Evaluation of the Potential for Soil Organic Carbon Content Monitoring With Farmers

Increasing soil organic carbon (SOC) content is crucial for soil quality and climate changemitigation. SOC monitoring is indispensable to the corresponding policies and shouldprovide results at farm scale to allow for incentives. In Switzerland, farmers performmandatory analyses of the SOC content of the 0–20 cm topsoil of every field, based ona composite sample, at least every 10 years. The corresponding results are stored in adatabase in canton of Geneva. These data may be relevant for topsoil SOC monitoring,in particular for carbon sequestration policies, provided that they show appropriatequality, which is analyzed in this study. The minimum detectable change (MDC) of pastresults calculated based on the observed SOC changes was 0.013% g g−1at cantonscale (2,700 fields). Based on extended sampling of three representative fields, differentsampling strategies were simulated to determine the best future sampling guidelines forfarmers. Collecting 20 aliquots with a gouge on the field diagonals was considered thebest sampling compromise with field MDC of∼0.1% g g−1and a sampling durationof 20 min. Compared to this procedure, former farmers’ sampling was not biased inaverage but showed a variance of 0.22% g g−1due to smaller number of aliquotsand varying sampling depths. Based on the best sampling results and assumptions onfarm-scale SOC variance or SOC differences, the MDCs at farm scale ranged from0.21 to 0.12% g g−1(5 fields) and 0.09 to 0.05% g g−1(30 fields), respectively.These MDCs are small compared to published monitoring networks MDCs and allowdetermining SOC change rates at farm scale, thus offering perspectives for inexpensiveand efficient monitoring in the frame of soil quality or climate mitigation incentives. Forthe latter, however, additional information with equivalent soil mass and deeper-layercarbon content would be necessary

Spatial variability and sampling requirements of the visual evaluation of soil structure in cropped fields

Scoring the structure quality of the soil with the Visual Evaluation of the Soil Structure (VESS) spade test receives growing interest due to its simplicity, reliability and the quality of the evaluation provided. Sampling requirements to achieve a predefined quality of estimation, however, were not defined. This paper aims at filling this knowledge gap. The spatial variability of VESS scoring is analysed in two field, one with homogeneous micro-topography and vegetation growth and one showing spots of two distinct states with respect to these criteria. The semi-variograms showed purely random distribution of the scores from 6–10 to > 150 m of inter distances between observations, thus allowing to calculate the number of spade tests required to reach objectives of accuracy or minimum detectable difference. The two zones of the heterogeneous field, and the homogeneous field, showed the same coeffcient of variation of 11%, which is small. Therefore, 5 spade tests only are enough to detect a 0.5 change of the scoring. This number is smaller than usually recommended or performed. The classical recommendation to sample in homogeneous vegetation growth is supported by our results. These results probably apply to most situations and can be used to design monitoring protocols

Apprentissage et pratique du test bêche VESS par application mobile

L’attention portée à la structure du sol connaît actuellement un regain d’intérêt, en relation avec l’évolution et la diversification des pratiques de travail du sol et la conception de systèmes de culture plus agro-écologiques. Les méthodes visuelles ont largement été utilisées pour évaluer et porter au champ un diagnostic sur la structure du sol. Dans ce numéro, nous passons en revue les principales méthodes et regardons leurs atouts et limites. Les méthodes se différencient en deux grands groupes suivant les modalités d’échantillonnage des volumes de sols observés : les méthodes basées sur la description de la face d’observation d’un profil de sol comme le profil cultural et les méthodes « bêche » basées sur la description de blocs de sol extraits de la parcelle. Elles utilisent toutes les mêmes types de critères pour observer l’état structural : l’analyse des vides du sol à travers une évaluation de la porosité visible à l’œil et l’organisation de la fraction solide (forme, distribution des tailles, le degré de cohésion des mottes, importance de la terre fine…). Cependant, la procédure d’évaluation de la structure est très différente suivant les méthodes. Le profil cultural privilégie une analyse spatiale de l’état structural pour inférer sur sa genèse alors que les méthodes « bêche » conduisent à une note globale de la qualité de la structure du sol. Cette revue montre que nous disposons de plusieurs méthodes fiables, qui peuvent être choisies en fonction des objectifs poursuivis et de leur plus ou moins grande facilité de mise en œuvre.The attention paid to soil structure is increasing in relation to the evolution and diversification of tillage systems and the move towards to more agro-ecological systems. Visual methods have been widely used to evaluate and make a diagnosis of soil structure in field conditions. In this issue we review the main methods and look at their strengths and limitations. The methods are divided into two main groups according to the sampling methods: the methods based on the description of the vertical face of a soil profile and the "spade" methods based on the description of soil blocks extracted from the plot. The different methods use the same type of criteria to evaluate the soil structure: the analysis of the voids of the soil through an evaluation of the visible porosity and the organization of the solid fraction (shape, the size distribution, the degree of cohesion of clods, proportion of fine aggregates, ...). However how to evaluate the soil structure is very different between methods. The “profil cultural” method favours a spatial analysis of the soil structure to infer on its genesis whereas the "spade" methods assign an overall score to evaluate the soil structure quality. The review shows there are several reliable visual methods, but the differences in the way to evaluate soil structure and in the implementation of the methods mean that the choice of methods must be made according to the objectives pursued and the users

Changes in topsoil organic carbon content in the Swiss leman region cropland from 1993 to present. Insights from large scale on-farm study

Increasing cropland topsoil organic carbon (SOC) content is a key goal for soil improving quality and adaptating soils to climate change. Moreover, the short term potential of climate mitigation by carbon sequestration is mostly attributed to increasing topsoil SOC content (Balesdent and Arrouays, 1999, Chambers et al., 2016; Minasny et al., 2017; Balesdent et al., 2018). However, the possibility to increase SOC content is highly disputed in current literature which is mostly based on field experiments. We quantified the on-farm SOC content deficit and SOC content change rate of cropland topsoil (0–20 cm) from western Switzerland using the data bases of Geneva and Vaud cantons containing more than 30,000 topsoil analyses, performed every ten years on every cultivated field of the region since 1993. SOC deficit was estimated as the amount of SOC necessary to reach the 0.1 SOC:clay ratio considered as the minimum required SOC amount for acceptable soil quality. Cropland topsoils of the Vaud and Geneva cantons displayed a 20% and 70% SOC content deficit, respectively. In both cantons, the range of observed rates of change in SOC content from 1993 to present was very large, from −30 to +30‰ per year, with a median value of 0. However, the time trends showed a highly significant linear increase of rates from −5‰ to +6‰ per year on average, in 1995 and 2015, respectively, with no change in SOC content reached by 2005–2007. These trends were attributed to the Swiss agri-environmental schemes applied at the end of 20th century, namely mandatory cover crops and minimum rotations of 4 crops. Further, SOC content increase was accordant with the continuing adoption of minimum tillage, conservation agriculture and multi-species intense cover crops. These findings oppose to those obtained in Swiss long-term experiments, which emphasizes the need to use on-farm information when adressing agriculture policy, climate mitigation or soil quality management issues

Indice de qualité des sols ::un outil d'aide à la décision pour la prise en compte de la qualité des sols pour la régulation hydrique d'un territoire

Les fonctions de régulations hydriques fournies par les sols sont très reconnues aujourd’hui et sollicitées pour contenir le ruissellement, en milieu urbain ou agricole. L’indice de qualité des sols (IQS) a pour ambition de pouvoir noter et représenter spatialement les fonctions des sols, dont celle de régulation hydrique, sur la base de l’analyse de propriété des sols. Cependant, la quantification des propriétés des sols n’est pas aisée principalement en raison du manque de données sur les propriétés des sols ou du coût de leur acquisition. La démarche IQSM ici présentée, résout cette difficulté en s’affranchissant de l’étape de campagne de mesure à l’échelle d’un territoire entier mais en tirant partie des informations géoréférencées existantes sur le territoire. De ces informations territoriales, des estimations de qualité des propriétés des sols sont attribuées par avis d’expert. Les informations analytiques sur les sols qui seraient déjà acquises sont intégrées. Un logigramme définit la prédominance d’une information sur une autre. Pour tenir compte des différents niveaux de fiabilités des informations délivrées, une note de fiabilité est associée à l’estimation, dont le calcul est lié au logigramme. Une représentation de la qualité fonction de régulation hydrique des sols est ainsi quantifiée à l’échelle d’un aménagement ou du territoire et permet l’aide à la décision quant aux aménagements futurs ou le renseignement d’un modèle hydrologique en l’absence de données d’infiltration ou de ruissellement.The water regulation functions provided by soils are very well recognised today and are called upon to contain runoff in urban and agricultural environments. The soil quality index (SQI) aims to record and spatially represent soil functions, including water regulation, on the basis of soil property analysis. However, the quantification of soil properties is not easy, mainly because of the lack of data on soil properties or the cost of acquiring them. The IQSM approach presented here solves this difficulty by avoiding the stage of a measurement campaign on the scale of an entire territory but by taking advantage of existing georeferenced information on the territory. From this territorial information, quality estimates of soil properties are assigned by expert opinion. Analytical soil information which has already been acquired is integrated. A flow chart defines the predominance of one piece of information over another. In order to take into account the different levels of reliability of the information provided, a reliability score is associated with the estimate, the calculation of which is linked to the flowchart. A representation of the quality function of soil water regulation is thus quantified at the scale of a development project or territory and allows decision support for future developments. It also allows coupling to a hydrological model in the absence of infiltration or runoff data

ACS et teneur en matière organique du sol ::quelques enseignements tirés de la région lémanique

L’agriculture de conservation des sols (ACS) n’est pas née avec la découverte du changement climatique. Elle est née de la volonté d’agriculteurs de préserver et de restaurer leurs sols et elle s’est développée en marge des politiques publiques. L’initiative 4-pour-mille (https://www.4p1000.org/fr) met en avant l’ACS comme solution de premier plan pour séquestrer le carbone dans les sols (figure 1), ce qui a suscité une grande production d’articles scientifiques et de prises de position, tant pour soutenir cette option que pour en souligner les faiblesses. Il en résulte des controverses et des affirmations probablement illisibles pour la plupart des acteurs. Cet article, s’appuyant sur les résultats de nos recherches, vise à apporter quelques éclaircissements sur les enjeux liés à la teneur en matière organique des sols en agriculture, notamment les enjeux climatiques, et les apports de l’ACS, à travers un retour d’expérience à grande échelle sur la région lémanique (cantons suisses de Vaud et Genève)

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

<jats:p>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.</jats:p&gt