Influence of drainage status on soil and water chemistry, litter decomposition and soil respiration in central Amazonian forests on sandy soils

Central Amazonian rainforest landscape supports a mosaic of tall terra firme rainforest and ecotone campinarana, riparian and campina forests, reflecting topography-induced variations in soil, nutrient and drainage conditions. Spatial and temporal variations in litter decomposition, soil and groundwater chemistry and soil CO2 respiration were studied in forests on sandy soils, whereas drought sensitivity of poorly-drained valley soils was investigated in an artificial drainage experiment. Slightly changes in litter decomposition or water chemistry were observed as a consequence of artificial drainage. Riparian plots did experience higher litter decomposition rates than campina forest. In response to a permanent lowering of the groundwater level from 0.1 m to 0.3 m depth in the drainage plot, topsoil carbon and nitrogen contents decreased substantially. Soil CO2 respiration decreased from 3.7±0.6 µmol m-2 s-1 before drainage to 2.5±0.2 and 0.8±0.1 µmol m-2 s-1 eight and 11 months after drainage, respectively. Soil respiration in the control plot remained constant at 3.7±0.6 µmol m-2 s-1. The above suggests that more frequent droughts may affect topsoil carbon and nitrogen content and soil respiration rates in the riparian ecosystem, and may induce a transition to less diverse campinarana or short-statured campina forest that covers areas with strongly-leached sandy soil

Measurements of soil respiration and simple models dependent on moisture and temperature for an Amazonian southwest tropical forest

Soil respiration plays a significant role in the carbon cycle of Amazonian tropical forests, although in situ measurements have only been poorly reported and the dependence of soil moisture and soil temperature also weakly understood. This work investigates the temporal variability of soil respiration using field measurements, which also included soil moisture, soil temperature and litterfall, from April 2003 to January 2004, in a southwest Brazilian tropical rainforest near Ji-Paraná, Rondônia. The experimental design deployed five automatic (static, semi-opened) soil chambers connected to an infra-red CO2 gas analyzer. The mean half-hourly soil respiration showed a large scattering from 0.6 to 18.9 µmol CO2 m-2 s-1 and the average was 8.0±3.4 µmol CO2 m-2 s-1. Soil respiration varied seasonally, being lower in the dry season and higher in the wet season, which generally responded positively to the variation of soil moisture and temperature year round. The peak was reached in the dry-to-wet season transition (September), this coincided with increasing sunlight, evapotranspiration and ecosystem productivity. Litterfall processes contributed to meet very favorable conditions for biomass decomposition in early wet season, especially the fresh litter on the forest floor accumulated during the dry season. We attempted to fit three models with the data: the exponential Q10 model, the Reichstein model, and the log-soil moisture model. The models do not contradict the scattering of observations, but poorly explain the variance of the half-hourly data, which is improved when the lag-time days averaging is longer. The observations suggested an optimum range of soil moisture, between 0.11

Topography induced spatial variations in diurnal cycles of assimilation and latent heat of Mediterranean forest

The aim of this study is to explain topography induced spatial variations in the diurnal cycles of assimilation and latent heat of Mediterranean forest. Spatial variations of the fluxes are caused by variations in weather conditions and in vegetation characteristics. Weather conditions reflect short-term effects of climate, whereas vegetation characteristics, through adaptation and acclimation, long-term effects of climate. In this study measurements of plant physiology and weather conditions are used to explain observed differences in the fluxes. A model is used to study which part of the differences in the fluxes is caused by weather conditions and which part by vegetation characteristics. Data were collected at four experimental sub-Mediterranean deciduous forest plots in a heterogeneous terrain with contrasting aspect, soil water availability, humidity and temperature. We used a sun-shade model to scale fluxes from leaf to canopy, and calculated the canopy energy balance. Parameter values were derived from measurements of light interception, leaf chamber photosynthesis, leaf nitrogen content and ¹³C isotope discrimination in leaf material. Leaf nitrogen content is a measure of photosynthetic capacity, and ¹³C isotope discrimination of water use efficiency. For validation, sap-flux based measurements of transpiration were used. The model predicted diurnal cycles of transpiration and stomatal conductance, both their magnitudes and differences in afternoon stomatal closure between slopes of different aspect within the confidence interval of the validation data. Weather conditions mainly responsible for the shape of the diurnal cycles, and vegetation parameters for the magnitude of the fluxes. Although the data do not allow for a quantification of the two effects, the differences in vegetation parameters and weather among the plots and the sensitivity of the fluxes to them suggest that the diurnal cycles were more strongly affected by spatial variations in vegetation parameters than by meteorological variables. This indicates that topography induced variations in vegetation parameters are of equal importance to the fluxes as topography induced variations in radiation, humidity and temperature

Soil CO2_2 efflux in Central Amazonia: Environmental and methodological effects

Soil respiration plays a significant role in the carbon cycle of Amazonian rainforests. Measurements of soil respiration have only been carried out in few places in the Amazon. This study investigated the effects of the method of ring insertion in the soil as well as of rainfall and spatial distribution on C

Net ecosystem exchange of carbon dioxide and water of far eastern Siberian Larch (Larix cajanderii) on permafrost.

Observations of the net ecosystem exchange of water and CO₂ were made during two seasons in 2000 and 2001 above a Larch forest in Far East Siberia (Yakutsk). The measurements were obtained by eddy correlation. There is a very sharply pronounced growing season of 100 days when the forest is leaved. Maximum half hourly uptake rates are 18 &micro;mol m^-2 s^-1; maximum respiration rates are 5 &micro;mol m^-2 s^-1. Net annual sequestration of carbon was estimated at 160 gCm^-2 in 2001. Applying no correction for low friction velocities added 60 g C m^-2. The net carbon exchange of the forest was extremely sensitive to small changes in weather that may switch the forest easily from a sink to a source, even in summer. June was the month with highest uptake in 2001. <P style='line-height: 20px;'> The average evaporation rate of the forest approached 1.46 mm day^-1 during the growing season, with peak values of 3 mm day^-1 with an estimated annual evaporation of 213 mm, closely approaching the average annual rainfall amount. 2001 was a drier year than 2000 and this is reflected in lower evaporation rates in 2001 than in 2000. <P style='line-height: 20px;'> The surface conductance of the forest shows a marked response to increasing atmospheric humidity deficits. This affects the CO₂ uptake and evaporation in a different manner, with the CO₂ uptake being more affected. There appears to be no change in the relation between surface conductance and net ecosystem uptake normalized by the atmospheric humidity deficit at the monthly time scale. The response to atmospheric humidity deficit is an efficient mechanism to prevent severe water loss during the short intense growing season. The associated cost to the sequestration of carbon may be another explanation for the slow growth of these forests in this environment

Soil Co2 efflux in central Amazonia: Environmental and methodological effects

Soil respiration plays a significant role in the carbon cycle of Amazonian rainforests. Measurements of soil respiration have only been carried out in few places in the Amazon. This study investigated the effects of the method of ring insertion in the soil as well as of rainfall and spatial distribution on CO2 emission in the central Amazon region. The ring insertion effect increased the soil emission about 13-20% for sandy and loamy soils during the firsts 4-7 hours, respectively. After rainfall events below 2 mm, the soil respiration did not change, but for rainfall greater than 3 mm, after 2 hours there was a decrease in soil temperature and respiration of about 10-34% for the loamy and sand soils, with emissions returning to normal after around 15-18 hours. The size of the measurement areas and the spatial distribution of soil respiration were better estimated using the Shuttle Radar Topographic Mission (SRTM) data. The Campina reserve is a mosaic of bare soil, stunted heath forest-SHF and tall heath forest-THF. The estimated total average CO2 emissions from the area was 3.08±0.8 µmol CO2 m-2 s-1. The Cuieiras reserve is another mosaic of plateau, slope, Campinarana and riparian forests and the total average emission from the area was 3.82±0.76 µmol CO2 m-2 s-1. We also found that the main control factor of the soil respiration was soil temperature, with 90% explained by regression analysis. Automated soil respiration datasets are a good tool to improve the technique and increase the reliability of measurements to allow a better understanding of all possible factors driven by soil respiration processes

Estimativa do índice de área Foliar (IAF) e biomassa em pastagem no estado de Rondônia, Brasil

Medidas mensais da altura da pastagem, biomassa total, variações de biomassa viva e morta, a área específica foliar (SLA) e o Índice de Área de Folha (IAF) de fevereiro de 1999 a janeiro de 2005 na Fazenda Nossa Senhora (FNS) e em Rolim de Moura (RDM) entre Fevereiro a Março de 1999, Rondônia, Brasil. A pastagem predominante é Urochloa brizantha (Hochst. ex A. Rich) R. D. Webster (99% na FNS e 76% em RDM), com pequenas manchas de Urochloa humidicula (Rendle). A altura média anual da grama foi de ~0,16 m. Com o pastejo, o mínimo mensal foi de 0,09 m (estação seca) e máximo de 0,3 m sem pastejo (estação úmida). O IAF, biomassa total, material morto, vivo e SLA tiveram valores médios de 2,5 m2 m-2 , 2202 kg ha-1, 2916 kg ha-1 e 19 m2 kg-1 respectivamente. A média mensal da biomassa foi 4224 kg ha-1 em 2002 e 6667 kg ha-1 em 2003. Grande variação sazonal do material vivo e morto, sendo mais alto o vivo durante a estação úmida (3229 contra 2529 kg ha-1), sendo o morto maior durante a seca (2542 contra 1894 kg ha-1). O nível de água no solo variou de -3,1 a -6,5 m durante as estações. Em médias anuais os IAF foram de 1,4 em 2000 a 2,8 em 2003 e o SLA entre 16,3 m2 kg-1 em 1999 e 20,4 m2 kg-1 em 2001. As observações do Albedo variaram de 0,18 para 0,16 em relação aos altos valores de IAF