Using stable isotopes to determine soil carbon input differences under ambient and elevated atmospheric CO2 conditions

Quantitative estimates of soil C input under ambient (35 Pa) and elevated (60 Pa) CO₂-partial pressure (pCO₂) were determined in a Free-Air Carbon dioxide Enrichment (FACE) experiment. To facilitate ¹³C-tracing, Trifolium repens L. was grown in a soil with an initial δ¹³C distinct by at least 5‰ from the δ¹³C of T. repens grown under ambient or elevated pCO₂. A shift in δ¹³C of the soil organic C was detected after one growing season. Calculated new soil C inputs in soil under ambient and elevated pCO₂ were 2 and 3 t ha^–1, respectively. Our findings suggest that under elevated CO₂ conditions, soil C sequestration may be altered by changes in plant biomass production and quality.     

Elevated atmospheric CO2 does not affect per se the preference for symbiotic nitrogen as opposed to mineral nitrogen of Trifolium repens L.

The objective of this investigation was to examine the effect of an elevated atmospheric CO₂ partial pressure ( p CO₂) on the N-sink strength and performance of symbiotic N₂ fixation in Trifolium repens L. cv. Milkanova. After initial growth under ambient p CO₂ in a nitrogen-free nutrient solution, T. repens in the exponential growth stage was exposed to ambient and elevated p CO₂ (35 and 60 Pa) and two levels of mineral N (N-free and 7·5 mol m^–3 N) for 36 d in single pots filled with silica sand in growth chambers. Elevated p CO₂ evoked a significant increase in biomass production from day 12 after the start of CO₂ enrichment. For plants supplied with 7·5 mol m^–3 N, the relative contribution of symbiotically fixed N (%N_sym) as opposed to N assimilated from mineral sources (¹⁵N-isotope-dilution method), dropped to 40%. However, in the presence of this high level of mineral N, %N_sym was unaffected by atmospheric p CO₂ over the entire experimental period. In plants fully dependent on N₂ fixation, the increase in N yield reflects a stimulation of symbiotic N₂ fixation that was the result of the formation of more nodules rather than of higher specific N₂ fixation. These results are discussed with regard to physiological processes governing symbiotic N₂ fixation and to the response of symbiotic N₂ fixation to elevated p CO₂ in field-grown T. repens .     

CO2 effects on the water budget of grassland microcosm communities

Experimental grassland ecosystems, in microcosms 0.2 m in diameter and with a 0.95 m soil column, varied in their responses to elevated partial pressure of CO₂ (pCO₂) and altered moisture inputs. Ecosystems on moderately fertile sandstone soil and with a typical mix of moderately fast-growing sandstone species, responded to elevated pCO₂ with decreases in mid-season evapotranspiration of nearly 50%. This pattern reversed at the end of the growing season, and sandstone ecosystems under elevated pCO₂ continued active transpiration farther into the summer drought. The sandstone ecosystems appeared to convert mid-season water conservation into increased late-season growth. Effects of increased pCO₂ on ecosystem evapotranspiration were much smaller in ecosystems with very infertile serpentine soil and a diverse mixture of slow-growing serpentine species.     

Does photosynthetic acclimation to elevated CO2 increase photosynthetic nitrogen-use efficiency? A study of three native UK grassland species in open-top chambers

1. The photosynthetic response to elevated CO₂ and nutrient stress was investigated in Agrostis capillaris, Lolium perenne and Trifolium repens grown in an open-top chamber facility for 2 years under two nutrient regimes. Acclimation was evaluated by measuring the response of light-saturated photosynthesis to changes in the substomatal CO₂ concentration.
2. Growth at elevated CO₂ resulted in reductions in apparent Rubisco activity in vivo in all three species, which were associated with reductions of total leaf nitrogen content on a unit area basis for A. capillaris and L. perenne . Despite this acclimation, photosynthesis was significantly higher at elevated CO₂ for T. repens and A. capillaris , the latter exhibiting the greatest increase of carbon uptake at the lowest nutrient supply.
3. The photosynthetic nitrogen-use efficiency (the rate of carbon assimilation per unit leaf nitrogen) increased at elevated CO₂, not purely owing to higher values of photosynthesis at elevated CO₂, but also as a result of lower leaf nitrogen contents.
4. Contrary to most previous studies, this investigation indicates that elevated CO₂ can stimulate photosynthesis under a severely limited nutrient supply. Changes in photosynthetic nitrogen-use efficiency may be a critical determinant of competition within low nutrient ecosystems and low input agricultural systems.     

Mycorrhizal fungi have a potential role in soil carbon storage under elevated CO2 and nitrogen deposition

In this review, we discuss the potential for mycorrhizal fungi to act as a source or sink for carbon (C) under elevated CO₂ and nitrogen deposition. Mycorrhizal tissue has been estimated to comprise a significant fraction of soil organic matter and below-ground biomass in a range of systems. The current body of literature indicates that in many systems exposed to elevated CO₂, mycorrhizal fungi might sequester increased amounts of C in living, dead and residual hyphal biomass in the soil. Through this process, the fungi might serve as a negative feedback on the rise in atmospheric CO₂ levels caused by fossil fuel burning and deforestation. By contrast, a few preliminary studies suggest that N deposition might increase turnover rates of fungal tissue and negate CO₂ effects on hyphal biomass. If these latter responses are consistent among ecosystems, C storage in hyphae might decline in habitats surrounding agricultural and urban areas. When N additions occur without CO₂ enrichment, effects on mycorrhizal growth are inconsistent. We note that analyses of hyphal decomposition under elevated CO₂ and N additions are extremely sparse but are critical in our understanding of the impact of global change on the cycling of mycorrhizal C. Finally, shifts in the community composition of arbuscular and ectomycorrhizal fungi with increasing CO₂ or N availability are frequently documented. Since mycorrhizal groups vary in growth rate and tissue quality, these changes in species assemblages could produce unforeseeable impacts on the productivity, survivorship, or decomposition of mycorrhizal biomass.     

Stimulation of r- vs. K-selected microorganisms by elevated atmospheric CO2 depends on soil aggregate size

Increased root exudation under elevated atmospheric CO₂ and the contrasting environments in soil macro- and microaggregates could affect microbial growth strategies. We investigated the effect of elevated CO₂ on the contribution of fast- ( r -strategists) and slow-growing ( K -strategists) microorganisms in soil macro- and microaggregates. We fractionated the bulk soil from the ambient and elevated (for 5 years) CO₂ treatments of FACE-Hohenheim (Stuttgart) into large macro- (>2 mm), small macro- (0.25–2.00 mm), and microaggregates (<0.25 mm) using 'optimal moist' sieving. Microbial biomass (C_mic), the maximum specific growth rate (μ), growing microbial biomass (GMB) and lag-period ( t _lag) were estimated by the kinetics of CO₂ emission from bulk soil and aggregates amended with glucose and nutrients. Although C_org and C_mic were unaffected by elevated CO₂, μ values were significantly higher under elevated than ambient CO₂ for bulk soil, small macroaggregates, and microaggregates. Substrate-induced respiratory response increased with decreasing aggregate size under both CO₂ treatments. Based on changes in μ, GMB and lag period, we conclude that elevated atmospheric CO₂ stimulated the r- selected microorganisms, especially in soil microaggregates. Such an increase in r -selected microorganisms indicates acceleration of available C mineralization in soil, which may counterbalance the additional C input by roots in soils in a future elevated atmospheric CO₂ environment.     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

Effects of Free-Air CO2 Enrichment (FACE) on experimental grassland communities

1. Experimental grassland communities (turves) were exposed to elevated (60 Pa) and ambient (35 Pa) CO₂ partial pressures (pCO₂) in a Free-Air Carbon Dioxide Enrichment (FACE) experiment between 30 March 1995 and 4 July 1996. The vegetation was cut once during the experiment prior to the final harvest (harvest 2).
2. No significant treatment effects on total plant biomass at the whole turf level were detected, although biomass was typically about 25% higher under fumigation in year 1 and about 15% higher in year 2.
3. Biomass for two of the six sown species was significantly higher at harvest 2 than at harvest 1. There were no significant differences between individual species' biomass under the two CO₂ treatments at either harvest 1 or 2 or in terms of overall cumulative biomass. However, in four of the five sown species in both years biomass tended to be higher in the fumigated than in the control rings ( Cerastium holosteiodes, Phleum pratense, Plantago lanceolata and Poa trivialis ). In contrast, Lolium perenne showed increased biomass under the control treatment relative to the fumigated treatment in both years. Owing to the high variance both within and between rings for each of the two treatments the statistical power of most, but not all, of the analyses carried out was poor.
4. The relative proportions of each species in the turves under fumigated and control treatments was broadly similar after the first summer, with differences in the second year being mainly owing to the negative response of L. perenne to CO₂ fumigation.     

Wood composition and energy content in a poplar short rotation plantation on fertilized agricultural land in a future CO2 atmosphere

To study the influence of elevated CO₂ and nitrogen (N) fertilization on wood properties and energy, Populus × euramericana trees were exposed to ambient CO₂ (about 370 μmol mol⁻¹ CO₂) or elevated CO₂ (about 550 μmol mol⁻¹ CO₂) using Free Air CO₂ Enrichment (FACE) technology in combination with two N levels. Elevated CO₂ was maintained for 5 years. After three growing seasons, the plantation was coppiced, one half of each experimental plot was fertilized and secondary sprouts were harvested after two growing seasons. Fourier transform infrared (FT-IR) spectra of wood revealed significant effects of both elevated CO₂ and N fertilization on wood chemistry, in particular, significant increases in lignin and decreases in N content. These results were corroborated by chemical analysis. Neither elevated CO₂ nor N fertilization affected the calorific value of wood, which was 19.3 MJ kg⁻¹. N fertilization enhanced the energy production per land area by 16–69% because of higher aboveground woody biomass production than on nonfertilized land. Estimates indicate that high yielding poplar short rotation cultivation may significantly contribute as an alternative feedstock for energy production.     

Elevated atmospheric CO2 influences the interaction between the parasitic angiosperm Orobanche minor and its host Trifolium repens

The influence of the root holoparasitic angiosperm Orobanche minor Sm. on the biomass, photosynthesis, carbohydrate and nitrogen content of Trifolium repens L. was determined for plants grown at two CO₂ concentrations (350 and 550 μmol mol⁻¹). Infected plants accumulated less biomass than their uninfected counterparts, although early in the association there was a transient stimulation of growth. Infection also influenced biomass allocation both between tissues (infected plants had lower root:shoot ratios) and within tissues:infected roots were considerably thicker before the point of parasite attachment and thinner below. Higher concentrations of starch were also found in roots above the point of attachment, particularly for plants grown in elevated CO₂. Elevated CO₂ stimulated the growth of T. repens only during the early stages of development. There was a significant interaction between infection and CO₂ on growth, with infected plants showing a greater response, such that elevated CO₂ partly alleviated the effects of the parasite on host growth. Elevated CO₂ did not affect total O. minor biomass per host, the number of individual parasites supported by each host, or their time of attachment to the host root system. Photosynthesis was stimulated by elevated CO₂ but was unaffected by O. minor . There was no evidence of down-regulation of photosynthesis in T. repens grown at elevated CO₂ in either infected or uninfected plants. The data are discussed with regard to the influence of elevated CO₂ on other parasitic angiosperm-host associations and factors which control plant responses to elevated CO₂.     

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

Interactive effects of elevated atmospheric CO₂ and arbuscular mycorrhizal (AM) fungi on biomass production and N₂ fixation were investigated using black locust ( Robinia pseudoacacia ). Seedlings were grown in growth chambers maintained at either 350 μmol mol⁻¹ or 710 μmol mol⁻¹ CO₂. Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The ¹⁵N isotope dilution method was used to determine N source partitioning between N₂ fixation and inorganic fertilizer uptake. Elevated atmospheric CO₂ significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO₂ had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO₂ levels enhanced nodule and root mass production, as well as N₂ fixation rates, of non- mycorrhizal seedlings. However, the relative response of biomass production to CO₂ enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N₂-fixing tree species is affected by enriched atmospheric CO₂.     

蒙古栎叶片及其土壤碳、氮同位素自然丰度对大气CO2浓度升高的响应

大气CO₂浓度升高对土壤氮素转化过程产生重要影响,研究其变化有助于更好地预测陆地生态系统的固碳潜力.氮同位素自然丰度作为生态系统氮素循环过程的综合指标能够有效地指示CO₂浓度升高对土壤氮素转化过程的影响.本研究采用开顶箱CO₂ 熏蒸法研究连续10年的大气CO₂ 浓度升高对我国东北地区蒙古栎及其土壤和微生物生物量碳、氮同位素自然丰度的影响.结果表明: 大气CO₂浓度升高改变了土壤氮循环过程,增加了土壤微生物和植物叶片δ¹⁵N;促进了富¹³C土壤有机碳分解,中和了贫¹³C植物光合碳输入的效果,导致土壤可溶性有机碳和微生物碳δ¹³C在CO₂升高条件下没有发生显著变化.这些结果表明,CO₂浓度升高很可能促进了土壤有机质矿化过程,并加剧了系统氮限制的状态.     

水稻光合碳在植物-土壤系统中的分配及其对CO2升高和施氮的响应

采用¹³C-CO₂进行连续标记,研究水稻分蘖期和孕穗期光合碳在植株-土壤系统中的分配及其对大气CO₂浓度升高(800 μL·L^-1)和施氮(100 mg·kg^-1)的响应.结果表明: CO₂浓度升高显著提高分蘖期根系生物量和孕穗期地上部生物量,并使生物量根冠比在分蘖期增加,而在孕穗期减小.CO₂浓度升高条件下,施氮使水稻地上部分生物量增加,却显著降低了孕穗期水稻根系生物量.CO₂浓度升高使光合¹³C在孕穗期向土壤的输入显著增加,然而施肥并没有促进由CO₂浓度升高驱动的光合¹³C在土壤中的积累,而且还降低了土壤中的光合¹³C的分配比例.综上,CO₂浓度升高显著提高了稻田土壤光合碳输入,促进稻田有机碳周转;施氮促进了水稻地上部的生长,却降低了光合碳向地下的分配比例.     

Simulated chronic nitrogen deposition increases carbon storage in Northern Temperate forests

High levels of atmospheric nitrogen (N) deposition in Europe and North America were maintained throughout the 1990s, and global N deposition is expected to increase by a factor of 2.5 over the next century. Available soil N limits primary production in many terrestrial ecosystems, and some computer simulation models have predicted that increasing atmospheric N deposition may result in greater terrestrial carbon (C) storage in woody biomass. However, empirical evidence demonstrating widespread increases in woody biomass C storage due to atmospheric N deposition is uncommon. Increased C storage in soil organic matter due to chronic N inputs has rarely been reported and is often not considered in computer simulation models of N deposition effects. Since 1994, we have experimentally simulated chronic N deposition by adding 3 g N m⁻² yr⁻¹ to four different northern hardwood forests, which span a 500 km geographic gradient in Michigan. Each year we measured tree growth. In 2004, we also examined soil C content to a depth of 70 cm. When we compared the control treatment with the NO₃⁻ deposition treatment after a decade of experimentation, ecosystem C storage had significantly increased in both woody biomass (500 g C m⁻²) and surface soil (0–10 cm) organic matter (690 g C m⁻²). The increase in surface soil C storage was apparently driven by altered rates of organic matter decomposition, rather than an increase in detrital inputs to soil. Our results, for study locations stretching across hundreds of kilometers, support the hypothesis that chronic N deposition may increase C storage in northern forests, potentially contributing to a sink for anthropogenic CO₂ in the northern Hemisphere.     

The Effect of Elevated [CO2] on Uptake and Allocation of 13C and 15N in Beech (Fagus sylvatica L.) during Leafing

Abstract: A continuous dual ¹³CO₂ and ¹⁵NH₄¹⁵NO₃ labelling experiment was undertaken to determine the effects of ambient (350μmol mol^-1) or elevated (700μmol mol^-1) atmospheric CO₂ concentrations on C and N uptake and allocation within 3-year-old beech ( Fagus sylvatica L.) during leafing. After six weeks of growth, total carbon uptake was increased by 63 % (calculated on total C content) under elevated CO₂ but the carbon partitioning was not altered. 56 % of the new carbon was found in the leaves. On a dry weight basis was the content of structural biomass in leaves 10 % lower and the lignin content remained unaffected under elevated as compared to ambient [CO₂]. Under ambient [CO₂] 37 %, and under elevated [CO₂] 51 %, of the lignin C of the leaves derived from new assimilates. For both treatments, internal N pools provided more than 90 % of the nitrogen used for leaf-growth and the partitioning of nitrogen was not altered under elevated [CO₂]. The C/N ratio was unaffected by elevated [CO₂] at the whole plant level, but the C/N ratio of the new C and N uptake was increased by 32 % under elevated [CO₂].     

大气CO2浓度上升和氮添加对南亚热带模拟森林生态系统土壤碳稳定性的影响

大气CO₂浓度升高和氮(N)添加对土壤碳库的影响是当前国际生态学界关注的一个热点。为阐述土壤不同形态有机碳的抗干扰能力, 运用大型开顶箱, 研究了4种处理((1)高CO₂浓度(700 µmol·mol^-1)和高氮添加(100 kg N·hm^-2·a^-1) (CN); (2)高CO₂浓度和背景氮添加(CC); (3)高氮添加和背景CO₂浓度(NN); (4)背景CO₂和背景氮添加(CK))对南亚热带模拟森林生态系统土壤有机碳库稳定性的影响。近5年的试验研究表明: (1) CN处理能明显地促进各土层中土壤总有机碳含量的增加, 其中, 下层土壤(5-60 cm土层)中的响应达到统计学水平。(2)活性有机碳库各组分对处理的响应有所差异: 不同土层中微生物生物量碳(MBC)的含量对各处理的响应趋势基本一致, 各土层中的MBC含量均为CN > CC > NN > CK, 其中0-5 cm、5-10 cm、10-20 cm 3个土层的处理间差异都达到了显著水平; 10-20 cm与20-40 cm两个土层中的易氧化有机碳处理间有显著差异; 而对于各土层中水溶性有机碳, 处理间差异均不明显。(3)各团聚体组分中的有机碳含量的响应也有所差异: 20-40 cm与40-60 cm土层中250-2000 μm组分的有机碳含量存在处理间差异; 40-60 cm土层中53-250 μm组分的有机碳对各处理响应敏感, CC处理和NN处理都有利于该组分碳的深层积累, 尤其CN处理下的效果最为明显; 在各处理10-20 cm、20-40 cm及40-60 cm土壤中, < 53 μm组分中的碳含量间差异显著。大气CO₂浓度上升和N添加促进了森林生态系统中土壤有机碳的增加, 尤其有利于深层土壤中微团聚体与粉粒、黏粒团聚体等较稳定组分中有机碳的积累, 增加了土壤有机碳库的稳定性。     

The impact of nitrogen supply on the potential response of a noxious, invasive weed, Canada thistle (Cirsium arvense) to recent increases in atmospheric carbon dioxide

A recognized invasive weed, Canada thistle ( Cirsium arvense L. Scop.) was grown at ambient and pre-ambient concentrations of atmospheric carbon dioxide [CO₂] (373 and 287 μmol mol⁻¹, respectively) at three levels of supplemental nitrogen (N) (3, 6 and 14.5 m M ) from seeding until flowering [77 days after sowing (DAS)]. The primary objective of the study was to determine if N supply limited the potential photosynthetic and growth response of this species to the increase in atmospheric [CO₂] which occurred during the 20th century (i.e. approximately 290 to 370 μmol mol⁻¹ CO₂). Leaf photosynthesis increased both as a function of growth [CO₂] and N supply during the first 46 DAS. Although by 46 DAS photosynthetic acclimation was observed relative to a common measurement CO₂ concentration, there was no interaction with N supply. Both [CO₂] and N increased biomass, relative growth rates and leaf area whereas root : shoot ratio was increased by CO₂ and decreased by increasing N; however, N supply did not effect the relative response to [CO₂] for any measured vegetative parameter up to 77 DAS. Due to the relative stimulation of shoot biomass, total above-ground N increased at elevated [CO₂] for all levels of supplemental N, but nitrogen use efficiency (NUE) did not differ as a function of [CO₂]. Overall, these data suggest that any potential response to increased atmospheric [CO₂] in recent decades for this noxious weedy species was probably not limited by nitrogen supply.     

Does soil nitrogen influence growth,water transport and survival of snow gum (Eucalyptus pauciflora Sieber ex Sprengel.) under CO2 enrichment?

Eucalyptus pauciflora Sieber ex Sprengel. (snow gum) was grown under ambient (370  µ L L⁻¹) and elevated (700  µ L L⁻¹) atmospheric [CO₂] in open-top chambers (OTCs) in the field and temperature-controlled glasshouses. Nitrogen applications to the soil ranged from 0.1 to 2.75 g N per plant. Trees in the field at high N levels grew rapidly during summer, particularly in CO₂-enriched atmosphere, but suffered high mortality during summer heatwaves. Generally, wider and more numerous secondary xylem vessels at the root–shoot junction in CO₂-enriched trees conferred fourfold higher below-ground hydraulic conductance. Enhanced hydraulic capacity was typical of plants at elevated [CO₂] (in which root and shoot growth was accelerated), but did not result from high N supply. However, because high rates of N application consistently made trees prone to dehydration during heatwaves, glasshouse studies were required to identify the effect of N nutrition on root development and hydraulics. While the effects of elevated [CO₂] were again predominantly on hydraulic conductivity, N nutrition acted specifically by constraining deep root penetration into soil. Specifically, 15–40% shallower root systems supported marginally larger shoot canopies. Independent changes to hydraulics and root penetration have implications for survival of fertilized trees under elevated atmospheric [CO₂], particularly during water stress.     

Elevated CO2 and moisture effects on soil carbon storage and cycling in temperate grasslands

In grassland ecosystems, most of the carbon (C) occurs below-ground. Understanding changes in soil fluxes induced by elevated atmospheric CO₂ is critical for balancing the global C budget and for managing grassland ecosystems sustainably. In this review, we use the results of short-term (1–2 years) studies of below-ground processes in grassland communities under elevated CO₂ to assess future prospects for longer-term increases in soil C storage.
Results are broadly consistent with those from other plant communities and include: increases in below-ground net primary productivity and an increase in soil C cycling rate, changes in soil faunal community, and generally no increase in soil C storage. Based on other experimental data, future C storage could be favoured in soils of moderate nutrient status, moderate-to-high clay content, and low (or moderateIy high) soil moisture status. Some support for these suggestions is provided by preliminary results from direct measurements of soil C concentrations near a New Zealand natural CO₂-venting spring, and by simulations of future changes in grassland soils under the combined effects of CO₂ fertilization and regional climate change.
Early detection of any increase in soil C storage appears unlikely in complex grassland communities because of (a) the difficulty of separating an elevated CO₂ effect from the effects of soil factors including moisture status, (b) the high spatial variability of soil C and (c) the effects of global warming. Several research imperatives are identified for reducing the uncertainties in the effects of elevated atmospheric CO₂ on soil C.     

A review of nitrogen enrichment effects on three biogenic GHGs: the CO2 sink may be largely offset by stimulated N2O and CH4 emission

Anthropogenic nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application, alters biogeochemical cycling of ecosystems in a way that leads to altered flux of biogenic greenhouse gases (GHGs). Our meta-analysis of 313 observations across 109 studies evaluated the effect of N addition on the flux of three major GHGs: CO₂, CH₄ and N₂O. The objective was to quantitatively synthesize data from agricultural and non-agricultural terrestrial ecosystems across the globe and examine whether factors, such as ecosystem type, N addition level and chemical form of N addition influence the direction and magnitude of GHG fluxes. Results indicate that N addition increased ecosystem carbon content of forests by 6%, marginally increased soil organic carbon of agricultural systems by 2%, but had no significant effect on net ecosystem CO₂ exchange for non-forest natural ecosystems. Across all ecosystems, N addition increased CH₄ emission by 97%, reduced CH₄ uptake by 38% and increased N₂O emission by 216%. The net effect of N on the global GHG budget is calculated and this topic is reviewed. Most often N addition is considered to increase forest C sequestration without consideration of N stimulation of GHG production in other ecosystems. However, our study indicated that although N addition increased the global terrestrial C sink, the CO₂ reduction could be largely offset (53–76%) by N stimulation of global CH₄ and N₂O emission from multiple ecosystems.