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C Inputs (c + input)
Selected AbstractsAssessing the effect of elevated carbon dioxide on soil carbon: a comparison of four meta-analysesGLOBAL CHANGE BIOLOGY, Issue 8 2009BRUCE A. HUNGATE Abstract Soil is the largest reservoir of organic carbon (C) in the terrestrial biosphere and soil C has a relatively long mean residence time. Rising atmospheric carbon dioxide (CO2) concentrations generally increase plant growth and C input to soil, suggesting that soil might help mitigate atmospheric CO2 rise and global warming. But to what extent mitigation will occur is unclear. The large size of the soil C pool not only makes it a potential buffer against rising atmospheric CO2, but also makes it difficult to measure changes amid the existing background. Meta-analysis is one tool that can overcome the limited power of single studies. Four recent meta-analyses addressed this issue but reached somewhat different conclusions about the effect of elevated CO2 on soil C accumulation, especially regarding the role of nitrogen (N) inputs. Here, we assess the extent of differences between these conclusions and propose a new analysis of the data. The four meta-analyses included different studies, derived different effect size estimates from common studies, used different weighting functions and metrics of effect size, and used different approaches to address nonindependence of effect sizes. Although all factors influenced the mean effect size estimates and subsequent inferences, the approach to independence had the largest influence. We recommend that meta-analysts critically assess and report choices about effect size metrics and weighting functions, and criteria for study selection and independence. Such decisions need to be justified carefully because they affect the basis for inference. Our new analysis, with a combined data set, confirms that the effect of elevated CO2 on net soil C accumulation increases with the addition of N fertilizers. Although the effect at low N inputs was not significant, statistical power to detect biogeochemically important effect sizes at low N is limited, even with meta-analysis, suggesting the continued need for long-term experiments. [source] Increasing CO2 from subambient to elevated concentrations increases grassland respiration per unit of net carbon fixationGLOBAL CHANGE BIOLOGY, Issue 8 2006H. WAYNE POLLEY Abstract Respiration (carbon efflux) by terrestrial ecosystems is a major component of the global carbon (C) cycle, but the response of C efflux to atmospheric CO2 enrichment remains uncertain. Respiration may respond directly to an increase in the availability of C substrates at high CO2, but also may be affected indirectly by a CO2 -mediated alteration in the amount by which respiration changes per unit of change in temperature or C uptake (sensitivity of respiration to temperature or C uptake). We measured CO2 fluxes continuously during the final 2 years of a 4-year experiment on C3/C4 grassland that was exposed to a 200,560 ,mol mol,1 CO2 gradient. Flux measurements were used to determine whether CO2 treatment affected nighttime respiration rates and the response of ecosystem respiration to seasonal changes in net C uptake and air temperature. Increasing CO2 from subambient to elevated concentrations stimulated grassland respiration at night by increasing the net amount of C fixed during daylight and by increasing either the sensitivity of C efflux to daily changes in C fixation or the respiration rate in the absence of C uptake (basal ecosystem respiration rate). These latter two changes contributed to a 30,47% increase in the ratio of nighttime respiration to daytime net C influx as CO2 increased from subamient to elevated concentrations. Daily changes in net C uptake were highly correlated with variation in temperature, meaning that the shared contribution of C uptake and temperature in explaining variance in respiration rates was large. Statistically controlling for collinearity between temperature and C uptake reduced the effect of a given change in C influx on respiration. Conversely, CO2 treatment did not affect the response of grassland respiration to seasonal variation in temperature. Elevating CO2 concentration increased grassland respiration rates by increasing both net C input and respiration per unit of C input. A better understanding of how C efflux varies with substrate supply thus may be required to accurately assess the C balance of terrestrial ecosystems. [source] Plant and microbial N acquisition under elevated atmospheric CO2 in two mesocosm experiments with annual grassesGLOBAL CHANGE BIOLOGY, Issue 2 2005Shuijin Hu Abstract The impact of elevated CO2 on terrestrial ecosystem C balance, both in sign or magnitude, is not clear because the resulting alterations in C input, plant nutrient demand and water use efficiency often have contrasting impacts on microbial decomposition processes. One major source of uncertainty stems from the impact of elevated CO2 on N availability to plants and microbes. We examined the effects of atmospheric CO2 enrichment (ambient+370 ,mol mol,1) on plant and microbial N acquisition in two different mesocosm experiments, using model plant species of annual grasses of Avena barbata and A. fatua, respectively. The A. barbata experiment was conducted in a N-poor sandy loam and the A. fatua experiment was on a N-rich clayey loam. Plant,microbial N partitioning was examined through determining the distribution of a 15N tracer. In the A. barbata experiment, 15N tracer was introduced to a field labeling experiment in the previous year so that 15N predominantly existed in nonextractable soil pools. In the A. fatua experiment, 15N was introduced in a mineral solution [(15NH4)2SO4 solution] during the growing season of A. fatua. Results of both N budget and 15N tracer analyses indicated that elevated CO2 increased plant N acquisition from the soil. In the A. barbata experiment, elevated CO2 increased plant biomass N by ca. 10% but there was no corresponding decrease in soil extractable N, suggesting that plants might have obtained N from the nonextractable organic N pool because of enhanced microbial activity. In the A. fatua experiment, however, the CO2 -led increase in plant biomass N was statistically equal to the reduction in soil extractable N. Although atmospheric CO2 enrichment enhanced microbial biomass C under A. barbata or microbial activity (respiration) under A. fatua, it had no significant effect on microbial biomass N in either experiment. Elevated CO2 increased the colonization of A. fatua roots by arbuscular mycorrhizal fungi, which coincided with the enhancement of plant competitiveness for soluble soil N. Together, these results suggest that elevated CO2 may tighten N cycling through facilitating plant N acquisition. However, it is unknown to what degree results from these short-term microcosm experiments can be extrapolated to field conditions. Long-term studies in less-disturbed soils are needed to determine whether CO2 -enhancement of plant N acquisition can significantly relieve N limitation over plant growth in an elevated CO2 environment. [source] Temperature and soil moisture effects on dissolved organic matter release from a moorland Podzol O horizon under field and controlled laboratory conditionsEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 5 2007M. I. Stutter Summary Organic upland soils store large amounts of humified organic matter. The mechanisms controlling the leaching of this C pool are not completely understood. To examine the effects of temperature and microbial cycling on C leaching, we incubated five unvegetated soil cores from a Podzol O horizon (from NE Scotland), over a simulated natural temperature cycle for 1 year, whilst maintaining a constant soil moisture content. Soil cores were leached with artificial rain (177 mm each, monthly) and the leachates analysed for dissolved organic carbon (DOC) and their specific C-normalized UV absorbance determined (SUVA, 285 nm). Monthly values of respiration of the incubated soils were determined as CO2 efflux. To examine the effects of vegetation C inputs and soil moisture, in addition to temperature, we sampled O horizon pore waters in situ and collected five additional field soil cores every month. The field cores were leached under controlled laboratory conditions. Hysteresis in the monthly amount of DOC leached from field cores resulted in greater DOC on the rising, than falling temperature phases. This hysteresis suggested that photosynthetic C stimulated greater DOC losses in early summer, whereas limitations in the availability of soil moisture in late summer suppressed microbial decomposition and DOC loss. Greater DOC concentrations of in-situ pore waters than for any core leachates were attributed to the effects of soil drying and physico-chemical processes in the field. Variation in the respiration rates for the incubated soils was related to temperature, and respiration provided a greater pathway of C loss (44 g C m,2 year,1) than DOC (7.2 g C m,2 year,1). Changes in SUVA over spring and summer observed in all experimental systems were related to the period of increased temperature. During this time, DOC became less aromatic, which suggests that lower molecular weight labile compounds were not completely mineralized. The ultimate DOC source appears to be the incomplete microbial decomposition of recalcitrant humified C. In warmer periods, any labile C that is not respired is leached, but in autumn either labile C production ceases, or it is sequestered in soil biomass. [source] Respiratory carbon loss of calcareous grasslands in winter shows no effects of 4 years' CO2 enrichmentFUNCTIONAL ECOLOGY, Issue 2 2002M. Volk Summary 1CO2 exchange measurements in long-term CO2 -enrichment experiments suggest large net carbon gains by ecosystems during the growing season that are not accounted for by above-ground plant biomass. Considerable amounts of C might therefore be allocated below ground. 2Winter ecosystem respiration from temperate grasslands under elevated CO2 may account for the loss of a significant part of the extra C gained during the growing season. To test this hypothesis, dark respiration was assessed throughout the winter of the fourth year of CO2 enrichment in a calcareous grassland. 3Using these data, a model was parameterized to estimate whole-winter respiratory CO2 losses. From November to February, 154 9 g C m,2 were respired under elevated CO2 and 144 5 g C m,2 under ambient [CO2], with no significant difference between the CO2 treatments. 4We conclude that (i) wintertime respiration does not constitute a larger C loss from the ecosystem at elevated CO2; and (ii) the absence of respiratory responses implies no extra growing-season C inputs with month-to-year turnover times at elevated CO2. [source] Indirect effects of soil moisture reverse soil C sequestration responses of a spring wheat agroecosystem to elevated CO2GLOBAL CHANGE BIOLOGY, Issue 1 2010SVEN MARHAN Abstract Increased plant productivity under elevated atmospheric CO2 concentrations might increase soil carbon (C) inputs and storage, which would constitute an important negative feedback on the ongoing atmospheric CO2 rise. However, elevated CO2 often also leads to increased soil moisture, which could accelerate the decomposition of soil organic matter, thus counteracting the positive effects via C cycling. We investigated soil C sequestration responses to 5 years of elevated CO2 treatment in a temperate spring wheat agroecosystem. The application of 13C-depleted CO2 to the elevated CO2 plots enabled us to partition soil C into recently fixed C (Cnew) and pre-experimental C (Cold) by 13C/12C mass balance. Gross C inputs to soils associated with Cnew accumulation and the decomposition of Cold were then simulated using the Rothamsted C model ,RothC.' We also ran simulations with a modified RothC version that was driven directly by measured soil moisture and temperature data instead of the original water balance equation that required potential evaporation and precipitation as input. The model accurately reproduced the measured Cnew in bulk soil and microbial biomass C. Assuming equal soil moisture in both ambient and elevated CO2, simulation results indicated that elevated CO2 soils accumulated an extra ,40,50 g C m,2 relative to ambient CO2 soils over the 5 year treatment period. However, when accounting for the increased soil moisture under elevated CO2 that we observed, a faster decomposition of Cold resulted; this extra C loss under elevated CO2 resulted in a negative net effect on total soil C of ,30 g C m,2 relative to ambient conditions. The present study therefore demonstrates that positive effects of elevated CO2 on soil C due to extra soil C inputs can be more than compensated by negative effects of elevated CO2 via the hydrological cycle. [source] Changes in topsoil carbon stock in the Tibetan grasslands between the 1980s and 2004GLOBAL CHANGE BIOLOGY, Issue 11 2009YUANHE YANG Abstract Climate warming is likely inducing carbon loss from soils of northern ecosystems, but little evidence comes from large-scale observations. Here we used data from a repeated soil survey and remote sensing vegetation index to explore changes in soil organic carbon (SOC) stock on the Tibetan Plateau during the past two decades. Our results showed that SOC stock in the top 30 cm depth in alpine grasslands on the plateau amounted to 4.4 Pg C (1 Pg=1015 g), with an overall average of 3.9 kg C m,2. SOC changes during 1980s,2004 were estimated at ,0.6 g C m,2 yr,1, ranging from ,36.5 to 35.8 g C m,2 yr,1 at 95% confidence, indicating that SOC stock in the Tibetan alpine grasslands remained relatively stable over the sampling periods. Our findings are nonconsistent with previous reports of loss of soil C in grassland ecosystems due to the accelerated decomposition with warming. In the case of the alpine grasslands on the Tibetan Plateau studied here, we speculate that increased rates of decomposition as soils warmed during the last two decades may have been compensated by increased soil C inputs due to increased grass productivity. These results suggest that soil C stock in terrestrial ecosystems may respond differently to climate change depending on ecosystem type, regional climate pattern, and intensity of human disturbance. [source] Enhanced litter input rather than changes in litter chemistry drive soil carbon and nitrogen cycles under elevated CO2: a microcosm studyGLOBAL CHANGE BIOLOGY, Issue 2 2009LINGLI LIU Abstract Elevated CO2 has been shown to stimulate plant productivity and change litter chemistry. These changes in substrate availability may then alter soil microbial processes and possibly lead to feedback effects on N availability. However, the strength of this feedback, and even its direction, remains unknown. Further, uncertainty remains whether sustained increases in net primary productivity will lead to increased long-term C storage in soil. To examine how changes in litter chemistry and productivity under elevated CO2 influence microbial activity and soil C formation, we conducted a 230-day microcosm incubation with five levels of litter addition rate that represented 0, 0.5, 1.0, 1.4 and 1.8 × litterfall rates observed in the field for aspen stand growing under control treatments at the Aspen FACE experiment in Rhinelander, WI, USA. Litter and soil samples were collected from the corresponding field control and elevated CO2 treatment after trees were exposed to elevated CO2 (560 ppm) for 7 years. We found that small decreases in litter [N] under elevated CO2 had minor effects on microbial biomass carbon, microbial biomass nitrogen and dissolved inorganic nitrogen. Increasing litter addition rates resulted in linear increase in total C and new C (C from added litter) that accumulated in whole soil as well as in the high density soil fraction (HDF), despite higher cumulative C loss by respiration. Total N retained in whole soil and in HDF also increased with litter addition rate as did accumulation of new C per unit of accumulated N. Based on our microcosm comparisons and regression models, we expected that enhanced C inputs rather than changes in litter chemistry would be the dominant factor controlling soil C levels and turnover at the current level of litter production rate (230 g C m,2 yr,1 under ambient CO2). However, our analysis also suggests that the effects of changes in biochemistry caused by elevated CO2 could become significant at a higher level of litter production rate, with a trend of decreasing total C in HDF, new C in whole soil, as well as total N in whole soil and HDF. [source] Sequestration and turnover of plant- and microbially derived sugars in a temperate grassland soil during 7 years exposed to elevated atmospheric pCO2GLOBAL CHANGE BIOLOGY, Issue 2 2007MICHAEL BOCK Abstract Temperate grasslands contribute about 20% to the global terrestrial carbon (C) budget with sugars contributing 10,50% to this soil C pool. Whether the observed increase of the atmospheric CO2 concentration (pCO2) leads to additional C sequestration into these ecosystems or enhanced mineralization of soil organic matter (SOM) is still unclear. Therefore, the aim of the presented study was to investigate the impact of elevated atmospheric pCO2 on C sequestration and turnover of plant- (arabinose and xylose) and microbially derived (fucose, rhamnose, galactose, mannose) sugars in soil, representing a labile SOM pool. The study was carried out at the Swiss Free Air Carbon Dioxide Enrichment (FACE) experiment near Zurich. For 7 years, Lolium perenne swards were exposed to ambient and elevated pCO2 (36 and 60 Pa, respectively). The additional CO2 in the FACE plots was depleted in 13C compared with ambient plots, so that ,new' (<7 years) C inputs could be determined by means of compound-specific stable isotope analysis (13C : 12C). Samples were fractionated into clay, silt, fine sand and coarse sand, which yielded relatively stable and labile SOM pools with different turnover rates. Total sugar sequestration into bulk soil after 7 years of exposure to elevated pCO2 was about 28% compared with the control plots. In both ambient and elevated plots, total sugar concentrations in particle size fractions increased in the order sand Linkages between plant functional composition, fine root processes and potential soil N mineralization ratesJOURNAL OF ECOLOGY, Issue 1 2009Dario A. Fornara Summary 1Plant functional composition may indirectly affect fine root processes both qualitatively (e.g. by influencing root chemistry) and quantitatively (e.g. by influencing root biomass and thus soil carbon (C) inputs and the soil environment). Despite the potential implications for ecosystem nitrogen (N) cycling, few studies have addressed the linkages between plant functional composition, root decay, root detritus N dynamics and soil N mineralization rates. 2Here, using data from a large grassland biodiversity experiment, we first show that plant functional composition affected fine root mass loss, root detritus N dynamics and net soil N mineralization rates through its effects on root chemistry rather than on the environment of decomposition. In particular, the presence of legumes and non-leguminous forbs contributed to greater fine root decomposition which in turn enhanced root N release and net soil N mineralization rates compared with C3 and C4 grasses. 3Second, we show that all fine roots released N immediately during decomposition and showed very little N immobilization regardless of plant composition. As a consequence, there was no evidence of increased root or soil N immobilization rates with increased below-ground plant biomass (i.e. increased soil C inputs) even though root biomass negatively affected root decay. 4Our results suggest that fine roots represent an active soil N pool that may sustain plant uptake while other soil N forms are being immobilized in microbial biomass and/or sequestered into soil organic matter. However, fine roots may also represent a source of recalcitrant plant detritus that is returned to the soil (i.e. fine roots of C4 and C3 grasses) and that can contribute to an increase in the soil organic matter pool. 5Synthesis. An important implication of our study is that the simultaneous presence of different plant functional groups (in plant mixtures) with opposite effects on root mass loss, root N release and soil N mineralization rates may be crucial for sustaining multiple ecosystem services such as productivity and soil C and N sequestration in many N-limited grassland systems. [source] Balancing fertility management and economics in organic field vegetable rotationsJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 15 2007Ulrich Schmutz Abstract BACKGROUND: Organic field-scale vegetables are among the most profitable enterprises in organic farming systems. They are also some of the most nutrient-demanding crops, and many organic arable systems with field-scale vegetables are stockless. Without livestock manure inputs, nutrient supply depends on fertility-building crops, which generate only costs and no income. Different strategies of fertility management were compared on a central England research farm. Fertility management treatments consisted of different lengths of fertility building with green waste compost additions. Outputs and inputs in terms of nutrients and economics were monitored for 31 rotations during 1996,2002. RESULTS: N, P and K rotational nutrient balances, as well as C inputs, showed a negative relationship with rotational gross margins. Variable and allocated fixed costs of fertility building were low, between 2 and 5% of variable costs (£0.5,2 ha,1 for 1 kg N ha,1 supplied to the rotation). The intensity of vegetable cropping in these rotations was moderate (25,40% vegetable crops in the rotation) and balancing of fertility management and economics was possible at this intensity without livestock manure or other permitted fertiliser additions. CONCLUSION: Completely stockless systems (in analogy may be called vegan) are possible in organic vegetable production without compromising on fertility or economics. However, for a higher vegetable-cropping intensity (up to 90%) a more sophisticated mix of short-term fertility-building and N-trapping crops will be needed and such rotations may require further external addition of green waste or livestock manure. Copyright © 2007 Society of Chemical Industry [source]
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