Requirement of vegetation gaps for seedling establishment of two unpalatable grasses in a native grassland of central Argentina

Abstract The objective was to determine the effects of root and shoot competition on seedling establishment of the unpalatable grasses Stipa gynerioides and S. tenuissima in a native grassland of central Argentina dominated by the palatable grass S. clarazii. Seeds of the two unpalatable species were sown in natural occurring microsites with shoot and root competition from the palatable species, and in artificially created microsites without either shoot competition or shoot and root competition. In addition, fresh seeds of the unpalatable species were subjected to daily alternating temperatures under laboratory and field conditions to determine the effect on seed dormancy and germination. Seedling establishment of S. gynerioides and S. tenuissima occurred only in microsites without shoot and root competition. Also, the fluctuation of temperature near the soil surface in these microsites reduced dormancy and promoted rapid germination in both species. Our results support the hypothesis that, in swards dominated by palatable grasses, vegetation gaps of low competitive pressure favour seedling establishment of unpalatable grasses. It is suggested that the creation of these gaps by overgrazing can be an important mechanism in the process of species replacement in native grasslands.     

Effect of competition intensity on recruitment of palatable and unpalatable grasses

In this study, we made an attempt to reveal how competition intensity from established plants impacts on palatable and unpalatable grass seedlings recruitment, in a natural mesic grassland of central Argentina. Our objective was to assess the seedling recruitment of a palatable species (Chascolytrum subaristatum) and an unpalatable species (Nassella trichotoma) in microsites differing in competition intensity from established plants. Identity (C. subaristatum and N. trichotoma) and defoliation severity were used as surrogate for competition intensity. In March 2017, we permanently marked established individuals of N. trichotoma and C. subaristatum and placed two circular plots adjacent to each individual. In one plot we added seeds of N. trichotoma and in the other seeds of C. subaristatum. After seeding, established plants were randomly assigned to one of three level of defoliation: without defoliation, low defoliation severity and high defoliation severity. From April to November 2017 (i.e. over a complete annual growing cycle), we measured seedling density, recruitment and growth. Our results supported the hypothesis that seedlings of palatable grasses are more competitive than seedlings of unpalatable grasses. Seedling of the palatable grass C. subaristatum recruited successfully regardless the intensity of competition from established plants, whereas seedlings of the unpalatable grass N. trichotoma recruited better under low competitive pressure from established plants. Our results suggest that the availability of microsites with low competitive pressure from the established vegetation, created by selective grazing of palatable grasses, promotes the recruitment of unpalatable grass seedlings. This mechanism may contribute to the species replacement process commonly observed in heavy grazed grasslands.     

Competitive interactions between palatable and unpalatable grasses native to a temperate semi-arid grassland of Argentina

The argument that selective grazing leads to competitive replacement of palatable grasses by unpalatable grasses is based upon the assumption that the competitive ability of the palatable species is higher than the one of unpalatable species in the absence of grazing. In order to test this hypothesis we have compared the competitive ability of Stipa clarazii (palatable) and S. trichotoma (unpalatable) under field conditions, and S. clarazii and S. gynerioides (unpalatable) under greenhouse conditions. The three species are native to a temperate semi-arid grassland of central Argentina. In the field experiment, plants of both species were grown either independently or in pairs (palatable + unpalatable), protected from grazing. Shoot and seed production were measured at the end of the growing seasons of 1993, 1994 and 1995. In the greenhouse experiment, plants of both species were grown in pots, either in monoculture or in mixture, under conditions of high and low water and mineral nutrient availability. Total biomass and seed production were measured at the end of the experimental period. In both experiments the presence of the unpalatable species did not affect (P < 0.05) the productive responses of the palatable species. On the contrary, the presence of the palatable species significantly reduced (P < 0.05) the productive responses of the unpalatable species. Our results support the assumption, on which most interpretations of floristic changes induced by grazing are based, that the competitive ability of palatable grasses is higher than the one of unpalatable grasses in the absence of grazing.     

Soil Nitrogen Conditions Approach Preinvasion Levels following Restoration of Nitrogen-Fixing Black Locust (Robinia pseudoacacia) Stands in a Pine–Oak Ecosystem

Restoring native plant communities on sites formerly occupied by invasive nitrogen‐fixing species poses unique problems due to elevated soil nitrogen availability. Mitigation practices that reduce available nitrogen may ameliorate this problem. We evaluated the effects of tree removal followed by soil preparation or mulching on native plant growth and soil nitrogen transformations in a pine–oak system formerly occupied by exotic nitrogen‐fixing Black locust (Robinia pseudoacacia) trees. Greenhouse growth experiments with native grasses, Andropogon gerardii and Sorghastrum nutans, showed elevated relative growth rates in soils from Black locust compared with pine–oak stands. Field soil nutrient concentrations and rates of net nitrification and total net N‐mineralization were compared 2 and 4 years since Black locust removal and in control sites. Although soil nitrogen concentrations and total net N‐mineralization rates in the restored sites were reduced to levels that were similar to paired pine–oak stands after only 2 years, net nitrification rates remained 3–34 times higher in the restored sites. Other nutrient ion concentrations (Ca, Mg) and organic matter content were reduced, whereas phosphorus levels remained elevated in restored sites. Thus, 2–4 years following Black locust tree removal and soil horizon mixing achieved through site preparation, the concentrations of many soil nutrients returned to preinvasion levels. However, net nitrification rates remained elevated; cover cropping or carbon addition during restoration of sites invaded by nitrogen fixers could increase nitrogen immobilization and/or reduce nitrate availability, making sites more amenable to native plant establishment.     

Influence of climate variability on plant production and N‐mineralization in Central US grasslands

Abstract. We assessed the influence of annual and seasonal climate variability over soil organic matter (SOM), above‐ground net primary production (ANPP) and in situ net nitrogen (N) mineralization in a regional field study across the International Geosphere Biosphere Programme (IGBP) North American mid‐latitude transect (Koch et al. 1995). We hypothesized that while trends in SOM are strongly correlated with mean climatic parameters, ANPP and net N‐mineralization are more strongly influenced by annual and seasonal climate because they are dynamic processes sensitive to short‐term variation in temperature and water availability. Seasonal and monthly deviations from long‐term climatic means, particularly precipitation, were greatest at the semi‐arid end of the transect. ANPP is sensitive to this climatic variability, but is also strongly correlated with mean annual climate parameters. In situ net N‐mineralization and nitrification were weakly influenced by soil water content and temperature during the incubation and were less sensitive to seasonal climatic variables than ANPP, probably because microbial transformations of N in the soil are mediated over even finer temporal scales. We found no relationship between ANPP and in situ net N‐mineralization. These results suggests that methods used to estimate in situ net N‐mineralization are inadequate to represent N‐availability across gradients where microbial biomass, N‐immobilization or competition among plants and microbes vary.     

Sawdust Addition Reduces the Productivity of Nitrogen‐Enriched Mountain Grasslands

Anthropogenic nutrient enrichment of mountain grasslands has boosted grasses and fast‐growing unpalatable plants at the expense of slow‐growing species, resulting in a significant loss in biodiversity. A potential tool to reduce nutrient availability and aboveground productivity without destroying the perennial vegetation is carbon (C) addition. However, little is known about its suitability under severe climatic conditions. Here, we report the results of a 3‐year field study assessing the effects of sawdust addition on soil nutrients, aboveground productivity, and vegetational composition of 10 grazed and ungrazed mountain grasslands. Of particular interest was the effect of C addition on grasses and on the tall unpalatable weed Veratrum album. After 3 years, soil pH, ammonium, and plant‐available phosphorus were not altered by sawdust application, and nitrate concentrations were marginally higher in treatment plots. However, the biomass of grasses and forbs (without V. album) was 20–25% lower in sawdust‐amended plots, whereas the biomass of V. album was marginally higher. Sawdust addition reduced the cover of grasses but did not affect evenness, vegetation diversity, or plant species richness, although species richness generally increased with decreasing biomass at our sites. Our results suggest that sawdust addition is a potent tool to reduce within a relatively short time the aboveground productivity and grass cover in both grazed and ungrazed mountain grasslands as long as they are not dominated by tall unpalatable weeds. The technique has the advantage that it preserves the topsoil and the perennial soil seed bank.     

Nitrogen mineralization in heathland ecosystems dominated by different plant species

Net N mineralization rates were measured in heathlands still dominated by ericaceous dwarf shrubs (Calluna vulgaris or Erica tetralix) and in heathlands that have become dominated by grasses (Molinia caerulea or Deschampsia flexuosa). Net N mineralization was measuredin situ by sequential soil incubations during the year. In the wet area (gravimetric soil moisture content 74–130%), the net N mineralization rates were 4.4 g N m^–2 yr^–1 in the Erica soil and 7.8 g N m^–2 yr^–1 in the Molinia soil. The net nitrification rate was negligibly slow in either soil. In the dry area (gravimetric soil moisture content 7–38%), net N mineralization rates were 6.2 g N M-2 yr^–1 in the Calluna soil, 10.9 g N m^–2 yr^–1 in the Molinia soil and 12.6 g N m^–2 yr^–1 in the Deschampsia soil. The Calluna soil was consistently drier throughout the year, which may partly explain its slower mineralization rate. Net nitrification was 0.3 g N m^–2 yr^–1 in the Calluna soil, 3.6 g N m^–2 yr^–1 in the Molinia soil and 5.4 g N m^–2 yr^–1 in the Deschampsia soil. The net nitrification rate increased proportionally with the net N mineralization rate suggesting ammonium availability may control nitrification rates in these soils. In the dry area, the faster net N mineralization rates in sites dominated by grasses than in the site dominated by Calluna may be explained by the greater amounts of organic N in the soil of sites dominated by grasses. In both areas, however, the net amount of N mineralized per gram total soil N was greater in sites dominated by Molinia or Deschampsia than in sites dominated by Calluna or Erica. This suggests that in heathlands invaded by grasses the quality of the soil organic matter may be increased resulting in more rapid rates of soil N cycling.     

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

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

Warming and drought reduce temperature sensitivity of nitrogen transformations

Shifts in nitrogen (N) mineralization and nitrification rates due to global changes can influence nutrient availability, which can affect terrestrial productivity and climate change feedbacks. While many single‐factor studies have examined the effects of environmental changes on N mineralization and nitrification, few have examined these effects in a multifactor context or recorded how these effects vary seasonally. In an old‐field ecosystem in Massachusetts, USA, we investigated the combined effects of four levels of warming (up to 4 °C) and three levels of precipitation (drought, ambient, and wet) on net N mineralization, net nitrification, and potential nitrification. We also examined the treatment effects on the temperature sensitivity of net N mineralization and net nitrification and on the ratio of C mineralization to net N mineralization. During winter, freeze–thaw events, snow depth, and soil freezing depth explained little of the variation in net nitrification and N mineralization rates among treatments. During two years of treatments, warming and altered precipitation rarely influenced the rates of N cycling, and there was no evidence of a seasonal pattern in the responses. In contrast, warming and drought dramatically decreased the apparent Q₁₀ of net N mineralization and net nitrification, and the warming‐induced decrease in apparent Q₁₀ was more pronounced in ambient and wet treatments than the drought treatment. The ratio of C mineralization to net N mineralization varied over time and was sensitive to the interactive effects of warming and altered precipitation. Although many studies have found that warming tends to accelerate N cycling, our results suggest that warming can have little to no effect on N cycling in some ecosystems. Thus, ecosystem models that assume that warming will consistently increase N mineralization rates and inputs of plant‐available N may overestimate the increase in terrestrial productivity and the magnitude of an important negative feedback to climate change.     

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

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

Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semiarid grassland

Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4 years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption‐mediated, positive plant–soil feedback induced by N inputs. Furthermore, N : P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.     

Differential Controls of Water Input on Litter Decomposition and Nitrogen Dynamics in the Patagonian Steppe

Studies of the effects of precipitation on litter decomposition and nitrogen mineralization in arid and semiarid environments have demonstrated contradictory results. We conducted a manipulative experiment with rainout shelters in the semiarid Patagonian steppe, aimed at assessing the direct effects of water availability on litter decomposition and net nitrogen mineralization while isolating the indirect effects. We created four levels of precipitation input: control and three levels (30, 55 and 80%) of precipitation interception and we examined litter decomposition and nutrient release of a dominant grass species, Stipa speciosa, inorganic soil nitrogen, and in situ net nitrogen mineralization over two consecutive years. Litter decomposition rates (k, year⁻¹) varied significantly (P < 0.001) among precipitation interception treatments and were positively correlated with incoming annual precipitation (APPT, mm/year) (k = 0.0007 × APPT + 0.137). In contrast, net N mineralization was not correlated with incoming precipitation. Soil NO₃⁻ significantly decreased with increasing precipitation input, whereas soil NH₄⁺ concentration did not differ among precipitation interception treatments. Controls of water input on litter decomposition appear to be different from controls on N mineralization in the semiarid Patagonian steppe. We suggest that although water availability affects both the mineralization of C and N, it differentially affects the movement and fate of the inorganic products. A consequence of the accumulation of inorganic N during dry episodes is that periods of maximum water and soil nutrient availability may occur at different times. This asynchrony in the availability of N and water in the soil may explain the observed lags in the response of primary production to increases in water availability.     

Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO2 concentration (canopy FACE)

In a mature temperate forest in Hofstetten, Switzerland, deciduous tree canopies were subjected to a free‐air CO₂ enrichment (FACE) for a period of 8 years. The effect of this treatment on the availability of nitrogen (N) in the soil was assessed along three transects across the experimental area, one under Fagus sylvatica, one under Quercus robur and Q. petraea and one under Carpinus betulus. Nitrate, ammonium and dissolved organic N (DON) were analysed in soil solution obtained with suction cups. Nitrate and ammonium were also captured in buried ion‐exchange resin bags. These parameters were related to the local intensity of the FACE treatment as measured from the ¹³C depletion of dissolved inorganic carbon in the soil solution. Over the 8 years of experiment, the CO₂ enrichment reduced DON concentrations, did not affect ammonium, but induced higher nitrate concentrations, both in soil solution and resin bags. In the nitrate captured in the resin bags, the natural abundance of the isotope ¹⁵N increased strongly. This indicates that the CO₂ enrichment accelerated net nitrification, probably as an effect of the higher soil moisture resulting from the reduced transpiration of the CO₂‐enriched trees. It is also possible that N mineralization was enhanced by root exudates (priming effect) or that the uptake of inorganic N by these trees decreased slightly as the result of a reduced N demand for fine‐root growth. In this mature deciduous forest, we did not observe any progressive N limitation due to elevated atmospheric CO₂ concentrations; on the contrary, we observed an enhanced N availability over the 8 years of our measurements. This may, together with the global warming projected, exacerbate problems related to N saturation and nitrate leaching, although it is uncertain how long the observed trends will last in the future.     

Do grazers alter nitrogen dynamics on grazing lawns in a South African savannah?

The presence of grazers on grazing lawns in East Africa and North America often alters nitrogen cycling and availability. Grazing lawns can be defined as areas where grasses are kept in a short, actively growing, palatable state by the action of grazers. Our aim was to test whether lawns have enhanced leaf nitrogen (N) concentrations, total soil N and δ¹⁵N when compared to tall grass areas in a South African savannah. Previous studies have used ecosystem δ¹⁵N as a proxy of N availability, and enriched δ¹⁵N values have been suggested to indicate higher N availability or higher N transformation rates. Across all sites, foliar N concentrations (but not soil N) were higher when compared to tall grass areas, and evidence of enriched foliar and soil δ¹⁵N values was found on the lawns. These results suggest that grazers may be involved in altering the rates of N transformations directly on grazing lawns. Regardless of whether these N transformations included increased net N mineralization, higher N concentrations in above‐ground foliage attract grazers back to the lawns, encouraging their maintenance.     

Legumes regulate grassland soil N cycling and its response to variation in species diversity and N supply but not CO2

Legumes are an important component of plant diversity that modulate nitrogen (N) cycling in many terrestrial ecosystems. Limited knowledge of legume effects on soil N cycling and its response to global change factors and plant diversity hinders a general understanding of whether and how legumes broadly regulate the response of soil N availability to those factors. In a 17‐year study of perennial grassland species grown under ambient and elevated (+180 ppm) CO₂ and ambient and enriched (+4 g N m^?2 year^?1) N environments, we compared pure legume plots with plots dominated by or including other herbaceous functional groups (and containing one or four species) to assess the effect of legumes on N cycling (net N mineralization rate and inorganic N pools). We also examined the effects of numbers of legume species (from zero to four) in four‐species mixed plots on soil N cycling. We hypothesized that legumes would increase N mineralization rates most in those treatments with the greatest diversity and the greatest relative limitation by and competition for N. Results partially supported these hypotheses. Plots with greater dominance by legumes had greater soil nitrate concentrations and mineralization rates. Higher species richness significantly increased the impact of legumes on soil N metrics, with 349% and 505% higher mineralization rates and nitrate concentrations in four‐species plots containing legumes compared to legume‐free four‐species plots, in contrast to 185% and 129% greater values, respectively, in pure legume than nonlegume monoculture plots. N‐fertilized plots had greater legume effects on soil nitrate, but lower legume effects on net N mineralization. In contrast, neither elevated CO₂ nor its interaction with legumes affected net N mineralization. These results indicate that legumes markedly influence the response of soil N cycling to some, but not all, global change drivers.     

Invasion of Euryops floribundus in degraded South African communal grassland: Unpacking the invasion relationship with elevation,soil properties,the quality and quantity of the herbaceous layer

Shrub invasion is a recent occurrence in African grassland ecosystems. This study assessed the distribution of Euryops floribundus (a native shrub species) along an elevation and invasion intensity gradients, and its relationship with soil and herbaceous vegetation in a South African grassland. Shrub density increased (p < 0.05) from the uplands (2,301 plants/ha) to the bottomlands (4,888 plants/ha). Themeda triandra (a highly palatable grass) was dominant at the uplands and sloping sites, whereas Eragrostis plana and Sporobolus africanus (poorly palatable grasses) dominated the bottomlands. Upland soils had the highest organic carbon (OC), phosphorous (P), calcium and nitrogen (N) contents, but the lowest shrub density and herbaceous biomass. Canonical correspondence analysis indicated that elevation, soil OC and N contents determine important vegetation variables along the elevation gradient. In the shrub density gradient, soil P, OC and N responded positively to E. floribundus density. Grass biomass in the moderate and heavily invaded sites was high, but the abundance of palatable grasses declined, suggesting that E. floribundus invasion may be concomitant with a decrease in rangeland condition in terms of quality forage provision.     

Can Carbon Addition Increase Competitiveness of Native Grasses? A Case Study from California

There is growing interest in the addition of carbon (C) as sucrose or sawdust to the soil as a tool to reduce plant‐available nitrogen (N) and alter competitive interactions among species. The hypothesis that C addition changes N availability and thereby changes competitive dynamics between natives and exotics was tested in a California grassland that had experienced N enrichment. Sawdust (1.2 kg/m) was added to plots containing various combinations of three native perennial bunchgrasses, exotic perennial grasses, and exotic annual grasses. Sawdust addition resulted in higher microbial biomass N, lower rates of net N mineralization and net nitrification, and higher concentrations of extractable soil ammonium in the soil. In the first year sawdust addition decreased the degree to which exotic annuals competitively suppressed the seedlings of Nassella pulchra and, to a lesser extent, Festuca rubra, both native grasses. However there was no evidence of reduced growth of exotic grasses in sawdust‐amended plots. Sawdust addition did not influence interactions between the natives and exotic perennial grasses. In the second year, however, sawdust addition did not affect the interactions between the natives and either group of exotic grasses. In fact, the native perennial grasses that survived the first year of competition with annual grasses significantly reduced the aboveground productivity of annual grasses even without sawdust addition. These results suggest that the addition of sawdust as a tool in the restoration of native species in our system provided no significant benefit to natives over a 2‐year period.     

Experimentally increased nutrient availability at the permafrost thaw front selectively enhances biomass production of deep‐rooting subarctic peatland species

Climate warming increases nitrogen (N) mineralization in superficial soil layers (the dominant rooting zone) of subarctic peatlands. Thawing and subsequent mineralization of permafrost increases plant‐available N around the thaw‐front. Because plant production in these peatlands is N‐limited, such changes may substantially affect net primary production and species composition. We aimed to identify the potential impact of increased N‐availability due to permafrost thawing on subarctic peatland plant production and species performance, relative to the impact of increased N‐availability in superficial organic layers. Therefore, we investigated whether plant roots are present at the thaw‐front (45 cm depth) and whether N‐uptake (¹⁵N‐tracer) at the thaw‐front occurs during maximum thaw‐depth, coinciding with the end of the growing season. Moreover, we performed a unique 3‐year belowground fertilization experiment with fully factorial combinations of deep‐ (thaw‐front) and shallow‐fertilization (10 cm depth) and controls. We found that certain species are present with roots at the thaw‐front (Rubus chamaemorus) and have the capacity (R. chamaemorus, Eriophorum vaginatum) for N‐uptake from the thaw‐front between autumn and spring when aboveground tissue is largely senescent. In response to 3‐year shallow‐belowground fertilization (S) both shallow‐ (Empetrum hermaphroditum) and deep‐rooting species increased aboveground biomass and N‐content, but only deep‐rooting species responded positively to enhanced nutrient supply at the thaw‐front (D). Moreover, the effects of shallow‐fertilization and thaw‐front fertilization on aboveground biomass production of the deep‐rooting species were similar in magnitude (S: 71%; D: 111% increase compared to control) and additive (S + D: 181% increase). Our results show that plant‐available N released from thawing permafrost can form a thus far overlooked additional N‐source for deep‐rooting subarctic plant species and increase their biomass production beyond the already established impact of warming‐driven enhanced shallow N‐mineralization. This may result in shifts in plant community composition and may partially counteract the increased carbon losses from thawing permafrost.     

Soil ecosystem function under native and exotic plant assemblages as alternative states of successional grasslands

Old fields often become dominated by exotic plants establishing persistent community states. Ecosystem functioning may differ widely between such novel communities and the native-dominated counterparts. We evaluated soil ecosystem attributes in native and exotic (synthetic) grass assemblages established on a newly abandoned field, and in remnants of native grassland in the Inland Pampa, Argentina. We asked whether exotic species alter soil functioning through the quality of the litter they shed or by changing the decomposition environment. Litter decomposition of the exotic dominant Festuca arundinacea in exotic assemblages was faster than that of the native dominant Paspalum quadrifarium in native assemblages and remnant grasslands. Decomposition of a standard litter (Triticum aestivum) was also faster in exotic assemblages than in native assemblages and remnant grasslands. In a common garden, F. arundinacea showed higher decay rates than P. quadrifarium, which reflected the higher N content and lower C:N of the exotic grass litter. Soil respiration rates were higher in the exotic than in the native assemblages and remnant grasslands. Yet there were no significant differences in soil N availability or net N mineralization between exotic and native assemblages. Our results suggest that exotic grass dominance affected ecosystem function by producing a more decomposable leaf litter and by increasing soil decomposer activity. These changes might contribute to the extended dominance of fast-growing exotic grasses during old-field succession. Further, increased organic matter turnover under novel, exotic communities could reduce the carbon storage capacity of the system in the long term.     

Relationships among soil fertility,native plant diversity and exotic plant abundance inform restoration of forb‐rich eucalypt woodlands

Aim Water and nutrient availability are major limits to productivity in semi‐arid ecosystems; hence, ecological restoration often focuses on conserving or concentrating soil resources. By contrast, nutrient enrichment can promote invasion by exotic annuals, leading to restoration approaches that target reduction of soil nutrients. We aimed to explore potential biodiversity trade‐offs between these approaches by investigating relationships among soil nutrients, exotic annuals and native plant diversity and composition. In particular, we investigated the hypothesis that native plant diversity in semi‐arid to temperate woodlands reflects the productivity–diversity hypothesis, leading to hump‐backed relationships with soil nutrients such that (1) native plant diversity declines with increasing nutrient enrichment and (2) native diversity is limited at the lowest levels of soil fertility. Location Fragmented, long‐ungrazed Eucalyptus loxophleba subsp. loxophleba (York gum)–Acacia acuminata (jam) woodlands in the wheatbelt of South‐Western Australia. Methods We conducted stratified surveys of floristic composition and topsoil nutrient concentrations in 112 woodland patches. We used generalized linear models, structural equation models and ordinations to characterize relationships among soil nutrients, rainfall, exotic annuals and patch‐scale (100 m²) native plant composition and diversity. Results Patch‐scale native plant diversity declined strongly with increasing exotic abundance. This was partly related to elevated soil nutrient concentrations, particularly total nitrogen and available phosphorus. By contrast, there was little evidence for positive correlations between soil nutrients and native diversity, even at very low soil nutrient concentrations. Main conclusions Minimizing weed invasions is crucial for maximizing native plant diversity in E. loxophleba woodlands and could include nutrient‐depleting treatments without substantially compromising the functional capacity of soils to maintain native plant richness and composition. More broadly we emphasize that understanding relationships among ecosystem productivity, plant diversity and exotic invasions in the context of associated theoretical frameworks is fundamental for informing ecological restoration.