Utilising conservative tracers and spatial surveys to identify controls on pathways and DOC exports in an Arctic catchment

Dissolved organic carbon (DOC) is typically the predominant form of carbon exported from headwater streams, it therefore represents a major carbon export from Arctic catchments. The projected deepening of thaw depth in permafrost regions, due to an increase in air temperature, may have a significant effect on the amount of DOC exported from these systems. However, quantification of the impacts of climate driven changes on DOC export are still highly uncertain. Understanding the processes controlling DOC export is therefore crucial in predicting the potential impact of projected environmental changes. The controls of DOC production and transport are heavily influenced by soil and vegetation, which are highly variable across the landscape. To completely understand these systems information regarding spatial variability of plants, soils and thaw depths must be taken into account. In this study sub-weekly sampling of DOC was undertaken throughout 2014 in a headwater (<1 km2) catchment in the Northwest Territories, Canada. Spatial surveys of soil properties, active thaw depth and normalised difference vegetation index (NDVI) were collected and used in conjunction with conservative stable water isotopes tracers and major ions to understand sources, flow pathways and timing of DOC exports from the catchment. Stable isotope tracers act as fingerprints of water allowing sources and pathways to be assessed. Observations reveal changing DOC concentrations throughout the season as the active layer deepens and the connectivity of the soils to the stream network throughout the catchment increases. Linking the DOC data with the conservative tracer response improves the identification of carbon pathways and fluxes from the soils; preliminary analysis indicates DOC is being delivered via deeper more mineral soils later in the season. The results indicate that the active layer depth has a strong influence on the amount of DOC exported from the system, independent of the amount of carbon stored in these deeper soils

Dynamics and pathways of autotrophic and heterotrophic soil CO₂ efflux revealed by forest girdling

&lt;p&gt;1. Quantifying pathways and temporal dynamics of carbon (C) flux between plants and soil is critical to our understanding of the long-term fate of C stored in soils. The potential priming of old organic matter decomposition by fresh C input from plants means that the impact of environmental changes on the interactions between plant C allocation and soil C storage need to be better understood. We used forest girdling to investigate the partitioning of total soil CO2 efflux (RS) into autotrophic (RA) and heterotrophic (RH) flux components and their interaction with litter decomposition.&lt;/p&gt; &lt;p&gt;2. The reduction in RS in girdled plots stabilized within two weeks at 65% of control plot values, indicating that RS is dominated by RH, and that only a small pool of available non-structural C remains in roots in late summer to sustain rhizosphere metabolic processes. RA contributions declined from 35% late in the growing season to about 25% in winter.&lt;/p&gt; &lt;p&gt;3. Our results indicate that actual root respiration (RR) and respiration by ectyomycorrhizas and other rhizospheric organisms (RM) contribute c. 50% each to RA between September and early November. During winter, RA remained significantly greater than zero despite frequent sub-zero air temperatures, with RM being a dominant component of RA during this period.&lt;/p&gt; &lt;p&gt;4. Forest girdling significantly increased the age of C in soil-respired CO2, consistent with the removal of contemporary C derived from RA. Partitioning of soil CO2 efflux on the basis of 14C results shows good agreement with the flux reduction observed between girdled and control plots.&lt;/p&gt; &lt;p&gt;5.  Litter bag incubations indicate a promoting influence of an intact C supply to the rhizosphere on decomposition, indicating a positive rhizosphere priming effect.&lt;/p&gt; &lt;p&gt;6. Synthesis: Our results demonstrate significant contribution of mycorrhizas and other rhizosphere organisms to RS, and suggest a direct link between an intact rhizosphere and litter decomposition dynamics. These results highlight the tight coupling between autotroph activity and soil decomposition processes in forest soils, and add to the growing body of evidence that plant and soil processes cannot be treated separately.&lt;/p&gt

Litter decomposition and soil CO2 efflux on the Mediterranean island of Pianosa

Mediterranean ecosystems are particularly vulnerable to changes in climate and land use forecasted for the near future, with likely perturba- tions of the carbon cycle. The aim of our study was to quantify particular aspects of the carbon cycle in typical Mediterranean ecosystems, in particular (1) the decay rates of litter from common tree and shrub species, (2) the efflux of CO2 from the soil and its relation to soil and litter moisture, and (3) the dynamics of the stable isotope 13 C during litter decomposition. Field work was conducted on the island Pianosa, which comprises a range of common Mediterranean ecosystem types. Litter decay rates of three selected species (Cistus monspenliensis, Pistacia lentiscus and Juniperus phoenicia) were found to be low with an average of 70 % of initial mass remaining after 2 years of field incubation. Over the same period, all litter types showed only a slight (&lt;10 %) net loss of N. Despite relatively high initial N contents, litter decay rates were comparable to those reported in the literature, suggesting that C and N dynamics are decoupled during litter decomposition. Over the two years of incubation, 13 C dynamics were not unanimous between the three litter types, with only a slight enrichment in one species. Continuation of this ongoing experiment is likely to resolve the long term effects of decomposition on 13 C enrichment on litter. Soil CO2 efflux was found to be unusually high (peak rates of over 9 \ub5mol m-2 s-1 ), owing to both high soil water content and soil temperature during an intensive measuring campaign in October 2003. Mean daily fluxes in woodland ecosystems were significantly higher than in either macchia or ex agricultural ecosystems, exceeding the latter about twofold. However, when scaled to the relative surface representation on Pianosa, the highest contribution of daily soil CO2 efflux stems from Macchia type vegetation, followed by abandoned agricultural sites and woodland ecosystems (around 20, 22, and 8.5 t C d-1 , respectively). With the exception of one site, soil CO2 efflux correlated positi- vely with litter content at different sites across the island. Rather than causing the higher fluxes directly, higher litter contents are likely to indicate higher site productivity rates, resulting in higher CO2 turnover dynamics and hence higher overall soil CO2 efflux rates. Owing to the only small range of soil moisture conditions during the measuring campaign, no dependence of soil CO2 efflux on soil moisture could be detected. However, a range of moisture conditions between sites was noted, indicating the significance of site specific conditions also within the same ecosystem types

Taxa de emissão de CO2 de um latossolo fertirrigado com ácido fosfórico por gotejamento CO2 emission rate from a fertigated bare soil with phosphoric acid by dripping

A aplicação de fertilizantes fosfatados por meio de fertirrigação com sistemas de irrigação localizada pode causar obstrução de emissores. Para evitar esse problema, pode ser utilizado o ácido fosfórico como fonte de fósforo às plantas. Porém, têm sido pouco investigados os efeitos da irrigação relacionados às perdas de CO2 do solo para a atmosfera, em conseqüência da decomposição do carbono orgânico e da infiltração de água no solo. Neste trabalho, investigou-se, no período de um mês, o efeito da fertirrigação com ácido fosfórico nas taxas de emissão de CO2 de um latossolo desprovido de vegetação, na Área Experimental de Irrigação da UNESP, Câmpus de Jaboticabal - SP. Utilizou-se de um sistema de irrigação por gotejamento, com delineamento experimental em blocos casualizados, constando de cinco repetições e cinco tratamentos (0; 30; 60; 90 e 120 kg ha-1de P2O5), aplicados via fertirrigação com ácido fosfórico. Verificou-se que as taxas de emissão de CO2 aumentaram significativamente após as fertirrigações, porém não houve efeito da dose do ácido fosfórico sobre as taxas. A umidade do solo mostrou-se um fator importante na relação entre as variações das taxas de emissão e a temperatura do solo ao longo do período estudado.<br>The application of phosphoric fertilizers through fertigation, with localized irrigation systems, can cause emitters obstruction. In order to avoid this problem, the phosphoric acid can be used as phosphorus source to the plants. However, it has been little investigations on the effects of the irrigation practices, related to the CO2 transference to the atmosphere, due to organic matter decomposition in the soil and its water infiltration. At this work, the rates of emissions of CO2 from a bare soil without vegetation, and fertigated along one month were investigated. The experiment was conducted with randomized blocks design in São Paulo State University - UNESP, Jaboticabal, Brazil. The drip irrigation system was used, with five treatments and five replications. The treatments were constituted by five rates (0; 30; 60; 90 and 120 kg ha-1 of P2O5), applied by fertigation using phosphoric acid. By the results, it is possible to verify that the emissions increased significantly after the fertigation events, however, no effect of phosphoric acid added to water was observed on emissions. Soil moisture was a relevant factor in the relationship among the variations of the emission rates and the temperature of the soil along the studied period

An Analysis of Soil Respiration across Northern Hemisphere Temperate Ecosystems

Over two-thirds of terrestrial carbon is stored belowground and a significant amount of atmospheric CO₂ is respired by roots and microbes in soils. For this analysis, soil respiration (Rs) data were assembled from 31 AmeriFlux and CarboEurope sites representing deciduous broadleaf, evergreen needleleaf, grasslands, mixed deciduous/evergreen and woodland/savanna ecosystem types. Lowest to highest rates of soil respiration averaged over the growing season were grassland and woodland/savanna &lt deciduous broadleaf forests &lt evergreen needleleaf, mixed deciduous/evergreen forests with growing season soil respiration significantly different between forested and non-forested biomes (p &lt 0.001). Timing of peak respiration rates during the growing season varied from March/April in grasslands to July-September for all other biomes. Biomes with overall strongest relationship between soil respiration and soil temperature were from the deciduous and mixed forests (R⁲ ≥ 0.65). Maximum soil respiration was weakly related to maximum fine root biomass (R⁲ = 0.28) and positively related to the previous years' annual litterfall (R⁲ = 0.46). Published rates of annual soil respiration were linearly related to LAI and fine root carbon (R⁲ = 0.48, 0.47), as well as net primary production (NPP) (R⁲ = 0.44). At 10 sites, maximum growing season Rs was weakly correlated with annual GPP estimated from eddy covariance towersites (R⁲ = 0.29; p &lt 0.05), and annual soil respiration and total growing season Rs were not correlated with annual GPP (p &gt 0.1). Yet, previous studies indicate correlations on shorter time scales within site (e.g., weekly, monthly). Estimates of annual GPP from the Biome-BGC model were strongly correlated with observed annual estimates of soil respiration for six sites (R⁲ = 0.84; p &lt 0.01). Correlations from observations of Rs with NPP, LAI, fine root biomass and litterfall relate above and belowground inputs to labile pools that are available for decomposition. Our results suggest that simple empirical relationships with temperature and/or moisture that may be robust at individual sites may not be adequate to characterize soil CO₂ effluxes across space and time, agreeing with other multi-site studies. Information is needed on the timing and phenological controls of substrate availability (e.g., fine roots, LAI) and inputs (e.g., root turnover, litterfall) to improve our ability to accurately quantify the relationships between soil CO₂ effluxes and carbon substrate storage