K. Van Oost

Characteristics and controlling factors of old gullies under forest in a temperate humid climate: a case study from the Meerdaal Forest (Central Belgium)

Abstract In many forests of Northwestern Europe old gullies can be found, but few studies have reported their genesis and characteristics. This study investigates these old gullies under forest in the large case-study area of Meerdaal Forest, in the Central Belgian loess belt. The objectives are (1) to determine the spatial distribution of these gullies, (2) to measure their morphological and topographical characteristics and (3) to reconstruct the factors that led to their development. In the 1329-ha study area, 252 channel-like incisions were mapped. Different types of incisions could be distinguished. Besides small and large gullies, many incisions were sunken lanes or road gullies. These road gullies are aligned along north–south oriented lines, whereas the concentration of old gullies is strongly related to the distribution of archaeological sites. Out of the 252 mapped incisions, 43 large gullies and 21 representative road gullies were selected for detailed morphological and topographical measurements. The characteristics of these two types of incisions were compared with ephemeral gullies formed under nearby cropland. Significant differences in morphology between the three types could be demonstrated. Ephemeral gullies under cropland and large gullies under forest differ significantly in all measured parameters, except bottom width. Both the old gullies and road gullies under forest have a significantly larger cross section and total eroded volume compared with the ephemeral gullies observed under cropland. This indicates that once formed, the old gullies were not ploughed in nor were they filled by sediment originating in their drainage areas, because of limited sediment production. Comparing topographical characteristics (i.e. slope at the gully head and runoff contributing area) of forest gullies and ephemeral gullies that formed under cropland yields important indications about their formation. The larger sedimentation slope of forest gullies, compared with ephemeral gullies and road gullies, suggests that the forest gullies incised on vegetated slopes as a consequence of runoff from the adjacent plateau, where the forest cover was disturbed. For the old gullies under forest, no relation between slope at the gully head and runoff contributing area is observed, probably because most gullies occur on very steep slopes. When simulating arable land-use in the study area, zones where ephemeral gullies are expected to develop can be predicted using published topographical threshold relationships. Comparing the zones where ephemeral gullies are predicted with the position of old gullies under forest leads to the conclusion that gully incision was most probably not triggered by extreme rainfall events and that they are not of periglacial origin. The observed gully pattern can best be explained by local, anthropogenically determined land-use changes.

Identification of important factors in the process of tillage erosion: the case of mouldboard tillage

Abstract Most of the experimental studies on tillage erosion by mouldboard tillage have hitherto focused on the characterisation of the erosivity of a given tillage pass for specific operating conditions. The effect of variations in tillage speed and depth on soil translocation and the associated tillage erosion has largely been overlooked. This paper reports on the results of a series of tillage experiments that were set up to investigate the effect of variations in tillage speed and depth on net soil displacement and the associated tillage erosivity for mouldboard tillage carried out in two contrasting directions: parallel to the contour lines (contour tillage) or perpendicular to the contour lines (up- and downslope tillage). The experimental data clearly show that the average soil displacement distance is not only a function of slope gradient, but is also strongly affected by tillage speed and tillage depth. A model to predict the average displacement distance and the tillage transport coefficient k from slope gradient, tillage depth, tillage speed and tillage direction information was constructed. The effect of tillage depth on mouldboard tillage erosivity depends on tillage direction: k-values increase exponentially with tillage depth for up- and downslope tillage, while the increase is linear for contour tillage. On the other hand, the effect of tillage speed on tillage erosivity is similar for both tillage directions considered, but less pronounced. Validation of this model using data on mouldboard tillage erosion experiments available in the literature shows that the effect of variations in tillage depth on tillage erosivity could be predicted very well. Although absolute values were slightly underestimated, which is probably due to the large variation in soil characteristics and implements used, this indicates that tillage depth, tillage speed and tillage direction are the dominant controlling factors on the magnitude of the tillage transport coefficient. Using this model structure, a series of nomograms were constructed, that allow one to evaluate the effect of changes in tillage depth, speed and/or tillage direction on the soil erosivity of a given mouldboard tillage operation. These nomograms, whereby the tillage transport coefficient k is predicted as a function of tillage speed, tillage depth and tillage direction, are a useful tool in evaluating strategies to minimise tillage erosion.

A simple method for estimating the influence of eroding soil profiles on atmospheric CO2

Although soil erosion has often been considered a net source of atmospheric carbon (C), several recent studies suggest that erosion serves as a net C sink. We have developed a spreadsheet-based model of soil organic C dynamics within an eroding profile (Soil Organic Carbon, Erosion, Replacement, and Oxidation (SOrCERO)) that calculates effects of soil organic carbon (SOC) erosion and altered SOC oxidation and production on the net exchange of C between the eroding profile and atmosphere. SOrCERO suggests that erosion can induce a net C sink or source, depending on management practices, the extent to which SOC oxidation and production characteristics change with erosion, and the fate of eroded SOC. Varying these parameters generated a wide range of C source and sink estimates (maximum net source and sink of 1.1/3.1 Pg C yr(-1) respectively, applying results globally), highlighting research needs to constrain model estimates. We invite others to download SOrCERO (http://www.kbs.ku.edu/people/staff_www/billings/index.html) to test conceptual models and eroding soil profiles of interest in a consistent, comparable fashion.

Links among warming, carbon and microbial dynamics mediated by soil mineral weathering

Quantifying soil carbon dynamics is of utmost relevance in the context of global change because soils play an important role in land–atmosphere gas exchange. Our current understanding of both present and future carbon dynamics is limited because we fail to accurately represent soil processes across temporal and spatial scales, partly because of the paucity of data on the relative importance and hierarchical relationships between microbial, geochemical and climatic controls. Here, using observations from a 3,000-kyr-old soil chronosequence preserved in alluvial terrace deposits of the Merced River, California, we show how soil carbon dynamics are driven by the relationship between short-term biotic responses and long-term mineral weathering. We link temperature sensitivity of heterotrophic respiration to biogeochemical soil properties through their relationship with microbial activity and community composition. We found that soil mineralogy, and in particular changes in mineral reactivity and resulting nutrient availability, impacts the response of heterotrophic soil respiration to warming by altering carbon inputs, carbon stabilization, microbial community composition and extracellular enzyme activity. We demonstrate that biogeochemical alteration of the soil matrix (and not short-term warming) controls the composition of microbial communities and strategies to metabolize nutrients. More specifically, weathering first increases and then reduces nutrient availability and retention, as well as the potential of soils to stabilize carbon.Soil weathering, rather than short-term warming, controls microbial community composition, nutrient availability and soil carbon content, according to observations from a 3-Myr-old soil chronosequence preserved in river terraces in California.

A step towards a holistic assessment of soil degradation in Europe: Coupling on-site erosion with sediment transfer and carbon fluxes

ABSTRACT Soil degradation due to erosion is connected to two serious environmental impacts: (i) on‐site soil loss and (ii) off‐site effects of sediment transfer through the landscape. The potential impact of soil erosion processes on biogeochemical cycles has received increasing attention in the last two decades. Properly designed modelling assumptions on effective soil loss are a key pre‐requisite to improve our understanding of the magnitude of nutrients that are mobilized through soil erosion and the resultant effects. The aim of this study is to quantify the potential spatial displacement and transport of soil sediments due to water erosion at European scale. We computed long‐term averages of annual soil loss and deposition rates by means of the extensively tested spatially distributed WaTEM/SEDEM model. Our findings indicate that soil loss from Europe in the riverine systems is about 15% of the estimated gross on‐site erosion. The estimated sediment yield totals 0.164 ± 0.013 Pg yr−1 (which corresponds to 4.62 ± 0.37 Mg ha−1 yr−1 in the erosion area). The greatest amount of gross on‐site erosion as well as soil loss to rivers occurs in the agricultural land (93.5%). By contrast, forestland and other semi‐natural vegetation areas experience an overall surplus of sediments which is driven by a re‐deposition of sediments eroded from agricultural land. Combining the predicted soil loss rates with the European soil organic carbon (SOC) stock, we estimate a SOC displacement by water erosion of 14.5 Tg yr−1. The SOC potentially transferred to the riverine system equals to 2.2 Tg yr−1 (˜15%). Integrated sediment delivery‐biogeochemical models need to answer the question on how carbon mineralization during detachment and transport might be balanced or even off‐set by carbon sequestration due to dynamic replacement and sediment burial. HIGHLIGHTSWaTEM/SEDEM was applied to simulate soil loss and deposition rates at European scale.Our findings indicate that soil loss in the riverine systems is about 15% of RUSLE2015 estimates.The estimated sediment yield in Europe totals 0.164 ± 0.013 Pg yr−1.We estimate a SOC displacement by water erosion in Europe of 14.5 Tg yr−1.