P-poor Soil Research Articles

Phosphorus drives adaptive shifts in membrane lipid pools and synthesis between soils

Phospholipids are key constituents of microbial membranes and account for a substantial fraction of microbial P, while P-free betaine lipids provide a pathway for membrane synthesis independent of P. The aim of this study was to test if modulation of relative amounts of phospholipids vs P-free betaine lipids is a response to P availability. To examine responses of membrane lipids to P availability, we added P or P + C to an extremely P-poor sandstone-derived soil (total P = 41 μg g−1) and a higher P shale-derived soil (total P = 201 μg g−1). By using D2O labelling and LC-MS we were able to disentangle membrane lipid synthesis dependent on P (i.e. 2H incorporation into phospholipids) from membrane lipid synthesis independent of P (i.e. 2H incorporation into P-free betaine lipids). In unamended soils, P-free betaine lipids were almost 60% of the membrane lipid pool of the P-poor sandstone-derived soil, but only 10% of the higher-P shale-derived soil. Hence, to produce a unit of membrane lipid, the microbial population of sandstone soil required only 54% as much P as the microbial population of shale soil. Four days after adding P to the sandstone soil, phospholipids accounted for a larger proportion of the pool and synthesis of membrane lipids while pool and synthesis of P-free betaine lipids decreased. The increased allocation to phospholipids in sandstone soil amended with P likely reflected amelioration of P limitation given that P addition did not affect pools or synthesis of phospholipids versus betaine lipids in the higher-P shale soil. Collectively these data support the idea that having membranes with a high proportion of P-free betaine lipids is a key adaptive trait of microbial populations of P-poor soil, while the plasticity afforded by membrane lipid remodelling enables short-term fine-tuning of membrane lipid composition to P availability.

Leaf manganese and phenolics as proxies of soil acidification and phosphorus acquisition mechanisms in lentil cultivars on alkaline soil.

Leaf manganese (Mn) concentration has been used as a proxy for root exudation and phosphorus (P) uptake under controlled conditions, but there are limited field studies that confirm its validity. On an alkaline, P-poor soil, four lentil cultivars ('Samos', 'Thessaly', 'Flip', 'Algeria') received two P rates (0 and 26.2kgPha-1 ), for two growing seasons, to study whether aboveground assessments [leaf P, Mn, phenolic concentration (TPhe)] can approximate rhizosphere physiological traits related to P acquisition [soil acidification (ΔpH), arbuscular mycorrhizal fungi (AMF) colonisation, acid phosphatase activity (APase)]. Phosphorus addition had no effect on the determined traits. Cultivars differed in leaf P, Mn, TPhe and AMF, but there was no clear pattern relating aboveground traits to rhizosphere traits related to P acquisition, thus not confirming that leaf Mn can be a proxy of root exudation. Of three growth stages [V 7-8, R1 (first bloom), R4 (flat pod)], R1 seemed to be critical, showing the highest leaf P, ΔpH, AMF and TPhe. Precipitation and temperatures over the growing season were determinants of lentil responses affecting rhizosphere activity, soil P availability and finally leaf traits. In conclusion, in lentil on alkaline and P-limiting soils, high leaf Mn and phenolic concentration are not reliable indicators of rhizosphere P-acquiring mechanisms.

Weathering of calcareous bedrocks is strongly affected by the activity of soil microorganisms

Phosphorus (P) availability in calcareous forest soils is commonly low compared to siliceous soils. The main reason for this is that phosphate ions tend to precipitate with calcium (Ca). Weathering of calcareous rocks and the potential of microorganisms to dissolve calcareous parent material is not fully understood. Therefore, we examined microbial carbonate dissolution and the abundance of phosphorus-solubilizing bacteria in temperate forest soils with contrasting calcareous parent materials. We incubated soil extracts with weathered parent materials (i.e., dolomite and limestone) from two calcareous forest soils differing in P content and determined the rates of P and Ca solubilization. In addition, we determined the abundance of phosphorus-solubilizing bacteria (PSB). We found that the net Ca solubilization rate ranged from 8.8 to 511.1 nmol m−2 d−1 across both soils and depths. Calcium dissolution rates were negatively related to pH and positively related to the concentration of organic acids. The gross P solubilization rates were on average 63.6% higher from dolomite (P-poor soil) than from limestone (P-rich soil). The abundance of soil PSB ranged from 3.8 % at the limestone site (P-rich soil) to 24.4 % at the dolomite site (P-poor soil). The higher abundance of PSB in the soil derived from dolomite is in line with the high Ca and P solubilization rates at this site, indicating that PSB abundance is related to rock weathering rates from calcareous soils. Pseudomonadales and Enterobacteriales were by far the two most abundant bacterial orders in the PSB community of both soils and soil depths. In conclusion, this study shows, first, that weathering of calcareous bedrocks is strongly affected by the activity of soil microorganisms, and second, that there is likely a selective pressure in P-poor soils towards a higher abundance of PSB.

Microbial tradeoffs in internal and external use of resources regulated by phosphorus and carbon availability

A general strategy in modern agriculture to reduce phosphorus (P) fertilization is to rely on microbial efficiency of P acquisition and recycling from organic sources. However, this involves extracellular enzymes that require energy from ATP, so the process depends on the microbes’ physiological state and soil P availability. To elucidate the key relationships we compared P acquisition processes in P-poor soil (Cambisol) and links between C:P stoichiometry, enzyme activity, and ATP with microbial communities in contrasting activity states (dormancy, growth followed by starvation and gradually activated, respectively induced by no, single large (50 μg C g−1 soil) and multiple low (five days of 10 μg C g−1 soil day−1) additions of glucose as a carbon (C) source).A sole P input, without C addition, almost doubled microbial C (Cmic) contents, maintained stable phosphatase activity at 36 nmol h−1 per nmol ATP and raised microbial P (Pmic) 2.7-fold. In contrast, sole glucose addition increased Pmic by only 8%, confirming that P-limitation was much stronger than C limitation. Only 5–10 % of P potentially mineralized by phosphatase was recovered as microbial P. Cmic:Pmic ratios in microbial biomass <200 and >350 respectively reflected C starvation and strong P starvation. The ATP was a suitable predictor of microbial biomass in soil lacking fresh substrate, but weak predictor of microbial biomass after substrate input.Structural equation models revealed contrasting strategies of P utilization depending on microbial activity state. Dormant microorganisms (without glucose addition) invested most P to ATP production. In contrast, following substrate addition P-limited microorganisms accelerated phosphatase production, and hence capacity to mine P in organic sources. Thus, the P utilization/acquisition strategies depended on C accessibility and were modulated by P availability.

Fate of low-molecular-weight organic phosphorus compounds in the P-rich and P-poor paddy soils

Continuous application of organic fertilizers can cause accumulation of organic phosphorus (P) in soil, especially in the low-molecular-weight organic phosphorus (LMWOP) forms. This organic P pool represents a potentially important source of P for both plants and microorganisms. To understand the effect of long-term fertilization (30 years) (P-rich soil) vs. fallowing (P-poor soil) on the bioavailability and fate of LMWOP in subtropical paddy soils, we determined the sorption and mineralization of 14C-labeled adenosine, adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP) in each soil. The contents of carbon, nitrogen, and P in the P-rich soil were more than two times greater than those in the P-poor soil. The mineralization rates of the LMWOP compounds were faster in the P-rich soil compared to the P-poor soil, and followed the order AMP>ADP>ATP. Using sterilized soil, all forms of adenosine-P were strongly sorbed to the solid phase and reached saturation in a short time, with the adsorbance increasing with the number of phosphate groups. We concluded that the mineralization of LMWOP compounds was repressed slightly by sorption to the solid phase, but only in the short term. Thus, LMWOP compounds serve as readily available sources of C for microorganisms, making P available for themselves as well as for the plants. However, P accumulation and the progressive saturation of the P sorption sites in highly fertile soils may increase the potential risk of P runoff.

Leaching of Phosphomonoesterase Activities in Beech Forest Soils: Consequences for Phosphorus Forms and Mobility

Phosphomonoesterases play an important role in the soil phosphorus (P) cycle since they hydrolyze P monoester to phosphate. Their activity is generally measured in soil extracts, and thus, it remains uncertain how mobile these enzymes are and to which extent they can be translocated within the soil profile. The presence of phosphomonoesterases in soil solutions potentially affects the share of labile dissolved organic P (DOP), which in turn would affect P leaching. Our study aimed at assessing the production and leaching of phosphomonoesterases from organic layers and topsoil horizons in forest soils and its potential effect on dissolved P forms in leachates obtained from zero-tension lysimeters. We measured phosphomonoesterase activities in leached soil solutions and compared it with those in water extracts from litter, Oe/Oa, and A horizons of two beech forests with a contrasting nitrogen (N) and P availability, subjected to experimental N × P fertilization. In addition, we determined phosphate and DOP. In soil solutions leached from litter, Oe/Oa, and A horizons, phosphomonoesterase activities ranged from 2 to 8 μmol L–1 h–1 during summer, but remained below detection limits in winter. The summer values represent 0.1–1% of the phosphomonoesterase activity in soil extracts, indicating that enzymes can be translocated from organic layers and topsoils to greater soil depths. Activities of phosphomonoesterases obtained by water extracts were greater in the organic layer of the P-poor site, while activities of those in soil solutions were similar at the two sites. Nitrogen addition increased phosphomonoesterase activities in leached soil solutions of the organic layer of the N- and P-poor soil. Using a modeling approach, we estimated that approx. 76% of the initial labile DOP was hydrolyzed to dissolved inorganic P within the first 24 h. Back calculations from measured labile DOP revealed an underestimation of approx. 15% of total dissolved P, or 0.03 mg L–1. The observed leaching of phosphomonoesterases implies that labile organic P could be hydrolyzed in deeper soil horizons and that extended sample storage leads to an underestimation of the contribution of DOP to total dissolved P leaching. This has been neglected in the few field studies measuring DOP leaching.

Young navel orange rootstock improves phosphorus absorption from poorly soluble pools through rhizosphere processes

An increased plant capacity to efficiently use the poorly available pools will be very helpful in citrus production. Information on the various mechanisms evolved by citrus rootstocks to optimize soil phosphorus (P) access is limited. Thus, the objectives of this study were: To evaluate P acquisition efficiency (PAE) in Thomson Navel Orange (Citrus sinensis L. Osbeck.) plants grafted on three rootstocks and adaptive mechanisms developed by each to these rootstocks to increase P uptake in one calcareous soil. For this purpose, a pot experiment was conducted in a completely randomized design with three replications. Treatments consisted of three rootstocks of citrus, including Sour Orange (SO), Swingle Citrumelo (SC) and Troyer Citrange (TC). 18 months after planting seedlings in pots, plants were harvested and the rhizosphere soils were separated. Dissolved organic carbon (DOC), the population of fungi and bacteria, microbial biomass P (MBP), alkaline (AlP) and acid (AcP) phosphatases, available P (P-Olsen), and P fractionation according to the modified Hedley’ sequential fractionation procedure were determined in the rhizosphere soils. After the harvest, P acquisition efficiency of plants (PAE) was determined. The results of this study showed that SO promoted higher scion P uptake and increased PAE. The fungal and bacterial population, MBP, AcP in the SO rhizosphere soils was greater than the SC and TC rhizosphere soils (P < 0.05). P availability closes the root surface of SO rootstock was higher in comparison to the other rootstocks. Moreover, the effect of the rhizosphere on various fractions of P was under rootstock control. The highest amount of exchangeable P and Fe and Al soil P in the rhizosphere soils was found in SO grafting, while the lowest P associated with Ca compounds and residual P in the rhizosphere soils was found in SO-grafting. This necessarily implies the P mobilization of poorly available pools are vital mechanisms for P acquisition by plants on SO in this P-poor soil. In conclusion, the results of the present study showed that different PAE of rootstocks may be associated with the various mechanisms of action of each rootstock involved in P absorption.

The Importance of Root Interactions in Field Bean/Triticale Intercrops.

To highlight the contribution of belowground interactions to biomass and N and P yields, field bean and triticale were grown in a P-poor soil as sole crops and as replacement intercrops at two N levels. The shoots were always in contact, while the roots of adjacent rows were free to interact or were completely separated. This allowed simultaneous testing the intraspecific and interspecific competition between rows, which to our knowledge has not been studied before. Root biomass, distribution in soil, morphometry, and functional traits were determined, together with the nodule number and biomass. The Land Equivalent Ratio for shoot biomass and N and P yield were higher than 1 when roots were in contact, and markedly lower when they were separated. This demonstrates the positive contribution of root interactions, which in field bean, consisted of increased root elongation without changes in biomass and nutrient status; in triticale, of increased N and P uptake efficiency and reduced biomass partitioning to roots. The soil-plant processes underlying intercrop advantage led to complementarity in N sources with low N inputs and facilitated N and P uptake with high N inputs, which demonstrates that intercropping could be profitable in both low and high input agriculture.

Microbial solubilization of silicon and phosphorus from bedrock in relation to abundance of phosphorus-solubilizing bacteria in temperate forest soils

Biogeochemical weathering of bedrock is the most important input of silicon (Si) and phosphorus (P) to forest ecosystems. While soil microbes, and in particular P-solubilizing bacteria (PSB), are known to accelerate the solubilization of Si and P from silicate rocks, our understanding of the mechanisms driving biogenic weathering are still limited. To fill this gap, incubation experiments with weathered parent materials (i.e. basalt, andesite and paragneiss) of four soils and water extracts of the four soils, differing in P stocks, were conducted. We found that the net Si solubilization rate ranged from 5.0 (±1.8) to 91.0 (±2.4) nmol m−2 d−1 across all examined soils. The silicate dissolution rates were negatively related to the decrease in pH (ΔpH) and positively related to the amounts of organic acids released by microbes. We found that the gross P solubilization rates from the parent materials were ~11 times higher at the P-rich site (BBR) compared to the P-poor site (CON). In addition, we determined the abundance and the taxonomic composition of PSB communities in the four soils. The abundance of PSB ranged from 2% at the P-rich site to 22.1% at the P-poor site, indicating that a selective pressure exists in P-poor soils towards a higher abundance of P-solubilizers. Yet, despite the relative high abundance of PSB, the gross P solubilization rates were low in the soils derived from P-poor parent material. The genus Pseudomonas was found only in the PSB community at the P-poor site. Burkholderiales and Bacillales together were by far the two most abundant orders among the PSB communities in all soils and depths. In conclusion, this study shows that PSB are more abundant in P-poor soil than in P-rich soil, while the weathering rate seems to be mostly dependent on the bedrock.

In or Out of Equilibrium? How Microbial Activity Controls the Oxygen Isotopic Composition of Phosphate in Forest Organic Horizons With Low and High Phosphorus Availability

While there are estimates of the abiotic processes contribution to soil phosphorus (P) availability, less is known about the contribution of biological processes. Two main enzymatic processes involved in soil P cycling are known to alter the oxygen isotopic composition of phosphate (δ18O-P), each in a different way, through the cleavage of the P–O bond: the intracellular P turnover and the organic P hydrolysis. The former induces isotopic equilibration between phosphate and water and is considered the major process affecting soil available P via microbial P release. The latter induces depleted δ18O-P in the phosphate released from the mineralization of organic P. We studied P dynamics in organic horizons of two contrasting soils (low- and high-P availability) from temperate beech forests. We labelled the soil with 18O-enriched water and followed changes in the δ18O-P of different soil P pools in the presence or absence of added leaf litter during three months of incubation. δ18O-P values of almost all P pools progressively increased indicating oxygen incorporation from the enriched soil water into phosphate via the above-mentioned enzymatic processes. δ18O-P of available P increased more in the P-rich soil than in the P-poor soil and approached the isotopic equilibrium between phosphate and water, revealing the impact of microbial P release into the available P. However, in the P-poor soil, the available P brought the signature induced by phosphatase enzymes, indicating that it was mostly originated from the hydrolysis of organic P. Therefore, a below-equilibrium signature can be still an indicator of control of microbial P on the available P via Po hydrolysis reactions. Finally, two independent isotopic approaches with 33P and δ18O-P provided very similar estimates of P exchanged between the available P and other inorganic soil pools. This suggests that δ18O-P can be successfully used to trace P fluxes, provided that the underlying processes do not break the P – O bonds of the phosphate molecule.

Sulfur, Na2-EDTA and their mixture effects on phosphorus release from cow bone char in P-poor sandy soil

Abstract The high cost and environmental problems related to the production of phosphate fertilizers as well as their excessive and continuous applications represent great challenges in modern agriculture. Therefore, we conducted an incubation experiment using sandy soil to evaluate the effects of elemental sulfur, disodium ethylene diamine tetra-acetic acid (Na2-EDTA) and their mixture with cow bone char as well as incubation periods on changes of phosphorus (P) availability, water soluble P, soil pH, electrical conductivity (EC), soluble calcium (Ca) and soluble sulfate (SO4). Applying sulfur (S), Na2EDTA and their mixture with cow bone char improved significantly phosphorus availability in soil, Olsen-P value increased from 7.65 mg kg−1 (control) to 12.20 mg kg−1 (cow bone char + sulfur), 12.80 mg kg−1 (cow bone char + Na2-EDTA) and 13.07 mg kg−1 (cow bone char + sulfur + Na2-EDTA) at the end of incubation periods While, cow bone char + sulfur + Na2-EDTA treatment lead Olsen-P to decline significantly with increasing incubation periods. Amending soil by sulfur with cow bone char treatment caused significant increase in the Olsen-P with increasing incubation periods. Cow bone char + sulfur and cow bone char + Na2-EDTA showed non-significant differences in Olsen-P. Because of the increase in phosphate fertilizer prices, the results of this study demonstrated the great potential of cow bone char usage which is a promising way to improve phosphorus availability in P-poor soil as well as being a clean alternative and renewable source of phosphate fertilizer. Consequently, we recommend utilizing cow bone char with sulfur as slow release fertilizer with time due to their low cost and it is a safe source of phosphorus in sustainable agriculture, rather than using Na2-EDTA due to its high price.

C/P stoichiometry of dying rice root defines the spatial distribution and dynamics of enzyme activities in root-detritusphere

As the primary microbial substrate after shoot cutting, the element stoichiometry of root-detritus (dying or dead roots) influences the enzyme activity in root-detritusphere. However, the effect of the C/P ratio of root-detritus on the dynamics and distribution of enzyme activities is little revealed. We hypothesised that P fertilisation would decrease the C/P ratio of root-detritus, therefore affecting the hotspot areas and hot moments of C-acquiring and P-acquiring enzyme activities, as well as their activity ratio (C/P acquisition ratio). Root-detritus of low (59.0) and high (170.8) C/P ratios was produced in P-poor soil with and without P fertilisation, respectively. In situ soil zymography showed that the distribution of C-acquiring enzymes (β-glucosidase and cellobiohydrolase) was more associated with root-detritus than P-acquiring enzymes (acid and alkaline phosphomonoesterase). P fertilisation increased the hotspot areas of C-acquiring enzyme activities over the experiment, without influencing their temporal dynamics. However, its effect on phosphomonoesterase activities depended on the decomposition and delayed the appearance of the highest hotspot areas. P supply met the microbial demand in P-fertilised soil, with high C/P acquisition ratio and constant stoichiometry of microbial biomass C (MBC)/microbial biomass P (MBP). A low C/P acquisition ratio and high MBC/MBP in non-fertilised soil was observed, indicating P limitation for microorganisms. After the 150-day incubation, Olsen P significantly increased in P-fertilised soil (P < 0.05), whereas it decreased in the root-detritusphere of non-fertilised soil. We conclude that the decomposition of root-detritus with a low C/P ratio has potential to improve soil P availability; however, C-P imbalance may increase during the decomposition of root-detritus with a high C/P ratio.

Rare taxa of alkaline phosphomonoesterase-harboring microorganisms mediate soil phosphorus mineralization

As a homologous gene encoding microbial alkaline phosphomonoesterase, the expression of phoD is critically controlled by P availability and thus contributes to the mineralization of soil organic P under P-depleted condition. However, its role in the regulation of soil P turnover is largely unknown due to the complex coupling of physiochemical and biological processes in the P cycle, especially in paddy field. We hypothesized that 1) P fertilization would decrease the abundance of phoD gene and change the composition of phoD-harboring microbial community and 2) the high abundance of phoD-harboring microorganisms in P-poor soil would stimulate the synthesis of alkaline phosphomonoesterase, thus mitigating P limitation via the mineralization of organic P. After 42 days of rice growth, the phoD abundance negatively correlated with soil P availability, and it was significantly higher in non-fertilized treatments than in P-fertilized treatments for both rhizosphere and bulk soils. A stronger competition among phoD-harboring microorganisms was detected in non-fertilized soil than in P-fertilized soil, with Bradyrhizobium, Methylobacterium, and Methylomonas being the dominant taxa in all samples. However, the high phoD gene abundance under P-poor condition was mainly due to the growth of rare operational taxonomic units (OTUs) affiliated to Actinobacteria and Cyanobacteria (relative abundance < 3%). Consistent with our hypothesis, the growth of phoD-harboring microorganisms stimulated the hydrolysis of organic P in non-fertilized soil. However, in the P-fertilized treatments, the increase in OTU abundance was accompanied by the depletion of exchangeable P and accumulation of microbial biomass P. Our findings suggest that phoD-harboring microorganisms have the potential to immobilize P in biomass when the supply is sufficient while mineralize organic P under P-poor condition, during which the rare taxa play an important role.

Forest Soil Phosphorus Resources and Fertilization Affect Ectomycorrhizal Community Composition, Beech P Uptake Efficiency, and Photosynthesis.

Phosphorus (P) is an important nutrient, whose plant-available form phosphate is often low in natural forest ecosystems. Mycorrhizal fungi mine the soil for P and supply their host with this resource. It is unknown how ectomycorrhizal communities respond to changes in P availability. Here, we used young beech (Fagus sylvatica L.) trees in natural forest soil from a P-rich and P-poor site to investigate the impact of P amendment on soil microbes, mycorrhizas, beech P nutrition, and photosynthesis. We hypothesized that addition of P to forest soil increased P availability, thereby, leading to enhanced microbial biomass and mycorrhizal diversity in P-poor but not in P-rich soil. We expected that P amendment resulted in increased plant P uptake and enhanced photosynthesis in both soil types. Young beech trees with intact soil cores from a P-rich and a P-poor forest were kept in a common garden experiment and supplied once in fall with triple superphosphate. In the following summer, labile P in the organic layer, but not in the mineral top soil, was significantly increased in response to fertilizer treatment. P-rich soil contained higher microbial biomass than P-poor soil. P treatment had no effect on microbial biomass but influenced the mycorrhizal communities in P-poor soil and shifted their composition toward higher similarities to those in P-rich soil. Plant uptake efficiency was negatively correlated with the diversity of mycorrhizal communities and highest for trees in P-poor soil and lowest for fertilized trees. In both soil types, radioactive P tracing (H333PO4) revealed preferential aboveground allocation of new P in fertilized trees, resulting in increased bound P in xylem tissue and enhanced soluble P in bark, indicating increased storage and transport. Fertilized beeches from P-poor soil showed a strong increase in leaf P concentrations from deficient to luxurious conditions along with increased photosynthesis. Based on the divergent behavior of beech in P-poor and P-rich forest soil, we conclude that acclimation of beech to low P stocks involves dedicated mycorrhizal community structures, low P reserves in storage tissues and photosynthetic inhibition, while storage and aboveground allocation of additional P occurs regardless of the P nutritional status.

Dynamics of phosphorus nutrition, allocation and growth of young beech (Fagus sylvatica L.) trees in P-rich and P-poor forest soil.

To investigate how long-lived forest trees cope with low soil phosphorus (P) availabilities, we characterized P nutrition of beech (Fagus sylvatica, L.) in soils from P-rich and P-poor beech forests throughout an annual growth cycle. Young trees were excavated with intact soil cores in mono-specific beech forests, kept under common garden conditions, and used for 33P labeling, analyses of P uptake, P content and biomass during five phenological stages (dormancy in winter, bud swelling in early spring, mature leaves in early and late summer, and senescent leaves in fall). Seasonal allocation patterns showed that young, emerging leaves were preferred sinks for P under P-poor conditions, thereby keeping foliar P concentrations at levels similar to those of trees grown in P-rich soil. Phosphorus concentrations in stems and roots of trees from the P-poor conditions were lower than those from P-rich conditions. Coarse roots were the main P storage tissue, supplying inorganic P to newly formed leaves, originating from the inorganic and organic P pools under low and high P conditions, respectively. Beech trees in P-poor soil exhibited net biomass increment early in the annual growth along with a strong P deficit, which was replenished by enhanced uptake in late summer and fall. Trees in P-rich soil grew until late summer, and showed a moderate P decline in organic pools and recovery late in fall, which coincided with elevated P uptake from soil. Beech in P-poor soil produced more biomass per unit of P but at a slower growth rate than those in P-rich soil, thereby exhibiting similar P-use efficiencies. Temporal decoupling of growth and P acquisition in combination with internal P trade-off between storage tissues and leaves facilitated flexible acclimation of beech to a wide range of soil P availabilities.

Root-associated fungal microbiota of nonmycorrhizal Arabis alpina and its contribution to plant phosphorus nutrition

Most land plants live in association with arbuscular mycorrhizal (AM) fungi and rely on this symbiosis to scavenge phosphorus (P) from soil. The ability to establish this partnership has been lost in some plant lineages like the Brassicaceae, which raises the question of what alternative nutrition strategies such plants have to grow in P-impoverished soils. To understand the contribution of plant-microbiota interactions, we studied the root-associated fungal microbiome of Arabis alpina (Brassicaceae) with the hypothesis that some of its components can promote plant P acquisition. Using amplicon sequencing of the fungal internal transcribed spacer 2, we studied the root and rhizosphere fungal communities of A. alpina growing under natural and controlled conditions including low-P soils and identified a set of 15 fungal taxa consistently detected in its roots. This cohort included a Helotiales taxon exhibiting high abundance in roots of wild A. alpina growing in an extremely P-limited soil. Consequently, we isolated and subsequently reintroduced a specimen from this taxon into its native P-poor soil in which it improved plant growth and P uptake. The fungus exhibited mycorrhiza-like traits including colonization of the root endosphere and P transfer to the plant. Genome analysis revealed a link between its endophytic lifestyle and the expansion of its repertoire of carbohydrate-active enzymes. We report the discovery of a plant-fungus interaction facilitating the growth of a nonmycorrhizal plant under native P-limited conditions, thus uncovering a previously underestimated role of root fungal microbiota in P cycling.

Phosphorus resorption by young beech trees and soil phosphatase activity as dependent on phosphorus availability.

Motivated by decreasing foliar phosphorus (P) concentrations in Fagus sylvatica L. forests, we studied P recycling depending on P fertilization in mesocosms with juvenile trees and soils of two contrasting F. sylvatica L. forests in a greenhouse. We hypothesized that forests with low soil P availability are better adapted to recycle P than forests with high soil P availability. The P resorption efficiency from senesced leaves was significantly higher at the P-poor site (70%) than at the P-rich site (48%). P fertilization decreased the resorption efficiency significantly at the P-poor site to 41%, while it had no effect at the P-rich site. Both acid and alkaline phosphatase activity were higher in the rhizosphere of the P-poor than of the P-rich site by 53 and 27%, respectively, while the activities did not differ in the bulk soil. Fertilization decreased acid phosphatase activity significantly at the P-poor site in the rhizosphere, but had no effect on the alkaline, i.e., microbial, phosphatase activity at any site. Acid phosphatase activity in the P-poor soil was highest in the rhizosphere, while in the P-rich soil, it was highest in the bulk soil. We conclude that F. sylvatica resorbed P more efficiently from senescent leaves at low soil P availability than at high P availability and that acid phosphatase activity in the rhizosphere but not in the bulk soil was increased at low P availability. Moreover, we conclude that in the P-rich soil, microbial phosphatases contributed more strongly to total phosphatase activity than plant phosphatases.

Phosphorus addition enhances loss of nitrogen in a phosphorus-poor soil

Plants and microbes have limited stoichiometric flexibility to take up and store nitrogen (N) and phosphorus (P). Variation in the relative availability of N and P to plants and microbes may therefore affect how strongly N and P are held in terrestrial ecosystems with important implications for net primary productivity and carbon sequestration. We hypothesized that an increase in P availability in a P-poor soil would increase N uptake by plants and microbes thereby reducing N loss. We grew mixtures of the C3 grass Phalaris aquatica L. and the legume Medicago sativa L. in mesocosms with soils low in P availability and then used a novel technique by adding a 15N tracer with and without 1 g P m−2 to soil with different moisture and available N conditions, and measured the 15N recovery after 48 h in microbes, plants and soil. In contrast to our hypothesis, we found that P addition reduced 15N in microbes without water stress by 80% and also reduced total15N recovery, particularly without water stress. Water stress in combination with N addition further showed low total 15N recovery, possibly because of reduced plant uptake thereby leaving more 15N in the soil available for nitrification and denitrification. Our results suggest that P addition can result in large gaseous N loss in P-poor soils, most likely by directly stimulating nitrification and denitrification.

Heterogeneity in spatial P-distribution and foraging capability by Zea mays: effects of patch size and barriers to restrict root proliferation within a patch.

Localized proliferation of roots in nutrient-enriched patches seems to be an adaptive response in many plants, but its function is still debatable. To understand the efficiency and limitation of foraging behaviour, the impact of patch size and the presence or absence of a barrier to root proliferation within phosphorus (P)-enriched patches was examined. In pots filled with P-poor soil, six treatments of heterogeneous P supply were prepared: three patch sizes with or without a root barrier between patches. In addition, a homogeneous P supply treatment was also prepared. Irrespective of these treatments, each pot received the same total amount of P. Maize (Zea mays) was grown in each pot for 45 d in a greenhouse. P content and biomass were greatest in plants grown in the largest patch due to successful root proliferation, and were higher in the presence of a root barrier. Interestingly, plants preferentially developed adventitious nodal roots projecting from the stem into the P-enriched soil, particularly in the largest patch with a root barrier. Removal of the barrier reduced the P-uptake capacity per unit root surface area or volume in P-enriched patches, revealing that the P-uptake capacity per root can be suppressed even in P-rich soil if other portions on the root axis encounter P-poor conditions. The results suggest that the efficiency of root morphological plasticity is largely determined by the size of the P-enriched patch. Furthermore, the results imply a novel aspect of P-uptake physiology that roots in heterogeneous P cannot demonstrate their potential capacity, as would be observed in roots encountering P continuously; this effect is probably mediated by an internal root factor.

PHOSPHORUS EFFICIENCY OF BORNEAN RAIN FOREST PRODUCTIVITY: EVIDENCE AGAINST THE UNIMODAL EFFICIENCY HYPOTHESIS

Plant communities on nutrient-poor soils are thought to use nutrients more efficiently to produce biomass than plant communities on nutrient-rich soils. Yet, increased efficiency with declining soil nutrients has not been demonstrated empirically in lowland tropical rain forests, where plant growth is thought to be strongly limited by soil nutrients, especially phosphorus (P). We tested for higher P uptake and use efficiency across a 16-fold soil P gradient in lowland Borneo by measuring the P content of aboveground net primary productivity (fine litter production plus new tree growth; ANPP) for 24 months. Extractable soil P was positively related to litter production, tree growth, and ANPP. Efficiency of P response (ANPP/available soil P), uptake (P uptake/available soil P), and use (ANPP/P uptake) increased monotonically with declining soil P and was significantly higher on P-rich soil than P-poor soil. Increased P uptake and use efficiency with declining soil P enabled higher than expected plant produc...