This paper investigates the physical properties of soil samples repacked from aggregate beds and the potential for retrieving representative soil pore properties at the field scale based on shrinkage analysis of a repacked composite sample. This approach was tested on 60 arable fields in Switzerland presenting a large range of soil organic carbon (SOC) and texture. Soil cores constituted from composite samples were repacked at the observed field bulk density. Their pore properties were compared to undisturbed soil samples from the same fields. The soil pore properties were characterized using shrinkage analysis and correlated to soil texture and SOC. The repacking protocol successfully recreated structured samples. The <40‐µm equivalent diameter pore and structural pore size distributions were comparable to those of the undisturbed soils. For pores >40 µm, a larger porosity was measured for repacked samples compared to undisturbed ones, limiting the approach to assess the structure and dynamics of larger soil pores. Furthermore, soil clay content as well as SOC were strong predictors for the samples’ physical properties including pore space. The increase in structural porosity in the repacked sample was inversely proportional to the structural porosity already present in the undisturbed samples. Steeper regression slopes of the pore volume to SOC or clay relationship were observed for the repacked samples. These slope changes were comparable to those observed in soil structure restoration experiments and opposite to those observed in soil compaction trials. We conclude that repacking soil from a composite sample allows us to characterize the potential soil structure condition with respect to its SOC and clay content. The method can be used to assess soil structure quality in response to changes in soil management.

Soil carbon is an important component of the terrestrial carbon cycle and could be augmented through improved soil management to mitigate climate change. However, data gaps for numerous regions and a lack of understanding of the heterogeneity of biogeochemical processes across diverse soil landscapes hinder the development of large-scale representations of soil organic matter (SOM) dynamics. In this Perspective, we outline how understanding soil formation processes and complexity at the landscape scale can inform predictions of soil organic matter (SOM) cycling and soil carbon sequestration. Long-term alterations of the soil matrix caused by weathering and soil redistribution vary across climate zones and ecosystems, but particularly with the structure of landscapes at the regional scale. Thus, oversimplified generalizations that assume that the drivers of SOM dynamics can be scaled directly from local to global regimes and vice versa leads to large uncertainties in global projections of soil C stocks. Data-driven models with enhanced coverage of underrepresented regions, particularly where soils are physicochemically distinct and environmental change is most rapid, are key to understanding C turnover and stabilization at landscape scales to better predict global soil carbon dynamics. Soil carbon cycling is closely linked with landscape complexities in soil properties, climate and land use. This Perspective outlines how soil formation theory could provide insight on landscape-scale soil–carbon interactions as well as carbon sequestration and improve predictions of future soil organic matter dynamics.

### Competing Interest Statement The authors have declared no competing interest.

Soil organic matter (SOM) quantity drives soil bacterial community composition from the regional to global scale. Qualitative characteristics of SOM are known to affect soil bacterial communities in manipulation experiments. However, it remains unresolved how strongly SOM characteristics affect soil bacterial community composition at the macroscale. Here, we investigated how quantity versus qualitative characteristics of SOM shape community composition along a biogeochemical gradient of grassland soils. We assessed relative abundance patterns of soil bacteria and characterised SOM based on scalable methods. Soils with higher SOM content (along a continuum between 0.6% and 18.7% SOC) and acidic pH (along a continuum between pH 4.1-6.7) hosted fewer narrowly distributed taxa (i.e., taxa occurring in few sites) and therefore had lower bacterial alpha diversity. We could explain a larger fraction of bacterial community composition (up to 59.6% of 16S rRNA reads) in these soils. Consequently, we understand community composition in low-SOM soils less than in high-SOM soils, because the drivers of narrowly distributed taxa remain poorly understood. Qualitative SOM characteristics did not strongly affect biogeographical patterns of widely distributed soil bacterial taxa. This suggests that broad aspects of SOM quality do not dominate soil bacterial community composition at the investigated macroscale.

Agricultural terraces, being among the volumetrically largest and most common man-made landforms, have been widely implemented to support essential soil ecosystem services, e.g., erosion control, soil nutrient, and water retention, and have had an essential impact on soil organic carbon (SOC) stock and its exchange with the atmospheric C. However, the direction and magnitude of this impact remain highly uncertain. By integrating the broad-scale field observations of 14 terrace sites across the EU with a global data synthesis, we demonstrate that the effectiveness of terracing-driven SOC sequestration potential is intricately controlled by climate conditions that govern in-return soil properties. Our findings reveal that the terracing practices represent a promising land management strategy for enhancing SOC sequestration, but also that risks of SOC loss exist when building terraces under arid climate, where they could be potentially very beneficial to crop productivity and SOC storage. We recommend that future terrace construction should integrate water and nutrient recycling techniques to ensure soil moisture and nutrient availability, enhancing land productivity and maximizing SOC sequestration potential. Our data suggest that promoting the recovery of the lost topsoil C during terrace construction through increasing C inputs and C use efficiency, i.e., straw return and nutrient amendment is an efficient way to counteract initial SOC losses.

Secondary forests will increasingly dominate tropical forest landscapes in the decades to come. Understanding how local biogeochemistry impacts regrowth trajectories is paramount, especially if this biogeochemistry is impacted by land-use change. In this study, we employ three lines of evidence from central African secondary forest succession sequences that suggest that cations might be a limiting and vulnerable resource to sustain secondary forest regrowth in the tropics. First – along succussion, our analysis reveals that atmospheric phosphorus supply exceeds demand during forest succession, while plant base cation demands are met predominantly through depletion of soil stocks. As such, soil nutrient metrics indicate an increase in available phosphorus along the succession, contrasting with a decrease in available cations. Coincidentally, fine root, foliar, and litter stoichiometry collectively demonstrate a decline in tissue calcium concentrations relative to nitrogen and phosphorus during succession. These findings collectively suggest that calcium becomes a progressively scarce resource in central African forests during secondary succession. Second – also along succession, we show a substantial shift in ecosystem cation storage from soil to woody biomass over succession, while not for nitrogen or phosphorus, rendering it a vulnerable nutrient in the context of land-use change scenarios involving woody biomass export. From independent data, we also show that with increasing repeated clearing, the total soil cation stocks are depleted, while we see no such consistent effect for nitrogen or phosphorus. Coincidentally, at a catchment scale, we see that cation losses increase as catchments get increasingly impacted by land-use change. Third – based on a pot experiment, tree growth seems limited first and foremost by cations, and then by nitrogen. This again reiterates that cations might be an important and overlooked limiting element for sustaining plant growth in highly depleted tropical soils. Collectively, this work calls for an expanded perspective on nutrient dynamics and highlights the vulnerability of cations in the face of changing land-use scenarios, with potentially important sustainability issues in the long-term – especially if the secondary regrowth potential is lowered.

Intensifying effects of global climate change have spurred efforts to enhance carbon sequestration and the long-term storage of soil organic carbon (OC). Current soil carbon models predominantly assume that inputs of OC are biospheric, that is, primarily derived from plant decomposition. However, these overlook the contribution of OC from soil parent material, including petrogenic organic carbon (OCpetro) from OC-bearing (meta-)sedimentary bedrock. To our knowledge, no soil carbon model accounts for the inputs of OCpetro to soils, resulting in significant gaps in our understanding about the roles OCpetro plays in soils. Here, we call for cross-disciplinary research to investigate the transport and stability of OCpetro across the bedrock–soil continuum. We pose four key questions as motivation for this effort. Ignoring the inputs of OCpetro to soils has significant implications, including overestimating biospheric carbon stocks and turnover times. Furthermore, we lack information on the role that OCpetro may play in priming microbial communities, as well as the impacts of land management on OCpetro stocks.

Large areas of European peatlands have been drained for agriculture, but drained organic soils are a strong source of carbon dioxide (CO2). Reinstalling high water tables would inhibit further peat oxidation and reduce CO2 and nitrous oxide (N2O) emissions, but most cash crops do not grow in waterlogged conditions. Paddy rice cultivation could offer a new option for continuing the agricultural use of these soils under wet conditions. However, paddy rice cultivation is known to be a strong source of methane (CH4), which might cancel out the potential climate benefit from reduced CO2 and N2O emissions. The main aim of this study was, therefore, to quantify for the first time the greenhouse gas (GHG) balance of paddy rice grown on organic soil in the temperate climate zone of the Swiss Plateau.In an outdoor mesocosm experiment, we measured the greenhouse gases CO2, CH4, and N2O with manual chambers on a weekly to biweekly interval for one year. During the experiment, rice (Oryza sativa L.) was cultivated under flooded conditions with mid-season drainage on organic soil. As a reference treatment, ley was grown on drained organic soil (water table -100 cm).Preliminary results from the growing season (April - October) including planting and harvest suggest that the overall GHG balance of paddy rice cultivation on organic soil (9.3 ± 1.9 t CO2 eq. ha-1 including harvest exports) was significantly lower than of ley grown on drained organic soil (27.9 ± 5.0 t CO2 eq. ha-1 including harvest exports). This difference was mainly attributed to the strong reduction in ecosystem respiration under flooded conditions compared to ley on drained organic soil. Paddy rice cultivation was a source of methane (49.2 ± 19.7 kg CH4 ha-1), while the drained organic soil covered with ley was a CH4 sink (-0.6 ± 0.1 kg CH4 ha-1). The flooded conditions in the paddy rice mesocosms significantly lowered N2O emissions (0.7 ± 0.3 kg N2O ha-1) compared to drained grassland (4.7 ± 3.1 kg N2O ha-1). N2O and CH4 accounted for 16.0 ± 6.8 % of the total GHG balance in the rice on organic soil treatment, whereas it was only 4.9 ± 2.6 % in the ley on drained organic soil.Together, we show that paddy rice cultivation on organic soil is a valid alternative to upland agriculture in the temperate zone and offers significant GHG emission reduction potentials.

The recent expansion of commercial agriculture in the Miombo woodlands of central Africa has led to widespread levelling of termite mounds. These mounds contain significantly lower soil organic carbon (SOC) than surrounding soils, and their levelling could largely reduce SOC content in the plough layer, which remains understudied. We aim to investigate the effects of mound levelling on SOC of the plough layer in a 1.5 km2 plot used for commercial farming and quantify the contribution of pre-existing termite mounds to SOC variation in the levelled cropland. Before and after levelling, we conducted unmanned aerial vehicle (UAV) surveys with structure-from-motion (SfM) technique, and paired soil sampling (0−25 cm) in between-mound areas. Termite mounds were regularly distributed but morphologically heterogeneous in the plot, with volumes ranging from 7.2 m3 to 820.9 m3. Large termite mounds clustered in areas with higher topographic wetness index (TWI). Three years after levelling, SOC content in the plough layer of the plot overall reduced by 26

Deforestation for cropland expansion in the sloping landscapes along the East African Rift system causes severe soil erosion and thus the loss of fertile, organic rich topsoil. However, the varying effect of land degradation in the region on soils developed from different parent material - which may influence soil fertility and carbon stabilization - are still largely unknown. To examine these factors, we compared soil organic carbon (SOC) and soil fertility indicators in undisturbed forest topsoils with cropland hillslope topsoils along a chronosequence after deforestation (2–7, 10–20, 20–40, > 60 years of cropping, land abandonment) on mafic (South Kivu, Democratic Republic of Congo) and felsic parent material (western Uganda). From previous studies, we expected higher soil fertility and SOC contents and therefore slower degradation on mafic soils due to the higher amounts of clay and pedogenic metal phases which stabilize SOM and thus further maintain soil fertility. However, we found similar SOC contents on both parent materials and a consistent decrease with time after deforestation. SOC values were significantly lower in soils that were cleared more than 60 years ago, compared to cropland which was cleared 2–7 years ago and nearby undisturbed forest topsoils (0–10 cm soil depth). While the effective cation exchange capacity (ECEC) positively correlated with SOC in soils on felsic parent material, this was not observed in soils with mafic parent material, where it correlated with mineralogical proxies (total reserves in bases). In both regions, SOC did not correlate with clay content. Mid-Holocene carbonate volcanism appears to have offset soil degradation in the felsic region, contributing to higher pH and ECEC and impeding land abandonment due to the maintenance of acceptable soil fertility levels. Surprisingly, abandoned cropland sites in the mafic region still had an average SOC content of 14–29 g kg-1 in topsoils, likely due to strong fixation of SOC with reactive metal phases; however, they were characterized by extremely low pH values and high Al3+ mobility, combined with low available nutrient status. Our results emphasize that soil fertility and carbon stabilization are reliant on the mineral composition of the underlying parent material, even in deeply weathered soils of the humid tropics. Soil organic matter in degraded tropical cropland soils does not appear to be a reliable indicator of soil fertility.

Paddock trails offer horses the possibility to follow their natural urge to move and to behave interactively in a group association. To create appropriate conditions all year round, the installation of paddock grids is a common solution to avoid muddy trails and to prevent horses from injuries. The impact of horses on the soil on those trails is relatively unknown. In this study, we quantified the impact of horses kept on paddock trails on key soil quality indicators (soil bulk density, microbial biomass and soil organic carbon (SOC)) of the topsoil (0–0.3 m depth), and evaluated possible protective effects of paddock grids in an on‐farm study across 17 sites. We found significantly higher soil bulk density at 0–0.1 m depth, significantly lower soil microbial biomass in the 0–0.2 m layer, and significantly lower SOC contents at 0–0.2 m depth in paddock trails compared to ungrazed control sites. Comparing trails with and without paddock grids showed no significant difference in soil bulk density at any sampling depth, but significantly lower soil microbial biomass and a significantly higher soil organic carbon to nitrogen ratio (soil CN‐ratio) in trails with paddock grids compared to trails without grids. We could not find any impact of soil texture on the response ratio of the measured soil quality indicators, regardless of the type of trail (with or without paddock grids). Although we found overall lower mean soil bulk densities in trails with paddock grids, the difference to trails without paddock grids was not significant. A trend of an increase in bulk density over time was found for trails without paddock grids but not for trails with paddock grids, indicating that paddock grids might have a protective effect over time. In summary, our results suggest that soil quality is negatively affected by horses on paddock trails but that the effects are restricted to the top 0.2 m of soil. Furthermore, the results indicate that paddock grids were not able to prevent the negative effects of horses trampling but weakened them.

Soil microbes perform important functions in the soil organic carbon (SOC) cycle and soil microbial decomposition activity is a major determinant of the carbon budget of a soil. It is well-established that soil microbial physiology is directly affected by temperature and moisture. However, it is less clear to what extent the environmental setting (i.e. long-term climatic conditions, soil physicochemistry) vs. the microbial actors (i.e. soil bacterial and fungal community composition) control the cycling of SOC in the absence of strong direct physiological constraints such as temperature and moisture limitation.To address this knowledge gap, we used 35 grassland topsoils (0 – 10 cm) from 10 WRB major soil groups along a north-south transect in Chile, which ranged from arid steppe to tundra. We compiled climatic data and relevant physicochemical soil properties, together with an in depth characterization of OM quality. We then incubated the soils for 1 week in conditions favorable for microbial activity (20 °C, 50 % of water holding capacity). After incubation, we quantified soil microbial carbon and nitrogen, enzyme kinetics of three groups of relevant extracellular enzymes, basal heterotrophic respiration as well as microbial growth rates and carbon use efficiencies by incorporation of 18O into DNA. In addition, we characterized the microbial actors by DNA extraction and Illumina barcoding of a region of the 16S rRNA gene (bacteria) and a section of the ITS region (fungi). Finally, to investigate how strongly the measured microbial SOC functions were linked with the environmental setting vs. the microbial actors, we applied three different cross-validated regression approaches.The resulting data highlights the links between environment, microbial community composition and SOC cycle functions under conditions without direct temperature and moisture limitation. Our findings show that the environmental setting controlled the amount of microbial biomass, and in extension biomass dependent SOC cycle functions such as heterotrophic respiration. In contrast, microbial community composition was a better predictor of SOC cycle functions that are independent of microbial biomass such as carbon use efficiency and relative microbial growth rates. These insights help to disentangle the roles of the environmental setting and the microbial actors in the context of microbial SOC cycle functions.

Tropical Africa is globally one of the most sensitive regions for soil erosion and is characterised by an important yield gap. Rapid population growth is expected to triple food demands in Sub-Saharan Africa by 2050. These rising food demands need to be met by cropland that is highly prone to soil erosion. In particular, the White Nile-Congo ridge region between the DR Congo and Uganda is a hotspot for issues relating to food security linked to massive soil degradation due to steep terrain, highly erosive rainfall and low soil cover conditions. Despite its importance, most soil erosion studies in the region are based on plot or large-scale modelling studies. Both approaches lack information on inter-field connectivity processes, which are especially important in smallholder farming structures where the average field size is 0.1 ha. To address this, a UAV-based monitoring campaign was carried out over different cropland sites (two in the DR Congo and two in Uganda) at high spatial and temporal resolution. The campaign covered more than 500 individual fields which were monitored twice per month (for two years) using UAV-based aerial photography to get insights into event-based rill erosion processes and the role of landscape connectivity. The aerial photography data was classified according to its field condition: (i) vegetation covered, (ii) bare soil without signs of rill erosion, (iii) field with rill erosion. The results highlight the relevance of land cover patchiness due to smallholder farming structures on inter-field connectivity with rill erosion often discontinuing downslope across field boundaries. Therefore, rill development is highly localised and affects individual fields. We further conclude that rill erosion in the White Nile-Congo ridge region is not an episodic process but takes place regularly during the rainy season as a result of high frequency erosive rainfall (on average 20 erosive rainfall events per rainy season) falling on bare soil in fields that are left fallow for individual cultivation periods. Soil erosion dynamics in the study area are complex and controlled by processes that are challenging to be represented in large scale predictions on soil degradation.

Abstract Timely crop monitoring and yield prediction are essential in guiding management decision making. The aim of the study was to estimate the agronomic traits of paddy rice (Oryza sativa L.) using unmanned aerial vehicle (UAV)‐multispectral imaging. A randomized complete block design field experiment with a split–split plot arrangement was set up in the Ruzizi plain, Democratic Republic of Congo (DRC). Spectral imaging data were collected at rice tillering and panicle initiation stages. Predictive analysis of rice agronomic traits was performed using linear and decision tree‐based machine learning techniques. Paddy rice trait predictions were critically sensitive to the timing of image acquisition but not largely affected by the model. The most accurate predictions were made at rice panicle initiation stage, with R2 values of 0.62, 0.65, and 0.75 for yield, aboveground biomass, and plant nitrogen (N) uptake, respectively. The visible atmospherically resistant index (VARI), modified chlorophyll absorption in reflective index, and ratio vegetation index, along with near infrared and green bands, played a critical role in predicting paddy rice N uptake and yield. The same spectral features associated with crop height and canopy data were essential for predicting paddy rice aboveground biomass. UAV‐multispectral data were able to assess agricultural intensification strategies at field/landscape scale irrespective of soil types, watering regimes, and cultivars. Special consideration should be attributed to VARI, as it enables economical prediction of paddy rice traits. The UAV technologies are therefore reliable tools for monitoring rice production and can be applied in agricultural extension in the DRC.

Phosphorus (P) is crucial for ecosystem functioning, yet primary productivity in many tropical regrowth forests on highly weathered soils is assumed to be limited by P availability. Here, we used an isotope pool dilution (IPD) technique to quantify gross inorganic P (Pi) transformation rates along secondary forest succession trajectories in the central Congo Basin to assess land-use change effects on soil P cycling. We considered gross Pi desorption and gross organic P (PO) mineralization together as a joint influx of Pi into the bicarbonate-extractable Pi pool (BIC-extractable Pi; PBIC), termed "gross mobilization", while gross Pi sorption and gross microbial Pi uptake were treated as a joint efflux of Pi from the PBIC pool, referred to as "gross immobilization". Average gross fluxes ranged between 0.11 and 0.26 mu g P g-1 d-1, which is at the lower end of globally observed soil gross P cycling rates. No significant trends in soil BIC-extractable Pi or Pi fluxes were found along secondary forest succession trajectories, and PBIC showed a rapid turnover time (ca. 2 days). Environmental controls on gross Pi transformation rates varied between sites with the highest gross rates noted for sandy soils, while heavier, clayey soils resulted in lower rates, as well as lower PBIC. Significant predictors for gross Pi fluxes included variables related to microbial activity (tracer recovery in microbial biomass and microbial biomass carbon), as well as the PBIC pool size and dissolved organic carbon, reflecting both biotic and abiotic controls. These findings highlight the importance of microbial and physicochemical characteristics for P cycling in tropical forest soils, especially in landscapes impacted by slash-and-burn agriculture.

Soil organic carbon (SOC) dynamics in temperate regions are highly affected by lateral soil fluxes induced by soil erosion. SOC dynamics in eroding tropical cropland systems characterized by deeply weathered soils and heterogeneous small-scale subsistence farming structures, however, are not well understood yet. Along topographic gradients in the East African Albertine rift region, we investigated the differences in SOC stocks and persistence for the upper meter of tropical soils developed from geochemically distinct parent materials and cultivated by subsistence farmers. We show that SOC stocks and persistence do not follow topography-driven patterns expected from research on less weathered, more fertile soils of temperate climate zones and more large-scale farming systems. At all investigated topographic positions, the SOC stocks were low compared to temperate regions while variability of stocks was high in both top- and subsoil. Full profile (0 - 100 cm) SOC stocks ranged from 256.1 t C ha(-1) to 297.3 t C ha(-1) at plateaus, from 224.4 t C ha(-1) to 276.1 t C ha(-1) at slopes and from 305.1 t C ha(-1) to 366.0 t C ha(-1) at footslopes. Independent of soil parent material and unless situated on very steep slopes (>15 % slope steepness), SOC stocks in eroding positions are, therefore, similar to those in non-eroding landscape positions and stable at a low level despite heavy erosion. Our results further suggest that deposition of eroded topsoil material at footslopes only slightly increases SOC stocks. Therefore, SOC stocks in this rapidly eroding tropical systems seem not to be heavily altered by soil redistribution while other soil parameters indicate heavy signs of soil disturbance. Tropical soil features and the distinctiveness of small scale subsistence farming practices create an extremely patchy and variable distribution of SOC, which requires us to rethink the way these landscapes can be modelled to represent C dynamics.

The concept of soil organic carbon (SOC) saturation emerged a bit more than 2 decades ago as our mechanistic understanding of SOC stabilization increased. Recently, the further testing of the concept across a wide range of soil types and environments has led some people to challenge the fundamentals of soil C saturation. Here, we argue that, to test this concept, one should pay attention to six fundamental principles or “rights” (R's): the right measures, the right units, the right dispersive energy and application, the right soil type, the right clay type, and the right saturation level. Once we take care of those six rights across studies, we find a maximum of C stabilized by minerals and estimate based on current data available that this maximum stabilization is around 82 ± 4 g C kg−1 silt + clay for 2 : 1-clay-dominated soils while most likely being only around 46 ± 4 g C kg−1 silt + clay for 1 : 1-clay-dominated soils. These estimates can be further improved using more data, especially for different clay types across varying environmental conditions. However, the bigger challenge is a matter of which C sequestration strategies to implement and how to implement them in order to effectively reach this 82/46 g C kg−1 silt + clay in soils across the globe.

Human excreta-derived fertilizers (HEDFs) are organic fertilizers made from human excreta sources such as urine and feces. HEDFs can contribute to a sustainable and circular agriculture by reuse of valuable nutrients that would otherwise be discarded. However, HEDFs may contain contaminants such as pharmaceuticals, persistent organic compounds, heavy metals and pathogens which can negatively affect plant, water and soil quality. Moreover, consumer prejudice, farmer hesitance and strict regulations can discourage utilization of HEDFs. Here, we conducted a thorough review of published literature to explore the opportunities and challenges of using HEDFs in agricultural systems by evaluating the suitability of human excreta as a nutrient source, their typical contaminant composition, how they affect the quality of crops, soils and water and their societal impact and acceptance. We found that HEDFs are suitable nutrient-rich fertilizers, but may contain contaminants. Processing treatments increase the fertilizer quality by reducing these contaminants, but they do not remove all contaminants completely. Regarding the environmental impacts of these fertilizers, we found overall positive effects on crop yield, soil nutrients, plant-soil-microbe interactions and plant pathogen suppression. The use of HEDFs reduces water contamination from sewage waste dumping, but nutrient leaching dependent on soil type may still affect water quality. We found no increased risks with human pathogens compared to inorganic fertilizers but identified processing treatment as well as crop and soil type significantly affect these risks. Lastly, we found that public acceptance is possible with clear regulations and outreach to inform consumers and farmers of their multi-faceted benefits and safe usage after processing treatments. In summary, this review emphasizes the great potential of HEDFs and its positive impacts on society, especially in regions where conventional fertilizers are scarce, while also stressing the need for adaptation to specific soils and crops.