The importance of soil aggregation in determining the dynamics of soil organic carbon (SOC) during erosion, transportation and deposition is poorly understood. Particularly, we do not know how ...aggregation contributes to the often-observed accumulation of SOC at depositional sites. Our objective was to assess how aggregation affects SOC stabilization in comparison to interactions of SOC with minerals. We determined and compared aggregate size distributions, SOC distribution in density fractions, and lignin-derived phenols from aggregated soil samples at both eroding and depositional sites. The stabilization effect of aggregation was quantified by comparing mineralization from intact and crushed macro-aggregates. Deposition of eroded soil material resulted in carbon (C) enrichment throughout the soil profile. Both macro-aggregate associated SOC and C associated with minerals (heavy fraction) increased in their importance from the eroding to the depositional site. In the uppermost topsoil (0–5 cm), SOC mineralization from intact aggregates was larger at the depositional site than at the eroding site, reflecting the large input of labile organic matter (plant residues) promoting aggregation. Contrastingly, in the subsoil, mineralization rates were lower at the depositional site because of effective stabilization by interactions with soil minerals. Aggregate crushing increased SOC mineralization by 10–80% at the eroding site, but not at the depositional site. The content of lignin-derived phenols did not differ between eroding and depositional sites in the topsoil (24.6–30.9 mg per g C) but was larger in the subsoil of the eroding site, which was accompanied by higher lignin oxidation. Lignin data indicated minor effects of soil erosion and deposition on the composition of SOC. We conclude that SOC is better protected in aggregates at the eroding than at the depositional site. During transport disaggregation and consequently SOC mineralization took place, while at the depositional site re-aggregation occurred mainly in the form of macro-aggregates. However, this macro-aggregation did not result in a direct stabilization of SOC. We propose that the occlusion of C inside aggregates serves as a pathway for the eroded C to be later stabilized by organo-mineral interaction.
•SOC was better protected in aggregates at eroding than at depositional sites.•Macro-aggregation did not lead to a direct SOC stabilization at depositional sites.•Stabilization was triggered by the subsequent formation of mineral-associated SOC.•Lignin data showed minor effects of erosion and deposition on composition of SOC.
Green water, or plant‐available soil moisture, is a substantial subset of terrestrial fresh water. Land‐use change (LUC) alters green water dynamics through interactions on the micro‐level (i.e., ...between the soil and vegetation) and on the macro‐level (i.e., between the land surface and atmosphere). Ongoing global deforestation, and growing interest in reforestation projects, begs the question whether such large‐scale LUCs have major eco‐hydrological impacts via the process of terrestrial moisture recycling. This requires a systematic, mechanistic understanding of green water dynamics in relation to LUC. Hence, this literature review addresses the above question via a scoping review that draws from papers covering empirical observations and simulated approximations on the hydrological effects of LUC from different parts of the world. The results show that some regions are more vulnerable to LUC than others and can affect local as well as distant hydrology of landscapes. Furthermore, we find that many studies focus on the global level or on tropical rainforests, through which we identify a knowledge gap for temperate regions and drylands. We derive analytical tools and directions for further research that can improve understanding of the effects of LUC on moisture recycling patterns to minimize unexpected hydrological impacts for nature and society.
Key Points
Advanced understanding of the effects of land‐use change on moisture recycling patterns demands an overarching review on this issue
Spatial and temporal patterns of moisture recycling are highly variable, but the hydroclimatic effects of land‐use changes on these patterns remain—although sensible considering the processes of scale and uncertainties due to water's active role in the atmosphere—under‐researched
There is a need to increase our understanding of context‐specific land‐use change effects on moisture recycling dynamics via case study research to evaluate potential hydroclimatic effects and prevent unintended consequences on water resources
Semiarid climate conditions hamper natural re-vegetation, leaving the soil vulnerable to erosion after the cessation of agriculture. Therefore, soil and landscape protective measures, especially ...afforestations, have been implemented in the Mediterranean region since the early 20th century. This study aims to determine the long term impact of afforestation on soil functioning, in comparison with natural re-vegetation (secondary succession) on abandoned fields and semi-natural vegetation.
A comparison of secondary succession and afforestation with the present traditional rain fed cereal fields and semi-natural (open) forest, including natural resource islands, was made as well. Composite soil samples were taken to study the physical (i.e. texture, aggregate stability) and chemical (i.e. carbon content, nutrient availability) soil characteristics after 20 and 40 years of afforestation and secondary natural succession. To take into account the resource island effect, the spatial heterogeneity induced by differences in plant cover, samples were taken both below and in between the tree canopy of the semi-natural and afforested Pinus halepensis trees.
Our results indicate that under secondary succession on abandoned fields, soil quality improves non-linearly and only marginally over a time of 40 years. The afforestation showed a much more pronounced linear increase for most soil quality indicators, resulting in soil conditions comparable to what can be found under the semi-natural forest vegetation. Site preparation might have been a crucial factor for the success of ecosystem restoration in the studied dry land area as it improved water availability for the afforestation.
Display omitted
•Secondary succession caused soil degradation during the first 20 years.•Afforesting improved most studied soil quality indicators.•Natural resource islands were formed by afforesting after 40 years.
Soil organic carbon (SOC) is the largest pool of non-sedimentary terrestrial carbon (C) and small changes in vertical SOC fluxes in the erosion-transport-deposition system could have a significant ...effect on atmospheric C levels. The ongoing sink/source discussion related to SOC seems to depend on which mechanisms are dominant during each of the three stages of soil erosion: detachment, transportation and deposition. Understanding C dynamics during each phase of soil erosion is essential to accurately assess the net effect of erosion. Currently, there is a knowledge gap when it comes to the movement of mobilized SOC from the site of detachment to the depositional site. This review provides an overview on the current understanding of the fate of eroded SOC during the transport phase of soil erosion. The stability of SOC appears to be a logistic interplay between SOC accessibility, presence of decomposers and suitable abiotic conditions. The main protection mechanisms of SOC are physical and/or chemical protection, which both make SOC inaccessible to decomposers and hence prevent mineralization. Transport subjects the SOC to disturbances and changing environmental conditions which interfere with the effectiveness of the protection mechanisms. The vulnerability of these mechanisms to erosive transport are not yet known. Increased physical impact is associated with disaggregation which releases previously protected SOC. Changes in geochemical composition of soil potentially changes the extent of organo-mineral bindings and hence either strengthen or weaken chemical protection. Complex chemical structures might result in more resistant C called biochemical stable SOC and is vulnerable to destabilization during transport due to changes in (a)biotic conditions along the trajectory. A complete understanding of the fate of mobilized SOC during transportation is essential to assess the net effect of soil erosion under different conditions. Standardization of both methodology and terminology in the field of soil erosion will further contribute to resolving the controversy on the net effect of erosion. The focus for future research should be on documenting the different interacting processes active during erosive transport and their effect on SOC fluxes.
Land degradation affects natural and cultivated socio-ecosystems worldwide. Soil erosion is one of the main processes leading to land degradation, and the process is accelerated by human actions. ...Spain is dealing with extensive land degradation caused by land use and land cover change (LULCC), for instance by land abandonment, and local geo-ecological conditions. Recent land abandonment in Spain can be largely related to changing policies and international market development, which have resulted in the reallocation of agriculture and a shift from traditional rainfed crops to intensification of irrigated agriculture. This radical change in LULCC by land abandonment resulted in two opposite trends, towards greening-up and towards land degradation, each with enormous consequences for the environment by its effects on soil hydrology, runoff, sediment sources, soil erosion, fluvial channel adjustments and forest fire risks. To mitigate negative effects, appropriate management and conservation strategies are necessary.
In this review, we analyze the top-down policy framework of soil conservation in Spain, with a specific focus on the Region of Murcia. We found that multiple international policies, i.e. the United Nations Convention to Combat Desertification, the European Water Framework Directive and the European Common Agricultural Policy (CAP), contribute to soil and water conservation at the national level, where the national administration selects most appropriate measures according to the country's current state. These measures are incorporated in national policies, such as the National Action Plan to Combat Desertification and the National Rural Development Programme. In case of the CAP, agro-environmental subsidies are an instrument to promote soil protection at a national level. Regionally adjusted sub-measures, based on regional environmental characteristics, are then integrated in the Regional Rural Development Programmes of the Spanish Autonomous Communities. The application of subsidies, related to soil protection, is found to be controversial, as studies do not agree upon its effect on soil erosion and land degradation control. To improve decision-making, concepts such as the ecosystem service approach and nature-based solutions are suggested to be included in future policies, as these concepts aim to improve the status of the entire ecosystem in a more holistic manner than is currently the case.
Forest fires can be a source of contamination because, among others, of the use of chemicals to their extinction (flame retardants, FRs), or by the production of Polycyclic Aromatic Hydrocarbons ...(PAHs) derived from high temperature alteration of organic matter. Up to our knowledge, this study is the first to assess the direct (PAHs 16 on the USA EPA's priority list), and indirect tri- to hepta- brominated diphenyl ethers (PBDEs), organophosphorus flame retardants (PFRs) and perfluoroalkyl substances (PFASs) contamination related to forest fires. The abundance and distribution of these contaminants were monitored on two Mediterranean hillslopes, one burned and one unburned, near Azuébar (SE Spain). Samples were taken in the foot, middle, and top of the slope, at two depths, and in two environments (under canopy and bare soil). Sediments were collected from sediment fences after erosive rainfall events. Most of the screened compounds were found in both, burned and control hillslopes, though significant differences were found between both. In burned soil, low concentrations of PBDEs (maximum ΣPBDEs: 7.3ngg−1), PFRs (664.4ngg−1) and PFASs (56.4ngg−1) were detected in relation to PAHs (Σ16 PAHs=1255.3ngg−1). No significant influence of the hillslope position was observed for any of the contaminants but differences based on depth and vegetation presence tended to be significant, particularly for the PAHs. After the first erosive event, concentrations of PBDEs and PAHs were higher in sediment than in soil (ΣPBDEs: 17.8ngg−1 and Σ16 PAHs=3154.2ngg−1) pointing out the importance of connectivity processes, especially shortly after fire.
Display omitted
•Assessment of PBDEs, PAHs, PFRs and PFASs distribution in burned and unburned hillslopes•BDE-85 concentrations were highest so no PBDE mixture was present in the fire extinguisher.•The fire added significant PAH amounts into the soil (1256ngg−1), mainly in the upper 2cm.•PFRs and PFASs were found in both hillslopes with values up to 352ngg−1 and 17ngg−1, respectively.•Contaminants transport downslope in the erosion events depends on the compound nature.
Recent studies show increasing evidence for perseveration of soil organic matter (SOM) controlled by interactions with the soil matrix (i.e. mineral surfaces and aggregates) rather than chemical ...recalcitrance of the SOM. However, a consensus is still absent for potential controls of SOM chemical composition on SOM stabilization and persistence. Soil fatty acids (FAs), which form an important SOM component, can be used to investigate the effects of chemical properties on SOM stabilization because they are easily degraded by microorganisms but can be stabilized by the soil matrix against decomposition. Here we investigated whether inherent molecular properties of FAs control their stability in soils and their interactions with the soil matrix. Soil samples were collected from alpine grasslands of the Peruvian Andes (Andosols, Umbrisols and Phaeozems), as they are characterized by high carbon stocks and abundant aliphatics. We applied pyrolysis - gas chromatography/mass spectrometry analyses assisted by tetramethylammonium hydroxide (TMAH-pyrolysis-GC/MS) to determine the chemical composition of bulk SOM and FAs before and after a 76-day incubation experiment, comparing a situation with intact versus crushed soil aggregates. The results showed that the TMAH-pyrolysis-GC/MS yielded a large proportion of FAs (>60% relative abundance of identified compounds), with a major contribution of free FAs. FA stability was controlled by the presence of double bonds (unsaturated vs. saturated FAs) and carbon chain length. Unsaturated FAs significantly (P < 0.05) predicted soil organic carbon mineralization rates and were more depleted after the incubation compared to saturated FAs. The depletion of unsaturated FAs is likely explained by their easier degradation compared to saturated FAs. The easier degradation might be explained by the smaller extent of stabilization through association with mineral surfaces and/or chemical properties rather than stabilization through occlusion in aggregates. In terms of carbon chain length, FA stability decreased from short-chain to long-chain FAs. A possible explanation for this is that short-chain FAs received more protection by occlusion in aggregates compared to long-chain FAs or that short-chain FAs were produced during the incubation as a result of microbial transformation of FAs. Such microbial transformation has limited effects on the prediction of FA stability using double bonds and carbon chain length. However, we observed that soil types and horizons did influence the controls of double bonds and carbon chain length on FA stability. Our results corroborate the hypothesis that the inherent properties of soil FAs control their interactions with the soil matrix and indirectly govern their stabilization and persistence in the Peruvian Andean soils under study.
Human‐induced afforestation has been one of the main policies for environmental management of farmland abandonment in Mediterranean areas. Over the last decades, several studies have reviewed the ...impact of afforestation activities on geomorphological and hydrological responses and soil properties, although few studies have evaluated the effects on water table dynamics. In parallel to human‐induced afforestation activities, natural revegetation occurred in abandoned fields and in fields where the intensity of human activity declined, driving the expansion of shrubs. This research addresses the spatial and temporal variability of water table dynamics in a small afforested sub‐catchment located in the Central Spanish Pyrenees. Differences between afforestation (Pinus nigra and Pinus sylvestris) and natural plant colonization (shrubs, mainly Genista scorpius, Buxus sempervirens, and Juniperus communis) and early abandoned meadows (G. scorpius), are analysed in terms of runoff generation and seasonal water table depth dynamics. Precipitation, runoff and water table datasets recorded for the 2014–2019 period are used. Results show a high temporal and spatial variability with large fluctuations in discharge and water table. Groundwater dynamics varied markedly over the year, identifying a wet and dry period with different responses suggesting different runoff generation processes (Hortonian flow during dry and wet periods, and saturation excess runoff during wet conditions). Furthermore, important differences are noted among the various land cover types: (i) in the natural revegetation area (shrubland and meadows) a marked seasonal cycle was observed with short saturation periods during winter and spring; and (ii) in the afforestation areas, the water table dynamics showed a seasonal cycle with a high variability, with fast responses and rapid oscillations. Likewise, the relationship between the depth of water table and hydrological variables was not straightforward, suggesting complex hydrological behaviour.
Spatial and temporal analysis of water table dynamics in afforestation and natural plant colonization areas in the Central Spanish Pyrenees.
Natural revegetation areas have a marked seasonal cycle with short saturation periods during winter and spring.
Afforestation areas have a seasonal cycle with fast responses and rapid oscillations.
Forest expansion in Mediterranean mountain areas is a widespread phenomenon resulting from the abandonment of agricultural and pastoral activities during the last century. Therefore, knowledge of the ...long-term storage capacity of soil organic carbon (SOC) in Mediterranean forests is of great interest in the context of global change. However, the effects of these land uses and covers (natural secondary forest, afforestation with conifers and silvo-pastoral ecosystems (dehesas)) on SOC dynamics are still uncertain. The main objectives of this study were to evaluate physico-chemical soil properties, SOC and nitrogen stocks, and SOC fractions in Mediterranean forests and to assess the effects of tree species, the soil environment (acidic and alkaline), and land management. We selected five land uses and land covers: managed and non-managed afforestation and dehesa (except for alkaline dehesa) and a stage of succession when tree species begin to become established after abandonment. This study concludes that although total SOC stocks are higher in afforested systems with conifers, SOC is stored in less stable carbon pools than in broadleaf forests. In addition, this study confirms that there are marked differences in the results between acidic and alkaline environments. Finally, the management system is also a significant factor, particularly for afforested sites.
PDF
