Utilization of near infrared reflectance spectroscopy (NIRS) to quantify the impact of earthworms on soil and carbon erosion in steep slope ecosystem : a study case in Northern Vietnam

This work focuses on a new approach to quantify the effects of above-ground earthworm's activity on soil erosion in steep slope ecosystems such as in Northern Vietnam. In these areas and in many others in the world, soil erosion becomes a major issue while the factors that determine it are still misunderstood. Earthworm's activity is believed to influence soil erosion rate, but we are still unable to precisely quantify their contribution to soil erosion. In this study, we used Near Infrared Reflectance Spectroscopy (NIRS) to quantify the proportion of soil aggregate in eroded soil coming from earthworm activity. This was done by generating NIRS signatures corresponding to different soil surface aggregates (above-ground soil casts produced by earthworms vs. surrounding topsoil). In order to test the proposed approach, we compared the NIRS-signature of eroded soil sediments to those of earthworms' casts and of the surrounding soils. Our results strongly supported that NIRS spectra might be used as "fingerprints" to identify the origin of soil aggregates. Although earthworms are generally assumed to play a favorable role in promoting soil fertility and ecosystem services, this method shows that cast aggregates constitute about 36 and 77% of sediments in two tropical plantations, Paspalum atratum and Panicum maximum plantations, respectively. In light with these results, we estimated that earthworms led to an annual loss of 3.3 and 15.8 kg of carbon ha(-1) yr(-1), respectively in P. atratum and P. maximum agroecosystems

Earthworms accelerate soil porosity turnover under watering conditions

Endogeic earthworms significantly modify soil aggregation and porosity, which in turn control water flow in soil. This study aimed to determine how the earthworm casting activity influences soil porosity and its dynamics. The main hypothesis was that the deposition of belowground water-stable casts increases soil porosity and its water stability. First we quantified cast production by the endogeic earthworm species Metaphire posthuma under laboratory conditions for 15 days. Secondly, casts and the bulk soil were analysed for structural stability to water and were packed in soil and subjected to wetting under various conditions and energy levels. The shape and size of pores were measured by image analysis. Almost all casts (98%) were produced belowground. M. posthuma produced approximately five times its own weight per day. Casts were depleted in C and were more easily disaggregated by water than the bulk soil. Although casts initially led to larger soil porosity (on average 50%), their structure was unstable. As a consequence, water inputs led to a faster decrease in soil porosity in the presence of casts. Large pores in between casts were rapidly replaced by small elongated and rounded pores. These results suggest that cast lifespan and associated porosity are of primary importance in the regulation of soil porosity turnover and the ecological functions that are under its control. Our findings suggest that in the field, the low stability of casts is likely to lead to a rapid compaction of the soil after rainfall events. However, high levels of cast production may prevent soil porosity from being broken down. Soil structural porosity thus depends on the balance between the production and degradation of casts. Improvements to the soil structure will occur when the former predominates

Dissimilarity between dust, heat, and momentum turbulent transports during aeolian soil erosion

International audienceMeasuring accurately size-resolved dust flux near the surface is crucial for better quantifying dust losses by semiarid soils. Dust fluxes have been usually estimated from the flux-gradient approach, assuming similarity between dust and momentum turbulent transport. This similarity has, however, never been verified. Here we investigate the similarity between dust (0.3 to 6.0 m in diameter), momentum, and heat fluxes during aeolian erosion events. These three fluxes were measured by the Eddy Covariance technique during the WIND-O-V (WIND erOsion in presence of sparse Vegetation's) 2017 field experiment over an isolated erodible bare plot in South Tunisia. Our measurements confirm the prevalence of ejection and sweep motions in transporting dust as for heat and momentum. However, our measurements also reveal a different partition of the dust flux between ejection and sweep motions and between eddy time scales compared to that of momentum and heat fluxes. This dissimilarity results from the intermittency of the dust emission compared to the more continuous emission (absorption) of heat (momentum) at the surface. Unlike heat emission and momentum absorption, dust release is conditioned by the wind intensity to initiate sandblasting. Consequently, ejection motions do not carry dust as often as heat and low momentum from the surface. This dissimilarity diminishes with increasing wind intensity as saltation patterns, and thus dust emission through sandblasting, become spatially more frequent. Overall, these findings may have implications on the evaluation of dust flux from techniques based on similarity with momentum or heat turbulent transport

Influence of Atmospheric Stability on the Size Distribution of the Vertical Dust Flux Measured in Eroding Conditions Over a Flat Bare Sandy Field

International audienceIn spite of their importance for the modeling of the atmospheric cycle of mineral dust, measurements of the intensity and size-distribution of the dust emission flux produced by wind erosion in natural conditions remain rare. During the WIND-O-V's (WIND erOsion in presence of sparse Vegetation) 2017 experiment, 8 major erosion events having occurred on a sandy flat field of southern Tunisia were documented. Consistent with the small size (90 µm) of the erodible sand grains and the low aerodynamic roughness length (Z 0 < 0.079 cm), the threshold for wind erosion was low (= 22 cm s-1). The classical gradient method was applied to assess the size-resolved vertical dust flux, and the stability of the atmosphere quantified by the means of the Richardson number (Ri) as well as of its shear stress () and thermal gradient () components. The vertical dust flux increased with following a power law but the number size- distribution of the dust flux was found to be significantly richer in submicron particles in thermally unstable than in stable periods. This challenges the usual assumption that, independently of their size, the particles smaller than 10 µm follow equally the movements of the air masses in which they are embedded and that the thermal stratification of the surface layer does not affect the size-distribution of the surface flux when measured a few meters above the ground. Finally, we propose a simple empirical method for taking this influence of the thermal instability into account

Aerodynamic parameters over an eroding bare surface: reconciliation of the law of the wall and eddy covariance determinations

International audienceAssessing accurately the surface friction velocity is a key issue for predicting and quantifying aeolian soil erosion. This is usually done either indirectly from the law of the wall (LoW) of the mean wind velocity profile or directly from eddy covariance (EC) of the streamwise and vertical wind velocity fluctuations. However, several recent experiments have reported inconsistency between friction velocities deduced from both methods. Here we reinvestigate the determination of aerodynamic parameters (friction velocity and surface roughness length) over an eroding bare surface and look at the possible reasons for observing differences on these parameters following the method. For that purpose a novel field experiment was performed in South Tunisia under the research program WIND-O-V (WIND erOsion in presence of sparse Vegetation). We find no significant difference between friction velocities obtained from both law of the wall and EC approaches when the friction velocity deduced from the EC method was extrapolated to the surface. Surface roughness lengths show a clear increase with wind erosion, with more scattered values when deduced from the EC friction velocity. Our measurements further suggest an average value of the von Karman constant of 0.407±0.002, although individual wind events lead to different average values due probably to the definition of the ground level or to the stability correction. Importantly, the von Karman constant was found independent of the wind intensity and thus of the wind soil erosion intensity. Finally, our results lead to several recommendations for estimating the aerodynamic parameters over bare surface in order to evaluate saltation and dust fluxes