Increased precipitation modulates the influence of nitrogen and litter inputs on the nutrient resorption proficiency rather than efficiency of <Emphasis Type="Italic">Leymus chinensis</Emphasis>

Resorption of nutrients from senescing organs is an important conservation mechanism that is usually influenced by the supply of soil nutrients and plant growth requirements. Therefore, it is likely that increases in nitrogen (N), precipitation, and litter could lead to changes in nutrient resorption because of changes in nutrients in the soil and accelerated plant growth in response to the alleviation of water limitations in arid and semiarid environments. In the current study, we investigated the effects of water, N, and litter addition on the nutrient resorption efficiency and proficiency of N and phosphorus (P) in leaves and stems of Leymus chinensis in Inner Mongolia, China. Our results showed that N addition significantly decreased the N resorption efficiency in leaves under water addition, and increased P resorption efficiency under ambient precipitation conditions. There was no apparent influence of either litter or water addition on N and P resorption efficiencies. However, N and litter addition significantly altered N and P resorption proficiencies, and these effects were modulated by water availability. Furthermore, changes in resorption proficiencies were mainly associated with alterations in the nutritional status of green organs in response to water, N and litter addition, except for leaf P. Our findings highlight the importance of increased precipitation in modulating the nutrient resorption proficiency of plants under potentially increased nutrient availability in semiarid grasslands. Therefore, global changes in precipitation and N, and corresponding litter changes could result in complex effects on plant nutrient economies and, in turn, could influence the return of nutrients to the soil.     

Changing leaf litter feedbacks on plant production across contrasting sub-arctic peatland species and growth forms

Plant species and growth forms differ widely in litter chemistry, which affects decay and may have important consequences for plant growth via e.g. the release of nutrients and growth-inhibitory compounds. We investigated the overall short-term (9.5 months) and medium-term (21.5 months) feedback effects of leaf litter quality and quantity on plant production, and tested whether growth forms can be used to generalise differences among litter species. Leaf litter effects of 21 sub-arctic vascular peatland species on Poa alpina test plants changed clearly with time. Across all growth forms, litter initially reduced plant biomass compared with untreated plants, particularly litters with a high decomposition rate or low initial lignin/P ratio. In the second year, however, litter effects were neutral or positive, and related to initial litter N concentration (positive), C/N, polyphenol/N and polyphenol/P ratios (all negative), but not to decomposability. Differences in effect size among several litter species were large, while differences in response to increasing litter quantities were not significant or of similar magnitude to differences in response to three contrasting litter species. Growth forms did not differ in initial litter effects, but second-year plant production showed a trend (P < 0.10) for differences in response to litters of different growth forms: evergreen shrubs < graminoids or deciduous shrubs < forbs. While long-persisting negative litter effects were predominant across all growth forms, our data indicate that even within nutrient-constrained ecosystems such as northern peatlands, vascular plant species, and possibly growth forms, differ in litter feedbacks to plant growth. Differences in the composition of undisturbed plant communities or species shifts induced by external disturbance, such as climate change, may therefore feedback strongly to plant biomass production and probably nutrient cycling rates in northern peatlands. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Depth‐based differentiation in nitrogen uptake between graminoids and shrubs in an Arctic tundra plant community

Questions

The rapid climate warming in tundra ecosystems can increase nutrient availability in the soil, which may initiate shifts in vegetation composition. The direction in which the vegetation shifts will co‐determine whether Arctic warming is mitigated or accelerated, making the understanding of successional trajectories urgent. One of the key factors influencing the competitive relationships between plant species is their access to nutrients, depending on the depth where they take up most nutrients. However, nutrient uptake at different soil depths by tundra plant species that differ in rooting depth is unclear.

Location

Kytalyk Nature Reserve, northeast Siberia, Russia.

Methods

We injected ¹⁵N to 5 cm, 15 cm and the thaw front of the soil in a moist tussock tundra. The absorption of ¹⁵N by grasses, sedges, deciduous shrubs and evergreen shrubs from the three depths was compared.

Results

The results clearly show a vertical differentiation of N uptake by these plant functional types, corresponding to their rooting strategy. Shallow‐rooting dwarf shrubs were more capable of absorbing nutrients from the upper soil than from deeper soil. Deep‐rooting grasses and sedges were more capable of absorbing nutrients from deeper soil than the dwarf shrubs. The natural ¹⁵N abundances in control plants also indicate that graminoids can absorb more nutrients from the deeper soil than dwarf shrubs.

Conclusions

Our results show that graminoids and shrubs in the Arctic differ in their N uptake strategies, with graminoids profiting from nutrients released at the thaw front, while shrubs mainly forage in upper soil layers. Our results suggest that tundra vegetation will become graminoid‐dominated as permafrost thaw progresses and nutrient availability increases in the deep soil.     

Reindeer influence on ecosystem processes in the tundra

Reindeer have been recorded to increase nutrient cycling rate and primary production in studies from fences almost 40 years old that separate areas with different grazing regimes in northern Fennoscandia. To further understand the mechanism behind the effects of herbivores on primary production, we measured the size of the major C and N pools, soil temperature, litter decomposition rate and N mineralization rate in lightly, moderately and heavily grazed areas along two of these fences.
Plant N found in new biomass, indicative of plant N assimilation, was significantly higher in moderately and heavily grazed areas than in lightly grazed areas, which corresponded to a decreased amount of N in old plant parts. The amount of N found in plant litter or organic soil layer did not differ between the grazing treatments. Together with soil N concentrations and litter decomposition rates, soil temperatures were significantly higher in moderately and heavily grazed areas.
We conclude that the changes in soil temperature are important for the litter decomposition rate and thus on the nutrient availability for plant uptake. However, the changes in plant community composition appear to be more important for the altered N pools and thus the enhanced primary production. The results provide some support for the keystone herbivore hypothesis, which states that intensive grazing can promote a transition from moss-rich tundra heath to productive grasslands. Grazing altered N fluxes and pools, but the total N pools were similar in all grazing treatments. Our study thus indicates that grazing can increase the primary production through enhancing the soil nutrient cycling rate, even in a long term perspective on an ecological timescale.     

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.     

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.     

Phosphorus conservation during post‐fire regeneration in a Chilean temperate rainforest

Nitrogen and phosphorus are the main elements limiting net primary production in terrestrial ecosystems. When growing in nutrient‐poor soils, plants develop physiological mechanisms to conserve nutrients, such as reabsorbing elements from senescing foliage (i.e. nutrient retranslocation). We investigated the changes in soil N and P in post‐fire succession in temperate rainforests of southern Chile. In this area, forest recovery often leads to spatially scattered, discrete regeneration with patches varying in age, area, species richness and tree cover, representing different degrees of recovery from disturbance. We hypothesized that soil nutrient concentrations should differ among tree regenerating patches depending on the progress of forest regeneration and that nutrient resorption should increase over time as colonizing trees respond to limited soil nutrients. To evaluate these hypotheses, we sampled 40 regeneration patches in an area of 5 ha, spanning a broad range of vegetation complexity, and collected soil, tree foliage and litter samples to determine N and P concentrations. Nutrient concentrations in leaf litter were interpreted as nutrient resorption proficiency. We found that soil P was negatively correlated with all the indicators of successional progress, whereas total soil N was independent of the successional progress. Foliar N and P were unrelated to soil nutrient concentrations; however, litter N was negatively related to soil N, and litter P was positively related with soil P. Finally, foliar N:P ratios ranged from 16 to 25, which suggests that P limitation can hamper post‐fire regeneration. We provide evidence that after human‐induced fires, succession in temperate forests of Chile can become nutrient limited and that high nutrient retranslocation is a key nutrient conservation strategy for regenerating tree communities.     

Direct and indirect control by snow cover over decomposition in alpine tundra along a snowmelt gradient

We assessed direct and indirect effects of snow cover on litter decomposition and litter nitrogen release in alpine tundra. Direct effects are driven by the direct influence of snow cover on edaphoclimatic conditions, whereas indirect effects result from the filtering effect of snow cover on species’ abundance and traits. We compared the in situ decomposition of leaf litter from four dominant plant species (two graminoids, two shrubs) at early and late snowmelt locations using a two-year litter-bag experiment. A seasonal experiment was also performed to estimate the relative importance of winter and summer decomposition. We found that growth form (graminoids vs. shrubs) are the main determinants of decomposition rate. Direct effect of snow cover exerted only a secondary influence. Whatever the species, early snowmelt locations showed consistently reduced decomposition rates and delayed final stages of N mineralization. This lower decomposition rate was associated with freezing soil temperatures during winter. The results suggest that a reduced snow cover may have a weak and immediate direct effect on litter decomposition rates and N availability in alpine tundra. A much larger impact on nutrient cycling is likely to be mediated by longer term changes in the relative abundance of lignin-rich dwarf shrubs.     

Earthworms,Collembola and residue management change wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>) and herbivore pest performance (Aphidina: <Emphasis Type="Italic">Rhophalosiphum padi</Emphasis>)

Management practices of arable systems determine the distribution of soil organic matter thereby changing decomposer animal activity and their impact on nutrient mineralization, plant growth and plant-herbivore interactions. Decomposer-mediated changes in plant growth and insect pest performance were investigated in wheat-aphid model systems in the greenhouse. Three types of litter distribution were established: litter patch at the soil surface (simulating mulching), litter patch deeper in soil (simulating ploughing) and litter homogeneously mixed into soil (simulating disk cultivation). The litter was labelled with (15)N to follow the mineralization and uptake of nutrients by the plants. Earthworms (Aporrectodea caliginosa) and Collembola (Protaphorura armata) were included as representatives of major functional groups of decomposers. Wheat (Triticum aestivum) was planted and aphids (Rhophalosiphum padi) were introduced to leaves as one of the most important pests. Earthworms, Collembola and litter distribution affected plant growth, N acquisition and aphid development in an interactive way. Earthworms and Collembola increased biomass of seeds, shoots and roots of wheat. Increased plant growth by earthworms and Collembola was mainly due to increased transfer of N from soil (rather than litter) into plants. Despite increasing plant growth, earthworms reduced aphid reproduction. Aphid reproduction was not correlated closely with plant N concentrations, but rather with the concentration of litter N in wheat. Unexpectedly, both Collembola and earthworms predominantly affected the mobilization of N from soil organic matter, and by altering the distribution of litter earthworms reduced infestation of crops by aphids via reducing plant capture of litter N, in particular if the litter was concentrated deeper in soil. The results suggest that management practices stimulating a continuous moderate increase in nutrient mobilization from soil organic matter rather than nutrient flushes from decomposing fresh organic matter result in maximum plant growth with minimum plant pest infestation.     

Interactive Effects Between Reindeer and Habitat Fertility Drive Soil Nutrient Availabilities in Arctic Tundra

Herbivores impact nutrient availability and cycling, and the net effect of herbivory on soil nutrients is generally assumed to be positive in nutrient-rich environments and negative in nutrient-poor ones. This is, however, far from a uniform pattern, and there is a recognized need to investigate any interactive effects of herbivory and habitat fertility (i.e., plant C/N ratios) on soil nutrient availabilities. We determined long-term effects of reindeer on soil extractable nitrogen (N) and phosphorus (P) and their net mineralization rates along a fertility gradient of plant carbon (C) to N and P ratios in arctic tundra. Our results showed that reindeer had a positive effect on soil N in the more nutrient-poor sites and a negative effect on soil P in the more nutrient-rich sites, which contrasts from the general consensus. The increase in N availability was linked to a decrease in plant and litter C/N ratios, suggesting that a shift in vegetation composition toward more graminoids favors higher N cycling. Soil P availability was not as closely linked to the vegetation and is likely regulated more by herbivore-induced changes in soil physical and chemical properties. The changes in soil extractable N and P resulted in higher soil N/P ratios, suggesting that reindeer could drive the vegetation toward P-limitation. This research highlights the importance of including both the elements N and P and conducting studies along environmental gradients in order to better understand the interactive effects of herbivory and habitat fertility on nutrient cycling and primary production.     

Variable Responses of Lowland Tropical Forest Nutrient Status to Fertilization and Litter Manipulation

Predicting future impacts of anthropogenic change on tropical forests requires a clear understanding of nutrient constraints on productivity. We compared experimental fertilization and litter manipulation treatments in an old-growth lowland tropical forest to distinguish between the effects of inorganic nutrient amendments and changes in nutrient cycling via litterfall. We measured the changes in soil and litter nutrient pools, litterfall, and fine root biomass in plots fertilized with nitrogen (N), phosphorus (P), or potassium (K), and in litter addition and litter removal treatments during 7 years. Soil inorganic N and litter N increased in double-litter plots but not in N-fertilized plots. Conversely, litter P and soil pools of P and K increased in fertilized plots but not in the double-litter plots. Soil and litter pools of N and K decreased in the no-litter plots. Changes in litterfall with added nutrients or litter were only marginally significant, but fine root biomass decreased with both the litter and the K addition. Differences between the two experiments are mostly attributable to the coupled cycling of carbon and nutrients in litter. Increased nutrient inputs in litter may improve plant uptake of some nutrients compared to fertilization with similar amounts. The litter layer also appears to play a key role in nutrient retention. We discuss our findings in the context of possible impacts of anthropogenic change on tropical forests.     

Determinants of Leaf Litter Nutrient Cycling in a Tropical Rain Forest: Soil Fertility Versus Topography

We investigated the influence of landscape-level variation in soil fertility and topographic position on leaf litter nutrient dynamics in a tropical rain forest in Costa Rica. We sampled across the three main edaphic conditions (ultisol slope, ultisol plateau, and inceptisol) to determine the effect of soil nutrients on leaf litter nutrient concentrations while controlling for topography, and to examine topographic effects while controlling for soil nutrients. Both leaf litter macronutrient [phosphorus (P), nitrogen (N), sulfur (S), calcium (Ca), potassium (K), magnesium (Mg)] and micronutrient concentrations were quantified throughout a 4-year period. Leaf litter [P], [N] and [K] varied significantly among soil types. The variation in [P], [N], and [K] was explained by soil fertility alone. Leaf litter [S], [Ca], and [Mg] did not vary among the three soil types. Macronutrient (P, K, Mg, S, Ca) concentrations in the leaf litter were much less variable than those of Fe and Al. Lower variability in essential plant nutrients suggests a great deal of plant control over the amount of nutrients resorbed before senescense. Leaf litter macronutrient concentrations varied significantly over the 4-year period, but the temporal variation did not differ among the three edaphic types as anticipated. Hence, although the magnitude of nutrient fluxes may be controlled by local factors such as soil fertility, temporal patterns are likely regulated by a common environmental variable such as precipitation or temperature.     

Nutrient content and dynamics in north Swedish shrub tundra areas

Forbs and leaves of deciduous shrubs had high concentrations of Ca, Mg, K, P, and N. Deciduous dwarf shrubs had intermediate concentrations but higher than evergreen dwarf shrubs. Monocots, cryptogams, woody and belowground tissues had low concentrations.
Plant nutrient concentrations and nutrient content in soil organic matter increased from dry towards moist tundra areas.
The residence time of nutrients was considerably less than ten years in surface litter, but several decades or centuries in total organic matter. The longest residence time was found in the moist part of the tundra.
N, K, and P in Betula nana leaves were translocated to a great extent prior to leaf fall, whereas Mg and particularly Ca were only slightly translocated. As on other tundra areas shortage of nutrients probably limits plant growth. In that case short supply of N and F seems most probable due to retention in litter and soil organic matter.     

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.     

Controls of primary productivity and nutrient cycling in a temperate grassland with year‐round production

Abstract Net primary production (NPP) and nutrient dynamics of grasslands are regulated by different biotic and abiotic factors, which may differentially affect functional plant groups. Most studies have dealt with grasslands that have extremely low or zero production over a significant period of the year. Here we explore the relative importance of a few environmental factors as controls of aerial and below‐ground plant biomass production and nutrient dynamics in a grassland that is active throughout the year. We investigate their effect on the response of three main plant functional groups (warm‐ and cool‐season graminoids and forbs). We conducted a factorial experiment in a continuously grazed site in the Flooding Pampa grassland (Argentina). Factors were seasons (summer, autumn, winter and spring), and environmental agents (mowing, shade, addition of phosphorus [P] and nitrogen [N]). N addition had the largest and most extended impact: it tripled aerial NPP in spring and summer but had no effect on below‐ground biomass. This positive effect was accompanied by higher N acquisition and higher soil N availability. Mowing increased aerial NPP in winter, increased root biomass in the first 10 cm during autumn and winter and promoted N and P uptake by plants. Shading did not affect aerial NPP, but stimulated N and P uptake by plants. P addition had no effect on aerial NPP, but increased shallow root biomass and its N content in spring, and tripled P accumulation in plant biomass. The three plant functional groups differentially accounted for these ecosystem‐level responses. Graminoids explained the greater biomass production of N‐fertilized plots and mowing tended to promote forbs. These results suggest that the environmental controls of aerial NPP in this grassland vary among seasons, differentially impact the major floristic groups, and affect the energy and nutrient transfer to herbivores.     

Nutrient conservation strategies in native Andean‐Patagonian forests

Abstract. Nutrient conservation in vegetation affects rates of litter decomposition and soil nutrient availability. Although resorption has been traditionally considered one of the most important plant strategies to conserve nutrients in temperate forests, long leaf life‐span and low nutrient requirements have been postulated as better indicators. We aimed at identifying nutrient conservation strategies within characteristic functional groups of NW Patagonian forests on Andisols. We analysed C‐, N‐, P‐, K‐ and lignin‐concentrations in mature and senescent leaves of ten native woody species within the functional groups: broad‐leaved deciduous species, broad‐leaved evergreens and conifers. We also examined mycorrhizal associations in all species. Nutrient concentration in mature leaves and N‐ resorption were higher in broad‐leaved deciduous species than in the other two functional groups. Conifers had low mature leaf nutrient concentrations, low N‐resorption and high lignin/N ratios in senescent leaves. P‐ and K‐resorptions did not differ among functional groups. Broad‐leaved evergreens exhibited a species‐dependent response. Nitrogen in mature leaves was positively correlated with both N resorption and soil N‐fertility. Despite the high P‐retention capacity of Andisols, N appeared to be the more limiting nutrient, with most species being proficient in resorbing N but not P. The presence of endomycorrhizae in all conifers and the broad‐leaved evergreen Maytenus boaria, ectomycorrhizae in all Nothofagus species (four deciduous, one evergreen), and cluster roots in the broad‐leaved evergreen Lomatia hirsuta, would be possibly explaining why P is less limiting than N in these forests.     

Original Articles: Differential Influence of Plant Species on Soil Nitrogen Transformations Within Moist Meadow Alpine Tundra

Plant species can influence nitrogen (N) cycling indirectly through the feedbacks of litter quality and quantity on soil N transformation rates. The goal of this research was to focus on small-scale (within-community) variation in soil N cycling associated with two community dominants of the moist meadow alpine tundra. Within this community, the small-scale patchiness of the two most abundant species (Acomastylis rossii and Deschampsia caespitosa) provides natural variation in species cover within a relatively similar microclimate, thus enabling estimation of the effects of plant species on soil N transformation rates. Monthly rates of soil N transformations were dependent on small-scale variation in both soil microclimate and species cover. The relative importance of species cover compared with soil microclimate increased for months 2 and 3 of the 3-month growing season. Growing-season net N mineralization rates were over ten times greater and nitrification rates were four times greater in Deschampsia patches than in Acomastylis patches. Variability in litter quality [carbon:nitrogen (C:N) and phenolic:N], litter quantity (aboveground and fine-root production), and soil quality (C:N) was associated with three principal components. Variability between the species in litter quality and fine-root production explained 31% of the variation in net N mineralization rates and 36% of net nitrification rates. Site variability across the landscape in aboveground production and soil C:N explained 33% of the variation in net N mineralization rates and 21% of net nitrification rates. Within the moist meadow community, the high spatial variability in soil N transformation rates was associated with differences in the dominant species' litter quality and fine-root production. Deschampsia-dominated patches consistently had greater soil N transformation rates than did Acomastylis-dominated patches across the landscape, despite site variability in soil moisture, soil C:N, and aboveground production. Plant species appear to be an important control of soil N transformation in the alpine tundra, and consequently may influence plant community structure and ecosystem function.     

Spatial patterns of total and available N and P at alpine treeline

Background and aims

Vegetation can have direct and indirect effects on soil nutrients. To test the effects of trees on soils, we examined the patterns of soil nutrients and nutrient ratios at two spatial scales: at sites spanning the alpine tundra/subalpine forest ecotone (ecotone scale), and beneath and beyond individual tree canopies within the transitional krummholz zone (tree scale).

Methods

Soils were collected and analyzed for total carbon (C), nitrogen (N), and phosphorus (P) as well as available N and P on Niwot Ridge in the Colorado Rocky Mountains.

Results

Total C, N, and P were higher in the krummholz zone than the forest or tundra. Available P was also greatest in the krummholz zone while available N increased from the forest to the tundra. Throughout the krummholz zone, total soil nutrients and available P were higher downwind compared to upwind of trees.

Conclusions

The krummholz zone in general, and downwind of krummholz trees in particular, are zones of nutrient accumulation. This pattern indicates that the indirect effects of trees on soils are more important than the direct effects. The higher N:P ratios in the tundra suggest nutrient dynamics differ from the lower elevation sites. We propose that evaluating soil N and P simultaneously in soils may provide a robust assay of ecosystem nutrient limitation.     

长白山苔原带凋落物生态化学计量特征及其对模拟氮沉降的响应

长白山苔原是我国乃至欧亚大陆东部独有的高山苔原,根据前人调查植被以灌木苔原为主要类型。在全球变暖背景下,近30年来,长白山岳桦林下的草本植物侵入苔原带,原生灌木苔原分化为灌木苔原、灌草苔原和草本苔原,形成了灌木、灌草混合和草本3种不同类型的凋落物,凋落物数量和质量发生显著改变。与此同时长白山苔原氮沉降量也在逐年增加,导致了土壤中氮的累积,势必影响凋落物的分解。凋落物作为连接植物和土壤的纽带,其分解过程中碳（C）、氮（N）、磷（P）等化学组分和化学计量比的变化直接和间接影响着土壤养分有效性和植物养分利用策略。为揭示氮沉降增加对长白山苔原带不同类型凋落物化学组分及生态化学计量特征早期变化的影响,开展了为期8个月的模拟氮沉降室内凋落物分解实验。在苔原带采集灌木优势种牛皮杜鹃和草本优势种小叶章的凋落物带回实验室,模拟灌木牛皮杜鹃群落、灌草混合的牛皮杜鹃-小叶章群落和草本小叶章群落的3种不同类型凋落物,设置三个施氮处理：对照（CK,0 g N m^-2 a^-1）、低氮（LN,10 g N m^-2 a^-1）、高氮（HN,20 g N m^-2 a^-1）。研究表明：（1）不施氮处理时,3种凋落物的C、P均呈释放状态,木质素（Li）呈先累积再略有降解趋势;牛皮杜鹃凋落物的N元素富集而其余两种凋落物N元素呈释放状态;灌草混合和草本凋落物比原生的灌木凋落物C和N元素释放快、Li累积少;而灌木凋落物的P释放略快于灌草和草本凋落物。3种植被类型凋落物的C/N、C/P、Li/N大小表现为：牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物>小叶章凋落物;N/P表现为：小叶章凋落物>牛皮杜鹃凋落物>牛皮杜鹃-小叶章混生群落凋落物。（2）氮沉降促进3种类型凋落物分解过程中C、N和P化学组分的释放,且氮浓度越高促进作用越显著。在牛皮杜鹃凋落物分解过程中,氮素添加到达某一阈值后,其C/N、C/P、N/P、Li/N的降幅最大,后续若再增加氮素,其对化学计量比的影响均会减弱;本实验中的氮素添加量增加促进了小叶章凋落物的C/N、Li/N下降。（3）草本植物入侵引起凋落物类型的变化带来凋落物分解加快,将导致长白山苔原带养分循环的变化;氮沉降增加对小叶章凋落物化学组分的释放及C/N、Li/N的下降更为促进,小叶章凋落物内难分解化合物减少,分解受到促进。高氮沉降加快了小叶章凋落物与土壤、草本植物之间的养分循环。因此,随着未来苔原带氮沉降量的增加,将更有利于小叶章在与牛皮杜鹃的竞争中获胜,使苔原带呈现草甸化趋势。     

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。