八島 未和

This study aims to characterize soil chemical properties and microbial biomass, greenhouse gas production, and organic matter dynamics in upland rice field as affected by the long-term fertilizer managements in Uganda. Soil total C (TC) and N (TN) contents were in the relatively smaller range under different fertilizer treatments, even after 20 crop seasons. However, available phosphate contents showed positive correlation with average yield of upland rice. Incubation experiments were conducted under aerobic or under flooding conditions to measure CO2, methane, and nitrous oxide productions. After the incubation, soil samples were extracted to quantify nitrification rate for aerobic condition and ammonification rate for flooding condition. Soil microbial biomass carbon (MBC) and nitrogen were measured. Stable isotope ratio of C-13 and N-15 were also determined for the soil samples. CO2 production potential under aerobic condition was higher than the flooding condition. The qCO(2) (CO2/MBC) in the treatment applied with compost tended to be higher than the other treatments. Positive correlation between nitrous oxide production and nitrification was found. The delta C-13 values of the soil samples indicated that the effect of C4 plants before rice cultivation still remained, while the contribution of biological N-2 fixation was little according to delta N-15 values. These results indicate that soil microbial biomass in upland rice field of the long-term fertilizer experiment in Uganda was characterized with higher qCO(2). Greenhouse gas production was affected by fertilizer management, while soil organic C before the long-term experiment still remained in the experiment.

Enhancing crop yield response to elevated CO2 concentrations (E-[CO2]) is an important adaptation measure to climate change. A high-yielding indica rice cultivar "Takanari" has recently been identified as a potential candidate for high productivity in E-[CO2] resulting from its large sink and source capacities. To fully utilize these traits, nitrogen should play a major role, but it is unknown how N levels influence the yield response of Takanari to E-[CO2]. We therefore compared grain yield and quality of Takanari with those of Koshihikari, a standard japonica cultivar, in response to Free-Air CO2 enrichment (FACE, +200 mu mol mol(-1)) under three N levels (0, 8, and 12g m(-2)) over three seasons. The biomass of both cultivars increased under E-[CO2] at all N levels; however, the harvest index decreased under E-[CO2] in the N-limited treatment for Koshihikari but not for Takanari. The decreased harvest index of Koshihikari resulted from limited enhancement of spikelet number under N-limitation. In contrast, spikelet number increased in E-[CO2] in Takanari even without N application, resulting in significant yield enhancement, averaging 18% over 3 years, whereas Koshihikari exhibited virtually no increase in yield in E-[CO2] under the N-limited condition. Grain appearance quality of Koshihikari was severely reduced by E-[CO2], most notably in N-limited and hot conditions, by a substantial increase in chalky grain, but chalky grain % did not increase in E-[CO2] even without N fertilizer. These results indicated that Takanari could retain its high yield advantage over Koshihikari with limited increase in chalkiness even under limited N conditions and that it could be a useful genetic resource for improving N use efficiency under E-[CO2].

Methane is one of the greenhouse gases emitted from paddy soil ecosystems and may induce global warming and climate change; therefore, mitigation options are urgently required to establish mitigation technology to reduce methane emission without affecting rice production. Methane is produced by a balance between oxidizing agents (such as iron) and reducing agents (easily decomposable soil organic matter), according to the so-called Takai theory. To evaluate options for mitigating methane production potential and to examine the applicability of the Takai theory in Southeast Asian paddy soils, 23 soil samples were collected from Thailand, Indonesia, Philippines, and Vietnam. These soil samples were anaerobically incubated to measure their methane production potential and examined to see whether their chemical properties, such as the ferrous, total iron, and organic matter contents, were correlated. We found a significant negative correlation between the methane production potential and the total iron content, and a positive correlation between the methane production potential and the hexose content, as an index for a soil's easily decomposable organic matter content. The methane-C/CO2-C production ratio was also positively correlated with the mineralizable nitrogen/ferrous contents ratio, which indicated that the Takai theory, established for Japanese paddy soils, is also useful in Southeast Asian paddy soils and that the soil's iron content is important to estimate the methane production potential.

To better predict forest productivity under rising atmospheric CO2 concentration ([CO2]), it is critical to understand how intrinsic water-use efficiency (WUEi) and its relationship with tree growth are affected by the concomitant changes in environmental conditions such as precipitation, temperature, and air pollution that either enhance or undermine any potential CO2 fertilization effect. We investigated changes in delta C-13 and WUEi in annual rings and basal area increment (BAI) of Pinus densiflora (from 1968 to 2007) and Quercus variabilis (from 1970 to 2007) trees in relation to precipitation, temperature, and air pollution in a humid temperate forest. The WUEi of P. densiflora increased by 39.9%, whereas that of Q. variabilis did not change over time in the study period. The WUEi was not affected by precipitation for both species but increased (P < 0.001) with temperature for P. densiflora and with SO2 emissions for both species. Multiple regression models suggested that the effect of [CO2] on tree growth was much higher than temperature; however, for the period (1998 to 2007) when SO2 emissions data were available, SO2 emission was the driver of changes in BAI and WUEi, and temperature effects became stronger than [CO2]. Overall, BAI and WUEi were positively (P < 0.001) correlated for P. densiflora, but not for Q. variabilis. We conclude that temperature and air pollution rather than precipitation were key determinants of WUEi at the study site and that the two species had contrasting responses to environmental changes. (C) 2015 Elsevier B.V. All rights reserved.

Sand mixing effects on chloroform fumigation-extraction (CFE) efficiency in the determination of soil microbial biomass carbon (MBC) of water-saturated soils were investigated in two soils with different soil organic C (SOC) contents. Sand mixing increased (P < 0.001) MBC by up to 20% and 107% for the soil with low and high SOC values, respectively, suggesting that the creation of water-empty macropores by sand mixing improved chloroform fumigation efficiency. This study demonstrates that sand mixing is a feasible measure to improve CFE efficiency for the determination of the MBC of water-saturated soils, particularly for soils with a high SOC.

Canada bluejoint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] outcompetes overstory tree species such as white spruce [Picea glauca (Moench) Voss] by creating a thick litter layer and competing for the available nitrogen (N). This study was conducted to investigate the effects of bluejoint litter layer (with or without litter removal) and N fertilization on soil water and N availabilities using principal component analysis (PCA) and foliar delta N-15 and delta C-13 of bluejoint in a plantation in north-central Alberta, Canada. PCA using soil properties and understory growth data demonstrated that N fertilization was more effective in changing the soil environment and resource availabilities for bluejoint growth than litter layer removal. The increase in soil N availability by N fertilization was linked with increased bluejoint foliar delta N-15 (by around 3 parts per thousand) in fertilized plots, as a result of greater N isotopic fractionation in the fertilized plots. The more negative delta C-13 (by around 1 parts per thousand) of bluejoint in litter layer-removed plots suggested that litter layer removal increased soil water availability, indicating that the litter layer reduced soil water availability on the site. Therefore, results from this and previous studies showed that the litter layer decreased both soil water and N availabilities. Although the exact mechanisms of the benefit of the litter layer for bluejoint remains unknown, bluejoint likely adversely impacted tree growth by competing for N due to its strong N acquisition ability under soil resource-limiting conditions.

The aim of this study was to evaluate the cumulative net free-living nitrogen (N) fixation and its response to elevated atmospheric carbon dioxide (CO2) in the submerged surface of paddy soil. The study site was an actual paddy area in central Japan that was equipped with a free-air CO2 enrichment (FACE) facility and was composed of four bays, where each bay had an elevated CO2 plot (FACE plot) and an ambient plot. Field incubation was conducted at subplots without N fertilizer application from May 28 to August 18, 2012 (82 days). The CO2 enrichment was an average of +528 ppm during the study period at the subplots. The changes in total N (TN) content in the surface soil (0-1 cm) were determined in comparison to the initial TN content. The cumulative net N fixations during the 82-day study period were 47.1 +/- A 3.7 and 43.3 +/- A 5.8 (standard error) kg N ha(-1) in the FACE and ambient plots, respectively. The difference between the FACE and ambient plots was not significant (p = 0.05). However, these values were partly affected by the charophyte blooms, which are not involved in N fixation. The results are not conclusive, and the one bay without charophyte blooming displayed a significant increase in the cumulative net N fixation (approximately 10 kg N ha(-1)) with CO2 enrichment (p < 0.001).

Land-use type and nitrogen (N) addition strongly affect nitrous oxide (N2O) and carbon dioxide (CO2) production, but the impacts of their interaction and the controlling factors remain unclear. The aim of this study was to evaluate the effect of both factors simultaneously on N2O and CO2 production and associated soil chemical and biological properties. Surface soils (0-10 cm) from three adjacent lands (apple orchard, grassland and deciduous forest) in central Japan were selected and incubated aerobically for 12 weeks with addition of 0, 30 or 150 kg N ha(-1) yr(-1). Land-use type had a significant (p < 0.001) impact on the cumulative N2O and CO2 production. Soils from the apple orchard had higher N2O and CO2 production potentials than those from the grassland and forest soils. Soil net N mineralization rate had a positive correlation with both soil N2O and CO2 production rates. Furthermore, the N2O production rate was positively correlated with the CO2 production rate. In the soils with no N addition, the dominant soil properties influencing N2O production were found to be the ammonium-N content and the ratio of soil microbial biomass carbon to nitrogen (MBC/MBN), while those for CO2 production were the content of nitrate-N and soluble organic carbon. N2O production increased with the increase in added N doses for the three land-use types and depended on the status of the initial soil available N. The effect of N addition on CO2 production varied with land use type; with the increase of N addition doses, it decreased for the apple orchard and forest soils but increased for the grassland soils. This difference might be due to the differences in microbial flora as indicated by the MBC/MBN ratio. Soil N mineralization was the major process controlling N2O and CO2 production in the examined soils under aerobic incubation conditions.

Historical growth response of Quercus variabilis, which is the most important deciduous timber species in Korea, was investigated using the width and C isotope ratio (C-13/C-12 denoted as delta C-13) of the annual ring from 1975 to 2007. Tree disks were collected from three Q. variabilis trees with different growth statuses from a site in the Mt. Naejang area, and analyzed for annual ring width and delta C-13. Basal area increment (BAI) of the annual ring was calculated from the width data, and carbon isotope discrimination (Delta) was calculated using delta C-13. The inter-correlations among BAI, Delta, and environmental variables were explored. The BAT was positively (p < 0.001) correlated with atmospheric CO2 concentration ([CO2]), reflecting increased net photosynthesis with [CO2], whereas the negative correlations of BAT with either NO2 (p < 0.05) or O-3 (p < 0.05) concentrations suggested that atmospheric pollution might have restricted tree growth to some degree. The Delta was positively correlated with both temperature (p < 0.05) and [CO2] (p < 0.001), and BAT was also positively correlated with Delta (p < 0.001). However, precipitation was correlated with neither BAT nor Delta, indicating that the precipitation amount is sufficient for tree growth in the study site. Such relationships suggest that stomatal rather than non-stomatal control is the predominant mechanism of photosynthetic acclimation of Q. variabilis under changing environmental conditions in the study site where water availability is not limited.

Only limited information is available in the research area on the effect of elevated CO2 concentration ([CO2]) and air temperature (T-air) on the fertilizer N uptake by rice. This study was conducted to investigate changes in rice uptake of N derived from fertilizer (NDFF) and soil (NDFS) as well as fertilizer N uptake efficiency (FUE) with elevated [CO2] and T-air in two soils with different fertility.Rice (Oryza sativa L.) plants were grown with N-15-urea for two growing seasons (2007 in the less fertile and 2008 in the more fertile soil) in temperature gradient chambers under two (ambient and elevated) levels of [CO2] and T-air regimes. At harvest, dry matter (DM) and N uptake amount of rice compartments (root, shoot, and grain) were determined.The DM of whole rice increased (P < 0.01) with co-elevation of [CO2] and T-air in both years (by 28.0 % in 2007 and by 27.4 % in 2008). The DM in 2008 was greater than that in 2007 by 48.1 to 63.1 % probably due to better soil fertility as well as longer sunshine hours (456 h vs. 568 h). Co-elevation of [CO2] and T-air increased total N uptake, NDFF, and NDFS by 19.4 to 29.1 % in general compared to the ambient conditions. The FUE increased with co-elevation of [CO2] and T-air from 46.5 to 59.5 % in 2007 and from 36.7 to 43.8 % in 2008.The projected global warming with elevated [CO2] is expected to increase FUE via enhanced DM accumulation with less increments in the soils that have higher indigenous soil N availabilities.

The production/consumption of greenhouse gases (GHG) in soils are of great importance in global warming, but the involved soil physico-chemical and biological characteristics affecting GHG production and consumption potentials are poorly understood in different land-use types. Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) production/consumption potentials from four land-use types and 10 soil types in central Japan and eastern Hungary, and their relationships with soil characteristics, were investigated. The average of CO2 production in Japanese soils was significantly higher than that of Hungarian soils due to the relatively higher microbial biomass carbon (MBC) content. N2O production from both countries' soils did not exhibit a significant difference. Most soils except Japanese paddy and soybean soils showed the potentials for CH4 consumption. Forest and grassland soils had relatively higher CO2 and N2O production than orchard and cropland soils for both countries. From regression analyses, it could be concluded that soil total nitrogen (TN) and ammonium-nitrogen (NH4 (+)-N) account for 40.8% and 25.5% variations of the soils' CO2 and N2O productions, respectively. The CH4 consumption was positively correlated with soil carbon/nitrogen (C/N) ratio, while soil MBC availability could account for 15% variation of CH4 consumption under aerobic conditions.

Soil acidification is known to be one of the constraints of tree growth; however, it is unclear how it affects tree growth at photosynthesis level (i.e., through affecting stomatal conductance vs. carboxylation rate) during the growth of trees. This paper studied the effects of soil acidification on Pinus densiflora foliar chemistry and tree ring C isotope ratio (C-13/C-12, expressed as delta C-13) and their relationship with tree growth.Tree growth (diameter, annual growth ring area, and root biomass), soil chemistry (pH, mineral N, and exchangeable Ca and Al), foliage chemistry (N, Ca/Al, and delta C-13), and tree ring delta C-13 in P. densiflora stands along a soil pH gradient (from 4.38 to 4.83, n = 9) in southern Korea were investigated.Overall, trees with relatively poor growth under more acidic soil conditions (low pH and Ca/Al) had lower values of foliar N concentration and delta C-13 and tree ring delta C-13, suggesting that restricted N uptake under more acidic soil conditions caused N limitation for photosynthesis, leading to poor tree growth. In addition, relationships between mean annual area increment and carbon isotope discrimination of tree rings at five-yr intervals from 1968 to 2007 revealed that the impact of soil acidification on tree growth became severer during the last 15 yrs as negative correlations between them became significant after 1993.Reduced N uptake under acidic soil conditions resulted in lower radial growth of P. densiflora via non-stomatal limitation of photosynthesis.

Background and aims Canada bluejoint grass is a strong competitor for water and N against young white spruce trees in regenerating boreal forests in moist habitats. We investigated foliar delta C-13 and delta N-15 responses of white spruce to understory removal and N fertilization to infer treatment effects on soil water and N dynamics.Methods An experiment with understory removal and N fertilization as the main and split-plot treatments was conducted in a 13-year-old white spruce plantation in the boreal region for 2 years. Soil N availability, gravimetric soil moisture content, tree growth, foliar C and N concentrations, and foliar delta C-13 and delta N-15 values were determined.Results Foliar delta C-13 was increased by understory removal in the second year and by N fertilization in the first year. There was a positive (P < 0.01) relationship between foliar delta C-13 and N concentration, suggesting that understory removal and N fertilization improved tree growth via increased N availability and thus increased carboxylation rate of photosynthesis. The positive correlation (P < 0.01) of foliar delta N-15 with foliar N concentration and with available soil N indicated that understory removal and N fertilization facilitated N isotope fractionation accompanying increased soil N turnover.Conclusions Foliar delta C-13 and its relationship with other variables suggested that understory removal and N fertilization improved white spruce growth mainly through enhanced carboxylation rate due to greater N availability rather than increased stomatal conductance. Specifically, understory removal and N fertilization increased soil N availability and subsequently induced N loss from the system, leading to a higher N concentration and delta N-15 in the foliage. Control of competing vegetation and N fertilization could be used as management tools to reduce N limitation.

The effects of different land-use histories on contents of soil carbon (C) and nitrogen (N) and fluxes of greenhouse gases [carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)] measured using the closed chamber method were investigated in the Arkaim museum reserve located in the South Ural of Russia. A natural forest site (NF) and two grassland sites that had different land-use histories (CL: cropland until 1991; PST: pasture until 1991; both sites have been fallow for 18 years) were selected for soil sampling and gas flux measurements. The vegetation in NF was mainly Betula pendula Roth. with steppe cherry and grassy cover. Perennial grasses (Stipa spp., Festuca spp. and others) have been planted in CL and PST since 1991 to establish reserve mode, and the projective cover of these plants were > 90% in both sites in 2009. Soil samples were taken from the A horizon in the three sites, and additionally samples of the O horizon were taken from NF. The contents of soil C and N [total C, total N, soluble organic C, soluble N and microbial biomass C (MBC)] in the O horizon of NF were the largest among all investigated soils (p < 0.05). Additionally, the total C, total N and MBC in PST were significantly larger than in CL (p < 0.05). Positive CO2 fluxes (i.e., CO2 efflux) in all three investigated sites were observed. The CO2 efflux in NF was significantly larger than in CL and PST (129, 30 and 25 mg C m(-2) hour(-1), respectively, p < 0.05), although there was no significant difference in values of CO2 efflux between CL and PST. There were no significant differences in the fluxes of CH4 and N2O among NF, CL and PST (p > 0.05). Our current research indicated that, in soils of the Eurasian steppe zone of Russia, total C, total N and MBC were affected not only by current land-use (i.e., fallow grassland vs. natural forest) but also by past (until 18 years ago) land-use.

Quantification of rhizodeposition (root exudates and root turnover) represents a major challenge for understanding the links between above-ground assimilation and below-ground anoxic decomposition of organic carbon in rice paddy ecosystems. Free-air CO2 enrichment (FACE) fumigating depleted (CO2)-C-13 in rice paddy resulted in a smaller C-13/C-12 ratio in plant-assimilated carbon, providing a unique measure by which we partitioned the sources of decomposed gases (CO2 and CH4) into current-season photosynthates (new C) and soil organic matter (old C). In addition, we imposed a soil-warming treatment nested within the CO2 treatments to assess whether the carbon source was sensitive to warming. Compared with the ambient CO2 treatment, the FACE treatment decreased the C-13/C-12 ratio not only in the rice-plant carbon but also in the soil CO2 and CH4. The estimated new C contribution to dissolved CO2 was minor (ca. 20%) at the tillering stage, increased with rice growth and was about 50% from the panicle-formation stage onwards. For CH4, the contribution of new C was greater than for heterotrophic CO2 production; ca. 40-60% of season-total CH4 production originated from new C with a tendency toward even larger new C contribution with soil warming, presumably because enhanced root decay provided substrates for greater CH4 production. The results suggest a fast and close coupling between photosynthesis and anoxic decomposition in soil, and further indicate a positive feedback of global warming by enhanced CH4 emission through greater rhizodeposition.

Upland blueberry (Vaccinium ashei Read.) requires acidic and organic matter rich soils for its cultivation. Recently, the raised bed cultivation system has widely been used for blueberry cultivation, but it has a potential to cause nitrogen (N) losses via leaching and nitrous oxide (N2O) emission especially with high rate of N application. We investigated the effects of organic amendments on N losses; via leaching ammonium ([image omitted]) and nitrate ([image omitted]) and N2O emission under acid soil conditions. The treatments were (i) ammonium sulfate application (AS), (ii) AS with peat moss (AS + PM), and (iii) AS with sawdust compost (AS + SD). A container (surface area: 47 cm x 40 cm = 0.188 m2) experiment to simulate raised bed cultivation system using an ochric Cambisol was conducted at Chiba University, Japan, during 2008 and 2009. A blueberry plant in each container received N fertilizer three times: 14.8, 23.8, and 14.8 g N m-2 in July 2008, March, and July 2009, respectively. Incorporation of peat moss and sawdust compost with ferrous sulfate effectively decreased soil pH to its optimal ranges (4.0-5.2) and resulted in increased growth and yield of blueberry (berry yield of 2009: 93, 285, and 348 g plant-1 in AS, AS + PM, and AS + SD treatment, respectively) due to increased soil porosity and consequently enhanced plant root biomass. Concurrently, the AS + SD treatment resulted in the least N losses, followed by AS + PM and AS treatment both through leaching and N2O emission. The emitted N2O amount over about two-year blueberry cultivation were 1.28, 0.80, and 0.58 g N m-2 for AS, AS + PM, and AS + SD treatments, respectively. These results indicated that application of organic amendments decreased N losses to the environment most likely due to increased plant N uptake, microbial N immobilization, and consequent reduction in nitrification, denitrification, and leaching.

Azolla filiculoides is a floating aquatic fern growing in tropical and temperate freshwater ecosystems. As A. filiculoides has symbiotic nitrogen-fixing cyanobacteria (Anabaena azollae) within its leaf cavities, it is cultivated in rice paddies to improve N availability and suppress other wetland weeds. To understand how C assimilation and N accumulation in A. filiculoides respond to elevated atmospheric carbon dioxide concentration (CO2) in combination with P addition and higher temperatures, we conducted pot experiments during the summer of 2007 and 2008. In 2007, we grew A. filiculoides in pots at two treatment levels of added P fertilizer and at two levels of [CO2] (380 ppm for ambient and 680 ppm for elevated [CO2]) in controlled-environment chambers. In 2008, we grew A. filiculoides in four controlled-environment chambers at two [CO2] levels and two temperature levels (34/26A degrees C (day/night) and 29/21A degrees C). We found that biomass and C assimilation by A. filiculoides were significantly increased by elevated [CO2], temperature, and P level (all P < 0.01), with a significant interaction between elevated [CO2] and added P (P < 0.01). Tissue N content was decreased by elevated [CO2] and increased by higher temperature and P level (all P < 0.01). The acetylene reduction assay showed that the N-fixation activity of A. filiculoides was not significantly different under ambient and elevated [CO2] but was significantly stimulated by P addition. N-fixation activity decreased at higher temperatures (34/26A degrees C), indicating that 29/21A degrees C was more suitable for A. azollae growth. Therefore, we conclude that the N accumulation potential of A. filiculoides under future climate warming depends primarily on the temperature change and P availability, and C assimilation should be increased by elevated [CO2].

When synthetic fertilizer is co-applied with composted manure (compost), the high urease activity of compost may stimulate urea hydrolysis, thus increasing NH3 volatilization when urea is co-applied as a synthetic fertilizer. In this study, the interactive effects of compost type (low vs. high urease activity, referred to as CL and CH, respectively) and synthetic fertilizer form (urea vs. ammonium sulfate) were assessed in a 60-day greenhouse study. The compost was applied as a basal fertilization only at the initiation of the experiment, and the fertilizers (N-15 labeled or not) were split into basal and additional fertilization. During the 10 days after the basal fertilization, co-application of CL with urea did not increase NH3 volatilization as compared with urea alone treatment. However, co-application of CH with urea resulted in a significant increase in NH3 volatilization by more than 3-fold, not only from applied fertilizer-N-15 but also from compost and/or soil over the other two (urea alone and urea with CL) treatments. Meanwhile, when ammonium sulfate was co-applied with CH, NH3 volatilization from fertilizer became 3-fold lower than that when urea was co-applied. During the second 10 days after the application of urea as an additional fertilizer, the amount of NH3 volatilization from CH-treated soil did not differ from that observed with CL, and this is probably attributable to a reduction in urease activity in soils over time. These results indicate that not only the combination of compost and synthetic fertilizer, but also the time difference between the application of compost and synthetic fertilizer, are crucial to reducing NH3 volatilization.

A laboratory incubation was conducted to evaluate nitrous oxide (N2O) production during nitrification and the effect of a nitrification inhibitor on N2O production from different profiles in a Japanese orchard Andosol. Soils were collected from five profiles: A1, A2, Bw1, Bw2, and BC. The soils were treated with ammonium sulfate at the rate of 200mgNkg-1 with or without dicyandiamide (DCD) and incubated under aerobic conditions for 32 days. The net nitrification rate without DCD during the first 8 days was greater in the surface soils than in the subsurface soils. Accordingly, the surface soils showed a greater cumulative N2O production than the subsurface soils. Application of DCD significantly reduced the nitrification rate and thus N2O production from any depths of soils by 33.8 to 62.9%. Our study showed that substantial N2O was produced from the subsurface soil, although the amount was less than from the surface soils.

How resource availabilities affect the competitiveness of Canada bluejoint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] is poorly understood. Bluejoint is a widespread grass species in boreal forests and competes with tree species such as white spruce [Picea glauca (Moench) Voss] for belowground resources (e.g., soil N and water) when their supply is limited. In this green house-based study, we tested the following hypotheses: (i) bluejoint competition reduces white spruce growth when belowground resource availabilities are limited; (ii) greater N and water availabilities may increase bluejoint competition and its adverse effects on white spruce growth; and (iii) white spruce foliar delta C-13 and delta N-15 are affected by soil N and water availabilities and bluejoint competition. A 2 x 2 x 2 (competition x N availability x water availability) factorial experiment was conducted using pots of planted white spruce seedlings with or without bluejoint. Bluejoint competition reduced the volume index (diameter(2) x height) of white spruce by 50%. The competitiveness of bluejoint appeared to be independent of resource availabilities, but bluejoint had greater growth response to increased N availability than white spruce. Bluejoint competition depleted white spruce foliar delta C-13 and delta N-15 by 1.2 and 1.2%o, respectively, even under adequate water supply, indicating that N deficiency caused by bluejoint competition had a C-13 discrimination during photosynthesis) compared with the potential effect of drought stress on foliar delta C-13, and that strong NH4 uptake by bluejoint may have prevented significant soil N losses and N-15 enrichment through nitrification and subsequent denitrification.

Canada bluejoint grass [Calamagrostis canadensis (Michx.) Beauv., referred to as bluejoint below] is a competitive understory species widely distributed in the boreal region in North America and builds up a thick litter layer that alters the soil surface microclimate in heavily infested sites. This study examined the effects of understory removal, N fertilization, and litter layer removal on litter decomposition, soil microbial biomass N (MBN), and net N mineralization and nitrification rates in LFH (the sum of organic horizons of litter, partially decomposed litter and humus on the soil surface) and mineral soil (0-10 cm) in a 13-year-old white spruce [Picea glauca (Moench.) Voss] plantation infested with bluejoint in Alberta, Canada. Removal of the understory vegetation and the litter layer together significantly increased soil temperature at 10 cm below the mineral soil surface by 1.7 and 1.3 degrees C in summer 2003 and 2004, respectively, resulting in increased net N mineralization (by 1.09 and 0.14 mg N kg(-1)day(-1) in LFH and mineral soil, respectively, in 2004) and net nitrification rates (by 0.10 and 0.20 mg N kg(-1) day(-1) in LFH and mineral soil, respectively, in 2004). When the understory vegetation was intact, nitrification might have been limited by NH4+ availability due to competition for N from bluejoint and other understory species. Litter layer removal increased litter decomposition rate (percentage mass loss per month) from 2.6 to 3.0% after 15 months of incubation. Nitrogen fertilization did not show consistent effects on soil MBN, but increased net N mineralization and nitrification rates as well as available N concentrations in the soil. Clearly, understory removal combined with N fertilization was most effective in increasing rates of litter decomposition, net N mineralization and nitrification, and soil N availability. The management of understory vegetation dominated by bluejoint in the boreal region should consider the strong effects of understory competition and the accumulated litter layer on soil N cycling and the implications for forest management.

Canada bluejoint grass (Calamagrostis canadensis (Michx.) Beauv.) is a very competitive understory species and can create a thick litter layer that may decrease tree growth in white spruce (Picea glauca (Moench.) Voss) stands in the boreal region. We examined the effects of Canada bluejoint grass dominated understory competition, N fertilization, and litter layer removal on the nutrition and growth of white spruce, in relation to soil N availability in a 13-year-old plantation near Whitecourt, Alberta, Canada, in 2003 and 2004. We hypothesized that: (1) understory competition reduces white spruce growth as a result of competition for available N; (2) litter layer removal increases white spruce growth by increasing soil temperature and N availability; (3) N fertilization improves white spruce growth by increasing N availability. Soil gravimetric moisture content in LFH was reduced by understory competition and litter layer removal in 2003, which had a very dry summer. Understory removal increased white spruce diameter growth, 100-needle biomass, and needle N concentration and content. In 2003, N fertilization consistently increased 100-needle weight and needle N content when litter layer was removed, but decreased or did not affect those values without litter layer removal. Nitrogen fertilization increased needle N concentration in 2004. The inconsistent effects of N fertilization may be due to the short-term nature of fertilization effects and immobilization of N by organic matter. Rates of white spruce diameter growth and soil mineral N supply in 2004 were related (R-2 = 0.65, P < 0.001). Vector analysis showed that N fertilization or litter layer removal with intact understory vegetation either increased or did not affect needle N concentration but decreased tree biomass. We conclude that understory control will benefit tree growth in young white spruce stands in the boreal region that have passed the free-to-grow stage. However, N fertilization or litter layer removal alone in bluejoint infested sites may not benefit the trees. (c) 2006 Elsevier B.V. All rights reserved.