Vertical distribution of biological and geochemical phosphorus subcycles in two southern Appalachian forest soils

We measured Al, Fe, and P fractions by horizon in two southern Appalachian forest soil profiles, and compared solution PO₄ ^–1 removal in chloroform-sterilized and non-sterilized soils, to determine whether biological and geochemical P subcycles were vertically stratified in these soils. Because organic matter can inhibit Al and Fe oxide crystallization, we hypothesized that concentrations of non-crystalline (oxalate-extractable) Al (Al₀) and Fe (Fe₀), and concomitantly P sorption, would be greatest in near-surface mineral (A) horizons of these soils.Al₀ and Fe₀ reached maximum concentrations in forest floor and near-surface mineral horizons, declined significantly with depth in the mineral soil, and were highly correlated with P sorption capacity. Small pools of readily acid-soluble (AF-extractable) and readily-desorbable P suggested that PO₄ ^3– was tightly bound to Al and Fe hydroxide surfaces. P sorption in CHCl₃-sterilized mineral soils did not differ significantly from P sorption in non-sterilized soils, but CHCl₃ sterilization reduced P sorption 40–80% in the forest floor. CHCl₃ labile (microbial) P also reached maximum concentrations in forest floor and near-surface mineral horizons, comprising 31–35% of forest floor organic P. Combined with previous estimates of plant root distributions, data suggest that biological and geochemical P subcycles are not distinctly vertically stratified in these soils. Plant roots, soil microorganisms, and P sorbing minerals all reach maximum relative concentrations in near-surface mineral horizons, where they are likely to compete strongly for PO₄ ^3– available in solution.     

The effects of flooding for rice culture on soil chemical properties and subsequent maize growth

Summary The effects of flooding and lowland rice culture on soil chemical properties and subsequent maize growth were investigated in two contrasting rice soils of S.E. Australia. The effects of incorporating rice straw, either during or after flooding were also studied. The experiment was conducted in a glasshouse with the use of large intact soil cores.Previous flooding markedly reduced maize growth, leaf P concentration and P uptake, despite the application of a large quantity of P fertilizer after drainage. Soil analyses showed that previous flooding increased the Langmuir sorption terms for maximum P sorption and bonding energy. The availability of P was more closely related to the bonding energy between soil and P than to the capacity of the soils to sorb P. The increases, in the P sorption parameters, were associated with decreases in the crystallinity of the free iron oxides as determined by their oxalate solubility. It was concluded that depressed P supply to maize sown in previously flooded soils was due to stronger P sorption by the drained soils, rather than to P immobilization during flooding.Rice plants grown during flooding reduced the amount of N available to the subsequent maize crop, but did not significantly affect P availability. Rice straw added during flooding did not affect subsequent maize growth, but when added after flooding caused microbial immobilization of N.Salts, Fe or Mn from previous flooding did not affect maize growth.     

Microbial responses to P addition in six South African forest soils

Forests growing on highly weathered soils are often phosphorus (P) limited and competition between geochemical and biological sinks affects their soil P dynamics. In an attempt to elucidate the factors controlling the relative importance of these two sinks, we investigated the relationship of between soil microbial growth kinetics and soil chemical properties following amendments with C, N and P in six South African forest soils. Microbial growth kinetics were determined from respiration curves derived from measurements of CO₂ effluxes from soil samples in laboratory incubations. We found that microbial growth rates after C + N additions were positively related to NaOH-extractable P and decreased with soil depth, whereas the lag time (the time between substrate addition and exponential growth) was negatively related to extractable P. However, the growth rate and lag time were unrelated to the soil’s sorption properties or Al and Fe contents. Our results indicate that at least some of the NaOH-extractable inorganic P may be biologically available within a relatively short time (days to weeks) and might be more labile than previously thought. Our results also show that microbial utilization of C + N only seemed to be constrained by P in the deeper part of the soil profiles.     

Phosphorus management for perennial crops in central Amazonian upland soils

The present contribution discusses the soil P status of central Amazonian upland soils, the effects of tree crops on soil P availability and the factors controlling soil P cycling in land use systems with tree crops. Soil fertility management has to target the prevalent P deficiency by adequate P fertilization, especially in southern and northern municipalities of central Amazônia where the largest areas with severe P deficiency are found. P fixation to clay minerals is not a major obstacle for P management in the highly weathered upland soils of the central Amazon due to their low Al- and Fe-oxide contents. Low total soil P amounts are mainly responsible for low P availability. Tree crops are found to be especially suitable for land use under low-P-input conditions. Their large P return to soil by litterfall and pruning improves soil P availability. Additionally, litter quality affects P release and soil P availability. Both aspects, quantity and quality effects, are strongly dependent on tree species. Phosphorus sorption does not seem to be reduced by different litter types confirming earlier results that P fixation is not a major problem in central Amazonian upland soils. In conclusion, biological approaches are more important than physical approaches to improve soil P availability in central Amazonian Oxisols. With large P cycling through soil microbial biomass and between plant and soil, a higher availability of added P can be maintained and P applications only need to replenish P exports by harvest. Low P additions will improve productivity also for long-term uptake by trees. This is of high importance in regions with poor infrastructure and the lack of financial resources.     

Iron Reduction and Soil Phosphorus Solubilization in Humid Tropical Forests Soils: The Roles of Labile Carbon Pools and an Electron Shuttle Compound

The affinity of iron oxides and hydroxides for phosphorus is thought to contribute to phosphorus limitation to net primary productivity in humid tropical forests on acidic, highly weathered soils. Perennially warm, humid conditions and high biological activity in these soils can result in fluctuating redox potential that in turn leads to considerable iron reduction in the presence of labile carbon and humic substances. We investigated the effects of reducing conditions in combination with the addition of labile carbon substrates (glucose and acetate) and an electron shuttle compound on iron reduction and phosphorus release in a humid tropical forest soil. Glucose or acetate was added to soils as a single dose at the beginning of the experiment, and as pulsed inputs over time, which more closely mimics patterns in labile carbon availability. Iron reduction and phosphorus mobilization were weakly stimulated by a single low level addition of carbon, and the addition of the electron shuttle compound with or without added carbon. Pulsed labile carbon additions produced a significant increase in soil pH, soluble iron, and phosphorus concentrations. Pulsed labile carbon inputs also promoted the precipitation of ferrous hydroxide complexes which could increase the capacity for P sorption, although our results suggest that rates of P solubilization exceeded re-adsorption. Plant and microbial P demand are also likely to serve as an important sinks for released P, limiting the role of P re-adsorption. Our results suggest that reducing conditions coupled with periodic carbon inputs can stimulate iron reduction and a corresponding increase in soil phosphorus mobilization, which may provide a source of phosphorus to plants and microorganisms previously undocumented in these ecosystems.     

Leaf litter inputs decrease phosphate sorption in a strongly weathered tropical soil over two time scales

In strongly weathered soils, leaf litter not only returns phosphorus (P) to the soil environment, it may also modify soil properties and soil solution chemistry, with the potential to decrease phosphate sorption and increase plant available P. Using a radioactive phosphate tracer (³²P) and 1 h laboratory incubations we investigated the effect of litter inputs on phosphate sorption over two time scales: (1) long-term field litter manipulations (litter addition, control and litter removal) and (2) pulses of litter leachate (i.e. water extracts of leaf litter) from five species. Leachate pulse effects were compared to a simulated throughfall, which served as a control solution. Soil receiving long-term doubling of leaf litter maintained five-fold more phosphate in solution than the litter removal soil. In addition to the quantity of phosphate sorbed, the field litter addition treatment decreased the strength of phosphate sorption, as evaluated through extraction of sorbed ³²P using a weakly acidic ammonium fluoride solution (Bray 1). In litter removal soil, leachate pulses significantly reduced phosphate sorption in comparison to the throughfall control for all five species evaluated. However, the ability of leachate pulses to reduce phosphate sorption decreased when soil had received field litter inputs. Across soils the effect of leachate pulses on phosphate sorption increased with net sorption of dissolved organic C, with the exception of leachate from one species that had a higher index of aromatic C concentration. These results demonstrate that litter inputs, as both long-term inputs and short-term leachate pulses, can decrease the quantity and strength of phosphate sorption, which may increase the biological availability of this key nutrient.     

Organic matter turnover in a sagebrush steppe landscape

Laboratory incubations of¹⁵N-amended soils from a sagebrush steppe in south-central Wyoming indicate that nutrient turnover and availability have complex patterns across the landscape and between microsites. Total and available N and P and microbial C and N were highest in topographic depressions characterized by tall shrub communities. Net and gross N mineralization rates and respiration were also highest in these areas, but microbial efficiencies expressing growth relative to respiration cost were highest in soils of exposed ridgetop sites (prostrate shrub communities). Similar patterns occurred between shrub and intershrub soils, with greater nutrient availability under shrubs, but lower microbial efficiencies under shrubs than between. Surface soils had higher soil nutrient pools and N mineralization rates than subsurface soils, but N and C turnover and microbial efficiencies were lower in those surface soils. All soils decreased in respiration, mineralization, and immobilization rates during the 30-day incubation period, apparently approaching a steady-state substrate use. Soil microbial activity of the high organic matter accumulation areas was apparently more limited by labile substrate.     

Soil organic sulfur dynamics in a coniferous forest

Sulfate microbial immobilization and the mineralization of organic S were measured in vitro in soil horizons (LFH, Ae, Bhf, Bf and C) of the Lake Laflamme watershed (47°17 N, 71°14 O) using ³⁵SO₄. LFH samples immobilized from 23 to 77% of the added ³⁵SO₄ within 2 to 11 days. The ³⁵SO₄ microbial immobilization increased with temperature and reached an asymptote after a few days. The mineral soil generally immobilized less than 20% of the added ³⁵SO₄, and an asymptote was reached after 2 days. An isotopic equilibrium was rapidly reached in mineral horizons. A two-compartment (SO₄ and organic S) model adequately described ³⁵SO₄ microbial immobilization kinetics. The active organic reservoir in the whole soil profile represented less than 1% of the total organic S. The average concentrations of dissolved organic S (DOS) in the soil solutions leaving the LFH, Bhf and Bf horizons were respectively 334, 282 and 143 µgL^–1. Assuming that the DOS decrease with soil depth corresponded to the quantities adsorbed in the B horizons, we estimated that 12 800 kgha^–1 of organic S could have been formed since the last glaciation, which is about 13 times the size of the actual B horizons reservoirs. Our results suggest that the organic S reservoirs present in mineral forest soils are mostly formed by the DOS adsorption resulting from incomplete litter decomposition in the humus layer. The capability of these horizons to immobilize SO₄ from the soil solution would be restricted to a 1% active fraction composed of microorganisms. Despite their refractory nature, these reservoirs can, however, be slowly decomposed by microorganisms and contribute to the S-SO₄ export from the watershed in the long term.     

Microbial biomass and respiration responses to nitrogen fertilization in a polar desert

How microbial communities respond to increases in available nitrogen (N) will influence carbon (C) and nutrient cycles. Most studies addressing N fertilization focus on mid-latitude ecosystems, where complex aboveground–belowground interactions can obscure the response of the soil microbial community, and little is known about how soil microbial communities of polar systems, particularly polar deserts, will respond. The low C content and comparatively simpler (low biomass and biodiversity) soil communities of the McMurdo Dry Valleys of Antarctica may allow easier identification of the mechanisms by which N fertilization influences microbial communities. Therefore, we conducted a microcosm incubation using three levels of N fertilization, added in solution to simulate a pulse of increased soil moisture, and measured microbial biomass and respiration over the course of 4.5 months. Soil characteristics, including soil pH, conductivity, cation content, chlorophyll a, and organic C content were measured. Soils from two sites that differed in stoichiometry were used to examine how in situ C:N:P influenced the N-addition response. We hypothesized that negative influences of N enrichment would result from increased salinity and ion content, while positive influences would result from enhanced C availability and turnover. We observed that microbes were moderately influenced by N addition, including stimulation and inhibition with increasing levels of N. Mechanisms identified include direct inhibition due to N toxicity and stimulation due to release from N, rather than C, limitation. Our results suggest that, by influencing microbial biomass and activity, N fertilization will influence C cycling in soils with very low C content.     

Reversibility of phosphorus sorption by ferruginous nodules

Ferruginous nodules sorb significant amounts of available soil and fertiliser phosphate. The effect of this sorption on phosphorus availability of an agricultural soil was tested by sequential extraction and by exhaustive cropping with millet (Pennisetum typhoides) in a greenhouse trial following fertilisation of the original soil containing 70% nodules and of prepared samples containing various mixes of separated soil fines and nodules. Phosphorus sorption maxima by the soil fines and nodules were 190 mg kg⁻¹ and 380 mg kg⁻¹ respectively. Samples of fines and nodules which had sorbed 110 and 194 mg kg⁻¹ were submitted to 8 successive extractions with 0.01 M KCl, after which P desorption amounted to 117 mg kg⁻¹ and 103 mg kg⁻¹ respectively. Hysteresis between sorption and desorption was negligible for the soil fines and increased with increasing nodule content of the samples. In the greenhouse experiment, P uptake at the first cropping was highest in the soil fines at all levels of phosphorus applied. Subsequent croppings, however, showed higher P uptake in the concretionary soils. These results indicate a higher initial P release from the soil fines with cropping followed by an earlier exhaustion of phosphorus. At the end of the greenhouse experiment, yields were low in spite of the large quantities of P still remaining in the soils. Phosphorus fractionation showed that, of the P left in the soil after cropping 20% was in labile, 29% in Fe or Al-associated, and 51% in low-availability forms.     

Vegetation Leachate During Arctic Thaw Enhances Soil Microbial Phosphorus

Leachate from litter and vegetation penetrates permafrost surface soils during thaw before being exported to aquatic systems. We know this leachate is critical to ecosystem function downstream and hypothesized that thaw leachate inputs would also drive terrestrial microbial activity and nutrient uptake. However, we recognized two potential endpoint scenarios: vegetation leachate is an important source of C for microbes in thawing soil; or vegetation leachate is irrelevant next to the large background C, N, and P pools in thaw soil solution. We assessed these potential outcomes by making vegetation leachate from frozen vegetation and litter in four Arctic ecosystems that have a variety of litter quality and soil C, N, and P contents; one of these ecosystems included a disturbance recovery chronosequence that allowed us to test our second hypothesis that thaw leachate response would be enhanced in disturbed ecosystems. We added water or vegetation leachate to intact, frozen, winter soil cores and incubated the cores through thaw. We measured soil respiration throughout, and soil solution and microbial biomass C, N, and P pools and gross N mineralization immediately after a thaw incubation (?10 to 2°C) lasting 6 days. Vegetation leachate varied strongly by ecosystem in C, N, and P quantity and stoichiometry. Regardless, all vegetated ecosystems responded to leachate additions at thaw with an increase in the microbial biomass phosphate flush and an increase in soil solution carbon and nitrogen, implying a selective microbial uptake of phosphate from plant and litter leachate at thaw. This response to leachate additions was absent in recently disturbed, exposed mineral soil but otherwise did not differ between disturbed and undisturbed ecosystems. The selective uptake of P by microbes implies either thaw microbial P limitation or thaw microbial P uptake opportunism, and that spring thaw is an important time for P retention in several Arctic ecosystems.     

Effects of labile soil carbon on nutrient partitioning between an arctic graminoid and microbes

We measured partitioning of N and P uptake between soil microorganisms and potted Festuca vivipara in soil from a subarctic heath in response to factorial addition of three levels of labile carbon (glucose) combined with two levels of inorganic N and P. The glucose was added to either non-sterilized or sterilized (autoclaved) soils in quantities which were within the range of reported, naturally occurring amounts of C released periodically from the plant canopy. The aims were, firstly, to examine whether the glucose stimulated microbial nutrient uptake to the extent of reducing plant nutrient uptake. This is expected in nutrient-deficient soils if microbes and plants compete for the same nutrients. Secondly, we wanted to test our earlier␣interpretation that growth reduction observed in graminoids after addition of leaf extracts could be caused directly by labile carbon addition, rather than by phytotoxins in the extracts. Addition of high amounts of N did not affect the microbial N pool, whereas high amounts of added P significantly increased the microbial P pool, indicating a luxury P uptake in the microbes. Both plant N and in particular P uptake increased strongly in response to soil sterilization and to addition of extra N or P. The increased␣uptake led to enhanced plant growth when both elements were applied in high amounts, but only led to increased tissue concentrations without growth responses when the nutrients were added separately. Glucose had strong and contrasting effects on plant and microbial N and P uptake. Microbial N and P uptake increased, soil inorganic N and P concentrations were reduced and plant N and P uptake declined when glucose was added. The responses were dose-dependent within the range of 0–450 μg C g⁻¹ soil added to the non-sterilized soil. The opposite responses of plants and microbes showed that plant acquisition of limiting nutrients is dependent on release of nutrients from the soil microbes, which is under strong regulation by the availability and microbial uptake of labile C. Hence, we conclude, firstly, that the microbial populations can compete efficiently with plants for nutrients to an extent of affecting plant growth when the microbial access to labile carbon is high in nutrient deficient soils. We also conclude that reduced growth of plants after addition of leaf extracts to soil can be caused by carbon-induced shifts in nutrient partitioning between plants and microbes, and not necessarily by phytotoxins added with the extracts as suggested by some experiments. Received: 15 February 1997 / Accepted: 12 July 1997     

Soil phosphorus fractions,aluminum, and water retention as affected by microbial activity in an Ultisol

Increased organic matter input into weathered and infertile soils through agricultural techniques such as minimum tillage or agroforestry can improve P availability to crops. Organic matter is an energy source for microbes, and their activity may be responsible in part for increased levels of labile P. The objective of the work reported here was to examine, in a highly weathered Ultisol, the influence of microbial activity in mobilizing P, maintaining it in a plant-available state, and preventing its fixation, and the effect of N and biocides on these processes. Exchangeable aluminum and soil moisture were also determined, since they interact with microbes and soil P.Results showed that increased microbial activity reduced sorption of dissolved and organic P by soil, maintained inorganic P in soluble and labile pools, increased microbial P, decreased mineral P, increased exchangeable Al, and increased water retention. Additions of N and biocides had variable effects, probably due to complex interactions between N, degrading biocides, and microbial populations.     

The course of 15N-ammonium nitrate in a spring barley cropping system

¹⁵N-labelled ammonium nitrate was applied to spring barley growing on a Cambisol soil in western Switzerland. Immobilization, plant uptake and disappearance of inorganic nitrogen were followed at frequent intervals. Fertilizer nitrogen disappeared shortly after its application, mainly through immobilization by soil microorganisms and absorption by the crop. Some of the added nitrogen was probably denitrified as a result of humid conditions during the first days after fertilizer application. At the end of the growing season, 31% of the added nitrogen was recovered from the aerial barley plants, and 56% was immobilized by microorganisms. Most of the fertilizer nitrogen not used by the crop was immobilized in the upper 0–30 cm soil layer. This prevented downward movement of nitrate and limited nitrogen losses. Fertilizer efficiency was mainly determined by the competition between crop uptake and microbial immobilization. Careful consideration of the time of fertilization, taking into account plant growth and weather conditions, can result in an increase in fertilizer efficiency and minimal pollution.     

Effects of CO2 and N fertilization on decomposition and N immobilization in ponderosa pine litter

Naturally senesced needles from ponderosa pine (Pinus ponderosa Dougl.), grown from seed in open-top chambers under three levels of CO₂ (350, 525 and 700 μl l^-1) and three levels of N fertilization (0, 10 and 20 g N m^-2 yr^-1), were used in a field litterbag decomposition study and in a laboratory study on potential microbial and nonmicrobial N immobilization. The litterbag studies revealed no statistically significant effects of either CO₂ or N treatment on mass loss, N concentration, or N content over a 26-month period. The laboratory study of potential ¹⁵N immobilization revealed no statistically significant effects of CO₂ or N treatment on either total or microbial immobilization. Elevated (CO₂) did have a significant negative effect on nonmicrobial immobilization, however. Natural abundance of ¹⁵N was significantly greater with elevated (CO₂) in both live and naturally senesced needles under all N treatments. This pattern combined with ¹⁵N natural abundance in soils suggests that saplings grown under elevated (CO₂) were either taking up more N from surface horizons or from a more recalcitrant soil N pool in either horizon. This revised version was published online in June 2006 with corrections to the Cover Date.     

不同施肥模式下稻田土壤微生物生物量磷对土壤有机碳和磷素变化的响应

通过15年的红壤稻田长期定位试验,研究了不同施肥模式下土壤微生物生物量磷(MB-P)对土壤有机碳和磷素变化的响应.结果表明红壤稻田有机碳源的长期投入和土壤有机碳的逐年升高使土壤微生物生物量碳(MB-C)维持在较高水平(＞800 mg·kg-1),是稻田土壤MB-P(＞16.0 mg·kg-1)提高的主要原因.长期不施磷肥条件下,土壤全磷含量与试验前相比显著降低(P＜0.05),而土壤有机磷含量平均提高了29.3%;土壤亏损的磷形态主要是无机磷(Al-P、Fe-P、Ca-P和O-P),其中Al-P含量处于最低水平(平均0.5 mg·kg-1).另外,长期不施磷肥土壤的MB-P远高于Olsen法提取态磷(Olsen-P)(＜7.0 mg·kg-1),而稻田土壤MB-P与Al-P呈显著相关(P＜0.05),表明土壤微生物对稻田土壤Al-P、Fe-P、Ca-P和O-P的利用是促进其向有效磷方向转化的关键途径.磷肥配合有机养分循环利用不仅提高了土壤磷库的积累,而且通过土壤微生物的活化有效地提高了土壤磷的有效性.     

Impacts of chronic nitrogen additions vary seasonally and by microbial functional group in tundra soils

Previous studies have shown that fertilization with nitrogen depresses overall microbial biomass and activity in soil. In the present study we broaden our understanding of this phenomenon by studying the seasonality of responses of specific microbial functional groups to chronic nitrogen additions in alpine tundra soils. We measured soil enzyme activities, mineralization kinetics for 8 substrates, biomass of 8 microbial functional groups, and changes in N and carbon pools in the soil. Our approach allowed us to compare the ability of the soil microbial biomass to utilize various substrates in addition to allowing us to estimate changes in biomass of microbial functional groups that are involved in carbon and nitrogen cycling. Overall microbial activity and biomass was reduced in fertilized plots, whereas pools of N in the soil and microbial biomass N were higher in fertilized plots. The negative effects of N were most prominent in the summer. Biomass of the dominant microbial functional groups recovered in fertilized soils during the winter and nitrogen storage in microbial biomass was higher in fertilized soils in the autumn and winter than in the summer. Microbial immobilization of N may therefore be a significant sink for added N during autumn and winter months when plants are not active. One large microbial group that did not recover in the winter in fertilized soils was phenol mineralizers, possibly indicating selection against microbes with enzyme systems for the breakdown of phenolic compounds and complex soil organic matter. Overall, this work is a step towards understanding how chronic N additions affect the structure and biogeochemical functioning of soil microbial communities.     

Rapid abiotic transformation of nitrate in an acid forest soil

Nitrate immobilization into organic matter isthought to require catalysis by the enzymes ofsoil microorganisms. However, recent studiessuggest that nitrate added to soil isimmobilized rapidly and this process mayinclude abiotic pathways. We amended living andsterilized soil with ¹⁵N-labeled nitrateand nitrite to investigate biotic and abioticimmobilization. We report rapid transformationof nitrate in incubations of the O layer offorest soils that have been sterilized toprevent microbial activity and to denaturemicrobial enzymes. Approximately 30, 40, and60% of the ¹⁵N-labeled nitrate added tolive, irradiated, or autoclaved organic horizonsoil disappeared from the extractableinorganic-N pool in less than 15 minutes. About5% or less of the nitrate was recovered asinsoluble organic N in live and sterilizedsoil, and the remainder was determined to besoluble organic N. Added ¹⁵N-nitrite,however, was either lost to gaseous N orincorporated into an insoluble organic N formin both live and sterile organic soils. Hence,the fate and pathway of apparent abioticnitrate immobilization differs from thebetter-known mechanisms of nitrite reactionswith soil organic matter. Nitrate and nitriteadded to live A-horizon soil was largelyrecovered in the form added, suggesting thatrapid conversion of nitrate to solubleorganic-N may be limited to C-rich organichorizons. The processes by which this temperateforest soil transforms added nitrate to solubleorganic-N cannot be explained by establishedmechanisms, but appears to be due to abioticprocesses in the organic horizon.     

Novel P-Solubilizers from Calcium Bound Phosphate Rich Pine Forest of Lower Himalaya

Phosphorus, an essential element for life, is continuously depleting from soils and thus demands sustainable management particularly in agriculture and forestry. Inorganic P constitutes the major proportion as tricalcium phosphate in soils of lower Himalayan region of Pakistan. We sampled these soils and screened for P-solubilizing microbes. A range of culturable microbial community (bacteria and fungi) was isolated and molecularly characterized which make the P available from mineral phosphates. There was an increase in abundance of phosphate solubilizing bacteria (PSB) at a 6-inch depth of the pine rhizosphere compared to the surface soil samples. Moreover, the isolates from lower Himalaya have higher abundance and better efficiency to solubilize the inorganic P than the ones from non-Himalaya. Most likely the P-solubilization done by our P-solubilizing microbes is via acidification as we observed the decrease in pH of the medium of microbial growth. Furthermore, the majority of isolated PSB belong to gammaproteobacterial class of Gram negative bacteria. Most interestingly, 13% of our isolated PSB were psychrotolerant (physiologically active at cold environment, i.e., 4°C) and able to solubilize inorganic P as efficiently as at ambient temperature. This study is unique in reporting the P-solubilizing microbes, particularly the psychrotolerant bacterial strains, of Lower Himalaya. Therefore the isolated bacterial and fungal strains have potential and may serve as biofertilizers in the region to increase the P availability in soils.     

Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest

I analyzed the rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest. The sites differ in age, ranging from 400 to 4,100,000 yr, but are comparable in other variables (all at 1200 miasl with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5 °C, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5 °C under three treatments: 1) field-wet without amendments, 2) air dried to a permanent wilting point, and 3) fertilized with phosphate (NaH₂PO₄) at the rate of 50 g P per g dry soil. Both mineralization and nitrification rates varied significantly among the sites at the field temperature (p<.00001). Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-yr site before declining abruptly to a near constant value from the 9000 to the 1,400,000-yr sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH₄- and NO₃-N produced on an area basis (g m^-2 20 d^-1) increased sharply from 0.2 in the youngest site to 1.2 in the 5000-yr site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspond to the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, and poorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilization invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.