Fluvial geomorphological dynamics and land use changes: impact on the organic carbon stocks of soil and sediment

The drainage basin of the Turrilla river (SE of Spain) went through important land cover changes since 1950s; from mainly an agrarian scenario in 1956 to other depopulated and forested in 2015. This study analyzes the effects of land use changes on fluvial dynamics and their relationship with the organic carbon (OC) stock in fluvial sedimentary deposits as well as in the soil of the basin. Methods included a fluvial geomorphological analysis and a land use change analysis in combination with OC databases of soil and sediment. Results showed that the fluvial channel experienced important morphological changes related to different erosion processes and stabilization of fluvial deposits, induced by land use changes in the drainage area. The active channel decreased 63% in the study period, while bank erosion and gully erosion increased (34% and 21 %, respectively). Alluvial fans and alluvial plain were also extended (21% and 7 %, respectively) and alluvial bars were colonized by vegetation. Sediment was impoverished in OC compared to catchment soils (0.24 enrichment ratio sediment/soil). However the increase of OC stock (Mg ha-1) was very similar between soil (25 %) and sediment (23 %). The total reservoir of OC (Mg) increased 27% in sediments and 25% in the catchment soils. Results show the large influence of geomorphological dynamics on the OC reservoir at the catchment scale. A very high potential of fluvial sediments to increase OC sinks was observed, particularly in scenarios where the active channel is narrowed and the fluvial channel is encroached with vegetation, facilitating the input of OC in sediment. The potential of sediment to sequester organic carbon could be very useful in planning and management of fluvial sedimentary zones in climate change mitigation policies. © 2019, Universidad Austral de Chile. All rights reserved.Este estudio ha recibido apoyo financiero del proyecto DISECO (CGL2014-55405-R) del Plan Nacional de Ciencia del Ministerio de Economía y Competitividad de España, del proyecto SOGLO (P7/24 IAP BELSPO) del gobierno de Bélgica. AHM recibió apoyo financiero para una estancia en la Universidad Nacional de Córdoba (Argentina) del Banco de Santander mediante el Convenio Becas de Intercambio Latinoamérica (Programa ILA). CBF recibió apoyo financiero para dos estancias en el extranjero del programa Salvador de Madariaga 2017 (Ministerio de Educación, Cultura y Deporte, Gobierno de España) y del programa Jiménez de la Espada 2017 (Fundación Séneca, Agencia de Ciencia y Tecnología de la Región de Murcia). MAB fue parcialmente financiada por un contrato Juan de la Cierva-Incorporación (Ref: IJCI-2015-23500). Todas estas estancias permitieron el trabajo continuado en la redacción de este artículo

Eco-geomorphological connectivity and coupling interactions at hillslope scale in drylands: Concepts and critical examples

The diagnosis of land degradation requires a deep understanding of ecosystem functioning and evolution. In dryland systems, in particular, research efforts must address the redistribution of scarce resources for vegetation, in a context of high spatial heterogeneity and non-linear response. This fact explains the prevalence of eco-hydrological perspectives interested in runoff processes and, the more recent, focused on connectivity as an indicator of system resource optimisation. From a geomorphological perspective and reviewing the concepts of eco-hydro-geomorphological interactions operating in ecosystems, this paper explores the effects of erosion on vegetation configuration through two case studies at different spatio-temporal scales. We focus on the structure-function linkage, specifically on how morphological traits relate with different stages in the erosional sequence, both in the abiotic and the biotic domain. Results suggest that vegetation dynamics are affected by structural boundary conditions at both scales, i.e. by surface armouring related with rock fragments at the patch scale, and by the degree of hillslope-channel coupling at the hillslope scale. Our preliminary results can serve as new working hypotheses about the structure-function interplay on hillslopes. All this, taking advantage of the recent technological achievements for acquiring very high-resolution geospatial data that offer new analytical possibilities in a range of scales

Movilización de carbono orgánico por distintos procesos erosivos en la conexión ladera-cauce

With the purpose of analysing the type (labile or stable) and quantity of organic carbon (OC) mobilized by different erosive processes identified at the slope-bed connection, the erosion deposits of gullies, sheet erosion, bank erosion and tillage erosion were studied in a small catchment (10 ha) and compared to the characteristics of the catchment soils. Selectivity upon soil detachment and transport was associated to different OC content and types in the erosion deposits. Enrichment ratios of organic carbon sediment/soil were low (~0,40 ± 0,26), even though a slight enrichment was described for fine particles (positively correlated to CO). These results were attributed to mineralization processes prevailing over OC burial in a very active channel where depositional sites are scarce.Con el fin de caracterizar la cantidad y tipo (lábil o recalcitrante) de carbono orgánico (CO) movilizado por distintos procesos erosivos identificados en las conexiones ladera-cauce, se estudiaron las características de los depósitos de erosión concentrada en cárcavas, erosión hídrica laminar, erosión lateral-gravitacional y erosión por laboreo en el contacto ladera-cauce de una cuenca de pequeño tamaño (10 ha) y se relacionaron con las características de los suelos-fuentes originales de donde procedían. La selectividad en el arranque y transporte de suelo de los distintos procesos se pudo asociar a diferentes contenidos y tipos de CO en los depósitos. Las razones de enriquecimiento de carbono orgánico sedimento/suelo fueron bajas (~0,40 ± 0,26), a pesar de haber un ligero enriquecimiento en partículas finas (correlacionadas positivamente con el CO) en los depósitos. Todo ello se atribuyó a los efectos de la mineralización en un cauce muy activo con pocas zonas de deposición y abundantes procesos de erosión no selectiva

Long-term effectiveness of sustainable land management practices to control runoff, soil erosion, and nutrient loss and the role of rainfall intensity in Mediterranean rainfed agroecosystems

Mediterranean environments are especially susceptible to soil erosion and to inappropriate soil management, leading to accelerated soil loss. Sustainable Land Management (SLM) practices (such as reduced tillage, no-tillage, cover crops, etc.,) have the potential to reduce soil, organic carbon (OC), and nutrient losses by erosion. However, the effectivity of these practices is site-dependent and varies under different rainfall conditions. The objective of this paper was to evaluate the effects of SLM practices in two rainfed systems (a wheat field and an almond orchard) representative of a large area of the driest Mediterranean regions - on runoff, soil erosion, particle size distribution, and OC and nutrient (N and P) contents in sediments. The influence of the rainfall characteristics on the effectiveness of the SLM practices was also evaluated. The SLM implemented were: reduced tillage (RT) in the wheat field and almond orchard and reduced tillage combined with green manure (RTG) in the almond orchard; these were compared to conventional tillage, the usual practice in the area. Open erosion plots were set up to monitor the effects of SLM on soil carbon and nutrients and on soil erosion after each rainfall event over six years (2010 2016). The results show that the SLM practices evaluated resulted in increased organic carbon (OC) and nutrients (N and P) contents in the soil, and reduced runoff, erosion, and mobilization of organic carbon and nutrients in sediments. Reductions in runoff of 30% and 65% and decreases in erosion of 65 and 85% were found in the wheat field and almond orchards, respectively. In addition, the total OC, N, and P losses in the wheat field were reduced by 56%, 45%, and 64%, respectively, while in the almond field the OC, N, and P losses were reduced by 90% under RT and by 85% under RTG. The beneficial effect of the SLM practices on soil erosion was observed within 18 months of their implementation and continued throughout the six years of the study. Furthermore, the effectiveness of tillage reduction with respect to erosion control and carbon and nutrients mobilization was highest during the most intense rainfall events, which are responsible for the highest erosion rates in Mediterranean areas. Our results support the key role of SLM practices under semiarid conditions as useful tools for climate change mitigation and adaptation, given the expected increase in high-intensity rainfall events in semiarid areas. © 2019 The AuthorsThis study site has been funded by several national (CYCIT AGL201125069//CICYT AGL2010-20941//CGL2013-42009-R//CGL2014-55-405-R), Regional (Séneca Foundation: 08757/PI/08//19350/PI/14), and European Commission H2020 (F6 DG RTD 037046 and Grant 728003, DIVERFARMING projects). Joris de Vente acknowledges support from a Ramón y Cajal research grant (RYC-2012-10375) and María Almagro was supported by the Juan de la Cierva Program (IJCI-2015-23500)

Mechanisms of Degradation and Identification of Connectivity and Erosion Hotspots

The context of processes and characteristics of soil erosion and land degradation in Mediterranean lands is outlined. The concept of connectivity is explained. The remainder of the chapter demonstrates development of methods of mapping, analysis and modelling of connectivity to produce a spatial framework for development of strategies of use of vegetation to reduce soil erosion and land degradation. The approach is applied in a range of typical land use types and at a hierarchy of scale from land unit to catchment. Patterns of connectivity and factors influencing the location and intensity of processes are identified, including the influence of topography, structures such as agricultural terraces and check dams, and past land uses. Functioning of connectivity pathways in various rainstorms is assessed. Modes of terrace construction and extent of maintenance, as well as presence of tracks and steep gradients are found to be of importance. A method of connectivity modelling that incorporates effects of structure and vegetation was developed and has been widely applied subsequently

Modeling the interactions between river morphodynamics and riparian vegetation

The study of river-riparian vegetation interactions is an important and intriguing research field in geophysics. Vegetation is an active element of the ecological dynamics of a floodplain which interacts with the fluvial processes and affects the flow field, sediment transport, and the morphology of the river. In turn, the river provides water, sediments, nutrients, and seeds to the nearby riparian vegetation, depending on the hydrological, hydraulic, and geomorphological characteristic of the stream. In the past, the study of this complex theme was approached in two different ways. On the one hand, the subject was faced from a mainly qualitative point of view by ecologists and biogeographers. Riparian vegetation dynamics and its spatial patterns have been described and demonstrated in detail, and the key role of several fluvial processes has been shown, but no mathematical models have been proposed. On the other hand, the quantitative approach to fluvial processes, which is typical of engineers, has led to the development of several morphodynamic models. However, the biological aspect has usually been neglected, and vegetation has only been considered as a static element. In recent years, different scientific communities (ranging from ecologists to biogeographers and from geomorphologists to hydrologists and fluvial engineers) have begun to collaborate and have proposed both semiquantitative and quantitative models of river-vegetation interconnections. These models demonstrate the importance of linking fluvial morphodynamics and riparian vegetation dynamics to understand the key processes that regulate a riparian environment in order to foresee the impact of anthropogenic actions and to carefully manage and rehabilitate riparian areas. In the first part of this work, we review the main interactions between rivers and riparian vegetation, and their possible modeling. In the second part, we discuss the semiquantitative and quantitative models which have been proposed to date, considering both multi- and single-thread river