Soils In Denmark Research Articles

Importance of small geological features for simulated spatial patterns in tile drain flow in Fensholt catchment of Denmark

Study regionFensholt, Denmark. Study focusTile drains are commonly used in agricultural fields with loamy soils in Denmark to improve crop yield by removing excessive water. Spatial patterns of drain flows are controlled by the climate, geology, topography, and tile installation. We assessed the combined effect of topography and geology on spatial pattern of tile drain flows in 10 m resolution using numerical modelling. We developed three groundwater models using different geological models by integrating high-resolution data from geophysical methods with field estimated hydraulic conductivity. New insightsThe mapping and modelling revealed small geological features of higher hydraulic conductivity in clayey-till. The results showed that the spatial patterns of drain flows to recharge ratio (drainage fraction, DF) are driven by topography; the models had a high DF in local depressions and a low DF in local hills. The DF was related to the Topographical Position Index (TPI), suggesting that the DF is controlled by small-scale topography both upstream and downstream of the study area. We found that geology amplifies the spatial patterns of tile drain flows; a higher hydraulic conductivity relative to a lower hydraulic conductivity increases the change of tile drain flow for a one-unit change in the TPI. This was attributed to a change from small-scale flow systems to field-scale flow systems. The study suggested that topography helps to delineate high and low DF while geology controls the magnitude of DF. The study emphasized the importance of mapping and modeling of geology for managing moraine agricultural areas that can be found in parts of North America and Scandinavia for agricultural water management.

Limitations of farm management data in analyses of decadal changes in SOC stocks in the Danish soil‐monitoring network

AbstractChanges in soil organic carbon (SOC) storage in agricultural land are an important part of the Land Use, Land‐Use Change and Forestry component of national greenhouse gas emission inventories. Furthermore, as climate mitigation strategies and incentives for carbon farming are being developed, accurate estimates of SOC stocks are essential to verify any management‐induced changes in SOC. Based on agricultural mineral soils in the Danish soil‐monitoring network, we analysed management effects on SOC stocks using data from the two most recent surveys (2009 and 2019). Between 2009 and 2019, the average increase in SOC stock was 1.2 Mg C ha−1 for 0–50 cm despite a loss of 1.2 Mg C ha−1 from the topsoil (0–25 cm), stressing the importance of including deeper soil layers in soil‐monitoring networks. Comparing all four national surveys (1986, 1997, 2009, 2019), the mean SOC stock of mineral soils in Denmark appears stable. The change in SOC stock between 2009 and 2019 was analysed in detail in relation to management practices as reported by farmers. We found that the effects of single management factors were difficult to isolate from co‐varying factors including soil parameters and that the use of farm management data to explain changes in SOC stocks observed in soil‐monitoring networks appears limited. Uncertainty in SOC stock estimates also arises from low sampling frequency and statistical challenges related to regression to the mean. However, repeated stock measurements at decadal intervals still represent a benchmark for the overall development in regional and national SOC storage, as affected by actual farm management.

Quantification of CO2 uptake by enhanced weathering of silicate minerals applied to acidic soils

The application of ground silicate minerals to agricultural soils has been proposed as a method for taking up CO2 by enhancing the weathering rate of these minerals through their exposure to soil acids. Alternatively, glacial rock flour, a finely grained material which is abundantly available without the need for energy-intensive grinding, could be used. However, simple and inexpensive methods for determining the amount of CO2 taken up as a result of weathering of applied minerals are still needed, and the impact of non-carbonic acids on CO2 uptake has yet to be accounted for. Here, we present a protocol for correcting estimates of CO2 uptake due to enhanced mineral weathering to account for weathering by non-carbonic soil acids. We determine that soils with a pH below 6.3 need correction for weathering by other acids than carbonic acid and that, given the impact of non-carbonic acids, soils with a pH below 5.2 may not be ideal candidates for mineral applications aimed at CO2 uptake, depending on the pCO2. We report an estimated CO2 uptake of 728 kg CO2 ha−1 after the application of 50 tons ha−1 of Greenlandic glacial rock flour to an acidic, sandy soil in Denmark over 3 years.

An empirical model for prediction of topsoil deformation in field traffic

Agricultural topsoils are characterized by a heterogeneous mixture of structural units with a considerable difference in soil mechanical properties. Mechanistic modeling of compaction for this layer is challenging because of the need to parameterize model input parameters. This study quantified soil deformation during loading in semi-confined conditions and searched for drivers decisive for observed strains with no a priori assumptions of material properties. Undisturbed soil samples were collected in the plough layer of eleven sandy and loamy soils in Denmark. The samples were adjusted to either of the following matric potentials: − 30, − 50, − 75, − 100, − 160 and − 300 hPa (pF from ∼1.5 to ∼2.5). Cores at each water condition were loaded with an annulus covering one third of the soil surface at either of the following normal loads: 30, 60, 90, 120, 150, 180 kPa. Multiple regression was performed to estimate the best model describing the variation in soil compressibility. Measured strain, ε, decreased with increase in soil organic matter, bulk density and pF, but the effect of pF was affected by soil clay content. The model explained ∼84 % of the variation in data and predicted well measured strain for two independent data sets. The regression model is suggested for prediction of soil deformation of arable topsoils in field traffic. A procedure is described for the prediction, including calculation of the relevant stress based on loading characteristics.

The effect of catch crops in spring barley on nitrate leaching and their fertilizer replacement value

Catch crops have been proven effective in reducing nitrogen leaching in cropping systems due to their uptake of post-harvest soil mineral N prior to the main percolation season in winter. Information on the residual effect of catch crops, and their nitrogen fertilizer replacement value is scarce. To determine nitrogen fertilizer replacement values for different catch crops a field experiment was conducted over a four-year period on a sandy soil in Denmark, involving different catch crops and catch crop mixtures with non-legume and legume species. The residual effect of the catch crops was measured in spring barley grown in the residual year. For comparison, winter rye was also included. Over the four-year period the spring barley/fodder radish showed significantly lower leaching compared with the other catch crops, and the winter rye had the highest leaching. Regarding the residual effect, fodder radish and ryegrass, were most valuable, with nitrogen fertilizer replacement values ranging from 13 to 41 kg ha-1, based on fertilizer recovery efficiency of 50%. The process based APSIM (Agricultural Production Systems sIMulator) model was used to simulate the spring barley/catch crop rotations and the winter rye, and showed good model performance regarding N leaching and, crop yield and N uptake. The model was then run over a period of 20 years, to determine variations in N leaching and residual effects due to environmental conditions. The simulations showed that catch crops reduced N leaching by 38–64% when grown every year, and by 21–39% when grown only every second year. Nitrogen fertilizer replacement values ranged on average from 28 to 44 kg N ha-1, but showed high inter-annual variability. The nitrogen fertilizer replacement values showed only a weak relationship with the N uptake of the catch crop prior to termination, likely due to long term effects of N mineralisation from catch crop residues and pre-emptive competition for N by the catch crop.

Proper management of irrigation and nitrogen-application increases crop N-uptake efficiency and reduces nitrate leaching

ABSTRACT Irrigation is, on one hand, expected to increase the risk of nitrate leaching through increased rates of percolation, but, on the other hand, enhances plant nutrient uptake and growth, thereby limiting the risk of leaching. To investigate this dichotomy, we analysed the effects of irrigation at three nitrogen (N)-application rates in spring barley (Hordeum distichum L., two experiments with 50, 100, and 150 kg N ha−1) and winter oilseed rape (Brassica napus L., one experiment with 50, 150, and 250 kg N ha−1) on a coarse sandy soil in Denmark in a humid climate, which facilitates nitrate leaching. Analyses comprised grain/seed dry matter yield, N-uptake, nitrogen use efficiency (partial nitrogen budget, PNB, and partial-factor productivity, PFP) and nitrate leaching. For both crops, increasing N-application without consideration of the crops’ drought-stress responses lead to a relatively lower N-uptake in grain, lower yield, lower PNB and PFP and higher nitrate leaching, although responses were not proportionally to increasing N-application. The effect of irrigation at the lowest N-rates was limited. The non-irrigated treatments with the highest N-rates had a grain/seed yield of 3.2, 2.3 and 0.7 t ha−1 and nitrate leaching rates of 64, 72 and 127 kg N ha−1 compared to a grain/seed yield of 5.3, 5.0 and 2.6 kg N ha−1 and nitrate leaching rates of 61, 42 and 85 kg N ha−1 (for spring barley, spring barley and winter oilseed rape, respectively). These results show that synchronised management of both irrigation and N-application are essential for reducing the risk of nitrate leaching and to promote efficient crop N-uptake in periods of droughts.

Estimating organic carbon stocks of mineral soils in Denmark: Impact of bulk density and content of rock fragments

Management measures to reduce atmospheric carbon dioxide concentrations by increasing soil organic carbon (SOC) storage need verification, e.g., by periodic sampling of soils to estimate resulting changes in SOC stock. Estimates of SOC stocks are affected by content of rock fragments (systematic bias) and soil bulk density (random but significant effect), both of which may vary significantly between soils. We investigated the importance of using site-specific bulk density and correcting for rock fragment content on estimates of SOC stock in 0–50 cm depth of agricultural minerals soils, collected in 2019 in the Danish National Square Grid. We found that use of an average bulk density value for a given soil type category produced valid estimates of SOC stocks for regional/national inventories. However, large variations in bulk density were found within a given soil type category, which can result in over- or under-estimation at local sites. This calls for measurement of site-specific bulk density and rock fragment content to produce valid estimates of field-scale SOC stock, e.g., to be used in farm carbon credit schemes.

Estimating Organic Carbon Stocks of Mineral Soils in Denmark:The Impact of Bulk Density and Stone Content

Estimating Organic Carbon Stocks of Mineral Soils in Denmark:The Impact of Bulk Density and Stone Content

Diurnal and Seasonal Mapping of Water Deficit Index and Evapotranspiration by an Unmanned Aerial System: A Case Study for Winter Wheat in Denmark

Precision irrigation is a promising method to mitigate the impacts of drought stress on crop production with the optimal use of water resources. However, the reliable assessment of plant water status has not been adequately demonstrated, and unmanned aerial systems (UAS) offer great potential for spatiotemporal improvements. This study utilized UAS equipped with multispectral and thermal sensors to detect and quantify drought stress in winter wheat (Triticum aestivum L.) using the Water Deficit Index (WDI). Biennial field experiments were conducted on coarse sand soil in Denmark and analyses were performed at both diurnal and seasonal timescales. The WDI was significantly correlated with leaf stomatal conductance (R2 = 0.61–0.73), and the correlation was weaker with leaf water potential (R2 = 0.39–0.56) and topsoil water status (the highest R2 of 0.68). A semi-physical model depicting the relationship between WDI and fraction of transpirable soil water (FTSW) in the root zone was derived with R2 = 0.74. Moreover, WDI estimates were improved using an energy balance model with an iterative scheme to estimate the net radiation and land surface temperature, as well as the dual crop coefficient. The diurnal variation in WDI revealed a pattern of the ratio of actual to potential evapotranspiration, being higher in the morning, decreasing at noon hours and ‘recovering’ in the afternoon. Future work should investigate the temporal upscaling of evapotranspiration, which may be used to develop methods for site-specific irrigation recommendations.

Random forest regression results in accurate assessment of potato nitrogen status based on multispectral data from different platforms and the critical concentration approach

Remote sensing can be used for precision nutrient management to assess plant nitrogen (N) status in a spatially detailed and real-time manner. Despite recent advances in satellite- and drone technology and machine learning, neither differences between platforms nor methodological aspects for estimating plant N status have been sufficiently investigated. In this study, multispectral data obtained by ground (handheld Rapidscan), air- (unmanned aerial vehicle, UAV) and spaceborne (Sentinel-2) platforms were exploited to estimate plant N uptake (PNU), concentration (PNC) and N nutrition index (NNI). The test plant was potato grown for three years on a sandy soil in Denmark and the analysis was based on the critical N dilution curve. Parametric (PR) and non-parametric (random forest, RFR) regressions were conducted and compared in predicting mid-season PNU, PNC and NNI from band reflectances or vegetation indices (VIs) derived from each platform data. The results obtained by the UAV data had the highest accuracy, largely due to the fine spatial resolution. For both regression types, PNU and NNI correlated better than PNC to reflectance data. For the UAV data, validation Nash-Sutcliffe model efficiency (NSE) of PNU and NNI ranged between 0.64–0.95 and 0.41–0.92 respectively, with corresponding values for relative root mean square error (RRMSE) of 7.1–22% and 5.86–22%. The lower end of NSE and higher end RRMSE intervals systematically being from the PR, which demonstrates the robustness and the high accuracy of RFR in predicting plant N status. The other platforms resulted in acceptable results, with validation NSE and RRMSE for PNU and NNI of, respectively, 0.60–0.79 and 14–20%, 0.25–0.79 and 10–17% for Rapidscan, and 0.48–0.83 and 17–28%, 0.42–0.82 and 12–19% for Sentinel-2. The band reflectance and the VIs were equally suited as input predictors for the RFR algorithm. The N requirement calculated from all three datasets reflected the field observations well. The study reveals the potential of different regression methods for detailed spatial estimation of plant N status to guide in-season fertilization by matching the plant growth demands, emphasizing the strengths of the RFR. The procedure is helpful for the digital agriculture and the smart farming industry aiming to avoid excess application of N.

Environmental constraints to net primary productivity at northern latitudes: A study across scales of radiation interception and biomass production of potato

Intercepted photosynthetically active radiation (Ipar, MJ m-2 d-1) is a key biophysical variable governing plant photosynthetic rate and net primary productivity (NPP, g m-2 d-1). Under optimal growth conditions, Ipar scales proportionally to NPP by a factor termed ‘optimum radiation use efficiency’ (RUEopt). The Carnegie-Ames-Stanford approach (CASA) considers temperature and moisture constraints to RUEopt and has been widely used in remote sensing studies to estimate productivity of various ecosystems. However, scale effects on the CASA have not been sufficiently investigated nor quantified. In this study, data at scales of field (Rapidscan reflectance data), air- (unmanned aerial vehicle (UAV) imagery) and spaceborne (Sentinel-2 imagery), as well as for several environmental constraints, were utilized to develop and test the CASA. The test plant was potato grown for multiple years on sandy soils in Denmark. The results showed that data from all scales provided comparable estimates of daily fraction of Ipar (fIpar) and Ipar. Maximum air temperature and diffuse radiation were the most important environmental factors constraining RUEopt of potato. Taking these into account considerably improved the prediction of NPP with the CASA (R2 increase of 25–32 % compared to no constraints), whereas stress effects due to soil moisture and vapor pressure deficits were less important (relative improvement in R2 of 2–3 %). Hence, RUEopt was 4.66, 4.19 and 4.98 g MJ-1 for Rapidscan, UAV and Sentinel-2, respectively, which was about 47 %–59 % higher than the estimates of the observed actual radiation use efficiency. This study offers an operational method for deriving RUEopt at plant species level since remote sensing and meteorological data are both readily available, and demonstrates that environmental constraints of the CASA does vary, depending on the study region. The method can be used to predict dry matter production in-season for other crops as well, with potential for extending the approach to determine fertilization and irrigation demands.

Nitrous oxide emissions from oilseed rape cultivation were unaffected by flash pyrolysis biochar of different type, rate and field ageing

Nitrous oxide (N2O) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an N2O emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of N2O include the application of biochar, but efficacy and mechanisms of N2O suppression are elusive. We measured N2O emissions in a WOSR field trial on a sandy loam soil in Denmark over 402 days in 2017–2018, comparing biochar applications from two feedstocks (wheat straw and pig manure fibers), two application rates (1.5 and 15 Mg ha−1) and field ageing of up to three years. Further, a controlled incubation experiment was performed to examine the effect of biochar dose and ageing on N2O production and consumption by denitrification. Biochar treatments had no significant effects on cumulative N2O emissions (1.71–2.78 kg N ha−1 yr−1). Likewise, no significant effects were found on crop yield, yield-scaled N2O emission, soil mineral N content, gravimetric soil moisture or pH. The fertilizer induced EF was 0.51% which is well below the IPCC Tier 1 EF of 1%. High doses of fresh, but not field-aged biochar suppressed N2O production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate N2O emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha−1 flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant N2O mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their N2O mitigating effect. Yet, in relation to sustainability of WOSR cultivation for biodiesel, the current study suggests that C sequestration by biochar is not compromised by increased N2O emissions.

A Rare Thioquinolobactin Siderophore Present in a Bioactive Pseudomonas sp. DTU12.1.

Many of the soil-dwelling Pseudomonas species are known to produce secondary metabolite compounds, which can have antagonistic activity against other microorganisms, including important plant pathogens. It is thus of importance to isolate new strains of Pseudomonas and discover novel or rare gene clusters encoding bioactive products. In an effort to accomplish this, we have isolated a bioactive Pseudomonas strain DTU12.1 from leaf-covered soil in Denmark. Following genome sequencing with Illumina and Oxford Nanopore technologies, we generated a complete genome sequence with the length of 5,943,629 base pairs. The DTU12.1 strain contained a complete gene cluster for a rare thioquinolobactin siderophore, which was previously described as possessing bioactivity against oomycetes and several fungal species. We placed the DTU12.1 strain within Pseudomonas gessardii subgroup of fluorescent pseudomonads, where it formed a distinct clade with other Pseudomonas strains, most of which also contained a complete thioquinolobactin gene cluster. Only two other Pseudomonas strains were found to contain the gene cluster, though they were present in a different phylogenetic clade and were missing a transcriptional regulator of the whole cluster. We show that having the complete genome sequence and establishing phylogenetic relationships with other strains can enable us to start evaluating the distribution and evolutionary origins of secondary metabolite clusters.

Influence of long-term phosphorus fertilisation history on the availability and chemical nature of soil phosphorus

Data on the dynamics and fate of phosphorus (P) under low soil-test P (STP) conditions is essential for the development of cropping strategies with a high P use efficiency. This study examined the effects of long-term (20 years) mineral and organic fertiliser P applications on a P-depleted sandy loam soil in Denmark. The cycling of P was examined by use of P budgets (inputs-offtake), chemical P extractions, 33P isotopic exchange kinetics (IEK), and solution 31P nuclear magnetic resonance (NMR) spectroscopy on NaOH-EDTA extracts.Recovery of applied P in the topsoil was smaller for animal slurry P compared with mineral fertiliser P. The budgets suggest deeper soil layers play an important role for the cycling of P. Resin-extractable P (2 to 17 mg kg−1), Olsen-P (7 to 16 mg kg−1) and E1min (1 to 6 mg kg−1) were correlated with the P budgets. Between 63 and 77% of total inorganic P was not exchangeable in a period of three months (E>3months), with the lowest value observed in no-P treatment N1K1. The data show that a redistribution of exchangeable P had taken place under the influence of a strongly negative P budget. Microbial P (6 to 18 mg kg−1) increased under animal slurry inputs compared with mineral fertiliser applications (p < 0.05). All soils were dominated by phosphomonoesters and orthophosphate (98 to 99% of the NMR signal). Concentrations of orthophosphate (86 to 135 mg kg−1) varied significantly between treatments (p < 0.01), whereas forms of organic P remained largely unchanged.The results demonstrate that P applications increased the amount of P that is potentially available for plants, irrespective of input form. Nevertheless, most P applied in excess of crop uptake resulted in an increase of the amount of P that is slowly exchangeable. Under low soil test P conditions such as in the current trial, fertiliser P applied in excess of plant demand that accumulates in soil would thus only be partially available for crops in subsequent years. On the other hand the data suggests that soil P reserves may be utilised for crop growth, but at the low soil P intensity plant access to P will have to be managed carefully.

Complete Genome Sequence of a Bioactive Pseudomonas sp. Strain, DTU12.3, Isolated from Soil in Denmark.

Here, we report the complete annotated genome sequence of a Pseudomonas sp. strain, DTU12.3. It was isolated from leaf-covered soil in Denmark and potentially has bioactivity against certain plant pathogens.

Mapping potential acid sulfate soils in Denmark using legacy data and LiDAR-based derivatives

Leaching large amounts of acidity and metals into recipient watercourses and estuaries, acid sulfate (a.s.) soils constitute a substantial environmental issue worldwide. Mapping of these soils enables measures to be taken to prevent pollution in high risk areas. In Denmark, legislation prohibits drainage of areas classified as potential a.s. soils without prior permission from environmental authorities. The mapping of these soils was first conducted in the 1980’s. Wetlands, in which Danish potential a.s. soils mostly occur, were targeted and the soils were surveyed through conventional mapping. In this study, a probability map for potential a.s. soil occurrence was constructed for the wetlands located in Jutland, Denmark (c. 6500km2), using the digital soil mapping (DSM) approach. Among the variety of available DSM techniques, artificial neural networks (ANNs) were selected. More than 8000 existing soil observations and 16 environmental variables, including geology, landscape type, land use and terrain parameters, were available as input data within the modeling. Prediction models based on various network topologies were assessed for different selections of soil observations and combinations of environmental variables. The overall prediction accuracy based on a 30% hold-back validation data reached 70%. Furthermore, the conventional map indicated 32% of the study area (c. 2100km2) as having a high frequency for potential a.s. soils while the digital map displayed about 46% (c. 3000km2) as high probability areas for potential a.s. soil occurrence. ANNs, thus, demonstrated promising predictive classification abilities for the mapping of potential a.s. soils on a large extent.

The ability of cover crops to reduce nitrogen and phosphorus losses from arable land in southern Scandinavia and Finland

This review summarizes current knowledge from the literature and experimental studies on the role of cover crops (CCs) in reducing nitrogen (N) leaching and phosphorus (P) losses to waters under the marine and humid continental climate conditions of southern Scandinavia and Finland. Field leaching studies from 11 sites indicate that undersown ryegrass (Lolium spp.) CCs are robust, with average N uptake in aboveground CC biomass of 7 to 38 kg N ha−1 (6.2 to 34 lb N ac−1). Use of CCs sown at harvest (e.g., crucifers) is restricted to southern Scandinavia for climate reasons. The mean relative reduction in N leaching reported for all CCs investigated was 43%, but it ranged between 62% increase instead of a reduction after a red clover (Trifolium pratense) CC on a clay soil to a reduction of 85% to 89% with a perennial ryegrass CC on sandy soils in Denmark (36 to 51 kg ha−1 [32 to 46 lb ac−1]). The data indicate that CCs do not substantially reduce total P losses by runoff and leaching. The effects of CCs on total P leaching varied between a relative increase of 86% and a decrease of 43%. Climate conditions involving freezing-thawing during winter increased the risk of losses of dissolved P from CC biomass. CCs have been implemented to varying degrees into agri-environmental programs. They are mandatory in Denmark and subsidized in Norway, Sweden, and Finland. CCs are grown on 8% of arable land in Denmark, 5% in Sweden, 1% in Finland, and 0.5% in Norway, but CC area is now increasing dramatically in Finland due to a new subsidy program. In all countries there is a need, and potential, for increased use of CCs, but there are several constraints, particularly reduced interest among farmers. There is a clear need to identify CC systems and develop implementation strategies for appropriate distribution of CCs on different soils and regions with respect to required reductions in N leaching and P losses. For this, more knowledge is required, especially about the effect of CCs on P losses (e.g., the effect of species with different partitioning between shoot and root biomass and the effects of CC systems with harvesting of biomass). There is also a need to devise balanced solutions for maintaining and increasing the frequency of CCs in crop rotations to exploit the possible benefits of CCs in reducing nutrient losses.

Effects of wood chip ash fertilization on soil chemistry in a Norway spruce plantation on a nutrient-poor soil

Harvest of forest biomass for energy production may lead to export of nutrients from the forest. Recirculation of nutrients from wood chip combustion by ash spreading in forests has been proposed as a means for counteracting the nutrient export. This study was carried out to examine the effect of wood chip ash application on soil chemistry in a 44-year-old Norway spruce (Picea abies) plantation on a nutrient-poor soil in Denmark and to investigate the effect of applying different ash types and doses. Soil samples were collected and analyzed 2.5years (3 growing seasons) after ash application. This study shows that, regardless of ash formulation, preparation or dose, application of wood ash to forest soil has a liming effect in the O-horizon manifested as an increase in CECe, BS and pH. This effect was not seen in the mineral soil within the time frame of this study. At the same time, an increase in Cd was found in the O-horizon, corresponding to the amount added in the ashes. Generally, no other increase in soil contents of the heavy metals was seen. Hardening of the wood ash did not decrease the chemical impact on the soil chemistry as compared to non-treated ash whereas an increase in ash application dose increases the liming effect.

Biochar Effects on Soil Aggregate Properties Under No-Till Maize

Soil aggregates are useful indicators of soil structure and stability, and the impact on physical and mechanical aggregate properties is critical for the sustainable use of organic amendments in agricultural soil. In this work, we evaluated the short-term soil quality effects of applying biochar (0–10 kg m−2), in combination with swine manure (2.1 and 4.2 kg m−2), to a no-till maize (Zea mays L.) cropping system on a sandy loam soil in Denmark. Topsoil (0–20 cm) aggregates were analyzed for clay dispersibility, aggregate stability, tensile strength (TS), and specific rupture energy (SRE) using end-over-end shaking, a Yoder-type wet-sieving method, and an unconfined compression test in soil samples collected 7 and 19 months after final biochar application. The highest rates of biochar and swine manure application resulted in the highest aggregate stability and lowest clay dispersibility. Applying both amendments systematically increased TS and SRE for large aggregates (4–8 and 8–16 mm) but not for small aggregates (1–2 and 2–4 mm). Increased biochar application also decreased the friability index of soil aggregates. Based on X-ray visualization, it was found that aggregates containing larger amounts of biochar particles had higher TS and SRE probably because of bonding effects. Based on the improved soil aggregate properties, we suggest that biochar can be effective for increasing and sustaining overall soil quality, for example, related to minimizing the soil erosion potential.

The Effects of Moisture Conditions-From Wet to Hyper dry-On Visible Near-Infrared Spectra of Danish Reference Soils

Changes in soil water content are known to affect soil reflectance. Even though it was suggested some time ago that the phenomenon of increased forward scattering due to the presence of water in the soil is related to water film thickness and matric potential, there has been no detailed investigation of this in any studies. The effects of moisture conditions on visible near-infrared (vis-NIR) spectra of four representative soils in Denmark have been assessed as a function of both water film thickness (expressed as the number of molecular layers) and matric potential. Complete water retention curves, from wet (pF 0.3, pF = log(|j|), where φ is the matric potential in cm) to hyper dry end (oven-dried and freeze-dried soil), were obtained by initial wetting followed by successive draining and drying of soil samples, performing NIR measurements at each step. Soil reflectance was found to decrease systematically, yet not proportionally, with decreasing matric potential and increasing molecular layers. The changes in molecular layers were best captured by the soil reflectance of clay-rich soils. Here the largest increase in reflectance occurred between pF 3 and 4, caused by the shift from capillary to adsorptive surface forces. In support of this, the smallest changes in reflectance were seen in the sandiest soil. Freeze drying the soil highest in organic C increased reflectance, possibly due to an alteration in organic matter during freezing. The different reflectance behavior of soil with a higher organic C content may be linked to differences in the amount, but also the quality (higher hydrophobicity) of the organic matter. However, this needs to be confirmed in further studies.