Plant nitrogen dynamics and nitrogen-use strategies under altered nitrogen seasonality and competition

BACKGROUND AND AIMS: Numerous studies have examined the effects of climatic factors on the distribution of C(3) and C(4) grasses in various regions throughout the world, but the role of seasonal fluctuations in temperature, precipitation and soil N availability in regulating growth and competition of these two functional types is still not well understood. This report is about the effects of seasonality of soil N availability and competition on plant N dynamics and N-use strategies of one C(3) (Leymus chinensis) and one C(4) (Chloris virgata) grass species. METHODS: Leymus chinensis and C. virgata, two grass species native to the temperate steppe in northern China, were planted in a monoculture and a mixture under three different N seasonal availabilities: an average model (AM) with N evenly distributed over the growing season; a one-peak model (OM) with more N in summer than in spring and autumn; and a two-peak model (TM) with more N in spring and autumn than in summer. KEY RESULTS: The results showed that the altered N seasonality changed plant N concentration, with the highest value of L. chinensis under the OM treatment and C. virgata under the TM treatment, respectively. N seasonality also affected plant N content, N productivity and N-resorption efficiency and proficiency in both the C(3) and C(4) species. Interspecific competition influenced N-use and resorption efficiency in both the C(3) and C(4) species, with higher N-use and resorption efficiency in the mixture than in monoculture. The C(4) grass had higher N-use efficiency than the C(3) grass due to its higher N productivity, irrespective of the N treatment or competition. CONCLUSIONS: The observations suggest that N-use strategies in the C(3) and C(4) species used in the study were closely related to seasonal dynamics of N supply and competition. N seasonality might be involved in the growth and temporal niche separation between C(3) and C(4) species observed in the natural ecosystems.     

不同小麦品种氮效率与氮吸收对氮素供应的响应及生理机制

以具有典型特征的不同氮效率小麦品种为材料,研究了低氮和高氮条件下小麦的生物学性状、生理参数和氮同化代谢酶活性.结果表明:低氮条件下,不同氮效率小麦品种根系干质量、茎叶干质量、植株氮累积量基本上为氮高效品种>中效品种>低效品种.低氮条件下,氮吸收高效品种（冀97-6360）的根系活跃吸附面积、TTC还原力、叶片硝酸还原酶活性和叶片NO₃^-含量最大;生理高效品种(石新5418)具有较高的叶片亚硝酸还原酶活性和谷氨酰胺合成酶活性,较低的植株全氮含量、叶片NO₃^-含量和硝酸还原酶活性.低氮条件下植株氮利用效率与氮吸收系数显著相关.不同小麦品种在高氮条件下的生物学性状、生理参数和氮同化代谢酶活性与低氮条件下不尽一致.     

Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams

Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH₄⁺ uptake rates were measured in eight streams using short-term ¹⁵N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams.     

Soil nitrogen availability evaluations based on nitrogen mineralization potentials of soils and uptake of labeled and unlabeled nitrogen by plants

Summary Sudangrass [Sorghum sudanense (Piper) Stapf] was grown in a greenhouse pot experiment on 39 soils having a broad range of chemical and physical characteristics. Labelled N as sodium nitrate (9% excess N¹⁵) was applied at rates of 200 and 400 mg of N per pot (2kg of soil). After 6 weeks of growth, total N and N¹⁵ were determined on plant tops and roots and on the cropped soils. Maximum yield differed widely among the soils owing to variations in yield-limiting factors other than N. Despite the diversity of responses to N fertilizer, the experiment provided a meaningful basis for assessing soil nitrogen availability. Amounts of N taken up from soils were similar from pots receiving no fertilizer N and from pots receiving labeled N.Amounts of soil organic N mineralized during cropping plus the mineral N present initially in the soils correlated highly with amounts of soil N taken up by whole plants (tops and roots). Average recovery by whole plants of mineral N formed before and during the cropping period was about 85 per cent, a value corresponding closely to recovery of fertilizer N in this experiment. The similarity in recovery of N provided by soil and fertilizer suggests that mineral N from these sources comprised a common pool that behaved as an entity with respect to mineralization-immobilization relations or other reactions affecting N availability to plants.A-values, the amounts of soil N having an availability equivalent to that of applied fertilizer N, were similar for two levels of applied labeled N and for tops and whole plants. Moreover, A-values were similar to amounts of N mineralized before and during crop growth. This result is particularly significant, since amounts of N mineralized during crop growth were estimated from N mineralization potentials, taking into account the effects of temperature on the mineralization rate constant. Thus, the study provides preliminary evidence that the soil N mineralization potential offers a basis for reliably estimating amounts of soil N mineralized during selected periods of time under specified temperature regimes.     

Elevated CO2 increases nitrogen rhizodeposition and microbial immobilization of root-derived nitrogen

With this study, we aimed to determine how elevated CO(2) affects rhizodeposition and the cycling of rhizodeposited nitrogen (N) in the soil under C(3) and C(4) plants. In addition, we examined how cultivated genotypes of wheat (Triticum turgidum) and maize (Zea mays) responded to elevated CO(2) in comparison with their wild relatives. By constructing an N-transfer experiment we could directly assess cycling of the rhizodeposited N and trace the fate of rhizodeposited N in the soil and in receiver plants. Biomass production, rhizodeposition and cycling of root-borne N in maize genotypes were not affected by elevated CO(2). Elevated CO(2) stimulated above- and below-ground biomass production of the wheat genotypes on average by 38%, and increased rhizodeposition and immobilization of root-derived N on average by 30%. Concurrently, elevated CO(2) reduced mineral (15)N and re-uptake of the root-derived N by 50% in wheat. This study shows that elevated CO(2) may enhance N limitation by increasing N rhizodeposition and subsequent immobilization of the root-derived N.     

Soil mineralizable nitrogen and soil nitrogen supply under two-year potato rotations

Two-year potato rotations were evaluated for their effects on soil mineralizable N and soil N supply. Pre-plant soil samples (0–15 cm) collected from the potato year after seven rotation cycles were used to estimate soil mineralizable N using a 24 week aerobic incubation. Potentially mineralizable N (N ₀ ) ranged from 102 to 149 kg N ha^?1, and was greater after pea/white clover and oats/Italian ryegrass than after oats by an average of 35 and 22%, respectively. Labile, intermediate and stable mineralizable N pools were increased after pea/white clover compared with oats, whereas only the stable mineralizable N pool was increased after oats/Italian ryegrass. Potato plant N uptake with no fertilizer applied was greater in potato-pea/white clover compared with the three other rotations (126 vs. average of 67 kg N ha^?1). Choice of rotation crop in potato production influences both the quantity and quality of soil mineralizable N.     

Intercropping effect on root growth and nitrogen uptake at different nitrogen levels

Aims Intercropping legumes and non-legumes may affect the root growth of both components in the mixture, and the non-legume is known to be strongly favored by increasing nitrogen (N) supply. The knowledge of how root systems affect the growth of the individual species is useful for understanding the interactions in intercrops as well as for planning cover cropping strategies. The aim of this work was (i) to determine if different levels of N in the topsoil influence root depth (RD) and intensity of barley and vetch as sole crops or as an intercropped mixture and (ii) to test if the choice of a mixture or the N availability in the topsoil will influence the N uptake by deep roots.Methods In this study, we combined rhizotron studies with root extraction and species identification by microscopy with studies of growth, N uptake and 15 N uptake from deeper soil layers, for studying the root interactions of root growth and N foraging for barley (Hordeum vulgare L.) and vetch (Vicia sativa L.), frequently grown in mixtures as cover crops. N was added at 0 (N0), 50 (N1) and 150 (N2) kg N ha^-1. The roots discrimination relying on the anatomical and morphological differences observed between dicots and monocots proved to be a reliable method providing valuable data for the analysis.Important findings The intercrop and the barley attained slightly higher root intensity (RI) and RD than the vetch, with values around 150 crosses m^-1 and 1.4 m, respectively, compared to 50 crosses m^-1 and 0.9 m for the vetch. At deep soil layers, intercropping showed slightly larger RI values compared to the sole-cropped barley. The barley and the intercropping had larger root length density (RLD) values (200–600 m m ?3) than the vetch (25–130) at 0.8–1.2 m depth. The topsoil N supply did not show a clear effect on the RI, RD or RLD; however, increasing topsoil N favored the proliferation of vetch roots in the intercropping at deep soil layers, with the barley:vetch root ratio ranging from 25 at N0 to 5 at N2. The N uptake of the barley was enhanced in the intercropping at the expense of the vetch (from ~100mg plant^-1 to 200). The intercropped barley roots took up more labeled nitrogen (0.6mg 15 N plant^-1) than the sole-cropped barley roots (0.3mg 15 N plant^-1) from deep layers.     

供氮水平对爬山虎幼苗生长形态和氮分配的影响

通过水培试验,研究了不同氮素水平(0、0.15、0.3、0.45、0.6和0.75g.L-1)条件下爬山虎幼苗的生长形态和体内氮分配状况.结果表明:低氮处理的爬山虎侧枝发育较小,而较高氮处理的侧枝着生位置低,且长度显著高于低氮处理;供氮水平的提高能促进植株的生物量及茎、叶、根氮含量和氮累积量的增加,其中叶生物量占全株生物量的比例最高,在50%以上;叶的氮累积量占整个植株总氮量的60%～70%.     

不同供氮量对二月兰产量、土壤无机氮残留及氮平衡的影响

研究华北冬绿肥二月兰对不同供氮水平的响应特征,确定实现绿肥高产高效的土壤适宜供氮量,可为华北集约化农田最大化发挥绿肥生态效应和优化春玉米/冬绿肥轮作体系氮素管理提供理论依据和技术参考.选取多年不施肥试验地设置供氮梯度试验,研究了不同供氮水平对冬绿肥二月兰翻压前地上部生物量累积、氮素吸收、土壤无机氮残留和冬绿肥季土壤氮素平衡的影响.结果表明: 在土壤无机氮含量较低(0～90 cm土层15 kg·hm^-2)条件下,施氮显著提高二月兰生物量和吸氮量.其中,施氮90 kg·hm^-2处理表现最高,绿肥生物量(干质量)和吸氮量分别为2031.0和42.0 kg·hm^-2;土壤无机氮残留量随施氮量增加而增加,且在施氮量高于60 kg·hm^-2后呈现快速增加趋势;随施氮量增加二月兰生长季的表观氮平衡表现出由亏缺到盈余的变化特征,在施氮量为60～90 kg·hm^-2条件下氮收支基本平衡.土壤供氮量(绿肥播前0～90 cm土壤无机氮含量与施氮量之和)与二月兰生物量、吸氮量和绿肥翻压前土壤无机氮含量的关系可以分别用二次、线性加平台和指数方程进行模拟,依据模型计算二月兰生物量最高值(2010 kg·hm^-2)时的播前土壤供氮量和绿肥翻压前土壤无机氮残留量分别是136和78 kg·hm^-2;而在二月兰吸氮量最高值40 kg·hm^-2时,二月兰生物量为1919 kg·hm^-2,相当于最高生物量的95%,绿肥翻压前土壤残留无机氮降低至57 kg·hm^-2,与之对应的播前土壤供氮量为105 kg·hm^-2,该值与目前华北地区优化施氮下玉米收获后土壤残留无机氮推荐含量(100 kg·hm^-2)基本相当.综合考虑绿肥的农学和环境效应,春玉米/冬绿肥轮作体系中二月兰播前土壤供氮量应控制在100～105 kg·hm^-2.     

The effect of cropping and fertiliser nitrogen rates on nitrogen balance in soil

Summary Fertilizer/soil N balance of cropped and fallow soil has been studied in a pot experiment carried out with grey forest soil (southern part of Moscow region) at increasing rates of¹⁵N labelled ammonium sulfate (0; 8; 16; 32 mg N/100 g of soil). The fertilizer¹⁵N balance has been shown to depend upon its application rate and the presence of growing plants. Fertilizer N uptake efficiency was maximum (72.5%) and gaseous losses-minimum (12.5%) at the application rate of 16 mg N/100 g of soil. Fertilizer N losses from the fallow soil were 130–220% versus those from the cropped soil. At the application of fertilizer N the plant uptake of soil N was 170–240% and the amount of soil N as N–NH₄ exchangeable + N–NO₃ in fallow was 350–440% as compared to the control treatment without nitrogen (PK).After cropping without or with N fertilizer application at the rates of 8 and 32 mg N/100 g of soil, a positive nitrogen balance has been found which is likely due to nonsymbiotic (associative) N-fixation. It has been shown that biologically fixed nitrogen contributes to plant nutrition.     

Seasonal flooding,nitrogen mineralization and nitrogen utilization in a prairie marsh

Flooding can be an important control of nitrogen (N) biogeochemistry in wetland ecosystems. In North American prairie marshes, spring flooding is a dominant feature of the physical environment that increases emergent plant production and could influence N cycling. I investigated how spring flooding affects N availability and plant N utilization in whitetop (Scolochloa festucacea) marshes in Manitoba, Canada by comparing experimentally spring-flooded marsh inside an impoundment with adjacent nonflooded marsh. The spring-flooded marsh had net N mineralization rates up to 4 times greater than nonflooded marsh. Total growing season net N mineralization was 124 kg N ha^–1 in the spring-flooded marsh compared with 62 kg N ha^–1 in the nonflooded marsh. Summer water level drawdown in the spring-flooded marsh decreased net N mineralization rates. Net nitrification rates increased in the nonflooded marsh following a lowering of the water table during mid summer. Growing season net nitrification was 33 kg N ha^–1 in the nonflooded marsh but < 1 kg N ha^–1 in the spring-flooded marsh. Added NO₃ ^–1 induced nitrate reductase (NRA) activity in whitetop grown in pot culture. Field-collected plants showed higher NRA in the nonflooded marsh. Nitrate comprised 40% of total plant N uptake in the nonflooded marsh but <1% of total N uptake in the spring-flooded marsh. Higher plant N demand caused by higher whitetop production in the spring-flooded marsh approximately balanced greater net N mineralization. A close association between the presence of spring flooding and net N mineralization and net nitrification rates indicated that modifications to prairie marshes that change the pattern of spring inundation will lead to rapid and significant changes in marsh N cycling patterns.     

Root-induced nitrogen mineralisation: A nitrogen balance model

The possibility is examined that carbon (C) released into the soil from a root could enhance the availability of nitrogen (N) to plants by stimulating microbial activity. Two models are described, both of which assume that C released from roots is used by bacteria to mineralise and immobilise soil organic N and that immobilised N released when bacteria are grazed by bacterial-feeding nematodes or protozoa is taken up by the plant. The first model simulates the individual transformations of C and N and indicates that root-induced N mineralisation could supply only up to 10% of the plant's requirement, even if unrealistically ideal conditions are assumed. The other model is based on evidence that about 40% of immobilised N is subsequently taken up by the plant. A small net gain of N by the plant is shown (i.e. the plant takes up more N than it loses through exudation), although with exudate of up to C:N 33:1 less than 6% of the plant's requirement is supplied by root-induced N mineralisation. It is argued, however, that rhizosphere bacteria do not use plant-derived C to mineralise soil organic N to any great extent and that in reality root-induced N mineralisation is even less important than these models indicate.     

模拟氮沉降对杂草生长和氮吸收的影响

以杂草早熟禾、黑麦草、野燕麦、天蓝苜蓿、白车轴草、北美车前、婆婆纳、无芒稗、牛筋草和刺苋为试验材料,以4.0g·m^-2·yr^-1的N输入为模拟氮沉降浓度,研究了不同杂草功能类群对模拟氮沉降的响应.结果表明,模拟氮沉降处理下,杂草的生物量(总生物量、地上部分生物量、根生物量)呈增加趋势,但不同功能类群对氮增加的响应明显不同,C₄禾本科、C₃豆科及C₃禾本科植物的生物量受到氮沉降的显著促进,但C₃非禾本科和C₄非禾本科植物的生物量则受氮沉降的影响不显著;不同功能类群的根冠比、植株含氮及植株吸收氮的总量对模拟氮沉降的响应无明显规律,但物种间差异显著.氮沉降提高野燕麦和北美车前的生物量的根冠比,但对其他生物种类没有显著影响.没有发现氮沉降对植物体内的含氮量有显著的影响,但氮沉降却显著地提高了除刺苋、早熟禾及婆婆纳之外的所有杂草物种对N的摄收.由于物种对氮沉降的响应不同,未来氮沉降的增加将加速杂草群落组成的变化.     

Postanthesis remobilisation and losses of nitrogen in wheat in relation to applied nitrogen

A ¹⁵N leaf feeding technique was used to measure the extent of remobilisation and loss of nitrogen (N) that had been accumulated prior to anthesis in wheat plants that received three rates of N fertilizer. Uptake of postanthesis N to the heads was reduced as the quantity of applied N was increased. This reduction of postanthesis N uptake did not affect the quqntity of N in the heads because the loss of preanthesis accumulated N was reduced and the extent of remobilised preanthesis N increased at higher rates of N application. At the lowest rate of N application the increase in ¹⁵N in the heads of fertile shoots arose chiefly from the remobilisation of N in the stem supporting the head. At higher rates of N application this source of N was increasingly supplemented by N remobilised from infertile tillers and roots.     

Differential responses of growth and nitrogen uptake to organic nitrogen in four gramineous crops

The capability to utilize different forms of nitrogen (N) by sorghum (Sorghum bicolor), rice (Oryza sativa), maize (Zea mays), and pearl millet (Pennisetum glaucum) was determined in pot experiments. Seedlings were grown for 21 d without N, or with 500 mg N kg(-1) soil applied as ammonium nitrate, rice bran or a mixture of rice bran and straw. No treatment-dependent changes of root length, surface area, and fractal dimension were observed. Shoot growth and N uptake in maize and pearl millet correlated with the inorganic N (ammonium and nitrate) concentration in the soil, suggesting that these species depend upon inorganic N uptake. On the other hand, shoot growth and N uptake patterns in sorghum and rice indicated that these two species could compensate low inorganic N levels in the organic material treatments by taking up organic N (proteins). Analysis of N uptake rates in solution culture experiments confirmed that sorghum and rice roots have higher capabilities to absorb protein N than maize and pearl millet.     

Effects of enhanced nitrogen deposition and phosphorus limitation on nitrogen budgets of semi-natural grasslands

Increased reactive atmospheric N deposition has been implicated in floristic changes in species‐rich acidic and calcareous grasslands, but the fate of this pollutant N in these ecosystems is unknown. This paper reports the first analysis of N budgets and N fluxes for two grasslands in the White Peak area of Derbyshire, one of the most heavily N‐polluted locations in the UK. N fluxes were monitored in lysimeter cores (retaining the original turfs) taken from field plots of unimproved acidic and calcareous grasslands that had received (in addition to ambient N deposition) simulated enhanced N deposition treatments of 3.5 and 14 g N m^?2 yr^?1 for 6 years. The influence of reducing phosphorus limitation was assessed by factorial additions of P. Seasonal leached losses of nitrate, ammonia and organic N were monitored in detail along with estimates of N removal through simulated grazing and gaseous losses through denitrification and volatilization. The rates of N fluxes by these pathways were used to create N budgets for the grasslands. Both grasslands were found to be accumulating much of the simulated additional N deposition: up to 89% accumulated in the calcareous grassland and up to 38% accumulated in the acidic grassland. The major fluxes of N loss from these grasslands were by simulated grazing and leaching of soluble organic N (constituting 90% of leached N under ambient conditions). Leached inorganic N (mainly nitrate) contributed significantly to the output flux of N under the highest N treatment only. Loss of N through ammonia volatilization accounted for less than 6% of the N added as simulated deposition, while denitrification contributed significantly to output fluxes only in the acidic grassland during winter. The implications of the results for ecosystem N balances and the likely consequences of N accumulation on these grasslands are discussed.     

Immediate and long-term nitrogen oxide emissions from tropical forest soils exposed to elevated nitrogen input

Tropical nitrogen (N) deposition is projected to increase substantially within the coming decades. Increases in soil emissions of the climate‐relevant trace gases NO and N₂O are expected, but few studies address this possibility. We used N addition experiments to achieve N‐enriched conditions in contrasting montane and lowland forests and assessed changes in the timing and magnitude of soil N‐oxide emissions. We evaluated transitory effects, which occurred immediately after N addition, and long‐term effects measured at least 6 weeks after N addition. In the montane forest where stem growth was N limited, the first‐time N additions caused rapid increases in soil N‐oxide emissions. During the first 2 years of N addition, annual N‐oxide emissions were five times (transitory effect) and two times (long‐term effect) larger than controls. This contradicts the current assumption that N‐limited tropical montane forests will respond to N additions with only small and delayed increases in soil N‐oxide emissions. We attribute this fast and large response of soil N‐oxide emissions to the presence of an organic layer (a characteristic feature of this forest type) in which nitrification increased substantially following N addition. In the lowland forest where stem growth was neither N nor phosphorus (P) limited, the first‐time N additions caused only gradual and minimal increases in soil N‐oxide emissions. These first N additions were completed at the beginning of the wet season, and low soil water content may have limited nitrification. In contrast, the 9‐ and 10‐year N‐addition plots displayed instantaneous and large soil N‐oxide emissions. Annual N‐oxide emissions under chronic N addition were seven times (transitory effect) and four times (long‐term effect) larger than controls. Seasonal changes in soil water content also caused seasonal changes in soil N‐oxide emissions from the 9‐ and 10‐year N‐addition plots. This suggests that climate change scenarios, where rainfall quantity and seasonality change, will alter the relative importance of soil NO and N₂O emissions from tropical forests exposed to elevated N deposition.     

Responses of ecosystem nitrogen cycle to nitrogen addition: a meta-analysis

? Anthropogenic nitrogen (N) addition may substantially alter the terrestrial N cycle. However, a comprehensive understanding of how the ecosystem N cycle responds to external N input remains elusive. ? Here, we evaluated the central tendencies of the responses of 15 variables associated with the ecosystem N cycle to N addition, using data extracted from 206 peer-reviewed papers. ? Our results showed that the largest changes in the ecosystem N cycle caused by N addition were increases in soil inorganic N leaching (461%), soil NO?? concentration (429%), nitrification (154%), nitrous oxide emission (134%), and denitrification (84%). N addition also substantially increased soil NH?+ concentration (47%), and the N content in belowground (53%) and aboveground (44%) plant pools, leaves (24%), litter (24%) and dissolved organic N (21%). Total N content in the organic horizon (6.1%) and mineral soil (6.2%) slightly increased in response to N addition. However, N addition induced a decrease in microbial biomass N by 5.8%. ? The increases in N effluxes caused by N addition were much greater than those in plant and soil pools except soil NO??, suggesting a leaky terrestrial N system.     

Influence of a pulsed nitrogen supply on growth and nitrogen uptake in alpine graminoids

The supply of N in alpine soils is influenced by environmental factors (freeze-thaw, drying-rewetting, release of N from winter snowpack) which lead to a pulsed nature in plant N availability. To address the ability of alpine species to acquire N and grow when N is supplied in a pulsed manner, six alpine graminoid species, 3 sedges (Cyperaceae) and 3 grasses (Poaceae), were grown under 3 treatments: low and high N supply applied 3 times weekly, and a pulsed N supply applied once weekly at the same concentration as the high N treatment, but with the same total N supply as the low N treatment. Growth, biomass allocation, and N uptake were the same in all species for plants grown under a pulsed N treatment relative to a constant N supply with the same amount of total N. Root:shoot ratios and uptake of experimentally applied ¹⁵N indicated there were no adjustments in growth allocation or root uptake capacity in the plants to enhance the uptake of N when supplied in a pulsed relative to a more constant supply. The fertility of the site from which the graminoids were collected did not influence the plants' ability to respond to a high versus a low N supply, but instead growth form was more important. Grasses exhibited variation in growth, biomass allocation, and N uptake in response to changes in N supply, while sedges did not.     

Root responses to nitrogen pulse frequency under different nitrogen amounts

Responses of morphology and biomass allocation of roots to frequency of nitrogen (N) pulse potentially influence the fitness of plants, but such responses may be determined by root size. We grew 12 plant species of three functional groups (grasses, forbs, and legumes) under two N pulse frequencies (high vs. low supply frequency) and two N amounts (high vs. low supply amount). Compared to low-amount N supply, high-amount N supply stimulated biomass accumulation and root growth by either increasing the thickness and length of roots or decreasing the root mass fraction. Compared to low-frequency N supply, high-frequency N supply improved biomass accumulation and root growth in forbs or grasses, but not in legumes. Furthermore, the magnitude of the response to N frequency was significantly negatively correlated with root size at the species scale, but this was only true when the N amount was high. We conclude that root responses to N frequency are related to plant functional types, and non-legume species is more sensitive to N frequency than legume species. Our results also suggest that root size is a determinant of root responses to N frequency when N supply amount is high.