Sampling and analysis or visual examination of soil to assess its status and use potential is widely practiced from plot to national scales. However, the choice of relevant soil attributes and interpretation of measurements are not straightforward, because of the complexity and site-specificity of soils, legacy effects of previous land use, and trade-offs between ecosystem services. Here we review soil quality and related concepts, in terms of definition, assessment approaches, and indicator selection and interpretation. We identify the most frequently used soil quality indicators under agricultural land use. We find that explicit evaluation of soil quality with respect to specific soil threats, soil functions and ecosystem services has rarely been implemented, and few approaches provide clear interpretation schemes of measured indicator values. This limits their adoption by land managers as well as policy. We also consider novel indicators that address currently neglected though important soil properties and processes, and we list the crucial steps in the development of a soil quality assessment procedure that is scientifically sound and supports management and policy decisions that account for the multi-functionality of soil. This requires the involvement of the pertinent actors, stakeholders and end-users to a much larger degree than practiced to date.

Soil quality – A critical review

Biogeochemistry of paddy soils

Feeding the world population, 7.3 billion in 2015 and projected to increase to 9.5 billion by 2050, necessitates an increase in agricultural production of ~70% between 2005 and 2050. Soil degradation, characterized by decline in quality and decrease in ecosystem goods and services, is a major constraint to achieving the required increase in agricultural production. Soil is a non-renewable resource on human time scales with its vulnerability to degradation depending on complex interactions between processes, factors and causes occurring at a range of spatial and temporal scales. Among the major soil degradation processes are accelerated erosion, depletion of the soil organic carbon (SOC) pool and loss in biodiversity, loss of soil fertility and elemental imbalance, acidification and salinization. Soil degradation trends can be reversed by conversion to a restorative land use and adoption of recommended management practices. The strategy is to minimize soil erosion, create positive SOC and N budgets, enhance activity and species diversity of soil biota (micro, meso, and macro), and improve structural stability and pore geometry. Improving soil quality (i.e., increasing SOC pool, improving soil structure, enhancing soil fertility) can reduce risks of soil degradation (physical, chemical, biological and ecological) while improving the environment. Increasing the SOC pool to above the critical level (10 to 15 g/kg) is essential to set-in-motion the restorative trends. Site-specific techniques of restoring soil quality include conservation agriculture, integrated nutrient management, continuous vegetative cover such as residue mulch and cover cropping, and controlled grazing at appropriate stocking rates. The strategy is to produce “more from less” by reducing losses and increasing soil, water, and nutrient use efficiency.

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Restoring Soil Quality to Mitigate Soil Degradation

Summary Biodiversity is responsible for the provision of many ecosystem services; human well-being is based on these services, and consequently on biodiversity. In soil, earthworms represent the largest component of the animal biomass and are commonly termed ‘ecosystem engineers’. This review considers the contribution of earthworms to ecosystem services through pedogenesis, development of soil structure, water regulation, nutrient cycling, primary production, climate regulation, pollution remediation and cultural services. Although there has been much research into the role of earthworms in soil ecology, this review demonstrates substantial gaps in our knowledge related in particular to difficulties in identifying the effects of species, land use and climate. The review aims to assist people involved in all aspects of land management, including conservation, agriculture, mining or other industries, to obtain a broad knowledge of earthworms and ecosystem services.

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A review of earthworm impact on soil function and ecosystem services

Bacteria and fungi can contribute to nutrients bioavailability and aggregate formation in degraded soils.

Both soil quality degradation and climate change mitigation issues emphasize the need to increase, or at least stabilize, the topsoil organic carbon content (wt%) in arable land. This on-farm study aimed at measuring the impact of agricultural practices on changes in soil organic carbon (SOC) content over 10 years. A total of 120 fields belonging to 120 farms representative of the cropping systems and soil properties in Western Switzerland (Lake Geneva region) was randomly selected. The field 0–20 cm topsoil was sampled at a 10-years interval, and the corresponding cropping practices were gathered using farmer’s interviews and the mandatory records of yearly practices at field level in Swiss-farms. Only 1) organic matter inputs and 2) cover-crop intensity were significantly correlated to SOC increase while 3) the soil tillage intensity and 4) the soil saturation in carbon expressed as a SOC to clay content ratio were correlated to SOC decrease. Among others, temporary meadows were not correlated to changes in SOC content mainly due to increased tillage and decreased cover-crops between meadows. Organic farming did not correlate either with SOC changes due to the large tillage intensity applied for weed control. The observed SOC content changes ranged from −56‰ to +74‰ and were well explained by a linear regression model with additive effect of the four identified SOC change factors. The additivity of these factors means that farmers can emphasize the methods of their choice when regenerating their soils. This study advocates that strict no-till is not required at low carbon saturation level (small SOC:Clay ratio). However, as carbon saturation increases, conservation tillage and then no-till practices become necessary to further increase SOC contents. These findings are in accordance with previous studies showing that since 2015 SOC is increasing at more than +4‰ on average in the region and provide practical insights to further manage the transition of farming systems towards soil regeneration.

On-Farm Relationships Between Agricultural Practices and Annual Changes in Organic Carbon Content at a Regional Scale

Soils of the Senegal river middle valley: properties and evolution under irrigation

Alors que l‘irrigation parait etre le recours essentiel face aux besoins croissants de la population mondiale en nourriture, la degradation de la qualite des sols et des eaux annihile presque totalement les efforts d‘amenagement. Le suivi de l‘evolution des eaux et des sols sous culture apparait indispensable pour garantir une agriculture irriguee rentable et durable dans la zone. Dans cette region, les risques de degradation des sols resultent de deux processus : l‘alcalinisation et la salinisation des sols. Les parcelles etudiees sont mises en valeur par une culture de contre‐saison froide (novembre‐mars) de tomate et d‘oignon. Dans cet article, l‘etude des transferts d‘eau dans les parcelles irriguees par le biais d‘un suivi experimental in situ met en evidence l‘evolution des fronts d‘humidification due aux apports d‘eau et sa consequence eventuelle sur le comportement des sols. L‘etude a permis une meilleure comprehension de la dynamique hydrique au sein des parcelles equipees : saturation du profil et processus de recharge de la nappe. Elle a notamment permis de montrer que les transferts d‘eau vers la nappe pendant la periode d‘irrigation sont negligeables. L‘etude montre egalement que le lessivage des sols est quasi inexistant durant toute la campagne agricole, ce qui a pour consequence d‘entrainer une accumulation de sels dans le profil du sol. Cette derniere constitue une menace serieuse pour le comportement agricole du sol qui est appele a se degrader par alcalinisation.

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Suivi expérimental des transferts d‘eau provoqués par l‘irrigation dans le périmètre de Ouro Madiou en moyenne vallée du fleuve Sénégal

Role And Behaviour Of Clay Minerals In Alpine Debris Flows

Depuis J990, lecours dufleuve Sénégal est entièrement contrôlé, par un barrage réservoir en amont etpar un barrage anti-sel dans le delta. L'unedes perspectives offertes par ces aménagements est l'irrigation :jusqu 'à 240000hadeterres irrigables ont étérecensées sur la seule rive sénégalaise. La transformation des conditions d'accès à la ressource en eau s'accompagne deprofonds bouleversements, écologiques, maisaussiéconomiques et sociaux. Un programme de recherche pluridisciplinaire menéconjointementpar lOrstom etl 'lsra s'estfixépour objectifs d'étudier ces transformations, enparticulier au niveau des systèmes deproduction, etdedéfinir les conditions (t'évolution vers une agriculture irriguée durable. QUAND LES PLUIES TOMBAIENT

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Pascal Boivin

Papers

On-Farm Relationships Between Agricultural Practices and Annual Changes in Organic Carbon Content at a Regional Scale

Soils of the Senegal river middle valley: properties and evolution under irrigation

Suivi expérimental des transferts d‘eau provoqués par l‘irrigation dans le périmètre de Ouro Madiou en moyenne vallée du fleuve Sénégal

Role And Behaviour Of Clay Minerals In Alpine Debris Flows

Agriculture irriguée dans la vallée du fleuve Sénégal : une approche pluridisciplinaire