P-poor Soils Research Articles

Direct foliar acquisition of desert dust phosphorus fertilizes forest trees despite reducing photosynthesis.

Phosphorus (P) availability to forest trees is often limited by local soil conditions that increase its fixation to soil minerals. In certain regions, atmospheric-P inputs can compensate for low soil-P availability. Among atmospheric-P sources, desert dust is the most dominant. However, the effects of desert dust on P nutrition and its uptake mechanisms by forest trees are currently unknown. We hypothesized that forest trees that naturally grow on P-poor soils or soils with high soil-P fixation capacity can acquire P from desert dust deposited on their leaves via direct foliar uptake, bypassing the soil, thus promoting tree growth and productivity. We performed a controlled greenhouse experiment with three forest tree species: Palestine Oak (Quercus calliprinos) and Carob (Ceratonia siliqua), native to the NE edge of the Saharan desert, and Brazilian peppertree (Schinus terebinthifolius), native to the Atlantic Forest in Brazil, which is located on the western part of the trans-Atlantic Saharan dust route. To simulate natural dust deposition events, the trees had desert dust applied directly upon their foliage and were monitored for growth and final biomass, P levels, leaf surface pH and the rate of photosynthesis. The dust treatment increased the P concentration significantly by 33-37% in Ceratonia and Schinus trees. On the other hand, trees that received the dust displayed a 17-58% reduction in biomass, probably related to particle coverage of the leaf surface that inhibited photosynthesis by 17-30%. Overall, our findings show that direct P uptake from desert dust can be an alternative P uptake pathway for multiple tree species under P-deficient conditions, with implications for forest trees' P economy.

Variability in mycorrhizal status of plant species is much larger within than between plots in grassland and coastal habitats.

Community-level studies linking plant mycorrhizal status to environment usually do not account for within-plot mycorrhizal status variability; thus, patterns of plant mycorrhizal status diversity are largely unknown. Here, we assessed the relative importance of within- and between-plot variability components in mycorrhizal status and examined how plant mycorrhizal status diversity is related to soil nutrient availability. We hypothesised larger between-plot variability in mycorrhizal status and higher plant mycorrhizal status diversity in P-poor soils. To test these hypotheses, we used plant phylogenies, vegetation, soil and plant mycorrhizal status data from Czech semi-natural grasslands and Scottish coastal habitats. We divided plant mycorrhizal status diversity into divergence and evenness and tested their relations to soil P, K, Ca and Mg. Within-plot variability component of mycorrhizal status was always, on average, at least 2.2 times larger than between-plot variability in our datasets. Plant mycorrhizal status divergence was positively related to Ca (in both datasets) and Mg (only in grasslands and when accounting for phylogeny). In grasslands, the relationship between Mg and plant mycorrhizal status evenness was negative when accounting for phylogeny, while it was positive when not accounting for phylogeny. Plant mycorrhizal status diversity was not linked to P and its relation to K was inconsistent. Our results suggest that high Ca in the soil can promote coexistence of mycorrhizal, facultatively mycorrhizal and non-mycorrhizal plant species. We encourage future studies to also focus on within-plot variability in mycorrhizal status, because it appears to be highly relevant in herbaceous systems.

Seasonal variation in AMF colonisation, soil and plant nutrient content in gypsum specialist and generalist species growing in P-poor soils

AimsGypsum soils are P-limited atypical soils that harbour a rich endemic flora. These singular soils are usually found in drylands, where plant activity and soil nutrient availability are seasonal. No previous studies have analysed the seasonality of P nutrition and its interaction with the arbuscular mycorrhiza fungi (AMF) colonisation in gypsum plants. Our aim was to evaluate the seasonal changes in plant nutrient status, AMF colonisation and rhizospheric soil nutrient availability in gypsum specialist and generalist species.MethodsWe evaluated seasonal variation in the proportion of root length colonised by AMF structures (hyphae, vesicules and arbuscules), plant nutrient status (leaf C, N and P and fine root C and N) and rhizospheric soil content (P, organic matter, nitrate and ammonium) of three gypsum specialists and two generalists throughout a year.ResultsAll species showed arbuscules within roots, including species of Caryophyllaceae and Brassicaceae. Root colonisation by arbuscules (AC) was higher in spring than in other seasons, when plants showed high leaf P-requirements. Higher AC was decoupled from inorganic N and P availability in rhizospheric soil, and foliar nutrient content. Generalists showed higher AC than specialists, but only in spring.ConclusionsSeasonality was found in AMF colonisation, rhizospheric soil content and plant nutrient status. The mutualism between plants and AMF was highest in spring, when P-requirements are higher for plants, especially in generalists. However, AMF decoupled from plant demands in autumn, when nutrient availability increases in rhizospheric soil.

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.

Plant Species Rather than Elevated Atmospheric CO2 Impact Rhizosphere Properties and Phosphorus Fractions in a Phosphorus-Deficient Soil

By 2050, elevated atmospheric CO2 (eCO2) could stimulate plant growth, but dwindling phosphorus (P) stocks in the soil could limit growth. However, little is known about how eCO2 could affect soil P availability and dynamics in P-poor soils. Here, we conducted a 6-week pot experiment where three plant species were grown in a low-P soil under ambient (390 ppm) and eCO2 (700 ppm) to investigate plant growth, rhizosphere properties, and changes in soil P fractions. Our results showed that under P deficiency, plant biomass, P uptake, and rhizosphere properties did not respond to eCO2. Changes were noted by plant species. Compared to the control soil (unplanted pots), rhizosphere pH decreased the most under wheat, while microbial biomass P was higher under blue lupin. Among plant species, the blue lupin rhizosphere exhibited higher acid and alkaline phosphatase activity as well as organic anion release. Soil P fractions were impacted by plant species but similar across CO2 treatments. Blue lupin accumulated labile organic P while depleted moderately labile organic P. Accumulation of labile organic P could be ascribed to microbial P immobilisation, whereas the mineralization of moderately labile organic P was associated with higher phosphatase activity. Wheat depleted acid extractable inorganic P the most, probably due to soil acidification and higher root biomass. These results suggest that plants can mobilise different P fractions irrespective of their chemical availability using morphological and/or physiological adaptations. However, these adaptations to acquire P from a low-P soil were not affected by eCO2. This implies that current P fertiliser recommendations to boost or maintain crop production in low-P soils would remain unchanged under future eCO2.

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.

Organic Nutrients Induced Coupled C- and P-Cycling Enzyme Activities During Microbial Growth in Forest Soils

Besides environmental and soil physical drivers, the functional properties of microbial populations, i.e. growth rate, enzyme production and maintenance requirements are dependent on the microbes’ environment. The soil nutrition status and the quantity and quality of the substrate input, both infer different growth strategies of microorganisms. It is uncertain, how enzyme systems respond during the different phases of microbial growth and retardation in soil. The objective of this study was to uncover the changes of microbial functioning and their related enzyme systems in nutrient-poor and nutrient-rich beech forest soil during the phases of microbial growth. We determined microbial growth via kinetic approach by substrate-induced respiratory response of microorganisms, enabling the estimation of total and growing biomass of the microbial community. To induce microbial growth we used glucose, while yeast extract simulated additional input of nutrients and factors indicating microbial residues (i.e. necromass compounds). Microbial growth on glucose showed a 12–18 h delay in associated enzyme activity increase or the absence of distinct activity responses (Vmax). β-glucosidase and chitinase (NAG) demonstrated clear differences of Vmax in time and between P-rich and P-poor soils. However, during microbial growth on glucose + yeast extract, the exponential increase in enzymatic activity was clearly stimulated accompanied by a delay of 8–12 h, smoothing the differences in nutrient-acquisition dynamics between the two soils. Furthermore, cross-correlation of β-glucosidase and acid phosphatase between the two sites demonstrated harmonized time constraints, which reflected the establishment of comparable and balanced enzymatic systems within the decomposition network. The network accelerated nutrient acquisition to maintain microbial growth, irrespective of the contrasting soil properties and initial nutrient stocks, indicating similar tradeoffs of C- and P- cycling enzymes in both soils. This reflects comparable temporal dynamics of activities in nutrient-poor and nutrient-rich soil when the glucose + yeast extract was added. During lag phase and phase of exponential microbial growth, substrate turnover time of all enzymes was shortened in nutrient-poor forest soil exclusively, reflecting that the quality of the added substrate strongly matters during all stages of microbial growth in soil.

Fine Root Size and Morphology of Associated Hyphae Reflect the Phosphorus Nutrition Strategies of European Beech Forests

Roots are among the major controls of nutrient and C cycles and together with mycorrhizal fungi they are assumed to play a key role especially in P nutrition of forest ecosystems. Current publications emphasized that size distribution of fine roots reflects the crucial impact of roots on biogeochemical cycles. However, we know hardly anything about the spatial distribution of fine root size classes and their specific surface as well as distribution of mycorrhizal fungi among different size classes in undisturbed soils. We used a novel method based on epifluorescence microscopy to analyze fine roots in undisturbed soil samples. We expected that based on these analyses proposed differences between P-rich and P-poor soils get clearer than based on routine methods for fine root analysis. This was examined at two European beech forests in Germany at silicate rock that differ in P supply by the parent material. We analyzed the fine root frequency, size distribution and surface based on resin impregnated cross sections taken from undisturbed soil samples. Fine roots were classified according to their size and morphology of associated ectomycorrhiza with epifluorescence microscopy. More than 82% of the absorbing surface was associated with roots having a diameter smaller than 100µm. The fine root surface area present per square meter of soil was 388m² at the P-poor site and 220m² at the P-rich site. In addition, percentage of mycorrhization of fine root surface was 47% at P-poor site and only 38% at P-rich site. The biggest root length density and the highest absorbing fine root surface with mycorrhization and abundance of extramatrical mycelia was observed in forest floor at the P-poor site and in subsoil at P-rich site. Our results confirm that beech trees adapt their root traits according to P availability showing higher absorbing surface at the P-poor site compared to the P-rich site. In contrast, at the soil-profile scale, rooting density and mycorrhization increased with P availability thereby showing an efficient way of root and mycorrhizal fungi placement. Overall, distinct differences in fine root traits, between the P-rich and P-poor site were most evident for fine roots smaller than 100μm.

Soil microbial populations substitute phospholipids with betaine lipids in response to low P availability

Soil microbes can be limited by phosphorus availability. A significant proportion of cellular P is accounted for by the phospholipids that comprise membranes. Experiments with aquatic microbial communities and isolated microbes have shown one way microbes can reduce their cellular P requirements is by substituting phospholipids with P-free polar lipids. We do not know if soil microbial communities can also spare P by using P-free lipids instead of phospholipids.The aims of this study were to examine a range of contrasting soils and a) determine the profile of polar fatty-acid-based lipids; and b) examine if P-free polar lipids are more abundant in P-poor soils. To obtain a broad range in P availability we examined soils from 16 sites that encompassed a range of geologies and vegetation types. Intact lipids were identified and quantified using liquid-chromatography-mass spectrometry. An independent estimate of polar lipid classes was obtained by hydrolysing lipids and quantifying headgroups with capillary electrophoresis-mass spectrometry.Among sites soil P status varied from supra-optimal (N:P = 2.4 g g−1) through to severely deficient (N:P = 80 g g−1). The dominant polar lipids among sites were phosphatidylethanolamine (PE), phosphatidylcholine (PC), and the betaine lipid diacylglyceryl-N,N,N-trimethylhomoserine (DGTS). Among sites as P availability declined a larger proportion of polar lipids was accounted for by betaine lipids such that in the most severely P-deficient soils phospholipids were only 61% of total polar lipids. If we assume that phospholipids account for one-third of cellular P, the measured substitution of phospholipids with betaine lipids would reduce the amount of P required for growth by around 10%. The substantial P saving due to lipid substitution could contribute to the selection of taxa under P deficiency.

Glutamate dehydrogenase is essential in the acclimation of Virgilia divaricata, a legume indigenous to the nutrient-poor Mediterranean-type ecosystems of the Cape Fynbos

Glutamate dehydrogenase (NAD(H)- GDH, EC 1.4.1.2) is an important enzyme in nitrogen (N) metabolism. It serves as a link between C and N metabolism, in its role of assimilating ammonia into glutamine or deaminating glutamate into 2-oxoglutarate and ammonia. GDH may also have a key in the N assimilation of legumes growing in P-poor soils. Virgilia divaricata is such a legume, growing in the nutrient limited soils of the mediterranean-type Cape fynbos ecosystem. In order to understand the role of GDH in the nitrogen nutrition of V. divaricata, the aim of this study was to identify the GDH gene transcripts, their relative expressions and enzyme activity in P-stressed roots and nodules during N metabolism. During P deficiency there was a reduction in total plant biomass as well as total plant P concentration. The analysis of the GDH cDNA sequences in V. divaricata revealed the presence of GHD1 and GHD2 subunits, these corresponding to the GDH1, GDH-B and GDH3 genes of legumes and non-legume plants. The relative expression of GDH1 and GDH2 genes in the roots and nodules, indicates that two the subunits were differently regulated depending on the organ type, rather than P supply. Although both transcripts appeared to be ubiquitously expressed in the roots and nodules, the GDH2 transcript evidently predominated over those of GDH1. Furthermore, the higher expression of both GDH transcripts in the roots than nodules, suggests that roots are more reliant on on GDH in P-poor soils, than nodules. With regards to GHD activity, both aminating and deaminating GDH activities were differently affected by P deficiency in roots and nodules. This may function to assimilate N and regulate internal C and N in the roots and nodules. The variation in GDH1 and GDH2 transcript expression and GDH enzyme activities, indicate that the enzyme may be regulated by post-translational modification, instead of by gene expression during P deficiency in V. divaricata.

Arbuscular mycorrhizae and absence of cluster roots in the Brazilian Proteaceae Roupala montana Aubl.

Plants growing on soils poor in phosphorus (P) develop P-acquisition strategies such as symbiotic associations with arbuscular mycorrhizal fungi (AMF). In very poor soils, cluster roots, a non-symbiotic alternative strategy enables plants to extract P uptake by developing modified roots. The latter strategy is characteristic (if not a derived trait) of the Southern Hemisphere Proteaceae, which are thus non-mycorrhizal. The Proteaceae have been studied mainly in Australia, where they are very diverse, especially on very P-poor soils. We investigated the presence of cluster roots and/or AMF in the Proteaceae Roupala montana Aubl. from three areas of the Brazilian Cerrado. This is, a seasonal neotropical savanna on highly weathered soils characterised by high aluminium content, low pH, and very low available P. We discovered that R. montana forms arbuscular mycorrhiza and no cluster roots were observed. All the plantlets collected were mycorrhizal. We also evaluated the fertility of the soil (especially the P availability). It was found that R. montana grows in soils containing more than 220 mg kg−1 total P. Thus, they are, more fertile than in most of Australian soils and likely have sufficient available P to support plant nutrition by way of mycorrhizae. Further research should investigate whether other Brazilian, and more generally non-Australian, Proteaceae species can establish associations with AMF, and the link with soil P availability. Our findings have implications for the phylogenetic patterns of loss of symbiosis with AMF within the Proteaceae.

Embolism resistance drives the distribution of Amazonian rainforest tree species along hydro-topographic gradients.

Species distribution is strongly driven by local and global gradients in water availability but the underlying mechanisms are not clear. Vulnerability to xylem embolism (P50 ) is a key trait that indicates how species cope with drought and might explain plant distribution patterns across environmental gradients. Here we address its role on species sorting along a hydro-topographical gradient in a central Amazonian rainforest and examine its variance at the community scale. We measured P50 for 28 tree species, soil properties and estimated the hydrological niche of each species using an indicator of distance to the water table (HAND). We found a large hydraulic diversity, covering as much as 44% of the global angiosperm variation in P50 . We show that P50 : contributes to species segregation across a hydro-topographic gradient in the Amazon, and thus to species coexistence; is the result of repeated evolutionary adaptation within closely related taxa; is associated with species tolerance to P-poor soils, suggesting the evolution of a stress-tolerance syndrome to nutrients and drought; and is higher for trees in the valleys than uplands. The large observed hydraulic diversity and its association with topography has important implications for modelling and predicting forest and species resilience to climate change.

Drying and rewetting foster phosphorus depletion of forest soils

Phosphorus (P) is a key nutrient but still we have a limited knowledge on the controls of mobilization and fluxes of P in forest soils. Our study explored the linkages between P mobilization in organic horizons and mineral soils and the P status of soils, as affected by two consecutive drying and rewetting (D/W) cycles. We sampled litter layers (Oi), mixed Oe-Oa horizons, and A horizons in three beech forests along a P availability gradient in Germany. Carbon mineralization and release of dissolved organic matter (DOC, DOP) and dissolved inorganic P (DIP) were studied in microcosms exposed to an initial harsh drying (40 °C for 72 h) and a moderate dry spell (1 month at 20 °C). In Oi horizons, net P mineralization decreased with decreasing P status despite a similar C mineralization at all sites. This supports the general concept that the stoichiometric difference between substrate and microbial biomass primarily drives P release from decomposing organic matter. Counterintuitively, P mobilization per unit soil P increased towards P-poor sites in the mineral soil, likely due to decreasing contents of reactive secondary minerals and the consequently smaller P sorption. Drying and rewetting caused stronger mobilization of DIP and DOP (+108% on average) than of DOC (+51%). The parallel decline in specific UV absorptivity of DOM suggests that lysis of microbial cells drove the drought-induced P release. The D/W effects on P mobilization were particularly strong in P-poor soils, where greater portions of P are bound to microbial biomass, which are prone to become released upon rewetting. Since mobilized P can potentially be leached from soils, our findings indicate, that drought-induced P mobilization fosters the progressive P depletion of already P-poor soils. The possible P leaching losses from mineral soils seem rather be driven by soil mineralogy than by P status.

Relationship between acid phosphatase activity and P concentration in organs of Cyclopia and Aspalathus species, and a non-legume of the Cape Floristic Region

The role of tissue acid phosphatase (APase) activity of legumes and non-legumes in their P nutrition and adaptation to low-P soils is not well understood. To better understand this, a relationship between APase activity and P concentration in leaves, stems, roots and nodules of legumes, Cyclopia and Aspalathus and a non-legume, Leucadendron strictum, all native to the P-poor soils of the Cape fynbos biome, was assessed. Plants were collected and each separated into leaves, stems and roots. Phosphatase enzyme activity was assayed in soil using the p-nitrophenol method, while soil P and shoot P were measured using ICP-MS. To measure tissue APase activity, an acetate buffer was added into ground plant material and contents filtered. An acetate buffer and a p-nitrophenyl solution were added to the supernatant and contents incubated. After incubation, NaOH (0.5 M) was added and absorbance read at 405 nm. At Koksrivier, Cyclopia genistoides exhibited the highest leaf enzyme activity whilst Aspalathus aspalathoides showed the highest enzyme activity in the stems. At both Kleinberg and Kanetberg, Cyclopia subternata and Cyclopia longifolia showed the highest APase activity in leaves, followed by stems and lowest in roots. P concentration closely mirrored enzyme activity in organs of all test species from each site. APase activity positively correlated with P concentration in organs of all the test Cyclopia and Leucadendron species, indicating that intracellular APase activity is directly linked to P mobilization and translocation in these species. Percentage of N derived from fixation was positively correlated with tissue APase activity in C. genistoides (r = 0.911*), A. aspalathoides (r = 0.868*) and Aspalathus caledonensis (r = 0.957*), suggesting that APase activity could be directly or indirectly linked to symbiotic functioning in these fynbos legumes, possibly via increased P supply to sites of N₂ fixation.

Evaluating groundnut (<i>Arachis hypogaea</i> L.) performance due to genotype and phosphorus source on p-poor alfisols in two Nigerian agro-ecologies

Legumes require substantial quantities of phosphorus (P) for effective growth and yield. Under deficient conditions, P fertilisation will improve these. Field experiments were conducted in 2016 cropping season, at two Nigerian agro-ecologies, to evaluate effects of genotype and P source on yield of groundnuts grown on P-poor soils. The experiments were conducted at two experimental sites of the Institute for Agricultural Research, Ahmadu Bello University, Zaria (Minjibir and Samaru). The treatments consisted of 16 groundnut genotypes and three P sources. The P source was laid out in main- and genotype in sub-plot of a split plot design with four replicates. Data on pods, haulms and chlorophyll content (CC) were observed. There was no significant (P>0.05) difference between the genotypes in terms of pod yield, although higher pod yield was recorded at Samaru than Minjibir. SAMNUT 22 recorded the highest haulm yield (27.85 g plant-1) among all the genotypes, while SAMNUT 21 (18.57 g plant-1) and SAMNUT 24 (18.18 g plant-1), which were statistically similar, recorded the least haulm yield. Highest CC was recorded in SAMNUT 23 (40.39 mg plant-1), which was at par with ICGV-IS 07083 (39.06 mg plant-1) while Kwankwaso had the lowest (34.18 mg plant-1) CC. Higher CC and haulm yield were recorded at Minjibir than at Samaru. The P source significantly contributed to dry haulm (P<0.05) and pod (P<0.01) yields, but not in terms of CC (P>0.05).Keywords: Alfisols, genotype, haulm yield, Nigerian agro-ecologies, pod yield, P poor

Effect of P stoichiometry on the abundance of nitrogen-cycle genes in phosphorus-limited paddy soil

Previous studies have shown that phosphorus addition to P-limited soils increases gaseous N loss. A possible explanation for this phenomenon is element stoichiometry (specifically of C:N:P) modifying linked nutrient cycling, leading to enhanced nitrification and denitrification. In this study, we investigated how P stoichiometry influenced the dynamics of soil N-cycle functional genes. Rice seedlings were planted in P-poor soils and incubated with or without P application. Quantitative PCR was then applied to analyze the abundance of ammonia-oxidizing (amoA) and denitrifying (narG nirK, nirS, nosZ) genes in soil. P addition reduced bacterial amoA abundance but increased denitrifying gene abundance. We suggest this outcome is due to P-induced shifts in soil C:P and N:P ratios that limited ammonia oxidization while enhancing P availability for denitrification. Under P application, the rhizosphere effect raised ammonia-oxidizing bacterial abundance (amoA gene) and reduced nirK, nirS, and nosZ in rhizosphere soils. The change likely occurred through greater C input and O2 release from roots, thus altering C availability and redox conditions for microbes. Our results show that P application enhances gaseous N loss potential in paddy fields mainly through stimulating denitrifier growth. We conclude that nutrient availability and elemental stoichiometry are important in regulating microbial gene responses, thereby influencing key ecosystem processes such as denitrification.

Substrate Chemistry and Rainfall Regime Regulate Elemental Composition of Tree Leaves in Karst Forests

Forests on calcareous substrates constitute a large fraction of the vegetation in Puerto Rico. Plant growth on these substrates may be affected by nutrient deficiencies, mainly P and Fe, resulting from high pH and formation of insoluble compounds of these elements. The occurrence of these forests in humid and dry areas provides an opportunity to compare nutrient relations, water use efficiency, and N dynamics, using biogeochemical parameters. We selected sites under humid climate in the north, and dry climate in the southwest of Puerto Rico. Adult, healthy leaves of species with high importance values were collected at each site and analyzed for their elemental composition and the natural abundance of C and N isotopes. Calcium was the dominant cation in leaf tissues, explaining over 70% of the ash content variation, and Al and Ca concentration were positively correlated, excepting only two Al-accumulating species. Karst vegetation consistently showed high N/P ratios comparable to forests on P-poor soils. Dry karst sites had significantly higher δ13C and δ15N ratios. We conclude that forests on karst are mainly limited by P availability, and that mechanisms of nutrient uptake in the rhizosphere lead to linear correlations in the uptake of Ca and Al. Isotope ratios indicate higher water use efficiency, and predominant denitrification in dry karst forest sites.

Phosphorus in Preferential Flow Pathways of Forest Soils in Germany

The transport of nutrients in forest soils predominantly occurs along preferential flow pathways (PFP). This study investigated the composition of phosphorus (P) forms in PFPs and soil matrix in several temperate beech forests with contrasting soil P contents in Germany. The PFPs were visualized using dye tracer experiments. Stained and unstained soil was sampled from three profile cuts per plot and analyzed for P fractions. The results show that labile P concentrations were highest in the O-layer and had the same range of values at all sites (240–320 mg·kg−1), although total P (TP) differed considerably (530–2330 mg·kg−1). The ratio of labile P to TP was significantly lower in the P-rich soil compared to the medium and P-poor soils. By contrast, the ratio of moderately labile P to TP was highest at the P-rich site. The shifts in P fractions with soil depth were generally gradual in the P-rich soil, but more abrupt at the others. The contents of labile and moderately labile P clearly differed in PFPs compared to soil matrix, but not statistically significant. The studied soils are characterized by high stone contents with low potential for P sorption. However, indications were found that labile organically bound P accumulates in PFPs such as biopores.

Denitrification and associated N2O emissions are limited by phosphorus availability in a grassland soil

Effects of phosphorus (P) availability on nitrogen (N) loss through microbial nitrification and denitrification processes remain poorly understood. We conducted an incubation experiment to study the responses of N2O production after P (0 and 3mgPkg−1 soil) addition with and without NH4+ or NO3− (0 and 15mgNkg−1 soil) in a high P-fixing grassland soil. The N addition included a 15N tracer (1mg15Nkg−1). In a separate experiment, we also investigated the effect of P availability on potential nitrification and denitrification. We hypothesised that the addition of P in the soil would increase N2O emission from microbial nitrification and denitrification and thereby less soil 15N-NH4+ and 15N-NO3− would be recovered. We found that P addition only significantly increased cumulative N2O emission and respiration in the NO3− treatment, suggesting stimulated activities of denitrifying microorganisms due to their relief from P limitation. However, a decrease in soil 15N recovery after P addition in the NO3− treatment was not detected because of the very small loss of N as N2O (<0.3% of the applied 15N tracer). Potential denitrification marginally increased with P addition, while potential nitrification was not affected by P addition. We conclude that P addition may increase gaseous loss of N as N2O in P-poor soils through microbial denitrification rather than nitrification when inorganic N is abundant.

Variable P supply affects N metabolism in a legume tree, Virgilia divaricata, from nutrient-poor Mediterranean-type ecosystems.

Virgilia divaricata Adamson is a forest margin legume that is known to invade the N- and P-poor soils of the mature fynbos, implying that it tolerates variable soil N and P levels. It is not known how the legume uses inorganic N from soil and atmospheric sources under variable P supply. Little is known about how P deficiency affects the root nodule metabolic functioning of V. divaricata and the associated energy costs of N assimilation. This study aimed to determine whether P deficiency affects the metabolic status of roots and nodules, and the impact on the routes of N assimilation in V. divaricata.V. divaricata had reduced biomass, plant P concentration and biological nitrogen fixation during P deficiency. Based on adenylate data, P-stressed nodules maintained their P status better than P-stressed roots. V. divaricata was able to alter C and N metabolism differently in roots and nodules under P stress. This was achieved via internal P cycling by possible replacement of membrane phospholipids with sulfolipids and galactolipids, and increased reliance on the pyrophosphate (PPi)-dependent metabolism of sucrose via UDP-glucose (UDPG) and to fructose-6-phosphate (Fru-6-P). P-stressed roots mostly exported ureides as organic N and recycled amino acids via deaminating glutamate dehydrogenase. In contrast, P-stressed nodules largely exported amino acids. Compared with roots, nodules showed more P conservation during low P supply. The roots and nodules of V. divaricata metabolised N differently during P stress, meaning that these organs may contribute differently to the success of this plant in soils from forest to fynbos.