Soil Nitrogen Availability Affects Belowground Carbon Allocation and Soil Respiration in Northern Hardwood Forests of New Hampshire

Nitrogen deposition raises temperature sensitivity of soil organic matter decomposition in subtropical forest

Subtropical ecosystems are strongly affected by nitrogen (N) deposition, impacting soil organic matter (SOM) availability and stocks. Here we aimed to reveal the effects of N deposition on i) the structure and functioning of microbial communities and ii) the temperature sensitivity (Q10) of SOM decomposition. Phosphorus (P) limited evergreen forest in Guangdong Province, southeastern China, was selected, and N deposition (factor level: N (100 kg N ha-1 y-1 (NH4NO3)) and control (water), arranged into randomized complete block design (n = 3)) was performed during 2.5 y. After that soils from 0 to 20 cm were collected, analyzed for the set of parameters and incubated at 15, and 25, and 35 °C for 112 days. N deposition increased the microbial biomass N and the content of fungal and Gram-positive bacterial biomarkers; activities of beta-glucosidase (BG) and acid phosphatase (ACP) also increased showing the intensification of SOM decomposition. The Q10 of SOM decomposition under N deposition was 1.66 and increased by 1.4 times than under control. Xylosidase (BX), BG, and ACP activities increased with temperature under N but decreased with the incubation duration, indicating either low production and/or decomposition of enzymes. Activities of polyphenol-(PPO) and peroxidases (POD) were higher under N than in the control soil and were constant during the incubation showing the intensification of recalcitrant SOM decomposition. At the early incubation stage (10 days), the increase of Q10 of CO2 efflux was explained by the activities of BX, BQ, ACP, and POD and the quality of the available dissolved organic matter pool. At the later incubation stages (112 days), the drop of Q10 of CO2 efflux was due to the depletion of the labile organic substances and the shift of microbial community structure to K-strategists. Thus, N deposition decoupled the effects of extracellular enzyme activities from microbial community structure on Q10 of SOM decomposition in the subtropical forest soil.

Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China

Background

Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO₃ ^- loss potential following secondary succession are not well understood in karst ecosystems.

Methods

In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and ¹⁵N natural abundance (δ ¹⁵N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ ¹⁵N compositions of soil organic nitrogen (SON) in aggregates and soil to plant ¹⁵N enrichment factor (EF = δ ¹⁵N_leaf -δ ¹⁵N_soil), combined with soil inorganic N contents and δ ¹⁵N compositions were used to indicate the alterations of soil N availability and NO₃ ^-loss potential following secondary succession.

Results

Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The δ ¹⁵N values of SON also significantly decreased, mainly affected by plant δ ¹⁵N composition and N mineralization. SON content in macro-aggregates and soil NH₄ ⁺ content significantly increased while δ ¹⁵N values of NH₄ ⁺ decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ ¹⁵N values, the EF values, and the (NO₃ ^--N)/(NH₄ ⁺-N) ratio gradually decreased, indicating reduced NO₃ ^- loss following secondary succession.

Conclusions

Soil N availability improves and NO₃ ^- leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.

Nitrogen Addition Decreases Rhizodepositionby Chinese Fir (Cunninghamia lanceolata (Lamb.) Hook) Seedlings and Its Distribution in Soil Aggregates

Rhizodeposition-derived carbon plays an important role in plant nutrient acquisition and soil carbon sequestration. However, how nitrogen deposition affects the distribution of rhizodeposition-derived carbon into aggregate classes (macrogagregates, microaggregates, and silt and clay) is unclear. We conducted a nitrogen addition experiment on Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) seedlings with continuously labeled 13CO2 for 120 days. Plant growth and the distribution of rhizodeposition-derived carbon into aggregate classes were assessed. Results showed that nitrogen additionconsiderably increased the ratio of aboveground to belowground biomass, but not aboveground and belowground biomass. Compared with the control, nitrogen addition resulted in a significantdecreaseby 52%inrhizodeposition-derived carbon in bulk soil.We found that more rhizodeposition-derived carbon was incorporated into macroaggregate, followed by microaggregate, and silt and clay regardless of nitrogen addition. The rhizodeposition-derived carbon was significantly decreased by 40% in macroaggregate, 60%in microaggregate, and 61%in silt and clayafter nitrogenaddition. Nitrogen addition and aggregate classes had no interactive effect on the rhizodeposition-derived carbon. Our results suggest that nitrogen deposition decreases the rhizodeposition of Chinese fir and its distributionin aggregate classes.

Stable nitrogen and carbon isotope compositions in plant-soil systems under different land-use types in a red soil region, Southeast China

Background

Stable N isotope compositions in plant-soil systems have been widely used to indicate soil N transformation and translocation processes in ecosystems. However, soil N processes and nitrate ( NO 3 - ) loss potential under different land-use types are short of systematic comparison in the red soil region of Southeast China.

Methods

In the present study, the stable N and C isotope compositions (δ ¹⁵N and δ ¹³C) of soil and leaf were analyzed to indicate soil N transformation processes, and the soil to plant ¹⁵N enrichment factor (EF) was used to compare soil NO 3 - loss potential under different land-use types, including an abandoned agricultural land, a natural pure forest without understory, and a natural pure forest with a simple understory.

Results

The foliar δ ¹⁵N value (-0.8‰) in the abandoned agricultural land was greater than those of the forest lands (ranged from -2.2‰ to -10.8‰). In the abandoned agricultural land, δ ¹⁵N values of soil organic nitrogen (SON) increased from 0.8‰ to 5.7‰ and δ ¹³C values of soil organic carbon (SOC) decreased from -22.7‰ to -25.9‰ with increasing soil depth from 0-70 cm, mainly resulting from SON mineralization, soil organic matter (SOM) decomposition, and C₄ plant input. In the soils below 70 cm depth, δ ¹⁵N values of SON (mean 4.9‰) were likely affected by microbial assimilation of ¹⁵N-depleted NO 3 - . The variations in δ ¹⁵N values of soil profiles under the two forests were similar, but the EF values were significant different between the pure forest with a simple understory (-10.0‰) and the forest without understory (-5.5‰).

Conclusions

These results suggest that soil to plant ¹⁵N enrichment factor have a great promise to compare soil NO 3 - loss potential among different ecosystems.

N Addition Overwhelmed the Effects of P Addition on the Soil C, N, and P Cycling Genes in Alpine Meadow of the Qinghai-Tibetan Plateau

Although human activities have greatly increased nitrogen (N) and phosphorus (P) inputs to the alpine grassland ecosystems, how soil microbial functional genes involved in nutrient cycling respond to N and P input remains unknown. Based on a fertilization experiment established in an alpine meadow of the Qinghai-Tibetan Plateau, we investigated the response of the abundance of soil carbon (C), N, and P cycling genes to N and P addition and evaluated soil and plant factors related to the observed effects. Our results indicated that the abundance of C, N, and P cycling genes were hardly affected by N addition, while P addition significantly increased most of them, suggesting that the availability of P plays a more important role for soil microorganisms than N in this alpine meadow ecosystem. Meanwhile, when N and P were added together, the abundance of C, N, and P cycling genes did not change significantly, indicating that the promoting effects of P addition on microbial functional genes abundances were overwhelmed by N addition. The Mantel analysis and the variation partitioning analysis revealed the major role of shoot P concentration in regulating the abundance of C, N, and P cycling genes. These results suggest that soil P availability and plant traits are key in governing C, N, and P cycling genes at the functional gene level in the alpine grassland ecosystem.

Foliar nutrient concentrations of six northern hardwood species responded to nitrogen and phosphorus fertilization but did not predict tree growth

Foliar chemistry can be useful for diagnosing soil nutrient availability and plant nutrient limitation. In northern hardwood forests, foliar responses to nitrogen (N) addition have been more often studied than phosphorus (P) addition, and the interactive effects of N and P addition have rarely been described. In the White Mountains of central New Hampshire, plots in ten forest stands of three age classes across three sites were treated annually beginning in 2011 with 30 kg N ha^-1 y^-1 or 10 kg P ha^-1 y^-1 or both or neither-a full factorial design. Green leaves of American beech (Fagus grandifolia Ehrh.), pin cherry (Prunus pensylvanica L.f.), red maple (Acer rubrum L.), sugar maple (A. saccharum Marsh.), white birch (Betula papyrifera Marsh.), and yellow birch (B. alleghaniensis Britton) were sampled pre-treatment and 4-6 years post-treatment in two young stands (last cut between 1988-1990), four mid-aged stands (last cut between 1971-1985) and four mature stands (last cut between 1883-1910). In a factorial analysis of species, stand age class, and nutrient addition, foliar N was 12% higher with N addition (p < 0.001) and foliar P was 45% higher with P addition (p < 0.001). Notably, P addition reduced foliar N concentration by 3% (p = 0.05), and N addition reduced foliar P concentration by 7% (p = 0.002). When both nutrients were added together, foliar P was lower than predicted by the main effects of N and P additions (p = 0.08 for N × P interaction), presumably because addition of N allowed greater use of P for growth. Foliar nutrients did not differ consistently with stand age class (p ≥ 0.11), but tree species differed (p ≤ 0.01), with the pioneer species pin cherry having the highest foliar nutrient concentrations and the greatest responses to nutrient addition. Foliar calcium (Ca) and magnesium (Mg) concentrations, on average, were 10% (p < 0.001) and 5% lower (p = 0.01), respectively, with N addition, but were not affected by P addition (p = 0.35 for Ca and p = 0.93 for Mg). Additions of N and P did not affect foliar potassium (K) concentrations (p = 0.58 for N addition and p = 0.88 for P addition). Pre-treatment foliar N:P ratios were high enough to suggest P limitation, but trees receiving N (p = 0.01), not P (p = 0.64), had higher radial growth rates from 2011 to 2015. The growth response of trees to N or P addition was not explained by pre-treatment foliar N, P, N:P, Ca, Mg, or K.

Nonlinear responses of ecosystem carbon fluxes to nitrogen deposition in an old-growth boreal forest

Nitrogen (N) deposition is known to increase carbon (C) sequestration in N-limited boreal forests. However, the long-term effects of N deposition on ecosystem carbon fluxes have been rarely investigated in old-growth boreal forests. Here we show that decade-long experimental N additions significantly stimulated net primary production (NPP) but the effect decreased with increasing N loads. The effect on soil heterotrophic respiration (Rh) shifted from a stimulation at low-level N additions to an inhibition at higher levels of N additions. Consequently, low-level N additions resulted in a neutral effect on net ecosystem productivity (NEP), due to a comparable stimulating effect on NPP and Rh, while NEP was increased by high-level N additions. Moreover, we found nonlinear temporal responses of NPP, Rh and NEP to low-level N additions. Our findings imply that actual N deposition in boreal forests likely exerts a minor contribution to their soil C storage.

Quantifying uncertainty in annual runoff due to missing data

Long-term streamflow datasets inevitably include gaps, which must be filled to allow estimates of runoff and ultimately catchment water budgets. Uncertainty introduced by filling gaps in discharge records is rarely, if ever, reported. We characterized the uncertainty due to streamflow gaps in a reference watershed at the Hubbard Brook Experimental Forest (HBEF) from 1996 to 2009 by simulating artificial gaps of varying duration and flow rate, with the objective of quantifying their contribution to uncertainty in annual streamflow. Gaps were filled using an ensemble of regressions relating discharge from nearby streams, and the predicted flow was compared to the actual flow. Differences between the predicted and actual runoff increased with both gap length and flow rate, averaging 2.8% of the runoff during the gap. At the HBEF, the sum of gaps averaged 22 days per year, with the lowest and highest annual uncertainties due to gaps ranging from 1.5 mm (95% confidence interval surrounding mean runoff) to 21.1 mm. As a percentage of annual runoff, uncertainty due to gap filling ranged from 0.2–2.1%, depending on the year. Uncertainty in annual runoff due to gaps was small at the HBEF, where infilling models are based on multiple similar catchments in close proximity to the catchment of interest. The method demonstrated here can be used to quantify uncertainty due to gaps in any long-term streamflow data set, regardless of the gap-filling model applied.

Variation in plant belowground resource allocation across heterogeneous landscapes: implications for post-fire resprouting

PREMISE

Resource availability affects biomass allocation in ways that could influence plant responses to disturbance such as fire. This is important because fire also varies across landscapes in ways that are correlated to resource availability. We hypothesized that plants growing in landscape microsites with a shortage of nutrients and water allocate more biomass and resources to belowground structures (and thus promote traits that enhance post-fire resprouting ability) than plants in more mesic sites.

METHODS

We selected sites in three contrasting topographies (3 gullies, 3 midslopes, and 3 ridges) that supported different vegetation types and fire regimes, in Jalisco, Mexico. At each site, we measured soil nutrient and water content and light availability. Then we sampled biomass and root starch allocation in three post-fire resprouting shrubs that grow across a wide range of microenvironmental conditions.

RESULTS

The ridges showed the highest values of solar radiation and the lowest of soil N and water content. Overall, we found a significant tendency for higher root-to-shoot (R/S) ratios, greater fine root biomass, and higher root starch content, in individuals growing in ridges or midslopes compared to the values of the plants living in gullies.

CONCLUSIONS

Plants located in open canopy sites, characterized by a shortage of nutrients and water, tend to allocate more biomass belowground than plants in wet and fertile sites. Thus, plants in wet and fertile forests should be more vulnerable to increased disturbance such as wildfires.

Nonlinear responses of total belowground carbon flux and its components to increased nitrogen availability in temperate forests

With the increased interest in allocating more carbon (C) belowground for C sequestration, total belowground C flux (TBCF) and its dynamics have become an important research topic. However, it remains uncertain whether TBCF responds nonlinearly to increased nitrogen (N) availability and how its main components (root and ectomycorrhizal (ECM) fungi) contribute to TBCF. We established a four-year N addition experiment with control, low-N, medium-N, and high-N fertilization treatments in a N-limited temperate forest in northern China. We measured TBCF and its three main components including root respiration, ECM fungal respiration, and root production. The involved edaphic and plant factors were also measured. TBCF showed a nonlinear response to the increasing amounts of N addition, accelerated by 10.37% in low-N addition and restrained by 10.29% in high-N addition. Contrasting patterns of the contributions of root respiration and ECM fungal respiration to TBCF implies different strategies of investment in roots and ECM fungi under the different N-availability statuses. The ratio of production and respiration in roots under N addition was nearly 1:2, which indicated that when soil N availability increases, roots prefer to lose C by overflow respiration rather than fix C in new biomass. The low-N addition increased TBCF by directly increasing root respiration and indirectly increasing coarse root biomass. The medium-N addition positively affected TBCF by increasing root respiration but this positive effect was cancelled out by the significantly negative effect of the increased soil total N concentration. The decrease in soil pH was the most effective pathway to decrease TBCF in high-N addition. Because of a large-scale reforestation program for C sink management in recent years, our findings of the nonlinear response of TBCF to different N fertilization treatments could provide insight for predicting belowground C sequestration potential and its response to atmospheric N deposition.

Microbe-mediated attenuation of soil respiration in response to soil warming in a temperate oak forest

Soil respiration (Rs) in response to climate warming received a wide concern due to its important role in terrestrial ecosystem carbon (C) cycling, but the warming-induced effects of soil microbes on soil respiration are still less understood, especially over time. Our study aims to understand the long-term warming induced effects of soil microbes on Rs. A field soil warming experiment using a completely randomized design was conducted in a naturally regenerated oak forest (Quercus aliena) in central China. Soil warming was executed by infrared heater throughout the period from 2011 to 2015. Our results showed that soil temperature was a main factor in regulating Rs in a temperate oak forest throughout the experiment, while soil water content determined Rs only when a naturally dry year occurred. The positive effect of soil warming on Rs that was observed (i.e., 37.5 to 42.0% in the first two years) gradually diminished in the following three years (i.e., 0.9 to 15.4%). Significant positive warming effects on the temperature sensitivity of Rs (Q₁₀) only occurred in the second year. Continuous soil warming caused the decline in nitrogen (N) availability, with a significant increase in microbial biomass-specific enzyme activities for N-acquisition. The attenuation of microbial biomass increment and the decreased ratio of enzymatic C:N acquisition contributed to the diminished warming effect on Rs over time. Our study suggests that microbe-mediated attenuation of Rs, accompanied by the concomitant decline in soil N availability in response to warming, should be taken into consideration in global C cycle modeling.

Litter Management as a Key Factor Relieves Soil Respiration Decay in an Urban-Adjacent Camphor Forest under a Short-Term Nitrogen Increment

Increases in bioavailable nitrogen (N) level can impact the soil carbon (C) sequestration in many forest ecosystems through its influences on litter decomposition and soil respiration (Rs). This study aims to detect whether the litter management can affect the influence of N addition on Rs. We conducted a one-year field experiment in a camphor forest of central-south China to investigate the responses of available N status and soil Rs to N addition and litter manipulation. Four N addition plots (NH4NO3; 0, 5, 15, 30 g N m−2 year−1 as N0, N1, N2, N3, respectively) were established with three nested litter treatments: natural litter input (CK), double litter input (LA), and non-litter input (LR). We found a short-lived enhancement effect of N addition on soil (NO3-N) and net nitrification (RN), but not on (NH4-N), net ammonification (RA), or mineralization (RM). N addition also decreased Rs in CK spots, but not in LA or LR spots, in which the negative effects of N additions on Rs were alleviated by either litter addition or reduction. A priming effect was also observed in LA treatments. A structural equation modeling analysis showed that litter treatments had direct positive effects on soil available N contents and Rs, which suggested that litter decomposition may benefit from litter management when N is not a limiting factor in subtropical forests.

Inorganic Nitrogen Addition Affects Soil Respiration and Belowground Organic Carbon Fraction for a Pinus tabuliformis Forest

The capability of forest ecosystems to sequester carbon from the atmosphere largely depends on the interaction of soil organic matter and nitrogen, and thus, this process will be greatly influenced by nitrogen deposition under the future scenario of global change. To clarify this interaction, the current study explored the variations in soil carbon fraction and soil respiration with different levels of nitrogen deposition. NH4NO3 was added at concentrations of 0, 50, 100, 200, and 400 kg N ha−1 year−1 separately on twenty 100 m2 plots in a Pinus tabuliformis Carr forest in northern China. Soil samples were analyzed for their nutrient content and biophysical properties two years after nitrogen application, and the soil respiration rate was measured every month during the study period. Seasonal variation and nitrogen addition significantly affected soil respiration rate. On average, nitrogen addition significantly reduced the annual soil respiration rate by 23.74%. Fine root biomass significantly decreased by an average of 43.55% in nitrogen treatment plots compared to the control plot. However, the average proportions of autumn and winter soil respiration rates out of the annual cumulative soil respiration rate greatly increased from 23.57% and 11.04% to 25.90% and 12.18%, respectively. The soil microbial biomass carbon content in the control plot was 342.39 mg kg−1, 23.50% higher than the average value in nitrogen treatment plots. The soil dissolved organic carbon was reduced by 22.60%, on average, following nitrogen addition. Significant correlations were detected between fine root biomass and the annual cumulative soil respiration rate, soil microbial biomass carbon content, and soil dissolved organic carbon content. This demonstrates that nitrogen addition affects soil organic carbon transformation and carbon emission, mainly by depressing fine root production.

Interactions among plants, bacteria, and fungi reduce extracellular enzyme activities under long-term N fertilization

Atmospheric nitrogen (N) deposition has enhanced soil carbon (C) stocks in temperate forests. Most research has posited that these soil C gains are driven primarily by shifts in fungal community composition with elevated N leading to declines in lignin degrading Basidiomycetes. Recent research, however, suggests that plants and soil microbes are dynamically intertwined, whereby plants send C subsidies to rhizosphere microbes to enhance enzyme production and the mobilization of N. Thus, under elevated N, trees may reduce belowground C allocation leading to cascading impacts on the ability of microbes to degrade soil organic matter through a shift in microbial species and/or a change in plant-microbe interactions. The objective of this study was to determine the extent to which couplings among plant, fungal, and bacterial responses to N fertilization alter the activity of enzymes that are the primary agents of soil decomposition. We measured fungal and bacterial community composition, root-microbial interactions, and extracellular enzyme activity in the rhizosphere, bulk, and organic horizon of soils sampled from a long-term (>25 years), whole-watershed, N fertilization experiment at the Fernow Experimental Forest in West Virginia, USA. We observed significant declines in plant C investment to fine root biomass (24.7%), root morphology, and arbuscular mycorrhizal (AM) colonization (55.9%). Moreover, we found that declines in extracellular enzyme activity were significantly correlated with a shift in bacterial community composition, but not fungal community composition. This bacterial community shift was also correlated with reduced AM fungal colonization indicating that declines in plant investment belowground drive the response of bacterial community structure and function to N fertilization. Collectively, we find that enzyme activity responses to N fertilization are not solely driven by fungi, but instead reflect a whole ecosystem response, whereby declines in the strength of belowground C investment to gain N cascade through the soil environment.

Nitrogen and phosphorus enrichment effects on CO2 and methane fluxes from an upland ecosystem

Reactive nitrogen (N) deposition can affect many ecosystem processes, particularly in oligotrophic habitats, and is expected to affect soil C storage potential through increases in microbial decomposition rate as a consequence of greater N availability. Increased N availability may also result in changes in the principal limitations on ecosystem productivity. Phosphorus (P) limitation may constrain productivity in instances of high N deposition, yet ecosystem responses to P availability are poorly understood. This study investigated CO₂ and CH₄ flux responses to N and P enrichment using both short- (1year) and long-term (16year) nutrient addition experiments. We hypothesised that the addition of either N or P will increase CO₂ and CH₄ fluxes, since both plant production and microbial activity are likely to increase with alleviation from nutrient limitation. This study demonstrated the modification of C fluxes from N and P enrichment, with differing results subject to the duration of nutrient addition. On average, relative to control, the addition of N alone inhibited CO₂ flux in the short-term (-9%) but considerably increased CO₂ emissions in the long-term (+35%), reduced CH₄ uptake in the short term (-90%) and reduced CH₄ emission in the long term (-94%). Phosphorus addition increased CO₂ and CH₄ emission in the short term (+20% and +184% respectively), with diminishing effect into the long term, suggesting microbial communities at these sites are P limited. Whilst a full C exchange budget was not examined in the experiment, the potential for soil C storage loss with long-term nutrient enrichment is demonstrated and indicates that P addition, where P is a limiting factor, may have an adverse influence on upland soil C content.

Six-Year Nitrogen-Water Interaction Shifts the Frequency Distribution and Size Inequality of the First-Order Roots of Fraxinus mandschurica in a Mixed Mature Pinus koraiensis Forest

The variation in fine root traits in terms of size inequality at the individual root level can be identified as a strategy for adapting to the drastic changes in soil water and nutrient availabilities. The Gini and Lorenz asymmetry coefficients have been applied to describe the overall degree of size inequality, which, however, are neglected when conventional statistical means are calculated. Here, we used the Gini coefficient, Lorenz asymmetry coefficient and statistical mean in an investigation of Fraxinus mandschurica roots in a mixed mature Pinus koraiensis forest on Changbai Mountain, China. We analyzed 967 individual roots to determine the responses of length, diameter and area of the first-order roots and of branching intensity to 6 years of nitrogen addition (N), rainfall reduction (W) and their combination (NW). We found that first-order roots had a significantly greater average length and area but had smaller Gini coefficients in NW plots compared to in control plots (CK). Furthermore, the relationship between first-order root length and branching intensity was negative in CK, N, and W plots but positive in NW plots. The Lorenz asymmetry coefficient was >1 for the first-order root diameter in NW and W plots as well as for branching intensity in N plots. The bimodal frequency distribution of the first-order root length in NW plots differed clearly from the unimodal one in CK, N, and W plots. These results demonstrate that not only the mean but also the variation and the distribution mode of the first-order roots of F. mandschurica respond to soil nitrogen and water availability. The changes in size inequality of the first-order root traits suggest that Gini and Lorenz asymmetry coefficients can serve as informative parameters in ecological investigations of roots to improve our ability to predict how trees will respond to a changing climate at the individual root level.

Fast-growing Acer rubrum differs from slow-growing Quercus alba in leaf, xylem and hydraulic trait coordination responses to simulated acid rain

We investigated the effects of historic soil chemistry changes associated with acid rain, i.e., reduced soil pH and a shift from nitrogen (N)- to phosphorus (P)-limitation, on the coordination of leaf water demand and xylem hydraulic supply traits in two co-occurring temperate tree species differing in growth rate. Using a full-factorial design (N × P × pH), we measured leaf nutrient content, water relations, leaf-level and canopy-level gas exchange, total biomass and allocation, as well as stem xylem anatomy and hydraulic function for greenhouse-grown saplings of fast-growing Acer rubrum (L.) and slow-growing Quercus alba (L.). We used principle component analysis to characterize trait coordination. We found that N-limitation, but not P-limitation, had a significant impact on plant water relations and hydraulic coordination of both species. Fast-growing A. rubrum made hydraulic adjustments in response to N-limitation, but trait coordination was variable within treatments and did not fully compensate for changing allocation across N-availability. For slow-growing Q. alba, N-limitation engendered more strict coordination of leaf and xylem traits, resulting in similar leaf water content and hydraulic function across all treatments. Finally, low pH reduced the propensity of both species to adjust leaf water relations and xylem anatomical traits in response to nutrient manipulations. Our data suggest that a shift from N- to P-limitation has had a negative impact on the water relations and hydraulic function of A. rubrum to a greater extent than for Q. alba We suggest that current expansion of A. rubrum populations could be tempered by acidic N-deposition, which may restrict it to more mesic microsites. The disruption of hydraulic acclimation and coordination at low pH is emphasized as an interesting area of future study.