Goswin Heckrath

Rewetting of organic lowland soils may result in a large and prolonged phosphorus (P) load to the aquatic environment as legacy iron (Fe) is reductively dissolved and the associated P released. We hypothesize that the remaining P sorption capacity in anaerobic soils determines P mobilization. To study P mobilization and transport under steady-state flow, a convective discharge experiment was conducted in the laboratory at 10 °C. Sixty undisturbed soil columns were taken from two different soil depths (5-25 cm, 25-50 cm) in six Danish lowlands characterized by wide variability in P, as well as Fe and aluminum oxide contents. The column experiment used oxygen-free deionized water flowing at a rate of 1 mm per hour over a period of 28 days. The cumulated effluent was analyzed for different P and Fe forms, dissolved organic carbon (OC), ammonium, nitrate, and other solutes on day 5, 14 and 28 during the experiment. Active flow volume and non-equilibrium flow conditions were determined with the help of a tritium tracer. Upon completing the leaching experiments, the soil columns were dismantled for determination of relevant soil properties.Across the six sampling sites, OC content varied strongly with subsoil OC% (1 – 49%) consistently exceeding topsoil OC% (1 – 44%). Molybdate reactive P (MRP) leaching generally followed site-specific OC patterns, indicating a strong link between C availability and P mobilization. Sites with higher OC showed elevated MRP leaching rates, and higher MRP release, especially in topsoils. In subsoils, MRP leaching was lower and less variable across sites. Columns remained strongly anaerobic during the experiment. Iron-poor sites showed higher MRP leaching. Release rates of MRP declined sharply with increasing molar ratios of bicarbonate-dithionite or oxalate-extractable Fe and P (Fe:P) indicating strong sorption control by Fe oxides. This effect was much more pronounced in topsoils. Saturated hydraulic conductivity also varied substantially among and within sites, ranging from 0.04 to 156 cm d⁻¹. Hydrological conditions further influenced P mobilization: higher flow rates and short residence times caused limited reductive Fe(III) dissolution and MRP release, whereas prolonged residence under low-flow conditions enhanced Fe(II) and MRP release. Additionally, P release to the aqueous phase remained low when the soil’s residual sorption capacity (RSC) exceeded 100 mmol kg⁻¹. In general, we observed lower P release rates compared to those typically reported for batch experiments with similar soils. We expect that our findings will support improved modeling of P export from rewetted organic lowland soils.

The EU’s proposed Soil Monitoring Law recognizes soil erosion as major threat to soil health and requests Member States to monitor different soil erosion processes. Amongst the major soil erosion processes tillage erosion has received rather limited attention. Unlike water and wind erosion, whose effects are often easily visible in the landscape, the extent and severity of erosion caused directly by soil tillage only become evident after decades of tillage through spatial variations in soil properties. Tillage redistributes large amounts of soil from convexities to concavities within fields of rolling topography thus driving a spatially heterogeneous evolution of soil nutrient and carbon stocks. Currently, the few tillage erosion models available are based on a modest number of field surveys and tillage tracer experiments, and we are lacking operational tools for monitoring tillage erosion and its impact on soil health in the long-term.To explore the potential impact of tillage erosion on the crop land in Denmark we have done a scenario analysis comparing the output from a robust soil redistribution model with remote sensing data of topsoil carbon contents. Our study aimed to (1) map tillage-induced soil redistribution across Denmark at a 10-meter resolution and (2) assess its impact on estimating topsoil organic carbon (SOC) content on arable land. Running the WaTEM model with a LiDAR-derived DEM smoothed to different degrees and assuming a typical tillage intensity, we estimated tillage-induced annual soil redistribution rates. We then used Sentinel 2A-derived bare soil composites and other co-variates together with the soil redistribution rates as predictors for mapping SOC via machine learning.Our modelling results showed that without smoothing the 10-m resolution DEM, 23% of the arable land in Denmark had tillage-induced soil loss rates >2.5 t ha-1 a-1 while 12% exceeded 5 t ha-1 a-1. At the highly eroding sites, measured plough layer SOC contents obtained from a national survey tended to be lowest. Soil redistribution modelled with the less smoothed DEM showed stronger correlations with the bare soil composite bands in erosional zones. While the bare soil composite was the main predictor for SOC contents, tillage erosion rate was the only other important predictor at a national scale.In lack of a mechanistic model for mapping the effect of tillage on SOC stock evolution and other soil properties on the arable land across Denmark, our scenario analysis highlights the unsustainability of current intensive tillage practices. To comply with the Soil Monitoring Law the development of operational tools for mapping actual tillage erosion and its impact on soil health must be prioritized.

Macropore transport is an important process of phosphorus (P) loss from tile-drained agricultural land to surface waters where P inputs may cause accelerated eutrophication. Many laboratory experiments or plot studies have shown that P loss by macropore transport increases with increasing concentrations of mobilizable P in the topsoil. However, operational models that quantify the risk of P losses by macropore transport based on typically available information on soil properties, including P status and soil hydrological properties, are currently lacking.This study has collated and analyzed comprehensive existing data from standardized column-leaching experiments with 193 topsoils from different locations in Denmark. In addition to general physical and chemical soil properties including soil P pools, water, and P transport were measured on the large undisturbed soil columns. This data has been used to investigate relationships between P loss and soil properties under varying degrees of macropore transport. Specifically, we have used two statistical methods to analyze relationships between variables and to explore predictive models – multiple linear mixed models (MLMM) and structural equation modeling (SEM). The latter technique allows for testing complex causal relationships among observed and latent variables.Our SEM approach has so far yielded rather poor model fits, and the model structures for estimating the loss of dissolved and particulate P from the columns were characterized by low significance. This was partly due to missing data. In contrast, different MLMM fitted the measured dissolved and particulate P losses satisfactorily. Water-extractable P and saturated hydraulic conductivity were the most important variables for estimating dissolved P losses, while colloid mobilization in soils and tritium leaching breakthrough time explained particulate P losses to a large degree.Our initial statistical analyses show that P loss in dissolved and particulate form from large columns under macropore runoff scenarios can be reasonably explained by soil properties that are typically mapped in Denmark. This approach could bridge empirical and mechanistic modeling and facilitate mapping the risk of P loss by macropore transport.

Intensive agriculture and artificial drainage have transformed natural peatlands into significant sources of greenhouse gas emissions as well as phosphorus and nitrogen pollution, the latter leading to an increased risk of eutrophication of adjacent water bodies. To address this issue and comply with the EU Water Framework Directive, restoring peatlands is a key strategy involving widespread rewetting to re-establish their roles as nutrient and carbon sinks. To achieve restoration objectives, it is essential to analyse and understand the temporal and spatial variability in porewater composition within natural and rewetted peatlands, with a specific focus on the influence of peatland type (bog or fen) and peat degradation status. Hence, an extensive international field survey of soil water and bulk soil was conducted from 1997 to 2017 on 60 natural and rewetted peatlands (both bogs and fens) in Germany, Poland, Estonia, Sweden, and the United Kingdom. The anoxic porewater samples were collected from water-saturated soil layers between 0 and 0.6 m depth. To quantify the concentrations of various chemical compounds in porewater at each location, at least three samples were taken using dialysis samplers within a spatial range of 5 to 10 meters. Selected sites were monitored for seasonal changes over a post-wetting period of 10 to 20 years. The results show significant differences in peat characteristics of upper soil layers from rewetted peatlands and natural peatlands, with the lowest values for nutrient contents, particularly in bogs. Notably, rewetted peatlands did not consistently display higher pore water concentrations of dissolved compounds compared to natural peatlands. However, in heavily drained and rewetted fens, anoxic pore waters exhibited concentrations of soluble reactive phosphorus (SRP), ammonium, and dissolved organic carbon one to two orders of magnitude higher than those in their natural counterparts. For example, SRP concentration in highly degraded peatlands ranged from 0.54 mg/L to 18.9 mg/L compared to 0.01 mg/L to and 3.6 mg/L in the natural peatlands. Weakly drained peatlands had, in some cases, slightly higher concentrations of dissolved substances compared to natural peatlands, but the differences were not statistically significant. Therefore, the research highlights the importance of porewater composition concerning the type of peatland, its degradation status, and spatial-temporal fluctuations in restored peatlands.

Macropore flow in structured soils is an important process in relation to the transport of water, contaminants and nutrients in the soil. A close relation exists between hydraulic conductivity K(h) near saturation and the potential of macropore flow. At the same time, tracer breakthrough experiments in the laboratory can determine the degree of macropore flow. In this study, we aim to investigate a direct link between tracer breakthrough characteristics and soil hydraulic properties (SHPs) of structured soils, which may explain spatial variation of solute transport in soils. We used SHPs and tracer breakthrough characteristics for 71 undisturbed topsoil columns (19 cm height, 20 cm diameter) from Denmark. We defined K10 (near-saturated hydraulic conductivity) as K(h) at a matric potential (h) of −10 cm and used logarithmic transformation, logK10. On the same soil columns, we calculated the 5%, 25%, and 50 % arrival times (AT) as the percentage of the cumulative relative mass of tracer percolating through the bottom of the soil column. The regression analyses proved significant positive relation between logK10 and 5% AT, 25% AT, and 50 % AT. The saturated hydraulic conductivity appeared to be less critical for the shape of the tracer breakthrough curves. In addition, the 5% AT, 25% AT, and 5 0%AT correlated with soil pF values at 1.7, 2.0, and 2.5 (volumetric water content at h equal to −100 cm, −300 cm, and −500 cm, respectively) showing significant negative correlations. Linking SHPs with tracer breakthrough characteristics on large intact columns thus proves highly useful for characterizing soil macropore functions.

Phosphorus (P) losses from artificially drained agricultural land can locally contribute to surface water eutrophication. These losses are a function of hydrological processes and long-term P accumulation in soils due fertilization practices, which is why agronomic mitigation options tend to be ineffective. However, as subsurface drainage systems concentrate water flow spatially, drainage filter technologies represent a potentially cost-effective end-of-pipe mitigation practice for P losses. The aim of this study was to test such a compact, full-scale P filter system under field conditions.The system is located in the Fensholt catchment, Denmark, and has two main compartments: a sediment filter for retaining particulate P and a porous reactive filter consisting of iron-coated sand (ICS) for dissolved P. A pump feeds drainage water from an adjacent ditch into the filter system at flow rates of typically 1-1.5 l s-1. Hydraulic loading of the system and drainage water composition were monitored continuously on a daily basis to evaluate system performance. Measurements of total P (TP), total dissolved P (TDP) and turbidity (NTU) were done. Suspended sediment was estimated in all water samples from turbidity measurements.During the runoff season October 2020 to June 2021 the hydraulic load to the filter system was 18000 m3 corresponding to an average hydraulic retention time (HRT) for the sediment filter of 92 minutes. Total P concentrations in drainage water at the system inlet varied substantially between 0.03 and 2.47 mg P l-1, while TDP varied between 0.04 and 0.84 mg P l-1. On average TDP represented 60% of TP. The sediment filter retained 71% and 64% of the estimated sediment and PP load, respectively. However, occasionally TDP was remobilized from the sediment filter in late spring due to chemical and biological processes. The TDP retention in the ICS filter averaged 51% for the drainage season. On a monthly basis TP retention in the filter system varied between -33% and 88% averaging 61% in 2020/21. This compares positively with other end-of-pipe solutions such as constructed wetlands which tend to have lower TP retention efficiencies under Danish conditions. However, the effective storage capacity of the compact P filter system has to be better understood including the mechanisms of potential P release processes and the required frequency of filter cleaning.

Conventional wet chemical methods for the determination of soil phosphorus (P) pools, relevant for environmental and agronomic purposes, are labor-intensive. Therefore, alternative techniques are needed, and a combination of the spectroscopic techniques—in this case, laser-induced breakdown spectroscopy (LIBS)—and visible near-infrared spectroscopy (vis-NIRS) could be relevant. We aimed at exploring LIBS, vis-NIRS and their combination for soil P estimation. We analyzed 147 Danish agricultural soils with LIBS and vis-NIRS. As reference measurements, we analyzed water-extractable P (Pwater), Olsen P (Polsen), oxalate-extractable P (Pox) and total P (TP) by conventional wet chemical protocols, as proxies for respectively leachable, plant-available, adsorbed inorganic P, and TP in soil. Partial least squares regression (PLSR) models combined with interval partial least squares (iPLS) and competitive adaptive reweighted sampling (CARS) variable selection methods were tested, and the relevant wavelengths for soil P determination were identified. LIBS exhibited better results compared to vis-NIRS for all P models, except for Pwater, for which results were comparable. Model performance for both the LIBS and vis-NIRS techniques as well as the combined LIBS-vis-NIR approach was significantly improved when variable selection was applied. CARS performed better than iPLS in almost all cases. Combined LIBS and vis-NIRS models with variable selection showed the best results for all four P pools, except for Pox where the results were comparable to using the LIBS model with CARS. Merging LIBS and vis-NIRS with variable selection showed potential for improving soil P determinations, but larger and independent validation datasets should be tested in future studies.