Quantitative estimation and vertical partitioning of the soil carbon dioxide fluxes at the hillslope scale on a loess soil

Abstract

Both modelling and experimental approaches have been applied to assess C exchange fluxes at large spatial scales. Yet, these approaches are subjected to substantial limitations and uncertainties. Here, we aim to highlight two key mechanisms able to improve the estimation of the hillslope aggregated CO 2 fluxes: (i) the persistence of soil organic 5 carbon (OC) in deep colluvium deposits; and (ii) the physical controls on CO 2 fluxes along soil profiles. This study focuses on a sloping cropland in the central loess belt of Belgium. On two contrasted soil types along the studied hillslope, we recorded timeseries of CO 2 concentration, water content and temperature along 1 m long soil profiles during two periods of 6 months. Then, we calculated profiles of CO 2 fluxes using the 10 gradient method. To extrapolate these fluxes to entire yearly periods (2011-2013), we performed simulation using the SOILCO 2 RothC model.The vertical partitioning of the soil CO 2 fluxes shows that ca. 90 to ca. 95 % of the surface CO 2 fluxes originates from the 10 first centimeters of the soil profile at the footslope. We show that high water filled pore space at this slope position disables the 15 transfer of biotic CO 2 along the soil profile. However, the total annual flux averaged along 3 years of simulation show that the top soil layer (0-10 cm) of the footslope generates CO 2 fluxes (870 ± 64 CO 2 -C m −2 year −1 ) which exceed those observed at the summit position (583 ± 61 CO 2 -C m −2 year −1 ). Hence, our results reconcile two seemingly contradictory hypotheses, i.e. (i) these support that soil OC at such a footslope 20 is stored along the main part of the soil profile and submitted to a long-term stabilization, and (ii) at the same time these support that the depositional footslope profile emits more CO 2 than the summit, due to its high amount and quality of OC. Our results support the need to consider slopes when modeling soil-atmosphere C exchanges. If landscapes dynamic processes are not accounted for, we pointed out a risk to under-25 estimate annual soil-atmosphere CO 2 exchanges by ca. 20 %.

Effect of soil salinity on the wheat and bean root respiration rate at low matric suctions

“…We used gradient method to measure soil respiration rate, r g , following Wiaux et al (2014). The soils were packed into 280 plastic pots (with 0.27 m height, L, and 0.26 m diameter) with an initial field bulk density, and a closed chamber with volume of 5 9 10 -4 m 3 was connected with each pot.…”

“…The CO 2 emission rate was measured with method suggested by Norman et al (1997) and Wiaux et al (2014) using Lambda device (Lambda, ADC Company, UK) every 3 days under greenhouse conditions (with *8-h darkness and *16-h lighting, relative humidity between 55 and 80%). Then Rr value was calculated by:…”

Soil Respiration in the Profiles of Forest Soils in Inland Dunes

“…There have been many laboratory investigations of soil (soil samples) and field investigations of the soil surface in different ecosystems, and these have shown that a large portion of the organic carbon in soil is found in the surface horizon [1] [7]. However, to understand overall carbon cycling in an ecosystem it is necessary to understand the dynamics of carbon in the deeper soil horizons [6] [8].…”