Proportion Of Net Primary Production Research Articles

Agricultural Land‐Use Increases Carbon Yields in Lowland Streams of the Congo Basin

AbstractAs the dominant mode of deforestation in the Congo Basin, shifting agriculture is expected to increase with the projected four‐fold population growth for the region by 2,100. To assess how this land‐use change will affect the export of carbon (C) to rivers in a typical lowland forest ecosystem, we studied paired watersheds near Kisangani, Democratic Republic of the Congo. Two streams, one draining an intact forest (Forest) and one draining an agricultural landscape (Ag), were gauged, equipped with sensors, and sampled fortnightly for one year. Annual average specific discharge was 1.4 mm d−1 (+76%) higher in the Ag compared to the Forest. Average annual dissolved organic C (DOC) and particulate organic C (POC) concentrations were 5.2 mg L−1 (+163%) and 1.3 mg L−1 (+81%) higher in the Ag stream, which, along with the higher discharge, resulted in 8.3 (+410%) and 2.4 g C m−2 yr−1 (+97%) larger C yields, respectively. Baseflow dissolved inorganic carbon, carbon dioxide, and methane yields were also higher in the Ag stream. Despite the higher yields of organic C (OC), the composition of OC did not differ significantly. Carbon to nitrogen ratios, along with isotopic signatures, revealed that both streams contained young, semi‐degraded organic matter derived from C3 vegetation. Correspondingly, biodegradable DOC (BDOC) proportions did not differ between the streams, although the Ag stream yielded more total BDOC. These results show that agricultural land‐use likely exports a greater proportion of Net Primary Productivity (NPP) to aquatic ecosystems, which may affect both C storage in soils and the proportion of gross PP that is ultimately respired.

The Outsized Role of Salps in Carbon Export in the Subarctic Northeast Pacific Ocean.

Periodic blooms of salps (pelagic tunicates) can result in high export of organic matter, leading to an "outsized" role in the ocean's biological carbon pump (BCP). However, due to their episodic and patchy nature, salp blooms often go undetected and are rarely included in measurements or models of the BCP. We quantified salp-mediated export processes in the northeast subarctic Pacific Ocean in summer of 2018 during a bloom of Salpa aspera. Salps migrated from 300 to 750m during the day into the upper 100m at night. Salp fecal pellet production comprised up to 82% of the particulate organic carbon (POC) produced as fecal pellets by the entire epipelagic zooplankton community. Rapid sinking velocities of salp pellets (400-1,200md-1) and low microbial respiration rates on pellets (<1% of pellet C respired day-1) led to high salp pellet POC export from the euphotic zone-up to 48% of total sinking POC across the 100m depth horizon. Salp active transport of carbon by diel vertical migration and carbon export from sinking salp carcasses was usually <10% of the total sinking POC flux. Salp-mediated export markedly increased BCP efficiency, increasing by 1.5-fold the proportion of net primary production exported as POC across the base of the euphotic zone and by 2.6-fold the proportion of this POC flux persisting 100m below the euphotic zone. Salps have unique and important effects on ocean biogeochemistry and, especially in low flux settings, can dramatically increase BCP efficiency and thus carbon sequestration.

Foliar C:N:P stoichiometric traits of herbaceous synusia and the spatial patterns and drivers in a temperate desert in Central Asia

Herbaceous synusia is an important component of shrub community/ecosystem in the Junggar Desert and contributes a large proportion of net primary productivity in spring and early summer. Soil in the Junggar Desert is seriously deficient in nitrogen (N), but herbaceous plants possess high N concentration and N:P (phosphorous) ratio at the individual and interspecies levels; however, the community-level leaf stoichiometric characteristics of herbaceous synusia, spatial distribution patterns, and the driving factors at the regional scale remain unclear. In this study, 79 sampling plots were set up across the entire Junggar Desert. Foliar samples were collected by whole-plant harvesting method in each sampling plot, and the status of foliar carbon (C), N, and P concentrations, stoichiometric ratios, interrelationships, spatial patterns, and influencing factors were systematically analyzed. Results showed that the average contents of foliar C, N, and P in herbaceous synusia were 331.47, 25.85, and 1.54 mg g−1, respectively, and foliar C:N, C:P, and N:P were 13.76, 225.91, and 17.30, respectively. In contrast to most terrestrial ecosystems and global vegetation, herbaceous synusia was characterized by low C, C:N, and C:P and high N and N:P. Foliar C, N, and P were linearly and allometrically correlated with one another significantly. The foliar N–C, P–C, and N–P scaling exponents were 1.987, 1.770, and 1.122, respectively, thus the order of accumulation rates of foliar C, N, and P was N > P > C. Foliar C, N, C:N, and N:P presented significant differences among different soil habitats and shrub communities. Foliar stoichiometric traits showed obvious and different spatial heterogeneity at the regional scale. Multi-environmental factors jointly influenced foliar stoichiometric traits, and climate and geographical factors presented greater impact than soil. In conclusion, the foliar C:N:P stoichiometric traits might be the unique and inherent property of desert herbaceous species in this study area formed by long-term adaptation to the arid and barren environments in Central Asia, and this finding should be fully considered in regional ecological conservation and management.

Impact Mechanism of the Ecological Vulnerability of Highly Developed Islands Based on the Bayesian Network Model-Applied to the Changshan Islands.

Islands are one of the most sensitive interfaces between global changes and land and sea dynamic effects, with high sensitivity and low stability. Therefore, under the dynamic coupling effect of human activities and frequent natural disasters, the vulnerability of the ecological environment of islands shows the characteristics of complexity and diversity. For the protection of island ecosystems, a system for the assessment of island ecosystems and studies on the mechanism of island ecological vulnerability are highly crucial. In this study, the North and South Changshan Islands of China were selected as the study area. Considering various impact factors of island ecological vulnerability, the geographical information systems (GIS) spatial analysis, field surveys, data sampling were used to evaluate island ecological vulnerability. The Bayesian network model was used to explore the impact mechanism of ecological vulnerability. The results showed that the ecological vulnerability of the North Changshan Island is higher than that of the South Changshan Island. Among all the indicators, the proportion of net primary productivity (NPP) and the steep slope has the strongest correlation with ecological vulnerability. This study can be used as references in the relevant departments to formulate management policies and promote the sustainable development of islands and their surrounding waters

Effects of tree species richness on fine root production varied with stand density and soil nutrients in subtropical forests

Fine root production accounts for a large proportion of net primary production (NPP) in forest ecosystems that is highly responsive to environmental and biotic changes. The underlying mechanisms of the relationship between tree species richness and fine root production have not been fully examined. Here we hypothesized that: (i) the relationship between aboveground species richness and fine root production could be attributable to belowground spatial resource partitioning; (ii) either symmetrical or asymmetrical root proliferation to obtain nutrients leads to increased fine root production; and (iii) stand density affects the relationship between species richness and fine root production. We used an ingrowth core method to estimate fine root production coupled to molecular approaches for identifying the tree species of sampled fine roots within each ingrowth core. There was a significant and positive relationship between aboveground species richness and fine root production. The increase in fine root production might partially be attributed to asymmetrical root proliferation rather than belowground spatial resource partitioning. A piecewise structural equation model (SEM) linking stand density and soil nutrients revealed that both factors play dominant roles in mediating the effects of aboveground species richness on fine root production. Moreover, fine root production and relative abundance of fine root distribution within-layers both depended on the effects of aboveground species richness × stand density × soil phosphorus (P) interactions. Therefore, soil P concentration and stand density partially explained the positive aboveground species richness–fine root production relationship.

Novel bacterial lineages associated with boreal moss species.

Mosses are critical components of boreal ecosystems where they typically account for a large proportion of net primary productivity and harbour diverse bacterial communities that can be the major source of biologically-fixed nitrogen in these ecosystems. Despite their ecological importance, we have limited understanding of how microbial communities vary across boreal moss species and the extent to which local site conditions may influence the composition of these bacterial communities. We used marker gene sequencing to analyze bacterial communities associated with seven boreal moss species collected near Fairbanks, AK, USA. We found that host identity was more important than site in determining bacterial community composition and that mosses harbour diverse lineages of potential N2 -fixers as well as an abundance of novel taxa assigned to understudied bacterial phyla (including candidate phylum WPS-2). We performed shotgun metagenomic sequencing to assemble genomes from the WPS-2 candidate phylum and found that these moss-associated bacteria are likely anoxygenic phototrophs capable of carbon fixation via RuBisCo with an ability to utilize byproducts of photorespiration from hosts via a glyoxylate shunt. These results give new insights into the metabolic capabilities of understudied bacterial lineages that associate with mosses and the importance of plant hosts in shaping their microbiomes.

Carbon dynamics of mature and regrowth tropical forests derived from a pantropical database (TropForC-db).

Tropical forests play a critical role in the global carbon (C) cycle, storing ~45% of terrestrial C and constituting the largest component of the terrestrial C sink. Despite their central importance to the global C cycle, their ecosystem-level C cycles are not as well-characterized as those of extra-tropical forests, and knowledge gaps hamper efforts to quantify C budgets across the tropics and to model tropical forest-climate interactions. To advance understanding of C dynamics of pantropical forests, we compiled a new database, the Tropical Forest C database (TropForC-db), which contains data on ground-based measurements of ecosystem-level C stocks and annual fluxes along with disturbance history. This database currently contains 3568 records from 845 plots in 178 geographically distinct areas, making it the largest and most comprehensive database of its type. Using TropForC-db, we characterized C stocks and fluxes for young, intermediate-aged, and mature forests. Relative to existing C budgets of extra-tropical forests, mature tropical broadleaf evergreen forests had substantially higher gross primary productivity (GPP) and ecosystem respiration (Reco), their autotropic respiration (Ra) consumed a larger proportion (~67%) of GPP, and their woody stem growth (ANPPstem) represented a smaller proportion of net primary productivity (NPP, ~32%) or GPP (~9%). In regrowth stands, aboveground biomass increased rapidly during the first 20 years following stand-clearing disturbance, with slower accumulation following agriculture and in deciduous forests, and continued to accumulate at a slower pace in forests aged 20-100 years. Most other C stocks likewise increased with stand age, while potential to describe age trends in C fluxes was generally data-limited. We expect that TropForC-db will prove useful for model evaluation and for quantifying the contribution of forests to the global C cycle. The database version associated with this publication is archived in Dryad (DOI: 10.5061/dryad.t516f) and a dynamic version is maintained at https://github.com/forc-db.

The decomposition of fine and coarse roots: their global patterns and controlling factors.

Fine root decomposition represents a large carbon (C) cost to plants, and serves as a potential soil C source, as well as a substantial proportion of net primary productivity. Coarse roots differ markedly from fine roots in morphology, nutrient concentrations, functions, and decomposition mechanisms. Still poorly understood is whether a consistent global pattern exists between the decomposition of fine (<2 mm root diameter) and coarse (≥2 mm) roots. A comprehensive terrestrial root decomposition dataset, including 530 observations from 71 sampling sites, was thus used to compare global patterns of decomposition of fine and coarse roots. Fine roots decomposed significantly faster than coarse roots in middle latitude areas, but their decomposition in low latitude regions was not significantly different from that of coarse roots. Coarse root decomposition showed more dependence on climate, especially mean annual temperature (MAT), than did fine roots. Initial litter lignin content was the most important predictor of fine root decomposition, while lignin to nitrogen ratios, MAT, and mean annual precipitation were the most important predictors of coarse root decomposition. Our study emphasizes the necessity of separating fine roots and coarse roots when predicting the response of belowground C release to future climate changes.

Midsummer spatial variation in methane efflux from stands of littoral vegetation in a boreal meso‐eutrophic lake

Summary1. Spatial variation of methane (CH4) efflux from the littoral zone of a meso‐eutrophic boreal lake was studied with a closed‐chamber technique for three summer days in 22 vegetation stands, consisting of three emergent and three floating‐leaved species.2. Between‐species differences in CH4 emission were significant. The highest emissions were measured from the emergent Phragmites australis stands (0.5–1.7 mmol m−2 h−1), followed by Schoenoplectus lacustris &gt; Equisetum fluviatile &gt; Nuphar lutea &gt; Sparganium gramineum &gt; Potamogeton natans. Within‐species differences between stands were not significant.3. In P. australis stands, the stand‐specific mean CH4 emission was significantly correlated with solar radiation, probably indicating the role of effective pressurised ventilation on CH4 fluxes. The proportion of net primary production emitted as CH4 was significantly higher in P. australis stands (7.4%) than in stands of S. lacustris and E. fluviatile (both 0.5%).4. In N. lutea stands, CH4 efflux was negatively correlated with the mean fetch and positively with the percentage cover of leaves on the water surface. There were no differences in CH4 efflux between intact N. lutea leaves and those grazed by coleopteran Galerucella nymphaeae. In S. graminaeum and P. natans stands, CH4 effluxes were not related to any of the measured environmental variables.5. For all vegetation stands, the biomass above water level explained about 60% of the observed spatial variation in CH4 emission, indicating the important role of plants as gas conduits and producers of substrates for methanogens in the anoxic sediment.

Effects of sea-ice extent and krill or salp dominance on the Antarctic food web

Krill (Euphausia superba) provide a direct link between primary producers and higher trophic levels in the Antarctic marine food web1,2,3,4,5,6. The pelagic tunicate Salpa thompsoni can also be important during spring and summer through the formation of extensive and dense blooms6,7,8,9. Although salps are not a major dietary item for Antarctic vertebrate predators7,10, their blooms can affect adult krill reproduction and survival of krill larvae. Here we provide data from 1995 and 1996 that support hypothesized relationships between krill, salps and region-wide sea-ice conditions11,12. We have assessed salp consumption as a proportion of net primary production, and found correlations between herbivore densities and integrated chlorophyll-a that indicate that there is a degree of competition between krill and salps. Our analysis of the relationship between annual sea-ice cover and a longer time series of air temperature measurements12,13 indicates a decreased frequency of winters with extensive sea-ice development over the last five decades. Our data suggest that decreased krill availability may affect the levels of their vertebrate predators. Regional warming and reduced krill abundance therefore affect the marine food web and krill resource management.

A study of the sources and sinks of methane and methyl chloroform using a global three‐dimensional Lagrangian tropospheric tracer transport model

Sources and sinks of methane and methyl chloroform are investigated using a global three‐dimensional Lagrangian tropospheric tracer transport model with parameterized hydroxyl and temperature fields. By comparison with methyl chloroform observations a global average tropospheric hydroyl radical concentration of 6.4×105 cm−3 was found to be consistent with published methyl chloroform emission data for the year 1980. Published methyl chloroform emissions data for 1981–1984 were found to be inconsistent with the observed methyl chloroform concentration increases. A large decrease in hydroxyl radical concentrations could explain the disagreement between the emission data and atmospheric methyl chloroform concentrations, but this is unlikely. Using the hydroxyl radical field calibrated to the methyl chloroform observations, the globally averaged release of methane and its spatial and temporal distribution were investigated. Two source function models of the spatial and temporal distribution of the flux of methane to the atmosphere were developed. The first model was based on the assumption that methane is emitted as a proportion of net primary productivity (NPP). With the average hydroxyl radical concentration fixed, the methane source term was computed as ∼623 Tg CH4, giving an atmospheric lifetime for methane ∼8.3 years. The second model identified source regions for methane from rice paddies, wetlands, enteric fermentation, termites, and biomass burning based on high‐resolution land use data. This methane source distribution resulted in an estimate of the global total methane source of ∼611 Tg CH4, giving an atmospheric lifetime for methane ∼8.5 years. The most significant difference between the two models were predictions of methane fluxes over China and South East Asia, the location of most of the world's rice paddies, indicating that either the assumption that a uniform fraction of NPP is converted to methane is not valid for rice paddies, or that NPP is underestimated for rice paddies, or that present methane emission estimates from rice paddies are too high. Using a recent measurement of the reaction rate of hydroxyl radical and methane by Vaghjiani and Ravishankara (G. L. Vaghjiani and A. R. Ravishankara, Rate coefficient for the reaction of OH and CH4: Implications to the atmospheric lifetime and budget of methane, submitted to Nature, 1990) (hereinafter referred to as Vaghjiani and Ravishankara, 1990) leads to estimates of the global total methane source for SF1 of ∼524 Tg CH4 giving an atmospheric lifetime of ∼10.0 years and for SF2 ∼514 Tg CH4 yielding a lifetime of ∼10.2 years. These results are provisional pending any revision of the reaction rate for hydroxyl radical and methyl chloroform.

Response of Baetis Mayflies (Ephemeroptera) to Catchment Logging

Following clear-cutting of a southern Appalachian hardwood catchment, standing stock densities and biomass of Baetis spp. increased in four stream substrate types (rock face, cobble riffle, pebble riffle and sandy reach) compared to those of a nearby undisturbed reference stream. Baetis production in the stream draining the clear-cut catchment averaged 17.6 X higher than that of the reference stream, and up to 27.6 X that of the reference stream in the more physically stable rock-face substrates. Food preferences and food-specific bioenergetic efficiencies were used to estimate the amount of food consumed and the amount of Baetis production attributable to each food category. Diatoms comprised the most important food in each stream. Analyses of gut contents of Baetis following logging indicate a significant increase in diatom consumption in the stream draining the clear-cut catchment. Diatom consumption in the reference stream (0.234 g ash-free dry mass [AFDM]/m2) and treatment stream (5.788 g AFDM/m2) comprised 9.0 and 7.4%, respectively, of the net primary production of each stream during the 1st year following the clear cut. These suggest that a similar proportion of net primary production was harvested by Baetis in each stream. Although the two Baetis populations consumed the same proportion of net primary production, Baetis in the clear-cut catchment stream had 1.22.0 X more diatoms in their guts while maintaining a standing stock biomass that exceeded that of the reference stream by 10-30 X, suggesting a much higher rate of periphyton harvesting in the clear-cut catchment. Sampling 4 and 5 years following the clear cut indicates significant declines in Baetis populations of the clear-cut stream that coincided with a 10-fold decrease in primary productivity. Although characterized by short, multivoltine life cycles and high fecundity, Baetis spp. comprise a minor component of the standing stock biomass in most headwater streams of the region. However, with disturbances such as clear-cutting, they respond quickly to exploit increases in autochthonous production, and assume major roles in energy processing. In contrast, larger, less fecund univoltine and semivoltine species lack the ability to respond quickly and exploit the relatively short-lived increases in primary production.

Biomass distribution and above- and below-ground production in young and mature Abiesamabilis zone ecosystems of the Washington Cascades

Biomass distribution and above- and below-ground net primary production were determined for 23- and 180-year-old Abiesamabilis (Dougl.) Forbes ecosystems growing at 1200-m elevation in the western Washington Cascade Range. Total organic matter accumulations were 427.0 t•ha−1 in the young stand, and 1247.1 t•ha−1 in the mature stand. Aboveground tree and detritus biomass were 49.0 t•ha−1 and 130.2 t•ha−1, respectively, in the young stand compared with 445.5 t•ha−1 and 389.4 t•ha−1 in the mature stand. Net primary production (NPP) was 18.3 t•ha−1 in the young stand and 16.8 t•ha−1 in the mature stand. Belowground dry matter production was 65% of total net production in the young stand and 73% of total net production in the mature stand. Conifer fine root production was 35.9% of NPP in the young and 66.4% of NPP in the mature stand. This apparent shift in fine root production as a proportion of NPP may be related to detritus accumulation.