend either on improving P acquisition efficiency or increasing P utilization. The various phosphate transporters (PTs) are involved in the uptake and transport of P from the soil to various plant cellular organelles. A plethora of regulatory elements including transcription factors, microRNAs and several proteins play a critical role in the regulation of coordinated cellular P homeostasis. Among these, the well-established P starvation signaling pathway comprising of central transcriptional factor phosphate starvation response (PHR), microRNA399 (miR399) as a long-distance signal molecule, and PHOSPHATE 2 (PHO2), an E2 ubiquitin conjugase is crucial in the regulation of phosphorus starvation responsive genes. Under PHR control, several classes of PHTs, microRNAs, and proteins modulate root architecture, and metabolic processes to enable plants to adapt to low P. Even though sucrose and inositol phosphates are known to influence the phosphorus starvation response genes, the exact mechanism of regulation is still unclear. In this review, a basic understanding of P homeostasis under low P in plants and all the above aspects are discussed.

Intercropping of Leguminous and Non-Leguminous Desert Plant Species Does Not Facilitate Phosphorus Mineralization and Plant Nutrition

More efficient use of soil resources, such as nitrogen (N) and phosphorus (P), can improve plant community resistance and resilience against drought in arid and semi-arid lands. Intercropping of legume and non-legumes can be an effective practice for enhancing P mineralization uptake, and plant nutrient status. However, it remains unclear how intercropping systems using desert plant species impact soil-plant P fractions and how they affect N and water uptake capacity. Alhagi sparsifolia (a legume) and Karelinia caspia (a non-legume) are dominant plant species in the Taklamakan Desert in Xinjiang Province, China. However, there is a lack of knowledge of whether these species, when intercropped, can trigger synergistic processes and mechanisms that drive more efficient use of soil resources. Thus, in a field experiment over two years, we investigated the impact of monoculture and intercropping of these plant species on soil-plant P fractions and soil-plant nutrients. Both plant species’ foliar nutrient (N, P, and K) concentrations were higher under monoculture than intercropping (except K in K. caspia). Nucleic acid P was higher in the monoculture plots of A. sparsifolia, consistent with higher soil labile P, while metabolic P was higher in monoculture K. caspia, associated with higher soil moderately labile Pi. However, both species had a higher residual P percentage in the intercropping system. Soils from monoculture and intercropped plots contained similar microbial biomass carbon (MBC), but lower microbial biomass N:microbial biomass phosphorus (MBN:MBP) ratio associated with reduced N-acetylglucosaminidase (NAG) activity in the intercropped soils. This, together with the high MBC:MBN ratio in intercropping and the lack of apparent general effects of intercropping on MBC:MBP, strongly suggest that intercropping improved microbe N- but not P-use efficiency. Interestingly, while EC and SWC were higher in the soil of the K. caspia monoculture plots, EC was significantly lower in the intercropped plots. Plants obtained better foliar nutrition and soil P mineralization in monocultures than in intercropping systems. The possible positive implications of intercropping for reducing soil salinization and improving soil water uptake and microbial N-use efficiency could have advantages in the long term and its utilization should be explored further in future studies.

The role of biodiversity in mitigating the effects of nutrient limitation and short-term rotations in plantations of subtropical China

Species diversity plays an essential role in enhancing ecosystem functions (EF) in both natural and plantation forests. However, we do not fully understand whether species diversity could maintain the sustainability of EFs in multiple-rotation plantations. Here, we hypothesized that tree species mixtures could mitigate declines in EFs along successive rotations, but could not maintain ecosystem multifunctionality. To test our hypothesis, we examined the effects of species diversity on four EFs, i.e., aboveground biomass (AGB), soil available nitrogen (SAN) and phosphorus (SAP), and soil organic matter (SOM), based on pure model simulation in plantations of subtropical China. The model fusion framework was set up by the integration of the process-based FORECAST and Multivariate Diversity-Interactions models. In the simulation, four local typical plantation tree species (two conifers, one evergreen broadleaf, and one deciduous N-fixing broadleaf) were selected and combined to form four monoculture and 11 mixture stands, and for each stand, the simulation was made for four 25-year rotations. The results showed that all the four EFs declined with the progress of rotations in both monoculture and mixtures, and the declining range was larger in monoculture than in mixtures in each rotation. Particularly, SAP significantly decreased while AGB, SAN, and SOM increased with diversity evenness from 0 (monoculture) to 1 (four species being equal abundant in the mixture). Overall, SAP and AGB displayed higher sensitivity to the disturbance of successive rotations compared with SAN and SOM. These results suggest that mixing species could not maintain EFs along with successive rotations because it could not alleviate SAP deficiencies in the soils resulted from the disturbances of silvicultural measures.

Correlations between allocation to foliar phosphorus fractions and maintenance of photosynthetic integrity in six mangrove populations as affected by chilling

Chilling restrains the distribution of mangroves. We tested whether foliar phosphorus (P) fractions and gene expression are associated with cold tolerance in mangrove species. We exposed seedlings of six mangrove populations from different latitudes to favorable, chilling and recovery treatments, and measured their foliar P concentrations and fractions, photochemistry, nighttime respiration, and gene expression. A Kandelia obovata (KO; 26.45°N) population completely and a Bruguiera gymnorhiza (Guangxi) (BGG; 21.50°N) population partially (30%) survived chilling. Avicennia marina (24.29°N), and other B. gymnorhiza (26.66°N, 24.40°N, and 19.62°N) populations died after chilling. Photosystems of KO and photosystem I of BGG were least injured. During chilling, leaf P fractions, except nucleic acid P in three populations, declined and photoinhibition and nighttime respiration increased in all populations, with the greatest impact in B. gymnorhiza. Leaf nucleic acid P was positively correlated with photochemical efficiency during recovery and nighttime respiration across populations for each treatment. Relatively high concentrations of nucleic acid P and metabolite P were associated with stronger chilling tolerance in KO. Bruguiera gymnorhiza exhibited relatively low concentrations of organic P in favorable and chilling conditions, but its partially survived population showed stronger compensation in nucleic acid P and Pi concentrations and gene expression during recovery.

Global variation in the fraction of leaf nitrogen allocated to photosynthesis

Plants invest a considerable amount of leaf nitrogen in the photosynthetic enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO), forming a strong coupling of nitrogen and photosynthetic capacity. Variability in the nitrogen-photosynthesis relationship indicates different nitrogen use strategies of plants (i.e., the fraction nitrogen allocated to RuBisCO; fLNR), however, the reason for this remains unclear as widely different nitrogen use strategies are adopted in photosynthesis models. Here, we use a comprehensive database of in situ observations, a remote sensing product of leaf chlorophyll and ancillary climate and soil data, to examine the global distribution in fLNR using a random forest model. We find global fLNR is 18.2 ± 6.2%, with its variation largely driven by negative dependence on leaf mass per area and positive dependence on leaf phosphorus. Some climate and soil factors (i.e., light, atmospheric dryness, soil pH, and sand) have considerable positive influences on fLNR regionally. This study provides insight into the nitrogen-photosynthesis relationship of plants globally and an improved understanding of the global distribution of photosynthetic potential.

Plant phosphorus-acquisition and -use strategies affect soil carbon cycling

Increased anthropogenic nitrogen (N) deposition is driving N-limited ecosystems towards phosphorus (P) limitation. Plants have evolved strategies to respond to P limitation which affect N cycling in plant-soil systems. A comprehensive understanding of how plants with efficient P-acquisition or -use strategies influence carbon (C) and N cycling remains elusive. We highlight how P-acquisition/-use strategies, particularly the release of carboxylates into the rhizosphere, accelerate soil organic matter (SOM) decomposition and soil N mineralisation by destabilising aggregates and organic-mineral associations. We advocate studying the effects of P-acquisition/-use strategies on SOM formation, directly or through microbial turnover.

Global patterns and drivers of rainfall partitioning by trees and shrubs

Spatiotemporal redistribution of incident rainfall in vegetated ecosystems results from the partitioning by plants into intercepted, stemflow, and throughfall fractions. However, variation in patterns and drivers of rainfall partitioning across global biomes remains poorly understood, which limited the ability of climate models to improve the predictions of biome hydrological cycle under global climate change scenario. Here, we synthesized and analyzed the partitioning of incident rainfall into interception, stemflow, and throughfall by trees and shrubs at the global scale using 2430 observations from 236 independent publications. We found that (1) globally, median levels of relative interception, stemflow, and throughfall accounted for 21.8%, 3.2%, and 73.0% of total incident rainfall, respectively; (2) rainfall partitioning varied among different biomes, due to variation in plant composition, canopy structure, and macroclimate; (3) relative stemflow tended to be driven by plant traits, such as crown height:width ratio, basal area, and height, while relative interception and throughfall tended to be driven by plant traits as well as meteorological variables. Our global assessment of patterns and drivers of rainfall partitioning underpins the role of meteorological factors and plant traits in biome-specific ecohydrological cycles. We suggest to include these factors in climate models to improve the predictions of local hydrological cycles and associated biodiversity and function responses to changing climate conditions.

A research framework for projecting ecosystem change in highly diverse tropical mountain ecosystems

Tropical mountain ecosystems are threatened by climate and land-use changes. Their diversity and complexity make projections how they respond to environmental changes challenging. A suitable way are trait-based approaches, by distinguishing between response traits that determine the resistance of species to environmental changes and effect traits that are relevant for species' interactions, biotic processes, and ecosystem functions. The combination of those approaches with land surface models (LSM) linking the functional community composition to ecosystem functions provides new ways to project the response of ecosystems to environmental changes. With the interdisciplinary project RESPECT, we propose a research framework that uses a trait-based response-effect-framework (REF) to quantify relationships between abiotic conditions, the diversity of functional traits in communities, and associated biotic processes, informing a biodiversity-LSM. We apply the framework to a megadiverse tropical mountain forest. We use a plot design along an elevation and a land-use gradient to collect data on abiotic drivers, functional traits, and biotic processes. We integrate these data to build the biodiversity-LSM and illustrate how to test the model. REF results show that aboveground biomass production is not directly related to changing climatic conditions, but indirectly through associated changes in functional traits. Herbivory is directly related to changing abiotic conditions. The biodiversity-LSM informed by local functional trait and soil data improved the simulation of biomass production substantially. We conclude that local data, also derived from previous projects (platform Ecuador), are key elements of the research framework. We specify essential datasets to apply this framework to other mountain ecosystems.

Legacy effects of nitrogen and phosphorus additions on vegetation and carbon stocks of upland heaths

Soil carbon (C) pools and plant community composition are regulated by nitrogen (N) and phosphorus (P) availability. Atmospheric N deposition impacts ecosystem C storage, but the direction of response varies between systems. Phosphorus limitation may constrain C storage response to N, hence P application to increase plant productivity and thus C sequestration has been suggested. We revisited a 23-yr-old field experiment where N and P had been applied to upland heath, a widespread habitat supporting large soil C stocks. At 10 yr after the last nutrient application we quantified long-term changes in vegetation composition and in soil and vegetation C and P stocks. Nitrogen addition, particularly when combined with P, strongly influenced vegetation composition, favouring grasses over Calluna vulgaris, and led to a reduction in vegetation C stocks. However, soil C stocks did not respond to nutrient treatments. We found 40% of the added P had accumulated in the soil. This study showed persistent effects of N and N + P on vegetation composition, whereas effects of P alone were small and showed recovery. We found no indication that P application could mitigate the effects of N on vegetation or increase C sequestration in this system.

Low phosphorus supply constrains plant responses to elevated CO2 : A meta-analysis

Phosphorus (P) is an essential macro-nutrient required for plant metabolism and growth. Low P availability could potentially limit plant responses to elevated carbon dioxide (eCO₂ ), but consensus has yet to be reached on the extent of this limitation. Here, based on data from experiments that manipulated both CO₂ and P for young individuals of woody and non-woody species, we present a meta-analysis of P limitation impacts on plant growth, physiological, and morphological response to eCO₂ . We show that low P availability attenuated plant photosynthetic response to eCO₂ by approximately one-quarter, leading to a reduced, but still positive photosynthetic response to eCO₂ compared to those under high P availability. Furthermore, low P limited plant aboveground, belowground, and total biomass responses to eCO₂ , by 14.7%, 14.3%, and 12.4%, respectively, equivalent to an approximate halving of the eCO₂ responses observed under high P availability. In comparison, low P availability did not significantly alter the eCO₂ -induced changes in plant tissue nutrient concentration, suggesting tissue nutrient flexibility is an important mechanism allowing biomass response to eCO₂ under low P availability. Low P significantly reduced the eCO₂ -induced increase in leaf area by 14.3%, mirroring the aboveground biomass response, but low P did not affect the eCO₂ -induced increase in root length. Woody plants exhibited stronger attenuation effect of low P on aboveground biomass response to eCO₂ than non-woody plants, while plants with different mycorrhizal associations showed similar responses to low P and eCO₂ interaction. This meta-analysis highlights crucial data gaps in capturing plant responses to eCO₂ and low P availability. Field-based experiments with longer-term exposure of both CO₂ and P manipulations are critically needed to provide ecosystem-scale understanding. Taken together, our results provide a quantitative baseline to constrain model-based hypotheses of plant responses to eCO₂ under P limitation, thereby improving projections of future global change impacts.

Long-term nitrogen loading alleviates phosphorus limitation in terrestrial ecosystems

Increased human-derived nitrogen (N) deposition to terrestrial ecosystems has resulted in widespread phosphorus (P) limitation of net primary productivity. However, it remains unclear if and how N-induced P limitation varies over time. Soil extracellular phosphatases catalyze the hydrolysis of P from soil organic matter, an important adaptive mechanism for ecosystems to cope with N-induced P limitation. Here we show, using a meta-analysis of 140 studies and 668 observations worldwide, that N stimulation of soil phosphatase activity diminishes over time. Whereas short-term N loading (≤5 years) significantly increased soil phosphatase activity by 28%, long-term N loading had no significant effect. Nitrogen loading did not affect soil available P and total P content in either short- or long-term studies. Together, these results suggest that N-induced P limitation in ecosystems is alleviated in the long-term through the initial stimulation of soil phosphatase activity, thereby securing P supply to support plant growth. Our results suggest that increases in terrestrial carbon uptake due to ongoing anthropogenic N loading may be greater than previously thought.

Effects of elevated carbon dioxide and elevated temperature on morphological, physiological and anatomical responses of Eucalyptus tereticornis along a soil phosphorus gradient

Eucalypts are likely to play a critical role in the response of Australian forests to rising atmospheric CO2 concentration ([CO2]) and temperature. Although eucalypts are frequently phosphorus (P) limited in native soils, few studies have examined the main and interactive effects of P availability, [CO2] and temperature on eucalypt morphology, physiology and anatomy. To address this issue, we grew seedlings of Eucalyptus tereticornis Smith across its P-responsive range (6-500 mg kg-1) for 120 days under two [CO2] (ambient: 400 μmol mol-1 (Ca) and elevated: 640 μmol mol-1 (Ce)) and two temperature (ambient: 24/16 °C (Ta) and elevated: 28/20 °C (Te) day/night) treatments in a sunlit glasshouse. Seedlings were well-watered and supplied with otherwise non-limiting macro- and micro-nutrients. Increasing soil P supply increased growth responses to Ce and Te. At the highest P supplies, Ce increased total dry mass, leaf number and total leaf area by ~50%, and Te increased leaf number by ~40%. By contrast, Ce and Te had limited effects on seedling growth at the lowest P supply. Soil P supply did not consistently modify photosynthetic responses to Ce or Te. Overall, effects of Ce and Te on growth, physiological and anatomical responses of E. tereticornis seedlings were generally neutral or negative at low soil P supply, suggesting that native tree responses to future climates may be relatively small in native low-P soils in Australian forests.

Nitrogen and Phosphorus effect on Sun-Induced Fluorescence and Gross Primary Productivity in Mediterranean Grassland

Sun-Induced fluorescence at 760 nm (F760) is increasingly being used to predict gross primary production (GPP) through light use efficiency (LUE) modeling, even though the mechanistic processes that link the two are not well understood. We analyzed the effect of nitrogen (N) and phosphorous (P) availability on the processes that link GPP and F760 in a Mediterranean grassland manipulated with nutrient addition. To do so, we used a combination of process-based modeling with Soil-Canopy Observation of Photosynthesis and Energy (SCOPE), and statistical analyses such as path modeling. With this study, we uncover the mechanisms that link the fertilization-driven changes in canopy nitrogen concentration (N%) to the observed changes in F760 and GPP. N addition changed plant community structure and increased canopy chlorophyll content, which jointly led to changes in photosynthetic active radiation (APAR), ultimately affecting both GPP and F760. Changes in the abundance of graminoids, (%graminoids) driven by N addition led to changes in structural properties of the canopy such as leaf angle distribution, that ultimately influenced observed F760 by controlling the escape probability of F760 (Fesc). In particular, we found a change in GPP–F760 relationship between the first and the second year of the experiment that was largely driven by the effect of plant type composition on Fesc, whose best predictor is %graminoids. The P addition led to a statistically significant increase on light use efficiency of fluorescence emission (LUEf), in particular in plots also with N addition, consistent with leaf level studies. The N addition induced changes in the biophysical properties of the canopy that led to a trade-off between surface temperature (Ts), which decreased, and F760 at leaf scale (F760leaf,fw), which increased. We found that Ts is an important predictor of the light use efficiency of photosynthesis, indicating the importance of Ts in LUE modeling approaches to predict GPP.