Publications

<span class="bullet">1. </span>The effects of global change on below-ground processes of the nitrogen (N) cycle have repercussions for plant communities, productivity and trace gas effluxes. However, the interacting effects of different components of global change on nitrification or denitrification have rarely been studied <em>in situ</em>. <span class="bullet">2. </span>We measured responses of nitrifying enzyme activity (NEA) and denitrifying enzyme activity (DEA) to over 4 years of exposure to several components of global change and their interaction (increased atmospheric CO₂ concentration, temperature, precipitation and N addition) at peak biomass period in an annual grassland ecosystem. In order to provide insight into the mechanisms controlling the response of NEA and DEA to global change, we examined the relationships between these activities and soil moisture, microbial biomass C and N, and soil extractable N. <span class="bullet">3. </span>Across all treatment combinations, NEA was decreased by elevated CO₂ and increased by N addition. While elevated CO₂ had no effect on NEA when not combined with other treatments, it suppressed the positive effect of N addition on NEA in all the treatments that included N addition. We found a significant CO₂–N interaction for DEA, with a positive effect of elevated CO₂ on DEA only in the treatments that included N addition, suggesting that N limitation of denitrifiers may have occurred in our system. Soil water content, extractable N concentrations and their interaction explained 74% of the variation in DEA. <span class="bullet">4. </span>Our results show that the potentially large and interacting effects of different components of global change should be considered in predicting below-ground N responses of Mediterranean grasslands to future climate changes. &nbsp;

Elevated atmospheric carbon dioxide (CO₂) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long-term CO₂ exposure. We investigated the effects of elevated CO₂ on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO₂ enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO₂ would stimulate root growth; and (3) elevated CO₂ would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm⁻²) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO₂ significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO₂ stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO₂ had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO₂ hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient-poor soil – may explain the transient response to elevated CO₂.

Free air carbon dioxide enrichment (FACE) and open top chamber (OTC) studies are valuable tools for evaluating the impact of elevated atmospheric CO₂ on nutrient cycling in terrestrial ecosystems. Using meta-analytic techniques, we summarized the results of 117 studies on plant biomass production, soil organic matter dynamics and biological N₂ fixation in FACE and OTC experiments. The objective of the analysis was to determine whether elevated CO₂ alters nutrient cycling between plants and soil and if so, what the implications are for soil carbon (C) sequestration. Elevated CO₂ stimulated gross N immobilization by 22%, whereas gross and net N mineralization rates remained unaffected. In addition, the soil C : N ratio and microbial N contents increased under elevated CO₂ by 3.8% and 5.8%, respectively. Microbial C contents and soil respiration increased by 7.1% and 17.7%, respectively. Despite the stimulation of microbial activity, soil C input still caused soil C contents to increase by 1.2% yr⁻¹. Namely, elevated CO₂ stimulated overall above- and belowground plant biomass by 21.5% and 28.3%, respectively, thereby outweighing the increase in CO₂ respiration. In addition, when comparing experiments under both low and high N availability, soil C contents (+2.2% yr⁻¹) and above- and belowground plant growth (+20.1% and+33.7%) only increased under elevated CO₂ in experiments receiving the high N treatments. Under low N availability, above- and belowground plant growth increased by only 8.8% and 14.6%, and soil C contents did not increase. Nitrogen fixation was stimulated by elevated CO₂ only when additional nutrients were supplied. These results suggest that the main driver of soil C sequestration is soil C input through plant growth, which is strongly controlled by nutrient availability. In unfertilized ecosystems, microbial N immobilization enhances acclimation of plant growth to elevated CO₂ in the long-term. Therefore, increased soil C input and soil C sequestration under elevated CO₂ can only be sustained in the long-term when additional nutrients are supplied.

Stable isotope analysis is a powerful tool in the study of soil organic matter formation. It is often observed that more decomposed soil organic matter is ¹³C, and especially ¹⁵N-enriched relative to fresh litter and recent organic matter. We investigated whether this shift in isotope composition relates to the isotope composition of the microbial biomass, an important source for soil organic matter. We developed a new approach to determine the natural abundance C and N isotope composition of the microbial biomass across a broad range of soil types, vegetation, and climates. We found consistently that the soil microbial biomass was ¹⁵N-enriched relative to the total (3.2 ‰) and extractable N pools (3.7 ‰), and ¹³C-enriched relative to the extractable C pool (2.5 ‰). The microbial biomass was also ¹³C-enriched relative to total C for soils that exhibited a C3-plant signature (1.6 ‰), but ¹³C-depleted for soils with a C4 signature (−1.1 ‰). The latter was probably associated with an increase of annual C3 forbs in C4 grasslands after an extreme drought. These findings are in agreement with the proposed contribution of microbial products to the stabilized soil organic matter and may help explain the shift in isotope composition during soil organic matter formation.

The availability of C and N to the soil microbial biomass is an important determinant of the rates of soil N transformations. Here, we present evidence that changes in C and N availability affect the ¹⁵N natural abundance of the microbial biomass relative to other soil N pools. We analysed the ¹⁵N natural abundance signature of the chloroform-labile, extractable, NO₃^–, NH₄⁺ and soil total N pools across a cattle manure gradient associated with a water reservoir in semiarid, high-desert grassland. High levels of C and N in soil total, extractable, NO₃^–, NH₄⁺ and chloroform-labile fractions were found close to the reservoir. The δ¹⁵N value of chloroform-labile N was similar to that of extractable (organic + inorganic) N and NO₃^– at greater C availability close to the reservoir, but was ¹⁵N-enriched relative to these N-pools at lesser C availability farther away. Possible mechanisms for this variable ¹⁵N-enrichment include isotope fractionation during N assimilation and dissimilation, and changes in substrate use from a less to a more ¹⁵N-enriched substrate with decreasing C availability.

In Alaska, an outbreak of spruce beetles (<em>Dendroctonus rufipennis</em>) recently infested over one million hectares of spruce (<em>Picea</em> spp.) forest. As a result, land management agencies have applied different treatments to infested forests to minimize fire hazard and economic loss and facilitate forest regeneration. In this study we investigated the effects of high-intensity burning, whole-tree harvest, whole-tree harvest with nitrogen (N) fertilization, and conventional harvest of beetle-killed stands 4 years after treatment, as well as clear-cut salvage harvest 6 years after treatment. We measured available soil ammonium and nitrate and estimated N loss from leaching using in situ cation and anion resin exchange capsules. We also assessed spruce regeneration and responses of understory plant species. Availability and losses of N did not differ among any of the management treatments. Even a substantial application of N fertilizer had no effect on N availability. Spruce regeneration significantly increased after high-intensity prescribed burning, with the number of seedlings averaging 8.9 m⁻² in burn plots, as compared to 0.1 m⁻² in plots that did not receive treatment. Biomass of the pervasive grass bluejoint (<em>Calamagrostis canadensis</em>) was significantly reduced by burning, with burn plots having 9.5% of the <em>C. canadensis</em> biomass of plots that did not receive treatment. N fertilization doubled <em>C. canadensis</em> biomass, suggesting that N fertilization without accompanying measures to control <em>C. canadensis</em> is the least viable method for promoting rapid spruce regeneration.

Experimentally increasing atmospheric CO₂ often stimulates plant growth and ecosystem carbon (C) uptake. Biogeochemical theory predicts that these initial responses will immobilize nitrogen (N) in plant biomass and soil organic matter, causing N availability to plants to decline, and reducing the long-term CO₂-stimulation of C storage in N limited ecosystems. While many experiments have examined changes in N cycling in response to elevated CO₂, empirical tests of this theoretical prediction are scarce. During seven years of postfire recovery in a scrub oak ecosystem, elevated CO₂ initially increased plant N accumulation and plant uptake of tracer ¹⁵N, peaking after four years of CO₂ enrichment. Between years four and seven, these responses to CO₂ declined. Elevated CO₂ also increased N and tracer ¹⁵N accumulation in the O horizon, and reduced ¹⁵N recovery in underlying mineral soil. These responses are consistent with progressive N limitation: the initial CO₂ stimulation of plant growth immobilized N in plant biomass and in the O horizon, progressively reducing N availability to plants. Litterfall production (one measure of aboveground primary productivity) increased initially in response to elevated CO₂, but the CO₂ stimulation declined during years five through seven, concurrent with the accumulation of N in the O horizon and the apparent restriction of plant N availability. Yet, at the level of aboveground plant biomass (estimated by allometry), progressive N limitation was less apparent, initially because of increased N acquisition from soil and later because of reduced N concentration in biomass as N availability declined. Over this seven-year period, elevated CO₂ caused a redistribution of N within the ecosystem, from mineral soils, to plants, to surface organic matter. In N limited ecosystems, such changes in N cycling are likely to reduce the response of plant production to elevated CO₂.

The amount of carbon plants allocate to mycorrhizal symbionts exceeds that emitted by human activity annually. Senescent ectomycorrhizal roots represent a large input of carbon into soils, but their fate remains unknown. Here, we present the surprising result that, despite much higher nitrogen concentrations, roots colonized by ectomycorrhizal (EM) fungi lost only one-third as much carbon as non-mycorrhizal roots after 2 years of decomposition in a piñon pine (<em>Pinus edulis</em>) woodland. Experimentally excluding live mycorrhizal hyphae from litter, we found that live mycorrhizal hyphae may alter nitrogen dynamics, but the afterlife (litter-mediated) effects of EM fungi outweigh the influences of live fungi on root decomposition. Our findings indicate that a shift in plant allocation to mycorrhizal fungi could promote carbon accumulation in soil by this pathway. Furthermore, EM litters could directly contribute to the process of stable soil organic matter formation, a mechanism that has eluded soil scientists.

<div class="page" title="Page 1"> <div class="layoutArea"> <div class="column"> 1. We examined the relative importance of litter quality and stream characteristics in determining decomposition rate and the macroinvertebrate assemblage living on autumn- shed leaves. 2. We compared the decomposition rates of five native riparian tree species (Populus fremontii, Alnus oblongifolia, Platanus wrightii, Fraxinus velutina and Quercus gambelii) across three south-western streams in the Verde River catchment (Arizona, U.S.A.). We also compared the decomposition of three- and five-species mixtures to that of single species to test whether plant species diversity affects rate. 3. Decomposition rate was affected by both litter quality and stream. However, litter quality accounted for most of the variation in decomposition rates. The relative importance of litter quality decreased through time, explaining 97% of the variation in the first week but only 45% by week 8. We also found that leaf mixtures decomposed more quickly than expected, when all the species included were highly labile or when the stream environment led to relatively fast decomposition. 4. In contrast to decomposition rate, differences in the invertebrate assemblage were more pronounced across streams than across leaf litter species within a stream. We also found significant differences between the invertebrate assemblage colonising leaf mixtures compared with that colonising pure species litter, indicating non-additive properties of litter diversity on stream invertebrates. 5. This study shows that leaf litter diversity has the capacity to affect in-stream decomposition rates and stream invertebrates, but that these effects depend on both litter quality and stream characteristics. </div> </div> </div>

<div class="page" title="Page 1"> <div class="layoutArea"> <div class="column"> Travertine deposits of calcium carbonate can dominate channel geomorphology in streams where travertine deposition creates a distinct morphology characterized by travertine terraces, steep waterfalls, and large pools. Algae and microorganisms can facilitate travertine deposition, but how travertine affects material and energy flow in stream ecosystems is less well understood. Nearly a century of flow diversion for hydropower production has decimated the natural travertine formations in Fossil Creek, Arizona. The dam will be decommissioned in 2005. Returning carbonate-rich spring water to the natural stream channel should promote travertine deposition. How will the recovery of travertine affect the ecology of the creek? To address this question, we compared primary production, decomposition, and the abundance and diversity of invertebrates and fish in travertine and riffle/run reaches of Fossil Creek, Arizona. We found that travertine supports higher primary productivity, faster rates of leaf litter decomposition, and higher species richness of the native invertebrate assemblage. Observations from snorkeling in the stream indicate that fish density is also higher in the travertine reach. We postulate that restoring travertine to Fossil Creek will increase stream productivity, rates of litter processing, and energy flow up the food web. Higher aquatic productivity could fundamentally shift the nature of the stream from a sink to a source of energy for the surrounding terrestrial landscape. </div> </div> </div>

<span class="paraNumber">[1]</span> Measuring the stable isotope composition of nitrous oxide (N₂O) evolved from soil could improve our understanding of the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N₂O formation in soil. However, interpretation of the isotopic data in N₂O is complicated by the lack of knowledge of fractionation parameters by different microbial processes responsible for N₂O production and consumption. Here we report isotopic enrichment for both nitrogen and oxygen isotopes in two stages of denitrification, N₂O production and N₂O reduction. We found that during both N₂O production and reduction, enrichments were higher for oxygen than nitrogen. For both elements, enrichments were larger for N₂O production stage than for N₂O reduction. During gross N₂O production, the ratio of <em>δ</em>¹⁸O-to-<em>δ</em>¹⁵N differed between soils, ranging from 1.6 to 2.7. By contrast, during N₂O reduction, we observed a constant ratio of <em>δ</em>¹⁸O-to-<em>δ</em>¹⁵N with a value near 2.5. If general, this ratio could be used to estimate the proportion of N₂O being reduced in the soil before escaping into the atmosphere. Because N₂O-reductase enriches N₂O in both isotopes, the global reduction of N₂O consumption by soil may contribute to the globally observed isotopic depletion of atmospheric N₂O.

<div data-canvas-width="401.6631666666664">The labeled atoms of carbon and nitrogen are widely used in biology, biochemistry, and soil science. Both radioactive (14C) and stable (12C, 13C,15N) isotopes are used as such labels. Recently, an increase in mass spec-trometry sensitivity made it possible to study natural ratios of carbon (13C/12C) and nitrogen (15N/14N) stableisotopes in various biological objects. Since most bio-logical processes discriminate isotopes, i.e., use lighterisotopes for fermentative reactions [10] and leave heavier isotopes in substrate, the study of isotopic com-positions of various habitats of organisms yields infor-mation on intensity and direction of biological pro-cesses. However, the interpretation of carbon and nitro-gen isotopic compositions in complex objects, such as soil, meets difficulties because of the far too great num-ber of factors that control isotope fractionation [2, 6,11, 13]. The forest soils of the Yenisei meridian signif-icantly vary in terms of environmental factors, and thisvariability opens up possibilities to reveal the most important factors that determine carbon and nitrogen isotopic compositions of soils. In this work, the distri-bution of carbon and nitrogen isotopes in the forestsoils of the Yenisei region of Siberia was investigated for the first time. It has been shown that these isotopes are good indicators of (a) the intensity of organic matter (OM) mineralization, (b) the contribution of nitrogen fixation to the nitrogen status of ecosystems, and (c) the</div> <div data-canvas-width="287.4446666666667">provision of ecosystems with moisture.</div>

<span class="paraNumber">[1]</span> Nitrous oxide (N₂O) is an important greenhouse gas and participates in the destruction of stratospheric ozone. Soil bacteria produce N₂O through denitrification and nitrification, but these processes differ radically in substrate requirements and responses to the environment. Understanding the controls over N₂O efflux from soils, and how N₂O emissions may change with climate warming and altered precipitation, require quantifying the relative contributions from these groups of soil bacteria to the total N₂O flux. Here we used ammonium nitrate (NH₄NO₃, including substrates for both processes) in which the nitrate has been enriched in the stable isotope of oxygen, ¹⁸O, to partition microbial sources of N₂O, arguing that a molecule of N₂O carrying the ¹⁸O labeled will have been produced by denitrification. We compared the influences of six common tree species on the relative contributions of nitrification and denitrification to N₂O flux from soils, using soils from the Siberian afforestation experiment. We also altered soil water content, to test whether denitrification becomes a dominant source of N₂O when soil water content increases. Tree species altered the proportion of nitrifier and denitrifier-derived N₂O. Wetter soils produced more N₂O from denitrification, though the magnitude of this effect varied among tree species. This indicates that the roles of denitrification and nitrification vary with tree species, and, that tree species influence soil responses to increased water content.

<span class="hlFld-Abstract">Interactions involving carbon (C) and nitrogen (N) likely modulate terrestrial ecosystem responses to elevated atmospheric carbon dioxide (CO₂) levels at scales from the leaf to the globe and from the second to the century. In particular, response to elevated CO₂ may generally be smaller at low relative to high soil N supply and, in turn, elevated CO₂ may influence soil N processes that regulate N availability to plants. Such responses could constrain the capacity of terrestrial ecosystems to acquire and store C under rising elevated CO₂ levels. This review highlights the theory and empirical evidence behind these potential interactions. We address effects on photosynthesis, primary production, biogeochemistry, trophic interactions, and interactions with other resources and environmental factors, focusing as much as possible on evidence from long-term field experiments.</span>

Rising levels of atmospheric CO₂ are thought to increase C sinks in terrestrial ecosystems. The potential of these sinks to mitigate CO₂ emissions, however, may be constrained by nutrients. By using metaanalysis, we found that elevated CO₂ only causes accumulation of soil C when N is added at rates well above typical atmospheric N inputs. Similarly, elevated CO₂ only enhances N₂ fixation, the major natural process providing soil N input, when other nutrients (e.g., phosphorus, molybdenum, and potassium) are added. Hence, soil C sequestration under elevated CO₂ is constrained both directly by N availability and indirectly by nutrients needed to support N₂ fixation.