soil-N Availability Research Articles

Combined Use of Charcoal, Sago Bark Ash, and Urea Mitigate Soil Acidity and Aluminium Toxicity

Highly weathered tropical acidic soils are characterized by low pH, low organic matter, and aluminium and iron toxicity. These factors pose a challenge to achieving sustainable agriculture. The continued increase in the human population with the accompanied increasing food demand have negatively impacted the global N cycle partly because of excessive use N fertilizers particularly urea which is commonly used in agriculture. Ammonia volatilization from urea as an example, negatives the environmental quality. This study focuses on soil-N availability, pH, exchangeable acidity, Al3+, and H+ of a highly weathered acid soils (Bekenu series) through the combined use of charcoal, sago bark ash, and urea. To this end, an incubation study was conducted for 90 days through the combined use of charcoal, sago bark ash, and urea to determine if this approach could improve soil N availability and pH at the same time reducing exchangeable acidity, and Al3+, and H+ toxicity. The amount of urea used was fixed at 100% as the recommended rate. Charcoal and sago bark ash were varied by 25%, 50%, 75%, and 100%, respectively of the recommended rate. Selected soil physico-chemical properties were determined using standard procedures. This study revealed that combined use of charcoal, sago bark ash, and urea increased soil pH and base cations simultaneously the approach also reduced exchangeable acidity, exchangeable Al3+, and exchangeable H+. There were no significant differences in soil total N, exchangeable NH4+, and available NO3− for the combined use of charcoal, sago bark ash, and urea and urea alone because of the acid neutralizing effect of the amendments. Apart from the sago bark ash’s liming effect, the high affinity of the functional groups of the charcoal for Al3+ might have impeded Al3+ from undergoing hydrolysis to produce more H+ because a complete one mole of Al3+ hydrolysis produces three moles of H+. Thus, the combined use of charcoal and sago bark ash can mitigate soil acidity and aluminium toxicity, although this approach has minimal effect on-N.

Comparison of soil-N availability in a field cultivated with legume or non-legume plants during the winter in a Mediterranean environment

Comparison of soil-N availability in a field cultivated with legume or non-legume plants during the winter in a Mediterranean environment

Changes in litter chemistry associated with global change-driven forest succession resulted in time-decoupled responses of soil carbon and nitrogen cycles

Global change-driven forest succession may modify key soil processes with potentially important impacts over carbon (C) and nutrient cycling. We studied how changes in litter throughout the replacement of Pinus sylvestris by Quercus pyrenaica influence the structure and functioning of soil microbial communities and the capacity of soils to sequester C and retain nitrogen (N). We designed a microcosm experiment to simulate the chronological sequence from pine to oak forest conversion in Central Spain, using mixtures of senescent litter (oak leaves, pine needles and an equal mixture of needles:leaves) and soils (from pure oak, mixed and pure pine stands). We investigated changing patterns of soil C and N contents, microbial community structure (PLFA) and greenhouse gas fluxes (CO2, CH4, N2O) across the chronosequence. The succession from pine to oak forest was associated with substantial changes in microbial community structure and functioning. Soil-C sink capacity was reduced, although soil-N availability was enhanced. We further show how effects of secondary succession on the C cycle were mismatched with N dynamics in response to two chronologically decoupled facts. First, there was an acceleration in soil organic matter (SOM) turnover after microbial –especially bacterial– growth ceased to be so intensely inhibited by needle litter (ecotone soils), resulting in lower fungal to bacterial ratios; and second, N mineralization was stimulated once pine-derived SOM was no longer present in soils (pure oak forest soils), resulting in further acceleration of SOM turnover, suppression of CH4 consumption and an increase in gram-negative bacteria. Our findings suggest that different sensitivities of key mechanisms (SOM decomposition, N mineralization, CH4 consumption) to factors associated with succession (e.g. recalcitrance of pine SOM and allelopathic effects over bacteria) could have significant impacts on soil microbial ecology, C and nutrient cycling.

Effect of split application of nitrogen on nitrous oxide emissions from plastic mulching maize in the semiarid Loess Plateau

Applying nitrogen (N) using split application to improve temporal synchronicity between crop-N demand and soil-N availability is a key strategy for improving nitrogen use efficiency (NUE). Its effect on nitrous oxide (N2O) emissions has not been well evaluated, especially from plastic mulching maize (Zea mays L.) in semiarid farmland. We conducted a two-year experiment to investigate the effect of split fertilizer N application on N2O emissions. Three approaches each applying a total of 225kgNha−1 were tested either as a single application at sowing (N1), N applied with two splits at a ratio of 4:6 (N2), and N applied with three splits at a ratio of 4:3:3 (N3), a no-N application treatment was also included as a control (N0). Compared with the single application, split application of N significantly increased the grain yields by 4.7–9.1%. The cumulative N2O emissions significantly increased by 18.1% under the three splits application in the 2014–2015 season compared with the single application but not in the growing season of 2015, which followed the same pattern with yield-scaled N2O emissions. Fertilizer application combined with precipitation was the main driving factor for N2O emissions. Therefore, the higher frequency (three times) of fertilizer application had a high risk for increasing N2O emissions due to the relatively strong and frequent precipitation in maize mid and late growth period in this region; applying N with two splits is more suitable for this region which could produce higher grain yields, and lower risk of N2O emissions.

Deepened winter snow increases stem growth and alters stem δ13C and δ15N in evergreen dwarf shrub Cassiope tetragona in high-arctic Svalbard tundra

Deeper winter snow is hypothesized to favor shrub growth and may partly explain the shrub expansion observed in many parts of the arctic during the last decades, potentially triggering biophysical feedbacks including regional warming and permafrost thawing. We experimentally tested the effects of winter snow depth on shrub growth and ecophysiology by measuring stem length and stem hydrogen (δ2H), carbon (δ13C), nitrogen (δ15N) and oxygen (δ18O) isotopic composition of the circumarctic evergreen dwarf shrub Cassiope tetragona growing in high-arctic Svalbard, Norway. Measurements were carried out on C. tetragona individuals sampled from three tundra sites, each representing a distinct moisture regime (dry heath, meadow, moist meadow). Individuals were sampled along gradients of experimentally manipulated winter snow depths in a six-year old snow fence experiment: in ambient (c. 20 cm), medium (c. 100 cm), and deep snow (c. 150 cm) plots. The deep-snow treatment consistently and significantly increased C. tetragona growth during the 2008–2011 manipulation period compared to growth in ambient-snow plots. Stem δ15N and stem N concentration values were significantly higher in deep-snow individuals compared to individuals growing in ambient-snow plots during the course of the experiment, suggesting that soil N-availability was increased in deep-snow plots as a result of increased soil winter N mineralization. Although inter-annual growing season-precipitation δ2H and stem δ2H records closely matched, snow depth did not change stem δ2H or δ18O, suggesting that water source usage by C. tetragona was unaltered. Instead, the deep insulating snowpack may have protected C. tetragona shrubs against frost damage, potentially compensating the detrimental effects of a shortened growing season and associated phenological delay on growth. Our findings suggest that an increase in winter precipitation in the High Arctic, as predicted by climate models, has the potential to alter the growth and ecophysiology of evergreen shrub C. tetragona through changes in plant mineral nutrition and frost damage protection.

Evaluation of nitrogen availability indices and their relationship with plant response on acidic soils of India

Plant’s nitrogen (N) requirement that is not fulfilled by available N in soil has to be supplied externally through chemical fertilizers. A reliable estimate of soil N-supplying capacity (NSC) is therefore essential for efficient fertilizer use. In this study involving a pot experiment with twenty acidic soils varying widely in properties, we evaluated six chemical indices of soil N-availability viz. organic carbon (C<sub>org</sub>), total N (N<sub>tot</sub>), acid and alkaline-KMnO<sub>4</sub> extractable-N, hot KCl extractable-N (KCl-N) and phosphate-borate buffer extractable-N (PBB-N), based on their strength of correlation with available-N values obtained through aerobic incubation (AI-N) and anaerobic incubation (ANI-N), and also with the dry matter yield (DMY), N percentage and plant (maize) N uptake (PNU). In general, the soils showed large variability in NSC as indicated by variability in PNU which ranged from 598 to 1026 mg/pot. Correlations of the N-availability indices with AI-N and ANI-N decreased in the order: PBB-N (r = 0.784** and 0.901**) > KCl-N (r = 0.773** and 0.743**) > acid KMnO<sub>4</sub>-N (r = 0.575** and 0.651**) ≥ C<sub>org</sub> (r = 0.591** and 0.531**) ≥ alkaline KMnO<sub>4</sub>-N (r = 0.394** and 0.548**) > N<sub>tot</sub> (r = 0.297** and 0.273*). Of all the indices evaluated, PBB-N showed the best correlations with plant parameters as well (r = 0.790** and 0.793** for DMY and PNU, respectively). Based on the highest correlations of PBB-N with biological indices as well as plant responses, we propose PBB-N as an appropriate index of N-availability in the acidic soils of India and other regions with similar soils.

Effectiveness of soil N availability indices in predicting site productivity in the oil sands region of Alberta

Background and aims Quantitative relationships between soil N availability indices and tree growth are lacking in the oil sands region of Alberta and this can hinder the development of guidelines for the reclamationof the disturbed landscape after oil sands extraction. The aim of this paper was to establish quantitative relationships between soilN availability indices and tree growth in the oil sands region of Alberta. Methods In situ N mineralization rates, in situ N availabilitymeasuredinthefieldusingPlant RootSimulators (PRS™probes),laboratoryaerobicandanaerobicsoilN mineralization rates, and soil C/N and N content were determined for both the forest floor and the 0–20 cm mineral soil in eight jack pine (Pinus banksiana Lamb.) stands in the oil sands region in northern Alberta. Tree growth rates were determined based on changes in tree ring width in the last 6 years and as mean annual aboveground biomass increment. Results Soil N availability indices across those forest stands varied and for each stand it was several times higher in the forest floor than in the mineral soil. The in situ and laboratory aerobic and anaerobic soil N mineralization rates, soil mineralized N, in situ N availability measured using PRS probes, soil C/N ratio and N content in both the forest floor and mineral soil,

Relationships among main soil properties and three N availability indices

A biological (3 and 6 weeks aerobic incubation) and a chemical method [successive extractions with cold 0.1 (H1-N) and 0.5M HCl (H2-N)] were applied to 21 soils to determine: a) the potentially mineralizable-N; b) the most useful soil variables for predicting soil-N availability; and c) their usefulness for predicting N uptake by a greenhouse wheat crop. At t = 3, both net N mineralized (NNM) and net N mineralization rate (NNMR) were correlated: a) positively with SOM- and CEC-related variables; and b) positively with soil δ15N and negatively with soil pH, suggesting that N-mineralization, dominated by nitrification, is associated with NO3 −-N losses and soil acidification. At t = 6, all previously discussed variables were important for NNM, but not for NNMR, mainly controlled by the available-P content. The importance of H1-N increased with N2-inputs and decreased with NO3 − losses and soil-N. Relationships of H1-N and H2-N with soil CEC and texture showed the strong relations among nutrients content, biological activity and N mineralization, and the recalcitrance of clay-bounded SOM. Soil-N correlations with wheat-N in absolute amount (positive) and as percentage of soil-N (negative) showed an important available-N supply by N-rich soils, despite their slow N turnover. All best regression models for wheat-N included 1-2 main available nutrients. The percentage of soil-N exported to plant biomass was negatively correlated with non-crystalline Al compounds and soil δ15N. Mineralized-N and wheat-N pools did not share many correlations with soil properties and seemed to come from different sources; consequently, the mineralized-N, which only explained a quarter of wheat-N variance, was not more useful than soil-N for predicting it. Wheat-N and hydrolysable-N only shared a positive correlation with soil-N, highlighting that they are two mainly unrelated N pools. Nevertheless, half of wheat-N variation was explained by its negative relationship with the percentage of soil-N as (H1+H2)-N; a possible explanation is that (H1+H2)-N is biologically labile and it constitutes an important soil-N fraction only when a limiting factor for microbial N mineralization or plant growth allowed its cumulation.

Influência da adubação orgânica no crescimento de tangerineira cv Clemenules e nos atributos químicos e microbiológicos do solo

O estudo objetivou avaliar o efeito de adubos orgânicos de diferentes origens, empregados como fontes de nitrogênio, sobre a atividade vegetativa das plantas de tangerineira (cv Clemenules/Poncirus trifoliata) durante a fase inicial de desenvolvimento e atributos químicos e microbiológicos do solo, nas condições pedológicas e climáticas da região de Pelotas-RS. O experimento foi conduzido durante dois anos (2002-2003), utilizando-se de plantas de 18 meses de idade, em vasos de 35 kg de capacidade, irrigadas por gotejamento, com seis repetições, sendo cada unidade experimental constituída por uma planta. Os tratamentos confrontados foram os seguintes: Controle (sem adubação); Adubação mineral (AM); Vermicomposto (VC); Vermicomposto + sangue bovino seco (VC + SB) e Resíduo da indústria de transformação de sucos cítricos (RITC). Ao final de cada estação vegetativa, foram avaliados os seguintes parâmetros biométricos: área de seção do porta-enxerto, altura da copa, número de brotações, folhas totais produzidas por planta e massa fresca e seca das folhas. Ao fim do experimento, três plantas de cada tratamento foram desplantadas, separados os principais órgãos, determinando-se a área total das folhas produzidas, a massa fresca e seca dos ramos principal e secundários e das raízes, separadas em grossas e finas. Na mesma época, amostras de solo de cada vaso foram coletadas e utilizadas para as análises químicas e microbiológicas. Em ambos os anos, foram coletadas folhas avaliando-se a concentração dos macronutrientes. Os efeitos mais marcantes sobre o desenvolvimento das plantas foram observados naquelas em que, além do vermicomposto (VC), foi fornecido periodicamente sangue seco (VC + SB). No solo desses tratamentos, também foi observado aumento do teor de nitrogênio total e da atividade microbiana do solo, avaliada pela respiração basal. O aporte de sangue seco (VC + SB) supriu satisfatoriamente as necessidades de N ocorrido nas plantas fertilizadas com adubo orgânico mesmo que estas últimas tenham recebido, no biênio de experimentação, cerca de 40% a menos de nitrogênio em relação às plantas adubadas com a fonte mineral (AM). Por isso, os tratamentos VC e RITC induziram um efeito limitado no desenvolvimento das plantas, enquanto, provavelmente, estas duas últimas fontes, e nas quantidades utilizadas, não forneceram suficientemente nitrogênio disponível para suprir a demanda das plantas. De maneira geral, a concentração dos macronutrientes foliares foi adequada em relação ao P, K, Ca e Mg e abaixo do normal para o N, demonstrando a elevada exigência desse nutriente na fase inicial de crescimento e desenvolvimento de tangerineiras cv. Clemenules.

INSECTS MEDIATE THE EFFECTS OF PROPAGULE SUPPLY AND RESOURCE AVAILABILITY ON A PLANT INVASION

Invasive species are a global threat to biodiversity and the functioning of natural ecosystems. Here, we report on a two-year experiment aimed at elucidating the combined and relative effects of three key controls on plant invasions: propagule supply, soil nitrogen (N) availability, and herbivory by native insects. We focus on the exotic species Lespedeza cuneata, a Rank 1 invasive species. Propagule supply and soil N-availability interacted to control the density and foliar cover of L. cuneata. In low N plots, density and foliar cover of L. cuneata were higher in the propagule addition plots than in the plots to which propagules were not added. Surprisingly, this interaction was significant only when the abundance of herbivores was experimentally reduced. This experiment provides evidence that native insect herbivores mediate the interactive effects of propagule supply and resources on invasion by a widespread invasive plant species.

Effect of Nitrogen Topdressing at Anthesis and the Association of Flag-Leaf Nitrogen with Grain Protein Concentration in Irrigated Spring Wheat

ABSTRACT Grain protein is an important component affecting the market value of hard red spring wheat (Triticum aestivum). In high-yielding irrigated environments, consistently attaining desired protein levels is a chronic problem. Nitrogen (N) management has a strong effect on protein concentration. The objective of this experiment was to evaluate the robustness of using flag-leaf N concentration as a tool for guiding in-season N application in order to obtain high-protein wheat. Three on-farm trials were conducted (each location using a cultivar of the farmer's choice) where N rates were varied. Nitrogen rates evaluated were 0, 79, 157, 236, and 314 kg ha−1. To evaluate the benefit of topdressing, all N was applied basally, or 45 kg ha−1 N was reserved from the basal dose for application at heading. Yield and protein response to applied N were variable across the three sites. This study postulated that this response was a function of initial soil-N availability. Where there was a yield response to N, it appeared that reserving a portion of N for topdressing increased protein but tended to decrease yield. At levels of N where yield was not limited, reserving a portion of N for topdressing did not appear to affect yield or protein. Although a linear relationship between grain protein and flag-leaf N was obtained by pooling data across sites, there was enough variation in this relationship to limit its utility as a tool for guiding in-season N application.

Foliar uptake and partitioning of urea-N by strawberry plants as affected by timing of supply and plant N status

SummaryFoliar spraying with urea is not as common in strawberry as in many other perennial crops, but may be a good approach to improve plant N status if soil-N availability and/or root uptake are insufficient to meet short-term plant N demand.The aims of this work were to examine possible differences in uptake and partitioning of foliar-N applied to strawberry plants at anthesis (T0) or at first fruit ripening (FFR; T1), and to examine if the level of soil-N fertilisation in the Spring affected uptake and partitioning at FFR. Foliar-N was applied to both leaf surfaces as a 1% (w/v) urea solution, using urea enriched with 9.883 atomic % 15N. Measurements and analyses were carried out 5 d after application. At FFR, foliar-N was applied to plants that had received soil-N levels of 0, 55 or 110 kg ha–1 (N0, N1 or N2). There was an effect of leaf age on the amount of urea solution adsorbed by leaves, which was higher in older than in younger leaves. Leaf-uptake of N was lower under treatment N1 at anthesis than at FFR, when an average of 99% of applied N was taken up. This was due, principally, to differences in leaf adsorption of urea solution between the two periods, with a possible minor effect of the larger leaf area at FFR than at anthesis due to differences in accumulated soil-N of 55 kg ha–1 and 19 kg ha–1, applied by T1 and T0 in treatment N1 respectively. At FFR, soil-N supply had no effect on the ability of leaves to absorb foliar-N. Nitrogen absorbed by leaves at the beginning of fruit harvest (FFR) was partitioned, preferentially, into the fruits; while significant amounts were retained in the leaves at anthesis. In conclusion, foliar spraying with urea represents a useful technique to increase strawberry N-content in the short period reported here. The high recovery of sprayed N indicates that this is an effective way to fertilise strawberry plants as an alternative or complement to soil-N supply.

ATMOSPHERIC CO2, SOIL-N AVAILABILITY, AND ALLOCATION OF BIOMASS AND NITROGEN BYPOPULUS TREMULOIDES

Our ability to predict whether elevated atmospheric CO2 will alter the cycling of C and N in terrestrial ecosystems requires understanding a complex set of feedback mechanisms initiated by changes in C and N acquisition by plants and the degree to which changes in resource acquisition (C and N) alter plant growth and allocation. To gain further insight into these dynamics, we grew six genotypes of Populus tremuloides Michx. that differ in autumnal senescence (early vs. late) under experimental atmospheric CO2 (35.7 and 70.7 Pa) and soil-N availability (low and high) treatments. Atmospheric CO2 concentrations were manipulated with open-top chambers, and soil-N availability was modified in open-bottom root boxes by mixing different proportions of native A and C horizon soil. Net N mineralization rates averaged 61 ng N·g−1·d−1 in low-N soil and 319 ng N·g−1·d−1 in high-N soil. After 2.5 growing seasons, we harvested above- and belowground plant components in each chamber and determined total biomass, N concentration, N content, and the relative allocation of biomass and N to leaves, stems, and roots. Elevated CO2 increased total plant biomass 16% in low-N soil and 38% in high-N soil, indicating that the growth response of P. tremuloides to elevated CO2 was constrained by soil-N availability. Greater growth under elevated CO2 did not substantially alter the allocation of biomass to above- or belowground plant components. At both levels of soil-N availability, elevated CO2 decreased the N concentration of all plant tissues. Despite declines in tissue N concentration, elevated CO2 significantly increased whole-plant N content in high-N soil (ambient = 137 g N/chamber; elevated = 155 g N/chamber), but it did not influence whole-plant N content in low-N soil (36 g N/chamber). Our results indicate that plants in high-N soil obtained greater amounts of soil N under elevated CO2 by producing a proportionately larger fine-root system that more thoroughly exploited the soil. The significant positive relationship between fine-root biomass and total-plant N content we observed in high-N soil further supports this contention. In low-N soil, elevated CO2 did not increase fine-root biomass or production, and plants under ambient and elevated CO2 obtained equivalent amounts of N from soil. In high-N soil, it appears that greater acquisition of soil N under elevated CO2 fed forward within the plant to increase rates of C acquisition, which further enhanced plant growth response to elevated CO2.

INTERACTIVE EFFECTS OF ATMOSPHERIC CO2AND SOIL-N AVAILABILITY ON FINE ROOTS OFPOPULUS TREMULOIDES

The objective of this experiment was to understand how atmospheric carbon dioxide (CO2) and soil-nitrogen (N) availability influence Populus tremuloides fine-root growth and morphology. Soil-N availability may limit the growth response of forests to elevated CO2 and interact with atmospheric CO2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO2 × N factorial field experiment and grew six genotypes of P. tremuloides for 2.5 growing seasons in 20 large open-top chamber/root-box experimental units at the University of Michigan Biological Station in northern lower Michigan (USA). In this paper we describe an integrated examination of how atmospheric CO2 and soil-N availability influence fine-root morphology, growth, mortality, and biomass. We also studied the relationship between root biomass and total soil respiration. Over 80% of the absorbing root length of P. tremuloides was accounted for by roots <0.4 mm in diameter, and specific root length (100–250 m/g) was much greater than reports for other temperate and boreal deciduous trees. Elevated atmospheric CO2 increased the diameter and length of individual roots. In contrast, soil N had no effect on root morphology. Fine-root length production and mortality, measured with minirhizotrons, was controlled by the interaction between atmospheric CO2 and soil N. Rates of root production and mortality were significantly greater at elevated CO2 when trees grew in high-N soil, but there were no CO2 effects at low soil N. Fine-root biomass increased 137–194% in high-N compared to low-N soil, and elevated atmospheric CO2 increased fine-root biomass (52%) in high soil N, but differences in low soil N were not significant. Across all treatments, dynamic estimates of net fine-root production were highly correlated with fine-root biomass (soil cores; r = 0.975). Mean rates of soil respiration were more than double in high-N compared to low-N soil, and elevated atmospheric CO2, when compared to ambient atmospheric CO2, increased mean rates of soil respiration 19% in 1995 and 25% in 1996. Across all treatments, total root biomass was linearly related to mean rates of soil respiration (r2 = 0.96). Our results indicate that atmospheric CO2 and soil-N availability strongly interact to influence P. tremuloides fine-root morphology, growth, and C turnover. Aspen-dominated ecosystems of the future are likely to have greater productivity fueled by greater nutrient uptake due to greater root length production. Further, it appears that elevated atmospheric CO2 will result in greater C inputs to soil through greater rates of fine-root production and turnover, especially in high-fertility soils. Increased C inputs to soil result in greater rates of soil respiration. At this time, it is not clear what effects increased rates of root turnover will have on C storage in the soil.

ATMOSPHERIC CO2AND THE COMPOSITION AND FUNCTION OF SOIL MICROBIAL COMMUNITIES

Elevated atmospheric CO2 has the potential to increase the production and alter the chemistry of organic substrates entering soil from plant production, the magnitude of which is constrained by soil-N availability. Because microbial growth in soil is limited by substrate inputs from plant production, we reasoned that changes in the amount and chemistry of these organic substrates could affect the composition of soil microbial com- munities and the cycling of N in soil. We studied microbial community composition and soil-N transformations beneath Populus tremuloides Michx. growing under experimental atmospheric CO2 (35.7 and 70.7 Pa) and soil-N-availability (low N 5 61 ng N·g 21 ·d 21 and high N 5 319 ng N·g 21 ·d 21 ) treatments. Atmospheric CO2 concentration was modified in large, open-top chambers, and we altered soil-N availability in open-bottom root boxes by mixing different proportions of A and C horizon material. We used phospholipid fatty-acid analysis to gain insight into microbial community composition and coupled this analysis to measurements of soil-N transformations using 15 N-pool dilution techniques. The infor- mation presented here is part of an integrated experiment designed to elucidate the phys- iological mechanisms controlling the flow of C and N in the plant-soil system. Our ob- jectives were (1) to determine whether changes in plant growth and tissue chemistry alter microbial community composition and soil-N cycling in response to increasing atmospheric CO2 and soil-N availability and (2) to integrate the results of our experiment into a synthesis of elevated atmospheric CO2 and the cycling of C and N in terrestrial ecosystems. After 2.5 growing seasons, microbial biomass, gross N mineralization, microbial im- mobilization, and nitrification (gross and net) were equivalent at ambient and elevated CO2, suggesting that increases in fine-root production and declines in fine-root N concentration were insufficient to alter the influence of native soil organic matter on microbial physiology; this was the case in both low- and high-N soil. Similarly, elevated CO2 did not alter the proportion of bacterial, actinomycetal, or fungal phospholipid fatty acids in low-N or high-N soil, indicating that changes in substrate input from greater plant growth under elevated CO2 did not alter microbial community composition. Our results differ from a substantial number of studies reporting increases and decreases in soil-N cycling under elevated CO 2. From our analysis, it appears that soil-N cycling responds to elevated atmospheric CO 2 in experimental situations where plant roots have fully colonized the soil and root-associated C inputs are sufficient to modify the influence of native soil organic matter on microbial physiology. In young developing ecosystems where plant roots have not fully exploited the soil, microbial metabolism appears to be regulated by relatively large pools of soil organic matter, rather than by the additional input of organic substrates under elevated CO 2.

Atmospheric CO 2 , Soil-N Availability, and Allocation of Biomass and Nitrogen by Populus tremuloides

Atmospheric CO 2 , Soil-N Availability, and Allocation of Biomass and Nitrogen by Populus tremuloides

GAS EXCHANGE, LEAF NITROGEN, AND GROWTH EFFICIENCY OFPOPULUS TREMULOIDESIN A CO2-ENRICHED ATMOSPHERE

Predicting forest responses to rising atmospheric CO2 will require an understanding of key feedbacks in the cycling of carbon and nitrogen between plants and soil microorganisms. We conducted a study for 2.5 growing seasons with Populus tremuloides grown under experimental atmospheric CO2 and soil-N-availability treatments. Our objective was to integrate the combined influence of atmospheric CO2 and soil-N availability on the flow of C and N in the plant–soil system and to relate these processes to the performance of this widespread and economically important tree species. Here we consider treatment effects on photosynthesis and canopy development and the efficiency with which this productive capacity is translated into aboveground, harvestable yield. We grew six P. tremuloides genotypes at ambient (35 Pa) or elevated (70 Pa) CO2 and in soil of low or high N mineralization rate at the University of Michigan Biological Station, Pellston, Michigan, USA (45°35′ N, 84°42′ W). In the second year of growth, net CO2 assimilation rate was significantly higher in elevated-CO2 compared to ambient-CO2 plants in both soil-N treatments, and we found little evidence for photosynthetic acclimation to high CO2. In the third year, however, elevated-CO2 plants in low-N soil had reduced photosynthetic capacity compared to ambient-CO2, low-N plants. Plants in high-N soil showed the opposite response, with elevated-CO2 plants having higher photosynthetic capacity than ambient-CO2 plants. Net CO2 assimilation rate was linearly related to leaf N concentration (log:log scale), with identical slopes but different intercepts in the two CO2 treatments, indicating differences in photosynthetic N-use efficiency. Elevated CO2 increased tissue dark respiration in high-N soil (+22%) but had no significant effect in low-N soil (+9%). There were no CO2 effects on stomatal conductance. At the final harvest, stem biomass and total leaf area increased significantly due to CO2 enrichment in high-N but not in low-N soil. Treatment effects on wood production were largely attributable to changes in leaf area, with no significant effects on growth efficiency. We conclude that harvest intervals for P. tremuloides on fertile sites will shorten with rising atmospheric CO2, but that tree size at canopy closure may be unaffected.

Dynamics of nitrogen and dry-matter partitioning and accumulation in broccoli (Brassica oleracea var. italica) in relation to extractable soil inorganic nitrogen

The effects of six rates of N fertilization (0, 125, 250, 375, 500 and 625 kg N ha−1) on the dynamics of N utilization relative to extractable inorganic N in the soil profile were determined for broccoli in three growing seasons. The amount of pre-existing extractable inorganic N in the soil was lowest for the spring planting, followed by the early-summer then late-summer plantings. During the first 2 wk after transplanting, plant dry-matter (DM) and N accumulation rates were low, and because of the mineralization of soil organic N the extractable soil inorganic N increased over that added as fertilizer, especially in the top 30 cm. From 4 wk after transplanting until harvest, DM and N accumulation in the plants was rapid and corresponded to a rapid depletion of extractable inorganic N from the soil. At high N-fertilization rates, leaf and stem DM and N accumulations at harvest were similar among the three plantings. However, the rates of accumulation in the two summer plantings were higher before and lower after inflorescence initiation than those in the spring planting. Under N treatments of 0 and 125 kg ha−1, total N in leaf tissue and the rate of leaf DM accumulation decreased while inflorescences developed. There was little extractable inorganic soil-N during inflorescence development in plots receiving no N fertilizer, yet inflorescence dry weights and N contents were ≥50 and ≥30%, respectively, of the maxima achieved with N fertilization. These results indicate that substantial N is translocated from leaves to support broccoli inflorescence growth under conditions of low soil-N availability. Key words: N translocation, N fertilizer

Clonal variation in above- and below-ground growth responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability

Trembling aspen (Populus tremuloides Michx.) is the most widely distributed tree species in North America making it important to terrestrial carbon and nutrient cycles. Due to anthropogenic climate change high latitude temperatures are expected to increase, making it necessary to assess the feedback between above- and below-ground carbon pools to increased temperature at sites of both high and low N-availability. We grew four clones of aspen at two levels of soil temperature and two levels of soil N-availability for 98 days and quantified photosynthesis, growth, biomass allocation, and root length production and mortality. High soil temperature increased rates of photosynthesis (65%), resulting in greater whole-plant growth (37%) through increases in roots, stems, and foliage; however these increases generally occurred only in soil of high N-availability. Root:shoot biomass allocation varied between clones but was unaffected by the soil temperature or N-availability treatments. Root length production and mortality increased at elevated soil temperature, but this response was modified by soil N-availability. At high soil temperature, soil N-availability had little effect on root dynamics, while at low soil temperature, high soil N-availability increased both the production and mortality (turnover) of roots. We conclude that trembling aspen has the potential for substantially greater growth and root turnover under conditions of warmer soil at sites of both high and low N-availability, but that allometric patterns of growth are under strong genetic, rather than environmental control.

Effects of balsam poplar (Populus balsamifera) tannins and low molecular weight phenolics on microbial activity in taiga floodplain soil: implications for changes in N cycling during succession

The transition from alder (Alnus tenuifolia) to balsam poplar (Populus balsamifera) is a critical turning point in primary succession on river floodplains in interior Alaska. Associated with the change in plant species are large changes in N cycling. N-fixation and nitrification decrease and the system becomes N-limited, with NH4+ dominating the inorganic N pool. Balsam poplar leaves contain large quantities of tannins and low molecular weight phenolic compounds. We evaluated the effect of these compounds on microbial respiration and N cycling in laboratory assays on soils from an alder-dominated site. Plant compounds were purified and applied to silica gel as an inert carrier. Both tannins and phenolics caused net N-immobilization over a 30-day assay. However, tannins inhibited respiration while phenolics stimulated it. There were no specific effects on nitrification. Thus, tannins acted as a general microbial inhibitor, while phenolics acted as a growth substrate. By inhibiting mineralization while stimulating immobilization, poplar secondary compounds may reduce soil N-availability during the transition betwen alder and poplar stages in succession. Keywords: respiration, mineralization, tannins, secondary chemicals, succession, plant–microbe interactions.