Functional differences between woodland savannas and seasonally dry forests from south‐eastern Brazil: Evidence from 15N natural abundance studies

Nitrogen availability and N‐cycling dynamics across ecosystems play a critical role in plant functioning and species distribution. Measurements of ¹⁵N natural abundance provides a way to assess ecosystem N dynamics, and the range of nitrogen stable isotope values (δ¹⁵N) for plants in an ecosystem can indicate divergent strategies for N uptake. We tested the hypotheses that the N‐rich seasonally dry forest would have higher soil and leaf δ¹⁵N and a smaller range of leaf δ¹⁵N values compared to the N‐poor cerradão (savanna woodland). We measured N concentration and δ¹⁵N in two soil depths and leaves of 27 woody species in cerradão and 26 in seasonally dry forest. As expected, total soil N concentration decreased while soil δ¹⁵N value increased with soil depth. Regardless of soil depth, seasonally dry forest soils had higher δ¹⁵N and total N concentration compared to cerradão soils. Foliar δ¹⁵N values varied from ?6.4‰ to 5.9‰ in cerradão and from ?2.3‰ to 8.4‰ in seasonally dry forest plants. Phylogenetically independent contrasts analysis and comparisons of δ¹⁵N mean values of the most abundant species and species co‐occurring in both sites confirmed the hypothesis of higher δ¹⁵N for seasonally dry forest in comparison to cerradão. These results corroborate the expectation of higher soil and leaf δ¹⁵N values in sites with higher soil N availability. However, except for the most abundant species, no across‐site leaf–soil (δ¹⁵N leaf –δ¹⁵N soil) differences (Δδ¹⁵N) were found suggesting that differences in leaf δ¹⁵N between cerradão and seasonally dry forest are driven by differences in soil δ¹⁵N. Variation of leaf δ¹⁵N was large in both sites and only slightly higher in cerradão, suggesting high diversity of N use strategies for both cerradão and seasonally dry forest communities.     

Fire affects the taxonomic and functional composition of soil microbial communities,with cascading effects on grassland ecosystem functioning

Fire is a crucial event regulating the structure and functioning of many ecosystems. Yet few studies have focused on how fire affects taxonomic and functional diversities of soil microbial communities, along with changes in plant communities and soil carbon (C) and nitrogen (N) dynamics. Here, we analyze these effects in a grassland ecosystem 9 months after an experimental fire at the Jasper Ridge Global Change Experiment site in California, USA. Fire altered soil microbial communities considerably, with community assembly process analysis showing that environmental selection pressure was higher in burned sites. However, a small subset of highly connected taxa was able to withstand the disturbance. In addition, fire decreased the relative abundances of most functional genes associated with C degradation and N cycling, implicating a slowdown of microbial processes linked to soil C and N dynamics. In contrast, fire stimulated above‐ and belowground plant growth, likely enhancing plant–microbe competition for soil inorganic N, which was reduced by a factor of about 2. To synthesize those findings, we performed structural equation modeling, which showed that plants but not microbial communities were responsible for significantly higher soil respiration rates in burned sites. Together, our results demonstrate that fire ‘reboots’ the grassland ecosystem by differentially regulating plant and soil microbial communities, leading to significant changes in soil C and N dynamics.     

Combustion influences on natural abundance nitrogen isotope ratio in soil and plants following a wildfire in a sub-alpine ecosystem

This before-and-after-impact study uses the natural abundance N isotope ratio (δ¹⁵N) to investigate the effects of a wildfire on sub-alpine ecosystem properties and processes. We measured the ¹⁵N signatures of soil, charred organic material, ash and foliage in three sub-alpine plant communities (grassland, heathland and woodland) in south-eastern Australia. Surface bulk soil was temporarily enriched in ¹⁵N immediately after wildfire compared to charred organic material and ash in all plant communities. We associated the enrichment of bulk soil with fractionation of N during combustion and volatilization of N, a process that also explains the sequential enrichment of ¹⁵N of unburnt leaves > ash > charred organic material in relation to duration and intensity of heating. The rapid decline in ¹⁵N of bulk soil to pre-fire values indicates that depleted ash, containing considerable amounts of total N, was readily incorporated into the soil. Foliar δ¹⁵N also increased with values peaking 1 year post-fire. Foliar enrichment was foremost coupled with the release of enriched NH₄ ⁺ into the soil owing to isotopic discrimination during volatilization of soluble N and combustion of organic material. The mode of post-fire regeneration influenced foliar ¹⁵N enrichment in two species indicating use of different sources of N following fire. The use of natural abundance of ¹⁵N in soil, ash and foliage as a means of tracing transformation of N during wildfire has established the importance of combustion products as an important, albeit temporary source of inorganic N for plants regenerating after wildfire.     

Post-Fire Spatial Patterns of Soil Nitrogen Mineralization and Microbial Abundance

Stand-replacing fires influence soil nitrogen availability and microbial community composition, which may in turn mediate post-fire successional dynamics and nutrient cycling. However, fires create patchiness at both local and landscape scales and do not result in consistent patterns of ecological dynamics. The objectives of this study were to (1) quantify the spatial structure of microbial communities in forest stands recently affected by stand-replacing fire and (2) determine whether microbial variables aid predictions of in situ net nitrogen mineralization rates in recently burned stands. The study was conducted in lodgepole pine (Pinus contorta var. latifolia) and Engelmann spruce/subalpine fir (Picea engelmannii/Abies lasiocarpa) forest stands that burned during summer 2000 in Greater Yellowstone (Wyoming, USA). Using a fully probabilistic spatial process model and Bayesian kriging, the spatial structure of microbial lipid abundance and fungi-to-bacteria ratios were found to be spatially structured within plots two years following fire (for most plots, autocorrelation range varied from 1.5 to 10.5 m). Congruence of spatial patterns among microbial variables, in situ net N mineralization, and cover variables was evident. Stepwise regression resulted in significant models of in situ net N mineralization and included variables describing fungal and bacterial abundance, although explained variance was low (R²<0.29). Unraveling complex spatial patterns of nutrient cycling and the biotic factors that regulate it remains challenging but is critical for explaining post-fire ecosystem function, especially in Greater Yellowstone, which is projected to experience increased fire frequencies by mid 21^st Century.     

Annual burning of a tallgrass prairie inhibits C and N cycling in soil,increasing recalcitrant pyrogenic organic matter storage while reducing N availability

Grassland ecosystems store an estimated 30% of the world's total soil C and are frequently disturbed by wildfires or fire management. Aboveground litter decomposition is one of the main processes that form soil organic matter (SOM). However, during a fire biomass is removed or partially combusted and litter inputs to the soil are substituted with inputs of pyrogenic organic matter (py‐OM). Py‐OM accounts for a more recalcitrant plant input to SOM than fresh litter, and the historical frequency of burning may alter C and N retention of both fresh litter and py‐OM inputs to the soil. We compared the fate of these two forms of plant material by incubating ¹³C‐ and ¹⁵N‐labeled Andropogon gerardii litter and py‐OM at both an annually burned and an infrequently burned tallgrass prairie site for 11 months. We traced litter and py‐OM C and N into uncomplexed and organo‐mineral SOM fractions and CO₂ fluxes and determined how fire history affects the fate of these two forms of aboveground biomass. Evidence from CO₂ fluxes and SOM C:N ratios indicates that the litter was microbially transformed during decomposition while, besides an initial labile fraction, py‐OM added to SOM largely untransformed by soil microbes. Additionally, at the N‐limited annually burned site, litter N was tightly conserved. Together, these results demonstrate how, although py‐OM may contribute to C and N sequestration in the soil due to its resistance to microbial degradation, a long history of annual removal of fresh litter and input of py‐OM infers N limitation due to the inhibition of microbial decomposition of aboveground plant inputs to the soil. These results provide new insight into how fire may impact plant inputs to the soil, and the effects of py‐OM on SOM formation and ecosystem C and N cycling.     

Effects of Formica podzolica ant colonies on soil moisture,nitrogen, and plant communities near nests

1. Ants are widely regarded as ‘ecosystem engineers’ because their nest construction and contributions to nutrient cycling change the biological, chemical, and physical properties of the soil around their nests. Despite increasing attention to ant manipulation of soil ecosystems, the extent to which many common species influence soil properties, as well as nutrient uptake and community composition of plants near nests, is still unknown. 2. This study tested hypotheses that activities of a common subalpine ant, Formica podzolica, alter soil moisture and pH, redistribute nitrogen around nests, and affect plant species abundance and ground cover. 3. A combination of field sampling techniques showed that distance from a nest had a positive relationship with soil moisture and a negative relationship with plant abundance next to and downhill from nests. Slope aspect also affected plant communities, with downhill transects having higher plant cover and above‐ground biomass than uphill transects. A stable isotope analysis did not reveal that plants near nests had enriched ¹⁵N, but there were substantial differences in ¹⁵N among sites. 4. Overall, this study uncovers significant impacts of F. podzolica on the subalpine microhabitats directly surrounding their nests.     

Carbon (δ13C) and nitrogen (δ15N) stable isotope composition in plant and soil in Southern Patagonia's native forests

Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ¹³C and δ¹⁵N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ¹³C values ranged from ?35.4‰ to ?27.7‰, and correlated positively with foliar δ¹⁵N values, ranging from ?3.7‰ to 5.2‰. Soil δ¹³C and δ¹⁵N values reflected the isotopic trends of the foliar tissues and ranged from ?29.8‰ to ?25.3‰, and ?4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ¹³C and δ¹⁵N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ¹³C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ¹⁵N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems.     

Sources of increased N uptake in forest trees growing under elevated CO2: results of a large‐scale 15N study

Nitrogen availability in terrestrial ecosystems strongly influences plant productivity and nutrient cycling in response to increasing atmospheric carbon dioxide (CO₂). Elevated CO₂ has consistently stimulated forest productivity at the Duke Forest free‐air CO₂ enrichment experiment throughout the decade‐long experiment. It remains unclear how the N cycle has changed with elevated CO₂ to support this increased productivity. Using natural‐abundance measures of N isotopes together with an ecosystem‐scale ¹⁵N tracer experiment, we quantified the cycling of ¹⁵N in plant and soil pools under ambient and elevated CO₂ over three growing seasons to determine how elevated CO₂ changed N cycling between plants, soil, and microorganisms. After measuring natural‐abundance ¹⁵N differences in ambient and CO₂‐fumigated plots, we applied inorganic ¹⁵N tracers and quantified the redistribution of ¹⁵N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO₂, consistent with deeper rooting and enhanced N mineralization. After tracer application, ¹⁵N was initially retained in the organic and mineral soil horizons. Recovery of ¹⁵N in plant biomass was 3.5 ± 0.5% in the canopy, 1.7 ± 0.2% in roots and 1.7 ± 0.2% in branches. After two growing seasons, ¹⁵N recoveries in biomass and soil pools were not significantly different between CO₂ treatments, despite greater total N uptake under elevated CO₂. After the third growing season, ¹⁵N recovery in trees was significantly higher in elevated compared to ambient CO₂. Natural‐abundance ¹⁵N and tracer results, taken together, suggest that trees growing under elevated CO₂ acquired additional soil N resources to support increased plant growth. Our study provides an integrated understanding of elevated CO₂ effects on N cycling in the Duke Forest and provides a basis for inferring how C and N cycling in this forest may respond to elevated CO₂ beyond the decadal time scale.     

Distribution of ecosystem C and N within contrasting vegetation types in a semiarid rangeland in the Great Basin,USA

Semiarid sagebrush ecosystems are being transformed by wildfire, rangeland improvement techniques, and exotic plant invasions, but the effects on ecosystem C and N dynamics are poorly understood. We compared ecosystem C and N pools to 1 m depth among historically grazed Wyoming big sagebrush, introduced perennial crested wheatgrass, and invasive annual cheatgrass communities, to examine whether the quantity and quality of plant inputs to soil differs among vegetation types. Natural abundance δ¹⁵N isotope ratios were used to examine differences in ecosystem N balance. Sagebrush-dominated sites had greater C and N storage in plant biomass compared to perennial or annual grass systems, but this was predominantly due to woody biomass accumulation. Plant C and N inputs to soil were greatest for cheatgrass compared to sagebrush and crested wheatgrass systems, largely because of slower root turnover in perennial plants. The organic matter quality of roots and leaf litter (as C:N ratios) was similar among vegetation types, but lignin:N ratios were greater for sagebrush than grasses. While cheatgrass invasion has been predicted to result in net C loss and ecosystem degradation, we observed that surface soil organic C and N pools were greater in cheatgrass and crested wheatgrass than sagebrush-dominated sites. Greater biomass turnover in cheatgrass and crested wheatgrass versus sagebrush stands may result in faster rates of soil C and N cycling, with redistribution of actively cycled N towards the soil surface. Plant biomass and surface soil δ¹⁵N ratios were enriched in cheatgrass and crested wheatgrass relative to sagebrush-dominated sites. Source pools of plant available N could become ¹⁵N enriched if faster soil N cycling rates lead to greater N trace gas losses. In the absence of wildfire, if cheatgrass invasion does lead to degradation of ecosystem function, this may be due to faster nutrient cycling and greater nutrient losses, rather than reduced organic matter inputs.     

Woody Plant Encroachment by <Emphasis Type="Italic">Juniperus virginiana</Emphasis> in a Mesic Native Grassland Promotes Rapid Carbon and Nitrogen Accrual

The cover and abundance of Juniperus virginiana L. in the U.S. Central Plains are rapidly increasing, largely as a result of changing land-use practices that alter fire regimes in native grassland communities. Little is known about how conversion of native grasslands to Juniperus-dominated forests alters soil nutrient availability and ecosystem storage of carbon (C) and nitrogen (N), although such land-cover changes have important implications for local ecosystem dynamics, as well as regional C and N budgets. Four replicate native grasslands and adjacent areas of recent J. virginiana encroachment were selected to assess potential changes in soil N availability, leaf-level photosynthesis, and major ecosystem C and N pools. Net N mineralization rates were assessed in situ over two years, and changes in labile soil organic pools (potential C and N mineralization rates and microbial biomass C and N) were determined. Photosynthetic nitrogen use efficiencies (PNUE) were used to examine differences in instantaneous leaf-level N use in C uptake. Comparisons of ecosystem C and N stocks revealed significant C and N accrual in both plant biomass and soils in these newly established forests, without changes in labile soil N pools. There were few differences in monthly in situ net N mineralization rates, although cumulative annual net N mineralization was greater in forest soils compared to grasslands. Conversely, potential C mineralization was significantly reduced in forest soils. Encroachment by J. virginiana into grasslands results in rapid accretion of ecosystem C and N in plant and soil pools with little apparent change in N availability. Widespread increases in the cover of woody plants, like J. virginiana, in areas formerly dominated by graminoid species suggest an increasing role of expanding woodlands and forests as regional C sinks in the central U.S.     

Resistance to wildfire and early regeneration in natural broadleaved forest and pine plantation

The response of an ecosystem to disturbance reflects its stability, which is determined by two components: resistance and resilience. We addressed both components in a study of early post-fire response of natural broadleaved forest (Quercus robur, Ilex aquifolium) and pine plantation (Pinus pinaster, Pinus sylvestris) to a wildfire that burned over 6000 ha in NW Portugal. Fire resistance was assessed from fire severity, tree mortality and sapling persistence. Understory fire resistance was similar between forests: fire severity at the surface level was moderate to low, and sapling persistence was low. At the canopy level, fire severity was generally low in broadleaved forest but heterogeneous in pine forest, and mean tree mortality was significantly higher in pine forest. Forest resilience was assessed by the comparison of the understory composition, species diversity and seedling abundance in unburned and burned plots in each forest type. Unburned broadleaved communities were dominated by perennial herbs (e.g., Arrhenatherum elatius) and woody species (e.g., Hedera hibernica, Erica arborea), all able to regenerate vegetatively. Unburned pine communities presented a higher abundance of shrubs, and most dominant species relied on post-fire seeding, with some species also being able to regenerate vegetatively (e.g., Ulex minor, Daboecia cantabrica). There were no differences in diversity measures in broadleaved forest, but burned communities in pine forest shared less species and were less rich and diverse than unburned communities. Seedling abundance was similar in burned and unburned plots in both forests. The slower reestablishment of understory pine communities is probably explained by the slower recovery rate of dominant species. These findings are ecologically relevant: the higher resistance and resilience of native broadleaved forest implies a higher stability in the maintenance of forest processes and the delivery of ecosystem services.     

Factors Regulating Nitrogen Retention During the Early Stages of Recovery from Fire in Coastal Chaparral Ecosystems

Fire is a fundamental reorganizing force in chaparral and other Mediterranean-type ecosystems. Postfire nutrient redistribution and cycling are frequently invoked as drivers of ecosystem recovery. The extent to which N is transported from slopes to streams following fire is a function of the balance between the rate at which soil microbes retain and metabolize N into forms that readily dissolve or leach, and how rapidly recovering plants sequester this mobilized N. To better understand how fire impacts this balance, we sampled soil and plant N dynamics in 17 plots distributed across two burned, chaparral-dominated watersheds in Santa Barbara County, California. We measured a variety of ecosystem properties in both burned and unburned plots on a periodic basis for 2 years, including soil water content, pH, soil and plant carbon and nitrogen, extractable inorganic nitrogen, dissolved organic nitrogen, and microbial biomass. In burned plots, nitrification was significantly enhanced relative to rates measured in unburned plots. Ephemeral herbs established quickly following the first postfire rain events. Aboveground plant biomass assimilated N commensurate with soil net mineralization, implying tight N cycling during the early stages of recovery. Microbial biomass N, on the other hand, remained low throughout the study. These findings highlight the importance of herbaceous species in conserving ecosystem nutrients as shrubs gradually recover.     

Short-term Nitrogen Fixation by Legume Seedlings and Resprouts After Fire in Mediterranean Old-fields

Fires may greatly alter the N budget of a plant community. During fire nitrogen is lost to the atmosphere. Although high light availability after fire promotes N₂-fixation, the presumably high soil N availability could limit N₂-fixation activity. The latter limitation might be particularly acute in legume seedlings compared with resprouts, which have immediate access to belowground stored carbon. We wished to learn whether early post-fire conditions were conducive to N₂-fixation in leguminous seedlings and resprouts in two types of grassland and in a shrubland and whether seedlings and resprouts incurred different amounts of N₂-fixation after fire. We set 18 experimental fires in early autumn on 6 plots, subsequently labelling 6 subplots (2 × 2 m²) in each community with ¹⁵NH₄⁺-N (99 atom % excess). For 9 post-fire months we measured net N mineralisation in the top 5 cm of soil and we calculated the fraction of legume N derived from the atmosphere (%Ndfa) in seedlings and resprouts. We used two independent estimates of the amounts of N derived from non-atmospheric sources in potentially N₂-fixing plants: mean soil pool abundance and the ¹⁵N-enrichment of non-legumes. Despite substantial soil net N mineralisation in all burned community types (about 2.6 g Nm⁻² during the first nine months after fire), the %Ndfa of various legume species was 52–99%. Legumes from both grasslands showed slightly higher N₂-fixation values than shrubland legumes. As grassland legumes grew in more belowground dense communities than shrubland legumes, we suggest that higher competition for soil resources in well established grass-resprouting communities may enhance the rate of N₂-fixation after fire. In contrast to our hypothesis, legume seedlings and resprouts from the three plant communities studied, had similar %Ndfa and apparently acquired most of their N from the atmosphere rather than from the soil.     

Nitrogen-15 signals of leaf-litter-soil continuum as a possible indicator of ecosystem nitrogen saturation by forest succession and N loads

Understanding forest carbon cycling responses to atmospheric N deposition is critical to evaluating ecosystem N dynamics. The natural abundance of ¹⁵N (??¹⁵N) has been suggested as an efficient and non-invasive tool to monitor N pools and fluxes. In this study, three successional forests in southern China were treated with four levels of N addition. In each treatment, we measured rates of soil N mineralization, nitrification, N₂O emission and inorganic N leaching as well as N concentration and ?? ¹⁵N of leaves, litters and soils. We found that foliar N concentration and ??¹⁵N were higher in the mature broadleaf forest than in the successional pine or mixed forests. Three-year continuous N addition did not change foliar N concentration, but significantly increased foliar ?? ¹⁵N (p < 0.05). Also, N addition decreased the ?? ¹⁵N of top soil in the N-poor pine and mixed forests and significantly increased that of organic and mineral soils in N-rich broadleaf forests (p < 0.05). In addition, the soil N₂O emission flux and inorganic N leaching rate increased with increasing N addition and were positively correlated with the ¹⁵N enrichment factor (?? _p/s) of forest ecosystems. Our study indicates that ?? ¹⁵N of leaf, litter and soil integrates various information on plant species, forest stand age, exogenous N input and soil N transformation and loss, which can be used to monitor N availability and N dynamics in forest ecosystems caused by increasing N deposition in the future.     

Soil nitrogen dynamics three years after a severe Araucaria–Nothofagus forest fire

Wildfires have shaped the biogeography of south Chilean Araucaria–Nothofagus rainforest vegetation patterns, but their impact on soil properties and associated nutrient cycling remains unclear. Nitrogen (N) availability shows a site‐specific response to wildfire events indicating the need for an increased understanding of underlying mechanisms that drive changes in soil N cycling. In this study, we selected unburned and burned sites in a large area of the National Park Tolhuaca that was affected by a stand‐replacing wildfire in February 2002. We conducted net N cycling flux measurements (net ammonification, net nitrification and net N mineralization assays) on soils sampled 3 years after fire. In addition, samples were physically fractionated and natural abundance of C and N, and ¹³C‐NMR analyses were performed. Results indicated that standing inorganic N pools were greater in the burned soil, but that no main differences in net N cycling fluxes were observed between unburned and burned sites. In both sites, net ammonification and net nitrification fluxes were low or negative, indicating N immobilization. Multiple linear regression analyses indicated that soil N cycling could largely be explained by two parameters: light fraction (LF) soil organic matter N content and aromatic Chemical Oxidation Resistant Carbon (COREC_arom), a relative measure for char. The LF fraction, a strong NH₄⁺ sink, decreased as a result of fire, while COREC_arom increased in the burned soil profile and stimulated NO₃^‐ production. The absence of increased total net nitrification might relate to a decrease in heterotrophic nitrification after wildfire. We conclude that (i) wildfire induced a shift in N transformation pathways, but not in total net N mineralization, and (ii) stable isotope measurements are a useful tool to assess post‐fire soil organic matter dynamics.     

Plant invasion alters nitrogen cycling by modifying the soil nitrifying community

Plant invasions have dramatic aboveground effects on plant community composition, but their belowground effects remain largely uncharacterized. Soil microorganisms directly interact with plants and mediate many nutrient transformations in soil. We hypothesized that belowground changes to the soil microbial community provide a mechanistic link between exotic plant invasion and changes to ecosystem nutrient cycling. To examine this possible link, monocultures and mixtures of exotic and native species were maintained for 4 years in a California grassland. Gross rates of nitrogen (N) mineralization and nitrification were quantified with ¹⁵N pool dilution and soil microbial communities were characterized with DNA‐based methods. Exotic grasses doubled gross nitrification rates, in part by increasing the abundance and changing the composition of ammonia‐oxidizing bacteria in soil. These changes may translate into altered ecosystem N budgets after invasion. Altered soil microbial communities and their resulting effects on ecosystem processes may be an invisible legacy of exotic plant invasions.     

Impacts of Soil Nitrogen and Carbon Additions on Forest Understory Communities with a Long Nitrogen Deposition History

Rates of nitrogen (N) deposition have been historically high throughout much of the northeastern United States; thus, understanding the legacy of these high N loads is important for maintaining forest productivity and resilience. Though many studies have documented plant invasions due to N deposition and associated impacts on ecosystems, less is known about whether invasive plants will continue to increase in dominance with further shifting nutrient regimes. Using soil N and carbon additions, we examined the impact of both increasing and decreasing soil N on native and invasive understory plant dynamics over 4 years in a northeastern deciduous forest with a long history of N deposition. Despite applying large quantities of N, we found no difference in soil nitrate (NO₃) or ammonium (NH₄ ⁺) pools in N addition plots over the course of the study. Indicative of the potential N saturation in these forest soils, resin-available NO₃ ^? and NH₄ ⁺ showed evidence that the added N was rapidly moving out of the soil in N addition plots. Accordingly, we also found that adding N to soil altered neither invasive nor native plant abundance, though adding N temporally increased invasive plant richness. Carbon additions decreased soil N availability seasonally, but did not alter the total percent cover of invasive or native plants. Rather than being suppressed by excess N availability, native plant species in this ecosystem are primarily inhibited by the invasive species, which now dominate this site. In conclusion, understory plant communities in this potentially N-saturated ecosystem may be buffered to future alterations in N availability.     

川西亚高山不同森林生态系统碳氮储量及其分配格局

森林采伐和恢复是影响森林碳氮储量的重要因素。以川西亚高山岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林为研究对象,采用样地调查和生物量实测的方法,研究了不同森林生态系统各组分碳、氮储量及其分配特征。结果表明岷江冷杉原始林、粗枝云杉阔叶林、天然次生林和粗枝云杉人工林生态系统碳储量分别为611.18、252.31、363.07 tC/hm~2和239.06 tC/hm~2;氮储量分别为16.44、12.11、15.48 tN/hm~2和8.92 tN/hm~2。恢复林分与原始林碳储量在土壤—植被的分配格局发生了变化,而氮储量未发生变化。岷江冷杉原始林以植被碳储量为主,恢复林分以土壤为主,氮储量均以土壤为主。乔木层碳储量分别占生态系统总储量的56.65%、17.63%、13.57%和22.05%,土壤层(0—80 cm)分别占32.03%、69.87%、76.20%和72.12%;土壤层氮储量占生态系统总储量的76.80%—92.58%。植物残体碳氮储量分别占生态系统总储量的4.40%—9.83%和2.94%—7.08%,林下植被所占比例最小。空间格局上,岷江冷杉原始林植被部分具有较高的碳储量,应进行保护。3种恢复林分具有较高的碳汇潜力,且地上/地下碳储量较低,表明其碳汇潜力尤其表现在地上部分。天然次生林利于土壤有机碳的积累,而人工林乔木层碳储量较高。     

The effect of fire on nutrients in a pine forest soil

The effect of a hot summer fire on soil nutrient contents in the upper 2 cm of Aleppo pine forest with a dense woody understory was studied from September 1985 to May 1986. In comparison with the adjacent unburned forest, total nitrogen decreased by 25% but available forms of nitrogen were much higher. In burned and unburned soils there was a similar trend to increase and decrease in NH ₄ ⁺ −N, However, while (NO ₂ ⁻ +NO ₃ ⁻ −N decreased in the unburned soil it rose rapidly in the burned ash soil. Total phosphorus increased by 300% after the fire but decreased again 2 months later. Also water-soluble P increased up to November and then decreased to the levels of the unburned soils. The same was true for electrical conductivity and pH, increasing immediately after the fire and then leveling off again. This increase in nutrient levels in the “ash soil” was reflected in the striking increase in shoot and root biomass and in the content of N, P, Mg, K, Ca, Zn and Fe in wheat and clover plants grown in pots in these soils. These nutrient levels were much higher in the wheat plants, which also produced 12 times more seeds in the “ash soil.” It seems that fire in these pine forests causes a short-term flush of the mineral elements in the upper “ash soil” layer which is reverted gradually via the herbaceous post-fire to the ecosystem.     

Inhibition of Nitrification Alters Carbon Turnover in the Patagonian Steppe

Human activities are altering biodiversity and the nitrogen (N) cycle, affecting terrestrial carbon (C) cycling globally. Only a few specialized bacteria carry out nitrification—the transformation of ammonium (NH ₄ ⁺ ) to nitrate (NO ₃ ⁻ ), in terrestrial ecosystems, which determines the form and mobility of inorganic N in soils. However, the control of nitrification on C cycling in natural ecosystems is poorly understood. In an ecosystem experiment in the Patagonian steppe, we inhibited autotrophic nitrification and measured its effects on C and N cycling. Decreased net nitrification increased total mineral N and NH ₄ ⁺ and reduced NO ₃ ⁻ in the soil. Plant cover (P < 0.05) and decomposition (P < 0.0001) decreased with inhibition of nitrification, in spite of increases in NH ₄ ⁺ availability. There were significant changes in the natural abundance of δ¹⁵N in the dominant vegetation when nitrification was inhibited suggesting that a switch occurred in the form of N (from NO ₃ ⁻ to NH ₄ ⁺ ) taken up by plants. Results from a controlled-condition experiment supported the field results by showing that the dominant plant species of the Patagonian steppe have a marked preference for nitrate. Our results indicate that nitrifying bacteria exert a major control on ecosystem functioning, and that the inhibition of nitrification results in significant alteration of the C cycle. The interactions between the C and N cycles suggest that rates of C cycling are affected not just by the amount of available N, but also by the relative availability for plant uptake of NH ₄ ⁺ and NO ₃ ⁻ .