Determination of respiration and photosynthesis fractionation coefficients for atmospheric dioxygen inferred from a vegetation-soil-atmosphere analog of the terrestrial biosphere in closed chambers

The isotopic composition of dioxygen in the atmosphere is a global tracer which depends on the biosphere flux of dioxygen toward and from the atmosphere (photosynthesis and respiration) as well as exchanges with the stratosphere. When measured in fossil air trapped in ice cores, the relative concentration of 16O, 17O and 18O of O2 can be used for several applications such as ice core dating and past global productivity reconstruction. However, there are still uncertainties about the accuracy of these tracers as they depend on the integrated isotopic fractionation of different biological processes of dioxygen production and uptake, for which we currently have very few independent estimates. Here we determined the respiration and photosynthesis fractionation coefficients for atmospheric dioxygen from experiments carried out in a replicated vegetation-soil-atmosphere analog of the terrestrial biosphere in closed chambers with growing Festuca arundinacea. The values for 18O discrimination during soil respiration and dark respiration in leave are equal to −12.3 ± 1.7 ‰ and −19.1 ± 2.4 ‰, respectively. We also found a value for terrestrial photosynthetic fractionation equal to +3.7 ± 1.3 ‰. This last estimate suggests that the contribution of terrestrial productivity in the Dole effect may have been underestimated in previous studies.

Grazing Reduces the Soil-Atmosphere Exchange of Greenhouse Gases During Freeze-Thaw Cycles in Meadow Steppes in Inner Mongolia

Both livestock grazing and soil freeze-thaw cycles (FTCs) can affect the soil-atmosphere exchange of greenhouse gases (GHGs) in grasslands. However, the combined effects of grazing and FTCs on GHG fluxes in meadow steppe soils remain unclear. In this study, we collected soils from paired grazing and enclosed sites and conducted an incubation experiment to investigate the effect of grazing on soil GHG fluxes in the meadow steppes of Inner Mongolia during three FTCs. Our results showed that FTCs substantially stimulated the emissions of soil N2O and CO2 and the uptake of CH4 in the meadow steppes. However, compared with enclosure treatments, grazing significantly reduced the cumulative N2O, CO2 and CH4 fluxes by 13.3, 14.6, and 26.8%, respectively, during the entire FTCs experiment. The soil dissolved organic carbon (DOC) and nitrogen (DON), NH4+-N and NO3–-N, significantly increased after three FTCs and showed close correlations with N2O and CO2 emissions. Structural equation modeling (SEM) revealed that the increase in NO3–-N induced by FTCs dominated the variance in N2O emissions and that DOC strongly affected CO2 emissions during thawing periods. However, long-term grazing reduced soil substrate availability and microbial activity and increased soil bulk density, which in turn decreased the cumulative GHG fluxes during FTCs. In addition, the interaction between grazing and FTCs significantly affected CO2 and CH4 fluxes but not N2O fluxes. Our results indicated that livestock grazing had an important effect on soil GHG fluxes during FTCs. The combined effect of grazing and FTCs should be taken into account in future estimations of GHG budgets in both modeling and experimental studies.

Observations on soil-atmosphere interactions after long-term monitoring at two sample sites subjected to shallow landslides

Soil-atmosphere interaction has implications in different scientific research contexts and is increasingly investigated through field measurements. This paper reports a detailed description of interaction between shallow soil and atmosphere at two test sites in Oltrepò Pavese area (Northern Italy). The two test sites are in the same climatic area but are characterised by different geological features. In fact, the first objective is to compare the behaviour of two different soils, namely a clayey-sandy silt (CL) and a silty clay (CH), under similar meteorological events. Soil-atmosphere interaction is studied on the basis of long-term (about 87 and 42 months for the two test sites, respectively) monitoring data of both volumetric water content and soil water potential, recorded at different depths along two vertical soil profiles in the first two metres from ground level. Field measurements, together with meteorological data such as precipitation and air temperature, allow for clear identification of the seasonal fluctuations of unsaturated soil hydraulic properties. To infer detailed information, the recorded data were processed and relationships between soil water potential and water content were investigated. Different time spans, from several months to a few days, even including single rainy events, are considered to show the hydraulic soil behaviour. The hysteretic cycles of water content with respect to soil water potential and non-equilibrium flow are highlighted. In particular, the measured soil water potential is in the range of 0–800 kPa and of 0–1500 kPa for the CL and CH soil, respectively. At both sites, the observed hysteretic cycles are more frequent in the hot season (summer) than in the cold season (winter) and tend to reduce with depth. The experimental results are compared with the soil water characteristic curves (SWCCs) to assess whether and to what extent the SWCCs are reliable in modelling the hydraulic behaviour of partially saturated soils, under atmospheric forcing, at least in the considered climatic contexts.

Capturing the Scale Dependency of Erosion-Induced Variation in CO2 Emissions on Terraced Slopes

Net soil CO2 emissions are not independent of topography but tend to decline with increasing slope gradients. Such decline has been attributed to increased runoff and greater soil loss on steep slopes, leaving the soil less habitable for microorganisms. However, the specific variations of slope gradients and thus the associated soil properties relevant for CO2 emissions, especially from terraced slopes, are often disguised by the coarse resolution of digital terrain models (DTMs) based on commonly available open-source data. Such misrepresentation of the relationship between topography and soil CO2 emissions carries the risk of a wrong assessment of soil-atmosphere interaction. By applying a slope dependent soil CO2 emission model developed from erosion plots to nearby sloping and partially terraced cropland using two DTMs of different spatial resolutions, this study tested the significance of these resolution-induced errors on CO2 emission estimates. The results show that the coarser-resolution Shuttle Radar Topography Mission (SRTM) underestimated CO2-C emission by 27% compared to the higher-resolution DTM derived from Unmanned Aerial Vehicles (UAV) imagery. Such difference can be mostly attributed to a better representation of the proportion of flat slopes in the high-resolution DTM. Although the observations from erosion plots cannot be directly extrapolated to a larger scale, the 27% underestimation using the coarser-resolution SRTM DTM emphasizes that it is essential to represent microreliefs and their impact on runoff and erosion-induced soil heterogeneity at an appropriate scale. The widespread impact of topography on erosion and deposition on cropland, and the associated slope-dependent heterogeneity of soil properties, may lead to even greater differences than those observed in this study. The greatly improved estimation on CO2 emissions by the UAV-derived DTM also demonstrates that UAVs have a great potential to fill the gap between conventional field investigations and commonly applied coarse-resolution remote sensing when assessing the impact of soil erosion on global soil-atmosphere interaction.

Integrating beef cattle on cropland affects net global warming potential

Recent interest in integrated crop-livestock (ICL) systems has prompted numerous investigations to quantify ecosystem service tradeoffs associated with management. However, few investigations have quantified ICL management effects on net global warming potential (GWP), particularly in semiarid regions. Therefore, we determined net GWP for grazed and ungrazed cropland in a long-term ICL study near Mandan, ND USA. Factors evaluated for their contribution to net GWP included carbon dioxide (CO2) emissions associated with production inputs and field operations, methane (CH4) emissions from enteric fermentation by beef cattle, change in soil carbon stocks, and soil-atmosphere CH4 and nitrous oxide (N2O) fluxes. Net GWP was significantly greater for grazed cropland (946 kg CO2equiv. ha-1 yr-1) compared to ungrazed cropland (200 kg CO2equiv. ha-1 yr-1) (P=0.0331). The difference in net GWP between treatments was largely driven by emissions from enteric fermentation (602 kg CO2equiv. ha-1 yr-1). Among other contributing factors, CO2 emissions associated with seed production and field operations were lower under ungrazed cropland (P = 0.0015 and 0.0135, respectively), while soil CH4 uptake was greater under grazed cropland (P = 0.0102). Soil-atmosphere N2O flux from each system negated nearly all the CO2equiv. sink capacity accrued from soil carbon stock change. As both production systems resulted in net greenhouse gas (GHG) emissions to the atmosphere, novel practices that constrain GHG sources and boost GHG sinks under semiarid conditions are recommended.

Monitoring soil retention properties in a riverbank susceptible to fluvial erosion

<p>One of the principal source of vulnerability for riverbanks is given by slopes instabilities, which is triggered on the riverside by fluvial erosion. In order to mitigate such erosion, the establishment of a dense herbaceous cover aims at promoting the slope protection and reducing the likelihood of embankment failure. In fact, the aerial parts of vegetation reduce the mechanical impact of river level fluctuations and rainfall on the embankment and retain sediment transported, while the belowground parts reinforce mechanically the materials forming the top of the embankment, facilitating drainage in the topmost layers and promoting plant water uptake, thus contributing to the regulation of the drying/wetting cycle.</p><p>Plating deep-rooting perennial, herbaceous species on earth embankments therefore represent a sustainable, green intervention for the protection of a riverbank susceptible to fluvial erosion, contributing to the preservation of the fluvial ecosystem environment and avoiding a wide use of grey solutions. The European research project OPERANDUM is testing also this typology of NBS, with an experimental site selected on the river Panaro, one of the main tributary of the main Po River, Italy. To investigate the effect of vegetation on the riverbank soil, a monitoring system has been installed at shallow depths. The system estimates soil water content, matric suction and pore water pressure, in order to quantify the effects of the growth of different vegetation species, which have been recently seeded on site, for analyzing the plant-soil-atmosphere interaction. The work will present the site preparation and the system implementation. The analysis of the first collected data and the outcomes of the preliminary investigations, including site and laboratory experiments, will then be discussed. Monitoring data collected along the entire vegetation growth cycle, that is expected to take around two years, will allow to quantify the influence of vegetation in the soil-atmosphere interaction processes and, on the long-term, verify its effective contribution in riverbank protection.</p>