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Carbon emissions from tropical land-use change are a major uncertainty in the global carbon cycle. In African woodlands, small-scale farming and the need for fuel are thought to be reducing vegetation carbon stocks, but quantification of these processes is hindered by the limitations of optical remote sensing and a lack of ground data. Here, we present a method for mapping vegetation carbon stocks and their changes over a 3-year period in a > 1000 km2 region in central Mozambique at 0.06 ha resolution. L-band synthetic aperture radar imagery and an inventory of 96 plots are combined using regression and bootstrapping to generate biomass maps with known uncertainties. The resultant maps have sufficient accuracy to be capable of detecting changes in forest carbon stocks of as little as 12 MgC ha−1 over 3 years with 95% confidence. This allows characterization of biomass loss from deforestation and forest degradation at a new level of detail. Total aboveground biomass in the study area was reduced by 6.9 ± 4.6% over 3 years: from 2.13 ± 0.12 TgC in 2007 to 1.98 ± 0.11 TgC in 2010, a loss of 0.15 ± 0.10 TgC. Degradation probably contributed 67% (96.9 ± 91.0 GgC) of the net loss of biomass, but is associated with high uncertainty. The detailed mapping of carbon stock changes quantifies the nature of small-scale farming. New clearances were on average small (median 0.2 ha) and were often additions to already cleared land. Deforestation events reduced biomass from 33.5 to 11.9 MgC ha−1 on average. Contrary to expectations, we did not find evidence that clearances were targeted towards areas of high biomass. Our method is scalable and suitable for monitoring land cover change and vegetation carbon stocks in woodland ecosystems, and can support policy approaches towards reducing emissions from deforestation and degradation ( REDD). [ABSTRACT FROM AUTHOR]

Three field manipulation experiments were carried out during 1993–1995 on the Northern Pennines to investigate the influences of temperature, solar radiation and rainfall on the release of dissolved organic carbon (DOC) from vegetated soil cores using zero-tension lysimeters. The cores were manipulated by being translocated to four sites down a climatic gradient, by artificial soil heating or being exposed to double normal rainfall. In each experiment three soil types, a brown earth, a micropodzol and a peaty gley, with differing organic matter content and distribution within the profile, were studied. DOC data, expressed as mg C m−2 day−1, were analysed following log10 transformation, by a repeated measures analysis of variance procedure, using climatic variables measured concurrently with sampling, and 1 and 2 months before sampling. DOC release was dominated by rainfall but was also associated with solar radiation and temperature. With each of the three climatic variables, rainfall, solar radiation and temperature, both positive and negative effects on DOC release have been found significant, indicating that the concurrent and delayed effects of the same variable may be different. DOC release was positively related with all three soils to concurrent rainfall, indicating rainfall's primary leaching action. DOC release was also negatively related to rainfall of the previous month indicating that its action depletes the leachable pool of DOC in the soil. DOC release was positively associated with solar radiation 2 months earlier, indicating that DOC's main source is that of primary production; DOC peaks closely followed peaks of annual primary production. DOC release was linked with temperature, the strongest association being with temperature 2 months earlier, indicating that temperature effects both primary production and DOC regeneration via organic matter decomposition. A conceptual model, relating our findings to those processes known to govern DOC release from soils, has been presented. [ABSTRACT FROM AUTHOR]

Cloud cover increases the proportion of diffuse radiation reaching the Earth's surface and affects many microclimatic factors such as temperature, vapour pressure deficit and precipitation. We compared the relative efficiencies of canopy photosynthesis to diffuse and direct photosynthetic photon flux density (PPFD) for a Norway spruce forest (25-year-old, leaf area index 11 m2 m−2) during two successive 7-day periods in August. The comparison was based on the response of net ecosystem exchange (NEE) of CO2 to PPFD. NEE and stomatal conductance at the canopy level ( Gcanopy) was estimated from half-hourly eddy-covariance measurements of CO2 and H2O fluxes. In addition, daily courses of CO2 assimilation rate ( AN) and stomatal conductance ( Gs) at shoot level were measured using a gas-exchange technique applied to branches of trees. The extent of spectral changes in incident solar radiation was assessed using a spectroradiometer. We found significantly higher NEE (up to 150%) during the cloudy periods compared with the sunny periods at corresponding PPFDs. Prevailing diffuse radiation under the cloudy days resulted in a significantly lower compensation irradiance (by ca. 50% and 70%), while apparent quantum yield was slightly higher (by ca. 7%) at canopy level and significantly higher (by ca. 530%) in sun-acclimated shoots. The main reasons for these differences appear to be (1) more favourable microclimatic conditions during cloudy periods, (2) stimulation of photochemical reactions and stomatal opening via an increase of blue/red light ratio, and (3) increased penetration of light into the canopy and thus a more equitable distribution of light between leaves. Our analyses identified the most important reason of enhanced NEE under cloudy sky conditions to be the effective penetration of diffuse radiation to lower depths of the canopy. This subsequently led to the significantly higher solar equivalent leaf area compared with the direct radiation. Most of the leaves in such dense canopy are in deep shade, with marginal or negative carbon balances during sunny days. These findings show that the energy of diffuse, compared with direct, solar radiation is used more efficiently in assimilation processes at both leaf and canopy levels. [ABSTRACT FROM AUTHOR]

The study investigates the effect of land-use change on nitrous oxide (N2O) and methane (CH4) fluxes from soil, in savanna ecosystems of the Orinoco region (Venezuela). Gas fluxes were measured by closed static chambers, in the wet and dry season, in representative systems of land management of the region: a cultivated pasture, an herbaceous savanna, a tree savanna and a woodland (control site). Higher N2O emissions were observed in the cultivated pasture and in the herbaceous savanna compared with the tree savanna and the woodland, and differences were mainly related to fine soil particle content and soil volumetric water content measured in the studied sites. Overall N2O emissions were quite low in all sites (0-1.58 mg N2O-N m-2 day-1). The cultivated pasture and the woodland savanna were on average weak CH4 sinks (-0.05 + 0.07 and -0.08 ± 0.05 mg CH4 m-2 day-1, respectively), whereas the herbaceous savanna and the tree savanna showed net CH4 production (0.23 ± 0.05 and 0.19 ± 0.05 mg CH4 m day respectively). Variations of CH4 fluxes were mainly driven by variation of soil water-filled pore space (WFPS), and a shift from net CH4 consumption to net CH4 production was observed at around 30% WFPS. Overall, the data suggest that conversion of woodland savanna to managed landscape could alter both CH4 and N2O fluxes; however, the magnitude of such variation depends on the soil characteristics and on the type of land management before conversion. [ABSTRACT FROM AUTHOR]

This study investigated the spatial and temporal variation in soil carbon dioxide (CO2) efflux and its relationship with soil temperature, soil moisture and rainfall in a forest near Manaus, Amazonas, Brazil. The mean rate of efflux was 6.45±0.25 SE μmol CO2 m−2s−1 at 25.6±0.22 SE°C (5 cm depth) ranging from 4.35 to 9.76 μmol CO2 m−2s−1; diel changes in efflux were correlated with soil temperature ( r2=0.60). However, the efflux response to the diel cycle in temperature was not always a clear exponential function. During period of low soil water content, temperature in deeper layers had a better relationship with CO2 efflux than with the temperature nearer the soil surface. Soil water content may limit CO2 production during the drying-down period that appeared to be an important factor controlling the efflux rate ( r2=0.39). On the other hand, during the rewetting period microbial activity may be the main controlling factor, which may quickly induce very high rates of efflux. The CO2 flux chamber was adapted to mimic the effects of rainfall on soil CO2 efflux and the results showed that efflux rates reduced 30% immediately after a rainfall event. Measurements of the CO2 concentration gradient in the soil profile showed a buildup in the concentration of CO2 after rain on the top soil. This higher CO2 concentration developed shortly after rainfall when the soil pores in the upper layers were filled with water, which created a barrier for gas exchange between the soil and the atmosphere. [ABSTRACT FROM AUTHOR]

Abstract We investigated the effects of spring barley growth on nitrogen (N) transformations and rhizosphere microbial processes in a controlled system under elevated carbon dioxide (CO2 ) at two levels of N fertilization (applied with 15 N labelling). After 25 d, elevated CO2 (twice ambient) increased plant growth (dry weight, DW) by 141% at low-N fertilization and by 60% at high-N fertilization, but its positive effect on the root-to-shoot ratio was only significant at low-N input. As a result of this plant response, elevated CO2 caused a greater soil CO2 efflux, rhizosphere soil DW, and soil microbial biomass under N-limiting conditions than under high N availability. Elevated CO2 also caused a significant (P < 0.001) increase in the N recovered by the plant from both the labelled (Nf ) and unlabelled (Ns + Nuf ) N pools. The dynamics of N in the system as affected by elevated CO2 were driven principally by mineralization–immobilization turnover, with little loss by denitrification. Under N-limiting conditions, there is evidence to suggest enhanced nutrient release from soil organic matter (SOM) pools—a process which could be defined as priming. The results of our experiment did not indicate a direct plant-mediated effect of elevated CO2 on nitrous oxide (N2 O) fluxes or denitrification activity. [ABSTRACT FROM AUTHOR]