Trial factors included three soil tillage systems completely randomised in four replications in two parallel field experiments.Soil cores were transported from the field to the respective partner institution, stored in a cooling room until further steps and processed by each partner according to an internal standard protocol. After drying the whole sample at 40 ◦C until constant weight, the samples were sieved to 2 mm to separate the coarse and the fine fraction . Coarse soil aggregates were destroyed during this step. The masses of both fractions were determined separately for bulk density determination. An aliquot of 10 g was taken from the fine fraction, weighed, dried in an oven at 105 ◦C until the weight was constant, cooled in a desiccator and weighed again. The residual water content was used to calculate the difference between the 105 ◦C dry mass and the 40 ◦C mass. A representative sub-sample was taken from the 40 ◦C dried fine fraction and finely ground with a ball or mortar mill. The 40 ◦C dried, 2 mm sieved and ground sub-samples were sent to the Research Institute of Organic Agriculture FiBL for further analysis. Reduced tillage clearly increased topsoil SOC stocks and decreased SOC stocks in or just below the old plough layer compared with traditional ploughing. Such a SOC stratification effect can be linked to the lack of mixing of organic matter into deeper soil layers achieved by ploughing and is limited to the topsoil in the case of reduced tillage. It may also be related to a change in root growth. Higher root length densities in the topsoil and lower densities in the layers to 30 cm depth compared with ploughing was found across no-till studies globally and for reduced tillage at the French site of our study . There, hydroponic grow system root abundance under reduced tillage was higher in 0–6 cm and lower in 12–70 cm. This root growth effect was attributed to the stratification of nutrients and a change in bulk density.

Higher bulk densities in soil layers lower than 10/15 cm measured in this study confirm a potentially growth limiting factor for roots into subsoil layers. They also suggest that the SOC stock decrease below the upper soil layer and the large SOC stock increase in the upper soil layer despite no change in bulk densities was mainly driven by changes in SOC content. We found higher SOC stocks in 70–100 cm soil depth across seven sites. Sampling deeper than 70 cm was impossible at two sites due to shallow bedrock. Stone and soil carbonate content increased with depth at some sites and, at the NL site, spots of peat were recorded in the subsoil. This introduces more spatial heterogeneity than in upper soil layers, which is known to challenge interpretation . Roots and macropores of anecic earthworms may be carbon pathways into deeper soil layers. Yet, root abundance was lower under reduced tillage than under ploughing in lower soil layers at the French site and earthworm monitoring in some of our studied sites did not show an effect of tillage systems on anecic species . Whether our observation of higher SOC stocks in 70–100 cm is an effect of spatial heterogeneity or tillage management cannot be answered within this study and offers opportunities for further research. Overall, the SOC stock profile distribution measured in our study resembles the SOC stock distribution of the well assessed no-till – ploughing comparisons compiled by several meta-analyses, e.g. by Luo et al. or Ogle et al. . Regarding the topsoil SOC enrichment, it can be assumed that soil erosion control is also achieved by reduced tillage in organic farming, which has been confirmed with direct erosion measurements at the CH-1 site . Since conservation tillage systems are discussed as climate change mitigation measures, original studies were repeatedly summarised by meta-analyses. Selecting for ones that also include subsoils, Luo et al. reported an insignificant 2.8% increase by no-till globally and Meurer et al. an insignificant increase by 1.2–1.3 Mg C ha− 1 or 0.1 Mg C ha− 1 yr− 1 by no-till in temperate climates. Our study, with a total SOC stock increase by reduced tillage of on average 1.7% after 8–21 years, equivalent to 1.5 Mg C ha− 1 or 0.09 Mg C ha− 1 yr− 1 in 0–50 cm shows a similar SOC sequestration for reduced tillage systems in organic farming. A comparison with other reduced tillage studies is more difficult since they mostly did not sample subsoils.

For instance, the review of K¨ ampf et al. on SOC distribution in topsoils in temperate climates indicates that SOC stocks under reduced tillage are intermediate between no-till and ploughing. Blanco-Canqui et al. sampled two long-term trials in Nebraska after 34 and 39 years under climatic conditions similar to our study. While the 39-year-old site showed an increase in total SOC stocks from ploughing to reduced tillage to no-till, SOC stocks at the 34-year-old site were 22% higher under reduced tillage than ploughing but similar to no-till. In our study after a trial duriation of 8–21 years, total SOC stocks in 0–100 cm accounted for 3.6% or 4.0 Mg C ha− 1 higher SOC stocks with reduced tillage resulting in a far lower increase in SOC stocks than in the study of Blanco-Canqui et al. . We, therefore, confirm that a certain SOC sequestration can be achieved by reduced tillage, even though tillage operations are not entirely stopped. The subsoil SOC processes urgently need further attention in future research. To assess the overall impact of tillage systems regarding climate change mitigation, other aspects like N2O emissions or fuel consumption need to be considered as well , since those emissions continue while a SOC steady state is reached after a certain period. In summary, reduced tillage systems under organic farming conditions can provide some SOC sequestration without the use of herbicides which are of increasing concern . This is an important outlook regarding future efforts to reduce pesticide inputs in conventional agriculture. Yields are, however, most likely lower than in conservation tillage systems with herbicide use, as indicated in the CH-1 field experiment . Cumulative SOC stocks were overall increased with reduced tillage, suggesting that SOC was enriched or losses reduced. The amount of SOC in soils is regulated by SOC turnover and stabilisation and there is evidence that ploughing disrupts soil aggregates exposing SOC to microbial consumption . Measurements in some of our studied sites support the lower level of soil disturbance in reduced tilled soils. At CH-1, a sandy loam, a change in microbial composition favouring fungi in combination with increased aggregate stability was determined.

Though aggregate stability was similar between tillage systems at CH-3 due to the high clay content , fungi in general and arbuscular mycorrhizal fungi that are sensitive to disturbance were more abundant in reduced tilled plots. Tillage systems also impact above ground carbon input, which was assessed by Virto et al. who positively related changes in crop C input to SOC stocks . In our study, regressions of tillage system differences in total crop biomass and weed biomass were slightly but not significantly linked to the SOC stock changes observed. In fact, reduced tillage in the organic farming context of this study decreased crop biomass yields by on average 8% in comparison with ploughing. Such a yield gap was reported by Cooper et al. and is related to increased weed pressure and plant nutrition issues. This is the main difference to conservation tillage practices in conventional farming, where herbicides and mineral fertilisers can sustain productivity. The decrease in crop biomass in our study led to a 0.2 Mg C ha− 1 yr− 1 lower total above ground C input. The lower input may have been outbalanced by i) higher weed pressure which was estimated to provide 0.3 Mg C ha− 1 yr− 1 from above ground biomass or ii) changes in below ground C inputs, which were not assessed in our study. It should be noted that weed data in this study are based on a limited dataset from four out of nine sites. This is, therefore, only a preliminary comparison that indicates that weeds may be an important source of C input which needs future research. Interestingly, tillage induced SOC stock changes were not related to trial duration in our study. Therefore, it seems that site-specific pedoclimatic conditions interact with management practices in a more complex way. Trends in SOC stock drivers were more apparent when sites were compared: Soil texture, especially clay contents, is often identified as an important predictor of SOC as fine mineral particles associate with SOC . However, in our study, silt content was also well correlated to SOC stocks. The clay/silt ratio finally had the best correlation and could be a good predictor of SOC stocks, representing the soil texture triangle in a condensed form. Soil pH ranged from 5.9–7.2 across sites. The positive correlation with SOC stocks may hint to the availability of exchangeable cations that were argued to impact SOM stabilisation and clay mineral behaviour . Apart from soil parameters,indoor garden the positive SOC stock correlation with precipitation is commonly found .

SOC stocks also increased with the amount of organic amendments used.Our study compared SOC stocks that were calculated on a site specific fixed depth approach with stocks that were further modeled on an equivalent soil mass . Site specificity means that we considered a priori knowledge on current and historic tillage depths during sampling and that the layers in the Ap horizon thus differ between sites. The equivalent soil mass approach is widely discussed as the more appropriate method as tillage or other soil management practices influence bulk densities and therefore soil masses. There is, however, no standardised protocol of the modelling procedure, and approaches vary considerably . In our study, the two approaches yielded the same outcome. As the ESM approach relies on the choice of input variables and the quality of the modeled cubic spline fit, we feel that there are more uncertainties added. Beyond, we have seen, just as Blanco-Canqui et al. , that soil sampling depth has a huge impact when assessing tillage system differences on SOC stocks. This suggests that sampling deep enough is more important than the method used to calculate SOC stocks. As farm machinery grows bigger and heavier in pursuit of economies of scale, traffic-induced soil compaction has become widespread. ESDAC defined soil compaction as “. a form of physical degradation resulting in densification and distortion of the soil where biological activity, porosity and permeability are reduced, strength is increased, and soil structure is partly destroyed”. Manifestations of soil compaction are multifaceted . Soil compaction causes a loss of nitrogen from soil resulting in a reduction of soil nitrogen uptake by plants . Yamulki and Jarvis found that compaction had a more profound effect than tillage on the release of gaseous emissions from agriculture. Tullberg et al. 2018 found evidence that trafficked soils have significantly higher N2O emissions than non-trafficked soils . Pangnakorn et al. documented significant difference in earthworm populations between compacted and non-compacted soils. Subsoil compaction can persist over a long time and is costly to eliminate, if elimination is possible at all . Hence, there is a need for smart agricultural techniques to avoid compaction . Managing machinery traffic in terms of: the placement of machinery traffic pathways; the axle loads, tyre sizes and inflation pressures used; and the soil conditions under which trafficking is allowed, can contribute to compaction avoidance. One such approach is to confine field traffic to permanent tracks that are maintained year after year, referred to as Controlled Traffic Farming . Earlier deployment of CTF technology relied on marking permanent tracks and frequently involved the deployment of gantries. The advent of high precision positioning and auto-steering systems, by avoiding the need to physically mark and manually steer along pathways, makes CTF a promising technology in future agriculture . In Chamen , CTF is labeled as “precision farming at its most efficient”. In its strict sense, CTF requires all machinery operations to be in permanent tracks.