Above- and Belowground Net Primary Production in a Temperate Mixed Deciduous Forest

Our current ability to detect and predict changes in forest ecosystem productivity is constrained by several limitations. These include a poor understanding of belowground productivity, the short duration of most analyses, and a need for greater examination of species- or community-specific variability in productivity studies. We quantified aboveground net primary productivity (ANPP) over 3 years (1999–2001), and both belowground NPP (BNPP) and total NPP over 2 years (2000–2001) in both mesic and xeric site community types of the mixed mesophytic forest of southeastern Kentucky to examine landscape variability in productivity and its relation with soil resource [water and nitrogen (N)] availability. Across sites, ANPP was significantly correlated with N availability (R² = 0.58, P = 0.028) while BNPP was best predicted by soil moisture content (R² = 0.72, P = 0.008). Because of these offsetting patterns, total NPP was unrelated to either soil resource. Interannual variability in growing season precipitation during the study resulted in a 50% decline in mesic site litter production, possibly due to a lag effect following a moderate drought year in 1999. As a result, ANPP in mesic sites declined 27% in 2000 compared to 1999, while xeric sites had no aboveground production differences related to precipitation variability. If global climate change produces more frequent occurrences of drought, then the response of mesic sites to prolonged moisture deficiency and the consequences of shifting carbon (C) allocation on C storage will become important questions.     

Coupled Nitrogen and Calcium Cycles in Forests of the Oregon Coast Range

Nitrogen (N) is a critical limiting nutrient that regulates plant productivity and the cycling of other essential elements in forests. We measured foliar and soil nutrients in 22 young Douglas-fir stands in the Oregon Coast Range to examine patterns of nutrient availability across a gradient of N-poor to N-rich soils. N in surface mineral soil ranged from 0.15 to 1.05% N, and was positively related to a doubling of foliar N across sites. Foliar N in half of the sites exceeded 1.4% N, which is considered above the threshold of N-limitation in coastal Oregon Douglas-fir. Available nitrate increased five-fold across this gradient, whereas exchangeable magnesium (Mg) and calcium (Ca) in soils declined, suggesting that nitrate leaching influences base cation availability more than soil parent material across our sites. Natural abundance strontium isotopes (⁸⁷Sr/⁸⁶Sr) of a single site indicated that 97% of available base cations can originate from atmospheric inputs of marine aerosols, with negligible contributions from weathering. Low annual inputs of Ca relative to Douglas-fir growth requirements may explain why foliar Ca concentrations are highly sensitive to variations in soil Ca across our sites. Natural abundance calcium isotopes (δ⁴⁴Ca) in exchangeable and acid leachable pools of surface soil measured at a single site showed 1 per mil depletion relative to deep soil, suggesting strong Ca recycling to meet tree demands. Overall, the biogeochemical response of these Douglas-fir forests to gradients in soil N is similar to changes associated with chronic N deposition in more polluted temperate regions, and raises the possibility that Ca may be deficient on excessively N-rich sites. We conclude that wide gradients in soil N can drive non-linear changes in base-cation biogeochemistry, particularly as forests cross a threshold from N-limitation to N-saturation. The most acute changes may occur in forests where base cations are derived principally from atmospheric inputs.     

Nitrogen availability patterns in white-sand vegetations of Central Brazilian Amazon

Addressing spatial variability in nitrogen (N) availability in the Central Brazilian Amazon, we hypothesized that N availability varies among white-sand vegetation types (campina and campinarana) and lowland tropical forests (dense terra-firme forests) in the Central Brazilian Amazon, under the same climate conditions. Accordingly, we measured soil and foliar N concentration and N isotope ratios (δ¹⁵N) throughout the campina-campinarana transect and compared to published dense terra-firme forest results. There were no differences between white-sand vegetation types in regard to soil N concentration, C:N ratio and δ¹⁵N across the transect. Both white-sand vegetation types showed very low foliar N concentrations and elevated foliar C:N ratios, and no significant difference between site types was observed. Foliar δ¹⁵N was depleted, varying from −9.6 to 1.6‰ in the white-sand vegetations. The legume Aldina heterophylla had the highest average δ¹⁵N values (−1.5‰) as well as the highest foliar N concentration (2.1%) while the non-legume species had more depleted δ¹⁵N values and the average foliar N concentrations varied from 0.9 to 1.5% among them. Despite the high variation in foliar δ¹⁵N among plants, a significant and gradual ¹⁵N-enrichment in foliar isotopic signatures throughout the campina–campinarana transect was observed. Individual plants growing in the campinarana were significantly enriched in ¹⁵N compared to those in campina. In the white-sand N-limited ecosystems, the differentiation of N use seems to be a major cause of variations observed in foliar δ¹⁵N values throughout the campina–campinarana transect.     

N and P in New Zealand Soil Chronosequences and Relationships with Foliar N and P

The growth of forest species in soil development chronosequences becomes increasingly phosphorus (P)-limited with time, as P is weathered, eroded and leached from soil. Foliar nitrogen (N) concentrations also tend to decrease with soil age when vegetation may be limited in both N and P. Here we report on soil development in temperate rain forests along three New Zealand chronosequences that have minimal pollution and disturbance from human activities, at Franz Josef, Waitutu and Central Volcanic Plateau, and on factors influencing soil net N mineralization (aerobic; 56 days) and foliar N and P concentrations. Except in very young soils (<500 years), at least 85% of total-P in mineral soil (0–10 cm) was transformed to organic-P. In each chronosequence, total-P declined with time, and foliar N:P ratios (mass) generally increased from 8 to 15–18, suggesting P was more limiting than N in the oldest soils of the chronosequence. There was a negative relationship between net N mineralization and C:N ratio for mineral soil. For the FH (organic) layer, net N mineralization had the strongest relationships with total-N concentration (positively) and C:organic-P ratio (negatively); however, relationships varied with forest group, suggesting that other factors were also important. Foliar P of kamahi (Weinmannia racemosa Linn. f.), a dominant canopy species, was related to soil organic-P, suggesting mineralization was an important process for tree nutrition.Foliar N was positively related to N concentration in the FH layer, but was not significantly related to any measured property in mineral soil, possibly because of the wide range of soils. The consistent declines in both soil and foliar P across the contrasting chronosequences strongly suggest that vegetation becomes progressively P-limited during long-term ecosystem development.     

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ¹⁵N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ¹⁵N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ¹⁵N varied widely, between 0 and –2‰ for nitrogen-fixing species, between 0 and –7‰ for deciduous non-fixing species, and between 0 and –11‰ for coniferous species. Relatively constant δ¹⁵N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ¹⁵N differences among species at individual sites. In fact, the largest variation among plant δ¹⁵N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ¹⁵N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ¹⁵N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T _r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ¹⁵N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ¹⁵N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T _r. Received: 14 December 1998 / Accepted: 16 August 1999     

Influence of Precipitation on Soil and Foliar Nutrients Across Nine Costa Rican Forests

We explored patterns of soil and foliar nutrients across nine mature forest sites in Costa Rica, where mean annual precipitation (MAP) ranged between 3500 and 5500 mm, altitude ranged between 200 and 1200 m, and species composition varied among sites. Our objective was to investigate the relationship between rainfall and plant or soil nutrient characteristics to better understand the potential long‐term effects that alterations in MAP could have on the nutrient dynamics of wet forest plant communities. Indicators of soil N availability (net mineralization and nitrification) decreased with MAP but were not related to foliar N. Soil and foliar P, by contrast, were not correlated with MAP but were positively correlated with each other. Thus, across our gradient foliar P was a better predictor of soil nutrient availability than foliar N. There were wide differences in foliar nutrient concentrations and N:P ratios among species within sites. At each site, legumes had higher mean percent N than nonlegumes, resulting in higher N:P ratios for legumes. Taken together, these data suggest that, at least in these forests, a climate‐driven decrease in MAP could cause an increase in net N mineralization and nitrification for the wetter sites. However, this may not affect productivity at the community level because of low P availability, complex feedbacks between soil and foliar nutrients, and interactions with other biological and environmental factors such as elevation.     

Relationships among net primary productivity, nutrients and climate in tropical rain forest: a pan-tropical analysis

Tropical rain forests play a dominant role in global biosphere-atmosphere CO(2) exchange. Although climate and nutrient availability regulate net primary production (NPP) and decomposition in all terrestrial ecosystems, the nature and extent of such controls in tropical forests remain poorly resolved. We conducted a meta-analysis of carbon-nutrient-climate relationships in 113 sites across the tropical forest biome. Our analyses showed that mean annual temperature was the strongest predictor of aboveground NPP (ANPP) across all tropical forests, but this relationship was driven by distinct temperature differences between upland and lowland forests. Within lowland forests (相似文献   

Resource manipulation effects on net primary production,biomass allocation and rain-use efficiency of two semiarid grassland sites in Inner Mongolia,China

Productivity of semiarid grasslands is affected by soil water and nutrient availability, with water controlling net primary production under dry conditions and soil nutrients constraining biomass production under wet conditions. In order to investigate limitations on plants by the response of root–shoot biomass allocation to water and nitrogen (N) availability, a field experiment, on restoration plots with rainfed, unfertilized control plots, fertilized plots receiving N (25 kg urea-N ha⁻¹) and water (irrigation simulating a wet season), was conducted at two sites with different grazing histories: moderate (MG) and heavy (HG) grazing. Irrigation and N addition had no effect on belowground biomass. Irrigation increased aboveground (ANPP) and belowground net primary production (BNPP) and rain-use efficiency based on ANPP (RUE_ANPP), whereas N addition on rainfed plots had no effect on any of the measured parameters. N fertilizer application on irrigated plots increased ANPP and RUE_ANPP and reduced the root fraction (RF: root dry matter/total dry matter), resulting in smaller N effects on total net primary production (NPP) and rain-use efficiency based on NPP. This suggests that BNPP should be included in evaluating ecosystem responses to resource availability from the whole-plant perspective. N effects on all measured parameters were similar on both sites. However, site HG responded to irrigation with higher ANPP and a lower RF when compared to site MG, indicating that species composition had a pronounced effect on carbon allocation pattern due to below- and aboveground niche complementarity.     

Controls of Bedrock Geochemistry on Soil and Plant Nutrients in Southeastern Utah

The cold deserts of the Colorado Plateau contain numerous geologically and geochemically distinct sedimentary bedrock types. In the area near Canyonlands National Park in Southeastern Utah, geochemical variation in geologic substrates is related to the depositional environment with higher concentrations of Fe, Al, P, K, and Mg in sediments deposited in alluvial or marine environments and lower concentrations in bedrock derived from eolian sand dunes. Availability of soil nutrients to vegetation is also controlled by the formation of secondary minerals, particularly for P and Ca availability, which, in some geologic settings, appears closely related to variation of CaCO₃ and Ca-phosphates in soils. However, the results of this study also indicate that P content is related to bedrock and soil Fe and Al content suggesting that the deposition history of the bedrock and the presence of P-bearing Fe and Al minerals, is important to contemporary P cycling in this region. The relation between bedrock type and exchangeable Mg and K is less clear-cut, despite large variation in bedrock concentrations of these elements. We examined soil nutrient concentrations and foliar nutrient concentration of grasses, shrubs, conifers, and forbs in four geochemically distinct field sites. All four of the functional plant groups had similar proportional responses to variation in soil nutrient availability despite large absolute differences in foliar nutrient concentrations and stoichiometry across species. Foliar P concentration (normalized to N) in particular showed relatively small variation across different geochemical settings despite large variation in soil P availability in these study sites. The limited foliar variation in bedrock-derived nutrients suggests that the dominant plant species in this dryland setting have a remarkably strong capacity to maintain foliar chemistry ratios despite large underlying differences in soil nutrient availability.     

Sapling growth as a function of light and landscape-level variation in soil water and foliar nitrogen in northern Michigan

Interspecific differences in sapling growth responses to soil resources could influence species distributions across soil resource gradients. I calibrated models of radial growth as a function of light intensity and landscape-level variation in soil water and foliar N for saplings of four canopy tree species, which differ in adult distributions across soil resource gradients. Model formulations, characterizing different resource effects and modes of influencing growth, were compared based on relative empirical support using Akaike’s Information Criterion. Contrary to expectation, the radial growth of species associated with lower fertility (Acer rubrum and Quercus rubra) was more sensitive to variation in soil resources than the high fertility species Acer saccharum. Moreover, there was no species tradeoff between growth under high foliar N versus growth under low foliar N, which would be expected if growth responses to foliar N mediated distributions. In general, there was functional consistency among species in growth responses to light, foliar N, and soil water availability, respectively. Foliar N influenced primarily high-light growth in F. grandifolia, A. rubrum, and Q. rubra (but was not significant for A. saccharum). In A. saccharum and A. rubrum, for which soil water availability was a significant predictor, soil water and light availability simultaneously limited growth (i.e., either higher light or water increased growth). Simple resource-based models explained 0.74–0.90 of growth variance, indicating a high degree of determinism. Results suggest that nitrogen effects on forest dynamics would be strongest in high-light early successional communities but that water availability influences growth in both early successional and understory environments.     

Foliage litter quality and annual net N mineralization: comparison across North American forest sites

The feedback between plant litterfall and nutrient cycling processes plays a major role in the regulation of nutrient availability and net primary production in terrestrial ecosystems. While several studies have examined site-specific feedbacks between litter chemistry and nitrogen (N) availability, little is known about the interaction between climate, litter chemistry, and N availability across different ecosystems. We assembled data from several studies spanning a wide range of vegetation, soils, and climatic regimes to examine the relationship between aboveground litter chemistry and annual net N mineralization. Net N mineralization declined strongly and non-linearly as the litter lignin:N ratio increased in forest ecosystems (r ² = 0.74, P < 0.01). Net N mineralization decreased linearly as litter lignin concentration increased, but the relationship was significant (r ² = 0.63, P < 0.01) only for tree species. Litterfall quantity, N concentration, and N content correlated poorly with net N mineralization across this range of sites (r ² < 0.03, P = 0.17–0.26). The relationship between the litter lignin:N ratio and net N mineralization from forest floor and mineral soil was similar. The litter lignin:N ratio explained more of the variation in net N mineralization than climatic factors over a wide range of forest age classes, suggesting that litter quality (lignin:N ratio) may exert more than a proximal control over net N mineralization by influencing soil organic matter quality throughout the soil profile independent of climate. Received: 16 December 1996 / Accepted: 8 February 1997     

Forest Productivity and Efficiency of Resource Use Across a Chronosequence of Tropical Montane Soils

We tested the hypothesis that plants adjust to nutrient availability by altering carbon allocation patterns and nutrient-use efficiency (NUE = net primary production [NPP] per unit nutrient uptake), but are constrained by a trade-off between NUE and light-use efficiency () = NPP per unit intercepted light). NPP, NUE and ) were measured in montane Metrosideros polymorpha forest across a 4.1 x 10⁶ yr space for time substitution chronosequence in which available soil N and P pools change with site age. Although the range of N and P availability across sites was broad, there was little difference in NPP between sites, and in contrast to theories of carbon allocation relative to limiting resources, we found no consistent relationships in production allocation to leaves, fine roots or wood. However, canopy nutrient pools and fluxes were correlated with the mass of fine roots per unit soil volume and there was a weak but positive correlation of NPP with LAI. Patterns of ) and NUE across the soil developmental sequence were opposite to each other. ) increased as nutrient availability and nutrient turnover increased, while NUE decreased in response to the same influences but reached its highest values where either N or P availability and turnover of both N and P were low. A negative correlation between ) and NUE supports the hypothesis that a trade-off exists between ) and leaf characteristics affecting NUE.     

Heterogeneity of Soils and Vegetation in an Eastern Amazonian Rain Forest: Implications for Scaling Up Biomass and Production

Transferring fine-scale ecological knowledge into an understanding of earth system processes presents a considerable challenge to ecologists. Our objective here was to identify and quantify heterogeneity of, and relationships among, vegetation and soil properties in terra firme rain forest ecosystems in eastern Amazonia and assess implications for generating regional predictions of carbon (C) exchange. Some of these properties showed considerable variation among sites; soil textures varied from 11% to 92% clay. But we did not find any significant correlations between soil characteristics (percentage clay, nitrogen [N], C, organic matter) and vegetation characteristics (leaf area index [LAI], foliar N concentration, basal area, biomass, stem density). We found some evidence for increased drought stress on the sandier sites: There was a significant correlation between soil texture and wood δ¹³C (but not with foliar δ¹³C); volumetric soil moisture was lower at sandier sites; and some canopy foliage had large, negative dawn water potentials (ψ_ld), indicating limited water availability in the rooting zone. However, at every site at least one foliage sample indicated full or nearly full rehydration, suggesting significant interspecific variability in drought vulnerability. There were significant differences in foliar δ¹⁵N among sites, but not in foliar % N, suggesting differences in N cycling but not in plant access to N. We used an ecophysiological model to examine the sensitivity of gross primary production (GPP) to observed inter- and intrasite variation in key driving variables—LAI, foliar N, and ψ_ld. The greatest sensitivity was to foliar N; standard errors on foliar N data translated into uncertainty in GPP predictions up to ±10% on sunny days and ±5% on cloudy days. Local variability in LAI had a minor influence on uncertainty, especially on sunny days. The largest observed reductions in ψ_ld reduced GPP by 4%–6%. If uncertainty in foliar N estimates is propagated into the model, then GPP estimates are not significantly different among sites. Our results suggest that water restrictions in the sandier sites are not enough to reduce production significantly and that texture is not the key control on plant access to N. Received 28 June 2001; accepted 13 March 2002.     

Soil Phosphorus Fractionation and Phosphorus-Use Efficiency of a Bornean Tropical Montane Rain Forest During Soil Aging With Podozolization

We compared phosphorus (P) dynamics and plant productivity in two montane tropical rain forests (Mount Kinabalu, Borneo) that derived from similar parent materials (largely sedimentary rocks) and had similar climates but differed in terms of soil age. The younger site originated from Quaternary colluvial deposits, whereas the older site had Tertiary-age material. The older site had a distinctive spodic horizon, reduced levels of labile inorganic soil P, higher concentrations of recalcitrant organic soil P, and lower rates of net soil N mineralization. P fertilization led to soil nitrogen (N) immobilization in the P-deficient soil, indicating that soil N mineralization was limited by P at the P-deficient older site. Mean foliar nutrient concentration (on both a weight and an area basis) was similar at the two sites for all elements except P, which was lower at the older site. Aboveground net primary production (ANPP) was lower at the older site than at the younger one; this difference could be explained by the reduced availability of P and N (as down-regulated by P) at the older site. The relatively ample allocation of P and N to leaves, despite the reduced availability at the P-deficient old site, was attributable to its high resorption efficiency. High resorption resulted in lower concentrations of elements in leaf litter—that is, less decomposable low-quality litter. On the other hand, the concentration of leaf litter lignin was considerably lower at the older site; this appeared to be a de facto adaptive mechanism to avoid retarding litter decomposition.     

Primary production and nitrogen allocation of field grown sugar maples in relation to nitrogen availability

Above ground net primary production (NPP), nitrogen (N) allocation, and retranslocation from senescing leaves were measured in 7 sugar-maple dominated sites having annual net N mineralization rates ranging from 26 to 94 kg · ha^–1 · yr^–1. The following responses were observed: (1) Green sun leaves on richer sites had higher N mass per unit leaf area than sun leaves on poorer sites; (2) Total canopy N varied much less than annual net mineralization, ranging from 81 to 111 kg · ha^–1; (3) This was due to the existence of a large and relatively constant pool of N which was retranslocated from senescing leaves for use the following year (54 to 80 kg · ha^–1); (4) The percentage of canopy N retranslocated by sugar maple was also relatively constant, but was slightly higher on the richer sites. Percent N in leaf litter did not change across the gradient; (5) Above ground NPP increased linearly in relation to N allocated above ground. Therefore, N use efficiency, expressed as above ground NPP divided by N allocated above ground was constant; (6) N use efficiency expressed as (NPP above ground/total N availability) was a curvilinear function of N availability; and (7) This pattern reflected a decreasing apparent allocation of N below ground with decreasing N availability.     

Nutrient dynamics on a precipitation gradient in Hawai'i

We evaluated soil and foliar nutrients in five native forests in Hawai'i with annual rainfall ranging from 500 mm to 5500 mm. All of the sites were at the same elevation and of the same substrate age; all were native-dominated forests containing Metrosiderospolymorpha Gaud. Soil concentrations of extractable NO₃-N and PO₄-P, as well as major cations (Ca, Mg, and K), decreased with increasing annual precipitation, and δ¹⁵N values became more depleted in both soils and vegetation. For M.polymorpha leaves, leaf mass per area (LMA) and lignin concentrations increased significantly, while δ¹³C values became more depleted with increasing precipitation. Foliar phosphorus, and major cation (Ca, Mg, and K) concentrations for M.polymorpha all decreased significantly with increasing precipitation. For other native forest species, patterns of LMA, δ¹³C, and δ¹⁵N generally mirrored the pattern observed for M. polymorpha. Decreasing concentrations of available rock-derived nutrients in soil suggest that the effect of increased rainfall on leaching outweighs the effect of increasing precipitation on weathering. The pattern of decreased foliar nutrient concentrations per unit leaf area and of increased lignin indicates a shift from relatively high nutrient availability to relatively high carbon gain by producers as annual precipitation increases. For nitrogen cycling, the pattern of higher inorganic soil nitrogen concentrations in the drier sites, together with the progressively depleted δ¹⁵N signature in both soils and vegetation, suggests that nitrogen cycling is more open at the drier sites, with smaller losses relative to turnover as annual precipitation increases. Received: 24 March 1997 / Accepted: 19 September 1997     

Nitrogen Controls on Fine Root Substrate Quality in Temperate Forest Ecosystems

Nitrogen controls on fine root substrate quality (that is, nitrogen and carbon-fraction concentrations) were assessed using nitrogen availability gradients in the Harvard Forest chronic nitrogen addition plots, University of Wisconsin Arboretum, Blackhawk Island, Wisconsin, and New England spruce-fir transect. The 27 study sites encompassed within these four areas collectively represented a wide range of nitrogen availability (both quantity and form), soil types, species composition, aboveground net primary production, and climatic regimes. Changes in fine root substrate quality among sites were most frequently and strongly correlated with nitrate availability. For the combined data set, fine root nitrogen concentration increased (adjusted R ² = 0.46, P < 0.0001) with increasing site nitrate availability. Fine root “extractive” carbon-fraction concentrations decreased (adjusted R ² = 0.32, P < 0.0002), “acid-soluble” compounds increased (adjusted R ² = 0.35, P < 0.0001), and the “acid-insoluble” carbon fraction remained relatively high and stable (combined mean of 48.7 ± 3.1% for all sites) with increasing nitrate availability. Consequently, the ratio of acid-insoluble C–total N decreased (adjusted R ² = 0.40, P < 0.0001) along gradients of increasing nitrate availability. The coefficients of determination for significant linear regressions between site nitrate availability and fine root nitrogen and carbon-fraction concentrations were generally higher for sites within each of the four study areas. Within individual study sites, tissue substrate quality varied between roots in different soil horizons and between roots of different size classes. However, the temporal variation of fine root substrate quality indices within specific horizons was relatively low. The results of this study indicate that fine root substrate quality increases with increasing nitrogen availability and thus supports the substrate quality component of a hypothesized conceptual model of nitrogen controls on fine root dynamics that maintains that fine root production, mortality, substrate quality, and decomposition increase with nitrogen availability in forest ecosystems in a manner that is analogous to foliage.     

Are patterns in nutrient limitation belowground consistent with those aboveground: results from a 4 million year chronosequence

Accurately predicting the effects of global change on net carbon (C) exchange between terrestrial ecosystems and the atmosphere requires a more complete understanding of how nutrient availability regulates both plant growth and heterotrophic soil respiration. Models of soil development suggest that the nature of nutrient limitation changes over the course of ecosystem development, transitioning from nitrogen (N) limitation in ‘young’ sites to phosphorus (P) limitation in ‘old’ sites. However, previous research has focused primarily on plant responses to added nutrients, and the applicability of nutrient limitation-soil development models to belowground processes has not been thoroughly investigated. Here, we assessed the effects of nutrients on soil C cycling in three different forests that occupy a 4 million year substrate age chronosequence where tree growth is N limited at the youngest site, co-limited by N and P at the intermediate-aged site, and P limited at the oldest site. Our goal was to use short-term laboratory soil C manipulations (using ¹⁴C-labeled substrates) and longer-term intact soil core incubations to compare belowground responses to fertilization with aboveground patterns. When nutrients were applied with labile C (sucrose), patterns of microbial nutrient limitation were similar to plant patterns: microbial activity was limited more by N than by P in the young site, and P was more limiting than N in the old site. However, in the absence of C additions, increased respiration of native soil organic matter only occurred with simultaneous additions of N and P. Taken together, these data suggest that altered nutrient inputs into ecosystems could have dissimilar effects on C cycling above- and belowground, that nutrients may differentially affect of the fate of different soil C pools, and that future changes to the net C balance of terrestrial ecosystems will be partially regulated by soil nutrient status.     

Soil moisture condition and soil nitrogen dynamics in a pure Alnus japonica forest in Korea

This study was conducted to examine the influences of soil-moisture conditions on soil nitrogen (N) dynamics, including in situ soil N mineralization, N availability, and denitrification in a pure Alnus japonica forest located in Seoul, central Korea. The soil N mineralization, N availability, and denitrification were determined using the buried bag incubation method, ion exchange resin bag method, and acetylene block method, respectively. The annual net N mineralization rate (kg N ha⁻¹ year⁻¹) and annual N availability (mg N bag⁻¹) were 40.26 and 80.65 in the relatively dry site, −5.43 and 45.39 in the moist site, and 7.09 and 39.17 in the wet site, respectively. The annual net N mineralization rate and annual N availability in the dry site were significantly higher than those in the moist and wet sites, whereas there was no significant difference between the moist and wet sites. The annual mean denitrification rate (kg N ha⁻¹ year⁻¹) in the dry, moist, and wet sites was 2.37, 2.76, and 1.59, respectively. However, there was no significant difference among sites due to the high spatial and temporal variations. Our results indicate that soil-moisture condition influenced the in situ N mineralization and resin bag N availability in an A. japonica forest, and treatments of proper drainage for poorly drained sites would increase soil N mineralization and N availability and consequently be useful to conserve and manage the A. japonica forest.     

Response of Douglas-fir and amabilis fir to split-root nutrition with inorganic and organic nitrogen

There is limited understanding of the spatial plasticity of conifer root growth in response to inorganic and organic nitrogen (N). In this study, slow-growing amabilis fir and fast-growing Douglas-fir, and slow- and fast-growing seedlots of the latter species were examined for their ability to proliferate roots preferentially in compartments of sand/peat medium enriched in organic and inorganic forms of N. In one experiment, N was supplied as 7.1 or 0.71 mM ammonium, nitrate and ammonium nitrate, and in a second experiment, N was supplied as ammonium or glycine. The seedlings’ ability to compensate for the starvation of a portion of the root system was assessed by measuring biomass of leaves, stems and roots, and foliar N concentration. Both fast- and slow-growing seedlots of Douglas-fir and slow-growing amabilis fir were able to proliferate roots in compartments of soil enriched with inorganic and organic N. In the first experiment, whole plant and root biomass was greatest when N was provided as ammonium followed by nitrate, and in the second experiment, seedling whole and root biomasses did not differ between ammonium and glycine treatments. All seedlings were able to compensate for the starvation of a portion of the root system, thus total plant biomass did not differ between split-root treatments; however, foliar N contents were lower in the 7.1/0.71 mM inorganic N split-root treatments. Foliar N concentrations were also lower in seedlings supplied with glycine.