Carbon cycling in cultivated land and its global significance

Long-term data from Sanborn Field, one of the oldest experimental fields in the USA, were used to determine the direction of soil organic carbon (SOC) dynamics in cultivated land. Changes in agriculture in the last 50 years including introduction of more productive varieties, wide scale use of mineral fertilizers and reduced tillage caused increases in total net annual production (TNAP), yields and SOC content. TNAP of winter wheat more than doubled during the last century, rising from 2.0–2.5 to 5–6 Mg ha^–1 of carbon, TNAP of corn rose from 3–4 to 9.5–11.0 Mg ha^–1 of carbon. Amounts of carbon returned annually with crop residues increased even more drastically, from less than 1 Mg ha^–1 in the beginning of the century to 3–3.5 Mg ha^–1 for wheat and 5–6 Mg ha^–1 for corn in the 90s. These amounts increased in a higher proportion because in the early 50s removal of postharvest residues from the field was discontinued. SOC during the first half of the century, when carbon input was low, was mineralized at a high rate: 89 and 114 g m^–2 y^–1 under untreated wheat and corn, respectively. Application of manure decreased losses by half, but still the SOC balance remained negative. Since 1950, the direction of the carbon dynamics has reversed: soil under wheat monocrop (with mineral fertilizer) accumulated carbon at a rate about 50 g m^–2 y^–1, three year rotation (corn/wheat/clover) with manure and nitrogen applications sequestered 150 g m² y^–1 of carbon. Applying conservative estimates of carbon sequestration documented on Sanborn Field to the wheat and corn production area in the USA, suggests that carbon losses to the atmosphere from these soils were decreased by at least 32 Tg annually during the last 40–50 years. Our computations prove that cultivated soils under proper management exercise a positive influence in the current imbalance in the global carbon budget.     

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.     

Anaerobic mineralization of marine sediment organic matter: Rates and the role of anaerobic processes in the oceanic carbon economy

Abstract

This paper addresses three related questions: (1) What factors control the efficiency of carbon burial in sediments? (2) Are rates of anaerobic organic matter degradation intrinsically lower than aerobic rates? (3) How important are anaerobic processes in the global marine sediment carbon economy?

Carbon burial efficiency (the ratio of the carbon burial rate and the carbon flux to the sediment surface) was estimated from literature data for a range of environments and was shown to be a function of sedimentation rate. No difference independent of sedimentation rate was found between aerobic and anaerobic sediments.

A review of recent microcosm and laboratory studies shows that anaerobic rates are not intrinsically lower than aerobic rates; fresh organic matter degrades at similar rates under oxic and anoxic conditions. Aerobic decomposition rates near the sediment surface are typically greater than anaerobic rates at depth because the most labile carbon is consumed before it can be buried in the anoxic zone.

A model approach was taken in estimating the importance of anaerobic processes in the global marine sediment economy, instead of extrapolating measured rates as done previously. The result, 150 Tg C yr^?1, is two to nine times lower than previous estimates. This rate is about 9% of the global aerobic carbon oxidation rate and is about equal to the rate of long‐term carbon burial. The importance of anaerobic processes in marine sediments lies in their role in determining the amount of carbon preserved, not in the amount of carbon remineralized overall.     

Phosphorus control of soil organic matter accumulation and cycling

The present level of understanding of P controls on organic matter accumulation and cycling in a pedological context owes much to soil sequence studies, and the hypotheses that emerged from them to explain the variation of P compounds with soil type and development. It is now widely accepted that nutrient transformations in soil are closely linked through a more dynamic biological cycle in which microorganisms have a central role. Concepts developed to account for the effects of P on N cycling, and for interpreting inter-relationships of C, N, P and S in both a pedological and biological context have yet to be tested adequately across different ecosystems. These concepts are discussed, in relation to some recent supporting evidence.     

Paradigm shifts in soil organic matter research affect interpretations of aquatic carbon cycling: transcending disciplinary and ecosystem boundaries

New conceptual models that highlight the importance of environmental, rather than molecular, controls on soil organic matter affect interpretations of organic matter (OM) persistence across terrestrial and aquatic boundaries. We propose that changing paradigms in our thinking about OM decomposition explain some of the uncertainties surrounding the fate of land-derived carbon (C) in marine environments. Terrestrial OM, which historically has been thought to be chemically recalcitrant to decay in soil and aquatic environments, dominates inputs to rivers yet is found in trace amounts in the ocean. We discuss three major transformations in our understanding of OM persistence that influence interpretations of the fate of aquatic OM: (1) a shift away from an emphasis on chemical recalcitrance as a primary predictor of turnover; (2) new interpretations of radiocarbon ages, which affect predictions of reactivity; and (3) the recognition that most OM leaving soils in dissolved form has been microbially processed. The first two explain rapid turnover for terrigenous OM in aquatic ecosystems once it leaves the soil matrix. The third suggests that the presence of terrestrial OM in aquatic ecosystems may be underestimated by the use of plant biomarkers. Whether these mechanisms occur in isolation of each other or in combination, they provide insight into the missing terrestrial C signature in the ocean. Spatially and temporally varying transformations of OM along land–water networks require that common terrestrial source indicators be interpreted within specific environmental contexts. We identify areas of research where collaborations between aquatic and terrestrial scientists will enhance quantification of C transfer from soils to inland water bodies, the ocean, and the atmosphere. Accurate estimates of OM processing are essential for improving predictions of the response of vulnerable C pools at the interface of soil and water to changes in climate and land use.     

陆地碳循环研究中的模型方法

陆地碳循环是全球变化研究中的重要内容,碳循环模型已成为研究陆地碳循环的必要方法．其中气候变化、大气ＣＯ２浓度上升以及人类活动引起的土地利用和土地覆盖变化导致陆地生态系统在结构、功能、组成和分布等方面的变化及其反馈关系对陆地碳循环的影响是模型模拟的关键问题．生物地理模型和生物地球化学模型是碳循环模型的两大类型,建模方法、模型性质、特点和应用范围各异．碳循环模型的发展方向是综合两类模型的特点,建立全球动态碳循环模型．     

Electron transfer of dissolved organic matter and its potential significance for anaerobic respiration in a northern bog

We investigated electron transfer processes of dissolved organic matter (DOM) and their potential importance for anaerobic heterotrophic respiration in a northern peatland. Electron accepting and donating capacities (EAC, EDC) of DOM were quantified using dissolved H₂S and ferric iron as reactants. Carbon turnover rates were obtained from porewater profiles (CO₂, CH₄) and inverse modeling. Carbon dioxide was released at rates of 0.2–5.9 mmol m⁻² day⁻¹ below the water table. Methane (CH₄) formation contributed <10%, and oxygen consumption 2% to 40%, leaving a major fraction of CO₂ production unexplained. DOM oxidized H₂S to thiosulfate and was reduced by dissolved ferric iron. Reduction with H₂S increased the subsequently determined EDC compared to untreated controls, indicating a reversibility of the electron transfer. In situ redox capacities of DOM ranged from 0.2 to 6.1 mEq g⁻¹ C (EAC) and from 0.0 to 1.4 mEq g⁻¹ C (EDC), respectively. EAC generally decreased with depth and changed after a water table drawdown and rebound by 20 and −45 mEq m⁻², respectively. The change in EAC during the water table fluctuation was similar to CH₄ formation rates. In peatlands, electron transfer of DOM may thus significantly contribute to the oxidation of reduced organic substrates by anaerobic heterotrophic respiration, or by maintaining the respiratory activity of sulfate reducers via provision of thiosulfate. Part of the anaerobic electron flow in peat soils is thus potentially diverted from methanogenesis, decreasing its contribution to the total carbon emitted to the atmosphere.     

Role of lakes for organic carbon cycling in the boreal zone

We calculated the carbon loss (mineralization plus sedimentation) and net CO₂ escape to the atmosphere for 79 536 lakes and total running water in 21 major Scandinavian catchments (size range 437–48 263 km²). Between 30% and 80% of the total organic carbon that entered the freshwater ecosystems was lost in lakes. Mineralization in lakes and subsequent CO₂ emission to the atmosphere was by far the most important carbon loss process. The withdrawal capacity of lakes on the catchment scale was closely correlated to the mean residence time of surface water in the catchment, and to some extent to the annual mean temperature represented by latitude. This result implies that variation of the hydrology can be a more important determinant of CO₂ emission from lakes than temperature fluctuations. Mineralization of terrestrially derived organic carbon in lakes is an important regulator of organic carbon export to the sea and may affect the net exchange of CO₂ between the atmosphere and the boreal landscape.     

Resource quality affects carbon cycling in deep-sea sediments

Deep-sea sediments cover ∼70% of Earth''s surface and represent the largest interface between the biological and geological cycles of carbon. Diatoms and zooplankton faecal pellets naturally transport organic material from the upper ocean down to the deep seabed, but how these qualitatively different substrates affect the fate of carbon in this permanently cold environment remains unknown. We added equal quantities of ¹³C-labelled diatoms and faecal pellets to a cold water (−0.7 °C) sediment community retrieved from 1080 m in the Faroe-Shetland Channel, Northeast Atlantic, and quantified carbon mineralization and uptake by the resident bacteria and macrofauna over a 6-day period. High-quality, diatom-derived carbon was mineralized >300% faster than that from low-quality faecal pellets, demonstrating that qualitative differences in organic matter drive major changes in the residence time of carbon at the deep seabed. Benthic bacteria dominated biological carbon processing in our experiments, yet showed no evidence of resource quality-limited growth; they displayed lower growth efficiencies when respiring diatoms. These effects were consistent in contrasting months. We contend that respiration and growth in the resident sediment microbial communities were substrate and temperature limited, respectively. Our study has important implications for how future changes in the biochemical makeup of exported organic matter will affect the balance between mineralization and sequestration of organic carbon in the largest ecosystem on Earth.     

Mast cell ionic channels: significance for stimulus-secretion coupling.

The activation of mast cells (MC) due to immunological stimulation causes an immediate and dramatic inflammatory response. We review current evidence indicating that the membrane permeabilities for calcium, chloride, sodium, and potassium have a significant role in the activation of these cells, and in some cases, specific ionic channels have been identified. Moreover, a number of intracellular mechanisms controlling these channels are pointed out, including different classes of G proteins, intracellular calcium, cAMP, and products of phosphoinositol breakdown. However, the interplay between factors controlling membrane conductances for different ions is not currently understood. The diversity of ionic effects on MC activation is depicted, illustrating that the ionic mechanisms of MC activation are specific for different MC types. Since nerve/mast cell interaction is a key element in the burgeoning field of neuroimmunology, we discuss the role of ionic channels as targets of neurotransmitter action in MC activation.     

Ectomycorrhizal fungi contribute to soil organic matter cycling in sub-boreal forests

Soils of northern temperate and boreal forests represent a large terrestrial carbon (C) sink. The fate of this C under elevated atmospheric CO₂ and climate change is still uncertain. A fundamental knowledge gap is the extent to which ectomycorrhizal fungi (EMF) and saprotrophic fungi contribute to C cycling in the systems by soil organic matter (SOM) decomposition. In this study, we used a novel approach to generate and compare enzymatically active EMF hyphae-dominated and saprotrophic hyphae-enriched communities under field conditions. Fermentation-humus (FH)-filled mesh bags, surrounded by a sand barrier, effectively trapped EMF hyphae with a community structure comparable to that found in the surrounding FH layer, at both trophic and taxonomic levels. In contrast, over half the sequences from mesh bags with no sand barrier were identified as belonging to saprotrophic fungi. The EMF hyphae-dominated systems exhibited levels of hydrolytic and oxidative enzyme activities that were comparable to or higher than saprotroph-enriched systems. The enzymes assayed included those associated with both labile and recalcitrant SOM degradation. Our study shows that EMF hyphae are likely important contributors to current SOM turnover in sub-boreal systems. Our results also suggest that any increased EMF biomass that might result from higher below-ground C allocation by trees would not suppress C fluxes from sub-boreal soils.     

Loss of extrafloral nectary on an oceanic island plant and its consequences for herbivory

Two Hibiscus (Malvaceae) species coexist on the oceanic Bonin (Ogasawara) Islands: Hibiscus glaber (an endemic species) and H. tiliaceus (the ancestral non-endemic species). Hibiscus tiliaceus produces extrafloral nectar from the sepals, while H. glaber does not. To clarify the effects of extrafloral nectar loss on Hibiscus-insect relationships, we examined herbivory and insect communities on flower buds of H. glaber and H. tiliaceus. Larvae of the endemic moth Rehimena variegata (Lepidoptera: Pyralidae) attacked 20% of the flower buds on H. glaber, while less than 0.2% of buds on H. tiliaceus were attacked. Introduced species of ants frequently visited the flower buds of H. tiliaceus to collect extrafloral nectar from the sepal, while they rarely visited those of H. glaber. Therefore, extrafloral nectar on H. tiliaceus sepals may function as a facultative defense against flower bud herbivory. The loss of extrafloral nectaries of H. glaber sepals may be related to the original paucity of native herbivores and ants on the Bonin Islands.     

Irrigation and enhanced soil carbon input effects on below-ground carbon cycling in semiarid temperate grasslands

Global climate change is generally expected to increase net primary production, resulting in increased soil carbon (C) inputs. To gain an understanding of how such increased soil C inputs would affect C cycling in the vast grasslands of northern China, we conducted a field experiment in which the responses of plant and microbial biomass and respiration were studied. Our experiment included the below-ground addition of particulate organic matter (POM) at rates equivalent to 0, 60, 120 and 240 g C m(-2), under either natural precipitation or under enhanced precipitation during the summer period (as predicted for that region in recent simulations using general circulation models). We observed that addition of POM had a large effect on soil microbial biomass and activity and that a major part of the added C was rapidly lost from the system. This suggests that microbial activity in the vast temperate grassland ecosystems of northern China is energy-limited. Moreover, POM addition (and the associated nutrient release) affected plant growth much more than the additional water input. Although we performed no direct fertilization experiments, the response of plant productivity to POM addition (and associated release of nutrients) leads us to believe that plant productivity in the semiarid grassland ecosystems of northern China is primarily limited by nutrients and not by water.     

Dynamic coupling of regulated binding sites and cycling myosin heads in striated muscle

In an activated muscle, binding sites on the thin filament and myosin heads switch frequently between different states. Because the status of the binding sites influences the status of the heads, and vice versa, the binding sites and myosin heads are dynamically coupled. The functional consequences of this coupling were investigated using MyoSim, a new computer model of muscle. MyoSim extends existing models based on Huxley-type distribution techniques by incorporating Ca²⁺ activation and cooperative effects. It can also simulate arbitrary cross-bridge schemes set by the researcher. Initial calculations investigated the effects of altering the relative speeds of binding-site and cross-bridge kinetics, and of manipulating cooperative processes. Subsequent tests fitted simulated force records to experimental data recorded using permeabilized myocardial preparations. These calculations suggest that the rate of force development at maximum activation is limited by myosin cycling kinetics, whereas the rate at lower levels of activation is limited by how quickly binding sites become available. Additional tests investigated the behavior of transiently activated cells by driving simulations with experimentally recorded Ca²⁺ signals. The unloaded shortening profile of a twitching myocyte could be reproduced using a model with two myosin states, cooperative activation, and strain-dependent kinetics. Collectively, these results demonstrate that dynamic coupling of binding sites and myosin heads is important for contractile function.     

Stimulation of grassland nitrogen cycling under carbon dioxide enrichment

 Nitrogen (N) limits plant growth in many terrestrial ecosystems, potentially constraining terrestrial ecosystem response to elevated CO₂. In this study, elevated CO₂ stimulated gross N mineralization and plant N uptake in two annual grasslands. In contrast to other studies that have invoked increased C input to soil as the mechanism altering soil N cycling in response to elevated CO₂, increased soil moisture, due to decreased plant transpiration in elevated CO₂, best explains the changes we observed. This study suggests that atmospheric CO₂ concentration may influence ecosystem biogeochemistry through plant control of soil moisture. Received: 18 December 1995 / Accepted: 19 June 1996     

Net production and carbon cycling in a bamboo Phyllostachys pubescens stand

Phyllostachys pubescens Mazel ex Houzeau de Lehaie is one of the largest bamboo species with a leptomorph root system in the world. The species originates in China and has been naturalized in the neighboring countries. It was introduced in 1746 into Japan because of the economic value of the young sprouts and culm woods. It escaped from the planted areas and expanded by invading the original vegetation. In order to clarify the basic ecological characteristics of the species, carbon fixation and cycling were determined in a stand of Phyllostachys pubescens. The standing culm density and average DBH in 1991 were 7100 ha^(-1) and 11.3 cm, respectively. The above-ground biomass was 116.5 t ha^(-1) for culms, 15.5 t ha^(-1) for branches, 5.9 t ha^(-1) for leaves and 137.9 t ha^(-1) in total. The total above-ground biomass was one of the largest among the world's bamboo communities. The biomasses of rhizomes and fine roots were 16.7 t ha^(-1) and 27.9 t ha^(-1), respectively. Annual soil respiration was 52.3 t CO⁽²⁾ ha^(-1) yr^(-1), the highest among those determined in Japan. The gross production was high: 32.8 t C ha^(-1) yr^(-1). Allocation of the products to its root system was also high: 34% to gross production and 46% to the fluxes out of the leaves into other compartments of the ecosystem. This resulted in the reduced above-ground net production of 18.1 t ha^(-1) yr^(-1), which fell within the average range of productivity of forests under similar climate conditions. This paper discusses the correspondence of the allocation pattern with the successful range expansion.