Responses of CO2 efflux from an alpine meadow soil on the Qinghai Tibetan Plateau to multi-form and low-level N addition

Aims

To assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO₂) exchange to N addition.

Methods

Based on a multi-form, low-level N addition experiment, soil CO₂ effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO₂ efflux.

Results

The results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH ₄ ⁺ -N pool. N applications significantly increased aboveground biomass and soil CO₂ efflux; moreover, this effect was more significant from NH ₄ ⁺ -N than from NO ₃ ^? -N fertilizer. In addition, the soil CO₂ efflux was mainly driven by soil temperature, followed by aboveground biomass and NH ₄ ⁺ -N pool.

Conclusions

These results suggest that chronic atmospheric N deposition will stimulate soil CO₂ efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth.     

Occurrence and intensity of wild boar disturbances, effects on the physical and chemical soil properties of alpine grasslands

Background and aims

Physical and chemical soil properties determine local plant conditions and resources, affecting plants’ ability to respond to disturbances. In alpine grasslands, wild boar disturbances occur at different intensities, what may affect differently their soil properties. Alpine soils from five contrasted plant communities were explored within and outside disturbances, accounting for an overall and community scale effect. Additionally, we analysed the effect of disturbance intensity on soil NO₃ ^--N and NH₄ ⁺-N.

Methods

Soils were analyzed for physical (bulk density, moisture content and electrical conductivity), and chemical properties (pH, total N and C, oxidizable C, C:N ratio, available K, P, Ca²⁺, Na⁺ and Mg²⁺). Resin bags were used to compare the effect of the disturbance occurrence and intensity on soil NO₃ ^--N and NH₄ ⁺-N.

Results

Bulk density, total N and NO₃ ^--N concentration were significantly higher in disturbed areas, while soil moisture, C:N, NH₄ ⁺-N, Na⁺, Mg²⁺ and Ca²⁺ concentrations were significantly lower. However, low disturbance intensity reduced NO₃ ^--N and increased NH₄ ⁺-N concentrations.

Conclusions

Wild boar occurrence and intensity strongly alter physical and chemical conditions of alpine soils, increasing soil compaction, and altering the availability of N forms. These changes may affect most plant species, thus affecting the structure and dynamics of alpine plant communities.     

Respiration costs associated with nitrate reduction as estimated by 14CO2 pulse labeling of corn at various growth stages

Utilization of nitrogen in the form of either nitrate (NO ₃ ^? ) or ammonium (NH ₄ ⁺ ) ions may affect the carbohydrate metabolism and energy budget of plants. Recent studies showed that greater expenses of NO ₃ ^? to NH ₄ ⁺ reduction mostly occur in the roots and during darkness. Fertilization of corn with ¹⁵N-labeled nitrate and ammonium, combined with pulse labeling of plants in a ¹⁴CO₂ atmosphere at the V6 and V8 growth stages, allowed us to evaluate the effect of N form on the CO₂ efflux from soil. NH ₄ ⁺ oxidation was inhibited by adding dicyandiamide. In respect to ammonium, nitrate addition increased root-derived CO₂ efflux from corn by 2.6 times at stage V6 and by 1.8 times at stage V8. The time of peak ¹⁴CO₂ efflux from soil also differed between two growing stages: at V6, efflux peaked only on the second day after pulse labeling, while at V8 this occurred within the first 6 h. The strong effect of NO ₃ ^? and NH ₄ ⁺ on root respiration requires considering the N form in the soil and the nitrate reduction site location in a plant when modeling soil respiration changes and when separately estimating individual CO₂ sources that contribute to the total soil CO₂ efflux.     

Importance of the nitrogen source in the grass species Brachiaria brizantha responses to sulfur limitation

Background and aims

Plant responses to S supply are highly dependent on N nutrition. We investigated the effect of S status on metabolic, nutritional, and production variables in Brachiaria brizantha treated with different N forms. Additionally, ¹⁵N and ³⁴S root influx were determined in plants under short- and long-term S deprivation.

Methods

Plants were submitted to soil fertilization treatments consisted of combinations of N forms [without N, ammonium (NH₄ ⁺), nitrate (NO₃ ^?) or NH₄ ⁺+NO₃ ^?] at S rates (0, 15, 30, or 45 mg dm^?3). N and S influx capacity was determined in hydroponically-grown plants.

Results

Shoot production due to S supply increased 53, 145 and 196 % with NH₄ ⁺, NH₄ ⁺+NO₃ ^? and NO₃ ^? treatments, respectively. No or low S impaired protein synthesis and led to high accumulation of N-NO₃ ^? and asparagine in NO₃ ^?-fed plants, both alone and with NH₄ ⁺. Proline accumulation was observed in NH₄ ⁺-fed plants. Short- and long-term S deprivation did not promote considerable changes in ¹⁵N influx. ³⁴S absorption decreased depending on the N form provided: NH₄ ⁺+NO₃ ^? > only NH₄ ⁺ > only NO₃ ^? > low N.

Conclusions

Including both NH₄ ⁺ and NO₃ ^? forms in fertilizer increases N and S intake potential and thereby enhances plant growth, nutritional value and production.     

Precipitation variability does not affect soil respiration and nitrogen dynamics in the understorey of a Mediterranean oak woodland

Background and aims

Future climate scenarios for the Mediterranean imply increasing precipitation variability. This study presents a large-scale water manipulation experiment simulating changes in precipitation variability, aiming at a better understanding of the effects of rainfall patterns on soil C and N cycling and understorey productivity in a Mediterranean oak woodland.

Methods

We used rain-out shelters to achieve (1) a normal dry period (7 days), and (2) a dry period increased three-fold (21 days), without altering total annual precipitation inputs.

Results

The temporal patterns of soil respiration (R _s) and soil inorganic N were not affected by treatment. However, water infiltration and N leaching increased with large infrequent watering events. R _s and soil NH₄ ⁺-N correlated with soil temperature, with soil NO₃ ^?-N being influenced by leaching.

Conclusions

The lack of significant treatment effects on either R _s or soil inorganic N can be explained by (1) minor differences in plant productivity between the treatments, suggesting equal plant N demand, and (2) the absence of moisture dependence of R _s and soil NH₄ ⁺-N. Increased N leaching with large infrequent precipitation events may have longer-term consequences for ecosystem functioning. Our results contribute to an improved understanding of possible climate change effects on key ecosystem processes in Mediterranean ecosystems.     

Small differences in ombrotrophy control regional-scale variation in methane cycling among <Emphasis Type="Italic">Sphagnum</Emphasis>-dominated peatlands

Across northern Alberta, Canada, bogs experience periodic wildfire and, in the Fort McMurray region, are exposed to increasing atmospheric N deposition related to oil sands development. As the fire return interval shortens and/or growing season temperatures increase, the regional peatland CO₂–C sink across northern Alberta will likely decrease, but the magnitude of the decrease could be diminished if increasing atmospheric N deposition alters N cycling in a way that stimulates post-fire successional development in bogs. We quantified net ammonification, nitrification, and dissolved organic N (DON) production in surface peat along a post-fire chronosequence of five bogs where we also experimentally manipulated N deposition (no water controls plus 0, 10, and 20 kg N ha^?1 yr^?1 simulated deposition, as NH₄NO₃). Initial KCl-extractable NH₄⁺–N, NO₃^?–N and DON averaged 176?±?6, 54?±?0.2, and 3580?±?40 ng N cm^?3, respectively, with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Net ammonification, nitrification, and DON production averaged 3.8?±?0.3, 1.6?±?0.2, and 14.3?±?2.0 ng N cm^?3 d^?1, also with no consistent changes as a function of time since fire and no consistent effects of experimental N addition. Our hypothesis that N mineralization would be stimulated after fire because root death would create a pulse of labile soil organic C was not supported, most likely because ericaceous plant roots typically are not killed in boreal bog wildfires. The absence of any N mineralization response to experimental N addition is most likely a result of rapid immobilization of added NH₄⁺–N and NO₃^?–N in peat with a wide C:N ratio. In these boreal bogs, belowground N cycling is likely characterized by large DON pools that turn over relatively slowly and small DIN pools that turn over relatively rapidly. For Alberta bogs that have persisted at historically low N deposition values and begin to receive higher N deposition related to anthropogenic activities, peat N mineralization processes may be largely unaffected until the peat C:N ratio reaches a point that no longer favors immobilization of NH₄⁺–N and NO₃^?–N.     

Effects of elevated CO2 and nitrogen addition on soil organic carbon fractions in a subtropical forest

Aims

The aim of this study was to investigate the effects of elevated CO₂ concentration and nitrogen addition on soil organic carbon fractions in subtropical forests where the ambient N deposition was high.

Methods

Seedlings of typical subtropical forest ecosystems were transplanted in ten open-top chambers and grown under CO₂ and nitrogen treatments. The treatments included: 1) elevated CO₂ (700?μmol?mol^-1); 2) N addition of 100?kg NH₄NO₃ ha^-1?yr^-1; 3) combined elevated CO₂ and N addition; and 4) control. We measured soil total organic carbon (TOC), particulate organic carbon (POC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC).

Results

Results showed that elevated CO₂ alone did not significantly affect soil TOC, POC and ROC after 4?years of treatment, but increased soil MBC and soil respiration compared to the control. N addition alone had no significant effect neither on soil TOC, POC and ROC, but decreased MBC and soil respiration over time. However, the elevated CO₂ and N addition together significantly increased soil POC and ROC, and had no significant effect on soil MBC.

Conclusions

This study indicated that even in N-rich subtropical forest ecosystems, inputs of N are still needed in order to sustain soil C accumulation under elevated CO₂.     

青藏高原高寒草甸土壤CO2排放对模拟氮沉降的早期响应

研究大气氮沉降输入对青藏高原高寒草甸土壤-大气界面CO₂交换通量的影响,对于准确评价全球变化背景下区域碳平衡至关重要。通过构建多形态、低剂量的增氮控制试验,利用静态箱-气相色谱法测定土壤CO₂排放通量,同时测定相关土壤变量和地上生物量,分析高寒草甸土壤CO₂排放特征及其主要驱动因子。研究结果表明:低、高剂量氮输入倾向于消耗土壤水分,而中剂量氮输入有利于土壤水分的保持;施氮初期总体上增加了土壤无机氮含量,铵态氮累积效应更为显著;施氮显著增加地上生物量和土壤CO₂排放通量,铵态氮的促进效应显著高于硝态氮。另外,土壤CO₂排放通量主要受土壤温度驱动,其次为地上生物量和铵态氮储量。上述结果反映了氮沉降输入短期内可能刺激了植物生长和土壤微生物活性,加剧了土壤-大气界面CO₂排放。     

Integrated soil and plant phosphorus management for crop and environment in China. A review

In semi-arid grassland ecosystems, soil biogeochemical processes are controlled by seasonal and inter-annual rainfall variation and temperature, which may override the long-term impact of grazers on N availability and N dynamics. In a three-year (2004?C2006) case study of an Inner Mongolian grassland, we analysed time-integrated (ion-exchange resins) and instantaneous (soil mineral N extractions) inorganic N availability at three sites of varying grazing intensities and combined these data with information on soil water content (SWC), aboveground net primary productivity (ANPP) and plant N uptake. Additionally, the effects of rainfall and grazing on N-form availability (NO ₃ ^? -N, NH ₄ ⁺ -N) were considered. Grazing had less impact on N availability compared to seasonal and annual rainfall distribution. One of the three study years (2004) showed a grazing effect with higher resin-N availability at the ungrazed site compared to the heavily grazed site. Inorganic N availability was low in the driest year (2005) and highest in a year of average rainfall amount and favourable distribution (2004). In general, we found a positive relationship between inorganic N availability and both plant productivity and plant N uptake. Rainfall also controlled the plant available NO ₃ ^? -N and NH ₄ ⁺ -N pools; NH ₄ ⁺ -N dominated the available inorganic N-form in times of low SWC, while the available NO ₃ ^? -N increased with SWC. We observed N availability and plant productivity in a temporal synchronized pattern. Increased rainfall variability and land-use practices affecting SWC will likely alter N availability dynamics (and the relation of N-forms) and, therefore, important processes of semi-arid natural grassland carbon and N cycling.     

Bracken fern (Pteridium aquilinum L. kuhn) promotes an open nitrogen cycle in heathland soils

Background and Aims

In spite of the broad array of studies conducted on the ecology of bracken fern (Pteridium aquilinum (L.) kuhn), there is currently only a limited understanding of how P. aquilinum alters the soil environment in which it succeeds. P. aquilinum is one of the world’s most aggressive invasive species and is known to effectively invade conservation priority habitats such as Calluna vulgaris (L.) heathland. The aim of this study was to evaluate differences in soil properties between intact stands of C. vulgaris and neighboring P. aquilinum to assess how P. aquilinum alters soil N transformations in a manner that might promote its success.

Methods

Replicate plots in five independently paired stands of P. aquilinum and C. vulgaris were established on land in which P. aquilinum is actively invading. Soils under the two plant types were evaluated for total N, mineralisable N, net nitrification, nitrifier activity, denitrification enzyme activity, polyphenol N complexing capacity, and resin sorption of inorganic N.

Results

Soils under P. aquilinum were consistently higher in NO₃ ^- and NH₄ ⁺ concentrations compared to C. vulgaris. Extractable organic and inorganic N concentrations for soil under P. aquilinum were respectively 65 %, 77 % and 358 % greater in amino N NH₄ ⁺-N and NO₃ ^--N compared to that under C. vulgaris. In-situ net nitrification (NO₃ ^- sorption to ionic resins) was found to be nearly 300 times greater under P. aquilinum than under C. vulgaris.

Conclusions

P. aquilinum alters the soil environment as to create an inorganic N-rich environment that is favorable to its growth and development.     

Decay rates and nitrogen dynamics of decomposing watercress (Nasturtium officinale R.Br.)

We examined the decomposition of watercress in the laboratory at 10° and 20 °C, and in the field. Rates varied from 0.058 g g^?1 day^?1 in the laboratory to 0.115 g g^?1 day^?1 in the field. There was a rapid generation of particles of size <500 µm. It is thought that washout of these from the litterbags in the field accounted for high field decomposition rate. Formation of dissolved nitrogen compounds during decomposition followed a time series from NH _inf4 ^sup+ to NO _in2 ^sup? to NO _inf3 ^sup? withdissolved organic nitrogen accumulating at the end of decomposition. Ammonification rates were 480 and 657 g NH₄-N g^?1 (dry wt) day^?1 and nitrification rates on the decomposing tissue were 640 and 571 µg NO₃-N g^?1 (dry wt) day^?1 at 10° and 20 °C respectively. Fifty-six per cent of the initial plant N was regenerated as NO₃-N 21% as DON and 25% remained as refractory particulate N.     

Spatial and temporal variation of atmospheric nitrogen deposition in the North China Plain

A monitoring network of nine sites was established to determine the spatial and temporal variation of atmospheric nitrogen (N) deposition in the North China Plain (NCP) over a two-year period. The annual bulk deposition of inorganic N in the North China Plain ranged from 18.4 to 38.5 kg/hm² and averaged 28.0 kg/hm². The concentration of NH₄⁺-N and NO₃^?-N in rainwater averaged 3.76 and 1.85 mg/L, respectively, which were significantly higher than the values at background sites in China (normally less than 0.5 mg/L). Annual bulk deposition of inorganic N in the Beijing area (32.5 kg/hm²) was higher than that in Shandong and Hebei provinces (21.2 kg/hm² on an average). Also bulk N deposition was much greater in Dongbeiwang and Fangshan than in Yanqing and Shunyi counties. Significant spatial variation of bulk deposition was observed in the Beijing area because of variation of precipitation, and 60% of bulk deposition occurred from June to September. Bulk deposition of NH₄⁺-N was 2.0 times that of NO₃^?-N deposition at the rural monitoring sites. However, the situation was reversed at the Beijing Academy of Agricultural-Forestry Sciences (BAAFS), the unique urban monitoring site. The results suggest that reduced N in precipitation is dominant in rural regions, but oxidized N is the major form in urban regions. The positive relationship between inorganic N deposition and precipitation can be fitted by a power equation (r²= 0.67), showing an increase of NH₄⁺-N and NO₃^?-N inputs with increased precipitation. Wet deposition of N accounted for 73% of the bulk deposition, implying that dry deposition of N, particularly NH₄⁺-N from dust, is important in the North China Plain.     

The influence of catchment characteristics on the seasonality of carbon and nitrogen species concentrations in upland rivers of Northern Scotland

Data from 13 catchments with no arable land in Northern Scotland were used to develop empirical linear regression models of average monthly NO₃ ^? concentrations and average summer and winter concentrations for NH₄ ⁺, dissolved organic N (DON) and dissolved organic carbon (DOC) as a function of catchment characteristics. All catchments displayed a pronounced seasonal NO₃ ^? cycle. Variation in monthly mean NO₃ ^? concentration within and between catchments could be predicted from mean monthly air temperature using separate regression equations for temperatures < and ≥ 5 °C. Soil type, climate and land use influenced NH₄ ⁺ concentrations. In summer, concentrations of NH₄ ⁺ were largest in catchments with extensive areas of brown forest soils, which are less acidic and more base-rich than other upland soils. However, concentrations declined with increasing conifer cover and summer rainfall. In winter, however, % conifer cover had a positive effect, while higher temperature and higher humus iron podzol cover had negative influences. DON concentration decreased with increasing catchment elevation in both summer and winter. Surprisingly, concentrations of DON only displayed a positive relationship with percentage peat cover in the summer. The most important factor controlling DOC concentration was soil type, with a positive relationship being observed between DOC and peat and humus iron podzol coverage. Elevation was also important, but only in the winter when concentrations were negatively correlated with maximum catchment elevation. Overall, multivariate regression equations explained the spatial and seasonal variability in N species concentrations over a range of catchments within Northern Scotland.     

Effects of forest clear-cutting on the carbon and nitrogen fluxes through podzolic soil horizons

Effects of clear-cutting on the dissolved fluxes of organic C (DOC), organic N (DON), NO₃ ^– and NH₄ ⁺ through surface soil horizons were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall and soil water from below the organic (including understorey vegetation and, after clear-cutting, also logging residues), eluvial and illuvial horizons were sampled weekly from 1993 to 1999. Clear-cutting was carried out in September 1996. The removal of the tree canopy decreased the deposition of DOC and DON to the forest floor and increased that of NH₄ ⁺ and NO₃ ^– but did not affect the deposition of total N (DTN, <3 kg ha^–1 a^–1). The leaching of DOC and DON from the organic horizon increased over twofold after clear-cutting (fluxes were on an average 168 kg C and 3.3 kg N ha^–1 a^–1), but the increased outputs were effectively retained in the surface mineral soil horizons. Inorganic N deposition was mainly retained by the logging residues and organic horizon indicating microbial immobilization. Increased NO₃ ^– formation reflected as elevated concentrations in the percolate from below the mineral soil horizons were observed especially in the third year after clear-cutting. However, the changes were small and the increased leaching of DTN from below the illuvial horizon remained small (<0.4 kg ha^–1 a^–1) and mainly DON. Effects of clear-cutting on the transport of C and N to surface waters will probably be negligible.     

氮肥配施生化抑制剂组合对黄泥田土壤氮素淋溶特征的影响

揭示尿素类肥料添加生化抑制剂组合后,在黄泥田土壤中硝态氮(NO~-_3-N)和铵态氮(NH~+_4-N)的淋溶损失规律。采用室内土柱淋溶培养试验,研究脲酶抑制剂N-丁基硫代磷酰三胺(NBPT)和硝化抑制剂2-氯-6-(三氯甲基)吡啶(CP)单独添加及配合施用对尿素和尿素硝铵(300 kg N/hm~2)中氮(N)素在土体中淋溶损失的影响。结果表明:尿素和尿素硝铵处理淋溶液中NH~+_4-N和NO~-_3-N浓度均呈先升后降的变化趋势,而出峰时间不一。NH~+_4-N和NO~-_3-N淋失量随着时间的延长,处理间差异逐渐变大。NBPT处理可以减缓尿素水解,有效抑制NH~+_4-N生成,延缓其出峰时间,减少NH~+_4-N流失;CP处理可以有效抑制NH~+_4-N向NO~-_3-N转化,减少NO~-_3-N流失。与单独添加NBPT和CP处理相比,两者配施对N素淋溶损失有明显的协同抑制效果在黄泥田土壤中,既能减缓尿素水解,保持土壤中较高NH~+_4-N含量,又能降低淋溶液中NO~-_3-N浓度。培养结束时(第72天),UAN处理中NO~-_3-N、NH~+_4-N、矿质态N淋失总量及硝化率较U处理高34.39%、5.32%、31.72%和15.71%。U+NBPT、U+CP和U+NBPT+CP处理较U处理分别显著降低NO~-_3-N淋失总量达15.58%、114.77%和73.45%;UAN+NBPT、UAN+CP和UAN+NBPT+CP处理较UAN处理分别显著降低达15.88%、54.87%和37.46%。不同处理NO~-_3-N淋失总量大小表现为:UAN UAN+NBPT U UAN+NBPT+CP U+NBPT UAN+CP U+NBPT+CP U+CP CK。在一定施肥量条件下,NBPT和CP单独施用或配施均可降低黄泥田土壤中NO~-_3-N累积淋失量。对各处理淋溶液中NO~-_3-N淋失量(y)随时间(x)的变化进行拟合,其中以线性方程(y=ax+b)的拟合度较高,且各抑制剂处理a、b值均存在明显差异。总体认为,在黄泥田土壤中施用CP及其与NBPT配施可以显著降低土壤NO~-_3-N淋溶损失,减少N素淋失风险,提高肥料利用率。     

氮素类型和剂量对寒温带针叶林土壤N2O排放的影响

大气氮沉降输入会增加森林生态系统氮素有效性,进而改变土壤N_2O产生与排放,然而有关不同氮素离子(氧化态NO_3~--N与还原态NH_4~+-N)沉降对土壤N_2O排放的影响知之甚少。以大兴安岭寒温带针叶林为研究对象,构建了3种类型(NH_4Cl、KNO_3、NH_4NO_3)和4个施氮水平(0、10、20、40 kg N hm~(-2)a~(-1))的增氮控制试验,利用流动化学分析仪和静态箱-气相色谱法4次/月测定凋落物层和矿质层土壤无机氮含量、土壤-大气界面N_2O净交换通量以及相关环境因子,分析施氮类型和剂量对土壤氮素有效性、土壤N_2O通量的影响探讨氮素富集条件下土壤N_2O通量的环境驱动机制。结果表明:施氮类型和剂量均显著影响土壤无机氮含量,土壤NH_4~+-N的积累效应显著高于NO_3~--N。施氮一致增加寒温带针叶林土壤N_2O排放,NH_4NO_3促进效应最为明显,增幅为442%-677%,高于全球平均水平(134%)。土壤N_2O通量与土壤温度、凋落物层NH_4~+-N含量正相关,且随着施氮水平增加而增加。结果表明大气氮沉降短期内不会导致寒温带针叶林土壤NO_3~--N大量流失,但会显著促进土壤N_2O的排放。此外,外源性NH_4~+和NO_3~-输入对土壤N_2O排放的促进作用具有协同效应,在未来森林生态系统氮循环和氮平衡研究中应该区分对待。     

Effect of elevated CO2 concentration and nitrate: ammonium ratios on gas exchange and growth of cassava (Manihot esculenta Crantz)

Aims

This study evaluated how different nitrogen forms affect growth and photosynthetic responses of cassava to CO₂ concentration.

Methods

Cassava was grown in 14-L pots in a greenhouse at 390 or 750 ppm of CO₂. Three nitrogen treatments were applied: (a) 12?mM NO₃ ^?, (b) 6?mM NO₃ ^??+?6?mM NH₄ ⁺, and (c) 12?mM NH₄ ⁺.

Results

Thirty-six days after treatments began, plants grown under elevated CO₂ and fertilized only with NO₃ ^? (750_NO₃ ^?) had photosynthetic rates similar to plants grown under 390_NO₃ ^?, indicating significant photosynthetic acclimation to CO₂. In contrast, photosynthetic rates at elevated CO₂ increased as NH₄ ⁺ increased in the nutrient solution, such that photosynthetic acclimation was reduced for plants fertilized with only NH₄ ⁺. However, this positive effect of NH₄ ⁺ on photosynthesis was not observed in more advanced growth stages, and the toxic effects of NH₄ ⁺ severely reduced total dry mass for these plants measured at the end of the experiment.

Conclusions

Our results indicate that cassava will respond with increased biomass accumulation in response to raising atmospheric CO₂ levels, and that N form can have an important impact on the photosynthetic response. However, the positive effect of NH₄ ⁺ fertilization on cassava photosynthetic CO₂ response eventually led to a toxicity problem that reduced biomass production. The challenge is to determine how to manage NH₄ ⁺ fertilization so that the photosynthetic benefit observed in the initial phase may persist throughout the crop cycle.     

城乡交错带土壤氮素空间分布及其影响因素

城乡交错带土壤氮素是城乡生态系统中最重要的氮源与氮汇,但是城市化下的土壤氮素分布及其影响机制还不清楚,基于3S平台研究了土壤氮素在成都西郊城乡交错带的空间分布特征及城市化对土壤氮素的影响。结果表明,研究区内土壤全氮(STN)、硝态氮(NO_3~--N)和铵态氮(NH_4~+-N)含量均值分别为(1.46±0.06)g/kg、(50.04±3.59)mg/kg和(6.72±0.53)mg/kg。区内土壤氮素含量从近郊向远郊逐渐增高,STN和NO_3~--N含量为中部高于南北部,NH_4~+-N含量则由西北部和东南部向中部递增。方差分析表明,区内不同土地利用方式下STN、NO_3~--N和NH_4~+-N含量差异显著(P0.05)。回归分析显示STN含量与建筑密度(BD)、道路密度(RD)均呈现显著线性负相关(P0.05),NO_3~--N含量与道路密度呈极显著线性负相关(P=0.001),与建筑密度关系不明显(P=0.217)。土壤NH_4~+-N与建筑密度呈显著负线性相关(P=0.001),与道路密度呈显著指数相关关系(P=0.021)。研究结果显示城市发展使得城乡交错带土壤氮素含量降低,这种影响伴随着建筑面积的增加,道路长度的增加而加强。     

Nitrogen deposition promotes ecosystem carbon accumulation by reducing soil carbon emission in a subtropical forest

Background and aims

Tropical and subtropical forests are experiencing high levels of atmospheric nitrogen (N) deposition, but the responses of such forests ecosystems to N deposition remain poorly understood.

Methods

We conducted an 8-year field experiment examining the effect of experimental N deposition on plant growth, soil carbon dioxide efflux, and net ecosystem production (NEP) in a subtropical Chinese fir forest. The quantities of N added were 0 (control), 60, 120, and 240 kg ha^?1 year^?1.

Results

NEP was lowest under ambient conditions and highest with 240 kg of N ha^?1 year^?1 treatment. The net increase in ecosystem carbon (C) storage ranged from 9.2 to 16.4 kg C per kg N added in comparison with control. In addition, N deposition treatments significantly decreased heterotrophic respiration (by 0.69–1.85 t C ha^?1 year^?1) and did not affect plant biomass. The nitrogen concentrations were higher in needles than that in fine roots.

Conclusions

Our findings suggest that the young Chinese fir forest is carbon source and N deposition would sequester additional atmospheric CO₂ at high levels N input, mainly due to reduced soil CO₂ emission rather than increased plant growth, and the amount of sequestered C depended on the rate of N deposition.     

Nitrate-use traits of understory plants as potential regulators of vegetation distribution on a slope in a Japanese cedar plantation

Background and aims

Plant physiological traits and their relation to soil N availability was investigated as regulators of the distribution of understory shrub species along a slope in a Japanese cedar (Cryptomeria japonica) plantation in central Japan.

Methods

At the study site, previous studies demonstrated that both net and gross soil nitrification rates are high on the lower slope and there are dramatic declines in different sections of the slope gradient. We examined the distributions of understory plant species and their nitrate (NO ₃ ^? -N) use traits, and compared the results with the soil traits.

Results

Our results show that boundaries between different dominant understory species correspond to boundaries between different soil types. Leucosceptrum stellipilum occurs on soil with high net and gross nitrification rates. Hydrangea hirta is dominant on soil with high net and low gross nitrification rates. Pieris japonica occurs on soil with very low net and gross nitrification rates. Dominant understory species have species-specific physiological traits in their use of NO ₃ ^? -N. Pieris japonica lacks the capacity to use NO ₃ ^? -N as a N source, but other species do use NO ₃ ^? -N. Lindera triloba, whose distribution is unrelated to soil NO ₃ ^? -N availability, changes the extent to which it uses NO ₃ ^? -N in response to soil NO ₃ ^? -N availability.

Conclusions

Our results indicate that differences in the physiological capabilities and adaptabilities of plant species in using NO ₃ ^? -N as a N source regulate their distribution ranges. The identity of the major form of available soil N is therefore an environmental factor that influences plant distributions.