Interactive effects of elevated CO<Subscript>2</Subscript>, N deposition and climate change on plant litter quality in a California annual grassland

Although global changes can alter ecosystem nutrient dynamics indirectly as a result of their effects on plant litter quality, the interactive effects of global changes on plant litter remain largely unexplored in natural communities. We investigated the effects of elevated CO₂, N deposition, warming and increased precipitation on the composition of organic compounds in plant litter in a fully-factorial experiment conducted in a California annual grassland. While lignin increased within functional groups under elevated CO₂, this effect was attenuated by warming in grasses and by water additions in forbs. CO₂-induced increases in lignin within functional groups also were counteracted by an increase in the relative biomass of forbs, which contained less lignin than grasses. Consequently, there was no net change in the overall lignin content of senesced tissue at the plot level under elevated CO₂. Nitrate additions increased N in both grass and forb litter, although this effect was attenuated by water additions. Relative to changes in N within functional groups, changes in functional group dominance had a minor effect on overall litter N at the plot level. Nitrate additions had the strongest effect on decomposition, increasing lignin losses from Avena litter and interacting with water additions to increase decomposition of litter of other grasses. Increases in lignin that resulted from elevated CO₂ had no effect on decomposition but elevated CO₂ increased N losses from Avena litter. Overall, the interactions among elements of global change were as important as single-factor effects in influencing plant litter chemistry. However, with the exception of variation in N, litter quality had little influence on decomposition over the short term.     

Nutrient amendments in a temperate grassland have greater negative impacts on early season and exotic plant species

Among the greatest challenges currently facing terrestrial plant communities are anthropogenic nutrient amendments. Nutrient inputs can change community attributes, such as diversity and composition, and have been implicated in the increasing dominance of certain groups of species, like exotics. Overall effects of nutrients will likely be contingent on other factors in the community. One such factor is within-season change in plant communities, which has seldom been examined but may be of considerable importance. We examined within-season effects of nutrient manipulations (fertilization and litter removal) on overall community attributes and different species groups in a temperate grassland. We found the effects of nutrient amendments varied significantly over the growing season. Fertilization reduced species richness and diversity at all census points, but had the greatest effects on communities early and late in the growing season. There were few seasonal effects on species richness within functional groups, suggesting that nutrient effects were consistent and established early. Although fertilization reduced both native and exotic forb species richness throughout the season, exotics surprisingly suffered greater species losses than natives. Comparisons between within-season data and data from the end of season showed nutrient amendments have stronger community effects than previously recognized. We found that differences in cumulative species richness (all species detected during the growing season) between fertilized and unfertilized plots were greater than differences in species richness measures from any single census date. Our results suggest that predicting the overall response of plant communities to anthropogenic nutrient amendments will require thorough temporal assessments, particularly of early season species, which are generally overlooked in community studies.     

Aboveground productivity and root–shoot allocation differ between native and introduced grass species

Plant species in grasslands are often separated into groups (C₄ and C₃ grasses, and forbs) with presumed links to ecosystem functioning. Each of these in turn can be separated into native and introduced (i.e., exotic) species. Although numerous studies have compared plant traits between the traditional groups of grasses and forbs, fewer have compared native versus introduced species. Introduced grass species, which were often introduced to prevent erosion or to improve grazing opportunities, have become common or even dominant species in grasslands. By virtue of their abundances, introduced species may alter ecosystems if they differ from natives in growth and allocation patterns. Introduced grasses were probably selected nonrandomly from the source population for forage (aboveground) productivity. Based on this expectation, aboveground production is predicted to be greater and root mass fraction to be smaller in introduced than native species. We compared root and shoot distribution and tissue quality between introduced and native C₄ grass species in the Blackland Prairie region of Central Texas, USA, and then compared differences to the more well-studied divergence between C₄ grasses and forbs. Comparisons were made in experimental monocultures planted with equal-sized transplants on a common soil type and at the same density. Aboveground productivity and C:N ratios were higher, on average, in native grasses than in native forbs, as expected. Native and introduced grasses had comparable amounts of shallow root biomass and tissue C:N ratios. However, aboveground productivity and total N were lower and deep root biomass and root mass fraction were greater in native than introduced grasses. These differences in average biomass distribution and N could be important to ecosystems in cases where native and introduced grasses have been exchanged. Our results indicate that native–introduced status may be important when interpreting species effects on grassland processes like productivity and plant N accumulation.     

Plant community responses to increased precipitation and belowground litter addition: Evidence from a 5‐year semiarid grassland experiment

Global climate change is predicted to stimulate primary production and consequently increases litter inputs. Changing precipitation regimes together with enhanced litter inputs may affect plant community composition and structure, with consequent influence on diversity and ecosystem functioning. Responses of plant community to increased precipitation and belowground litter addition were examined lasting 5 years in a semiarid temperate grassland of northeastern China. Increased precipitation enhanced community species richness and abundance of annuals by 16.8% and 44%, but litter addition suppressed them by 25% and 54.5% after 5 years, respectively. During the study period, perennial rhizome grasses and forbs had consistent negative relationship under ambient plots, whereas positive relationship between the two functional groups was found under litter addition plots after 5 years. In addition, increased precipitation and litter addition showed significant interaction on community composition, because litter addition significantly increased biomass and abundance of rhizome grasses under increased precipitation plots but had no effect under ambient precipitation levels. Our findings emphasize the importance of water availability in modulating the responses of plants community to potentially enhanced litter inputs in the semiarid temperate grassland.     

Maintenance of leaf N controls the photosynthetic CO2 response of grassland species exposed to 9 years of free‐air CO2 enrichment

Determining underlying physiological patterns governing plant productivity and diversity in grasslands are critical to evaluate species responses to future environmental conditions of elevated CO₂ and nitrogen (N) deposition. In a 9‐year experiment, N was added to monocultures of seven C₃ grassland species exposed to elevated atmospheric CO₂ (560 μmol CO₂ mol^?1) to evaluate how N addition affects CO₂ responsiveness in species of contrasting functional groups. Functional groups differed in their responses to elevated CO₂ and N treatments. Forb species exhibited strong down‐regulation of leaf N_mass concentrations (?26%) and photosynthetic capacity (?28%) in response to elevated CO₂, especially at high N supply, whereas C₃ grasses did not. Hence, achieved photosynthetic performance was markedly enhanced for C₃ grasses (+68%) in elevated CO₂, but not significantly for forbs. Differences in access to soil resources between forbs and grasses may distinguish their responses to elevated CO₂ and N addition. Forbs had lesser root biomass, a lower distribution of biomass to roots, and lower specific root length than grasses. Maintenance of leaf N, possibly through increased root foraging in this nutrient‐poor grassland, was necessary to sustain stimulation of photosynthesis under long‐term elevated CO₂. Dilution of leaf N and associated photosynthetic down‐regulation in forbs under elevated [CO₂], relative to the C₃ grasses, illustrates the potential for shifts in species composition and diversity in grassland ecosystems that have significant forb and grass components.     

Simple plant traits explain functional group diversity decline in novel grassland communities of Texas

Previous research has found that plant diversity declines more quickly in exotic than native grassland plots, which offers a model system for testing whether diversity decline is associated with specific plant traits. In a common garden experiment in the Southern Great Plains in central Texas, USA, we studied monocultures and 9-species mixtures of either all exotic or all native grassland species. A total of 36 native and exotic species were paired by phylogeny and functional group. We used community-level measures (relative abundance in mixture) and whole-plant (height, aboveground biomass, and light capture) and leaf-level traits (area, specific leaf area, and C:N ratio) to determine whether trait differences explained native-exotic differences in functional group diversity. Increases in species’ relative abundance in mixture were correlated with high biomass, height, and light capture in both native and exotic communities. However, increasing exotic species were all C₄ grasses, whereas, increasing native species included forb, C₃ grass and C₄ grass species. Exotic C₄ grasses had traits associated with relatively high resource capture: greater leaf area, specific leaf area, height, biomass, and light capture, but similar leaf C:N ratios compared to native C₄ grasses. Leaf C:N was consistently higher for native than exotic C₃ species, implying that resource use efficiency was greater in natives than exotics. Our results suggest that functional diversity will differ between grasslands restored to native assemblages and those dominated by novel collections of exotic species, and that simple plant traits can help to explain diversity decline.     

Exotic grasses and feces deposition by an exotic herbivore combine to reduce the relative abundance of native forbs

Increased resource availability can facilitate establishment of exotic plant species, especially when coincident with propagule supply. Following establishment, increased resource availability may also facilitate the spread of exotic plant species if it enhances their competitive abilities relative to native species. Exotic Canada geese (Branta canadensis) introduce both exotic grass seed and nutrients to an endangered plant community on the Gulf Islands of southwestern British Columbia, Canada. I used greenhouse experiments to assess the competitive advantage of the exotic grasses relative to native and exotic forbs in this community and to test the impacts of nutrient addition from goose feces on competitive outcomes. I grew experimental communities varying in their proportion of forbs versus exotic grasses, and added goose feces as a nutrient source. I found that both native and exotic forbs produced significantly more biomass in competition with conspecifics than in competition with the grasses, and that the proportional abundance of two out of three native forbs was lowest in the combined presence of exotic grasses and nutrient addition. In a second experiment, I found that in monoculture all species of forbs and grasses showed equal growth responses to nutrients. The exotic species did not convert additional nutrients into additional biomass at a higher rate, but did germinate earlier and grow larger than the native species regardless of nutrient availability. This suggests that the exotic species may have achieved their competitive advantage partly by pre-empting resources in community mixtures. Small and late-germinating native forbs may be particularly vulnerable to competitive suppression from exotic grasses and forbs and may be at an even greater disadvantage if their competitors are benefiting from early access to additional nutrients. In combination, the input of exotic propagules and additional nutrients by nesting geese may compromise efforts to maintain native community composition in this system.     

Species richness loss after nutrient addition as affected by N:C ratios and phytohormone GA3 contents in an alpine meadow community

Aims From the light-competition hypothesis, competition for light is asymmetric and the observed increases in plant-size variability with increasingly denser canopies are primarily due to competition for light. Greater plant height provides pre-emptive access to light and produces increased height differences among species. The question is what produces these differences in plant height or height growth response among species in response to fertilization.Methods In 2009, a field experiment of N, P and N + P enrichments at three levels each was initiated in an alpine meadow on the northeast Qinghai-Tibet Plateau. Effects of fertilization on species richness, aboveground net primary production (ANPP), relative light intensity and plant height of different plant functional groups were determined. Festuca ovina (grass), Kobresia humilis (sedge), Oxytropis ochrocephala (legume), Taraxacum lugubre (rosette forb) and Geranium pylzowianum (upright forb) were selected as exemplars of each of the indicated functional groups. The N:C ratios in aboveground biomass, gibberellic acid (GA 3) concentrations in leaves, plant heights and height relative growth rate (RGR) of these exemplar species were analyzed in detail.Important findings Species richness of grasses significantly increased with increasing N + P levels. Species richness of legumes and upright forbs decreased after N and N + P additions. P addition had no significant effect on species richness. The effects of N + P addition on species richness and ANPP were consistently stronger than those of the single N or P fertilization. Reductions in species richness caused by nutrient addition paralleled the increases in ANPP and decreases in light intensity under the canopies, indicating indirect effect of nutrient addition on species richness via ANPP-induced light competition. The exemplar species that responded most positively to fertilization in height and RGR also displayed stronger increases in their GA 3 content and N:C ratios. GA 3 concentrations and N:C ratios were positively correlated with height RGR when the data were pooled for all species. The tallest and the fastest-growing grass, F. ovina, had the largest increase in N:C ratios and the highest leaf GA 3 concentrations after nutrient addition. These results indicated that differential responses of GA 3 concentrations and N:C ratios to fertilization were related to the inequality in plant heights among species.     

Grazing-induced changes in plant composition affect litter quality and nutrient cycling in flooding Pampa grasslands

Changes in plant community composition induced by vertebrate grazers have been found to either accelerate or slow C and nutrient cycling in soil. This variation may reflect the differential effects of grazing-promoted (G+) plant species on overall litter quality and decomposition processes. Further, site conditions associated with prior grazing history are expected to influence litter decay and nutrient turnover. We studied how grazing-induced changes in plant life forms and species identity modified the quality of litter inputs to soil, decomposition rate and nutrient release in a flooding Pampa grassland, Argentina. Litter from G+ forbs and grasses (two species each) and grazing-reduced (G−) grasses (two species) was incubated in long-term grazed and ungrazed sites. G+ species, overall, showed higher rates of decomposition and N and P release from litter. However, this pattern was primarily driven by the low-growing, high litter-quality forbs included among G+ species. Forbs decomposed and released nutrients faster than either G+ or G− grasses. While no consistent differences between G+ and G− grasses were observed, patterns of grass litter decay and nutrient release corresponded with interspecific differences in phenology and photosynthetic pathway. Litter decomposition, N release and soil N availability were higher in the grazed site, irrespective of species litter type. Our results contradict the notion that grazing, by reducing more palatable species and promoting less palatable ones, should decrease nutrient cycling from litter. Plant tissue quality and palatability may not unequivocally link patterns of grazing resistance and litter decomposability within a community, especially where grazing causes major shifts in life form composition. Thus, plant functional groups defined by species’ “responses” to grazing may only partially overlap with functional groups based on species “effects” on C and nutrient cycling.     

Effects of experimental season of prescribed fire and nutrient addition on structure and function of previously grazed grassland

Aims Understanding the drivers of grassland structure and function following livestock removal will inform grassland restoration and management. Here, we investigated the effects of fire and nutrient addition on structure and function in a subtropical semi-native grassland recently released from grazing in south-central Florida. We examined responses of soil nutrients, plant tissue nutrients, biomass of live, standing dead and litter, and plant species composition to experimental annual prescribed fire applied during different seasons (wet season vs. dry season), and nutrient additions (N, P and N + P) over 9 years.Methods Experimental plots were set up in a randomized block split-plot design, with season of prescribed fire as the main treatment and nutrient addition as the subplot treatment. Species cover data were collected annually from 2002 to 2011 and plant tissue and plant biomass data were collected in 2002–2006 and 2011. Soil nutrients were analyzed in 2004, 2006 and 2011.Important findings Soil total phosphorus (P) levels increased substantially with P addition but were not influenced by prescribed fire. Addition of P and N led to increased P and N concentrations in live plant tissues, but prescribed fire reduced N in live tissue. Levels of tissue N were higher in all plots at the beginning of the experiment, an effect that was likely due to grazing activity prior to removal of livestock. Plant tissue N steadily declined over time in all plots, with annually burned plots declining faster than unburned plots. Prescribed fire was an important driver of standing dead and litter biomass and was important for maintaining grass biomass and percent cover. Nutrient addition was also important: the addition of both N and P was associated with greater live biomass and woody forbs. Removal of grazing, lack of prescribed fire, and addition of N + P led to a reduction of grass biomass and a large increase in biomass of a woody forb. Annual prescribed fire promoted N loss from the system by reducing standing dead and litter, but maintained desirable biomass of grasses.     

How plant diversity and legumes affect nitrogen dynamics in experimental grassland communities

Positive relationships between species richness and ecosystem processes such as productivity or nitrogen cycling can be the result of a number of mechanisms. We examined how species richness, biomass, and legume presence, diversity, and abundance explained nitrogen dynamics in experimental grassland plots in northern Sweden. Nitrogen concentrations and '¹⁵N values were measured in plants grown in 28 mixtures (58 plots) including 1, 2, 4, 8 or 12 local grassland species over four years. Values for '¹⁵N declined over time for all three functional groups (grasses, legumes, and non-leguminous forbs), suggesting greater reliance on N fixed by legumes over time by all species. Above ground percent nitrogen (%N) also declined over time but root %N and total N did not. Path analysis of above ground data suggested that two main factors affected %N and the size of the N pool. First, higher plant diversity (species richness) increased total N through increased biomass in the plot. Although in the first two years of the experiment this was the result of a greater probability of inclusion of at least one legume, in the last two years diversity had a significant effect on biomass beyond this effect. Second, percent legumes planted in the plots had a strong effect on above ground %N and '¹⁵N, but a much smaller effect on above ground biomass. In contrast, greater plant diversity affected N in roots both by increasing biomass and by decreasing %N (after controlling for effects mediated by root biomass and legume biomass). Increased legume biomass resulted in higher %N and lower '¹⁵N for both non-legume forbs and grasses in the first year, but only for grasses in the third year. We conclude that a sampling effect (greater probability of including a legume) contributed towards greater biomass and total N in high-diversity communities early on in the experiment, but that over time this effect weakened and other positive effects of diversity became more important.     

Effects of Nutrient Addition on Community Productivity and Structure of Serpentine Vegetation

Abstract: The effects of a nutrient addition experiment on the plant biomass of garigue vegetation on ultramafic (serpentine) soils in Tuscany, Italy, were investigated. Although community composition was not significantly changed, fertilization had a significant positive influence on biomass production. The most affected species groups were woody species (chamaephytes and phanerophytes), annual grasses and annual forbs; the bio mass increase of the perennial grasses and perennial forbs was statistically not significant. Soil extractable elements differed for calcium and potassium in the plots where they were added; sodium and nickel extractabilities were reduced by calcium ad dition due to the increased soil pH. Biomass production was linked more to major nutrient addition than to reduced nickel extractability, confirming that serpentine vegetation of Tuscany is mainly affected by nutritional stress rather than soil heavy metal content. The addition of calcium had a low effect on pri mary production of these ultramafic soils.     

Effects of exotic annual grass litter and local environmental gradients on annual plant community structure

Exotic annual grasses have been introduced into many semi-arid ecosystems worldwide, often to the detriment of native plant communities. The accumulation of litter from these grasses (i.e. residual dry biomass) has been demonstrated to negatively impact native plant communities and promote positive feedbacks to exotic grass persistence. More targeted experiments are needed, however, to determine the relative impact of exotic grass litter on plant community structure across local environmental gradients. We experimentally added exotic grass litter to annual forb-dominated open woodland communities positioned along natural canopy cover gradients in southwest Western Australia. These communities are an important component of this region’s plant biodiversity hotspot and are documented to be under threat from exotic annual grasses. After a one-year treatment period, we measured the effects of exotic grass litter, soil properties, and canopy cover on native and exotic species richness and abundance, as well as common species’ biomass and abundances. Plant community structure was more strongly influenced by soil properties and canopy cover than by grass litter. Total plant abundances per plot, however, were significantly lower in litter addition plots than control plots, a trend driven by native species. Exotic grass litter was also associated with lower abundances of one very common native species: Waitzia acuminata. Our results suggest that exotic grass litter limits the establishment of some native species in this system. Over multiple years, these subtle impacts may contribute substantially to the successful advancement of exotic species into this system, particularly in certain microenvironments.     

Effects of elevated CO2 and nitrogen fertilization pretreatments on decomposition on tallgrass prairie leaf litter

Standing dead and green foliage litter was collected in early November 1990 from Andropogon gerardii (C₄), Sorghastrum nutans (C₄), and Poa pratensis (C₃) plants that were grown in large open-top chambers under ambient or twice ambient CO₂ and with or without nitrogen fertilization (45 kg N ha⁻¹). The litter was placed in mesh bags on the soil surface of pristine prairie adjacent to the growth treatment plots and allowed to decay under natural conditions. Litter bags were retrieved at fixed intervals and litter was analyzed for mass loss, carbon chemistry, and total Kjeldahl nitrogen and phosphorus. The results indicate that growth treatments had a relatively minor effect on the initial chemical composition of the litter and its subsequent rate of decay or chemical composition. This suggests that a large indirect effect of CO₂ on surface litter decomposition in the tallgrass prairie would not occur by way of changes in chemistry of leaf litter. However, there was a large difference in characteristics of leaf litter decomposition among the species. Poa leaf litter had a different initial chemistry and decayed more rapidly than C₄ grasses. We conclude that an indirect effect of CO₂ on decomposition and nutrient cycling could occur if CO₂ induces changes in the relative aboveground biomass of the prairie species.     

Plant community diversity and composition affect individual plant performance

Effects of plant community diversity on ecosystem processes have recently received major attention. In contrast, effects of species richness and functional richness on individual plant performance, and their magnitude relative to effects of community composition, have been largely neglected. Therefore, we examined height, aboveground biomass, and inflorescence production of individual plants of all species present in 82 large plots of the Jena Experiment, a large grassland biodiversity experiment in Germany. These plots differed in species richness (1–60), functional richness (1–4), and community composition. On average, in more species-rich communities, plant individuals grew taller, but weighed less, were less likely to flower, and had fewer inflorescences. In plots containing legumes, non-legumes were higher and weighed more than in plots without legumes. In plots containing grasses, non-grasses were less likely to flower than in plots without grasses. This indicates that legumes positively and grasses negatively affected the performance of other species. Species richness and functional richness effects differed systematically between functional groups. The magnitude of the increase in plant height with increasing species richness was greatest in grasses and was progressively smaller in legumes, small herbs, and tall herbs. Individual aboveground biomass responses to increasing species richness also differed among functional groups and were positive for legumes, less pronouncedly positive for grasses, negative for small herbs, and more pronouncedly negative for tall herbs. Moreover, these effects of species richness differed strongly between species within these functional groups. We conclude that individual plant performance largely depends on the diversity of the surrounding community, and that the direction and magnitude of the effects of species richness and functional richness differs largely between species. Our study suggests that diversity of the surrounding community needs to be taken into account when interpreting drivers of the performance of individual plants.     

A water-centred framework to assess the effects of salinity on the growth and yield of wheat and barley

We conducted a field experiment in two alpine meadows to investigate the short-term effects of nitrogen enrichment and plant litter biomass on plant species richness, the percent cover of functional groups, soil microbial biomass, and enzyme activity in two alpine meadow communities. The addition of nitrogen fertilizer to experimental plots over two growing seasons increased plant production, as indicated by increases in both the living plant biomass and litter biomass in the Kobresia humilis meadow community. In contrast, fertilization had no significant effect on the amounts of living biomass and litter biomass in the K. tibetica meadow. The litter treatment results indicate that litter removal significantly increased the living biomass and decreased the litter biomass in the K. humilis meadow; however, litter-removal and litter-intact treatments had no impact on the amounts of living biomass and litter biomass in the K. tibetica meadow. Litter production depended on the degree of grass cover and was also influenced by nitrogen enrichment. The increase in plant biomass reflects a strong positive effect of nitrogen enrichment and litter removal on grasses in the K. humilis meadow. Neither fertilization nor litter removal had any impact on the grass biomass in the K. tibetica meadow. Sedge biomass was not significantly affected by either nutrient enrichment or litter removal in either alpine meadow community. The plant species richness decreased in the K. humilis meadow following nitrogen addition. In the K. humilis meadow, microbial biomass C increased significantly in response to the nitrogen enrichment and litter removal treatments. Enzyme activities differed depending on the enzyme and the different alpine meadow communities; in general, enzyme activities were higher in the upper soil layers (0–10 cm and 10–20 cm) than in the lower soil layers (20–40 cm). The amounts of living plant biomass and plant litter biomass in response to the different treatments of the two alpine meadow communities affected the soil microbial biomass C, soil organic C, and soil fertility. These results suggest that the original soil conditions, plant community composition, and community productivity are very important in regulating plant community productivity and microbial biomass and activity.     

Growth rate, root development and nutrient uptake of 55 plant species from the Great Plains Grasslands, USA

Empirical and theoretical studies have highlighted that plantcompetition and species diversity are substantially affected by interactionsamong plant growth and nutrient uptake rates, root lateral spread, rootplasticity, and small scale soil nutrient heterogeneity. This study wasdesignedto (a) experimentally estimate parameters regarding rootscaling patterns, root biomass allocation, growth rates, nutrient productivity,and root nutrient influx rates of 55 plant species common to Great Plainsgrasslands; and (b) determine if grasses and forbs can beclassified into statistically distinct groups based on these characteristics.Wefound that: (1) In all species root lateral spread, rootlength, and root surface area had significant allometric scaling relationshipswith root biomass, but that the relationships were unaffected by Navailability.(2) Reductions in the supply of N increased the root:shootratio in 62% of the species. (3) The frequency distributionandmean values of maximum relative growth rates were very similar for grasses andforbs/shrubs, but mid successional grasses had a higher relative growth ratethan late successional ones. (4) In 78% of the species tested,N productivity was increased by reductions in the N supply.(5)When subjected to a high N supply, the N and P productivity of grasses was, onaverage, higher than that of forbs/shrubs, and the N and P productivity ofC₄ grasses was, on average, higher than that of C₃grasses. No differences were found under a low N supply. (6)Nodifferences on the average maximum N and P influx rates per unit of rootsurfacearea were found between grasses and forbs or between C₃ andC₄ grasses, but both were correlated with maximum relative growthrate. (7) The set of parameters we measured were able toseparate grasses and forbs/shrubs into statistically distinct groups that tendto follow in broad terms the coarse vs. fine scaleforaging strategies hypothesis.     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

The impact of elevated CO2, increased nitrogen availability and biodiversity on plant tissue quality and decomposition

Elevated CO₂, increased nitrogen (N) deposition and increasing species richness can increase net primary productivity (NPP). However, unless there are comparable changes in decomposition, increases in productivity will most likely be unsustainable. Without comparable increases in decomposition nutrients would accumulate in dead organic matter leading to nutrient limitations that could eventually prohibit additional increases in productivity. To address this issue, we measured aboveground plant and litter quality and belowground root quality, as well as decomposition of aboveground litter for one and 2‐year periods using in situ litterbags in response to a three‐way factorial manipulation of CO₂ (ambient vs. 560 ppm), N deposition (ambient vs. the addition of 4 g N m⁻² yr⁻¹) and plant species richness (one, four, nine and 16 species) in experimental grassland plots. Litter chemistry responded to the CO₂, N and plant diversity treatments, but decomposition was much less responsive. Elevated CO₂ induced decreases in % N and % lignin in plant tissues. N addition led to increases in % N and decreases in % lignin. Increasing plant diversity led to decreases in % N and % lignin and an increase in % cellulose. In contrast to the litter chemistry changes, elevated CO₂ had a much lower impact on decomposition and resulted in only a 2.5% decrease in carbon (C) loss. Detectable responses were not observed either to N addition or to species richness. These results suggest that global change factors such as biodiversity loss, elevated CO₂ and N deposition lead to significant changes in tissue quality; however, the response of decomposition is modest. Thus, the observed increases in productivity at higher diversity levels and with elevated CO₂ and N fertilization are not matched by an increase in decomposition rates. This lack of coupled responses between production and decomposition is likely to result in an accumulation of nutrients in the litter pool which will dampen the response of NPP to these factors over time.     

Plant Functional Group Diversity as a Mechanism for Invasion Resistance

A commonly cited mechanism for invasion resistance is more complete resource use by diverse plant assemblages with maximum niche complementarity. We investigated the invasion resistance of several plant functional groups against the nonindigenous forb Spotted knapweed (Centaurea maculosa). The study consisted of a factorial combination of seven functional group removals (groups singularly or in combination) and two C. maculosa treatments (addition vs. no addition) applied in a randomized complete block design replicated four times at each of two sites. We quantified aboveground plant material nutrient concentration and uptake (concentration × biomass) by indigenous functional groups: grasses, shallow‐rooted forbs, deep‐rooted forbs, spikemoss, and the nonindigenous invader C. maculosa. In 2001, C. maculosa density depended upon which functional groups were removed. The highest C. maculosa densities occurred where all vegetation or all forbs were removed. Centaurea maculosa densities were the lowest in plots where nothing, shallow‐rooted forbs, deep‐rooted forbs, grasses, or spikemoss were removed. Functional group biomass was also collected and analyzed for nitrogen, phosphorus, potassium, and sulphur. Based on covariate analyses, postremoval indigenous plot biomass did not relate to invasion by C. maculosa. Analysis of variance indicated that C. maculosa tissue nutrient percentage and net nutrient uptake were most similar to indigenous forb functional groups. Our study suggests that establishing and maintaining a diversity of plant functional groups within the plant community enhances resistance to invasion. Indigenous plants of functionally similar groups as an invader may be particularly important in invasion resistance.