Jorge Durán

Soil denitrification fluxes from three northeastern North American forests across a range of nitrogen deposition.

In northern forests, large amounts of missing N that dominate N balances at scales ranging from small watersheds to large regional drainage basins may be related to N-gas production by soil microbes. We measured denitrification rates in forest soils in northeastern North America along a N deposition gradient to determine whether N-gas fluxes were a significant fate for atmospheric N inputs and whether denitrification rates were correlated with N availability, soil O2 status, or forest type. We quantified N2 and N2O fluxes in the laboratory with an intact-core method and monitored soil O2, temperature and moisture in three forests differing in natural and anthropogenic N enrichment: Turkey Lakes Watershed, Ontario; Hubbard Brook Experimental Forest, New Hampshire; and Bear Brook Watershed, Maine (fertilized and reference plots in hardwood and softwood stands). Total N-gas flux estimates ranged from <1 in fertilized hardwood uplands at Bear Brook to >100xa0kgxa0Nxa0ha−1xa0year−1 in hardwood wetlands at Turkey Lakes. N-gas flux increased systematically with natural N enrichment from soils with high nitrification rates (Bear Brookxa0<xa0Hubbard Brookxa0<xa0Turkey Lakes) but did not increase in the site where N fertilizer has been added since 1989 (Bear Brook). Our results show that denitrification is an important and underestimated term (1–24xa0% of atmospheric N inputs) in N budgets of upland forests in northeastern North America, but it does not appear to be an important sink for elevated anthropogenic atmospheric N deposition in this region.

Winter climate change affects growing-season soil microbial biomass and activity in northern hardwood forests

Understanding the responses of terrestrial ecosystems to global change remains a major challenge of ecological research. We exploited a natural elevation gradient in a northern hardwood forest to determine how reductions in snow accumulation, expected with climate change, directly affect dynamics of soil winter frost, and indirectly soil microbial biomass and activity during the growing season. Soils from lower elevation plots, which accumulated less snow and experienced more soil temperature variability during the winter (and likely more freeze/thaw events), had less extractable inorganic nitrogen (N), lower rates of microbial N production via potential net N mineralization and nitrification, and higher potential microbial respiration during the growing season. Potential nitrate production rates during the growing season were particularly sensitive to changes in winter snow pack accumulation and winter soil temperature variability, especially in spring. Effects of elevation and winter conditions on N transformation rates differed from those on potential microbial respiration, suggesting that N-related processes might respond differently to winter climate change in northern hardwood forests than C-related processes.

Winter climate change effects on soil C and N cycles in urban grasslands

Despite growing recognition of the role that cities have in global biogeochemical cycles, urban systems are among the least understood of all ecosystems. Urban grasslands are expanding rapidly along with urbanization, which is expected to increase at unprecedented rates in upcoming decades. The large and increasing area of urban grasslands and their impact on water and air quality justify the need for a better understanding of their biogeochemical cycles. There is also great uncertainty about the effect that climate change, especially changes in winter snow cover, will have on nutrient cycles in urban grasslands. We aimed to evaluate how reduced snow accumulation directly affects winter soil frost dynamics, and indirectly greenhouse gas fluxes and the processing of carbon (C) and nitrogen (N) during the subsequent growing season in northern urban grasslands. Both artificial and natural snow reduction increased winter soil frost, affecting winter microbial C and N processing, accelerating C and N cycles and increasing soilxa0:xa0atmosphere greenhouse gas exchange during the subsequent growing season. With lower snow accumulations that are predicted with climate change, we found decreases in N retention in these ecosystems, and increases in N2 O and CO2 flux to the atmosphere, significantly increasing the global warming potential of urban grasslands. Our results suggest that the environmental impacts of these rapidly expanding ecosystems are likely to increase as climate change brings milder winters and more extensive soil frost.

Spatial variability of soil properties under Pinus canariensis canopy in two contrasting soil textures

Knowledge of the spatial pattern and scale of plant resources is important to aid in understanding the causes of this spatial pattern and their consequences on process at the population, community, and ecosystem levels. We tested whether the effect of individual plants on the soil properties beneath their canopies might be mediated by soil texture, since this soil property has great influence on the soil organic matter protection, the soil cation exchange capacity, and the nutrients diffusion rate. We hypothesize that variables directly related to organic matter (microbial biomass-N [MB-N] or dissolved organic-N [DON]), as well as soil nutrients interacting with soil secondary minerals (PO4-P and NH4-N), should more closely follow the plant canopy projection in sandy soils than loamy ones. We also expected a higher spatial range and dependence of NO3-N in sandy soils, although the spatial distribution should not necessarily be affected by the plant position. To test these hypotheses, we used square plots (8xa0mu2009×u20098xa0m or 6xa0mu2009×u20096xa0m) placed around isolated mature individuals of Pinus canariensis in both loamy and sandy soils in P. canariensis forests, with replicates in summer and winter. Spatial pattern and scale of MB-N, DON, and inorganic-N and -P were analyzed with geostatistical methods. In the summer sampling, all soil variables had lower spatial ranges in the loamy soil than the sandy soil. However, no clear trend was observed in the winter. The spatial dependence of NO3-N from the two sampling dates was higher for the sandy soil than the loamy soil. Kriged maps in the sandy soil revealed that the spatial distributions of the summer soil moisture, MB-N, DON, and PO4-P were all dependent on pine location. Our results suggested that the presence of P. canariensis individuals may be an important source of spatial heterogeneity in these forests. Soil texture may determine the magnitude of the pine canopy’s effect on the spatial distribution of chemical and biological soil properties when water content is scant, but it may have negligible effects under conditions of higher water availability.

Wildfire changes the spatial pattern of soil nutrient availability in Pinus canariensis forests

Abstract• Soil resources are heterogeneously distributed in terrestrial plant communities. This heterogeneity is important because it determines the availability of local soil resources. A forest fire may change the spatial distribution of soil nutrients, affecting nutrition and survival of colonizing plants. However, specific information on the effects of ecosystem disturbance on the spatial distribution of soil resources is scarce.• We hypothesized that, on a short-term basis, wildfire would change the spatial patterns of soil N and P availability. To test this hypothesis, we selected two Pinus canariensis forests burned in 2005 and 2000, respectively, and a third forest that was unburned since at least 1990 (unburned). We incubated ionic exchange membranes (IEMs) in replicated plots to estimate soil N and P availability and characterized the spatial pattern using SADIE (Spatial Analysis by Distance Indices).• Mineral N, NO3-N and PO4-P availability, and aggregation and cluster indices for all nutrients were higher in the 2005 wildfire plots than in the 2000 wildfire and unburned plots.• Our results suggest that surviving plants or new individuals becoming established in a burned area would find higher soil resources, but also higher small-scale heterogeneity in nutrients, which may have a major impact on the performance of individual plants and on the forest structure and dynamics.Résumé• Les ressources du sol sont distribuées de manière hétérogène dans les communautés végétales terrestres. Cette hétérogénéité est importante car elle détermine la disponibilité locale des ressources du sol. Un feu de forêt peut changer la répartition spatiale des éléments nutritifs du sol, affectant la nutrition et la survie des plantes colonisatrices. Cependant, des informations précises sur les effets des perturbations des écosystèmes sur la répartition spatiale des ressources du sol sont rares.• Nous avons émis l’hypothèse qu’à court terme, un feu de forêt pourrait modifier les modèles de répartition spatiale de disponibilité de N et P dans le sol. Pour tester cette hypothèse, nous avons sélectionné deux forêts de Pinus canariensis qui ont brûlé respectivement en 2005 et 2000, et une troisième forêt qui n’a brûlé depuis au moins 1990 (non brûlée). Nous avons incubé des membranes échangeuses d’ions (IEMs) dans plusieurs parcelles pour estimer la disponibilité du sol en N et P et nous avons caractérisé le modèle spatial en utilisant SADIE (Analyse spatiale en fonction d’indices de distance).• N-minéral, N-NO3, disponibilité en PO4-P, agrégation et indices de cluster ont été plus élevés dans les parcelles incendiées en 2005 que dans celles incendiées en 2000 et les parcelles non brûlées.• Nos résultats suggèrent que les plants survivants ou les plants en cours d’installation dans une zone brûlée, auront accès à des ressources plus abondantes, mais seront également confrontés à une hétérogénéité plus importante dans la disponibilité en éléments nutritifs. Cette dernière peut avoir un impact majeur sur la performance individuelle des plants et sur la structure et la dynamique forestières.

Soil Denitrification Fluxes in a Northern Hardwood Forest: The Importance of Snowmelt and Implications for Ecosystem N Budgets

Nitrogen (N) is the nutrient that most frequently limits the productivity of forest ecosystems. Understanding N cycling and forest response to altered N inputs and climate change is an ongoing research challenge. In several intensively studied forests in northeastern North America, well-characterized N inputs are not balanced by measured N losses, suggesting that an unmeasured N loss pathway such as denitrification may be important. We studied soil denitrification gas fluxes in northern hardwood forests at the Hubbard Brook long-term ecological research site in New Hampshire, USA, and found that denitrification in apparently oxic soils could account for N losses greater than half of annual atmospheric N inputs. Denitrification rates were strongly affected by elevation and season, with higher rates occurring at high elevation plots and during snowmelt. These results suggest that denitrification accounts for a major portion of the increasing amounts of “missing N” reported for this site, and that a significant amount of the anthropogenic N that enters terrestrial ecosystems in northeastern North America is returned to the atmosphere as N2. These dynamics are highly vulnerable to change, however, as soil moisture levels and conditions during snowmelt are changing rapidly along with climate.

Changes in net N mineralization rates and soil N and P pools in a pine forest wildfire chronosequence

The concern that climate change may increase fire frequency and intensity has recently heightened the interest in the effects of wildfires on ecosystem functioning. Although short-term fire effects on forest soils are well known, less information can be found on the long-term effects of wildfires on soil fertility. Our objective was to study the 17-year effect of wildfires on forest net mineralization rates and extractable inorganic nitrogen (N) and phosphorus (P) concentrations. We hypothesize that (1) burned forest stands should exhibit lower net mineralization rates than unburned ones; (2) these differences would be greatest during the growing season; (3) differences between soil variables might also be observed among plots from different years since the last fire; and (4) due to fire-resistant geochemical processes controlling P availability, this nutrient should recover faster than N. We used a wildfire chronosequence of natural and unmanaged Pinus canariensis forests in La Palma Island (Canary Islands). Soil samples were collected during winter and spring at 22 burned and unburned plots. We found significantly higher values for net N mineralization and extractable N pools in unburned plots. These differences were higher for the winter sampling date than for the spring sampling date. Unlike extractable N and N mineralization rates, extractable P levels of burned plots exhibited a gradual recovery over time after an initial decrease. These results demonstrate that P. canariensis forest soils showed low resilience after wildfires, especially for N, and that this disturbance might induce long-term changes in ecosystem functioning.

Changes in leaf nutrient traits in a wildfire chronosequence.

The effect of wildfire on ecosystem function is gaining interest since climate change is expected to increase fire frequency and intensity in many forest systems. Fire alters the nutritional status of forest ecosystems, affecting ecosystem function and productivity, but further studies evaluating changes in leaf nutrient traits induced by forest wildfires are still needed. We used a 17-year-old Pinus canariensis wildfire chronosequence to elucidate the nature of nutrient limitations in natural and unmanaged pine forest in the Canary Islands. Pine needles were sampled in winter and spring and analysed for N and P concentrations. As expected, we found the lowest leaf N and leaf P in recently burned plots. However, the leaf N:P ratio was higher in burned versus unburned plots, suggesting that the decrease in P availability due to the fire is larger than that of N. For all leaf traits and sampling dates, leaf trait values in burned plots matched those observed in unburned plots 17xa0years after a fire. The N:P ratio found in P. canariensis needles was one of the lowest values reported in the literature for woody species, and suggests that all pine trees in the chronosequence are unambiguously limited by low N availability. Our results show that these N-limited pine forests retained N more efficiently than P 4xa0years after a wildfire; however, leaf N recovery is slower than P recovery, suggesting that the mechanisms responsible for pine N limitation operate continuously in these forests.

Nitrogen supply modulates the effect of changes in drying-rewetting frequency on soil C and N cycling and greenhouse gas exchange

Climate change and atmospheric nitrogen (N) deposition are two of the most important global change drivers. However, the interactions of these drivers have not been well studied. We aimed to assess how the combined effect of soil N additions and more frequent soil drying-rewetting events affects carbon (C) and N cycling, soil:atmosphere greenhouse gas (GHG) exchange, and functional microbial diversity. We manipulated the frequency of soil drying-rewetting events in soils from ambient and N-treated plots in a temperate forest and calculated the Orwin & Wardle Resistance index to compare the response of the different treatments. Increases in drying-rewetting cycles led to reductions in soil NO3- levels, potential net nitrification rate, and soilxa0:xa0atmosphere GHG exchange, and increases in NH4+ and total soil inorganic N levels. N-treated soils were more resistant to changes in the frequency of drying-rewetting cycles, and this resistance was stronger for C- than for N-related variables. Both the long-term N addition and the drying-rewetting treatment altered the functionality of the soil microbial population and its functional diversity. Our results suggest that increasing the frequency of drying-rewetting cycles can affect the ability of soil to cycle C and N and soilxa0:xa0atmosphere GHG exchange and that the response to this increase is modulated by soil N enrichment.

Long-term decrease of organic and inorganic nitrogen concentrations due to pine forest wildfire

Abstract• Growing concerns about fires and the increase of fire frequency and severity due to climate change have stimulated a large number of scientific papers about fire ecology. Most researchers have focused on the short-term effects of fire, and the knowledge about the long-term consequences of fires on ecosystem nutrient dynamics is still scarce.• Our aim was to improve the existing knowledge about the long-term effects of wildfires on forestlabile N concentrations. We hypothesized that fires may cause an initial decline in organic and inorganic N availability, and in the amount of microbial biomass-N; this should be followed by the recovery of pre-fire N concentrations on a long-term basis. We selected a fire chronosequence in Pinus canariensis forests on La Palma Island (Canary Islands, Spain). These forests are under low anthropogenic atmospheric deposition, and forest management is completely lacking; wildfires are therefore the only significant disturbance. Soil samples were collected during the winter and spring at 22 burned and unburned plots.• Fire produced a significant decrease in microbial biomass N, mineral N and dissolved organic N. Almost 20 y after fire, pre-fire levels of N concentrations had not recovered.• These results demonstrate that P. canariensis forest soils have a lower resilience against fire than expected. The magnitude of these observed changes suggests that pine forest wildfires may induce long-term (2 decades) changes in soil and in plant primary production.Résumé• Les préoccupations grandissantes au sujet des incendies, de l’augmentation de leur fréquence et de leur gravité attribuable aux changements climatiques ont stimulé la production d’un grand nombre d’articles scientifiques sur l’écologie des incendies. La plupart des chercheurs ont mis l’accent sur les effets à court terme de l’incendie, et les connaissances sur les conséquences à long terme des incendies sur la dynamique des éléments nutritifs de l’écosystème sont encore rares.• Notre objectif est d’améliorer les connaissances actuelles sur les effets à long terme des incendies sur les concentrations labiles d’azote en forêt. Nous avons émis l’hypothèse que les incendies peuvent provoquer une baisse initiale de l’azote organique et de la disponibilité de l’azote inorganique, et de la quantité de biomasse microbienne azotée, ce qui devrait être suivie par la récupération des concentrations d’azote d’avant le feu sur une base de long terme. Nous avons sélectionné une chronoséquence d’incendies dans des forêts de Pinus canariensis sur l’île de La Palma (îles Canaries, Espagne). Ces forêts sont situées sous de faibles dépôts atmosphériques d’origine anthropique, et la gestion des forêts est totalement absente ; les feux de forêt sont donc les seules perturbations importantes. Des échantillons de sol ont été recueillis au cours de l’hiver et du printemps dans 22 parcelles brûlées et non brûlées.• L’incendie a produit une diminution significative de la biomasse microbienne azotée, de l’azote minéral et de l’azote organique dissous. Presque 20 ans après l’incendie, les niveaux de concentrations d’azote d’avant le feu n’ont pas été récupérés.• Ces résultats montrent que les sols forestiers de P. canariensis ont une résilience contre le feu plus faible que prévu. L’ampleur des changements observés suggère que les feux dans les forêts de pin peuvent induire des changements à long terme (2 décades) dans les sols et dans la production primaire des plants.