Combined effects of nitrogen addition and litter manipulation on nutrient resorption of Leymus chinensis in a semi‐arid grassland of northern China

Plant growth in semi‐arid ecosystems is usually severely limited by soil nutrient availability. Alleviation of these resource stresses by fertiliser application and aboveground litter input may affect plant internal nutrient cycling in such regions. We conducted a 4‐year field experiment to investigate the effects of nitrogen (N) addition (10 g N·m^?2·year^?1) and plant litter manipulation on nutrient resorption of Leymus chinensis, the dominant native grass in a semi‐arid grassland in northern China. Although N addition had no clear effects on N and phosphorus (P) resorption efficiencies in leaves and culms, N fertilisation generally decreased leaf N resorption proficiency by 54%, culm N resorption proficiency by 65%. Moreover, N fertilisation increased leaf P resorption proficiency by 13%, culm P resorption proficiency by 20%. Under ambient or enriched N conditions, litter addition reduced N and P resorption proficiencies in both leaves and culms. The response of P resorption proficiency to litter manipulation was more sensitive than N resorption proficiency: P resorption proficiency in leaves and culms decreased strongly with increasing litter amount under both ambient and enriched N conditions. In contrast, N resorption proficiency was not significantly affected by litter addition, except for leaf N resorption proficiency under ambient N conditions. Furthermore, although litter addition caused a general decrease of leaf and culm nutrient resorption efficiencies under both ambient and enriched N conditions, litter addition effects on nutrient resorption efficiency were much weaker than the effects of litter addition on nutrient resorption proficiency. Taken together, our results show that leaf and non‐leaf organs of L. chinensis respond consistently to altered soil N availability. Our study confirms the strong effects of N addition on plant nutrient resorption processes and the potential role of aboveground litter, the most important natural fertiliser in terrestrial ecosystems, in influencing plant internal nutrient cycling.     

Nutrient resorption patterns of plant functional groups in a tropical savanna: variation and functional significance

Green and senesced leaf nitrogen (N) and phosphorus (P) concentrations of different plant functional groups in savanna communities of Kruger National Park, South Africa were analyzed to determine if nutrient resorption was regulated by plant nutritional status and foliar N:P ratios. The N and P concentrations in green leaves and the N concentrations in senesced leaves differed significantly between the dominant plant functional groups in these savannas: fine-leaved trees, broad-leaved trees and grasses. However, all three functional groups reduced P to comparable and very low levels in senesced leaves, suggesting that P was tightly conserved in this tropical semi-arid savanna ecosystem. Across all functional groups, there was evidence for nutritional control of resorption in this system, with both N and P resorption efficiencies decreasing as green leaf nutrient concentrations increased. However, specific patterns of resorption and the functional relationships between nutrient concentrations in green and senesced leaves varied by nutrient and plant functional group. Functional relationships between N concentrations in green and senesced leaves were indistinguishable between the dominant groups, suggesting that variation in N resorption efficiency was largely the result of inter-life form differences in green leaf N concentrations. In contrast, observed differences in P resorption efficiencies between life forms appear to be the result of both differences in green leaf P concentrations as well as inherent differences between life forms in the fraction of green leaf P resorbed from senescing leaves. Our results indicate that foliar N:P ratios are poor predictors of resorption efficiency in this ecosystem, in contrast to N and P resorption proficiencies, which are more responsive to foliar N:P ratios.     

Effects of fire alone or combined with thinning on tissue nutrient concentrations and nutrient resorption in <Emphasis Type="Italic">Desmodium nudiflorum</Emphasis>

This study examined tissue nutrient responses of Desmodium nudiflorum to changes in soil total inorganic nitrogen (TIN) and available phosphorus (P) that occurred as the result of the application of alternative forest management strategies, namely (1) prescribed low-intensity fire (B), (2) overstory thinning followed by prescribed fire (T + B), and (3) untreated control C), in two Quercus-dominated forests in the State of Ohio, USA. In the fourth growing season after a first fire, TIN was significantly greater in the control plots (9.8 mg/kg) than in the B (5.5 mg/kg) and T + B (6.4 mg/kg) plots. Similarly, available P was greater in the control sites (101 μg/g) than in the B (45 μg/kg) and T + B (65 μg/kg) sites. Leaf phosphorus ([P]) was higher in the plants from control site (1.86 mg/g) than in either the B (1.77 mg/g) or T + B plants (1.73 mg/g). Leaf nitrogen ([N]) and root [N] showed significant site–treatment interactive effects, while stem [N], stem [P], and root [P] did not differ significantly among treatments. During the first growing season after a second fire, leaf [N], stem [N], litter [P] and available soil [P] were consistently lower in plots of the manipulated treatments than in the unmanaged control plot, whereas the B and T + B plots did not differ significantly from each other. N resorption efficiency was positively correlated with the initial foliar [N] in the manipulated (B and T + B) sites, but there was no such relation in the unmanaged control plots. P resorption efficiency was positively correlated with the initial leaf [P] in both the control and manipulated plots. Leaf nutrient status was strongly influenced by soil nutrient availability shortly after fire, but became more influenced by topographic position in the fourth year after fire. Nutrient resorption efficiency was independent of soil nutrient availability. These findings enrich our understanding of the effects of ecosystem restoration treatments on soil nutrient availability, plant nutrient relations, and plant–soil interactions at different temporal scales.     

N:P ratios, δN fractionation and nutrient resorption along a nitrogen to phosphorus limitation gradient in an oligotrophic wetland complex

The vegetation N:P ratio is thought to be a diagnostic indicator of the nature of nutrient limitation in wetland vegetation. It should therefore be closely linked to other indicators of nutrient acquisition and conservation, such as nitrogen stable isotope fractionation (δ¹⁵N), nutrient resorption efficiency (RE) and resorption proficiency (RP). However, the interrelationships among these traits and the N:P ratio remain unclear. We compared tissue nutrient concentrations, N:P ratios, δ¹⁵N fractionation, RE, and RP along an N to P limitation gradient in an oligotrophic wetland valley in the South Island of New Zealand. Within the valley, the soil TN:TP ratio increased from 1.3 to 18.0 in three discrete wetlands along the gradient. In pooled data from all vegetation communities within each site, the mass-based vegetation N:P ratio correlated significantly (r² = 0.35, P < 0.01) to soil TN:TP ratios and increased from 10.2 ± 2.7 to 13.5 ± 3.6 along the N to P limitation gradient. This was accompanied by an increase in tissue δ¹⁵N enrichment from 2.05 ± 1.12‰ to 6.27 ± 1.70‰, consistent with more open N cycling and lower N demand. These trends held within all vegetation types, but were particularly strong in a Typha orientalis (C-strategist) community (soil TN:TP vs vegetation N:P correlation r² = 0.78, P < 0.001; δ¹⁵N increase from 1.81 ± 0.44‰ to 7.73 ± 1.79‰). The individual N and P concentrations and retention patterns were more species-specific and less responsive to the nutrient limitation gradient. T. orientalis maximised N resorption as N limitation increased (increasing NRE from 50.8 ± 3.3% to 71.7 ± 7.4%; reducing NRP from 0.70 ± 0.12% to 0.36 ± 0.13%) but did not alter PRE or PRP, whereas the S-strategist Schoenus pauciflorus maximised P resorption as P limitation increased (increasing PRE from 48.0 ± 5.6% to 73.5 ± 10.1%; reducing PRP from 0.053 ± 0.008% to 0.015 ± 0.004%) but did not alter NRE or NRP. These results show that the tissue N:P ratio and its associated δ¹⁵N enrichment are highly responsive indicators of the relative availability of N and P at the site and community level. However, they are not indicators of species-specific physiological requirements for N and P, or of likely responses of individual species to N or P enrichment, which are better interpreted from indicators such as RE and RP that describe nutrient retention behaviour.     

去叶时间对半干旱草原植物养分回收和干草生产的影响

去叶时间对半干旱草原植物养分回收和干草生产的影响 养分回收是植物养分保存的重要策略,其对环境和管理变化的响应关系到生态系统的养分循环和生产。去叶(刈割)是影响草地植物养分回收和生产的重要途径,而去叶时间的影响尚不清楚。本研究以内蒙古典型草原生态系统为对象,设置早期去叶(生物量高峰期之前)、峰期去叶(生物量高峰期)、晚期去叶(养分回收开始后)和不去叶(对照)四个处理,探讨了去叶时间对植物养分回收和生产的影响。通过测定植物物种和群落水平氮(N)和磷(P)回收特征,量化了植物N、P回收通量以及凋落物归还通量和干草输出通量,并评估了不同去叶时间处理下割草草地系统干草产量和质量。研究结果显示,峰期和晚期去叶降低植物群落N、P回收度,而早期去叶则对二者无影响;不同去叶时间处理下植物N、P回收效率相对稳定,仅晚期去叶降低N回收效率。峰期和晚期去叶降低植物群落N、P回收通量和凋落物N、P归还通量,而早期去叶并不影响这些参数。去叶时间降低植物群落养分回收通量,但未改变植物根系养分储存,说明根系养分吸收增加可补偿养分回收通量的降低。草地干草产量和质量在峰期去叶处理下最高,晚期去叶处理下最低。本研究结果为割草草地生态系统养分循环提供了新见解,通过调整刈割时间可以平衡草原的保护与生产,在植物生物量高峰期之前割草可实现保护和可持续生产的双重目标。     

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.     

Nitrogen and phosphorus resorption in trees of a Mexican tropical dry forest

Resorption efficiency (RE) and proficiency, foliar nutrient concentrations, and relative soil nutrient availability were determined during 3 consecutive years in tree species growing under contrasting topographic positions (i.e., top vs. bottom and north vs. south aspect) in a tropical dry forest in Mexico. The sites differed in soil nutrient levels, soil water content, and potential radiation interception. Leaf mass per area (g m^–2) increased during the growing season in all species. Soil P availability and mean foliar P concentrations were generally higher at the bottom than at the top site during the 3 years of the study. Leaf N concentrations ranged from 45.4 to 31.4 mg g^–1. Leaf P varied from 2.3 to 1.8 mg g^–1. Mean N and P RE varied among species, occasionally between top and bottom sites, and were higher in the dry than in the wet years of study. Senesced-leaf nutrient concentrations (i.e., a measure of resorption proficiency) varied from 13.7 to 31.2 mg g^–1 (N) and 0.4 to 3.3 mg g^–1 (P) among the different species and were generally indicative of incomplete nutrient resorption. Phosphorus concentrations in senesced leaves were higher at the bottom than at the top site and decreased from the wettest to the the driest year. Soil N and P availability were significantly different in the north- and south-facing slopes, but neither nutrient concentrations of mature and senesced leaves nor RE differed between aspects. Our results suggest that water more than soil nutrient availability controls RE in the Chamela dry forest, while resorption proficiency may be interactively controlled by both nutrient and water availability.     

Nutrient resorption of two evergreen shrubs in response to long-term fertilization in a bog

Plant resorption of multiple nutrients during leaf senescence has been established but stoichiometric changes among N, P and K during resorption and after fertilization are poorly understood. We anticipated that increased N supply would lead to further P limitation or co-limitation with N or K [i.e. P-(co)limitation], decrease N resorption and increase P and K resorption, while P and K addition would decrease P and K resorption and increase N resorption. Furthermore, Ca would accumulate while Mg would be resorbed during leaf senescence, irrespective of fertilization. We investigated the effect of N, P and K addition on resorption in two evergreen shrubs (Chamaedaphne calyculata and Rhododendron groenlandicum) in a long-term fertilization experiment at Mer Bleue bog, Ontario, Canada. In general, N addition caused further P-(co)limitation, increased P and K resorption efficiency but did not affect N resorption. P and K addition did not shift the system to N limitation and affect K resorption, but reduced P resorption proficiency. C. calyculata resorbed both Ca and Mg while R. groenlandicum resorbed neither. C. calyculata showed a higher resorption than R. groenlandicum, suggesting it is better adapted to nutrient deficiency than R. groenlandicum. Resorption during leaf senescence decreased N:P, N:K and K:P ratios. The limited response of N and K and the response of P resorption to fertilization reflect the stoichiometric coupling of nutrient cycling, which varies among the two shrub species; changes in species composition may affect nutrient cycling in bogs.     

Resorption of foliar nutrients in a regenerating southern Appalachian forest

Summary Leaves were sampled in a successional, southern Appalachian forest to estimate autumn foliar nutrient dynamics. Resorption of N and P in a successional forest equaled, or exceeded, resorption estimates for a more mature control forest. Foliar nutrient leaching was not sufficient to account for changes in autumn leaf N, P, Ca and Mg concentrations. The resorption process conserves nutrients by reducing nutrient losses from leaching and litter-fall, thereby closing the nutrient cycle in successional forests. We hypothesize that rapid recovery of primary productivity early in forest regeneration is the result of maximum nutrient resorption of limiting nutrients. Implications of these results for successional nutrient cycling theory are discussed.     

Foliar nutrient resorption patterns of four functional plants along a precipitation gradient on the Tibetan Changtang Plateau

Nutrient resorption from senesced leaves as a nutrient conservation strategy is important for plants to adapt to nutrient deficiency, particularly in alpine and arid environment. However, the leaf nutrient resorption patterns of different functional plants across environmental gradient remain unclear. In this study, we conducted a transect survey of 12 communities to address foliar nitrogen (N) and phosphorus (P) resorption strategies of four functional groups along an eastward increasing precipitation gradient in northern Tibetan Changtang Plateau. Soil nutrient availability, leaf nutrient concentration, and N:P ratio in green leaves ([N:P]_g) were linearly correlated with precipitation. Nitrogen resorption efficiency decreased, whereas phosphorus resorption efficiency except for sedge increased with increasing precipitation, indicating a greater nutrient conservation in nutrient‐poor environment. The surveyed alpine plants except for legume had obviously higher N and P resorption efficiencies than the world mean levels. Legumes had higher N concentrations in green and senesced leaves, but lowest resorption efficiency than nonlegumes. Sedge species had much lower P concentration in senesced leaves but highest P resorption efficiency, suggesting highly competitive P conservation. Leaf nutrient resorption efficiencies of N and P were largely controlled by soil and plant nutrient, and indirectly regulated by precipitation. Nutrient resorption efficiencies were more determined by soil nutrient availability, while resorption proficiencies were more controlled by leaf nutrient and N:P of green leaves. Overall, our results suggest strong internal nutrient cycling through foliar nutrient resorption in the alpine nutrient‐poor ecosystems on the Plateau. The patterns of soil nutrient availability and resorption also imply a transit from more N limitation in the west to a more P limitation in the east Changtang. Our findings offer insights into understanding nutrient conservation strategy in the precipitation and its derived soil nutrient availability gradient.     

High‐frequency fire alters C : N : P stoichiometry in forest litter

Fire is a major driver of ecosystem change and can disproportionately affect the cycling of different nutrients. Thus, a stoichiometric approach to investigate the relationships between nutrient availability and microbial resource use during decomposition is likely to provide insight into the effects of fire on ecosystem functioning. We conducted a field litter bag experiment to investigate the long‐term impact of repeated fire on the stoichiometry of leaf litter C, N and P pools, and nutrient‐acquiring enzyme activities during decomposition in a wet sclerophyll eucalypt forest in Queensland, Australia. Fire frequency treatments have been maintained since 1972, including burning every 2 years (2yrB), burning every 4 years (4yrB) and no burning (NB). C : N ratios in freshly fallen litter were 29–42% higher and C : P ratios were 6–25% lower for 2yrB than NB during decomposition, with correspondingly lower 2yrB N : P ratios (27–32) than for NB (34–49). Trends in litter soluble and microbial N : P ratios were similar to the overall litter N : P ratios across fire treatments. Consistent with these, the ratio of activities for N‐acquiring to P‐acquiring enzymes in litter was higher for 2yrB than NB, whereas 4yrB was generally intermediate between 2yrB and NB. Decomposition rates of freshly fallen litter were significantly lower for 2yrB (72 ± 2% mass remaining at the end of experiment) than for 4yrB (59 ± 3%) and NB (62 ± 3%), a difference that may be related to effects of N limitation, lower moisture content, and/or litter C quality. Results for older mixed‐age litter were similar to those for freshly fallen litter although treatment differences were less pronounced. Overall, these findings show that frequent fire (2yrB) decoupled N and P cycling, as manifested in litter C : N : P stoichiometry and in microbial biomass N : P ratio and enzymatic activities. Furthermore, these data indicate that fire induced a transient shift to N‐limited ecosystem conditions during the postfire recovery phase.     

Response of leaf, sheath and stem nutrient resorption to 7 years of N addition in freshwater wetland of Northeast China

Background and Aims

Increased N availability induced by agricultural fertilization applications and atmospheric N deposition may affect plant nutrient resorption in temperate wetlands. However, the relationship between nutrient resorption and N availability is still unclear, and most studies have focused on leaf nutrient resorption only. The aim of our study was to examine the response of leaf and non-leaf organ nutrient resorption to N enrichment in a temperate freshwater wetland.

Methods

We conducted a 7-year N addition experiment to investigate the effects of increased N loading on leaf, sheath and stem nutrient (N and P) resorption of two dominant species (Deyeuxia angustifolia and Glyceria spiculosa) in a freshwater marsh in the Sanjiang Plain, Northeast China.

Results

Our results showed that, for both leaf and non-leaf organs (sheath and stem), N addition decreased N resorption proficiency and hence increased litter N concentration. Moreover, the magnitude of N addition effect on N resorption proficiency varied with fertilization rates for D. angustifolia sheaths and stems, and G. spiculosa leaves. However, increased N loading produced inconsistent impacts on N and P resorption efficiencies and P resorption proficiency, and the effects only varied with species and plant organs. In addition, N enrichment increased litter mass and altered litter allocation among leaf, sheath and stem.

Conclusions

Our results highlight that leaf and non-leaf organs respond differentially to N addition regarding N and P resorption efficiencies and P resorption proficiency, and also suggest that N enrichment in temperate freshwater wetlands would alter plant internal nutrient cycles and increase litter quality and quantity, and thus substantially influence ecosystem carbon and nutrient cycles.     

Nutrient conservation increases with latitude of origin in European<Emphasis Type="Italic"> Pinus sylvestris</Emphasis> populations

Nutrient availability varies across climatic gradients, yet intraspecific adaptation across such gradients in plant traits related to internal cycling and nutrient resorption remains poorly understood. We examined nutrient resorption among six Scots pine (Pinus sylvestris L.) populations of wide-ranging origin grown under common-garden conditions in Poland. These results were compared with mass-based needle N and P for 195 Scots pine stands throughout the species' European range. At the common site, green needle N (r(2)=0.81, P=0.01) and P (r(2)=0.58, P=0.08) concentration increased with increasing latitude of population origin. Resorption efficiency (the proportion of the leaf nutrient pool resorbed during senescence) of N and P of Scots pine populations increased with the latitude of seed origin (r(2) > or = 0.67, P < or = 0.05). The greater resorption efficiency of more northerly populations led to lower concentrations of N and P in senescent leaves (higher resorption proficiency) than populations originating from low latitudes. The direction of change in these traits indicates potential adaptation of populations from northern, colder habitats to more efficient internal nutrient cycling. For native Scots pine stands, results showed greater nutrient conservation in situ in cold-adapted northern populations, via extended needle longevity (from 2 to 3 years at 50 degrees N to 7 years at 70 degrees N), and greater resorption efficiency and proficiency, with their greater resorption efficiency and proficiency having genotypic roots demonstrated in the common-garden experiment. However, for native Scots pine stands, green needle N decreased with increasing latitude (r(2)=0.83, P=0.0002), and P was stable other than decreasing above 62 degrees N. Hence, the genotypic tendency towards maintenance of higher nutrient concentrations in green foliage and effective nutrient resorption, demonstrated by northern populations in the common garden, did not entirely compensate for presumed nutrient availability limitations along the in situ latitudinal temperature gradient.     

Fire affects the taxonomic and functional composition of soil microbial communities,with cascading effects on grassland ecosystem functioning

Fire is a crucial event regulating the structure and functioning of many ecosystems. Yet few studies have focused on how fire affects taxonomic and functional diversities of soil microbial communities, along with changes in plant communities and soil carbon (C) and nitrogen (N) dynamics. Here, we analyze these effects in a grassland ecosystem 9 months after an experimental fire at the Jasper Ridge Global Change Experiment site in California, USA. Fire altered soil microbial communities considerably, with community assembly process analysis showing that environmental selection pressure was higher in burned sites. However, a small subset of highly connected taxa was able to withstand the disturbance. In addition, fire decreased the relative abundances of most functional genes associated with C degradation and N cycling, implicating a slowdown of microbial processes linked to soil C and N dynamics. In contrast, fire stimulated above‐ and belowground plant growth, likely enhancing plant–microbe competition for soil inorganic N, which was reduced by a factor of about 2. To synthesize those findings, we performed structural equation modeling, which showed that plants but not microbial communities were responsible for significantly higher soil respiration rates in burned sites. Together, our results demonstrate that fire ‘reboots’ the grassland ecosystem by differentially regulating plant and soil microbial communities, leading to significant changes in soil C and N dynamics.     

Soil C and N responses to woody plant expansion in a mesic grassland

Changes in land management and reductions in fire frequency have contributed to increased cover of woody species in grasslands worldwide. These shifts in plant community composition have the potential to alter ecosystem function, particularly through changes in soil processes and properties. In semi-arid grasslands, the invasion of shrubs and trees is often accompanied by increases in soil resources and more rapid N and C cycling. We assessed the effects of shrub encroachment in a mesic grassland in Kansas (USA) on soil CO₂ flux, extractable inorganic N, and N mineralization beneath shrub communities (Cornus drummondii) and surrounding undisturbed grassland sites. In this study, a shift in plant community composition from grassland to shrubland resulted in a 16% decrease in annual soil CO₂ flux(4.78 kg CO₂ m^–2 year^–1 for shrub dominated sites versus 5.84 kg CO₂ m^–2 year^–1 for grassland sites) with no differences in total soil C or N or inorganic N. There was considerable variability in N mineralization rates within sites, which resulted in no overall difference in cumulative N mineralized during this study (4.09 g N m^–2 for grassland sites and 3.03 g N m^–2 for shrub islands). These results indicate that shrub encroachment into mesic grasslands does not significantly alter N availability (at least initially), but does alter C cycling by decreasing soil CO₂ flux.     

Fire effects on ecosystem nitrogen cycling in a Californian bishop pine forest

Fire can cause severe nitrogen (N) losses from grassland, chaparral, and temperate and boreal forest ecosystems. Paradoxically, soil ammonium levels are markedly increased by fire, resulting in high rates of primary production in re-establishing plant communities. In a manipulative experiment, we examined the influence of wild-fire ash residues on soil, microbial and plant N pools in a recently burned Californian bishop pine (Pinus muricata D. Don) forest. Ash stimulated post-fire primary production and ecosystem N retention through direct N inputs from ash to soils, as well as indirect ash effects on soil N availability to plants. These results suggest that redistribution of surface ash after fire by wind or water may cause substantial heterogeneity in soil N availability to plants, and could be an important mechanism contributing to vegetation patchiness in fire-prone ecosystems. In addition, we investigated the impact of fire on ecosystem N cycling by comparing ¹⁵N natural abundance values from recently burned and nearby unburned P. muricata forest communities. At the burned site, ¹⁵N natural abundance in recolonising species was similar to that in bulk soil organic matter. By contrast, there was a marked ¹⁵N depletion in the same species relative to the total soil N pool at the unburned site. These results suggest that plant uptake of nitrate (which tends to be strongly depleted in ¹⁵N because of fractionation during nitrification) is low in recently burned forest communities but could be an important component of eco- system N cycling in mature conifer stands. Received: 29 June 1999 / Accepted: 24 October 1999     

Effects of fertilisation and irrigation on ‘foliar afterlife’ in alpine tundra

Question: How do increases in soil nutrient and water availability alter the nutrient fluxes through the resorption and litter decomposition pathways and how do they affect litter nutrient pools in a low‐productive alpine tundra ecosystem? Location: An alpine lichen‐rich tundra on Mt. Malaya Khati‐para in the NW Caucasus, Russia (43°27’ N, 41°42’ E; altitude 2800 m a.s.l.). Methods: We conducted a 4‐year fertilisation (N, P, N+P, lime) and irrigation experiment, and analysed the responses of nutrient resorption from senescing leaves, leaf litter quality and decomposability of six pre‐dominant vascular plant species, total plant community litter production and litter (nutrient) accumulation. Results: Vascular plant litter [N] and [P] increased 1.5 and 10 fold in response to N and P additions, due to increased concentrations of the nutrients in fresh leaves and unchanged or reduced resorption efficiency. Litter decomposability was not affected by nutrient amendments. Fertilisation enhanced litter production (180%; N+P treatment) and litter accumulation (80%; N+P), owing to tremendously increased production and low decomposability of graminoids. Together with increased litter [N] and [P] this led to great increases in total litter nutrient pools. Conclusions: Due to increased production of graminoids, nutrients added to the alpine tundra soil were mostly immobilised in recalcitrant, nutrient‐rich litter. This suggests that changing species composition in low productive ecosystems may act as an internal buffer mechanism, which under increased soil nutrient availability prevents the community from rapidly acquiring features typical of a high productive ecosystem such as high decomposability and high nutrient availability.     

氮、磷和水分供给对羊草枯叶分解质量的影响

研究不同氮(N)、磷(P)和水分梯度上植物枯叶的N和P浓度及C : N和C : P的响应格局, 对于预测N沉降增加和降水格局改变对枯叶分解乃至养分循环的潜在影响具有一定的实践意义。该研究通过3个盆栽控制试验(N、P添加试验: 0、0.5、1.0、2.0、4.0、8.0、16.0、32.0、64.0和128.0 g N(P)∙m^-210个水平; 控水试验: 3 600、3 900、4 500、5 100、6 000、7 200、9 000、12 000、 18 000和36 000 mL∙pot^-110个水平), 探讨了这些环境因子改变对羊草(Leymus chinensis)枯叶分解质量的影响。结果表明, 在一定范围内, N添加提高了羊草枯叶的N浓度, 降低了C : N, 对P浓度和C : P没有明显的影响; P添加提高了枯叶的N和P浓度, 降低了C : N和C : P; 供水量增加显著降低了枯叶的N和P浓度, 提高了C : N和C : P。这表明, N、P和水分因子的改变影响了植物枯叶的性状, 且不同梯度的影响程度不同。因此, 全球气候变化可能影响植物枯叶的分解质量, 进而可能改变植被-土壤系统的养分循环。     

Leaf nutrient concentration as an indicator of Populus and Tamarix response to flooding

Plants growing in infertile environments are able to produce more biomass per unit of nutrient taken up than plants of fertile habitats, and also to minimize nutrients loss by resorbing them from senescing leaves. The leaf nutrient concentration variability of two co-existing riparian tree genera (Populus and Tamarix) along a flood inundation gradient was examined to infer nutrient limitation and to compare nutrient use strategies in the two genera. To that end, seasonal and spatial variability in leaf nitrogen (N) and phosphorus (P) concentration (i.e., % dry mass of N and P) were analyzed in 720 samples of leaves (2 tree genera × 3 seasons × 12 sites × 10 tree replicates). Both Populus and Tamarix showed strong seasonal variability in leaf N and P concentrations, with values decreasing throughout the growing season. However, while N:P atomic ratio remained seasonally constant in Populus (N:P = 33), Tamarix shifted from N:P = 29 in spring to N:P = 36 and 37 in summer and fall. %N, %P and N:P atomic ratios were also spatially variable, but leaf litter N and P concentration (i.e., nutrient resorption proficiency) and leaf litter N:P generally followed the local flood inundation gradient as shown by linear mixed effects models. In particular, nutrient resorption was usually less proficient (higher terminal nutrient concentrations) at higher flood durations (in gravel bars and natural levees), whereas N:P increased in the drier sites (floodplain terrace). At floodplain level, a P-limitation that is higher than N-limitation seems to characterize the plant nutrient circulation in the riparian ecosystem studied. Tamarix was slightly more proficient in P resorption than Populus. The study shows that leaf nutrient concentration (e.g., N and P) derived from nutrient availability is partly controlled by the flood inundation regime and can be used as an indicator of nutrient limitation in forested floodplains. Subtle differences between tree genera provide an additional, novel explanation for the recent expansion of Tamarix in many arid and semi-arid rivers with altered hydrogeomorphic regimes.