Carbon,Nitrogen, Phosphorus,and Potassium Stoichiometry in an Ombrotrophic Peatland Reflects Plant Functional Type

Ombrotrophic bog peatlands are nutrient-deficient systems and important carbon (C) sinks yet the stoichiometry of nitrogen (N), phosphorus (P) and potassium (K), essential for plant growth and decomposition, has rarely been studied. We investigated the seasonal variation in C, N, P, and K concentrations and their stoichiometric ratios in photosynthetically active tissues of 14 species belonging to five plant functional types (PFTs) (mosses, deciduous trees/shrubs, evergreen shrubs, graminoids, and forb) at Mer Bleue bog, an ombrotrophic peatland in eastern Ontario, Canada. Although we observed variations in stoichiometry among PFTs at peak growing season, there was convergence of C:N:P:K to an average mass ratio of 445:14:1:9, indicating N and P co-limitation. Nitrogen, P, and K concentrations and stoichiometric ratios showed little seasonal variation in mosses, evergreens, and graminoids, but in forb and deciduous species were the largest in spring and decreased throughout the growing season. Variations in nutrient concentrations and stoichiometric ratios among PFTs were greater than seasonal variation within PFTs. Plants exhibit N and P co-limitation and adapt to extremely low nutrient availability by maintaining small nutrient concentrations in photosynthetically active tissues, especially for evergreen shrubs and Sphagnum mosses. Despite strong seasonal variations in nutrient availabilities, few species show strong seasonal variation in nutrient concentrations, suggesting a strong stoichiometric homeostasis at Mer Bleue bog.     

Microbial Metabolic Potential for Carbon Degradation and Nutrient (Nitrogen and Phosphorus) Acquisition in an Ombrotrophic Peatland

This study integrated metagenomic and nuclear magnetic resonance (NMR) spectroscopic approaches to investigate microbial metabolic potential for organic matter decomposition and nitrogen (N) and phosphorus (P) acquisition in soils of an ombrotrophic peatland in the Marcell Experimental Forest (MEF), Minnesota, USA. This analysis revealed vertical stratification in key enzymatic pathways and taxa containing these pathways. Metagenomic analyses revealed that genes encoding laccases and dioxygenases, involved in aromatic compound degradation, declined in relative abundance with depth, while the relative abundance of genes encoding metabolism of amino sugars and all four saccharide groups increased with depth in parallel with a 50% reduction in carbohydrate content. Most Cu-oxidases were closely related to genes from Proteobacteria and Acidobacteria, and type 4 laccase-like Cu-oxidase genes were >8 times more abundant than type 3 genes, suggesting an important and overlooked role for type 4 Cu-oxidase in phenolic compound degradation. Genes associated with sulfate reduction and methanogenesis were the most abundant anaerobic respiration genes in these systems, with low levels of detection observed for genes of denitrification and Fe(III) reduction. Fermentation genes increased in relative abundance with depth and were largely affiliated with Syntrophobacter. Methylocystaceae-like small-subunit (SSU) rRNA genes, pmoA, and mmoX genes were more abundant among methanotrophs. Genes encoding N₂ fixation, P uptake, and P regulons were significantly enriched in the surface peat and in comparison to other ecosystems, indicating N and P limitation. Persistence of inorganic orthophosphate throughout the peat profile in this P-limiting environment indicates that P may be bound to recalcitrant organic compounds, thus limiting P bioavailability in the subsurface. Comparative metagenomic analysis revealed a high metabolic potential for P transport and starvation, N₂ fixation, and oligosaccharide degradation at MEF relative to other wetland and soil environments, consistent with the nutrient-poor and carbohydrate-rich conditions found in this Sphagnum-dominated boreal peatland.     

Bacterial and Fungal Communities in a Degraded Ombrotrophic Peatland Undergoing Natural and Managed Re-Vegetation

The UK hosts 15–19% of global upland ombrotrophic (rain fed) peatlands that are estimated to store 3.2 billion tonnes of carbon and represent a critical upland habitat with regard to biodiversity and ecosystem services provision. Net production is dependent on an imbalance between growth of peat-forming Sphagnum mosses and microbial decomposition by microorganisms that are limited by cold, acidic, and anaerobic conditions. In the Southern Pennines, land-use change, drainage, and over 200 years of anthropogenic N and heavy metal deposition have contributed to severe peatland degradation manifested as a loss of vegetation leaving bare peat susceptible to erosion and deep gullying. A restoration programme designed to regain peat hydrology, stability and functionality has involved re-vegetation through nurse grass, dwarf shrub and Sphagnum re-introduction. Our aim was to characterise bacterial and fungal communities, via high-throughput rRNA gene sequencing, in the surface acrotelm/mesotelm of degraded bare peat, long-term stable vegetated peat, and natural and managed restorations. Compared to long-term vegetated areas the bare peat microbiome had significantly higher levels of oligotrophic marker phyla (Acidobacteria, Verrucomicrobia, TM6) and lower Bacteroidetes and Actinobacteria, together with much higher ligninolytic Basidiomycota. Fewer distinct microbial sequences and significantly fewer cultivable microbes were detected in bare peat compared to other areas. Microbial community structure was linked to restoration activity and correlated with soil edaphic variables (e.g. moisture and heavy metals). Although rapid community changes were evident following restoration activity, restored bare peat did not approach a similar microbial community structure to non-eroded areas even after 25 years, which may be related to the stabilisation of historic deposited heavy metals pollution in long-term stable areas. These primary findings are discussed in relation to bare peat oligotrophy, re-vegetation recalcitrance, rhizosphere-microbe-soil interactions, C, N and P cycling, trajectory of restoration, and ecosystem service implications for peatland restoration.     

Estimating Annual Soil Carbon Loss in Agricultural Peatland Soils Using a Nitrogen Budget Approach

Around the world, peatland degradation and soil subsidence is occurring where these soils have been converted to agriculture. Since initial drainage in the mid-1800s, continuous farming of such soils in the California Sacramento-San Joaquin Delta (the Delta) has led to subsidence of up to 8 meters in places, primarily due to soil organic matter (SOM) oxidation and physical compaction. Rice (Oryza sativa) production has been proposed as an alternative cropping system to limit SOM oxidation. Preliminary research on these soils revealed high N uptake by rice in N fertilizer omission plots, which we hypothesized was the result of SOM oxidation releasing N. Testing this hypothesis, we developed a novel N budgeting approach to assess annual soil C and N loss based on plant N uptake and fallow season N mineralization. Through field experiments examining N dynamics during growing season and winter fallow periods, a complete annual N budget was developed. Soil C loss was calculated from SOM-N mineralization using the soil C:N ratio. Surface water and crop residue were negligible in the total N uptake budget (3 – 4 % combined). Shallow groundwater contributed 24 – 33 %, likely representing subsurface SOM-N mineralization. Assuming 6 and 25 kg N ha-1 from atmospheric deposition and biological N2 fixation, respectively, our results suggest 77 – 81 % of plant N uptake (129 – 149 kg N ha^-1) was supplied by SOM mineralization. Considering a range of N uptake efficiency from 50 – 70 %, estimated net C loss ranged from 1149 – 2473 kg C ha^-1. These findings suggest that rice systems, as currently managed, reduce the rate of C loss from organic delta soils relative to other agricultural practices.     

The Effects of Permafrost Thaw on Soil Hydrologic, Thermal, and Carbon Dynamics in an Alaskan Peatland

Recent warming at high-latitudes has accelerated permafrost thaw in northern peatlands, and thaw can have profound effects on local hydrology and ecosystem carbon balance. To assess the impact of permafrost thaw on soil organic carbon (OC) dynamics, we measured soil hydrologic and thermal dynamics and soil OC stocks across a collapse-scar bog chronosequence in interior Alaska. We observed dramatic changes in the distribution of soil water associated with thawing of ice-rich frozen peat. The impoundment of warm water in collapse-scar bogs initiated talik formation and the lateral expansion of bogs over time. On average, Permafrost Plateaus stored 137 ± 37 kg C m⁻², whereas OC storage in Young Bogs and Old Bogs averaged 84 ± 13 kg C m⁻². Based on our reconstructions, the accumulation of OC in near-surface bog peat continued for nearly 1,000 years following permafrost thaw, at which point accumulation rates slowed. Rapid decomposition of thawed forest peat reduced deep OC stocks by nearly half during the first 100 years following thaw. Using a simple mass-balance model, we show that accumulation rates at the bog surface were not sufficient to balance deep OC losses, resulting in a net loss of OC from the entire peat column. An uncertainty analysis also revealed that the magnitude and timing of soil OC loss from thawed forest peat depends substantially on variation in OC input rates to bog peat and variation in decay constants for shallow and deep OC stocks. These findings suggest that permafrost thaw and the subsequent release of OC from thawed peat will likely reduce the strength of northern permafrost-affected peatlands as a carbon dioxide sink, and consequently, will likely accelerate rates of atmospheric warming.     

Nutrient Input and Carbon and Microbial Dynamics in an Ombrotrophic Bog

Slow rates of plant production and decomposition in ombrotrophic bogs are believed to be partially the result of low nutrient availability. To test the effect of nutrient availability on decomposition, carbon dioxide (CO₂) flux dynamics, microbial biomass, and nutrients, we added nitrogen (N) with phosphorus (P) and potassium (K), to prevent limitation of the latter 2 nutrients, over 2 growing seasons to plots at Mer Bleue peatland, Ontario, Canada. After the first growing season, increasing N fertilization (with constant P and K) decreased in vitro CO₂ production potential and increased microbial biomass measured with a chloroform fumigation-extraction technique in the upper peat profile, while by the end of the second season, CO₂ production potential was increased in response to N plus PK treatment, presumably due to more easily decomposable newly formed plant material. In situ CO₂ fluxes measured using chamber-techniques over the second year corroborated this presumption, with greater photosynthetic CO₂ uptake and ecosystem respiration (ER) during high N plus PK treatments. The more efficient microbial community, with slower CO₂ production potential and larger biomass, after the first year was characterized by larger fungal biomass measured with signature phospholipid fatty acids. The majority of N was likely quickly sequestered by the vegetation and transferred to dissolved organic forms and microbial biomass in the upper parts of the peat profile, while additional P relative to controls was distributed throughout the profile, implying that the vegetation at the site was N limited. However, in situ CO₂ flux data suggested the possibility of P or NPK limitation. We hypothesize that nutrient deposition may lead to enhanced C uptake by altering the microbial community and decomposition, however this pattern disappears through subsequent changes in the vegetation and production of more readily decomposable plant tissues.     

Effects of Soil Carbon Amendment on Nitrogen Availability and Plant Growth in an Experimental Tallgrass Prairie Restoration

Restoration of tallgrass prairie on former agricultural land is often impeded by failure to establish a diverse native species assemblage and by difficulties with nonprairie, exotic species. High levels of available soil nitrogen (N) on such sites may favor fast‐growing exotics at the expense of more slowly growing prairie species characteristic of low‐N soils. We tested whether reducing N availability through soil carbon (C) amendments could be a useful tool in facilitating successful tallgrass prairie restoration. We added 6 kg/m² hardwood sawdust to experimental plots on an abandoned agricultural field in the Sandusky Plains of central Ohio, United States, increasing soil C by 67% in the upper 15 cm. This C amendment caused a 94% reduction in annual net N mineralization and a 27% increase in soil moisture but had no effect on total N or pH. Overall, plant mass after one growing season was reduced by 64% on amended compared with unamended soil, but this effect was less for prairie forbs (?34%) than for prairie grasses (?67%) or exotics (?62%). After the second growing season, only exotics responded significantly to the soil C amendment, with a 40% reduction in mass. The N concentration of green‐leaf tissue and of senescent leaf litter was also reduced by the soil C treatment in most cases. We conclude that soil C amendment imparts several immediate benefits for tallgrass prairie restoration––notably reduced N availability, slower plant growth, and lower competition from exotic species.     

Soil Nitrogen in Relation to Quality and Decomposability of Plant Litter in the Patagonian Monte,Argentina

In two consecutive years, we analysed the effect of litter quality, quantity and decomposability on soil N at three characteristic sites of the Patagonian Monte. We assessed (i) concentrations of N, C, lignin and total phenolics and the C/N ratio in senesced leaves as indicators of litter quality of three species of each dominant plant life form (evergreen shrubs and perennial grasses), and (ii) N, and organic-C concentrations, potential N-mineralisation and microbial-N flush in the soil beneath each species. Rate constants of potential decomposition of senesced leaves and N content in decaying leaves during the incubation period were assessed in composite samples of the three sites as indicators of litter decomposability. Further, we estimated for each species leaf-litter production, leaf-litter on soil, and the mass of standing senesced leaves during the senescence period. Senesced leaves of evergreen shrubs showed higher decomposability than those of perennial grasses. Leaf-litter production, leaf-litter on soil, and the mass of standing senesced leaves differed significantly among species. The largest variations in leaf-litter production and leaf-litter on soil were observed in evergreen shrubs. The mass of standing senesced leaves was larger in perennial grasses than in evergreen shrubs. Nitrogen, organic C and potential N-mineralisation in soil were higher underneath evergreen shrubs than beneath perennial grasses, while no significant differences were found in microbial-N flush among life forms. The initial concentrations of C, N and total phenolics of senesced leaves explained together 78% of the total variance observed in the dry mass loss of decaying leaves. Litter decomposition rates explained 98%, 98%, 73%, and 67% of the total variance of soil N, organic C, net-N mineralisation, and microbial-N flush, respectively. We concluded that leaf-litter decomposition rates along with leaf-litter production are meaningful indicators of plant local effects on soil N dynamics in shrublands of the Patagonian Monte, and probably in other similar ecosystem of the world dominated by slow growing species that accumulate a wide variety of secondary metabolites including phenolics. Indicators such as C/N or lignin concentration usually used to predict litter decomposability or local plant effects may not be adequate in the case of slow growing species that accumulate a wide range of secondary metabolites or have long leaf lifespan and low leaf-litter production.     

Influence of Tomato Plant Mycorrhization on Nitrogen Metabolism,Growth and Fructification on P-Limited Soil

Journal of Plant Growth Regulation - The application of mycorrhizal fungi in agricultural soils as bio-fertilizers is going to be established as an agronomic practice for enhancing crop nutrients...     

Methanogenesis and Methanogen Diversity in Three Peatland Types of the Discontinuous Permafrost Zone,Boreal Western Continental Canada

Because recent patterns of permafrost collapse in boreal peatlands appear to enhance emissions of CH ₄ to the atmosphere, we examined methanogenesis and methanogen diversity in peat soil from peatlands with and without permafrost in two peatland complexes situated in continental western Canada. Peat soil from the active layer of permafrost bogs had very low rates of CH ₄ production (ca. 10 nmol g ^?1 day ^?1 ), and we were unable to PCR-amplify 16s rRNA gene sequences using Archaea-specific primers in four peat samples. Surface peat soil from continental bogs with no permafrost supported moderate rates of CH ₄ production (20–600 nmol g ^?1 day ^?1 ), with maximum rates in soil located close to the mean water table level. Additions of ethanol stimulated CH ₄ production rates, suggesting metabolic substrate limitations. Peat from internal lawns, which have experienced surface permafrost degradation in the past 150 years, had very rapid rates of CH₄ production (up to 800 nmol g ^?1 day ^?1 ) occurring within the soil profile. Concomitant rates of anaerobic CO ₂ production were greater in continental bogs (ca. 6 μmol g ^?1 day ^?1 ) than in internal lawns (ca. 4 μ mol g ^?1 day ^?1 ) or in permafrost bogs (2.8 μ mol g ^?1 day ^?1 ). Analysis of the 16s rRNA gene for Archaea in the continental bog indicated mostly sequences associate with Methanobacteriales and RC-I with a Methanosarcinaceae sequence in the deepest peat soil. In the internal lawn, Methanosarcinaceae were most common in peat soil with a Methanosaetaceae sequence in the deepest peat soil. This study showed that patterns of discontinuous permafrost and ongoing permafrost degradation in boreal regions create patchy soil environments for methanogens and rates of methanogenesis.     

Impacts of Hemlock Loss on Nitrogen Retention Vary with Soil Nitrogen Availability in the Southern Appalachian Mountains

The impacts of exotic insects and pathogens on forest ecosystems are increasingly recognized, yet the factors influencing the magnitude of effects remain poorly understood. Eastern hemlock (Tsuga canadensis) exerts strong control on nitrogen (N) dynamics, and its loss due to infestation by the hemlock woolly adelgid (Adelges tsugae) is expected to decrease N retention in impacted stands. We evaluated the potential for site variation in N availability to influence the magnitude of effects of hemlock decline on N dynamics in mixed hardwood stands. We measured N pools and fluxes at three elevations (low, mid, high) subjected to increasing atmospheric N deposition where hemlock was declining or absent (as reference), in western North Carolina. Nitrogen pools and fluxes varied substantially with elevation and increasing N availability. Total forest floor and mineral soil N increased (P?<?0.0001, P?=?0.0017, resp.) and forest floor and soil carbon (C) to N ratio decreased with elevation (P?<?0.0001, P?=?0.0123, resp.), suggesting that these high elevation pools are accumulating available N. Contrary to expectations, subsurface leaching of inorganic N was minimal overall (<1?kg?ha^?1 9 months^?1), and was not higher in stands with hemlock mortality. Mean subsurface flux was 0.16?±?0.04 (SE) (kg?N?ha^?1 100?days^?1) in reference and 0.17?±?0.05 (kg?N?ha^?1 100?days^?1) in declining hemlock stands. Moreover, although subsurface N flux increased with N availability in reference stands, there was no relationship between N availability and flux in stands experiencing hemlock decline. Higher foliar N and observed increases in the growth of hardwood species in high elevation stands suggest that hemlock decline has stimulated N uptake and growth by healthy vegetation within this mixed forest, and may contribute to decoupling the relationship between N deposition and ecosystem N flux.     

Spatial and Temporal Variability in Growing-Season Net Ecosystem Carbon Dioxide Exchange at a Large Peatland in Ontario,Canada

We measured net ecosystem exchange of carbon dioxide (CO2) (NEE) during wet and dry summers (2000 and 2001) across a range of plant communities at Mer Bleue, a large peatland near Ottawa, southern Ontario, Canada. Wetland types included ombrotrophic bog hummocks and hollows, mineral-poor fen, and beaver pond margins. NEE was significantly different among the sites in both years, but rates of gross photosynthesis did not vary spatially even though species composition at the sites was variable. Soil respiration rates were very different across sites and dominated interannual variability in summer NEE within sites. During the dry summer of 2001, net CO2 uptake was significantly smaller, and most locations switched from a net sink to a source of CO2 under a range of levels of photosynthetically active radiation (PAR). The wetter areas--poor fen and beaver pond margin--had the largest rates of CO2 uptake and smallest rates of respiratory loss during the dry summer. Communities dominated by ericaceous shrubs (bog sites) maintained similar rates of gross photosynthesis between years; by contrast, the sedge-dominated areas (fen sites) showed signs of early senescence under drought conditions. Water table position was the strongest control on respiration in the drier summer, whereas surface peat temperature explained most of the variability in the wetter summer. Q 10 temperature-respiration quotients averaged 1.6 to 2.2. The ratio between maximum photosynthesis and respiration ranged from 3.7:1 in the poor fen to 1.2:1 at some bog sites; it declined at all sites in the drier summer owing to greater respiration rates relative to photosynthesis in evergreen shrub sites and a change in both processes in sedge sites. Our ability to predict ecosystem responses to changing climate depends on a more complete understanding of the factors that control NEE across a range of peatland plant communities.     

Nitrogen Addition Shapes Soil Phosphorus Availability in Two Reforested Tropical Forests in Southern China

Scant information is available on how soil phosphorus (P) availability responds to atmospheric nitrogen (N) deposition, especially in the tropical zones. This study examined the effect of N addition on soil P availability, and compared this effect between forest sites of contrasting land‐use history. Effects of N addition on soil properties, litterfall production, P release from decomposing litter, and soil P availability were studied in a disturbed (reforested pine forest with previous understory vegetation and litter harvesting) and a rehabilitated (reforested mixed pine/broadleaf forest with no understory vegetation and litter harvesting) tropical forest in southern China. Experimental N‐treatments (above ambient) were the following: Control (no N addition), N50 (50 kg N ha^?1 yr^?1), and N100 (100 kg N ha^?1 yr^?1). Results indicated that N addition significantly decreased soil P availability in the disturbed forest. In the rehabilitated forest, however, soil P availability was significantly increased by N addition. Decreases in soil P availability may be correlated with decreases in rates of P release from decomposing litter in the N‐treated plots, whereas the increase in soil P availability was correlated with an increase in litterfall production. Our results suggest that response of soil P availability to N deposition in the reforested tropical forests in southern China may vary greatly with temporal changes in tree species composition and soil nutrient status, caused by different land‐use practices.     

Mineral Nitrogen in Plant Physiology and Plant Nutrition

Nitrogen (N) nutrition enhances metabolic processes that influences the physicochemical environment at the soil-root interface, modifies rhizosphere conditions, interferes with the uptake of cations and anions, and enhances or represses the activity of several enzyme systems. Also, it affects growth patterns, protein content, and protein quality of seeds.

Ammonium (NH₄)-N nutrition increases anion uptake, free amino-N/protein ratios, and acidity of root free space; it reduces carbohydrate levels in plant tissues. NO₃-N nutrition results in higher cation uptake, higher carbohydrate content in tissues, and alkalinization of root free space. N-Assimilation interferes with the allocation of dry matter and energy, which causes different growth rates of plant parts.

In this article we review the effects of mineral-N nutrition on uptake of cations and anions, activity of enzymes, growth patterns of roots and shoots, and water use efficiency, protein content, and protein quality of seeds.     

Permafrost Distribution Drives Soil Organic Matter Stability in a Subarctic Palsa Peatland

Palsa peatlands, permafrost-affected peatlands characteristic of the outer margin of the discontinuous permafrost zone, form unique ecosystems in northern-boreal and arctic regions, but are now degrading throughout their distributional range due to climate warming. Permafrost thaw and the degradation of palsa mounds are likely to affect the biogeochemical stability of soil organic matter (that is, SOM resistance to microbial decomposition), which may change the net C source/sink character of palsa peatland ecosystems. In this study, we have assessed both biological and chemical proxies for SOM stability, and we have investigated SOM bulk chemistry with mid-infrared spectroscopy, in surface peat of three distinct peatland features in a palsa peatland in northern Norway. Our results show that the stability of SOM in surface peat as determined by both biological and chemical proxies is consistently higher in the permafrost-associated palsa mounds than in the surrounding internal lawns and bog hummocks. Our results also suggest that differences in SOM bulk chemistry is a main factor explaining the present SOM stability in surface peat of palsa peatlands, with selective preservation of recalcitrant and highly oxidized SOM components in the active layer of palsa mounds during intense aerobic decomposition over time, whereas SOM in the wetter areas of the peatland remains stabilized mainly by anaerobic conditions. The continued degradation of palsa mounds and the expansion of wetter peat areas are likely to modify the bulk SOM chemistry of palsa peatlands, but the effect on the future net C source/sink character of palsa peatlands will largely depend on moisture conditions and oxygen availability in peat.     

Soil Patches of Inorganic Nitrogen in Subtropical Brazilian Plant Communities with Araucaria angustifolia

One-year old tubers of two hybrid calla lily (calla) cultivars (Zantedeschia ‘Pot of Gold’ and ‘Majestic Red’) were inoculated with the arbuscular mycorrhizal fungus (AMF), Glomus intraradices, or not, and grown at three different rates of phosphorus (P) supply to asses the effects of AMF-inoculation on plant development (time of shoot emergence and flowering), flowering (number, length and rate of flowering), and tuber biomass and composition over two growing cycles (2002, 2003). Tubers and flowers of calla responded differently to AMF inoculation. Differences in mycorrhizal responsiveness between cultivars was related to differences in P requirements for flower and tuber production, and the influence of P supply on resource allocation to different reproductive strategies. Inoculation increased shoot production and promoted early flowering, particularly in 2003. Inoculated plants also produced larger tubers than non-inoculated plants, but only increased the number of flowers per plant in 2003. High P supply also increased tuber biomass, but decreased the number of flowers per plant in 2002. Plants grown at a moderate P-rate, produced the most flowers in 2003. For ‘Majestic Red’, benefits from AMF were primarily in terms of tuber yield and composition, and AMF effects on marketable flower production could potentially have negative impact on production strategies for growers. Inoculation of ‘Pot of Gold’ primarily influenced flower production and aspects of tuber quality that caused detectable enhancement of tuber yield and flowering in the second growing cycle following inoculation (2003). The results of this study show that the responses of calla to AMF are partially a function of how nutrient supply alters resource allocation to sexual and vegetative reproduction. Whether AMF-induced changes in resource allocation to flowering and tubers significantly alters commercial productivity and quality of calla depends on the crop production goals (e.g. tubers, cut flowers or potted plants). The U.S. Government’s right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Optimization of Dynamic Plant Nitrogen Uptake,Using Apriori Information of Plant Nitrogen Content

Emphasizing that model development should be in accordance with the context under consideration, a model for plant nitrogen uptake is developed for use in connection with soil nitrogen models. The aim of the modeling is to improve the accuracy in model simulations of plant nitrogen uptake by application of optimization theory and apriori information of plant nitrogen content. A simple soil nitrogen model is coupled to the plant nitrogen uptake model, and solutions, obtained by two different methods, are presented. Suggestion for how to use the modeling principles in connection with a more complex plant nitrogen uptake model, and apriori information of plant dry weight, is given. It is believed that the modeling principles can also be used on other types of dynamic models with given apriori information.     

Concentrations and fluxes of dissolved organic carbon in an age-sequence of white pine forests in Southern Ontario,Canada

We determined concentrations and fluxes of dissolved organic carbon (DOC) in precipitation, throughfall, forest floor and mineral soil leachates from June 2004 to May 2006 across an age-sequence (2-, 15-, 30-, and 65-year-old) of white pine (Pinus strobus L.) forests in southern Ontario, Canada. Mean DOC concentration in precipitation, throughfall, leachates of forest floor, Ah-horizon, and of mineral soil at 1 m depth ranged from ∼2 to 7, 9 to 18, 32 to 88, 20 to 66, and 2 to 3 mg DOC L⁻¹, respectively, for all four stands from April (after snowmelt) through December. DOC concentration in forest floor leachates was highest in early summer and positively correlated to stand age, aboveground biomass and forest floor carbon pools. DOC fluxes via precipitation, throughfall, and leaching through forest floor and Ah-horizon between were in the range of ∼1 to 2, 2 to 4, 0.5 to 3.5, and 0.1 to 2 g DOC m⁻², respectively. DOC export from the forest ecosystem during that period through infiltration and groundwater discharge was estimated as ∼7, 4, 3, and 2 g DOC m⁻² for the 2-, 15-, 30-, and 65-year-old sites, respectively, indicating a decrease with increasing stand age. Laboratory DOC sorption studies showed that the null-point DOC concentration fell from values of 15 to 60 mg DOC L⁻¹ at 0 to 5 cm to <15 mg DOC L⁻¹ at 50 cm. Specific ultraviolet light absorption at 254 nm (SUVA₂₅₄) increased from precipitation and throughfall to a maximum in forest floor and decreased with mineral soil depth. No age-related pattern was observed for SUVA₂₅₄ values. DOC concentration in forest floor soil solutions showed a positive exponential relationship with soil temperature, and a negative exponential relationship with soil moisture at all four sites. Understanding the changes and controls of DOC concentrations, chemistry, and fluxes at various stages of forest stand development is necessary to estimate and predict DOC dynamics on a regional landscape level and to evaluate the effect of land-use change.     

外来入侵植物薇甘菊的2种化感物质对土壤氮循环的影响

为揭示薇甘菊(Mikania micrantha)通过化感作用影响土壤养分循环的入侵机制,通过外源添加薇甘菊2种化感物质(绿原酸和β-石竹烯),测定其对土壤氮素和氮循环功能菌的影响。结果表明,添加绿原酸和β-石竹烯均显著降低了土壤铵态氮含量,添加绿原酸显著提高了土壤硝态氮含量,β-石竹烯则对土壤硝态氮含量无显著影响。其主要原因是绿原酸和β-石竹烯均显著抑制了自生固氮菌和氨化细菌的繁殖,绿原酸显著促进了氨氧化细菌和硝酸化细菌的繁殖,而β-石竹烯仅促进了氨氧化细菌的繁殖,对硝酸化细菌没有影响。因此,薇甘菊能通过化感物质绿原酸和β-石竹烯影响氮循环功能菌的繁殖从而影响土壤氮循环。     

旱地小麦深施磷肥对土壤水分及植株氮素吸收、利用的影响

为探明磷肥在旱地小麦生产上的作用,寻求旱地小麦最佳施磷方式,在山西省闻喜县进行了低磷(75kg·hm~(-2))、中磷(112.5 kg·hm~(-2))、高磷(150 kg·hm~(-2))3个施磷量条件下20 cm、40 cm 2个深度施磷的田间试验,研究其对旱地麦田土壤水分及植株氮素吸收、利用的影响。结果表明:增加施磷量,越冬期-孕穗期0~100 cm土层土壤蓄水量提高,且深层施磷效果较好,尤其有利于返青期土壤蓄水量提高。增加施磷量,各生育时期植株含氮率提高,各生育时期植株氮素积累量显著提高,且深层施磷效果较好,尤其开花期含氮率。增加施磷量,花前各器官氮素运转量显著提高,深层施磷叶片氮素运转量对籽粒的贡献率提高,成熟期叶片氮素积累量及其所占比例显著降低。40 cm深度施磷150 kg·hm~(-2)花后氮素积累量最高。此外,越冬-孕穗期0~100 cm土层土壤蓄水量与花前氮素运转量关系密切,尤其与叶片氮素运转量关系密切,开花期土壤水分与花后氮素积累量关系系数最大。总之,增加施磷量,有利于提高花前1 m内土壤水分,有利于促进植株氮素积累、运转,且深层施磷效果显著,尤其可促进叶片氮素转移到籽粒,有利于开花期含氮率提高,有利于花后氮素积累。最终,40 cm深度施磷150 kg·hm~(-2)可显著提高旱地小麦氮肥吸收效率、氮肥生产效率、氮素收获指数。