Impact of Temporal Macropore Dynamics on Infiltration : Field Experiments and Model Simulations

Macropores greatly affect water and solute transport in soils. Most macropores are of biogenic origin; however, the resulting seasonal dynamics are often neglected. Our study aimed to examine temporal changes in biopore networks and the resulting infiltration patterns. We performed infiltration experiments with Brilliant Blue on pastureland in the Luxembourgian Attert catchment (spring, summer, and autumn 2015). We developed an image-processing scheme to identify and quantify changes in biopores and infiltration patterns. Subsequently, we used image-derived biopore metrics to parameterize the ecohydrological model echoRD (ecohydrological particle model based on representative domains), which includes explicit macropore flow and interaction with the soil matrix. We used the model simulations to check whether biopore dynamics affect infiltration. The observed infiltration patterns revealed variations in both biopore numbers and biopore–matrix interaction. The field-observed biopore numbers varied over time, mainly in the topsoil, with the largest biopore numbers in spring and the smallest in summer. The number of hydrologically effective biopores in the topsoil seems to determine the number and thereby the fraction of effective biopores in the subsoil. In summer, a strong biopore–matrix interaction was observed. In spring, the dominant process was rapid drainage, whereas in summer and autumn, most of the irrigated water was stored in the examined profiles. The model successfully simulated infiltration patterns for spring, summer, and autumn using temporally different macropore setups. Using a static macropore parameterization the model output deviated from the observed infiltration patterns, which emphasizes the need to consider macropores and their temporal dynamics in soil hydrological modeling

A“Dirty” Footprint: Macroinvertebrate diversity in Amazonian Anthropic Soils

International audienceAmazonian rainforests, once thought to be pristine wilderness, are increasingly known to have been widely inhabited, modified, and managed prior to European arrival, by human populations with diverse cultural backgrounds. Amazonian Dark Earths (ADEs) are fertile soils found throughout the Amazon Basin, created by pre-Columbian societies with sedentary habits. Much is known about the chemistry of these soils, yet their zoology has been neglected. Hence, we characterized soil fertility, macroinvertebrate communities, and their activity at nine archeological sites in three Amazonian regions in ADEs and adjacent reference soils under native forest (young and old) and agricultural systems. We found 673 morphospecies and, despite similar richness in ADEs (385 spp.) and reference soils (399 spp.), we identified a tenacious pre-Columbian footprint, with 49% of morphospecies found exclusively in ADEs. Termite and total macroinvertebrate abundance were higher in reference soils, while soil fertility and macroinvertebrate activity were higher in the ADEs, and associated with larger earthworm quantities and biomass. We show that ADE habitats have a unique pool of species, but that modern land use of ADEs decreases their populations, diversity, and contributions to soil functioning. These findings support the idea that humans created and sustained high-fertility ecosystems that persist today, altering biodiversity patterns in Amazonia

Do earthworms and roots cooperate to build soil macroaggregates ? A microcosm experiment

Soil ecosystem engineers are major actors of soil macroaggregation, a process that drives the production of ecosystem services by soils. However, our inability to identify the origins of different types of macroaggregates found in soils is an obstacle to describing and modeling their dynamics and associated processes (C sequestration; hydraulic properties). This laboratory study investigated mechanisms of biological soil macroaggregation by two different earthworm species (Apporectodea caliginosa (Savigny) and Allolobophora chlorotica (Savigny) and two plant species (Trifolium pratense, Plantago lanceolata L), in isolation and in all possible combinations. Near infrared (NIR) spectral analysis significantly discriminated macroaggregates according to the organisms that created them since each organism produced macroaggregates with distinct NIR signals (p<0.001). The largest departure from the control signal was observed with T. pratense whereas earthworms and P. lanceolata specific signals were less contrasted. Macroaggregates formed in the presence of more than one ecosystem engineers had mixed signals showing that several actors had participated in their construction. This means that roots and earthworms did not produce macroaggregates in isolation and rather added their effects in building structures of mixed origins. Further studies based on the present methodology will tell us more on below ground behaviors of ecosystem engineers and their interactive building of soil habitats

Near infrared spectroscopy (NIRS) to estimate earthworm cast age

International audienceAs ecosystem engineers, earthworms are major actors of soil aggregation, a process that drives the delivery of ecosystem services by soils. However, persistence of soil aggregates produced by earthworms, their degradation rates, and their role in the dynamics of soil organic matter (SOM) and nutrients remain poorly known. In this experiment, near infrared (NIR) spectral signatures were measured in subterranean casts of the endogeic earthworm Aporrectodea caliginosa, incubated in controlled laboratory conditions for different periods of time. In parallel, dynamics of total amounts of C and N were assessed in ageing casts. As casts aged, NIR spectral signatures went through three main stages in the maturation process: (1) rapid changes in the NIR signal during the first 48 h, (2) a maturation period from days 3–30 with much slower change in NIR spectral signatures and (3) a further stage of maturation (days 45–90), where cast signals converged towards those of control soil. The first two axes of the PCA corresponded closely to the C and N content, respectively, of the casts. C and N contents in casts remained higher than those in control soil during the whole incubation time. Drying of casts halted the mineralization of organic matter, resulting in lack of change in the NIR spectral signal as long as casts were kept dry

Variability of earthworm-induced biopores and their hydrological effectiveness in space and time

Earthworms create biopores and thereby increase the susceptibility of soils to preferential flow which, on the one hand, reduces surface runoff and soil erosion, but, on the other hand, enhances vertical water and solute transport. Spatial and temporal variability in earthworm abundances might lead to spatial and temporal variability in biopore densities and even in the hydrological effectiveness of these pores. In this paper, we present a reproducible sampling design for simultaneous earthworm-biopore observations and analyze the temporal variability in earthworm abundances, biopore densities and hydrological effectiveness of biopores and its differences between grassland and arable land. During one year we performed six field campaigns where we sampled earthworms and performed dye tracer experiments in the small Luxembourgian Wollefsbach catchment (4.4 km²) on three arable land sites and three grasslands. We quantified how abundances of ecological groups of earthworms affect biopore densities and hydrological effectiveness. Finally, we applied piecewise structural equation modeling (piecewise SEM) to find the most probable structure of the ecohydrological system of soil moisture, clay content, earthworms, biopore densities and their hydrological effectiveness. Our results show that earthworm abundance and biopore density as well as their hydrological effectiveness vary strongly between seasons and between grassland and arable land (0–300 earthworms m−2, 0–500 biopores m−2, 0–100% hydrologically effective biopores). Piecewise SEMs indicate that the temporal variability in earthworm abundances, biopore densities and hydrological effectiveness is significantly affected by two-month averaged soil moisture variability. We also found a significant effect of earthworm abundances on biopore densities and their hydrological effectiveness in 3 and 10 cm depth. Based on these results we recommend the consideration of spatial and temporal variability in biopore densities and their hydrological effectiveness for the reliable representation of macropore flow and related solute transport in soil hydrological models

Unveiling the age and origin of biogenic aggregates produced by earthworm species with their NIRS fingerprint in a subalpine meadow of Central Pyrenees.

In this study the near-infrared reflectance (NIR) spectra signals (750-2,500 nm) of soil samples was compared with the NIR signals of the biogenic aggregates produced in the lab by three earthworm species, i.e., Aporrectodea rosea (Savigny 1826), Lumbricus friendi Cognetti, 1904 and Prosellodrilus pyrenaicus (Cognetti, 1904) from subalpine meadows in the Central Pyrenees. NIR spectral signatures of biogenic aggregates, root-aggregates, and non-aggregated soil were obtained together with soil carbon (C), nitrogen (N), [Formula: see text] and [Formula: see text] determinations. The concentrations of C, N and C:N ratio in the three types of soil aggregates identified were not statistically significant (ANOVA, p>0.05) although non-macroaggregated soil had slightly higher C concentrations (66.3 g kg-1 dry soil) than biogenic aggregates (earthworm- and root-aggregates, 64.9 and 63.5 g kg-1 dry soil, respectively), while concentrations of [Formula: see text] and [Formula: see text] were highest in the root-attached aggregates (3.3 and 0.31 mg kg dry soil-1). Total earthworm density and biomass in the sampled area was 137.6 ind. m-2, and 55.2 g fresh weight m-2, respectively. The biomass of aggregates attached to roots and non-macroaggregated soil was 122.3 and 134.8 g m-2, respectively, while biomass of free (particulate) organic matter and invertebrate biogenic aggregates was 62.9 and 41.7 g m-2, respectively. Multivariate analysis of NIR spectra signals of field aggregates separated root aggregates with high concentrations of [Formula: see text] and [Formula: see text] (41.5% of explained variance, axis I) from those biogenic aggregates, including root aggregates, with large concentrations of C and high C:N ratio (21.6% of total variability, axis II). Partial Least Square (PLS) regressions were used to compare NIR spectral signals of samples (casts and soil) and develop calibration equations relating these spectral data to those data obtained for chemical variables in the lab. After a derivatization process, the NIR spectra of field aggregates were projected onto the PLS factorial plane of the NIR spectra from the lab incubation. The projection of the NIR spectral signals onto the PLSR models for C, N, [Formula: see text] and [Formula: see text] from casts produced and incubated in the lab allowed us to identify the species and the age of the field biogenic aggregates. Our hypothesis was to test whether field aggregates would match or be in the vicinity of the NIR signals that corresponded to a certain species and the age of the depositions produced in the lab. A NIRS biogenic background noise (BBN) is present in the soil as a result of earthworm activity. This study provides insights on how to analyse the role of these organisms in important ecological processes of soil macro-aggregation and associated organic matter dynamics by means of analyzing the BBN in the soil matrix

Soil aggregation, ecosystem engineers and the C cycle

International audienceSoil aggregation and its effects on soil C storage have been addressed in thousands of research articles over the last 40 years. Research has been mostly focused on the resistance of aggregates to mechanical disruption and the role of organic matter in aggregate stabilization. On the other hand, relatively little attention has been paid to identifying the microbial, plant root and macro-invertebrate actors and physical processes that continuously create and destroy aggregates. The sum and dynamics of these processes determines the ability of soils to store and conserve C. Understanding the interactions between aggregation dynamics and C transformations in soils therefore requires a precise identification of the agents that produce aggregates and knowledge of the rates of formation and persistence in the pools thus identified.We propose to separate macro-aggregated components of different, physicogenic and biogenic origins from non-macro-aggregated soil on a morphological basis, using a simple visual technique. The specific biological or physico-chemical agent which produced each individual macro-aggregate can then be determined using Near Infrared Spectroscopy (NIRS). A general description of the distribution and quality of organic matter among the different groups of macro-aggregates can be made. Simple soil re-aggregation or dis-aggregation tests conducted in field conditions further measure the production of different macro-aggregates with time and their mean residence times in the studied soil. Respirometry measurements on each recognized category of macro-aggregates evaluate the respective C losses through respiration. The methods described here will allow the dominant pathways of C flow at a given site to be characterized and possible management options to increase C storage identified. We finally discuss the different assumptions made to build this simple model and offer ways to test the methodology under field conditions

Interpreting landscapes of pre-Columbian raised-field agriculture using high-resolution LiDAR topography

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Ecosystem Engineers in a Self-organized Soil: A Review of Concepts and Future Research Questions

Equipe EABIOmEAGROSUPSoils are self-organized ecological systems within which organisms interact within a nested suite of discrete scales. Microorganisms form communities and physical structures at the smallest scale (microns), followed by the community of their predators organized in microfoodwebs (tens of microns), the functional domains built by ecosystem engineers (centimeters to meters), ecosystems, and landscapes. Ecosystem engineers, principally plant roots, earthworms, termites, and ants, play key roles in creating habitats for other organisms and controlling their activities through physical and biochemical processes. The biogenic, organic, and organomineral structures that they produce accumulate in the soil space to form three-dimensional mosaics of functional domains, inhabited by specific communities of smaller organisms (microfauna and mesofauna, microorganisms) that drive soil processes through specific pathways. Ecosystem engineers also produce signaling and energy-rich molecules that act as ecological mediators of biological engineering processes. Energy-rich ecological mediators may selectively activate microbial populations and trigger priming effects, resulting in the degradation, synthesis, and sequestration of specific organic substrates. Signaling molecules inform soil organisms of their producers' respective presences and change physiologies by modifying gene expression and through eliciting hormonal responses. Protection of plants against pests and diseases is largely achieved via these processes. At the highest scales, the delivery of ecosystem services emerges through the functioning of self-organized systems nested within each other. The integrity of the different subsystems at each scale and the quality of their interconnections are a precondition for an optimum and sustainable delivery of ecosystem services. Lastly, we present seven general research questions whose resolution will provide a firmer base for the proposed conceptual framework while offering new insights for sustainable use of the soil resource

Pre-adult specimen of <i>Andiorrhinus</i> sp.

<p>Photos Doyle McKey 2012.</p