Tropical forests: Their past,present, and potential future role in the terrestrial carbon budget

In this paper we review results of research to summarize the state-of-knowledge of the past, present, and potential future roles of tropical forests in the global C cycle. In the pre-industrial period (ca. 1850), the flux from changes in tropical land use amounted to a small C source of about 0.06 Pg yr^?1. By 1990, the C source had increased to 1.7 ± 0.5 Pg yr^?1. The C pools in forest vegetation and soils in 1990 was estimated to be 159 Pg and 216 Pg, respectively. No concrete evidence is available for predicting how tropical forest ecosystems are likely to respond to CO₂ enrichment and/or climate change. However, C sources from continuing deforestation are likely to overwhelm any change in C fluxes unless land management efforts become more aggressive. Future changes in land use under a “business as usual” scenario could release 41–77 Pg C over the next 60 yr. Carbon fluxes from losses in tropical forests may be lessened by aggressively pursued agricultural and forestry measures. These measures could reduce the magnitude of the tropical C source by 50 Pg by the year 2050. Policies to mitigate C losses must be multiple and concurrent, including reform of forestry, land tenure, and agricultural policies, forest protection, promotion of on-farm forestry, and establishment of plantations on non-forested lands. Policies should support improved agricultural productivity, especially replacing non-traditional slash-and-burn agriculture with more sustainable and appropriate approaches.     

Boreal forests and tundra

The circumpolar boreal biomes coverca. 2 10⁹ ha of the northern hemisphere and containca. 800 Pg C in biomass, detritus, soil, and peat C pools. Current estimates indicate that the biomes are presently a net C sink of 0.54 Pg C yr^?1. Biomass, detritus and soil of forest ecosystems (includingca. 419 Pg peat) containca. 709 Pg C and sequester an estimated 0.7 Pg C yr^?1. Tundra and polar regions store 60–100 Pg C and may recently have become a net source of 0.17 Pg C yr^?1. Forest product C pools, including landfill C derived from forest biomass, store less than 3 Pg C but increase by 0.06 Pg C yr^?1. The mechanisms responsible for the present boreal forest net sink are believed to be continuing responses to past changes in the environment, notably recovery from the little ice-age, changes in forest disturbance regimes, and in some regions, nutrient inputs from air pollution. Even in the absence of climate change, the C sink strength will likely be reduced and the biome could switch to a C source. The transient response of terrestrial C storage to climate change over the next century will likely be accompanied by large C exchanges with the atmosphere, although the long-term (equilibrium) changes in terrestrial C storage in future vegetation complexes remains uncertain. This transient response results from the interaction of many (often non-linear) processes whose impacts on future C cycles remain poorly quantified. Only a small part of the boreal biome is directly affected by forest management and options for mitigating climate change impacts on C storage are therefore limited but the potential for accelerating the atmospheric C release are high.     

Offsetting global CO2 emissions by restoration of degraded soils and intensification of world agriculture and forestry

Increase in atmospheric concentration of CO₂ from 285 parts per million by volume (ppmv) in 1850 to 370 ppm in 2000 is attributed to emissions of 270 ± 30 Pg carbon (C) from fossil fuel combustion and 136 ± 55 Pg C by land‐use change. Present levels of anthropogenic emissions involve 6·3 Pg C by fossil fuel emissions and 1·8 Pg C by land‐use change. Out of the historic loss of terrestrial C pool of 136 ± 55 Pg, 78 ± 12 Pg is due to depletion of soil organic carbon (SOC) pool comprising 26 ± 9 Pg due to accelerated soil erosion. A large proportion of the historic SOC lost can be resequestered by enhancing the SOC pool through converting to an appropriate land use and adopting recommended management practices (RMPs). The strategy is to return biomass to the soil in excess of the mineralization capacity through restoration of degraded/desertified soils and intensification of agricultural and forestry lands. Technological options for agricultural intensification include conservation tillage and residue mulching, integrated nutrient management, crop rotations involving cover crops, practices which enhance the efficiency of water, plant nutrients and energy use, improved pasture and tree species, controlled grazing, and judicious use of inptus. The potential of SOC sequestration is estimated at 1–2 Pg C yr⁻¹ for the world, 0·3–0·6 Pg C yr⁻¹ for Asia, 0·2–0·5 Pg C yr⁻¹ for Africa and 0·1–0·3 Pg C yr⁻¹ for North and Central America and South America, 0·1–0·3 Pg C yr⁻¹ for Europe and 0·1–0·2 Pg C yr⁻¹ for Oceania. Soil C sequestration is a win–win strategy; it enhances productivity, improves environment moderation capacity, and mitigates global warming. Copyright © 2003 John Wiley & Sons, Ltd.     

Soil carbon sequestration in China through agricultural intensification,and restoration of degraded and desertified ecosystems

The industrial emission of carbon (C) in China in 2000 was about 1 Pg yr⁻¹, which may surpass that of the United States (1ċ84 Pg C) by 2020. China's large land area, similar in size to that of the United States, comprises 124 Mha of cropland, 400 Mha of grazing land and 134 Mha of forestland. Terrestrial C pool of China comprises about 35–60 Pg in the forest and 120–186 Pg in soils. Soil degradation is a major issue affecting 145 Mha by different degradative processes, of which 126 Mha are prone to accelerated soil erosion. Total annual loss by erosion is estimated at 5ċ5 Pg of soil and 15ċ9 Tg of soil organic carbon (SOC). Erosion‐induced emission of C into the atmosphere may be 32–64 Tg yr⁻¹. The SOC pool progressively declined from the 1930s to 1980s in soils of northern China and slightly increased in those of southern China because of change in land use. Management practices that lead to depletion of the SOC stock are cultivation of upland soils, negative nutrient balance in cropland, residue removal, and soil degradation by accelerated soil erosion and salinization and the like. Agricultural practices that enhance the SOC stock include conversion of upland to rice paddies, integrated nutrient management based on liberal use of biosolids and compost, crop rotations that return large quantities of biomass, and conservation‐effective systems. Adoption of recommended management practices can increase SOC concentration in puddled soil, red soil, loess soils, and salt‐affected soils. In addition, soil restoration has a potential to sequester SOC. Total potential of soil C sequestration in China is 105–198 Tg C yr⁻¹ of SOC and 7–138 Tg C yr⁻¹ for soil inorganic carbon (SIC). The accumulative potential of soil C sequestration of 11 Pg at an average rate of 224 Tg yr⁻¹ may be realized by 2050. Soil C sequestration potential can offset about 20 per cent of the annual industrial emissions in China. Copyright © 2002 John Wiley & Sons, Ltd.     

Modeling Regional Carbon Dynamics and Soil Erosion in Disturbed and Rehabilitated Ecosystems as Affected by Land Use and Climate

The quantification of carbon (C) and nitrogen (N) cycling inecosystems is important for (a) understanding changes inecosystem structure and function with changes in land use, (b)determining the sustainability of ecosystems, and (c) balancingthe global C budget as it relates to global climate change.A meso-scale study was conducted to determine regional effectsof climate change on C and N cycling within disturbedecosystems. Objectives of the research were to quantify (a)sediment yield, (b) current C storage in vegetation and soils,and (c) soil C efflux from both abandoned and rehabilitatedcoal surface-mined lands in Ohio. A dynamic model was developedto simulate sediment yield, grassland production, and C and Ncycling on surface-mined lands. Evaluation of plant productionand soil erosion submodels with data sets from surface-minedlands in the mid-western U.S. resulted in r² values of 0.99 and0.97, respectively. Depending on the initial values of soil organic carbon (SOC),model simulations estimated that unvegetated surface-mined landsin Ohio yield approximately 441,325 Mg yr^-1 of sediment andemit between 2,000–20,000 Mg yr^-1 of C to the atmosphere fromdecomposition of SOC. While rehabilitated lands had a higher Cefflux rate than barren lands, a positive C sequestration rateof 18.4 Mg km^-2 yr ^-1 was estimated as a result oforganic matter additions. This sequestion rate increasedconsiderably under projected climate change scenarios, while itdecreased when simulated rehabilitated grasslands were harvestedfor hay. Changes in land use and cover can cause surface-minedlands to be either a net sink or source for C. Successful rehabilitation of mined lands can decrease erosion and promotesoil C sequestration, while at the same time providingadditional lands for the management of natural resources.     

Total carbon and nitrogen in the soils of the world

The soil is important in sequestering atmospheric CO₂ and in emitting trace gases (e.g. CO₂, CH₄ and N₂O) that are radiatively active and enhance the ‘greenhouse’ effect. Land use changes and predicted global warming, through their effects on net primary productivity, the plant community and soil conditions, may have important effects on the size of the organic matter pool in the soil and directly affect the atmospheric concentration of these trace gases. A discrepancy of approximately 350 × 10¹⁵ g (or Pg) of C in two recent estimates of soil carbon reserves worldwide is evaluated using the geo-referenced database developed for the World Inventory of Soil Emission Potentials (WISE) project. This database holds 4353 soil profiles distributed globally which are considered to represent the soil units shown on a ¹/₂° latitude by ¹/₂° longitude version of the corrected and digitized 1:5 M FAO–UNESCO Soil Map of the World. Total soil carbon pools for the entire land area of the world, excluding carbon held in the litter layer and charcoal, amounts to 2157–2293 Pg of C in the upper 100 cm. Soil organic carbon is estimated to be 684–724 Pg of C in the upper 30 cm, 1462–1548 Pg of C in the upper 100 cm, and 2376–2456 Pg of C in the upper 200 cm. Although deforestation, changes in land use and predicted climate change can alter the amount of organic carbon held in the superficial soil layers rapidly, this is less so for the soil carbonate carbon. An estimated 695–748 Pg of carbonate-C is held in the upper 100 cm of the world's soils. Mean C: N ratios of soil organic matter range from 9.9 for arid Yermosols to 25.8 for Histosols. Global amounts of soil nitrogen are estimated to be 133–140 Pg of N for the upper 100 cm. Possible changes in soil organic carbon and nitrogen dynamics caused by increased concentrations of atmospheric CO₂ and the predicted associated rise in temperature are discussed.     

The interaction of climate and land use in future terrestrial carbon storage and release

The processes controlling total carbon (C) storage and release from the terrestrial biosphere are still poorly quantified. We conclude from analysis of paleodata and climate biome model output that terrestrial C exchanges since the last glacial maximum (LGM) were dominated by slow processes of C sequestration in soils, possibly modified by C starvation and reduced water use efficiency of trees during the LGM. Human intrusion into the C cycle was immeasurably small. These processes produced an averaged C sink in the terrestrial biosphere on the order of 0.05 Pg yr^?1 during the past 10,000 years. In contrast, future C cycling will be dominated by human activities, not only from increasing C release with burning of fossil fuels, and but also from indirect effects which increase C storage in the terrestrial biosphere (CO₂ fertilization; management of C by technology and afforestation; synchronous early forest succession from widespread cropland abandonment) and decrease C storage in the biosphere (synchronous forest dieback from climatic stress; warming-induced oxidation of soil C; slowed forest succession; unfinished tree life cycles; delayed immigration of trees; increasing agricultural land use). Comparison of the positive and negative C flux processes involved suggests that if the C sequestration processes are important, they likely will be so during the next few decades, gradually being counteracted by the C release processes. Based only on tabulating known or predicted C flux effects of these processes, we could not determine if the earth will act as a significant C source from dominance by natural C cycle processes, or as a C sink made possible only by excellent earth stewardship in the next 50 to 100 yrs. Our subsequent analysis concentrated on recent estimates of C release from forest replacement by increased agriculture. Those results suggest that future agriculture may produce an additional 0.6 to 1.2 Pg yr^?1 loss during the 50 to 100 years to CO₂ doubling if the current ratio of farmed to potentially-farmed land is maintained; or a greater loss, up to a maximum of 1.4 to 2.8 Pg yr^?1 if all potential agricultural land is farmed.     

Phosphorus in the soil microbial biomass

Phosphorus in the soil microbial biomass (biomass P) and soil biomass carbon (biomass C) were linearly related in 15 soils (8 grassland, 6 arable, 1 deciduous woodland), with a mean P concentration of 3.3% in the soil biomass. The regression accounted for 82% of the variance in the data. The relationship was less close than that previously measured between soil biomass C and soil ATP content and indicates that biomass P measurements can only provide a rough estimate of biomass C content. Neither P concentration in the soil biomass, nor the amount of biomass P in soil, were correlated with soil NaHCO₃-extractable inorganic, organic or total P.The calculated mean annual flux of P through the biomass (in a soil depth of 10 cm) in 8 grassland soils was large, 23 kg P ha^?1 yr^?1, and more than three times the mean annual P flux through 6 arable soils (7 kg P ha^?1 yr^?1), suggesting that biomass P could make a significant contribution to plant P nutrition in grassland.About 3% of the total soil organic P in the arable soils was in microbial biomass and from 5 to 24% in the grassland soils. The decline in biomass P when an old grassland soil was put into an arable rotation for about 20 yr was sufficient to account for about 50% of the decline in total soil organic P during this period. When an old arable soil reverted to woodland, soil organic P doubled in 100 yr; biomass P increased 11-fold during the same period.     

Modeling the effects of climatic and co2 changes on grassland storage of soil C

We present results from analyses of the sensitivity of global grassland ecosystems to modified climate and atmospheric CO₂ levels. We assess 31 grassland sites from around the world under two different General Circulation Models (GCM) double CO₂ climates. These grasslands are representative of mostly naturally occurring ecosystems, however, in many regions of the world, grasslands have been greatly modified by recent land use changes. In this paper we focus on the ecosystem dynamics of natural grasslands. The climate change results indicate that simulated soil C losses occur in all but one grassland ecoregion, ranging from 0 to 14% of current soil C levels for the surface 20 cm. The Eurasian grasslands lost the greatest amount of soil C (～1200 g C m^?2) and the other temperate grasslands losses ranged from 0 to 1000 g C m^?2, averaging approximately 350 g C m^?2. The tropical grasslands and savannas lost the least amount of soil C per unit area ranging from no change to 300 g C m^?2 losses, averaging approximately 70 g C m^?2. Plant production varies according to modifications in rainfall under the altered climate and to altered nitrogen mineralization rates. The two GCM's differed in predictions of rainfall with a doubling of CO₂, and these differences are reflected in plant production. Soil decomposition rates responded most predictably to changes in temperature. Direct CO₂ enhancement effects on decomposition and plant production tended to reduce the net impact of climate alterations alone.     

Responses of soil organic matter and greenhouse gas fluxes to soil management and land use changes in a humid temperate region of southern Europe

We studied the effects of soil management and changes of land use on soils of three adjacent plots of cropland, pasture and oak (Quercus robur) forest. The pasture and the forest were established in part of the cropland, respectively, 20 and 40 yr before the study began. Soil organic matter (SOM) dynamics, water-filled pore space (WFPS), soil temperature, inorganic N and microbial C, as well as fluxes of CO₂, CH₄ and N₂O were measured in the plots over 25 months. The transformation of the cropland to mowed pasture slightly increased the soil organic and microbial C contents, whereas afforestation significantly increased these variables. The cropland and pasture soils showed low CH₄ uptake rates (<1 kg C ha⁻¹ yr⁻¹) and, coinciding with WFPS values >70%, episodes of CH₄ emission, which could be favoured by soil compaction. In the forest site, possibly because of the changes in soil structure and microbial activity, the soil always acted as a sink for CH₄ (4.7 kg C ha⁻¹ yr⁻¹). The N₂O releases at the cropland and pasture sites (2.7 and 4.8 kg N₂O-N ha⁻¹ yr⁻¹) were, respectively, 3 and 6 times higher than at the forest site (0.8 kg N₂O-N ha⁻¹ yr⁻¹). The highest N₂O emissions in the cultivated soils were related to fertilisation and slurry application, and always occurred when the WFPS >60%. These results show that the changes in soil properties as a consequence of the transformation of cropfield to intensive grassland do not imply substantial changes in SOM or in the dynamics of CH₄ and N₂O. On the contrary, afforestation resulted in increases in SOM content and CH₄ uptake, as well as decreases in N₂O emissions.     

Potential for carbon sequestration in the drylands

Non-forested drylands occupy 43% of the world's land surface yet they are not currently regarded as important in sequestering carbon due to overuse and poor management. Seventy percent of drylands have already undergone moderate to severe desertification and an additional 3.5% drops out of economic production each year. Reversing the trend towards desertification through cultivation of halophytes on saline lands, revegetation of degraded rangelands and other innovative conservation measures could result in net C sequestration in dryland soils of 0.5–1.0 Gt yr^?1 at a cost of $10–18 t^?1 C, based on a 100 yr scenario. Investment in antidesertification measures in the world's drylands appears to be an economical method to mitigate CO₂ buildup in the atmosphere while accomplishing a major international objective of restoring dryland productivity.     

Erosion risk assessment: A contribution for conservation priority area identification in the sub-basin of Lake Tana,north-western Ethiopia

Soil erosion is a serious environmental problem arising from agricultural intensification and landscape changes. Improper land management coupled with intense rainfall has intricated the problem in most parts of the Ethiopian highlands. Soil loss costs a profound amount of the national GDP. Thus, quantifying soil loss and prioritizing areas for conservation is imperative for proper planning and resource conservation. Therefore, this study has modeled the mean soil loss and annual sediment yield of the Gumara watershed. Landsat 5 TM, Landsat 7ETM+, and Landsat 8 OLI were used for land use land cover (LULC) change analysis. Besides these, other datasets related to rainfall, digital soil map, Digital Elevation Model, reference land use, and cover (LULC) ground truth points were used to generate parameters for modeling soil loss. The watershed was classified into five major land-use classes (water body, cultivated land, grazing land, built-up and forest and plantation) using a maximum likelihood algorithm covering a period of the last 30 years (1988–2019). The mean annual soil loss and sediment yield were quantified using RUSLE, Sediment delivery ratio (SDR), and Sediment Yields models (SY). The analysis result unveils that within the past 30 years, the watershed has undergone significant LULC changes from forest & plantation (46.33%) and grazing land to cultivated land (31.59%) with the rate of ?1.42km²yr^-1 and -2.80km²yr^-1 respectively. In the same vein, the built-up area has expanded to cultivated and grazing land. Subsequently, nearly 15% (207 km²) of the watershed suffered from moderate to very severe soil loss. On average, the watershed losses 24.2 t ha?¹ yr?¹ of soil and yields 2807.02 t ha?¹ yr?¹ sediment. Annually, the watershed losses 385,157 t ha?¹ yr?¹ soil from the whole study area. Among the admirative districts, Farta (Askuma, Giribi, Mahidere Mariam and Arigo kebeles), Fogera (Gazen Aridafofota and Gura Amba kebeles), East Este (Witimera kebele), and Dera (Gedame Eyesus and Deriana Wechit kebeles) districts which cover 50% of the watershed were found severely affected by soil erosion. Thus, to curve back this scenario, soil and water conservation practices should prioritize in the aforementioned districts of the watersheds.     

Storage and dynamics of carbon and nitrogen in soil physical fractions following woody plant invasion of grassland

Woody plant invasion of grasslands is prevalent worldwide. In the Rio Grande Plains of Texas, subtropical thorn woodlands dominated by C₃ trees/shrubs have been replacing C₄ grasslands over the past 150 yr, resulting in increased soil organic carbon (SOC) storage and concomitant increases in soil total nitrogen (STN). To elucidate mechanisms of change in SOC and STN, we separated soil organic matter into specific size/density fractions and determined the concentration of C and N in these fractions. Soils were collected from remnant grasslands (Time 0) and woody plant stands (ages 10-130 yr). Rates of whole-soil C and N accrual in the upper 15 cm of the soil profile averaged 10-30 g C m⁻² yr⁻¹ and 1-3 g N m⁻² yr⁻¹, respectively, over the past 130 yr of woodland development. These rates of accumulation have increased soil C and N stocks in older wooded areas by 100-500% relative to remnant grasslands. Probable causes of these increased pool sizes include higher rates of organic matter production in wooded areas, greater inherent biochemical resistance of woody litter to decomposition, and protection of organic matter by stabilization within soil macro- and microaggregates. The mass proportions of the free light fraction (<1.0 g cm⁻³) and macroaggregate fraction (>250 μm) increased linearly with time following woody plant invasion of grassland. Conversely, the mass proportions of free microaggregate (53-250 μm) and free silt+clay (<53 μm) fractions decreased linearly with time after woody invasion, likely reflecting stabilization of these fractions within macroaggregate structures. Carbon and N concentrations increased in all soil fractions with time following woody invasion. Approximately half of the C and N accumulated in free particulate organic matter (POM) fractions, while the remainder accrued in stable macro- and microaggregate structures. Soil C/N ratios indicated that the organic C associated with POM and macroaggregates was of more recent origin (less decomposed) than C associated with the microaggregate and silt+clay fractions. Because grassland-to-woodland conversion has been geographically extensive in grassland ecosystems worldwide during the past century, changes in soil C and N storage and dynamics documented here could have significance for global cycles of those elements.     

After the Kyoto Protocol: Can soil scientists make a useful contribution?*

Abstract. Over 170 countries have ratified the UN Framework Convention on Climate Change (UNFCCC) which aims at ‘the stabilisation of greenhouse gases in the atmosphere at a level that will prevent dangerous anthropogenic interference with the climate system’. The Kyoto Protocol, signed in 1997, commits the developed (‘Annex 1′) countries to a reduction in gaseous emissions. The global increase in atmospheric CO₂, the main greenhouse gas, comes mainly from fossil fuels (6.5 Gt C yr^?1), together with about 1.6 Gt C yr^?1 from deforestation. The atmospheric increase is only 3.4 Gt C yr^?1, however, due to a net sink in terrestrial ecosystems of about 2 Gt C yr^?1, and another in the oceans. Increasing net carbon sequestration by afforestation of previously non-forested land is one way of reducing net national emissions of CO₂ that is permitted under the Kyoto Protocol. Future modifications may also allow the inclusion of carbon sequestration brought about by other forestry and agricultural land management practices. However, associated changes in net fluxes of two other greenhouse gases identified in the Protocol — nitrous oxide (N₂O) and methane (CH₄) — will have to be taken into account. Growth of biomass crops can increase N₂O emissions, and drainage of wetlands for forestry or agriculture also increases them, as well as emissions of CO₂, while decreasing those of CH₄. The problems of how to quantify these soil sources and sinks, to maximize soil C sequestration, and to minimize soil emissions of CH₄ and N₂O, will present a major scientific challenge over the next few years — one in which the soil science community will have a significant part to play.     

Managing soils for negative feedback to climate change and positive impact on food and nutritional security

ABSTRACT

The increase in atmospheric concentration of carbon dioxide from 278 ppm in the pre-industrial era to 405 ppm in 2018, along with the enrichment of other greenhouse gases, has already caused a global mean temperature increase of 1°C. Among anthropogenic sources, historic land use and conversion of natural to agricultural eco-systems has and continues to be an importance source. Global depletion of soil organic carbon stock by historic land use and soil degradation is estimated at 133 Pg C. Estimated to 2-m depth, C stock is 2047 Pg for soil organic carbon and 1558 Pg for soil inorganic carbon, with a total of 3605 Pg. Thus, even a small change in soil organic carbon stock can have a strong impact on atmospheric CO₂ concentration. Soil C sink capacity, between 2020 and 2100, with the global adoption of best management practice which creates a positive soil/ecosystem C budget, is estimated at 178 Pg C for soil, 155 Pg C for biomass, and 333 Pg C for the terrestrial biosphere with a total CO₂ drawdown potential of 157 ppm. Important among techniques of soil organic C sequestration are adoption of a system-based conservation agriculture, agroforestry, biochar, and integration of crops with trees and livestock. There is growing interest among policymakers and the private sector regarding the importance of soil C sequestration for adaptation and mitigation of climate change, harnessing of numerous co-benefits, and strengthening of ecosystem services.     

Nitrous oxide and methane emissions from grassland treated with bark- or sawdust-containing manure at different rates

On the main Japanese island of Honshu, bark or sawdust is often added to cattle excreta as part of the composting process. Dairy farmers sometimes need to dispose of manure that is excess to their requirements by spreading it on their grasslands. We assessed the effect of application of bark- or sawdust-containing manure at different rates on annual nitrous oxide (N₂O) and methane (CH₄) emissions from a grassland soil. Nitrous oxide and CH₄ fluxes from an orchardgrass (Dactylis glomerata L.) grassland that received this manure at 0, 50, 100, 200, or 300?Mg?ha^?1?yr^?1 were measured over a two-year period by using closed chambers. Two-way analysis of variance (ANOVA) was employed to examine the effect of annual manure application rates and years on annual N₂O and CH₄ emissions. Annual N₂O emissions ranged from 0.47 to 3.03?kg?N?ha^?1?yr^?1 and increased with increasing manure application rate. Nitrous oxide emissions during the 140-day period following manure application increased with increasing manure application rate, with the total nitrogen concentration in the manure, and with cumulative precipitation during the 140-day period. However, manure application rate did not affect the N₂O emission factors of the manure. The overall average N₂O emission factor was 0.068%. Annual CH₄ emissions ranged from ?1.12 to 0.01?kg?C?ha^?1?yr^?1. The annual manure application rate did not affect annual CH₄ emissions.     

Riverine transport of atmospheric carbon: Sources,global typology and budget

Atmospheric C (TAC) is continuously transported by rivers at the continents’ surface as soil dissolved and particulate organic C (DOC, POC) and dissolved inorganic C (DIC) used in rock weathering reactions. Global typology of the C export rates (g.m^?2.yr^?1) for 14 river classes from tundra rivers to monsoon rivers is used to calculate global TAC flux to oceans estimated to 542 Tg.yr^?1, of which 37 % is as DOC, 18 % as soil POC and 45 % as DIC. TAC originates mostly from humid tropics (46 %) and temperate forest and grassland (31 %), compared to boreal forest (14 %), savannah and sub-arid regions (5 %), and tundra (4 %). Rivers also carry to oceans 80 Tg. yr^?1 of POC and 137 TG.yr^?1 of DIC originating from rock erosion. Permanent TAC storage on land is estimated to 52 Tg.yr^?1 in lakes and 17 Tg.yr^?1 in internal regions of the continents.     

Carbon sinks in mangroves and their implications to carbon budget of tropical coastal ecosystems

Nearly 50% of terrigenous materials delivered to the world's oceans are delivered through just twenty-one major river systems. These river-dominated coastal margins (including estuarine and shelf ecosystems) are thus important both to the regional enhancement of productivity and to the global flux of C that is observed in land-margin ecosystems. The tropical regions of the biosphere are the most biogeochemically active coastal regions and represent potentially important sinks of C in the biosphere. Rates of net primary productivity and biomass accumulation depend on a combination of global factors such as latitude and local factors such as hydrology. The global storage of C in mangrove biomass is estimated at 4.03 Pg C; and 70% of this C occurs in coastal margins from 0° to 10° latitude. The average rate of wood production is 12.08 Mg ha^?1 yr^?1, which is equivalent to a global estimate of 0.16 Pg C/yr stored in mangrove biomass. Together with carbon accumulation in mangrove sediments (0.02 Pg C/yr), the net ecosystem production in mangroves is about 0.18 Pg C/yr. Global estimates of export from coastal wetlands is about 0.08 Pg C/yr compared to input of 0.36 Pg C/yr from rivers to coastal ecosystems. Total allochthonous input of 0.44 Pg C/yr is lower than in situ production of 6.65 Pg C/yr. The trophic condition of coastal ecosystems depends on the fate of this total supply of 7.09 Pg C/yr as either contributing to system respiration, or becoming permanently stored in sediments. Accumulation of carbon in coastal sediments is only 0.41 Pg C/yr; about 6% of the total input. The NEP of coastal wetlands also contribute to the C sink of coastal margins, but the source of this C is part of the terrestrial C exchange with the atmosphere. Accumulation of C in wood and sediments of coastal wetlands is 0.205 Pg C/yr, half the estimate for sequestering of C in coastal sediments. Burial of C in shelf sediments is probably underestimated, particularly in tropical river-dominated coastal margins. Better estimates of these two C sinks in the tropics, coastal wetlands and shelf sediments, is needed to better understand the contribution of coastal ecosystems to the global carbon budget.     

Atmospheric deposition to grassland canopies: Lysimeter budgets discriminating between interception deposition,mineral weathering and mineralization

Lysimeter experiments were used to determine atmospheric input to grassland canopies. The combined effect of interception deposition + mineral weathering + mineralization was calculated from input/output budgets. Four types of lysimeters were used, either filled with very pure quartz sand or chalk grassland soil, and either without vegetation or planted with Brachypodium pinnatum (L.) Beauv., Combination of budgets for these four types of lysimeters yielded separate estimates of interception deposition and mineral weathering + mineralization. Ratios between total deposition and bulk deposition were 1.74 and 1.93 for N and S, respectively. Sources and sinks of H⁺ for lysimeters with chalk grassland soil and planted with Brachypodium (abbrev. CP-lysimeters) were about 10 times larger than for lysimeters without plants and filled with quartz sand. The contribution of atmospheric input to total H⁺-sources was 80% for bare lysimeters filled with quartz sand, and only 12% for CP-lysimeters. Bulk deposition and total atmospheric deposition of N was 1.25 and 2.18 kmol ha^?1 yr^?1, respectively, whereas N mineralization of chalk grassland soil yielded 1.62 kmol ha^?1 yr^?1, ‘Acid rain’ has only a minor influence on H⁺-transformations within a chalk grassland ecosystem, but N cycling is seriously affected by atmospheric input.     

Local variation in climate and land use during the time of the major kingdoms of the Tigray Plateau in Ethiopia and Eritrea

The Tigray Plateau of Ethiopia and Eritrea is vulnerable to environmental change, yet environmental influences on the rise and fall of the civilizations that once existed there are almost unexplored. We sampled sections of gully walls for palaeoenvironmental proxies from two sites: 1) Adi Kolen on the southern outskirts of the Plateau's most developed former empire, the Aksumite, and 2) Adigrat near polities dating to at least ca. 3000 cal yr BP. A multi-proxy approach for examining local variation in palaeoenvironments was evaluated that included stable isotopic and elemental analyses (δ¹³C_SOM, δ¹⁵N, %TOC, and %TN) of soil, and charcoal identification. An increase in δ¹⁵N values from older soils in Adi Kolen (4400 cal yr BP) and Adigrat (2900 cal yr BP) until 1200 cal yr BP is not explained by changes in δ¹⁵N that occur with time in an unchanging environment. It may instead indicate an overall decrease in rainfall from the earlier times until 1200 cal yr BP. In one Adigrat section, the decreases in organic δ¹³C and increases in C/N molar ratios from older to younger soil could have resulted from changes that occur over time, per se. In the remaining sections, however, δ¹³C_SOM trends more likely reflect changes in the biomass of C₄ relative to C₃ plants (% C₄ biomass). Changes in% C₄ biomass may reflect climate and/or land use. Deciphering which may be aided by analyses of the other proxies. Identified charcoal suggests that both sites supported some juniper forest types until very recently but that forests may have been a more important and dynamic component of Adigrat's vegetation history than Adi Kolen's. If environment affected the trajectories of the kingdoms of the Tigray Plateau, these results suggest that the exact nature of the changes in climate differed among kingdoms. The kingdoms prior to 1200 cal yr BP may have been exposed to increasing aridity punctuated with relatively wetter intervals. Thereafter, general changes in climate are not apparent. Land clearing dynamics are likely to have had a more consistent effect on the trajectories of kingdoms than climate changes.