Stable forest–savanna mosaic in north‐western Tanzania: local‐scale evidence from δ13C signatures and 14C ages of soil fractions

Aim The spatio‐temporal dynamics of dry evergreen forest patches in the savanna biome of the Kagera region (north‐western Tanzania) are largely unknown owing to a lack of pollen and macrofossil evidence. Our aims were to reconstruct local‐scale shifts of the forest–savanna boundary in order to determine whether the forests have been expanding or retreating on a centennial and millennial time‐scale. Location The Kagera region of north‐western Tanzania, East Africa. Methods The vegetation reconstruction was based on analysing δ¹³C signatures in soils along a transect spanning both C₄ open savanna and C₃ forest vegetation. Furthermore, we fractionated soil organic matter (SOM) according to density and chemical stability to analyse δ¹³C values of soil fractions with distinct radiocarbon ages. Results We found sharp changes in δ¹³C signatures in bulk SOM from the forest to the savanna, within a few metres along the transect. The forest soil profiles carried a persistent C₃‐dominated signature. Radiocarbon dating of the oldest, most recalcitrant forest soil fraction yielded a mean age of 5500 cal. yr bp , demonstrating that the forest has existed since at least the mid‐Holocene. The savanna sites showed a typical C₄ isotopic signature in SOM of topsoils, but subsoils and more recalcitrant SOM fractions also contained signals of C₃ plants. The dense soil fraction (ρ > 1.6 g cm^?3) carrying a pure C₄ label had a mean age of c. 1200 cal. yr bp , indicating the minimum duration of the dominance of grass vegetation on the savanna site. At the forest edge, the older C₄ grass signature of SOM has steadily been replaced by the more negative δ¹³C fingerprint of the forest trees. As this replacement has occurred mainly in the 10‐m‐wide forest–savanna ecotone over the last c. 1200 years, the forest expansion must be very slow and is very likely less than 15 m century^?1. Main conclusions Our results suggest that forest patches in the Kagera savanna landscape are very stable vegetation formations which have persisted for millennia. During the last millennium, they have been expanding very slowly into the surrounding savanna at a rate of less than 15 m century^?1.     

Carbon sequestration and turnover in soil under the energy crop Miscanthus: repeated 13C natural abundance approach and literature synthesis

The stability and turnover of soil organic matter (SOM) are a very important but poorly understood part of carbon (C) cycling. Conversion of C₃ grassland to the C₄ energy crop Miscanthus provides an ideal opportunity to quantify medium‐term SOM dynamics without disturbance (e.g., plowing), due to the natural shift in the δ¹³C signature of soil C. For the first time, we used a repeated ¹³C natural abundance approach to measure C turnover in a loamy Gleyic Cambisol after 9 and 21 years of Miscanthus cultivation. This is the longest C₃–C₄ vegetation change study on C turnover in soil under energy crops. SOM stocks under Miscanthus and reference grassland were similar down to 1 m depth. However, both increased between 9 and 21 years from 105 to 140 mg C ha^?1 (P < 0.05), indicating nonsteady state of SOM. This calls for caution when estimating SOM turnover based on a single sampling. The mean residence time (MRT) of old C (>9 years) increased with depth from 19 years (0–10 cm) to 30–152 years (10–50 cm), and remained stable below 50 cm. From 41 literature observations, the average SOM increase after conversion from cropland or grassland to Miscanthus was 6.4 and 0.4 mg C ha^?1, respectively. The MRT of total C in topsoil under Miscanthus remained stable at ~60 years, independent of plantation age, corroborating the idea that C dynamics are dominated by recycling processes rather than by C stabilization. In conclusion, growing Miscanthus on C‐poor arable soils caused immediate C sequestration because of higher C input and decreased SOM decomposition. However, after replacing grasslands with Miscanthus, SOM stocks remained stable and the MRT of old C₃‐C increased strongly with depth.     

Soil carbon storage under simulated climate change is mediated by plant functional type

The stability of soil organic matter (SOM) pools exposed to elevated CO₂ and warming has not been evaluated adequately in long‐term experiments and represents a substantial source of uncertainty in predicting ecosystem feedbacks to climate change. We conducted a 6‐year experiment combining free‐air CO₂ enrichment (FACE, 550 ppm) and warming (+2 °C) to evaluate changes in SOM accumulation in native Australian grassland. In this system, competitive interactions appear to favor C₄ over C₃ species under FACE and warming. We therefore investigated the role of plant functional type (FT) on biomass and SOM responses to the long‐term treatments by carefully sampling soil under patches of C₃‐ and C₄‐dominated vegetation. We used physical fractionation to quantify particulate organic matter (POM) and long‐term incubation to assess potential decomposition rates. Aboveground production of C₄ grasses increased in response to FACE, but total root biomass declined. Across treatments, C : N ratios were higher in leaves, roots and POM of C₄ vegetation. CO₂ and temperature treatments interacted with FT to influence SOM, and especially POM, such that soil carbon was increased by warming under C₄ vegetation, but not in combination with elevated CO₂. Potential decomposition rates increased in response to FACE and decreased with warming, possibly owing to treatment effects on soil moisture and microbial community composition. Decomposition was also inversely correlated with root N concentration, suggesting increased microbial demand for older, N‐rich SOM in treatments with low root N inputs. This research suggests that C₃–C₄ vegetation responses to future climate conditions will strongly influence SOM storage in temperate grasslands.     

Interpretation of soil δ13C as an indicator of vegetation change in African savannas

Question: The relationship between carbon‐13 in soil organic matter and C₃ and C₄ plant abundance is complicated because of differential productivity, litter fall and decomposition. As a result, applying a mass balance equation to δ¹³C data from soils cannot be used to infer past C₃ and C₄ plant abundance; only the proportion of carbon derived from C₃ and C₄ plants can be estimated. In this paper, we compare δ¹³C of surface soil samples with vegetation data, in order to establish whether the ratio of C₃:C₄ plants (rather than the proportion of carbon from C₃ and C₄ plants) can be inferred from soil δ¹³C. Location: The Tsavo National Park, in southeastern Kenya. Methods: We compare vegetation data with δ¹³C of organic matter in surface soil samples and derive regression equations relating the δ¹³C of soil organic matter to C₃:C₄ plant abundance. We use these equations to interpret δ¹³C data from soil profiles in terms of changes in inferred C₃:C₄ plant ratio. We compare our method of interpretation with that derived from a mass balance approach. Results: There was a statistically significant, linear relationship between the δ¹³C of organic matter in surface soil samples and the natural logarithm of the ratio of C₃:C₄ plants in the 100m² surrounding the soil sample. Conclusions: We suggest that interpretation of δ¹³C data from organic matter in soil profiles can be improved by comparing vegetation surveys with δ¹³C of organic matter in surface soil samples. Our results suggest that past C₃ plant abundance might be under‐estimated if a mass balance approach is used.     

Using Soil 13C to Detect the Historic Presence of Schizachyrium scoparium (Little Bluestem) Grasslands on Martha's Vineyard

Abstract We used differences in soil carbon δ¹³C values between forested sites and grasslands dominated by the C₄ grass Schizachyrium scoparium (little bluestem) to detect the presence of former grasslands in the historical landscape of the coastal sand plain of Martha's Vineyard, Massachusetts, U.S.A. Soil δ¹³C was measured at (1) sites with long‐term forest or grassland vegetation and (2) sites with known histories where forest vegetation invaded grassland and where forest converted to grassland. The δ¹³C of soil under long‐term grassland was –24.1‰ at 0 to 2 cm depth and –23.4‰ at 2 to 10 cm and was enriched by 3.4‰ and 2.8‰ compared with soil under long‐term forest. In forests that invaded grasslands dominated by S. scoparium, soil δ¹³C decreased as C derived from trees replaced C from S. scoparium. This decline occurred faster in surface soils and in the light soil organic matter fraction than in the mineral soil. In forests that converted to grasslands, soil δ¹³C increased and the rate of increase was similar in surface and mineral soil and in the different soil organic matter fractions. Rates of change indicated that soil δ¹³C could be used to detect changes in vegetation involving the presence or absence of S. scoparium during the last 150 years. Application of this model to a potential grassland restoration site on Martha's Vineyard where the landscape history was not known indicated that the site was previously unoccupied by S. scoparium during this time. The δ¹³C of surface mineral soil can be useful for detecting the presence of historic S. scoparium grasslands but only in the period well after European settlement of these coastal sand plain landscapes.     

Climate controls on C3 vs. C4 productivity in North American grasslands from carbon isotope composition of soil organic matter

We analyzed the δ¹³C of soil organic matter (SOM) and fine roots from 55 native grassland sites widely distributed across the US and Canadian Great Plains to examine the relative production of C₃ vs. C₄ plants (hereafter %C₄) at the continental scale. Our climate vs. %C₄ results agreed well with North American field studies on %C₄, but showed bias with respect to %C₄ from a US vegetation database (statsgo ) and weak agreement with a physiologically based prediction that depends on crossover temperature. Although monthly average temperatures have been used in many studies to predict %C₄, our analysis shows that high temperatures are better predictors of %C₄. In particular, we found that July climate (average of daily high temperature and month's total rainfall) predicted %C₄ better than other months, seasons or annual averages, suggesting that the outcome of competition between C₃ and C₄ plants in North American grasslands was particularly sensitive to climate during this narrow window of time. Root δ¹³C increased about 1‰ between the A and B horizon, suggesting that C₄ roots become relatively more common than C₃ roots with depth. These differences in depth distribution likely contribute to the isotopic enrichment with depth in SOM where both C₃ and C₄ grasses are present.     

Feedback from plant species change amplifies CO2 enhancement of grassland productivity

Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO₂ enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO₂ stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C₄ grass species were grown for 5 years along a field CO₂ gradient (250–500 μL L^?1) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO₂ enrichment increased community ANPP by 0–117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO₂‐induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP‐CO₂ regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction. This feedback accounted for 30–60% of the CO₂‐mediated increase in ANPP on the upland and lowland clay soils during the final 3 years and 1 year of the experiment, respectively. By contrast, species change had little influence on the ANPP‐CO₂ response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally similar C₄ grass species. By favoring a mesic C₄ tall grass, CO₂ enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO₂‐stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.     

Effect of C3–C4 Vegetation Change on δ13C and δ15N Values of Soil Organic Matter Fractions Separated by Thermal Stability

Carbon isotopic composition of soils subjected to C₃–C₄ vegetation change can be used to estimate C turnover in bulk soil and in soil organic matter (SOM) pools with fast and intermediate turnover rates. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability, so that thermogravimetry can be used to separate SOM pools with contrasting turnover rates. Soil samples from a field plot cultivated for 10.5 years with the perennial C₄ plant Miscanthus×gigantheus were analyzed by thermogravimetry coupled with differential scanning calorimetry (DSC). Three SOM fractions were distinguished according to the differential weight losses and exothermic or endothermic reactions measured by DSC. The δ¹³C and δ¹⁵N values of these three fractions obtained by gradual soil heating were measured by IRMS. The weight losses up to 190 °C mainly reflected water evaporation because no significant C and N losses were detected and δ¹³C and δ¹⁵N values of the residual SOM remained unchanged. The δ¹³C values (−16.4‰) of SOM fraction decomposed between 190 and 390 °C (containing 79% of total soil C) were slightly closer to that of the Miscanthus plant tissues (δ¹³C = −11.8‰) compared to the δ¹³C values (−16.8‰) of SOM fraction decomposed above 390 °C containing the residual 21% of SOM. Thus, the C turnover in the thermally labile fraction was faster than that in thermally stable fractions, but the differences were not very strong. Therefore, in this first study combining TG-DSC with isotopic analysis, we conclude that the thermal stability of SOM was not very strongly related to biological availability of SOM fractions. In contrast to δ¹³C, the δ¹⁵N values strongly differed between SOM fractions, suggesting that N turnover in the soil was different from C turnover. More detailed fractionation of SOM by thermal analysis with subsequent isotopic analysis may improve the resolution for δ¹³C.     

Soil carbon dynamics following land‐use change varied with temperature and precipitation gradients: evidence from stable isotopes

Knowledge of soil organic matter (SOM) dynamics following deforestation or reforestation is essential for evaluating carbon (C) budgets and cycle at regional or global scales. Worldwide land‐use changes involving conversion of vegetation with different photosynthetic pathways (e.g. C₃ and C₄) offer a unique opportunity to quantify SOM decomposition rate and its response to climatic conditions using stable isotope techniques. We synthesized the results from 131 sites (including 87 deforestation observations and 44 reforestation observations) which were compiled from 36 published papers in the literatures as well as our observations in China's Qinling Mountains. Based on the ¹³C natural abundance analysis, we evaluated the dynamics of new and old C in top soil (0–20 cm) following land‐use change and analyzed the relationships between soil organic C (SOC) decomposition rates and climatic factors. We found that SOC decomposition rates increased significantly with mean annual temperature and precipitation in the reforestation sites, and they were not related to any climatic factor in deforestation sites. The mean annual temperature explained 56% of variation in SOC decomposition rates by exponential model (y = 0.0014e^0.1395x) in the reforestation sites. The proportion of new soil C increased following deforestation and reforestation, whereas the old soil C showed an opposite trend. The proportion of new soil C exceeded the proportion of old soil C after 45.4 years' reforestation and 43.4 years' deforestation, respectively. The rates of new soil C accumulation increased significantly with mean annual precipitation and temperature in the reforestation sites, yet only significantly increased with mean annual precipitation in the deforestation sites. Overall, our study provides evidence that SOC decomposition rates vary with temperature and precipitation, and thereby implies that global warming may accelerate SOM decomposition.     

The use of carbon isotope ratios to evaluate legume contribution to soil enhancement in tropical pastures

Soil carbon distribution with depth, stable carbon isotope ratios in soil organic matter and their changes as a consequence of the presence of legume were studied in three 12-year-old tropical pastures (grass alone —Brachiaria decumbens (C₄), legume alone —Pueraria phaseoloides (C₃) and grass + legume) on an Oxisol in Colombia. The objective of this study was to determine the changes that occurred in the¹³C isotope composition of soil from a grass + legume pasture that was established by cultivation of a native savanna dominated by C₄ vegetation. The¹³C natural abundance technique was used to estimate the amount of soil organic carbon originating from the legume. Up to 29% of the organic carbon in soil of the grass + legume pasture was estimated to be derived from legume residues in the top 0–2-cm soil depth, which decreased to 7% at 8–10 cm depth. Improvements in soil fertility resulting from the soil organic carbon originated from legume residues were measured as increased potential rates of nitrogen mineralization and increased yields of rice in a subsequent crop after the grass + legume pasture compared with the grass-only pasture. We conclude that the¹³C natural abundance technique may help to predict the improvements in soil quality in terms of fertility resulting from the presence of a forage legume (C₃) in a predominantly C₄ grass pasture.     

Soil carbon stocks and carbon sequestration rates in seminatural grassland in Aso region,Kumamoto, Southern Japan

Global soil carbon (C) stocks account for approximately three times that found in the atmosphere. In the Aso mountain region of Southern Japan, seminatural grasslands have been maintained by annual harvests and/or burning for more than 1000 years. Quantification of soil C stocks and C sequestration rates in Aso mountain ecosystem is needed to make well‐informed, land‐use decisions to maximize C sinks while minimizing C emissions. Soil cores were collected from six sites within 200 km² (767–937 m asl.) from the surface down to the k‐Ah layer established 7300 years ago by a volcanic eruption. The biological sources of the C stored in the Aso mountain ecosystem were investigated by combining C content at a number of sampling depths with age (using ¹⁴C dating) and δ¹³C isotopic fractionation. Quantification of plant phytoliths at several depths was used to make basic reconstructions of past vegetation and was linked with C‐sequestration rates. The mean total C stock of all six sites was 232 Mg C ha^?1 (28–417 Mg C ha^?1), which equates to a soil C sequestration rate of 32 kg C ha^?1 yr^?1 over 7300 years. Mean soil C sequestration rates over 34, 50 and 100 years were estimated by an equation regressing soil C sequestration rate against soil C accumulation interval, which was modeled to be 618, 483 and 332 kg C ha^?1 yr^?1, respectively. Such data allows for a deeper understanding in how much C could be sequestered in Miscanthus grasslands at different time scales. In Aso, tribe Andropogoneae (especially Miscanthus and Schizoachyrium genera) and tribe Paniceae contributed between 64% and 100% of soil C based on δ¹³C abundance. We conclude that the seminatural, C₄‐dominated grassland system serves as an important C sink, and worthy of future conservation.     

Natural abundance of 13C and 15N in C3 and C4 vegetation of southern Africa: patterns and implications

The mean annual rainfall in southern Africa is found to explain over half of the observed variance in the stable nitrogen (N) isotopic signatures of C₃ vegetation in southern Africa (r²=0.54, P<0.01). The inverse relationship between the stable N isotopic signatures of foliar samples from C₃ vegetation and long‐term southern African rainfall is found on a scale larger than previously observed. A modest relationship is found between stable carbon (C) isotopic signatures of C₃ vegetation and rainfall across the region (r²=0.20, P<0.01). No such relationship is found between stable C and N isotopic signatures of C₄ vegetation and rainfall. The explanation of the relationship between ¹⁵N in C₃ vegetation and the mean annual rainfall presented here is that nutrient availability varies inversely with water availability. This suggests that water‐limited systems in southern Africa are more open in terms of nutrient cycling and therefore the resulting natural abundance of foliar ¹⁵N in these systems is enriched. The use of this relationship may be of value to those researchers modeling both the dynamics of vegetation and biogeochemistry across this region. The use of the isotopic enrichment in C₃ vegetation as a function of rainfall may provide an insight into nutrient cycling across the semi‐arid and arid regions of southern Africa. This finding has implications for the study of global change, especially as it relates to vegetation responses to changing regional rainfall regimes over time.     

Decadal cycling within long-lived carbon pools revealed by dual isotopic analysis of mineral-associated soil organic matter

Long-lived soil organic matter (SOM) pools are critical for the global carbon (C) cycle, but challenges in isolating such pools have inhibited understanding of their dynamics. We physically isolated particulate (>53 μm), silt-, and clay-sized organic matter from soils collected over two decades from a perennial C₃ grassland established on long-term agricultural soil with a predominantly C₄ isotopic signature. Silt- and clay-sized fractions were then subjected to a sequential chemical fractionation (acid hydrolysis followed by peroxide oxidation) to isolate long-lived C pools. We quantified ¹⁴C and the natural ¹³C isotopic label in the resulting fractions to identify and evaluate pools responsible for long-lived SOM. After removal of particulate organic matter (~14% of bulk soil C) sequential chemical treatment removed 80% of mineral-associated C. In all mineral-associated fractions, at least 55% of C₄-derived C was retained 32 years after the switch to C₃ inputs. However, C₃–C increased substantially beginning ~25 years after the switch. Radiocarbon-based turnover times ranged from roughly 1200–3000 years for chemically resistant mineral-associated pools, although some pools turned over faster under C₃ grassland than in a reference agricultural field, indicating that new material had entered some pools as early as 14 years after the vegetation switch. These findings provide further evidence that SOM chemistry does not always reflect SOM longevity and resistance to microbial decomposition. Even measureable SOM fractions that have extremely long mean turnover times (>1500 years) can have a substantial component that is dynamic over much shorter timescales.     

Grass water stress estimated from phytoliths in West Africa

Aim This study calibrates the relationship between phytolith indices, modern vegetation structure, and a climate parameter (AET/PET, i.e. the ratio of annual actual evapotranspiration to annual potential evapotranspiration), in order to present new proxies for long‐term Quaternary climate and vegetation changes, and model/data comparisons. Location Sixty‐two modern soil surface samples from West Africa (Mauritania and Senegal), collected along a latitudinal transect across four bioclimatic zones, were analysed. Methods Two phytolith indices are defined as normalized data: (1) humidity‐aridity index [Iph (%) = saddle vs. cross + dumbbell + saddle], and (2) water stress index [fan‐shaped index (Fs) (%) = fan‐shaped vs. sum of characteristic phytoliths]. Vegetation structures are delimited according to Iph and Fs boundaries. Bootstrapped regression methods are used for evaluating the strength of the relationship between the two phytolith indices and AET/PET. Additional modern phytolith assemblages, from Mexico, Cameroon and Tanzania are extracted in order to test the calibration established from the West African samples. Accuracy of the AET/PET phytolith proxy is compared with equivalent pollen proxy from the same area. Results Characterization of the grass cover is accurately made through Iph. A boundary of 20 ± 1.4% discriminates tall grass savannas from short grass savannas. Water stress and transpiration experienced by the grass cover can be estimated through Fs. AET/PET is accurately estimated from phytoliths by a transfer function: AET/PET = ?0.605 Fs ? 0.387 Iph + 0.272 (Iph – 20)² (r = 0.80 ± 0.04) in the application domain (AET/PET ranging from 0.1 ± 0.04 to 0.45 ± 0.04). Phytolith and pollen estimate with similar precision (r_pollen = 0.84 ± 0.04) the AET/PET in the studied area. Conclusions This study demonstrates that we can rely on the phytolith indices Iph and Fs to distinguish the different grasslands in tropical areas. Moreover, a new phytolith proxy of AET/PET, linked to water availability, is presented. We suggest from these results that combining phytolith and pollen proxies of AET/PET would help to constrain this climate parameter better, especially when phytolith assemblages are dominated by Panicoideae and Chloridoideae C₄‐grass phytoliths, are devoid of Pooideae C₃‐grass phytoliths, and occur with a few tropical ligneous woody dicotyledon phytoliths. As AET/PET is a bioclimatic indicator commonly used in vegetation models, such a combination would help to make model/data comparisons more efficient.     

Vegetation dynamics in a Quercus‐Juniperus savanna: An isotopic assessment

Abstract. Woody plants are increasing in many grassland and savanna ecosystems around the world. As a case in point, the Edwards Plateau of Texas, USA, is a vast region (93 000 km²) in which rapid woody encroachment appears to be occurring. The native vegetation (prior to the Anglo‐European settlement 150–200 yr ago) and the biogeochemical consequences of woody encroachment in this region, however, are poorly understood. To assess these matters we measured plant and soil δ¹³C, soil organic C and soil N content from grasslands and two important woody patch types (mature Quercus virginiana clusters and Juniperus ashei woodlands) in this region. Soil δ¹³C values showed that relative productivity of C₃ species has increased in grassland and both woody habitats in recent times. δ¹³C of SOC in grasslands and Q. virginiana clusters increased with depth from the litter layer to 30 cm (grasslands =?21 to ?13‰Q. virginiana clusters =?27 to ?17‰) and were significantly different between habitats at all depths, indicating that Q. virginiana has been a long‐term component of the landscape. In J. ashei woodlands, soil δ¹³C values (at 20–30 cm depth) near the woodland edge (‐13‰) converged with those of an adjacent grassland (‐13‰) while those from the woodland interior (‐15‰) remained distinct, indicating that the woodland has been present for many years but has recently expanded. Concentrations and densities of SOC and total N were generally greater in woody patches than in grasslands. However, differences in the amount of SOC and N stored beneath the two woody patch types indicates that C and N sequestration potentials are species dependent.     

Within trophic level shifts in collagen–carbonate stable carbon isotope spacing are propagated by diet and digestive physiology in large mammal herbivores

Stable carbon isotope analyses of vertebrate hard tissues such as bones, teeth, and tusks provide information about animal diets in ecological, archeological, and paleontological contexts. There is debate about how carbon isotope compositions of collagen and apatite carbonate differ in terms of their relationship to diet, and to each other. We evaluated relationships between δ¹³C_collagen and δ¹³C_carbonate among free‐ranging southern African mammals to test predictions about the influences of dietary and physiological differences between species. Whereas the slopes of δ¹³C_collagen–δ¹³C_carbonate relationships among carnivores are ≤1, herbivore δ¹³C_collagen increases with increasing dietary δ¹³C at a slower rate than does δ¹³C_carbonate, resulting in regression slopes >1. This outcome is consistent with predictions that herbivore δ¹³C_collagen is biased against low protein diet components (¹³C‐enriched C₄ grasses in these environments), and δ¹³C_carbonate is ¹³C‐enriched due to release of ¹³C‐depleted methane as a by‐product of microbial fermentation in the digestive tract. As methane emission is constrained by plant secondary metabolites in browse, the latter effect becomes more pronounced with higher levels of C₄ grass in the diet. Increases in δ¹³C_carbonate are also larger in ruminants than nonruminants. Accordingly, we show that Δ¹³C_collagen‐_carbonate spacing is not constant within herbivores, but increases by up to 5 ‰ across species with different diets and physiologies. Such large variation, often assumed to be negligible within trophic levels, clearly cannot be ignored in carbon isotope‐based diet reconstructions.     

Root exudation (net efflux of amino acids) may increase rhizodeposition under elevated CO2

Increases in atmospheric CO₂ concentration ([CO₂]) can lead to global climate change and theoretically could enhance carbon (C) deposition in soil, but data on this complex issue are contradictory. One approach for clarifying the diverse forces influencing plant‐derived C in the rhizosphere involves defining how elevated [CO₂] alters the fundamental process of C transfer from plant roots to the soil. We examine here how a step increase in [CO₂] affects the innate influx and efflux components of root exudation in axenic plants, as one foundation for understanding how climate change may affect rhizodeposition. Increasing [CO₂] from 425 to 850 μmol mol^?1 during short‐term trials enhanced shoot and root dry weight (P<0.01) of annual rye grass (Lolium multiflorum Lam.) and medic (Medicago truncatula L.) but had no effect on growth of maize (Zea mays L.). Root amino‐acid flux in the same plants changed only in maize, which increased the efflux rate (nmol g root fresh weight^?1 h^?1) of six amino acids (arginine, alanine, proline, tyrosine, lysine and leucine) significantly (P<0.05) under elevated [CO₂]. None of the three plant species altered the steady‐state concentration of 16 amino acids released into a hydroponic solution with changing [CO₂], apparently because amino‐acid influx rates, measured at 2.5 μm , consistently exceeded efflux rates. Indeed, plants recovered amino acids at rates 94–374% higher than they were lost from roots regardless of [CO₂]. These results indicate that, in theory, any effect of [CO₂] doubling on amino‐acid efflux can be offset by innately higher rates of influx. In practice, however, higher rates of amino‐acid cycling (i.e., efflux+influx) for each root segment (in C₄ maize) or from more root tissue (in the two C₃ species) should increase root exudation by plants exposed to elevated [CO₂] as additional amino acids would be adsorbed to soil particles or be taken up by soil microorganisms.     

Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO2: impact on C cycling and modeling

Elevated atmospheric CO₂ may alter decomposition rates through changes in plant material quality and through its impact on soil microbial activity. This study examines whether plant material produced under elevated CO₂ decomposes differently from plant material produced under ambient CO₂. Moreover, a long‐term experiment offered a unique opportunity to evaluate assumptions about C cycling under elevated CO₂ made in coupled climate–soil organic matter (SOM) models. Trifolium repens and Lolium perenne plant materials, produced under elevated (60 Pa) and ambient CO₂ at two levels of N fertilizer (140 vs. 560 kg ha^?1 yr^?1), were incubated in soil for 90 days. Soils and plant materials used for the incubation had been exposed to ambient and elevated CO₂ under free air carbon dioxide enrichment conditions and had received the N fertilizer for 9 years. The rate of decomposition of L. perenne and T. repens plant materials was unaffected by elevated atmospheric CO₂ and rate of N fertilization. Increases in L. perenne plant material C : N ratio under elevated CO₂ did not affect decomposition rates of the plant material. If under prolonged elevated CO₂ changes in soil microbial dynamics had occurred, they were not reflected in the rate of decomposition of the plant material. Only soil respiration under L. perenne, with or without incorporation of plant material, from the low‐N fertilization treatment was enhanced after exposure to elevated CO₂. This increase in soil respiration was not reflected in an increase in the microbial biomass of the L. perenne soil. The contribution of old and newly sequestered C to soil respiration, as revealed by the ¹³C‐CO₂ signature, reflected the turnover times of SOM–C pools as described by multipool SOM models. The results do not confirm the assumption of a negative feedback induced in the C cycle following an increase in CO₂, as used in coupled climate–SOM models. Moreover, this study showed no evidence for a positive feedback in the C cycle following additional N fertilization.     

Assessing the impact of land-use change on soil C sequestration in agricultural soils by means of organic matter fractionation and stable C isotopes

Within the framework of the Kyoto Protocol, the potential mitigation of greenhouse gas emissions by terrestrial ecosystems has placed focus on carbon sequestration following afforestation of former arable land. Central to this soil C sequestration are the dynamics of soil organic matter (SOM). In North Eastern Italy, a mixed deciduous forest was planted on continuous maize field soil with a strong C₄ isotopic C signature 20 years ago. In addition, a continuous maize field and a relic of the original permanent grassland were maintained at the site, thus offering the opportunity to compare the impacts on soil C dynamics by conventional agriculture, afforestation and permanent grassland. Soil samples from the afforested, grassland and agricultured systems were separated in three aggregate size classes, and inter‐ vs. intra‐aggregate particulate organic matter was isolated. All fractions were analyzed for their C content and isotopic signature. The distinct ¹³C signature of the C derived from maize vegetation allowed the calculation of proportions of old vs. forest‐derived C of the physically defined fractions of the afforested soil. Long‐term agricultural use significantly decreased soil C content (?48%), in the top 10 cm, but not SOM aggregation, as compared to permanent grassland. After 20 years, afforestation increased the total amount of soil C by 23% and 6% in the 0–10 and in the 10–30 cm depth layer, respectively. Forest‐derived carbon contributed 43% and 31% to the total soil C storage in the afforested systems in the 0–10 and 10–30 cm depths, respectively. Furthermore, afforestation resulted in significant sequestration of new C and stabilization of old C in physically protected SOM fractions, associated with microaggregates (53–250 μm) and silt&clay (<53 μm).     

Enhanced terrestrial carbon uptake in the Northern High Latitudes in the 21st century from the Coupled Carbon Cycle Climate Model Intercomparison Project model projections

The ongoing and projected warming in the northern high latitudes (NHL; poleward of 60 °N) may lead to dramatic changes in the terrestrial carbon cycle. On the one hand, warming and increasing atmospheric CO₂ concentration stimulate vegetation productivity, taking up CO₂. On the other hand, warming accelerates the decomposition of soil organic matter (SOM), releasing carbon into the atmosphere. Here, the NHL terrestrial carbon storage is investigated based on 10 models from the Coupled Carbon Cycle Climate Model Intercomparison Project. Our analysis suggests that the NHL will be a carbon sink of 0.3 ± 0.3 Pg C yr^?1 by 2100. The cumulative land organic carbon storage is modeled to increase by 38 ± 20 Pg C over 1901 levels, of which 17 ± 8 Pg C comes from vegetation (43%) and 21 ± 16 Pg C from the soil (8%). Both CO₂ fertilization and warming enhance vegetation growth in the NHL. Although the intense warming there enhances SOM decomposition, soil organic carbon (SOC) storage continues to increase in the 21st century. This is because higher vegetation productivity leads to more turnover (litterfall) into the soil, a process that has received relatively little attention. However, the projected growth rate of SOC begins to level off after 2060 when SOM decomposition accelerates at high temperature and then catches up with the increasing input from vegetation turnover. Such competing mechanisms may lead to a switch of the NHL SOC pool from a sink to a source after 2100 under more intense warming, but large uncertainty exists due to our incomplete understanding of processes such as the strength of the CO₂ fertilization effect, permafrost, and the role of soil moisture. Unlike the CO₂ fertilization effect that enhances vegetation productivity across the world, global warming increases the productivity at high latitudes but tends to reduce it in the tropics and mid‐latitudes. These effects are further enhanced as a result of positive carbon cycle–climate feedbacks due to additional CO₂ and warming.