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231 results for “Nitrogen addition”
Nitrogen Fixation Responses in Sphagnum Mosses to N-Additions to an Alberta Poor Fen, 2012-2015
Development of the oil sands has led to increasing atmospheric N deposition, with values as high as 17 kg N ha-1 yr-1; regional background levels <2 kg N ha-1 yr-1. To examine responses to N deposition, over five years, we experimentally applied N (as NH4NO3) to a poor fen near Mariana Lake, Alberta, at rates of 0, 5, 10, 15, 20, and 25 kg N ha-1 yr-1, plus controls (no water or N addition). Between June and August of each year, we measured N2-fixation using the acetylene reduction technique between 2012 and 2015Averaged across all measurement dates, whether described by a linear or segmented regression, N2-fixation rates decreased with increasing N addition. N2-fixation rates were not different between the water addition only treatments and the control treatments (p = 0.44). While increasing N deposition may not substantively change total inputs of new N to bogs, the form of new N inputs shifts to inorganic N in deposition, rather than organic N produced by the microorganisms that are fixing N2.
Pore water concentrations of Nitrogen from N-Addition plots in an Alberta Poor Fen, 2011-2015
Development of the oil sands has led to increasing atmospheric N deposition, with values as high as 17 kg N ha-1 yr-1; regional background levels <2 kg N ha-1 yr-1. To examine responses to N deposition, over five years, we experimentally applied N (as NH4NO3) to a poor fen near Mariana Lake, Alberta, at rates of 0, 5, 10, 15, 20, and 25 kg N ha-1 yr-1, plus controls (no water or N addition). We collected surface pore water from all plots several times a year throughout the 5 year experiment. Over the 5 years of the study, porewater NH4+-N, NO3--N, and DON concentrations at the top of the poor fen water table were unaffected by N addition (p = 0.06, 0.30, 0.16, respectively). However, porewater NH4+-N, NO3--N, and DON concentrations were substantially higher in 2011 than in 2012-2015 (Fig. 11). Water addition alone had no significant effect on porewater concentrations of NH4+-N, NO3--N, or DON (p > 0.99). We hypothesized that as N deposition increases to a level that exceeds the capacity of the fen vegetation to take up N, net N mineralization in surface peat would be inhibited by higher NH4+-N availability, net nitrification would be stimulated by higher NH4+-N availability, and concentrations of DIN in porewater at the top of the water table would increase, as DIN bypasses interception by the ground layer vegetation. None of these hypotheses was supported with nitrogen being immediately taken up by vegetation. It is unclear if longer term study would reveal similar responses.
Plant biomass: The Effect of Nitrogen Addition and Irrigation in High Diveristy Prairies
Water and nitrogen are two potentially major limiting resources in prairie grasslands of the region, and thus might be added to restored high-diversity grasslands used for biofuel production. In this full-factorial experiment, 36 high-diversity 9 m x 9 m plots that were planted with 32 species in 1994 were randomly assigned to one of six treatments. Treatments were all combination of a water treatment (ambient rainfall or ambient rainfall plus ~ 2 cm/week of irrigation) and of a nitrogen treatment (annual nitrogen addition of 0, 7, or 14 grams of N - as ammonium nitrate - per square meter). Aboveground biomass is harvested each fall in each plot, dried, and weighed. These treatments are determining the importance of both water and N limitation in high-diversity restored prairie grassland, their interactive effects, and the year-to-year variation in yields and treatment effects. They are also determining the potential sustainability of such yields and the effects of the various treatments on plant diversity.
Data from: Addition of nitrogen to canopy versus understory has different effects on leaf traits of understory plants in a subtropical evergreen broad–leaved forest
<p>Atmospheric nitrogen (N) deposition has substantial effects on forest ecosystems. The effects of N deposition on understory plants have been simulated by spraying N on the forest floor. Such understory addition of N (UAN) might simulate atmospheric N deposition in a biased manner, because it bypasses the canopy.</p> <p>We compared the effects of UAN and canopy addition of N (CAN) at 0, 25, and 50 kg N ha<sup>–1</sup> year<sup>–1</sup> on specific leaf area (SLA), leaf construction costs (CC), concentrations of leaf carbon ([C]), nitrogen ([N]), phosphorus ([P]), minerals ([Mineral]), nitrate ([NO<sub>3</sub><sup>-</sup>]), lignin ([Lignin]), lipids ([Lipid]), organic acids ([OA]), soluble phenolics ([SP]), total non-structural carbohydrates ([TNC]), and total structural carbohydrates ([TSC]) in six dominant understory species in a subtropical evergreen forest after five years of N treatments.</p> <p>We found that leaf CC, [C], [Lignin], [OA], [TNC] and [TSC] were significantly affected by N-addition approach and rate, but leaf [P] and [Lipid] were affected by N-addition approach and N-addition rate, respectively; leaf CC, [C], [P], [OA], and [TNC] were significantly lower under UAN than under CAN, but leaf [TSC] and [Lignin] were significantly higher and lower, respectively, under UAN than under CAN at 50 kg N ha<sup>–1</sup> year<sup>–1</sup>; the decline of leaf [C] and [Lignin] contributed to the significantly lower leaf CC under UAN than under CAN.</p> <p><em>Synthesis</em>. We show that canopy and understory N addition exerted significantly different effects on leaf traits of understory plants. The results indicate that understory plants in subtropical forest respond differently to understory addition of N from those to atmospheric deposition of N. Further studies are warranted to evaluate the unbiased ecological processes and functions of forest ecosystem responding to atmospheric N deposition via both canopy and understory N addition experiments over a longer term.</p>
Long-term nitrogen addition alters the community and energy channel but not diversity of soil nematodes in a subtropical forest
Summary <ol> <li>Research has indicated that increases in nitrogen (N) deposition can greatly affect ecosystem processes and functions. There is limited information about the effects of long-term N addition on soil nematodes and their functional composition, although nematodes are the most abundant multicellular animals on Earth.</li> <li>We conducted a field experiment in 2004 with four levels of N addition (0, 60, 120, and 240 kg N ha<sup>-1 </sup>yr<sup>-1</sup>) in a subtropical <i>Cunninghamia lanceolata</i> forest. Soil samples with three depths (0-20, 20-40 and 40-60 cm) were collected and the community structure, diversity and trophic groups of soil nematodes were determined in 2014.</li> <li>N addition significantly increased the abundance of bacterial- and fungal-feeding nematodes, but decreased the abundance of plant-feeding nematodes at the 0-20 cm soil layer. Accordingly, the plant parasite index and enrichment index decreased but the basal index and channel index increased, which weaken the importance of the plant-based energy channel, but enhance the importance of the fungal-based energy channel. N addition had no effects on the diversity of soil nematodes in three soil depths. Structural equation modeling analysis indicated that N loading directly changed plant-feeding (total <i>r<sup>2</sup></i>=0.42) nematodes, or indirectly affected bacterial- (<i>r<sup>2</sup></i>=0.43), fungal- (<i>r<sup>2</sup></i>=0.31) and plant-feeding nematodes via change soil nutrients, soil water content and pH.</li> <li>These findings suggest that N addition can change the community structure and energy channels soil nematodes, which would affect soil processes and food web functions in forest soils under future environmental change scenarios.</li> </ol>
Impact of nitrogen and phosphorus addition on resident soil and root mycobiomes in beech forests
<p>The aim of the study was to investigate the influence of fertilizers on root-associated and soil residing fungi in beech forests. We report data from a fertilization experiment in three temperate beech forest with contrasting soil phosphorus concentrations. We used soil cores and the fractions of the organic layer and mineral topsoil separately of spring and fall 2018. We collected bulk soil and roots. We provide data on soil for pH, mineral elements, ammonium, nitrate, carbon, nitrogen. We provide data on root mineral nutrients, carbon and nitrogen. Illumina sequencing in soil and roots provides fungal amplicon sequence variant (ASV) and morphotyping of beech fine roots the active colonizing ectomycorrhizal fungi, including diversity parameters.</p>
Contrasting responses of fine root biomass and traits to large-scale nitrogen and phosphorus addition in tropical forests in the Guiana shield
<p>Fine roots mediate plant nutrient acquisition and growth. Depending on soil nutrient availability, plants can regulate fine root biomass and morphological traits to optimise nutrient acquisition. Little is known, however, about the importance of these parameters influencing forest functioning. In this study, we measured root responses to nutrient additions to gain a mechanistic understanding of plant adaptations to nutrient limitation in two tropical forests in French Guiana, differing two-fold in their soil nutrient statuses. We analysed the responses of root biomass, mean root diameter (RD), specific root length (SRL), specific root area (SRA), root tissue density (RTD) and carbon (C), nitrogen (N) and phosphorus (P) concentrations in roots down to 15 cm soil depth after three years of N and P additions. At the lower-fertility site Paracou, no changes in root biomass or morphological traits were detected with either N or P addition, although P concentrations in roots increased with P addition. In the higher fertility site, Nouragues, root biomass and P concentrations in roots increased with P addition, with no changes in morphological traits. In contrast, N addition shifted root traits from acquisitive to more conservative by increasing RTD. A significant interaction between N and P in Nouragues pointed to stronger responses to P addition in the absence of N. Our results suggest that the magnitude and direction of root biomass and trait expression were regulated by soil fertility, corroborated by the response to N or P additions. At low fertility sites, we found lower plasticity in root trait expression compared to more fertile conditions, where N and P additions caused stronger and antagonistic responses. Identifying the exact role of mechanisms affecting root nutrient uptake in Amazon forests growing in different soils will be crucial to foresee if and how rapid global changes can affect their carbon allocation.</p>
Vertical profiles of leaf photosynthesis and leaf traits, and soil nutrients in two tropical rainforests in French Guiana before and after a three-year nitrogen and phosphorus addition experiment
<p>We provide a comprehensive dataset of vertical profiles of photosynthetic capacity and important leaf traits, including leaf N and P concentrations, from two three-year, large-scale fertilisation experiments conducted in two tropical rainforests in French Guiana. These data present a unique source of information to further improve model representations of the roles of N, P, and other leaf nutrients, in photosynthesis in tropical forests. To further facilitate the use of our data in syntheses and model studies, we provide an elaborate list of ancillary data, including important soil properties and nutrients, along with the leaf data. As environmental drivers are key to improve our understanding of carbon (C)-nutrient cycle interactions, this comprehensive dataset will aid to further enhance our understanding of how nutrient availability interacts with C uptake in tropical forests.</p>
Intra- and interspecific variability of specific leaf area mitigate the reduction of community stability in response to warming and nitrogen addition
<p><span>Global environmental changes are reducing the diversity and affecting the functioning of natural ecosystems as well as their ability to reliably provide ecosystem functions and services to mankind. Many studies have shown that a greater plant diversity can stabilize community productivity against environmental fluctuations. However, most of these studies focused on plant species richness, thus overlooking the potential role of functional traits in stabilizing community productivity against environmental fluctuations.</span> <span>Whether and how functional trait mean and variability influence community stability in response to environmental changes and their relative contributions to community stability are largely unknown. Here, we used a 10-year experiment to investigate the role of species richness, as well as functional mean and intra- and interspecific variability of specific leaf area (SLA) of plants within- and among communities in driving community stability in response to nitrogen (N) addition and warming.</span> <span>We found that both N addition and warming reduced the temporal stability of community productivity by reducing species richness and its contribution to species asynchrony and species stability. In contrast, changes in the mean and variability of SLA in response to N addition and warming mitigated the reduction of community stability. Specifically, N addition reduced variation in SLA both by reducing interspecific differences in SLA within communities and differences in mean values of SLA among communities. Warming increased intraspecific differences in SLA among communities, leading to higher species stability that partly buffered the reduction of community stability.</span><span> Our study demonstrates the role of trait mean and variability in mitigating the reduction of community stability in response to two pervasive global environmental changes. Gaining a deeper understanding of the processes linking global changes and the stability of our ecosystems requires integrating both trait mean values and trait variability.</span></p>
Provenance rather than nitrogen addition dominate the phenotypes and plasticity of Ambrosia artemisiifolia in China
<p><span>Phenotypic plasticity, which differs between species, populations, individuals, even traits, is considered as one of the mechanisms of plant invasion. We conducted a common garden experiment with six provenances (HRB, CC, HG, ML, MZ, LB) of <em>Ambrosia artemisiifolia</em> and three nitrogen addition treatments to test their performance response. In order to explore the adaptation strategies of <em>A. artemisiifolia </em>towards heterogeneous habitats, phenotypic plasticity indexes were calculated to analyze among different provenances and traits. All traits were significantly influenced (<em>P</em><0.05) by provenance, except for number of inflorescences, inflorescences length and intercellular CO<sub>2</sub> concentration. Nitrogen addition notably affected on seed length, root biomass ratio, root-shoot ratio and four physiological traits (Pn, Tr, Gs, Ci). Among six provenances, the phenotypic plasticity index differed significantly (<em>P</em><0.05), but there were no significant differences of plasticity among nitrogen treatments. And biomass phenotypic plasticity was relatively higher both in response to prevenance and to nitrogen. We also found that only the plasticity of 100-seed weight was closely related to latitude, climate and genetic factors. In summary, provenances had a greater impact on the phenotype and plasticity of <em>A. artemisiifolia</em> than nitrogen addition, indicating the potential for further evolution of plasticity. Differences in plasticity among provenances and traits are important in predicting the invasion trend of <em>A. artemisiifolia</em> in the future.</span></p>
High-level nitrogen additions accelerate soil respiration reduction over time in a boreal forest
<p>Increased nitrogen (N) inputs are widely recognized to reduce soil respiration (Rs), but how N deposition affects the temporal dynamics of Rs remains unclear. Using a decade-long fertilization experiment in a boreal larch forest (Larix gmelini) in northeast China, we found that the effects of N additions on Rs showed a temporal shift from a positive effect in the short-term (increased by 8% on average in the first year) to a negative effect over the longer term (decreased by 21% on average in the eleventh year). The rates of decrease in Rs for the higher N-levels were almost twice as high as those of the low N-level. Our results suggest that the reduction in Rs in response to increased N input is accelerated by high-level N additions, and experimental high N applications are likely to overestimate the contribution of N deposition to soil carbon sequestration in boreal forest.</p>
Divergent trait responses to nitrogen addition in tall and short species
<p><span>Asymmetrical light competition and direct detrimental effect of nitrogen have been proposed as two main mechanisms driving species richness declines following nitrogen (N) addition. N addition is also known to alter functional trait composition towards increased dominance of tall and fast-growing species. However, whether trait changes vary between species and, in particular, how the traits of tall and short species respond to N addition has rarely been studied. Understanding whether different trait changes occur for tall and short species would provide insight into mechanisms underlying N addition effects.</span></p> <p><span>Based on a long-term N addition experiment, we measured the natural height of 44 plant species and the photosynthetic active radiation (PAR) at seven heights in the vegetation and identified a clear stratification of the plant community into tall species (>30cm tall), which experienced high light conditions, and short species that grew under the canopy. We also measured four functional traits, including maximum plant height, specific leaf area, leaf dry matter content and leaf thickness, on 35 species that occurred</span> <span>in fertilized and unfertilized plots. </span></p> <p><span>Structural equation modelling revealed that nitrogen (N) addition significantly reduced species richness by enhancing light asymmetry and had no direct effects, suggesting that N detrimental effects are negligible in our system. Consistent with this, we found different responses of traits and diversity for the tall and short species. Specifically, N addition reduced the number of short species but increased the number of tall species. In addition, specific leaf area increased, and leaf dry matter content decreased, for short species only, suggesting that they shifted to a fast growth strategy to cope with lower light levels. In contrast, tall species increased their height further to capture more light at the top of the canopy. </span><span> </span></p> <p><span><em>Synthesis</em>. </span><span>The divergent trait responses observed for tall and short species show that although certain traits, like height and leaf traits, show correlated responses at the whole community level, these correlations may not be consistent across all species. Together, our results highlight that light competition is the main mechanism driving species loss following N addition gradient.</span></p>
Nitrogen addition amplifies the nonlinear drought response of grassland productivity to extended growing-season droughts
<p>Understanding the response of grassland production and carbon exchange to intra-annual variation in precipitation and nitrogen addition is critical for sustainable grassland management and ecosystem restoration. We introduced growing-season drought treatments of different lengths (15, 30, 45 and 60 day drought) by delaying growing-season precipitation in a long-term nitrogen addition experiment in a low diversity meadow steppe in northeast China. Response variables included aboveground biomass (AGB), ecosystem net carbon exchange (NEE), and leaf net carbon assimilation rate (<i>A</i>). In unfertilized plots drought decreased AGB by 13.7% after a 45 day drought and 31.7% after a 60 day drought (47.6% in fertilized plots). Progressive increases in the drought response of NEE were also observed. The effects of N addition on the drought response of productivity increased as drought duration increased, and these responses were a function of changes in AGB and biomass allocation, particularly root to shoot ratio. However, no significant effects of drought occurred in fertilized or unfertilized plots in the growing season a year after the experiment, N addition did limit the recovery of AGB from severe drought during the remainder of the current growing season. Our results imply that chronic N enrichment could exacerbate the effects of growing-season drought on grassland productivity caused by altered precipitation seasonality under climate change, but that these effects do not carry over to the next growing season.</p>
Responses of soil temperature, moisture, and respiration to five-year warming and nitrogen addition in a semi-arid grassland
<p><span>How climate warming interacts with atmospheric nitrogen (N) deposition to affect carbon (C) release from soils remains largely elusive, posing a major challenge in projecting climate change‒terrestrial C feedback. As part of a five-year (2006–2010) field manipulative experiment, this study was designed to examine the effects of 24-hour continuous warming and N addition on soil respiration and explore the underlying mechanisms in a semi-arid grassland on the Mongolian Plateau, China. Across the five years and all plots, soil respiration was not changed under the continuous warming, but was decreased by 3.7% under the N addition. The suppression of soil respiration by N addition in the third year and later could be mainly due to the reductions in the forb-to-grass biomass ratios. Moreover, there were interactive effects between continuous warming and N addition on soil respiration. Continuous warming increased soil respiration by 5.8% in the ambient N plots, but reduced it by 6.3% in the enriched N plots. Soil respiration was unaffected by N addition in the ambient temperature plots yet decreased by 9.4% in the elevated temperature plots. Changes of soil moisture and the proportion of legume biomass in the community might be primarily responsible for the non-additive effects of continuous warming and N addition on soil respiration.</span> This study provides empirical evidence for the positive climate warming‒soil C feedback in the ambient N condition. However, N deposition reverses the positive warming‒soil C feedback into a negative feedback, leading to decreased C loss from soils under a warming climate. Incorporating our findings into C-cycling models could reduce the uncertainties of model projections for land C sink and global C cycling under multifactorial global change scenarios.</p>
Enhanced isohydric behavior decoupled the whole-tree sap flux response to leaf transpiration under nitrogen addition in a subtropical forest
<p><span>Anthropogenic nitrogen deposition has the potential to change the leaf water-use strategy in the subtropical region of China. Nevertheless, the whole-tree level response crucial for ecosystem functions has not been well addressed over the past decades. In this study, the stem sap flux density (J<sub>S</sub>) was monitored for the whole-tree water transport capacity in two dominant species (<em>Schima</em> <em>superba</em> and <em>Castanopsis</em> <em>chinensis</em>) in a subtropical forest. To simulate the increased nitrogen deposition, the NH<sub>4</sub>NO<sub>3</sub> solutions were sprayed onto the forest canopy at 25 kg </span><span>ha<sup>-1</sup> year<sup>-1</sup></span><span> (CAN25) and 50 kg ha<sup>-1</sup> year<sup>-1</sup> (CAN50), respectively, since April 2013. The </span><span>J<sub>S</sub></span><span> and microclimate (monitored since January 2014) derived from the whole-tree level </span><span>stomatal conductance </span><span>(G<sub>S</sub>) were used to quantify the stomatal behavior </span><span>(G<sub>S</sub> sensitive to</span><span> vapor pressure deficit</span><span>, G<sub>S-VPD</sub>)</span><span> in response to the added nitrogen. </span><span>The maximum shoot hydraulic conductance (Kshoot-max) was also measured for both species. After one year of monitoring</span><span> in January 2015, the </span><span>mid-day (J<sub>S-mid</sub>) and daily mean (J<sub>S-mean</sub>) sap flux rates did not change under all the nitrogen addition treatments (p > 0.05). A consistent</span><span> decline in the </span><span>G<sub>S-VPD</sub></span><span> indicated an enhanced isohydric behavior for both species. In addition, the G<sub>S-VPD</sub></span><span> in the wet season was much lower than that in the dry season. </span><span><em>S</em>. <em>superba</em> </span><span>had a lower </span><span>G<sub>S-VPD</sub></span><span> and decreased J<sub>S-mid</sub>/J<sub>S-mean</sub>, implying a stronger stomatal control under the fertilization, which might be attributed to the low efficient diffuse-porous conduits and a higher JS. In addition, the G<sub>S</sub> for </span><span><em>S</em>. <em>superba</em></span><span> decreased and the </span><span>G<sub>S-VPD</sub></span><span> increased more under CAN50 than that under CAN25, indicating that the high nitrogen dose restrains the extra nitrogen benefits. Our results indicate</span><span>d</span><span> that the J<sub>S</sub> for both species was decoupled from the leaf transpiration for both species due to an enhanced isohydric behavior, and a xylem anatomy difference and fertilization dose would affect the extent of this decoupling relation.</span></p>
Belowground net primary productivity stability in response to a nitrogen addition gradient in an alpine meadow
<p>1.Temporal stability of ecosystem productivity is important for providing reliable ecosystem services under global changes. Great efforts have been made to explore the response of aboveground net primary productivity (ANPP) stability to nitrogen (N) enrichment, yet how it affects belowground net primary productivity (BNPP) stability remains elusive, which hinders a comprehensive understanding of ecosystem stability from the view of a whole system.</p> <p>2. Here, using a field manipulative experiment with six N addition rates (0, 2, 4, 8, 16, 32 g N m-2 year-1), we explored the response patterns and drivers of BNPP stability in the topsoil (0-20 cm) and subsoil (20-40 cm) in the alpine meadow.</p> <p>3. The results showed that BNPP stability at both soil depths showed a unimodal response to increasing N addition rates. Specifically, for both the topsoil and subsoil, low-level N addition significantly promoted BNPP stability, while high doses of N addition had no significant impact on BNPP stability, suggesting that current low level of N deposition likely benefits the stable provision of belowground functioning. Furthermore, dominant species stability and species richness contributed most to the changes in BNPP stability in the topsoil, whereas only dominant species stability was the dominant driver of BNPP stability in the subsoil.</p> <p>4. This study is among the first to illuminate the main mechanisms underlying the responses of BNPP stability to N enrichment at various soil depths, which will advance our current understanding of N addition effects on belowground processes and benefit the sustainable provision of ecosystem functioning in the context of atmospheric N deposition.</p>
Data from: Explaining productivity variation in response to nitrogen addition and warming requires intraspecific variability
<p>1. Recent studies have shown that intraspecific trait variability is an important source of total trait variation. However, the contribution of intraspecific variability to ecosystem functions in the face of global change remains unknown.</p> <p>2. We measured plant height, leaf area, specific leaf area, leaf dry matter content and leaf thickness of 26 species at individual or leaf level and aboveground net primary productivity in 48 subplots subjected to 8 years of nitrogen addition and warming in a Tibetan alpine meadow. We split community weighted mean (CWM) and Rao's quadratic entropy (Rao) of individuals' traits within a given community into "fixed value" (only caused by interspecific difference of traits) and intraspecific variability (only caused by intraspecific difference), respectively, using a variance partitioning method.</p> <p>3. We found that productivity showed a humped back response to nitrogen addition: it was highest at intermediate levels of nitrogen fertilization. The response trend was mediated by the changes of plant functional structure. Productivity was positive with fixed Rao of plant height and intraspecific variability of leaf area, i.e. community having higher fixed variability of plant height and individuals producing bigger leaf area can increase productivity via niche complementary and dominance effect. Warming reduced productivity directly and marginally decreased individuals' leaf area which suppresses productivity indirectly.</p> <p>4. Our research suggests the non-negligible role of plant intraspecific trait variability in maintaining ecosystem functions, especially in the face of global change.</p>
Data from: Plant attributes interact with fungal pathogens and nitrogen addition to drive soil enzymatic activities and their temporal variation
<p>Nitrogen enrichment can alter soil communities and their functioning directly, via changes in nutrient availability and stoichiometry, or indirectly, by changing plant communities or the abundance of consumers. However, most studies have only focused on one of these potential drivers and we know little about the relative importance of the different mechanisms (changes in nutrient availability, in plant diversity or functional composition, or in consumer abundance) by which nitrogen enrichment affects soil functioning. In addition, soil functions could vary dramatically between seasons, however, they are typically measured only once during the peak growing season. We therefore know little about the drivers of intra-annual stability in soil functioning.</p> <p>In this study, we measured activities of β-glucosidase and acid phosphatase, two extracellular enzymes that indicate soil functioning. We did so in a large grassland experiment which tested the effects, and relative importance, of nitrogen enrichment, plant functional composition and diversity, and foliar pathogen presence (controlled by fungicide) on soil functioning. We measured the activity of the two enzymes across seasons and years to assess the stability and temporal dynamics of soil functioning.</p> <p>Overall β-glucosidase activity was slightly increased by nitrogen enrichment over time but did not respond to the other experimental treatments. Conversely, plant functional diversity, and interactions between plant attributes and fungicide application, were important drivers of mean acid phosphatase activity. The temporal stability of both soil enzymes was differently affected by two facets of plant diversity: species richness increased temporal stability and functional diversity decreased it; however, these effects were dampened when nitrogen and fungicide were added.</p> <p>Synthesis: The fungicide effects on soil enzyme activities suggest that foliar pathogens can also affect belowground processes and the interacting effect of fungicide and plant diversity suggests that these plant enemies can modulate the relationship between plant diversity and ecosystem functioning. The contrasting effects of our treatments on the mean versus stability of soil enzyme activities clearly show the need to consider temporal dynamics in belowground processes, to better understand the responses of soil microbes to environmental changes such as nutrient enrichment.</p>
Eighteen-year nitrogen addition does not increase plant phosphorus demand in a nitrogen-saturated tropical forest
<ol> <li><span>Nitrogen (N) deposition usually increases plant tissue N concentrations and thus phosphorus (P) demand in young and/or N-limited forests, but the N-deposition effect on plant P demand has rarely been assessed in N-saturated forests.</span></li> <li><span>Impacts of 18-year external N additions (Control: 0, Low N: 50, Moderate N:100, and High N: 150 kg N ha<sup>-1</sup> yr<sup>-1</sup>) on leaf P of four plant life-forms (tree, shrub, herb, and liana), P fractions of bulk and rhizosphere soils were examined in a N-saturated mature tropical forest in southern China. </span></li> <li><span>Leaf N, P, and N: P ratios of all plant life-forms remained stable under three N-additions. Among soil P fractions, moderate labile organic P increased by 25-33% across three N-additions; and soil total P was increased by 11.76 % under Low N, and 8.87% under High N, compared with the control. The PLS-PM results showed that the path coefficient of microbial community to available P significantly increased and of inorganic P to available P significantly decreased under N additions than control. N additions improved soil P availability through microbe-mediated P transformation: Low N significantly increased soil microbial taxonomic diversity, and a higher microbial diversity could enlarge the sources of nutrient acquisition and stimulate decomposition of recalcitrant organic matters; while High N significantly decreased soil microbial taxonomic diversity, the remaining microorganisms that were screened by N-rich environments had the characteristics of resisting the N-addition effects and maintained efficient P acquisition.</span></li> <li> <span><em>Synthesis</em>.</span><span> Our findings provide a novel line of evidence that long-term N deposition did not increase plant P demand in a N-saturated mature tropical forest. The underlying mechanism is that plants did not increase N uptakes therefore nor increase P uptakes (a stable leaf N: P stoichiometry) in an already N-saturated ecosystem. Different N addition rates regulated soil P transformation via microbial community transition. These findings help improve the understanding of plant P acquisition and modeling of biogeochemical N-P cycling and vegetation productivity in N-rich forest ecosystems, particularly considering the fact that chronic N deposition may likely lead to soil N richness and even saturation of many forests in the future.</span> </li> </ol>
Dataset for: Asymmetric response of aboveground and belowground temporal stability to nitrogen and phosphorus addition in a Tibetan alpine grassland
<p><span>Anthropogenic eutrophication is known to impair the stability of aboveground net primary productivity (ANPP), but its effects on the stability of belowground (BNPP) and total (TNPP) net primary productivity remain poorly understood. Based on a nitrogen and phosphorus addition experiment in a Tibetan alpine grassland, we show that nitrogen addition had little impact on the </span><span>temporal stability</span><span> of ANPP, BNPP and TNPP, whereas phosphorus addition reduced the </span><span>temporal stability</span><span> of BNPP and TNPP, </span><span>but not ANPP</span><span>. Significant interactive effects of nitrogen and phosphorus addition were observed on the stability of ANPP because of the opposite phosphorus effects under </span><span>ambient and enriched nitrogen conditions</span><span>. We found that the stability of TNPP was primarily driven by that of BNPP rather than that of ANPP. The responses of BNPP stability cannot be predicted by those of ANPP stability, as the variations in responses of ANPP and BNPP to enriched nutrients, with ANPP increased while BNPP remained unaffected, resulted in asymmetric responses in their stability. The dynamics of grasses, the most abundant plant functional group, instead of community species diversity, largely contributed to the ANPP stability. </span><span>Under the enriched nutrient condition, the synchronization of grasses reduced the grass stability, while the latter had a significant but weak negative impact on the BNPP stability. </span><span>These findings challenge the prevalent view that species diversity regulates the responses of ecosystem stability to nutrient enrichment. Our findings also suggest that the ecological consequences of nutrient enrichment on ecosystem stability cannot be accurately predicted from the responses of aboveground components, and highlight the need for a better understanding of the belowground ecosystem dynamics.</span></p>
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