Slopes: soil erosion

Significant developments in soil erosion research for the period 1950-2000 are reviewed. The main emphasis is on work in Western Europe and North America. We highlight work on process studies in splash, rill and gully erosion. Important developments also occurred in monitoring, measuring, and modelling erosion as well as recording and understanding rates of erosion. We concentrate on cultivated and bare soils and have included badlands and peatland erosion in our review.

On the Value of Sentinel-1 InSAR Coherence Time-Series for Vegetation Classification

Synthetic aperture radar (SAR) acquisitions are mainly deemed suitable for mapping dynamic land-cover and land-use scenarios due to their timeliness and reliability. This particularly applies to Sentinel-1 imagery. Nevertheless, the accurate mapping of regions characterized by a mixture of crops and grasses can still represent a challenge. Radar time-series have to date mainly been exploited through backscatter intensities, whereas only fewer contributions have focused on analyzing the potential of interferometric information, intuitively enhanced by the short revisit. In this paper, we evaluate, as primary objective, the added value of short-temporal baseline coherences over a complex agricultural area in the São Paulo state, cultivated with heterogeneously (asynchronously) managed annual crops, grasses for pasture and sugarcane plantations. We also investigated the sensitivity of the radar information to the classification methods as well as to the data preparation and sampling practices. Two supervised machine learning methods—namely support vector machine (SVM) and random forest (RF)—were applied to the Sentinel-1 time-series at the pixel and field levels. The results highlight that an improvement of 10 percentage points (p.p.) in the classification accuracy can be achieved by using the coherence in addition to the backscatter intensity and by combining co-polarized (VV) and cross-polarized (VH) information. It is shown that the largest contribution in class discrimination is brought during winter, when dry vegetation and bare soils can be expected. One of the added values of coherence was indeed identified in the enhanced sensitivity to harvest events in a small but significant number of cases.

Spectral mapping of soil organic carbon

This chapter first reviews recent pilot studies covering limited areas often with exposed bare soils. Then we focus on the challenges for large-scale application of spectral mapping when the soil and parent material are heterogeneous and surface conditions are unknown. In order to deal with these constraints we discuss i) calibration of spectral models based on large spectral libraries, ii) surface conditions that disturb the soil signal, and iii) time series of images in order to delimit cropland fields and increase the extent of bare soil that can be mapped. Finally, a case study deals with a SOC prediction map derived from the spectra of a Sentinel-2 image and calibrated using the LUCAS spectral library.

Analysis of Land Cover Change and Its Implication on the Environment around the Dja Biosphere Reserve, Southeastern Cameroon

Land cover change is a growing concern around the world. This is especially true for protected areas which are rapidly degrading owing to pressure from anthropogenic activities. The aim of this study was to analyze land cover change for the periods 1980, 2008 and 2020 and its implication on the environment in and around the Dja Biosphere Reserve in south eastern Cameroon. This was done using remote sensing and geographical information systems techniques to quantify and measure the extent of land cover change in the study area for forty years. Household surveys were equally undertaken through the administration of questionnaires to farmers in villages located within the Dja Biosphere Reserve. Collected data was analyzed through the use of GIS software as well as Microsoft Excel. From the land cover maps, four classes were found: dense forest, cultivated areas, water surface, and buildings and bare soils. The transition matrix between 1980 and 2008 showed that 6477.81 ha of dense forest was lost to cultivated areas and between 2008 and 2020, 722.84 ha of dense forest was lost. Between 1980 and 2008 cultivated areas lost 0.07% and gain 0.72% between 2008 and 2020. Building and bare soils increase by 0.28% between 1980 and 2020. The Kappa index of agreement was 0.91 % between 1980 and 2008 and 0.88% between 2008 and 2020. Slash and burn agriculture (43.3%), hunting (36.3%) and harvesting of tree-based products (20.3%) were identified by farmers as the human activities with the most negative impact on the reserve. Results revealed that, there are still opportunities to safe this vulnerable reserve from the negative effects of land cover change through the practice of agroforestry.

Drought-tolerant cyanobacteria and mosses as biotechnological tools to attain land degradation neutrality

The induction of biocrusts through inoculation-based techniques has gained increasing scientific attention in the last 2 decades due to its potential to address issues related to soil degradation and desertification. The technology has shown the most rapid advances in the use of biocrust organisms, particularly cyanobacteria and mosses, as inoculants and biocrust initiators. Cyanobacteria and mosses are poikilohydric organisms – i.e., desiccation-tolerant organisms capable of reactivating their metabolism upon rehydration – that can settle on bare soils in abiotically stressing habitats, provided that selected species are used and an appropriate and customized protocol is applied. The success of inoculation of cyanobacteria and mosses depends on the inoculant's physiology, but also on the ability of the practitioner to identify and control, with appropriate technical approaches in each case study, those environmental factors that most influence the inoculant settlement and its ability to develop biocrusts. This review illustrates the current knowledge and results of biocrust induction biotechnologies that use cyanobacteria or mosses as inoculants. At the same time, this review's purpose is to highlight the current technological gaps that hinder an efficient application of the technology in the field.

Actual evaporation from bare soils - A comparison of numerical modelling and field lysimeter data

<p>Predicting evaporation from drying soils under limited water supply conditions, where water transfer to the atmosphere is limited primarily by soil hydraulic conductivity, is challenging. The parameterization of soil hydraulic properties (SHP) plays a crucial role in reliable predictions of evaporation. In particular, there are expected differences between traditional functions that consider water flow only in capillaries and functions that additionally consider non-capillary processes, i.e., water storage and film flow on particle surfaces and in corners and channels of pores. The non-capillary processes in simulating evaporation from soil surfaces become more important when the soil dries.</p><p>The purpose of this study was to investigate the applicability of different soil hydraulic function types in modelling the actual evaporation under water-limited conditions. Data were obtained from a large bare-soil field lysimeter (2.5 m height; 1 m<sup>2</sup> surface area), where the lysimeter mass and outflow were measured in hourly time intervals. Precipitation and actual evaporation were calculated from the mass changes of the lysimeter, using a simplified version of the AWAT filter approach of Peters et al. (2017). Meteorological parameters to calculate the potential evaporation were taken from the nearest weather station. Potential evaporation rates were obtained by (i) using the FAO-56 version of the Penman-Monteith equation and (ii) scaling these values to match the bare soil potential evaporation.</p><p>The evaporation was simulated using two different models for soil hydraulic properties: i) van Genuchten Mualem (VGM) (only capillary storage and flow), and ii) Peters-Durner-Iden (PDI) (capillary and non-capillary storage and flow). The results show a systematic difference in evaporation prediction by applying the PDI and VGM models, with higher evaporation rates for the PDI model under dry conditions.</p>

Soil moisture estimation over cereals fields using l-band alos2 data (merguellil case – KAIROUAN)

<p>Soil moisture is a key component for water resources management especially for irrigation needs estimation. We analyze in the present study, the potential of L-band data, acquired by (Advanced Land Observing Satellite-2) ALOS-2, to retrieve soil moisture over bare soils and cereal fields located in semi-arid area in the Kairouan plain.</p><p>In this context, we evaluate radar signal sensitivity to roughness, soil moisture and vegetation biophysical parameters. Based on multi-incidence radar data (28°, 32.5° and 36°), high correlations characterize relationships between backscattering coefficients in dual-polarization (HH and HV) and root mean square of heights (Hrms) and Zs, parameters, Sensitivity of radar data to soil moisture was discussed for three classes of NDVI (less than 0.25 for bare soils and dispersed vegetation, between 0.25 and 0.5 for medium vegetation and greater than 0.5 for dense cereals). With vegetation development, where NDVI values are higher than 0.25, SAR signal remains sensitive to soil moisture in HH pol. This sensitivity to moisture disappears, in HV pol for dense vegetation. For covered fields, L-band signal is very sensitive to Vegetation Water Content (VWC), with R² values ranging between 0.76 and 0.61 in HH and HV polarization respectively.</p><p>Simulating signal behavior is carried out through various models over bare soils and covered cereal fields. Over bare soils, proposed empirical expressions, modified versions of Integral Equation Model (IEM-B) and Dubois models (Dubois-B) are evaluated, generally for HH and HV polarizations. Best consistency is observed between real data and IEM-B backscattering simulations in HH polarization. More discrepancies between real and modelled data are observed in HV polarization.</p><p>Furthermore, to simulate L-band signal behavior over covered fields, the inversion of Water Cloud Model (WCM) coupled to different bare soil models is realized through direct equations and Look-up tables. Two options of WCM, are tested (with and without soil-vegetation interaction scattering term). For the first option, results highlight the good performance of IEM-B coupled to WCM in HH polarization with RMSE value between estimated and in situ moisture measurements equal to 4.87 vol.%. By adding soil – cereal interaction term in the second option of WCM, results reveal a stable accuracy in HH polarization and an important improvement of soil moisture estimations in HV polarization, with RMSE values are ranging between 6 and 7 vol.%.</p>

Microenvironmental Conditions Drive the Differential Cyanobacterial Community Composition of Biocrusts from the Sahara Desert

The Sahara Desert is characterized by extreme environmental conditions, which are a unique challenge for life. Cyanobacteria are key players in the colonization of bare soils and form assemblages with other microorganisms in the top millimetres, establishing biological soil crusts (biocrusts) that cover most soil surfaces in deserts, which have important roles in the functioning of drylands. However, knowledge of biocrusts from these extreme environments is limited. Therefore, to study cyanobacterial community composition in biocrusts from the Sahara Desert, we utilized a combination of methodologies in which taxonomic assignation, for next-generation sequencing of soil samples, was based on phylogenetic analysis (16S rRNA gene) in parallel with morphological identification of cyanobacteria in natural samples and isolates from certain locations. Two close locations that differed in microenvironmental conditions were analysed. One was a dry salt lake (a “chott”), and the other was an extension of sandy, slightly saline soil. Differences in cyanobacterial composition between the sites were found, with a clear dominance of Microcoleus spp. in the less saline site, while the chott presented a high abundance of heterocystous cyanobacteria as well as the filamentous non-heterocystous Pseudophormidium sp. and the unicellular cf. Acaryochloris. The cyanobacteria found in our study area, such as Microcoleus steenstrupii, Microcoleus vaginatus, Scytonema hyalinum, Tolypothrix distorta, and Calothrix sp., are also widely distributed in other geographic locations around the world, where the conditions are less severe. Our results, therefore, indicated that some cyanobacteria can cope with polyextreme conditions, as confirmed by bioassays, and can be considered extremotolerant, being able to live in a wide range of conditions.

Successional Herbaceous Species Affect Soil Processes in a High-Elevation Alpine Proglacial Chronosequence

The study investigated plant-soil interactions along a proglacial chronosequence in the Italian Alps, with a specific focus on pioneer and grassland species structure and biogeochemical processes, with the aim to evaluate the biotic patterns in ecosystem development. We recorded vascular plant frequencies and the mean diameter of one pioneer and one grassland target species in 18 permanent plots distributed along six different stages encompassing a 170-years chronosequence in the Lauson Glacier forefield (NW Italy). We evaluated the main soil properties and measured the C:N:P stoichiometry in the biomass of pioneer and grassland target species and in the underlying soil. For comparative purposes, we analyzed also bare soils sampled near the sampled plant individuals. Pioneer species number and cover significantly increased 10 and 40 years after deglaciation respectively, while alpine grassland species cover and number peaked only after 65 and 140 years, respectively. Along the chronosequence, soils beneath vascular plants were enriched in nutrients, especially under individuals of alpine grassland species, with total organic C contents ranging between 1.3 and 8.9 g·kg−1 compared to 0.2 and 3.3 g·kg−1 in bare soils. Nitrogen content in bare soils was nearly undetectable, while it increased in the plant-affected soils, leading to a more balanced C:N:P stoichiometry in the oldest stages. The colonization of alpine grassland species started immediately, although species number and cover increased only when the soil acquired sufficient nutrient supply and functionality. Although the ecosystem remained C and N limited, the soil could provide adequate conditions for more competitive species establishment, as confirmed by the increasing number and cover of alpine grassland species. Thus, soil nutrient dynamics were strongly influenced by plants, with a major influence triggered by late-successional grassland species.