Temperature influences carbon accumulation in moist tropical forests

Evergreen broad-leaved tropical forests can have high rates of productivity and large accumulations of carbon in plant biomass and soils. They can therefore play an important role in the global carbon cycle, influencing atmospheric CO2 concentrations if climate warms. We applied meta-analyses to published data to evaluate the apparent effects of temperature on carbon fluxes and storages in mature, moist tropical evergreen forest ecosystems. Among forests, litter production, tree growth, and belowground carbon allocation all increased significantly with site mean annual temperature (MAT); total net primary productivity (NPP) increased by an estimated 0.2-0.7 Mg C x ha(-1) x yr(-1) x degrees C(-1). Temperature had no discernible effect on the turnover rate of aboveground forest biomass, which averaged 0.014 yr(-1) among sites. Consistent with these findings, forest biomass increased with site MAT at a rate of 5-13 Mg C x ha(-1) x degrees C(-1). Despite greater productivity in warmer forests, soil organic matter accumulations decreased with site MAT, with a slope of -8 Mg C x ha(-1) x degrees C(-1), indicating that decomposition rates of soil organic matter increased with MAT faster than did rates of NPP. Turnover rates of surface litter also increased with temperature among forests. We found no detectable effect of temperature on total carbon storage among moist-tropical evergreen forests, but rather a shift in ecosystem structure, from low-biomass forests with relatively large accumulations of detritus in cooler sites, to large-biomass forests with relatively smaller detrital stocks in warmer locations. These results imply that, in a warmer climate, conservation of forest biomass will be critical to the maintenance of carbon stocks in moist tropical forests.     

Necromass production: studies in undisturbed and logged Amazon forests.

Necromass stocks account for up to 20% of carbon stored in tropical forests and have been estimated to be 14-19% of the annual aboveground carbon flux. Both stocks and fluxes of necromass are infrequently measured. In this study, we directly measured the production of fallen coarse necromass (> or = 2 cm diameter) during 4.5 years using repeated surveys in undisturbed forest areas and in forests subjected to reduced-impact logging at the Tapajos National Forest, Belterra, Brazil (3.08 degrees S, 54.94 degrees W). We also measured fallen coarse necromass and standing dead stocks at two times during our study. The mean (SE) annual flux into the fallen coarse necromass pool in undisturbed forest of 6.7 (0.8) Mg x ha(-1) x yr(-1) was not significantly different from the flux under a reduced-impact logging of 8.5 (1.3) Mg x ha(-1) x yr(-1) With the assumption of steady state, the instantaneous decomposition constants for fallen necromass in undisturbed forests were 0.12 yr(-1) for large, 0.33 yr(-1) for medium, and 0.47 yr(-1) for small size classes. The mass weighted decomposition constant was 0.15 yr(-1) for all fallen coarse necromass. Standing dead wood had a residence time of 4.2 years, and approximately 0.9 Mg x ha(-1) x yr(-1) of this pool was respired annually to the atmosphere through decomposition. Coarse necromass decomposition at our study site accounted for 12% of total carbon remineralization, and total aboveground coarse necromass was 14% of the aboveground biomass. Use of mortality rates to calculate production of coarse necromass leads to an underestimation of coarse necromass production by 45%, suggesting that nonlethal disturbance such as branch fall contributes significantly to this flux. Coarse necromass production is an important component of the tropical forest carbon cycle that has been neglected in most previous studies or erroneously estimated.     

桂西北喀斯特区原生林与次生林凋落物及养分归还特征比较

森林凋落物是森林生态系统的重要组成成分,其养分归还量在一定程度上决定着土壤养分有效性的高低。在土层浅薄且土被很不连续的我国喀斯特区域进行凋落物生物量及养分归还研究对我们更深刻地了解该区养分循环具有至关重要的意义。本文比较分析了桂西北喀斯特区3种原生林与3种次生林的全年凋落物量、组成、月凋落物量动态及养分归还量与动态。结果表明,圆果化香(Platycarya longipes Wu)、大叶蚊母树(Distylium Sieb.et Zucc.)与青檀(Pteroceltis tatarinowii Maxim.)3种原生林的年凋落物总量分别为2342.16,4057.99和1834.36kg·hm-2,而圆叶乌桕(Sapium rotundifolium Hemsl.)、八角枫(Alangium chinense(Lour.) Harms)和黄荆(Vitex negundo L.)3种次生林的年凋落物总量分别为3192.82,3284.26,2469.90kg·hm-2,除大叶蚊母树外,次生林年凋落物总量大于原生林。凋落物的组成中,叶凋落生物量均占总凋落物量的80%左右,甚至更高,而圆叶乌桕、八角枫和黄荆3种次生林群落的叶凋落物量占总凋落物量的百分比大于圆果化香、大叶蚊母树以及青檀3种原生林。凋落物的养分归还量的月动态与凋落物量的月动态一致,原生林呈"U"形曲线,而次生林则呈"W"形曲线。原生林和次生林凋落物的年养分归还量均为C﹥N﹥K﹥P,且次生林的C、N、P养分的归还量大于原生林。     

The importance of agricultural lands for Himalayan birds in winter

The impacts of land‐use change on biodiversity in the Himalayas are poorly known, notwithstanding widespread deforestation and agricultural intensification in this highly biodiverse region. Although intact primary forests harbor many Himalayan birds during breeding, a large number of bird species use agricultural lands during winter. We assessed how Himalayan bird species richness, abundance, and composition during winter are affected by forest loss stemming from agriculture and grazing. Bird surveys along 12 elevational transects within primary forest, low‐intensity agriculture, mixed subsistence agriculture, and intensively grazed pastures in winter revealed that bird species richness and abundance were greatest in low‐intensity and mixed agriculture, intermediate in grazed pastures, and lowest in primary forest at both local and landscape scales; over twice as many species and individuals were recorded in low‐intensity agriculture than in primary forest. Bird communities in primary forests were distinct from those in all other land‐use classes, but only 4 species were unique to primary forests. Low‐, medium‐, and high‐intensity agriculture harbored 32 unique species. Of the species observed in primary forest, 80% had equal or greater abundance in low‐intensity agricultural lands, underscoring the value of these lands in retaining diverse community assemblages at high densities in winter. Among disturbed landscapes, bird species richness and abundance declined as land‐use intensity increased, especially in high‐intensity pastures. Our results suggest that agricultural landscapes are important for most Himalayan bird species in winter. But agricultural intensification—especially increased grazing—will likely result in biodiversity losses. Given that forest reserves alone may inadequately conserve Himalayan birds in winter, comprehensive conservation strategies in the region must go beyond protecting intact primary forests and ensure that low‐intensity agricultural lands are not extensively converted to high‐intensity pastures.     

Restoration of ecosystems for biodiversity and carbon sequestration: Simulating growth dynamics of brigalow vegetation communities in Australia

Restoration of abandoned and degraded ecosystems through enhanced management of mature remnant patches and naturally regenerating (regrowth) forests is currently being used in the recovery of ecosystems for biodiversity protection and carbon sequestration. Knowledge of long-term dynamics of these ecosystems is often very limited. Vegetation models that examine long-term forest growth and succession of uneven aged, mixed-species forest ecosystems are integral to the planning and assessment of the recovery process of biodiversity values and biomass accumulation. This paper examined the use of the Ecosystem Dynamics Simulator (EDS) in projecting growth dynamics of mature remnant brigalow forest communities and recovery process of regrowth brigalow thickets. We used data from 188 long-term monitored plots of remnant and regrowth forests measured between 1963 and 2010. In this study the model was parameterised for 34 tree and shrub species and tested with independent long-term measurements. The model closely approximated actual development trajectories of mature forests and regrowth thickets but some inaccuracies in estimating regeneration through asexual reproduction and mortality were noted as reflected in stem density projections of remnant plots that had a mean of absolute relative bias of 46.2 (±12.4)%. Changes in species composition in remnant forests were projected with a 10% error. Basal area values observed in all remnant plots ranged from 6 to 29 m² ha⁻¹ and EDS projections between 1966 and 2005 (39 years) were 68.2 (±10.9)% of the observed basal area. Projected live aboveground biomass of remnant plots had a mean of 93.5 (±5.9) t ha⁻¹ compared to a mean of 91.3 (±8.0) t ha⁻¹ observed in the plots. In regrowth thicket, the model produced satisfactory projections of tree density (91%), basal area (89%), height (87%) and aboveground biomass (84%) compared to the observed attributes. Basal area and biomass accumulation in 45-year-old regrowth plots was approximately similar to that in remnant forests but recovery of woody understorey was very slow. The model projected that it would take 95 years for the regrowth to thin down to similar densities observed in original or remnant brigalow forests. These results indicated that EDS can produce relatively accurate projections of growth dynamics of brigalow regrowth forests necessary for informing restoration planning and projecting biomass accumulation.     

Carbon sequestration in California agriculture, 1980-2000.

To better understand agricultural carbon fluxes in California, USA, we estimated changes in soil carbon and woody material between 1980 and 2000 on 3.6 x 10(6) ha of farmland in California. Combining the CASA (Carnegie-Ames-Stanford Approach) model with data on harvest indices and yields, we calculated net primary production, woody production in orchard and vineyard crops, and soil carbon. Over the 21-yr period, two trends resulted in carbon sequestration. Yields increased an average of 20%, corresponding to greater plant biomass and more carbon returned to the soils. Also, orchards and vineyards increased in area from 0.7 x 10(6) ha to 1.0 x 10(6) ha, displacing field crops and sequestering woody carbon. Our model estimates that California's agriculture sequestered an average of 19 g C x m(-2) x yr(-1). Sequestration was lowest in non-rice annual cropland, which sequestered 9 g C x m(-2) x yr(-1) of soil carbon, and highest on land that switched from annual cropland to perennial cropland. Land that switched from annual crops to vineyards sequestered 68 g C x m(-2) x yr(-1), and land that switched from annual crops to orchards sequestered 85 g C x m(-2) x yr(-1). Rice fields, because of a reduction in field burning, sequestered 55 g C x m(-2) x yr(-1) in the 1990s. Over the 21 years, California's 3.6 x 10(6) ha of agricultural land sequestered 11.0 Tg C within soils and 3.5 Tg C in woody biomass, for a total of 14.5 Tg C statewide. This is equal to 0.7% of the state's total fossil fuel emissions over the same time period. If California's agriculture adopted conservation tillage, changed management of almond and walnut prunings, and used all of its orchard and vineyard waste wood in the biomass power plants in the state, California's agriculture could offset up to 1.6% of the fossil fuel emissions in the state.     

Nutrient regulation of organic matter decomposition in a tropical rain forest

Terrestrial biosphere-atmosphere CO2 exchange is dominated by tropical forests, so understanding how nutrient availability affects carbon (C) decomposition in these ecosystems is central to predicting the global C cycle's response to environmental change. In tropical rain forests, phosphorus (P) limitation of primary production and decomposition is believed to be widespread, but direct evidence is rare. We assessed the effects of nitrogen (N) and P fertilization on litter-layer organic matter decomposition in two neighboring tropical rain forests in southwest Costa Rica that are similar in most ways, but that differ in soil P availability. The sites contain 100-200 tree species per hectare and between species foliar nutrient content is variable. To control for this heterogeneity, we decomposed leaves collected from a widespread neotropical species, Brosimum utile. Mass loss during decomposition was rapid in both forests, with B. utile leaves losing >80% of their initial mass in <300 days. High organic matter solubility throughout decomposition combined with high rainfall support a model of litter-layer decomposition in these rain forests in which rapid mass loss in the litter layer is dominated by leaching of dissolved organic matter (DOM) rather than direct CO2 mineralization. While P fertilization did not significantly affect mass loss in the litter layer, it did stimulate P immobilization in decomposing material, leading to increased P content and a lower C:P ratio in soluble DOM. In turn, increased P content of leached DOM stimulated significant increases in microbial mineralization of DOM in P-fertilized soil. These results show that, while nutrients may not affect mass loss during decomposition in nutrient-poor, wet ecosystems, they may ultimately regulate CO2 losses (and hence C storage) by limiting microbial mineralization of DOM leached from the litter layer to soil.     

Carbon sequestration of black locust forests in the Yellow River Delta region,China

The Yellow River Delta region in China is a land area of 1,200,000 ha with rich natural resources. Adverse environmental conditions, such as low rainfall and high salinity, promote the dominance of black locust trees for afforestation. With the increase of CO₂ in the atmosphere, this forest and others throughout the world have become valued for their ability to sequester and store carbon. Forests store carbon in aboveground biomass (i.e. trees), belowground biomass (i.e. roots), soils and standing litter crop (i.e. forest floor and coarse woody debris). There are well-developed methods to sample forest ecosystems, including tree inventories that are used to quantify carbon in aboveground tree biomass. Such inventories are used to estimate the types of roundwood products removed from the forest during harvesting. Based on standard plot inventories and stem analyses, carbon sequestration estimates of trees were 222.41 t ha^?1 for the Yellow River Delta region accounted for 67.12% of the whole forest. Similarly, carbon storage by herbaceous matter and soil was 0.50 and 50.34 t ha^?1, respectively. The results suggest that carbon sequestration in the forest ecosystem was performed by most of the forest, which plays an increasingly important role in sequestering carbon as the stand grows.     

Trade‐Offs between Cattle Production and Bird Conservation in an Agricultural Frontier of the Gran Chaco of Argentina

Intensification of food production in tropical landscapes in the absence of land‐use planning can pose a major threat to biological diversity. Decisions on whether to spatially integrate or segregate lands for production and conservation depend in part on the functional relations between biological diversity and agricultural productivity. We measured diversity, density, and species composition of birds along a gradient of production intensification on an agricultural frontier of the Argentine Chaco, where dry tropical forests are cleared for cattle production. Bird species diversity in intact forests was higher than in any type of cattle‐production system. Bird species richness decreased nonlinearly as cattle yield increased. Intermediate‐intensity silvopastoral systems, those in which forest understory is selectively cleared to grow pastures of non‐native plants beneath the tree canopy, produced 80% of the mean cattle yield obtained in pastures on cleared areas and were occupied by 70–90% of the number of bird species present in the nearest forest fragments. Densities of >50% of bird species were significantly lower in open pastures than in silvopastoral systems. Therefore, intermediate‐intensity silvopastoral systems may have the greatest potential to sustain cattle yield and conserve a large percentage of bird species. However, compared with low‐intensity production systems, in which forest structure and extent were intact, intermediate‐intensity silvopastoral systems supported significantly fewer forest‐restricted bird species and fewer frugivorous birds. These data suggest that the integration of production and conservation through intermediate‐intensity silvopastoral systems combined with the protection of forest fragments may be required to maintain cattle yield, bird diversity, and conservation of forest‐restricted species in this agricultural frontier. Compromisos entre la Producción de Ganado y la Conservación de Aves en una Frontera Agrícola del Gran Chaco de Argentina     

Nitrogen critical loads for alpine vegetation and terrestrial ecosystem response: are we there yet?

Increases in the deposition of anthropogenic nitrogen (N) have been linked to several terrestrial ecological changes, including soil biogeochemistry, plant stress susceptibility, and community diversity. Recognizing the need to identify sensitive indicators of biotic response to N deposition, we empirically estimated the N critical load for changes in alpine plant community composition and compared this with the estimated critical load for soil indicators of ecological change. We also measured the degree to which alpine vegetation may serve as a sink for anthropogenic N and how much plant sequestration is related to changes in species composition. We addressed these research goals by adding 20, 40, or 60 kg N x ha(-1) x yr(-1), along with an ambient control (6 kg N x ha(-1) x yr(-1) total deposition), to a species-rich alpine dry meadow for an eight-year period. Change in plant species composition associated with the treatments occurred within three years of the initiation of the experiment and were significant at all levels of N addition. Using individual species abundance changes and ordination scores, we estimated the N critical loads (total deposition) for (1) change in individual species to be 4 kg N x ha(-1) yr(-1) and (2) for overall community change to be 10 kg N x ha(-1) x yr(-1). In contrast, increases in NO3- leaching, soil solution inorganic NO3-, and net N nitrification occurred at levels above 20 kg N x ha(-1) x yr(-1). Increases in total aboveground biomass were modest and transient, occurring in only one of the three years measured. Vegetative uptake of N increased significantly, primarily as a result of increasing tissue N concentrations and biomass increases in subdominant species. Aboveground vegetative uptake of N accounted for <40% of the N added. The results of this experiment indicate that changes in vegetation composition will precede detectable changes in more traditionally used soil indicators of ecosystem responses to N deposition and that changes in species composition are probably ongoing in alpine dry meadows of the Front Range of the Colorado Rocky Mountains. Feedbacks to soil N cycling associated with changes in litter quality and species composition may result in only short-term increases in vegetation N pools.     

Changes in Forest Use Value through Ecological Succession and Their Implications for Land Management in the Peruvian Amazon

Abstract:  Research on local use values of forests across an ecological succession informs land-use decisions and conservation planning. I evaluated use values of three age classes of secondary forest: fallow fields (<5 years old, $8.20/ha/year), young secondary forest (5–20 years old, $20.60/ha/year), and old secondary forest (>20 years old, $6.80/ha/year). I quantified daily forest product use and calculated use values in dollars per hectare per year for three communities in the northern Peruvian Amazon. I made three comparisons between forest types: number of useful species, value based on different use categories, and overall use values. Old secondary forest had the greatest number of total species present and species collected. Wood, food, and medicine were the three most valuable use categories. The value different families extracted from local forests varied enormously, but median forest values were lower for all forest types than potential gains from agricultural land use (e.g., coffee $167/ha/year). Values of different-aged stands on privately owned lands in two communities did not differ significantly, whereas in the third community, young secondary forest had a significantly greater value than other forest types. Old secondary forests were the most valuable source of wood products, and wood was the only use category in which there was any difference in the value of products extracted from different-aged forest stands. The value of all three forest types on open-access (nonprivate) lands was minimal (mean in each forest type, $0/ha/year). Local people can utilize the valuation results to develop land-use strategies that balance forest product use, agricultural productivity, and biodiversity conservation.     

Conservation of Epiphyte Diversity in an Andean Landscape Transformed by Human Land Use

Abstract: Epiphytes are diverse and important elements of tropical forests, but as canopy‐dwelling organisms, they are highly vulnerable to deforestation. To assess the effect of deforestation on epiphyte diversity and the potential for epiphyte conservation in anthropogenically transformed habitats, we surveyed the epiphytic vegetation of an Ecuadorian cloud forest reserve and its surroundings. Our study was located on the western slopes of the Andes, a global center of biodiversity. We sampled vascular epiphytes of 110 study plots in a continuous primary forest; 14 primary forest fragments; isolated remnant trees in young, middle‐aged, and old pastures; and young and old secondary forests. It is the first study to include all relevant types of habitat transformation at a single study site and to compare epiphyte diversity at different temporal stages of fragmentation. Epiphyte diversity was highest in continuous primary forest, followed by forest fragments and isolated remnant trees, and lowest in young secondary forests. Spatial parameters of habitat transformation, such as fragment area, distance to the continuous primary forest, or distance to the forest edge from inside the forest, had no significant effect on epiphyte diversity. Hence, the influence of dispersal limitations appeared to be negligible or appeared to operate only over very short distances, whereas microclimatic edge effects acted only in the case of completely isolated trees, but not in larger forest fragments. Epiphyte diversity increased considerably with age of secondary forests, but species assemblages on isolated remnant trees were impoverished distinctly with time since isolation. Thus, isolated trees may serve for recolonization of secondary forests, but only for a relatively short time. We therefore suggest that the conservation of even small patches of primary forest within agricultural landscape matrices is essential for the long‐term maintenance of the high epiphyte diversity in tropical cloud forests.     

Carbon dynamics of forests in Washington, U.S.A.: 21st century projections based on climate-driven changes in fire regimes

During the 21st century, climate-driven changes in fire regimes will be a key agent of change in forests of the U.S. Pacific Northwest (PNW). Understanding the response of forest carbon (C) dynamics to increases in fire will help quantify limits on the contribution of forest C storage to climate change mitigation and prioritize forest types for monitoring C storage and fire management to minimize C loss. In this study, we used projections of 21st century area burned to explore the consequences of changes in fire regimes on C dynamics in forests of Washington State. We used a novel empirical approach that takes advantage of chronosequences of C pools and fluxes and statistical properties of fire regimes to explore the effects of shifting age class distributions on C dynamics. Forests of the western Cascades are projected to be more sensitive to climate-driven increases in fire, and thus projected changes in C dynamics, than forests of the eastern Cascades. In the western Cascades, mean live biomass C is projected to decrease by 24-37%, and coarse woody debris (CWD) biomass C by 15-25% for the 2040s. Loss of live biomass C is projected to be lower for forests of the eastern Cascades and Okanogan Highlands (17-26%), and CWD biomass is projected to increase. Landscape mean net primary productivity is projected to increase in wet low-elevation forests of the western Cascades, but decrease elsewhere. These forests, and moist forests of the Okanogan Highlands, are projected to have the greatest percentage increases in consumption of live biomass. Percentage increases in consumption of CWD biomass are greater than 50% for all regions and up to four times greater than increases in consumption of live biomass. Carbon sequestration in PNW forests will be highly sensitive to increases in fire, suggesting a cautious approach to managing these forests for C sequestration to mitigate anthropogenic CO2 emissions.     

Recovery of Faunal Communities During Tropical Forest Regeneration

Abstract:  As mature tropical forests are cleared, secondary forests may play an important role in the conservation of animal species, depending on how fast animal communities recover during forest regeneration. I reviewed published studies on the recovery of animal species richness and composition during tropical forest regeneration. In 38 of the 39 data sets I examined, conversion of forest to agriculture or pasture substantially reduced species richness. Given suitable conditions for forest recovery, the species richness of the animal taxa considered can be predicted to resemble that of mature forests roughly 20–40 years after land abandonment. At least for ants and birds, however, recovery of species composition appears to take substantially longer than recovery of species richness. Because species richness for many taxa appears to recover relatively rapidly in secondary forests, conservation of secondary forests may be an effective investment in future diversity. The slower recovery of species composition indicates, however, that some species will require stands of mature forest to persist.     

Interactions between Carbon Sequestration and Shade Tree Diversity in a Smallholder Coffee Cooperative in El Salvador

Agroforestry systems have substantial potential to conserve native biodiversity and provide ecosystem services. In particular, agroforestry systems have the potential to conserve native tree diversity and sequester carbon for climate change mitigation. However, little research has been conducted on the temporal stability of species diversity and aboveground carbon stocks in these systems or the relation between species diversity and aboveground carbon sequestration. We measured changes in shade‐tree diversity and shade‐tree carbon stocks in 14 plots of a 35‐ha coffee cooperative over 9 years and analyzed relations between species diversity and carbon sequestration. Carbon sequestration was positively correlated with initial species richness of shade trees. Species diversity of shade trees did not change significantly over the study period, but carbon stocks increased due to tree growth. Our results show a potential for carbon sequestration and long‐term biodiversity conservation in smallholder coffee agroforestry systems and illustrate the opportunity for synergies between biodiversity conservation and climate change mitigation. Interacciones entre el Secuestro de Carbono y la Diversidad de Árboles de Sombra en una Cooperativa de Café de Pequeños Agricultores en El Salvador     

Carbon sequestration potential of Rhizophora mucronata and Avicennia marina as influenced by age, season, growth and sediment characteristics in southeast coast of India

This work analysed the carbon sequestration potential in two species of mangroves (Rhizophora mucronata and Avicennia marina) along with their growth, biomass, sediment characteristics for four seasons of the year 2009–2010, in planted stands of different age (1–17.5 years) in the Vellar-Coleroon estuarine complex, India. The mangroves were recorded to store significant amount of biomass. Avicennia marina performed better to display 75 % higher rate of carbon sequestration than that in Rhizophora mucronata. This could be attributed to growth efficiency and high biomass production. For instance, Avicennia marina exhibited 2.7 fold higher girth, 24 % higher net canopy photosynthesis, 2 fold aboveground biomass (AGB), 40 % more belowground biomass (BGB) and 77.3 % higher total biomass, than R. mucronata did. Seasonally the rate of carbon sequestration was 7.3 fold higher in post-monsoon, 3.4 fold in monsoon, 73 % more in summer than that in pre-monsoon. The rate of carbon sequestration was positively correlated with age of planted site, tree height, tree diameter, net canopy photosynthesis, AGB, BGB, total biomass, carbon stock, growth efficiency, AGB/tree height tree girth, leaf area index, silt content (p?<?0.01). The carbon sequestration was negatively corrected with soil temperature and clay content (p?<?0.05). Mangroves were found to be a productive system and important sink of carbon in the tropical coastal zone, but increasing soil temperature due to global warming would have a negative impact on carbon sequestration potential of the mangroves.     

Changes in microbial community characteristics and soil organic matter with nitrogen additions in two tropical forests

Microbial communities and their associated enzyme activities affect the amount and chemical quality of carbon (C) in soils. Increasing nitrogen (N) deposition, particularly in N-rich tropical forests, is likely to change the composition and behavior of microbial communities and feed back on ecosystem structure and function. This study presents a novel assessment of mechanistic links between microbial responses to N deposition and shifts in soil organic matter (SOM) quality and quantity. We used phospholipid fatty acid (PLFA) analysis and microbial enzyme assays in soils to assess microbial community responses to long-term N additions in two distinct tropical rain forests. We used soil density fractionation and 13C nuclear magnetic resonance (NMR) spectroscopy to measure related changes in SOM pool sizes and chemical quality. Microbial biomass increased in response to N fertilization in both tropical forests and corresponded to declines in pools of low-density SOM. The chemical quality of this soil C pool reflected ecosystem-specific changes in microbial community composition. In the lower-elevation forest, there was an increase in gram-negative bacteria PLFA biomass, and there were significant losses of labile C chemical groups (O-alkyls). In contrast, the upper-elevation tropical forest had an increase in fungal PLFAs with N additions and declines in C groups associated with increased soil C storage (alkyls). The dynamics of microbial enzymatic activities with N addition provided a functional link between changes in microbial community structure and SOM chemistry. Ecosystem-specific changes in microbial community composition are likely to have far-reaching effects on soil carbon storage and cycling. This study indicates that microbial communities in N-rich tropical forests can be sensitive to added N, but we can expect significant variability in how ecosystem structure and function respond to N deposition among tropical forest types.     

Agricultural legacies in forest environments: tree communities, soil properties, and light availability.

Temperate deciduous forests across much of Europe and eastern North America reflect legacies of past land use, particularly in the diversity and composition of plant communities. Intense disturbances, such as clearing forests for agriculture, may cause persistent environmental changes that continue to shape vegetation patterns as landscapes recover. We assessed the long-term consequences of agriculture for environmental conditions in central New York forests, including tree community structure and composition, soil physical and chemical properties, and light availability. To isolate the effects of agriculture, we compared 20 adjacent pairs of forests that were never cleared for agriculture (primary forests) and forests that established 85-100 years ago on plowed fields (secondary forests). Tree communities in primary and secondary forests had similar stem density, though secondary forests had 14% greater basal area. Species composition differed dramatically between the two forest types, with primary forests dominated by Acer saccharum and Fagus grandifolia and secondary forests by Acer rubrum and Pinus strobus. Primary and secondary forests showed no consistent differences in soil physical properties or in the principal gradient of soil fertility associated with soil pH. Within stands, however, soil water content and pH were more variable in primary forests. Secondary forest soils had 15% less organic matter, 16% less total carbon, and 29% less extractable phosphorus in the top 10 cm than adjacent primary stands, though the ranges of the forest types mostly overlapped. Understory light availability in primary and secondary forests was similar. These results suggest that, within 100 years, post-agricultural stands have recovered conditions comparable to less disturbed forests in many attributes, including tree size and number, soil physical properties, soil chemical properties associated with pH, and understory light availability. The principal legacies of agriculture that remain in these forests are the reduced levels of soil organic matter, carbon, and phosphorus; the spatial homogenization of soil properties; and the altered species composition of the vegetation.     

Amazonian deforestation and soil biodiversity

Clearance and perturbation of Amazonian forests are one of the greatest threats to tropical biodiversity conservation of our times. A better understanding of how soil communities respond to Amazonian deforestation is crucially needed to inform policy interventions that effectively protect biodiversity and the essential ecosystem services it provides. We assessed the impact of deforestation and ecosystem conversion to arable land on Amazonian soil biodiversity through a meta-analysis. We analyzed 274 pairwise comparisons of soil biodiversity in Amazonian primary forests and sites under different stages of deforestation and land-use conversion: disturbed (wildfire and selective logging) and slash-and-burnt forests, pastures, and cropping systems. Overall, 60% and 51% of responses of soil macrofauna and microbial community attributes (i.e., abundance, biomass, richness, and diversity indexes) to deforestation were negative, respectively. We found few studies on mesofauna (e.g., microarthropods) and microfauna (e.g., protozoa and nematodes), so those groups could not be analyzed. Macrofauna abundance and biomass were more vulnerable to the displacement of forests by pastures than by agricultural fields, whereas microbes showed the opposite pattern. Effects of Amazonian deforestation on macrofauna were more detrimental at sites with mean annual precipitation >1900 mm, and higher losses of microbes occurred in highly acidic soils (pH < 4.5). Limited geographic coverage, omission of meso- and microfauna, and low taxonomic resolution were main factors impairing generalizations from the data set. Few studies assessed the impacts of within-forest disturbance (wildfires and selective logging) on soil species in Amazonia, where logging operations rapidly expand across public lands and more frequent severe dry seasons are increasing the prevalence of wildfires.     

Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.