Nutrient resorption response to fire and nitrogen addition in a semi-arid grassland

Fire and nitrogen (N) addition, both widely used grassland restoration strategies, strongly influence community composition and ecosystem functioning. However, little is known about their effects on plant nutrient resorption from senescing leaves, especially in semi-arid ecosystems. We evaluated the effects of fire, N addition (5.25 g N m⁻² yr⁻¹) and their potential interactions on nutrient resorption in five plant species in a semi-arid grassland in northern China. Foliar nutrient concentrations and resorption proficiencies and efficiencies varied substantially among species and functional groups. Fire increased green leaf N concentration ([N]g) and decreased N resorption proficiency (N RP), P resorption proficiency (P RP) and P resorption efficiency (P RE). N addition led to higher [N]g and lower N resorption, whereas it did not affect P related responses. There was no interaction between fire and N addition to affect all response variables except for green leaf P concentration ([P]g). These results suggest that fire and N addition can influence ecosystem nutrient cycling directly by changing resorption patterns and litter quality. Given the substantial interspecific variations in nutrient content and resorption and the potentially changing community composition, both fire and N addition may have indirect impacts on ecosystem nutrient cycling in this semi-arid grassland.     

Stream order controls geomorphic heterogeneity and plant distribution in a savanna landscape

We posed the question: does viewing a savanna as a network of streams linked to a matrix of terrestrial hillslopes provide a useful framework to research and understand plant distribution in these landscapes? Our study area, the Phugwane River network, lies in the semi‐arid savanna of Kruger National Park, South Africa. We examined changes in hillslope geomorphology from first‐, third‐ and fifth‐order hillslopes with regression equations. The distribution of geomorphic boundaries was enumerated by moving window analysis and the relationship between geomorphology and plant distribution was explored through ordination. First‐order hillslopes had a simple geomorphology, fewer geomorphic boundaries and a relatively homogeneous plant assemblage. By contrast, fifth‐order hillslopes were more complex in geomorphology, with more boundaries and a relatively heterogeneous vegetation pattern. Stream order classification of a savanna drainage network resulted in landscape units distinguishable by geomorphology, geomorphic boundaries and vegetation pattern. Therefore, the drainage network is a useful template to expose and organize the complexity in savanna landscapes into easily managed and researched units. This perspective should inform a shift from single‐scale phytosociological views of homogeneous vegetation units towards multi‐scale conceptualizations of savannas as water dependent ecosystems.     

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 concentration ratios and co-limitation in South African grasslands

*Assessing plant nutrient limitation is a fundamental part of understanding grassland dynamics. The ratio of concentrations of nitrogen (N) and phosphorus (P) in vegetation has been proposed as an index of the relative limitation of biomass production by N and P, but its utility has not been tested well in grasslands. *At five sites in Kruger National Park, South Africa, across soil and precipitation contrasts, N and P were added in a factorial design to grass-dominated plots. *Although the N:P ratio of unfertilized vegetation across all sites (5.8) would have indicated that production was N-limited, aboveground production was consistently co-limited by N and P. Aboveground production was still greater in plots fertilized with N and P than in those fertilized with just N, but the N:P ratio did not exceed standard thresholds for P limitation in N-fertilized vegetation. Comparisons among sites showed little pattern between site N:P ratio and relative responses to N and P. *When combined with results from other grassland fertilization studies, these data suggest that the N:P ratio of grasses has little ability to predict limitation in upland grasslands. Co-limitation between N and P appears to be much more widespread than would be predicted from simple assumptions of vegetative N:P ratios.     

The fate of nutrients during fires in a tropical savanna

Abstract The nutrient loads contained in the grassy fuel before fires, and of ash subsequently, were compared to determine the fluxes of macronutrients, copper and zinc during fires at Kapalga in Kakadu National Park. The fluxes were estimated in three vegetation types: forest, woodland and open woodland. The magnitudes of the fluxes were greatest in the forest community where grassy fuel loads were highest at about 6.3 t ha^?1. In these sites, 54–94% of all measured nutrients in the fuel were transferred to the atmosphere during the fires. For each nutrient, the proportion transferred to the atmosphere as entrained ash was calculated by assuming that calcium was not volatilized during the fires. If the transfer of entrained ash represents local redistribution only, then rainfall accession and the deposition of these particu-lates should replace most of the losses of all nutrients except nitrogen (N). Estimated rates of biological fixation of N appear to be insufficient to replace the annual losses of N. It is therefore concluded that a regime of annual fires that completely burn the available grassy fuel would deplete N reserves in these savannas, unless there are other sources of biologically fixed N, which are unknown at present.     

The representation of rainfall and fire intensity in fossil pollen and charcoal records from a South African savanna

Savanna ecosystems of southern Africa are strongly influenced by water availability and fire intensity, and this study aimed to show whether these two specific environmental variables are reflected in fossil pollen and charcoal records. Palaeoecological records of charcoal concentration from three short sedimentary sequences were used to reconstruct fire intensity (the rate of energy released along a fire front) over 50 yrs in the Kruger National Park (KNP), South Africa. Fossil pollen percentages from surface and core-sediment samples taken from water bodies were compared with the reconstructed fire intensity over space and time. Higher fire intensity led to increased herbaceous cover and decreased woody plant growth. Fossil pollen percentages and charcoal concentrations were also compared with rainfall records. Increased macroscopic charcoal abundances and percentages of Cyperaceae pollen corresponded to periods of increased rainfall. The results of this study have shown that fossil pollen and charcoal records from savanna environments can be used to reconstruct past fire intensity and its impact on terrestrial vegetation, as well as changes in rainfall.     

Nutrient dynamics and plant assemblages of Macrotermes falciger mounds in a savanna ecosystem

Termites through mound construction and foraging activities contribute significantly to carbon and nutrient fluxes in nutrient-poor savannas. Despite this recognition, studies on the influence of termite mounds on carbon and nitrogen dynamics in sub-tropical savannas are limited. In this regard, we examined soil nutrient concentrations, organic carbon and nitrogen mineralization in incubation experiments in mounds of Macrotermes falciger and surrounding soils of sub-tropical savanna, northeast Zimbabwe. We also addressed whether termite mounds altered the plant community and if effects were similar across functional groups i.e. grasses, forbs or woody plants. Mound soils had significantly higher silt and clay content, pH and concentrations of calcium (Ca), magnesium (Mg), potassium (K), organic carbon (C), ammonium (NH₄⁺) and nitrate (NO₃⁻) than surrounding soils, with marginal differences in phosphorus (P) and sodium (Na) between mounds and matrix soils. Nutrient enrichment increased by a factor ranging from 1.5 for C, 4.9 for Mg up to 10.3 for Ca. Although C mineralization, nitrification and nitrification fraction were similar between mounds and matrix soils, nitrogen mineralization was elevated on mounds relative to surrounding matrix soils. As a result, termite mounds supported unique plant communities rich and abundant in woody species but less diverse in grasses and forbs than the surrounding savanna matrix in response to mound-induced shifts in soil parameters specifically increased clay content, drainage and water availability, nutrient status and base cation (mainly Ca, Mg and Na) concentration. In conclusion, by altering soil properties such as texture, moisture content and nutrient status, termite mounds can alter the structure and composition of sub-tropical savanna plant communities, and these results are consistent with findings in other savanna systems suggesting that increase in soil clay content, nutrient status and associated changes in the plant community assemblage may be a general property of mound building termites.     

Concentrations and resorption patterns of 13 nutrients in different plant functional types in the karst region of south-western China

Background and Aims

Elucidating the stoichiometry and resorption patterns of multiple nutrients is an essential requirement for a holistic understanding of plant nutrition and biogeochemical cycling. However, most studies have focused on nitrogen (N) and phosphorus (P), and largely ignored other nutrients. The current study aimed to determine relationships between resorption patterns and leaf nutrient status for 13 nutrient elements in a karst vegetation region.

Methods

Plant and soil samples were collected from four vegetation types in the karst region of south-western China and divided into eight plant functional types. Samples of newly expanded and recently senesced leaves were analysed to determine concentrations of boron (B), calcium (Ca), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), molybdenum (Mo), N, sodium (Na), P, sulphur (S) and zinc (Zn).

Key Results

Nutrient concentrations of the karst plants were lower than those normally found in other regions of China and the rest of the world, and plant growth was mainly limited by P. Overall, four nutrients revealed resorption [N (resorption efficiency 34·6 %), P (48·4 %), K (63·2 %) and Mg (13·2 %)], seven nutrients [B (–16·1 %), Ca (–44·0 %), Cu (–14·5 %), Fe (–205·5 %), Mn (–72·5 %), Mo (–35·6 %) and Zn (–184·3 %)] showed accumulation in senesced leaves and two nutrients (Na and S) showed no resorption or accumulation. Resorption efficiencies of K and Mg and accumulation of B, Ca, Fe and Mn differed among plant functional types, and this strongly affected litter quality. Resorption efficiencies of N, P and K and accumulation of Ca and Zn increased with decreasing concentrations of these nutrients in green leaves. The N:P, N:K and N:Mg ratios in green leaves predicted resorption proficiency for N, K and Mg, respectively.

Conclusions

The results emphasize the fact that nutrient resorption patterns strongly depend on element and plant functional type, which provides new insights into plant nutrient use strategies and nutrient cycling in karst ecosystems.     

Stoichiometric and nutrient resorption characteristics of dominant tree species in subtropical Chinese forests

This study investigated seasonal patterns in stoichiometric ratios, nutrient resorption characteristics, and nutrient use strategies of dominant tree species at three successional stages in subtropical China, which have not been fully understood. Fresh leaf and leaf litterfall samples were collected in growing and nongrowing seasons for determining the concentrations of carbon (C), nitrogen (N), and phosphorus (P). Then, stoichiometric ratios (i.e., C:N, C:P, N:P, and C:N:P) and resorption parameters were calculated. Our results found that there was no consistent variation in leaf C:N and C:P ratios among different species. However, leaf N:P ratios in late‐successional species became significantly higher, indicating that P limitation increases during successional development. Due to the P limitation in this study area, P resorption efficiency and proficiency were higher than corresponding N resorption parameters. Dominant tree species at early‐successional stage adopted “conservative consumption” nutrient use strategy, whereas the species at late‐successional stage inclined to adopt “resource spending” strategy.     

Low leaf N and P resorption contributes to nutrient limitation in two desert shrubs

Both water and nutrients are limiting in arid environments, and desert plants have adapted to these limitations through numerous developmental and physiological mechanisms. In the Mono Basin, California, USA, co-dominant Sarcobatus vermiculatus and Chrysothamnus nauseosus ssp. consimilis are differentially N and P limited. We hypothesized that low leaf N resorption contributes to N-limitation in Sarcobatus and that low leaf P resorption contributes to P-limitation in Chrysothamnus. As predicted, Sarcobatus resorbed proportionally 1.7-fold less N than Chrysothamnus, but reduced leaf P in senescent leaves to lower levels than Chrysothamnus (8.0–10.8-fold lower based on leaf area or mass, respectively), consistent with N, but not P limitations in Sarcobatus. Again, as predicted, Chrysothamnus resorbed proportionally 2.0-fold less P than Sarcobatus yet reduced leaf N in senescent leaves to lower levels than Sarcobatus (1.8–1.3-fold lower based on leaf area or mass, respectively), consistent with P, but not N limitations in Chrysothamnus. Leaf N and P pools were approximately 50% of aboveground pools in both species during the growing season, suggesting leaf resorption can contribute significantly to whole plant nutrient retention. This was consistent with changes in leaf N vs. P concentration as plants grew from seedlings to adults. Our results support the conclusion that N-limitation in Sarcobatus and P-limitation in Chrysothamnus are in part caused by physiological (or other) constraints that prevent more efficient resorption of N or P, respectively. For these species, differential nutrient resorption may be a key physiological component contributing to their coexistence in this saline, low resource habitat.     

Is there a temporal niche separation in the leaf phenology of savanna trees and grasses?

Aim It has been proposed that, in tropical savannas, trees deploy their leaves earlier in the growing season and grasses deploy their leaves later. This hypothesis implies a mechanism that facilitates the coexistence of trees and grasses in savannas. If true, this hypothesis would also allow algorithms to use differences in the phenological timing of grass and tree leaves to partition the relative contribution of grasses and trees to net primary production. In this study we examine whether a temporal niche separation between grasses and trees exists in savanna. Location A semi‐arid, subtropical savanna, Kruger National Park, South Africa. Methods We use a multi‐spectral camera to track through an entire growing season the normalized difference vegetation index (NDVI) of individual canopies of grasses and trees at eight sites arranged along a precipitation and temperature gradient. Results Among trees, we identified two distinct phenological syndromes: an early flushing syndrome and a late‐flushing syndrome. Leaf flush in the tree strategies appears to pre‐empt rainfall, whereas grass leaf flush follows the rain. The growing season of trees is 20 (late‐flushing trees) to 27 (early flushing trees) days longer than that of the grasses. Main conclusions We show that grasses and trees have different leaf deployment strategies. Trees deployed leaves at lower temperatures than grasses and retained them for longer at the end of the growing season. The timing of the increase in NDVI is, however, similar between grasses and late‐flushing trees and this complicates the separation of grass and tree signals from multi‐spectral satellite imagery.     

Nutrient stoichiometry of aquatic hyphomycetes: Interstrain variation and ergosterol conversion factors

Ecological stoichiometry is a powerful concept. Rarely, however, has it been applied to fungi, despite their pivotal role in ecosystems. In view of the paucity of stoichiometric data, we grew 16 fungal isolates from streams in liquid culture (C:N:P = 160:16:1) and analysed them for nitrogen (N), phosphorus (P) and ergosterol as a fungal biomass marker. Interspecific differences explained up to 60% of the variation in N, P and ergosterol concentrations, and variation between strains of the same species accounted for up to another 16%. We found an average C:N:P of 136:10:1 in mycelia, while N:ergosterol and P:ergosterol ratios were 9.5 and 2.5, respectively. These ratios are an important step towards establishing reliable conversion factors to estimate the contribution of fungi to litter nutrient contents in complex field samples. Estimates could be further improved by applying the species-specific conversion factors we obtained. Additional analyses of fungal strains in conditions reflecting field situations are needed to strengthen the basis of such estimates of fungal nutrient pools in ecosystems; however, inherent variation within species limits the accuracy and precision that can be achieved.     

Changes in nitrogen resorption traits of six temperate grassland species along a multi-level N addition gradient

Nitrogen (N) resorption from senescing leaves is an important mechanism of N conservation for terrestrial plant species, but changes in N-resorption traits over wide-range and multi-level N addition gradients have not been well characterized. Here, a 3-year N addition experiment was conducted to determine the effects of N addition on N resorption of six temperate grassland species belonging to three different life-forms: Stipa krylovii Roshev. (grass), Cleistogenes squarrosa (T.) Keng (grass), Artemisia frigida Willd. (semishrub), Melissitus ruthenica C.W.Wang (semishrub and N-fixer), Potentilla acaulis L. (forb) and Allium bidentatum Fisch.ex Prokh. (forb). Generally, N concentrations in green leaves increased asymptotically for all species. N concentrations in senescent leaves for most species (5/6) also increased asymptotically, except that the N concentration in senescent leaves of A. bidentatum was independent of N addition. N-resorption efficiency decreased with increasing N addition level only for S. krylovii and A. frigida, while no clear responses were found for other species. These results suggest that long-term N fertilization increased N uptake and decreased N-resorption proficiency, but the effects on N-resorption efficiency were species-specific for different temperate grassland species in northern China. These inter-specific differences in N resorption may influence the positive feedback between species dominance and N availability and thus soil N cycling in the grassland ecosystem in this region.     

N limitation increases along a temperate forest succession: evidences from leaf stoichiometry and nutrient resorption

温带森林演替加剧了氮限制：来自叶片化学计量和养分重吸收的证据 森林生产力和碳汇功能在很大程度上取决于土壤氮和磷的有效性。然而,迄今为止,养分限制随森林演替的时间变化仍存在争议。叶片化学计量和养分重吸收是预测植物生长养分限制的重要指标。基于此,本研究测定了温带森林4个演替阶段所有木本植物叶片和凋落叶中氮和磷的含量,并分析了演替过程中非生物因子和生物因子如何影响叶片化学计量和养分重吸收。研究结果表明,在个体尺度上,叶片氮磷含量在演替末期显著增加,而叶片氮磷比无显著变化;氮的重吸收效率随演替显著增加,然而磷的重吸收效率先增加后减少;氮重吸收效率与磷重吸收效率的比值仅在演替末期显著增加。此外,植物氮素循环对土壤养分的响应比磷素循环更弱。在群落尺度上,叶片氮磷含量随森林演替呈现先降低后升高的趋势,主要受香农-维纳多样性指数和物种丰富度的影响;叶片氮磷比随演替而显著变化,主要由胸径的群落加权平均值决定;氮的重吸收效率增加,主要受物种丰富度和胸径的影响,而磷的重吸收效率相对稳定。因此,氮重吸收效率与磷重吸收效率的比值显著增加,表明随着温带森林演替,氮限制加剧。这些结果可能反映了较高生物多样性群落中物种间对有限资源的激烈竞争,强调了生物因子在驱动森林生态系统养分循环中的重要性,为中国温带和北方森林可持续经营的施肥管理提供了参考。