Simona Castaldi

Three decades of global methane sources and sinks

Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios-which differ in fossil fuel and microbial emissions-to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain.

Impact of biochar application to a Mediterranean wheat crop on soil microbial activity and greenhouse gas fluxes

Biochar has been recently proposed as a management strategy to improve crop productivity and global warming mitigation. However, the effect of such approach on soil greenhouse gas fluxes is highly uncertain and few data from field experiments are available. In a field trial, cultivated with wheat, biochar was added to the soil (3 or 6 kg m(-2)) in two growing seasons (2008/2009 and 2009/2010) so to monitor the effect of treatments on microbial parameters 3 months and 14 months after char addition. N(2)O, CH(4) and CO(2) fluxes were measured in the field during the first year after char addition. Biochar incorporation into the soil increased soil pH (from 5.2 to 6.7) and the rates of net N mineralization, soil microbial respiration and denitrification activity in the first 3 months, but after 14 months treated and control plots did not differ significantly. No changes in total microbial biomass and net nitrification rate were observed. In char treated plots, soil N(2)O fluxes were from 26% to 79% lower than N(2)O fluxes in control plots, excluding four sampling dates after the last fertilization with urea, when N(2)O emissions were higher in char treated plots. However, due to the high spatial variability, the observed differences were rarely significant. No significant differences of CH(4) fluxes and field soil respiration were observed among different treatments, with just few exceptions. Overall the char treatments showed a minimal impact on microbial parameters and GHG fluxes over the first 14 months after biochar incorporation.

Greenhouse gases (CO2, CH4 and N2O) in lowland springs within an agricultural impacted watershed (Po River Plain, northern Italy)

In the Po River Plain, nitrogen surplus in permeable soils results in elevated downward nitrogen fluxes, mostly as nitrate. Lowland springs, aligned along interfaces between gravel and sandy soils, recycle part of this nitrogen to the surface and we hypothesised that they may be hot spots of N2O and other greenhouse gases, due to incomplete denitrification in the suboxic environment. In early and late summer 2009, water flow was measured and water samples were collected at the outlet and ∼1 km downstream at 14 springs; physico-chemical parameters [temperature, pH, dissolved inorganic nitrogen (DIN) and dissolved gases (O2, N2O, CH4, CO2)] were analysed. All springs were characterised by elevated nitrate concentrations (154–1411 μ M) and recycled to the surface inorganic nitrogen (∼180 kg N on average). Spring waters were suboxic (40–60% of O2 saturation) and CO2, CH4 and N2O supersaturated (26.6–2399.0, 0.002–1.02 and 0.02–1.02 μ M, respectively). CO2 and N2O underwent a significant degassing process from the supersaturated waters to the atmosphere. Calculated N2O emissions (up to 0.646 g N 2O·m −2·d −1, among the highest reported for aquatic environments) highlight the role of lowland springs as hotspots of N2O. We conclude that lowland springs located in heavily impacted watersheds recycle groundwater nitrate and have an extremely elevated potential as greenhouse gas emitters.

Long tree-ring chronologies provide evidence of recent tree growth decrease in a Central African tropical forest.

It is still unclear whether the exponential rise of atmospheric CO2 concentration has produced a fertilization effect on tropical forests, thus incrementing their growth rate, in the last two centuries. As many factors affect tree growth patterns, short -term studies might be influenced by the confounding effect of several interacting environmental variables on plant growth. Long-term analyses of tree growth can elucidate long-term trends of plant growth response to dominant drivers. The study of annual rings, applied to long tree-ring chronologies in tropical forest trees enables such analysis. Long-term tree-ring chronologies of three widespread African species were measured in Central Africa to analyze the growth of trees over the last two centuries. Growth trends were correlated to changes in global atmospheric CO2 concentration and local variations in the main climatic drivers, temperature and rainfall. Our results provided no evidence for a fertilization effect of CO2 on tree growth. On the contrary, an overall growth decline was observed for all three species in the last century, which appears to be significantly correlated to the increase in local temperature. These findings provide additional support to the global observations of a slowing down of C sequestration in the trunks of forest trees in recent decades. Data indicate that the CO2 increase alone has not been sufficient to obtain a tree growth increase in tropical trees. The effect of other changing environmental factors, like temperature, may have overridden the fertilization effect of CO2.

The effect of different N substrates on biological N2O production from forest and agricultural light textured soils

N2O emissions from two slightly alkaline sandy soils, from arable land and a woodland, were determined in a laboratory experiment in which the soils were incubated with different sources of nitrogen, with or without glucose, and with 0, 1 and 100 mL C2H2 L-1. Large differences in the rate of N2O production were observed between the two soils and between the different N treatments. The arable soil showed very low N2O emissions derived from reduced forms of N as compared with the N2O which was produced when the soil was provided with NO2- or NO3- and a C source, suggesting a very active denitrifier population. In contrast, the woodland soil showed a very low denitrification activity and a much higher N2O production derived from the oxidation of NH4+ and reduction of NO2- by some processes probably mediated by autotrophic or heterotrophic nitrifiers or dissimilatory NO2- reducers. In both soils, the highest N2O emissions were induced by NO2- addition. Those emissions were demonstrated to have a biological origin, as no significant N2O emissions were measured when the soil was autoclaved.

Tree-ring carbon and oxygen isotopes indicate different water use strategies in three Mediterranean shrubs at Capo Caccia (Sardinia, Italy)

Key messageVariations in stable carbon and oxygen isotope compositions of co-occurring plant species reflect their different water use strategies and indicate the importance of screening species’ WUEito plan climate change adaptation strategies.AbstractThe different abilities of plant species to cope with drought have been associated with structural and ecophysiological constraints. In this paper, we evaluate interspecific differences in intrinsic water use efficiency (WUEi) and the ratio of photosynthesis (A) to stomatal conductance (gs) in three co-occurring Mediterranean shrubs: two broad-leaved evergreen (Pistacia lentiscus and Phillyrea angustifolia) and one needle-like-leaved evergreen (Juniperus phoenicea). We used δ13C in rings to assess inter-annual changes in WUEi while the influence of the stomatal conductance was explored through δ18O. Our results indicate consistent differences in WUEi in the three species, largely determined by leaf traits and differences in stomatal conductance control. Juniperus phoenicea could be the most threatened by the current trend of increasing temperature and summers drought. Phillyrea angustifolia and P. lentiscus seem to be less affected by drought stress because of their tighter stomatal control and high survival rate under field conditions. Our study shows that shrubs with different leaf traits employ different plant ecophysiological strategies under drought stress.

Fungi-to-bacteria ratio in soils of European Russia

The selective inhibition technique by specific antibiotics (streptomycin, cycloheximide) applied to substrate-induced respiration (SIR) measurement was used to test the relative contribution of fungi to bacteria (F/B ratio) to the overall microflora-induced activity in soils of European Russia. Investigated soils covered a wide climatic transect and different ecosystem types including managed vs. natural ecosystems. Before direct comparison among sites, the antibiotic inhibition technique was optimized for soil characteristics. Once the optimal concentration was set, the combined effect of the two antibiotics resulted in average 60% inhibition of SIR. The analyzed sites (in total 47) including various biomes (tundra, middle taiga, southern taiga, subtaiga, dark coniferous forests outside the boreal region, steppe, mountain forests and arable sites), were characterized by a wide range of soil pHw (3.95–7.95), soil organic carbon (0.69–24.08%), soil microbial biomass carbon (149–5028 µg C g−1 soil) and soil basal respiration (0.24–8.28 µg CO2-C g−1 soil h−1). In all the analyzed sites, a predominance of fungal over bacteria activity was observed with F/B ratios always higher than one (4.9 on average). Natural sites were characterized by higher F/B ratios (on average 5.6) compared to agricultural ones (on average 3.5).

Soil N2O emissions in a Mediterranean shrubland disturbed by experimental fires

In the present work, post-burning soil N2O fluxes and related microbial processes were investigated in a Mediterranean shrubland subjected to experimental fires. Nine plots were selected, of which three were used as controls, three were burned with low-intensity fire and three with higher intensity fire. N2O fluxes, soil humidity and temperature were measured starting 2 days before burning and for 1 year after fire. Potential net nitrification, denitrification enzyme activity, mineral N and organic C were measured from soil samples collected periodically after burning. Cumulative data indicate a doubling of N2O production in burned plots over 1 year. Burned plots showed an increase of frequency of hot spots of N2O production. A slight detrimental effect of fire on the analysed biological activities was detected only immediately after burning. After 3 months, both potential net nitrification and denitrification enzyme activity had mostly recovered and potential net nitrification further increased over control levels in the following months. Fire seemed to induce a change in the main source of N2O, which in control plots was represented by heterotrophic activity (50–75%), whereas in burned plots it was mostly of autotrophic origin, most probably due to the significant increase of soil NH4+ after burning.

N2O Emission Factors for Italian Crops

Nitrous oxide (N2O) emissions from several Italian croplands along a latitudinal gradient were analyzed and the fertilizer induced emission (FIE) factor, for each single fertilization event, was calculated. Data show that the average emission factor was between 0.7 and 0.3 %, hence much lower than the IPCC EF used for temperate croplands. The relationship between N2O production and applied N fertilization rate was exponential and not linear, although the rate of exponential increase was lower than previously reported. Maximum N2O emission rate was correlated with magnitude of the total FIE, whereas it was inversely related to the length of FIE, which varied from a minimum of 8–56 days. Overall data suggest that the internationally applied emission factors for temperate crops, which are empirically derived from sites with cooler and wetter climates than the Mediterranean, would overestimate N2O emissions for Italian crops, in particular those developing between spring and summer.