Press-pulse interactions: effects of warming, N deposition, altered winter precipitation, and fire on desert grassland community structure and dynamics

Global environmental change is altering temperature, precipitation patterns, resource availability, and disturbance regimes. Theory predicts that ecological presses will interact with pulse events to alter ecosystem structure and function. In 2006, we established a long-term, multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric nitrogen (N) deposition, and increased winter precipitation on plant community structure and aboveground net primary production (ANPP) in a northern Chihuahuan Desert grassland. In 2009, a lightning-caused wildfire burned through the experiment. Here, we report on the interactive effects of these global change drivers on pre- and postfire grassland community structure and ANPP. Our nighttime warming treatment increased winter nighttime air temperatures by an average of 1.1 °C and summer nighttime air temperature by 1.5 °C. Soil N availability was 2.5 times higher in fertilized compared with control plots. Average soil volumetric water content (VWC) in winter was slightly but significantly higher (13.0% vs. 11.0%) in plots receiving added winter rain relative to controls, and VWC was slightly higher in warmed (14.5%) compared with control (13.5%) plots during the growing season even though surface soil temperatures were significantly higher in warmed plots. Despite these significant treatment effects, ANPP and plant community structure were highly resistant to these global change drivers prior to the fire. Burning reduced the cover of the dominant grasses by more than 75%. Following the fire, forb species richness and biomass increased significantly, particularly in warmed, fertilized plots that received additional winter precipitation. Thus, although unburned grassland showed little initial response to multiple ecological presses, our results demonstrate how a single pulse disturbance can interact with chronic alterations in resource availability to increase ecosystem sensitivity to multiple drivers of global environmental change.

[Responses of soil organic carbon and its labile fractions to nitrogen and phosphorus additions in Cunninghamia lanceolata plantations in subtropical China.]

A series of nitrogen (N) and phosphorus (P) addition experiments using treatments of N₀(0 kg N·hm^-2·a^-1), N₁(50 kg N·hm^-2·a^-1), N₂(100 kg N·hm^-2·a^-1), P (50 kg P·hm^-2·a^-1), N₁P and N₂P were conducted at Cunninghamia lanceolata plantations in subtropical China. The responses of soil organic carbon (SOC), particulate organic carbon (POC) and water-soluble organic carbon (WSOC) to the nutrient addition treatments after 3 years were determined. The results showed that N and P additions had no significant effects on SOC concentration in 0-20 cm soil layer, while P addition significantly decreased soil POC content in 0-5 cm soil layer by 26.1%. The responses of WSOC to N and P addition were mainly found in 0-5 cm soil layer, and low level N and P addition significantly increased the WSOC content in 0-5 cm soil layer. Nitrogen addition had no significant effect on POC/SOC, while the POC/SOC significantly decreased by 15.9% in response to P addition in 0-5 cm soil layer. In 5-10 cm and 10-20 cm soil layers, POC/SOC was not significantly altered in N and P addition treatments. Therefore, the forest soil C stability was mainly controlled by P content in subtropical areas. P addition was liable to cause the decomposition of surface soil active organic C and increased the soil C stability in the short term treatment.

Aspen phenylpropanoid genes' expression levels correlate with genets' tannin richness and vary both in responses to soil nitrogen and associations with phenolic profiles

Condensed tannin (CT) contents of European aspen (Populus tremula L.) vary among genotypes, and increases in nitrogen (N) availability generally reduce plants' tannin production in favor of growth, through poorly understood mechanisms. We hypothesized that intrinsic tannin production rates may co-vary with gene expression responses to soil N and resource allocation within the phenylpropanoid pathway (PPP). Thus, we examined correlations between soil N levels and both expression patterns of eight PPP genes (measured by quantitative-reverse transcription PCR) and foliar phenolic compounds (measured by liquid chromatography-mass spectrometry) in young aspen genets with intrinsically extreme CT levels. Monitored phenolics included salicinoids, lignins, flavones, flavonols, CT precursors and CTs. The PPP genes were consistently expressed more strongly in high-CT trees. Low N supplements reduced expression of genes throughout the PPP in all genets, while high N doses restored expression of genes at the beginning and end of the pathway. These PPP changes were not reflected in pools of tannin precursors, but varying correlations between gene expression and foliar phenolic pools were detected in young and mature leaves, suggesting that processes linking gene expression and the resulting phenolics vary spatially and temporally. Precursor fluxes suggested that CT-related metabolic rate or sink controls are linked to intrinsic carbon allocation strategies associated with N responses. Overall, we found more negative correlations (indicative of allocation trade-offs) between PPP gene expression and phenolic products following N additions in low-CT plants than in high-CT plants. The tannin-related expression dynamics suggest that, in addition to defense, relative tannin levels may also be indicative of intraspecific variations in the way aspen genets respond to soil fertility.

Species decline under nitrogen fertilization increases community-level competence of fungal diseases

The artificial fertilization of soils can alter the structure of natural plant communities and exacerbate pathogen emergence and transmission. Although the direct effects of fertilization on disease resistance in plants have received some research attention, its indirect effects of altered community structure on the severity of fungal disease infection remain largely uninvestigated. We designed manipulation experiments in natural assemblages of Tibetan alpine meadow vegetation along a nitrogen-fertilization gradient over 5 years to compare the relative importance of direct and indirect effects of fertilization on foliar fungal infections at the community level. We found that species with lower proneness to pathogens were more likely to be extirpated following fertilization, such that community-level competence of disease, and thus community pathogen load, increased with the intensity of fertilization. The amount of nitrogen added (direct effect) and community disease competence (indirect effect) provided the most parsimonious combination of parameters explaining the variation in disease severity. Our experiment provides a mechanistic explanation for the dilution effect in fertilized, natural assemblages in a highly specific pathogen-host system, and thus insights into the consequences of human ecosystem modifications on the dynamics of infectious diseases.

[Effects of nitrogen addition and elevated CO2 concentration on soil dissolved organic carbon and nitrogen in rhizosphere and non-rhizosphere of Bothriochloa ischaemum]

A pot experiment was conducted to study soil dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in the rhizosphere and non-rhizosphere of Bothriochloa ischaemum in loess hilly-gully region under the different treatments of CO₂ concentrations (400 and 800 μmol·mol^-1) and nitrogen addition (0, 2.5, 5.0 g N·m^-2·a^-1). The results showed that eleva-ted CO₂ treatments had no significant effect on the contents of DOC, dissolved total nitrogen (DTN), DON, dissolved ammonium nitrogen (NH₄⁺-N) and dissolved nitrate nitrogen (NO₃^--N) in the soil of rhizosphere and non-rhizosphere of B. ischaemum. The contents of DTN, DON, and NO₃^--N in the rhizosphere soil were significantly increased with the nitrogen application and the similar results of DTN and NO₃^--N also were observed in the non-rhizosphere of B. ischaemum. Nitrogen application significantly decreased DOC/DON in the rhizosphere of B. ischaemum. The contents of DTN, NO₃^--N and DON in the soil of rhizosphere were significantly lower than that in the non-rhizosphere soil, and DOC/DON was significantly higher in the rhizosphere soil than that in the non-rhizosphere soil. It indicated that short-term elevated CO₂ concentration had no significant influence on the contents of soil dissolved organic carbon and nitrogen. Simulated nitrogen deposition, to some extent, increased the content of soil dissolved nitrogen, but it was still insufficient to meet the demand of dissolved nitrogen for plant growing.

[Responses of plant community structure and species composition to warming and N addition in an alpine meadow, northern Tibetan Plateau, China]

Global climate warming and increasing nitrogen (N) deposition, as controversial global environmental issues, may distinctly affect the functions and processes of terrestrial ecosystems. It has been reported that the Qinghai-Tibet Plateau has been experiencing significant warming in recent decades, especially in winter. Previous studies have mainly focused on the effects of warming all the year round; however, few studies have tested the effects of winter warming. To investigate the effects of winter warming and N addition on plant community structure and species composition of alpine meadow, long-term N addition and simulated warming experiment was conducted in alpine meadow from 2010 in Damxung, northern Tibet. The experiment consisted of three warming patterns: Year-round warming (YW), winter warming (WW) and control (NW), crossed respectively with five N gradients: 0, 10, 20, 40, 80 kg N·hm^-2·a^-1. From 2012 to 2014, both warming and N addition significantly affected the total coverage of plant community. Specifically, YW significantly decreased the total coverage of plant community. Without N addition, WW remarkably reduced the vegetation coverage. However, with N addition, the total vegetation coverage gradually increased with the increase of N level. Warming and N addition had different effects on plants from different functional groups. Warming significantly reduced the plant coverage of grasses and sedges, while N addition significantly enhanced the plant coverage of grasses. Regression analyses showed that the total coverage of plant community was positively related to soil water content in vigorous growth stages, indicating that the decrease in soil water content resulted from warming during dry seasons might be the main reason for the decline of total community coverage. As soil moisture in semi-arid alpine meadow is mainly regulated by rainfalls, our results indicated that changes in spatial and temporal patterns of rainfalls under the future climate change scenarios would dramatically influence the vegetation coverage and species composition. Additionally, the effects of increasing atmospheric N deposition on vegetation community might also depend on the change of rainfall patterns.

[Effects of nitrogen additions on soil hydrolase and oxidase activities in Pinus elliottii plantations.]

We evaluated responses of hydrolase and oxidase activities in a subtropical Pinus elliottii plantation through a nitrogen (N) addition field experiment (dosage level: 0, 40, 120 kg N·hm^-2·a^-1). The results showed that N additions significantly decreased the carbon, nitrogen and phosphorus related hydrolase and oxidase activities. The activities of β-1,4-glucosidase (BG), cellobiohydrolase (CBH), β-1,4-N-acetylglucosaminidase (NAG) and peroxidase (PER) activities were decreased by 16.5%-51.1% due to N additions, and the decrease was more remarkable in the higher N addition treatment. The activities of α-1,4-glucosidase (aG), β-1,4-xylosidase (BX), acid phosphatase (AP) and phenol oxidase (PPO) were decreased by 14.5%-38.6% by N additions, however, there was no significant difference among the different N addition treatments. Soil enzyme activities varied obviously in different seasons. The activities of BG, NAG, BX, CBH, AP and PPO were in the order of March > June > October, and aG and PER activities were in the order of October > March > June. Most of the soil hydrolase and oxidase activities were positively correlated with soil pH, but negatively with NO₃^--N content. It indicated that N additions inhibited soil hydrolase and oxidase activities by reducing soil pH and increasing soil nitrification. N additions inhibited the soil organic matter mineralization and turnover in the subtropical area, and the effects were obvious with the increasing dosage of N additions.

High retention of 15 N-labeled nitrogen deposition in a nitrogen saturated old-growth tropical forest

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem ¹⁵ N labeling experiment performed in a N-rich old-growth tropical forest in southern China. We added ¹⁵ N tracer monthly as ¹⁵ NH₄¹⁵ NO₃ for 1 year to control plots and to N-fertilized plots (N-plots, receiving additions of 50 kg N ha^-1  yr^-1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N-plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N-plots, respectively. Leaching below 20 cm was 50 kg N ha^-1  yr^-1 in the control plots and was close to the N input (51 kg N ha^-1  yr^-1 ), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha^-1  yr^-1 . However, of these only 7 and 23 kg N ha^-1  yr^-1 in the control and N-plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N-saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short-term N retention represent the main difference between temperate forests and the studied tropical forest.

Interactive effects of global change factors on soil respiration and its components: a meta-analysis

As the second largest carbon (C) flux between the atmosphere and terrestrial ecosystems, soil respiration (Rs) plays vital roles in regulating atmospheric CO2 concentration ([CO2 ]) and climatic dynamics in the earth system. Although numerous manipulative studies and a few meta-analyses have been conducted to determine the responses of Rs and its two components [i.e., autotrophic (Ra) and heterotrophic (Rh) respiration] to single global change factors, the interactive effects of the multiple factors are still unclear. In this study, we performed a meta-analysis of 150 multiple-factor (≥2) studies to examine the main and interactive effects of global change factors on Rs and its two components. Our results showed that elevated [CO2 ] (E), nitrogen addition (N), irrigation (I), and warming (W) induced significant increases in Rs by 28.6%, 8.8%, 9.7%, and 7.1%, respectively. The combined effects of the multiple factors, EN, EW, DE, IE, IN, IW, IEW, and DEW, were also significantly positive on Rs to a greater extent than those of the single-factor ones. For all the individual studies, the additive interactions were predominant on Rs (90.6%) and its components (≈70.0%) relative to synergistic and antagonistic ones. However, the different combinations of global change factors (e.g., EN, NW, EW, IW) indicated that the three types of interactions were all important, with two combinations for synergistic effects, two for antagonistic, and five for additive when at least eight independent experiments were considered. In addition, the interactions of elevated [CO2 ] and warming had opposite effects on Ra and Rh, suggesting that different processes may influence their responses to the multifactor interactions. Our study highlights the crucial importance of the interactive effects among the multiple factors on Rs and its components, which could inform regional and global models to assess the climate-biosphere feedbacks and improve predictions of the future states of the ecological and climate systems.

[Effects of soil warming and nitrogen addition on the length distributions of different diameter class fine roots of Chinese fir seedlings.]

In order to determine how the diameter class length distribution (DCLD) of fine roots of Chinese fir (Cunninghamia lanceolata) would be affected by soil warming, nitrogen addition and their interaction, a factorial experiment of soil warming (ambient, +5 ℃) and nitrogen addition (ambient, +4 and +8 g N·m^-2·a^-1) was carried out in the Chenda State-owned Forest Farm in Sanming, Fujian Province. An expanded extreme value model fitted the DCLD of roots of all the six treatments very well (R²=0.97). The model parameters showed that soil warming reduced the total root length, but its effect on root diameter was not significant. Nitrogen addition decreased both total root length and root diameter. The interaction of soil warming and nitrogen addition had significant effects on total root length, but had no significant effects on root diameter. DCLD of fine roots under the six treatments could be fitted well by the extreme value function (R²>0.98). The correlation analysis showed that specific root length for roots of 0-1 mm diameter was significantly negatively correlated with the parameter c, and the actual total root length was significantly positively correlated with the parameter b. It was concluded that the root morphology of Chinese fir seedlings would respond to both soil warming, nitrogen addition and their interaction, and these responses could be reflected by the changes in parameters of the extreme value model.

Nitrogen supply modulates the effect of changes in drying-rewetting frequency on soil C and N cycling and greenhouse gas exchange

Climate change and atmospheric nitrogen (N) deposition are two of the most important global change drivers. However, the interactions of these drivers have not been well studied. We aimed to assess how the combined effect of soil N additions and more frequent soil drying-rewetting events affects carbon (C) and N cycling, soil:atmosphere greenhouse gas (GHG) exchange, and functional microbial diversity. We manipulated the frequency of soil drying-rewetting events in soils from ambient and N-treated plots in a temperate forest and calculated the Orwin & Wardle Resistance index to compare the response of the different treatments. Increases in drying-rewetting cycles led to reductions in soil NO3- levels, potential net nitrification rate, and soil : atmosphere GHG exchange, and increases in NH4+ and total soil inorganic N levels. N-treated soils were more resistant to changes in the frequency of drying-rewetting cycles, and this resistance was stronger for C- than for N-related variables. Both the long-term N addition and the drying-rewetting treatment altered the functionality of the soil microbial population and its functional diversity. Our results suggest that increasing the frequency of drying-rewetting cycles can affect the ability of soil to cycle C and N and soil : atmosphere GHG exchange and that the response to this increase is modulated by soil N enrichment.

Asphalt-derived high surface area activated porous carbons for carbon dioxide capture

Research activity toward the development of new sorbents for carbon dioxide (CO2) capture have been increasing quickly. Despite the variety of existing materials with high surface areas and high CO2 uptake performances, the cost of the materials remains a dominant factor in slowing their industrial applications. Here we report preparation and CO2 uptake performance of microporous carbon materials synthesized from asphalt, a very inexpensive carbon source. Carbonization of asphalt with potassium hydroxide (KOH) at high temperatures (>600 °C) yields porous carbon materials (A-PC) with high surface areas of up to 2780 m(2) g(-1) and high CO2 uptake performance of 21 mmol g(-1) or 93 wt % at 30 bar and 25 °C. Furthermore, nitrogen doping and reduction with hydrogen yields active N-doped materials (A-NPC and A-rNPC) containing up to 9.3% nitrogen, making them nucleophilic porous carbons with further increase in the Brunauer-Emmett-Teller (BET) surface areas up to 2860 m(2) g(-1) for A-NPC and CO2 uptake to 26 mmol g(-1) or 114 wt % at 30 bar and 25 °C for A-rNPC. This is the highest reported CO2 uptake among the family of the activated porous carbonaceous materials. Thus, the porous carbon materials from asphalt have excellent properties for reversibly capturing CO2 at the well-head during the extraction of natural gas, a naturally occurring high pressure source of CO2. Through a pressure swing sorption process, when the asphalt-derived material is returned to 1 bar, the CO2 is released, thereby rendering a reversible capture medium that is highly efficient yet very inexpensive.

Nitrogen deposition contributes to soil acidification in tropical ecosystems

Elevated anthropogenic nitrogen (N) deposition has greatly altered terrestrial ecosystem functioning, threatening ecosystem health via acidification and eutrophication in temperate and boreal forests across the northern hemisphere. However, response of forest soil acidification to N deposition has been less studied in humid tropics compared to other forest types. This study was designed to explore impacts of long-term N deposition on soil acidification processes in tropical forests. We have established a long-term N-deposition experiment in an N-rich lowland tropical forest of Southern China since 2002 with N addition as NH4 NO3 of 0, 50, 100 and 150 kg N ha(-1)  yr(-1) . We measured soil acidification status and element leaching in soil drainage solution after 6-year N addition. Results showed that our study site has been experiencing serious soil acidification and was quite acid-sensitive showing high acidification (pH(H2O) <4.0), negative water-extracted acid neutralizing capacity (ANC) and low base saturation (BS,< 8%) throughout soil profiles. Long-term N addition significantly accelerated soil acidification, leading to depleted base cations and decreased BS, and further lowered ANC. However, N addition did not alter exchangeable Al(3+) , but increased cation exchange capacity (CEC). Nitrogen addition-induced increase in SOC is suggested to contribute to both higher CEC and lower pH. We further found that increased N addition greatly decreased soil solution pH at 20 cm depth, but not at 40 cm. Furthermore, there was no evidence that Al(3+) was leaching out from the deeper soils. These unique responses in tropical climate likely resulted from: exchangeable H(+) dominating changes of soil cation pool, an exhausted base cation pool, N-addition stimulating SOC production, and N saturation. Our results suggest that long-term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future.

Interactions between CO2 enhancement and N addition on net primary productivity and water-use efficiency in a mesocosm with multiple subtropical tree species

Carbon dioxide (CO2 ) enhancement (eCO2 ) and N addition (aN) have been shown to increase net primary production (NPP) and to affect water-use efficiency (WUE) for many temperate ecosystems, but few studies have been made on subtropical tree species. This study compared the responses of NPP and WUE from a mesocosm composing five subtropical tree species to eCO2 (700 ppm), aN (10 g N m(-2) yr(-1) ) and eCO2 × aN using open-top chambers. Our results showed that mean annual ecosystem NPP did not changed significantly under eCO2 , increased by 56% under aN and 64% under eCO2 × aN. Ecosystem WUE increased by 14%, 55%, and 61% under eCO2 , aN and eCO2 × aN, respectively. We found that the observed responses of ecosystem WUE were largely driven by the responses of ecosystem NPP. Statistical analysis showed that there was no significant interactions between eCO2 and aN on ecosystem NPP (P = 0.731) or WUE (P = 0.442). Our results showed that increasing N deposition was likely to have much stronger effects on ecosystem NPP and WUE than increasing CO2 concentration for the subtropical forests. However, different tree species responded quite differently. aN significantly increased annual NPP of the fast-growing species (Schima superba). Nitrogen-fixing species (Ormosia pinnata) grew significantly faster only under eCO2 × aN. eCO2 had no effects on annual NPP of those two species but significantly increased annual NPP of other two species (Castanopsis hystrix and Acmena acuminatissima). Differential responses of the NPP among different tree species to eCO2 and aN will likely have significant implications on the species composition of subtropical forests under future global change.

Linking ethylene to nitrogen-dependent leaf longevity of grass species in a temperate steppe

BACKGROUND AND AIMS

Leaf longevity is an important plant functional trait that often varies with soil nitrogen supply. Ethylene is a classical plant hormone involved in the control of senescence and abscission, but its role in nitrogen-dependent leaf longevity is largely unknown.

METHODS

Pot and field experiments were performed to examine the effects of nitrogen addition on leaf longevity and ethylene production in two dominant plant species, Agropyron cristatum and Stipa krylovii, in a temperate steppe in northern China.

KEY RESULTS

Nitrogen addition increased leaf ethylene production and nitrogen concentration but shortened leaf longevity; the addition of cobalt chloride, an ethylene biosynthesis inhibitor, reduced leaf nitrogen concentration and increased leaf longevity. Path analysis indicated that nitrogen addition reduced leaf longevity mainly through altering leaf ethylene production.

CONCLUSIONS

These findings provide the first experimental evidence in support of the involvement of ethylene in nitrogen-induced decrease in leaf longevity.

Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semiarid grassland

Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4 years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption-mediated, positive plant-soil feedback induced by N inputs. Furthermore, N : P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.

Litter quality and decomposability of species from a Mediterranean succession depend on leaf traits but not on nitrogen supply

BACKGROUND AND AIMS

The rate of plant decomposition depends on both the decomposition environment and the functional traits of the individual species (e.g. leaf and litter quality), but their relative importance in determining interspecific differences in litter decomposition remains unclear. The aims of this study were to: (a) determine if species from different successional stages grown on soils with low and high nitrogen levels produce leaf and litter traits that decompose differently under identical conditions; and (b) assess which trait of living leaves best relates to litter quality and litter decomposability

METHODS

The study was conducted on 17 herbaceous species representative of three stages of a Mediterranean successional sere of Southern France. Plants were grown in monocultures in a common garden under two nitrogen levels. To elucidate how different leaf traits affected litter decomposition a microcosm experiment was conducted to determine decomposability under standard conditions. Tests were also carried out to determine how successional stage and nitrogen supply affected functional traits of living leaves and how these traits then modified litter quality and subsequent litter decomposability.

KEY RESULTS

The results demonstrated that leaf traits and litter decomposability varied according to species and successional stage. It was also demonstrated that while nitrogen addition affected leaf and litter traits, it had no effect on decomposition rates. Finally, leaf dry matter content stood out as the leaf trait best related to litter quality and litter decomposability

CONCLUSIONS

In this study, species litter decomposability was affected by some leaf and litter traits but not by soil nitrogen supply. The results demonstrated the strength of a trait-based approach to predict changes in ecosystem processes as a result of species shifts in ecosystems.

Nitrogen cycling in a Norway spruce plantation in Denmark--a SOILN model application including organic N uptake

A dynamic carbon (C) and nitrogen (N) circulation model, SOILN, was applied and tested on 7 years of control data and 3 years of manipulation data from an experiment involving monthly N addition in a Norway spruce ( Picea abies, L. Karst) forest in Denmark. The model includes two pathways for N uptake: (1) as mineral N after mineralisation of organic N, or (2) directly from soil organic matter as amino acids proposed to mimic N uptake by mycorrhiza. The model was parameterised and applied to the data from the control plot both with and without the organic N uptake included. After calibration, the models performance was tested against data from the N-addition experiment by comparing model output with measurements. The model reproduced well the overall trends in C and N pools and the N concentrations in soil solutions in the top soil layers whereas discrepancies in soil-solution concentrations in the deeper soil layers are seen. In the control data, the needle-N concentration was well reproduced except for small underestimations in some years because of drought effects not included in the model. In the N-addition experiment, SOILN reproduces the observed changes; in particular, the changes in needle-N concentrations and the overall distribution within the ecosystem of the extra added 3.5 g N m(-2) year(-1) parallel the observations. When organic N uptake is included, the simulations indicate that in the control plot receiving c. 1.9 g N m(-2) year(-1), the organic N uptake in average supplies 35% of the total plant N uptake. By addition of an extra 35 kg N ha(-1) year(-1), the organic N uptake is reduced to 16% of the total N uptake. Generally, inclusion of the pathway for organic N uptake improves model performance compared with observations for both C and N. This is because mineral N uptake alone implies a larger mineralisation rate, leading to bigger concentrations of N in the soil and soil water, bigger N losses, and net loss of c. 100 kg C ha(-1) year(-1), thereby causing depletion of the organic soil layer.