Organic matter is a vital component of soils and decisive for soil health and sustainable food production. Agricultural soil use often decreases soil organic matter stocks and climate change may aggravate the situation, putting food security and ecosystem functioning at risk. Bio-based fertilizers (BBFs, most often waste or residue-derived) are a vital part of future circular economies focussing on closed nutrient cycles in food production and agriculture. It is unclear if BBFs have positive effects on soil organic carbon (SOC) because studies on BBFs were so far mostly focussed on plant nutrition and ran mainly as short-term pot experiments. We filtered, extended and reclassified the comprehensive meta-analysis of Chen et al. (2018) to elucidate the effects of bio-based fertilizers on SOC concentrations in European and North American long-term experiments. The revised data set consisting of 260 observations from 60 field trials (now including a minimum duration of 5 years) showed significant effects of BBF quality on SOC concentration when compared with minerally fertilized plots and elucidated the role of soil and site properties on the efficiency of BBFs to increase SOC. The extended data set showed that solid and carbon-rich BBFs were most efficient in increasing SOC concentrations. Less developed soils and loamy soils in dry climates showed the highest potential for carbon sequestration through BBF application. We stress the importance of carbon inputs for the maintenance of current SOC concentrations, thus demanding the cautious use of C-depleting steps during BBF processing, only to be applied if elimination of organic pollutants and pathogens is needed.

The use of biochar in agricultural soils is currently being researched worldwide. Previous research results suggest that biochar has a positive effect on soil quality, in particular through improved soil structure, water holding capacity and nutrient sorption, as well as the potential for carbon sequestration. In order to evaluate the benefits and risks of the application, biochar needs to be tested under field conditions for long-term effects within different climate zones and soil types. Here we present first results from a long-term field experiment in Switzerland, in which we comprehensively and continuously evaluate the effects of biochar application on soil quality and crop yield. The trial consists of five agricultural fields in Zürich, Switzerland, with a total area of 10.5 ha. Soil type is mostly clayey loam and fields are managed according to organic guidelines and with reduced tillage management. We applied biochar once in spring 2021 according to 8 t/ha on half of the area according to a split-plot design. All fields are monitored continuously until at least 2027 for soil carbon (total, carbonate and pyrogenic), total soil nitrogen, pH, cation exchange capacity, aggregate stability, water holding capacity and yields. In addition, water content and penetration resistance are measured during the vegetation period. First results indicate a positive effect of biochar application on soil organic and pyrogenic carbon, however no effects on the other measured parameters. Therefore, under the given soil and climatic conditions in temperate soils biochar application may be valid to increase long-term organic carbon contents and carbon sequestration. Other propagated effects of biochar application like improved water holding capacity, cation exchange capacity and yields require higher biochar application rates to be significantly affected.

Conventional laboratory analysis of soil properties is often expensive and requires much time if various soil properties are to be measured. Visual and near-infrared (vis–NIR) spectroscopy offers a complementary and cost-efficient way to gain a wide variety of soil information at high spatial and temporal resolutions. Yet, applying vis–NIR spectroscopy requires confidence in the prediction accuracy of the infrared models. In this study, we used soil data from six agricultural fields in eastern Switzerland and calibrated (i) field-specific (local) models and (ii) general models (combining all fields) for soil organic carbon (SOC), permanganate oxidizable carbon (POXC), total nitrogen (N), total carbon (C) and pH using partial least-squares regression. The 30 local models showed a ratio of performance to deviation (RPD) between 1.14 and 5.27, and the root mean square errors (RMSE) were between 1.07 and 2.43 g kg−1 for SOC, between 0.03 and 0.07 g kg−1 for POXC, between 0.09 and 0.14 g kg−1 for total N, between 1.29 and 2.63 g kg−1 for total C, and between 0.04 and 0.19 for pH. Two fields with high carbonate content and poor correlation between the target properties were responsible for six local models with a low performance (RPD < 2). Analysis of variable importance in projection, as well as of correlations between spectral variables and target soil properties, confirmed that high carbonate content masked absorption features for SOC. Field sites with low carbonate content can be combined with general models with only a limited loss in prediction accuracy compared to the field-specific models. On the other hand, for fields with high carbonate contents, the prediction accuracy substantially decreased in general models. Whether the combination of soils with high carbonate contents in one prediction model leads to satisfying prediction accuracies needs further investigation.

Cover crops are grown between two main crops to reduce periods of bare fallow. In highly diverse crop rotations, the lengths of break periods between two main crops vary highly over time and consequently the cover cropping management differs from year to year. Long-term field trials are thus of limited use because the same cover cropping approach only appears once in several years. This increases the need to better determine the immediate effects of different cover cropping strategies on soil properties. This study evaluated two cover cropping strategies and monitored the temporal development of several soil properties on six fields in Eastern Switzerland in the 9 months period between harvest of winter wheat and sowing of spring crops. The two tested strategies were (a) double cover cropping (DCC) where two cover crops mixtures were grown subsequently and shallowly (3 cm) incorporated into the topsoil and (b) permanent soil cover (PSC) with one grass-clover mixture, which was harvested and thus not incorporated into the soil. Soil samples at three different soil depths (0-5, 5-10 and 10-20 cm) were sampled four times in high spatial resolution and analysed using a combined approach of visible near infrared spectroscopy and conventional lab methods. Differences between the sampling times and field sites were stronger than effects of different treatments. For soil organic carbon (SOC), no significant difference was measured between treatments in 0-20 cm soil depth. Only when analysed per depth segment, the PSC treatment showed significantly higher SOC increase in 5-10 cm soil depth than the DCC treatment. This could be due to the longer soil cover and thereby associated longer root growth period in the PSC treatment, leading to higher below ground C inputs than in the DCC treatment. On the other hand, the DCC treatment showed generally higher increases in permanganate oxidizable carbon stocks (0-5 cm), microbial C (0-10 cm), microbial N (0-10 cm) and mineral N (0-10 cm) than the PSC treatment. We conclude that maximizing cover crop above ground biomass input by planting two cover crops (DCC) benefitted soil microorganisms on most fields but was less beneficial on SOC than permanent soil cover (PSC) in 5-10 cm soil depth.

Abstract. The application of visual and near infrared soil spectroscopy (vis–NIR) is an easy and cost-efficient way to gain a wide variety of soil information to cover high spatial and temporal resolution in large-scale soil surveys and in local field-scale studies. However, unlike for conventional methods, the prediction accuracy of vis–NIR spectral models cannot yet be estimated before the data collection, which hampers its application at the local scale where often a high precision is required (e.g., field experiments). In this study we used soil data from six agricultural fields in Eastern Switzerland and calibrated i) field-specific (local) models and ii) general models (combining all fields) for organic carbon, total carbon, total nitrogen, permanganate oxidizable carbon and pH using partial least squares regression. 24 out of 30 local models showed an accurate or even excellent performance (ratio of performance to deviation (RPD) > 2) and the root mean square errors (RMSE) of prediction were, except for pH, maximum five times higher than the lab measurement error. The variability of a specific soil property and the mean carbonate concentration in the dataset were the two factors influencing the performance of the local models. We found a significant relationship between the coefficient of variation in the dataset and the metrics for model performance (R2, percental RMSE and RPD). Starting from a tolerable prediction error for the spectral measurements, the regressions can be used to develop a sampling design that matches the corresponding target variability. The five inaccurately performing local models with RPD < 2 were on the two fields with highest carbonate content raising the question if local vis–NIR models are suitable for soils with high carbonate concentration. General models combining the datasets from all six fields showed an accurate overall performance but the RMSE on the field level were higher compared to the local models.

Soil organic matter (SOM) plays a vital role for soil quality, sustainable food production and climate change mitigation. It is common knowledge that SOM consists of different pools with varying qualities, quantities, and turnover times. However, it is still poorly understood how mineral and organic fertilization affects the formation and stabilization of mineral-associated organic matter (MAOM) and how long it can remain there. Here, we report on the long-term effects of different farming systems on the stability and turnover of the fine silt and clay-sized MAOM fraction (<6.3 mu m) of a Haplic Luvisol (0-20 cm) in the DOK long-term trial (Switzerland). We compared three farming systems with contrasting fertilization (CONMIN = pure mineral, CONFYM = mineral + organic, BIODYN = pure organic) with an unfertilized control (NOFERT) between 1982 and 2017. We performed specific surface area (SSA) measurements on fractionated MAOM samples (<6.3 mu m) from 1982 to 2017, before and after removal of OM, measured the C-14 activity of all samples during the entire period and estimated the mean residence time (MRT) with a model taking into account 'bomb C-14' and radioactive decay. We found constant MAOM-C contents under organic fertilization. Results of SSA analysis indicate best conditions for MAOM-C stabilization under organic fertilization and different sorption mechanisms in MAOM between farming systems with and without organic fertilization. The modelled MRTs were significantly higher in NOFERT (238 +/- 40 yrs) and CONMIN (195 +/- 27 yrs), compared to CONFYM (138 +/- 18 yrs) and BIODYN (140 +/- 19 yrs), implying a high C turnover (i.e. more active MAOM) at high C contents under organic fertilization. Our findings show that MAOM is not the dead OM but corroborates the concept of 'dynamic stability'. Continuous OM inputs from organic fertilizers and their rapid and constant turnover are needed to stabilize the "stable" MAOM-C fraction.

Increasing soil organic carbon (SOC) stocks in agricultural soils removes carbon dioxide from the atmosphere and contributes towards achieving carbon neutrality. For farmers, higher SOC levels have multiple benefits, including increased soil fertility and resilience against drought-related yield losses. However, increasing SOC levels requires agricultural management changes that are associated with costs. Private soil carbon certificates could compensate for these costs. In these schemes, farmers register their fields with commercial certificate providers who certify SOC increases. Certificates are then sold as voluntary emission offsets on the carbon market. In this paper, we assess the suitability of these certificates as an instrument for climate change mitigation. From a soils' perspective, we address processes of SOC enrichment, their potentials and limits, and options for cost-effective measurement and monitoring. From a farmers' perspective, we assess management options likely to increase SOC, and discuss their synergies and trade-offs with economic, environmental and social targets. From a governance perspective, we address requirements to guarantee additionality and permanence while preventing leakage effects. Furthermore, we address questions of legitimacy and accountability. While increasing SOC is a cornerstone for more sustainable cropping systems, private carbon certificates fall short of expectations for climate change mitigation as permanence of SOC sequestration cannot be guaranteed. Governance challenges include lack of long-term monitoring, problems to ensure additionality, problems to safeguard against leakage effects, and lack of long-term accountability if stored SOC is re-emitted. We conclude that soil-based private carbon certificates are unlikely to deliver the emission offset attributed to them and that their benefit for climate change mitigation is uncertain. Additional research is needed to develop standards for SOC change metrics and monitoring, and to better understand the impact of short term, non-permanent carbon removals on peaks in atmospheric greenhouse gas concentrations and on the probability of exceeding climatic tipping points.

Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0-20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state C-13 CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (< 6.3 mu m) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned.

Soil organic carbon (SOC) originates from a complex mixture of organic materials, and to better understand its role in soil functions, one must characterise its chemical composition. However, current methods, such as solid-state C-13 nuclear magnetic resonance (NMR) spectroscopy, are time-consuming and expensive. Diffuse reflectance spectroscopy in the visible, near infrared and mid-infrared regions (vis-NIR: 350-2500 nm; mid-IR: 4000-400 cm(-1)) can also be used to characterise SOC chemistry; however, it is difficult to know the frequencies where the information occurs. Thus, we correlated the C functional groups from the C-13 NMR to the frequencies in the vis-NIR and mid-IR spectra using two methods: (1) 2-dimensional correlations of C-13 NMR spectra and the diffuse reflectance spectra, and (2) modelling the NMR functional C groups with the reflectance spectra using support vector machines (SVM) (validated using 5 times repeated 10-fold cross-validation). For the study, we used 99 mineral soils from the agricultural regions of Sweden. The results show clear correlations between organic functional C groups measured with NMR and specific frequencies in the vis-NIR and mid-IR spectra. While the 2D correlations showed general relationships (mainly related to the total SOC content), analysing the importance of the wavelengths in the SVM models revealed more detail. Generally, models using mid-IR spectra produced slightly better estimates than the vis-NIR. The best estimates were for the alkyl C group (R-2 = 0.83 and 0.85, vis-NIR and mid-IR, respectively), and the O/N-alkyl C group was the most difficult to estimate (R-2 = 0.34 and 0.38, vis-NIR and mid-IR, respectively). Combining C-13 NMR with the cost-effective diffuse reflectance methods could potentially increase the number of measured samples and improve the spatial and temporal characterisation of SOC. However, more studies with a wider range of soil types and land management systems are needed to further evaluate the conditions under which these methods could be used. Highlights Diffuse reflectance spectroscopy was used to characterise and model SOC functional chemistry. NMR derived C functional groups could be modelled with vis-NIR and mid-IR diffuse reflectance spectra. The methods allow for characterisation of SOC chemical composition on whole mineral soil samples. The approach can improve the spatial and temporal characterisation of SOC composition.

Soils are the basis of life on land and the ways in which we manage them for crop production, impact their role, functions and quality. Conventional farming uses industrial inputs to a level that is economically justified, whilst organic farming systems avoid mineral fertilizers and synthetic chemical pesticides. This study investigates the long-term effect of organic and conventional farming systems on soil quality. The DOK trial (bioDynamic, bioOrganic, Konventionell (German for conventional)) running since 1978 in Therwil (CH), compares bioorganic (BIOORG), biodynamic (BIODYN), and conventional (CONFYM) farming systems at two farmyard manure intensities corresponding to 0.7 and 1.4 livestock units per hectare with a purely mineral fertilized system (CONMIN) and an unfertilized control (NOFERT). The treatments in the DOK trial vary in plant protection and receive system-specific organic matter inputs differing in rate and quality. With this work, we revisit the soil organic carbon (SOC) dynamics across 42 years and redefine the previous perception of mainly declining SOC contents after 21 years of organic and conventional management (Fliessbach et al. 2007). After 42 years, we found SOC contents to be increased in BIODYN 1.4 and to a lesser extent also in BIOORG 1.4. CONFYM 1.4 showed stable SOC contents, while systems fertilized with manure of 0.7 livestock units and CONMIN lost SOC. SOC loss was highest in NOFERT. Enhanced biological soil quality under organic and particularly biodynamic management highlights the close link between soil biology and SOC changes. The impact of farming systems on SOC was detectable after 2 decades of continuous management. We conclude that recycling manure at a level of 1.4 livestock units per hectare permits maintenance of SOC levels and that composting manure, as performed in BIODYN 1.4, helps to further increase SOC levels and improve biological soil quality.

The interface between decaying plant residues and soil minerals represents an essential soil microenvironment at which soil organic matter forms. The high amount of microbial products and residues within this hot spot of microbial activity fosters the formation of mineral-associated organic matter. Besides classical quantitative analyses, our understanding of processes controlling soil organic matter formation greatly benefits from microscopic observations and measurements, which provide spatially resolved information at a meaningful scale for microbial processes and for the association between organic and mineral particles. We studied carbon and nitrogen transfer from fresh-plant residues to the mineral soil, through a litter decomposition experiment in an artificial soil mixture. Needles of Norway spruce (Picea abies L.) were placed in microbatch containers filled with an artificial soil mixture free of soil organic matter. Containers were buried in fresh organic layer material from a Norway spruce stand and incubated for 14 and 42 days. We applied nanoscale secondary ion mass spectroscopy (NanoSIMS) to investigate the spatial distribution of mineral and organic compounds at the needle vicinity and into the mineral soil (0-550 mu m from the needle). After 14 days, we depicted the formation of mineral-associated organic matter in the surrounding of the decaying needles. After 42 days, we observed substantial colonization of the needles and the detritusphere by saprotrophic fungi. The fungal hyphae extended into the mineral matrix of the artificial soil acting as vectors for the transfer of litter-derived carbon and nitrogen into the bulk soil. This resulted in an increase of the area covered by organic matter in the detritusphere, with up to 10% of the total investigated area classified as organic matter closely associated with mineral surfaces. Our results provide evidence that the carbon and nitrogen derived from litter decomposition transformed by microorganisms is transferred as mineral-associated organic matter, heterogeneously distributed from the litter source, and still detected 550 mu m away from the latter. The close association of newly formed soil organic matter and fine sized minerals suggests that the formation of mineral-associated OM and likely also microaggregates is directly driven by microbial activity in the vicinity of hot spots for plant carbon input (e.g. the detritusphere).

Conservation tillage is suggested to sequester carbon although a stratification of soil organic carbon rather than a total increase is mostly observed. It is not clear whether conservation tillage in combination with organic farming practices has a higher potential. Beyond, many datasets are biased in terms of sampling depth. A joint sampling campaign in nine European long-term trials considered soil organic carbon stocks until 100 cm soil depth comparing reduced tillage with ploughing under organic farming conditions. First results show a significant increase of carbon stocks in 0-30 cm and also in 0-100 cm depth with the conversion to reduced tillage.

The biogeochemical functioning of soils (e.g., soil carbon stabilization and nutrient cycling) is determined at the interfaces of specific soil structures (e.g., aggregates, particulate organic matter (POM) and organo-mineral associations). With the growing accessibility of spectromicroscopic techniques, there is an increase in nano- to microscale analyses of biogeochemical interfaces at the process scale, reaching from the distribution of elements and isotopes to the localization of microorganisms. A widely used approach to study intact soil structures is the fixation and embedding of intact soil samples in resin and the subsequent analyses of soil cross-sections using spectromicroscopic techniques. However, it is still challenging to link such microscale approaches to larger scales at which normally bulk soil analyses are conducted. Here we report on the use of laboratory imaging Vis-NIR spectroscopy on resin embedded soil sections and a procedure for supervised image classification to determine the microscale soil structure arrangement, including the quantification of soil organic matter fractions. This approach will help to upscale from microscale spectromicroscopic techniques to the centimetre and possibly pedon scale. Thus, we demonstrate a new approach to integrate microscale soil analyses into pedon-scale conceptual and experimental approaches. Highlights Quantification of soil constituents using Vis-NIR spectroscopy. New approach to use resin embedded soil core sections with intact structure. Reproducible quantification of soil constituents important for soil carbon storage. Vis-NIR as promising tool for upscaling from microscale to pdeon scale.

Soils contain more carbon (C) in the form of organic matter (soil organic matter = SOM) than the entire atmosphere and global vegetation combined. They are a central component of the global C cycle and its largest dynamic reservoir. Smart agricultural practices are discussed, on the one hand, as a way to mitigate climate change because they can increase the amount of SOM and thus actively remove C from the atmosphere. On the other hand, all intensively used soils lose C in the long term. The scientific key questions in this context revolve around the extent and dynamics of C storage, as well as the associated stabilization mechanisms involved and effects of agricultural use on the C budget.The DOK experiment is a long-term agronomic field trial near Basel (Switzerland) that compares biodynamic, organic and conventional management systems since 40 years. Within the "DynaCarb" project, we investigate how the management systems affect SOM fractions during the 40-year experimental period. We compare the unfertilized control to a purely mineral, a purely organic, and a combined fertilized, mineral-organic variant (four field replicates each) during six crop rotation cycles. By using a combined density and particle size fractionation, the SOM is separated into particulate and mineral-associated fractions and their development is quantitatively investigated in archived samples from 1982, 1989, 1996, 2003, 2010, and 2017."DynaCarb" investigates the medium- and long-term effects of different agricultural systems on SOM. These results are of great importance for the evaluation of the C sequestration potential of agricultural soils and for the identification of suitable management and fertilization strategies.

Organic matter is an important constituent of soils that controls many soil functions and is of vital importance for ecosystem services like climate regulation and food security. Soil organic matter (SOM consists of a wide spectrum of different organic substances that are highly heterogeneous in terms of chemical composition, stability against microbial decomposition and turnover time. SOM is heterogeneously distributed in the soil profile impeding its fast assessment. A technique to accurately measure SOM quality and quantity with a high spatial resolution in the soil profile is presently lacking. Imaging visible light and near infrared spectroscopy (imVisIR) is a promising technique for the fast and spatially resolved assessment of SOM quality and quantity. In this study, we evaluate the potential of imVisIR to quantitatively map the labile particulate organic matter fraction in undisturbed cores from mineral soils.

In soil, carbonates are important inorganic binding agents, controlling aggregate formation and soil structural stability. Until today, a clear understanding of the aggregate forming mechanisms in calcareous soil is missing. The objective of this study was to elucidate the initial aggregate formation in calcareous loess from the point when soil organic matter content was low and soil properties were mostly controlled by the parent material. We used a space-for-time chronosequence approach on agriculturally reclaimed loess soils in an open-cast lignite mining area in Garzweiler, Germany. We selected six time points from 0 to 24 years after reclamation in order to investigate the early processes of soil aggregate formation. Samples from two sampling depths (1-5 cm and 16-20 cm) were wet sieved into four size classes: silt- and clay-sized fraction (<63 mu m), large microaggregates (63-200 mu m), small macroaggregates (200-630 mu m), and large macroaggregates (>630 mu m). Each aggregate size class was characterized for soil organic carbon, total nitrogen and CaCO3 content. Organic matter amendment induced the formation of large macroaggregates (>630 mu m), but we did not detect an accumulation of soil organic carbon neither in microaggregates nor in the silt- and clay-sized fraction. The silt- and clay-sized fraction contained large amounts of CaCO3 throughout the whole space-for-time chronosequence. This finding shows that in undeveloped loess material, microaggregation is controlled by cementation by carbonates inherited from the loess. These carbonates were likely easily dissolved during contact with water, therefore, aggregates in the early stage of soil development were not water-stable. Soil aggregate formation on young calcareous soils consists of two main mechanisms, 1. fresh soil organic matter predominantly contributes to macroaggregate formation, and 2. intrinsic cementing of loess through carbonates affects microaggregate stability. Therefore, the early stages of soil development in loess material do not show an aggregate hierarchy as we would expect in soils with high silt and clay content, such as Cambisols and Luvisols.