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Sustainable agricultural land use requires an assessment of degradable soil organic matter (SOM) because of its key function for soil fertility and plant nutrition. Such an assessment for practical land use should consider transformation processes of SOM and its sources of different origin. In this study, we combined a 120‐day incubation experiment with thermal decay dynamics of agricultural soils altered by added organic amendments. The aim was to determine the abilities and limits of thermal analysis as a rapid approach revealing differences in the degradability of SOM. The carried out experiments based on two independent sampling sets. The first sample set consisted of soil samples taken from non‐fertilized plots of three German long‐term agricultural field experiments (LTAEs), then artificially mixed with straw, farmyard manure, sheep faeces, and charcoal equal to 60 Mg ha⁻¹ under laboratory conditions. The second sample set based on soil samples of different treatments (e.g., crop type, fertilization, cultivation) in LTAEs at Bad Lauchstadt and Muncheberg, Germany. Before and after the incubation experiment, thermal mass losses (TML) at selected temperatures were determined by thermogravimetry indicating the degradability of organic amendments mixed in soils. The results confirmed different microbial degradability of organic amendments and SOM under laboratory conditions. Thermal decay dynamics revealed incubation‐induced changes in the artificial soil mixtures primarily at TML around 300°C in the case of applied straw and sheep faeces, whereas farmyard manure showed mainly changes in TML around 450°C. Charcoal did not show significant degradation during incubation, which was confirmed by TML. Detailed analyses of the artificial soil mixtures revealed close correlations between CO₂‐C evolution during incubation and changes in TML at 300°C with R² > 0.96. Results of the soils from LTAEs showed similar incubation‐induced changes in thermal decay dynamics for fresh plant residues and farmyard manure. We conclude that the practical assessment of SOM could be facilitated by thermal decay dynamics if modified sample preparation and evaluation algorithms are used beyond traditional peak analysis.

For more than 40 years, farmers in Germany have used the fertilization recommendation schema provided by the Association of German Agricultural Investigation and Research Institutions (VDLUFA) to quantify the required lime (CaO) demand of arable mineral soils. To be applicable as guidelines in practice, the results of 30 years of fertilization experiments that studied the correlation between crop yields and the actual soil pH, the soil texture, and the soil organic matter (SOM) content were finally condensed into a look-up table system. However, because the original experimental data are no longer accessible, the purpose of this study is to reconstruct the interaction between the three soil parameters and their appropriate lime demands. Therefore, the class-based, stepwise approach of the look-up table system is transferred into a continuous, stepless approach using mathematical models. Under the precondition to preserve the pH-, texture- and SOM-dependent CaO amounts recommended in the look-up system (n = 317) to the greatest extent possible, the algorithm was successful, more than 99% of their variability could be explained by the models. This adaptation helps to meet the accuracy of present-day requirements of precision farming.

In 1978, after >70 years of continuous management of the long-term fertilization experiment in Bad Lauchstadt, a general change of soil management was implemented for several treatments. Experimental plots which had previously received a high input of organic matter (OM) now received only low OM input and vice versa. The resulting changes of SOC concentrations on different treatments vary from −0.1 to +0.1 g kg−1 yr−1. These trends were analyzed in order to explain the observed variations with the mean carbon input together with the quality of the different sources to build up new SOC. The results showed that this quality related carbon flux from fresh organic matter (FOM) into SOC, here defined as “carbon reproduction flux” (Crep), is a useful indicator to explain changes in SOC trends. Moreover, it can be used to predict the amount of FOM carbon input that is needed to achieve a given sequestration rate. The results revealed that carbon sequestration requires less effort on plots that had already higher carbon input rates in the past because the SOC storage, still far from the equilibrium state, had already a positive trend. Therefore, it is important to consider not only SOC stock but also the direction of the current trend when selecting measures that increase SOC to achieve the goal of the “4‰ Initiative: soils for food security and climate” which was launched during the COP21 conference.

Crop yield variations are strongly influenced by the spatial and temporal availabilities of water and nitrogen in the soil during the crop growth season. To estimate the quantities and distributions of water and nitrogen within a given soil, process-oriented soil models have often been used. These models require detailed information about the soil characteristics and profile architecture (e.g., soil depth, clay content, bulk density, field capacity and wilting point), but high resolution information about these soil properties, both vertically and laterally, is difficult to obtain through conventional approaches. However, on-the-go electrical resistivity tomography (ERT) measurements of the soil and data inversion tools have recently improved the lateral resolutions of the vertically distributed measurable information. Using these techniques, nearly 19,000 virtual soil profiles with defined layer depths were successfully created for a 30 ha silty cropped soil over loamy and sandy substrates in Central Germany, which were used to initialise the CArbon and Nitrogen DYnamics (CANDY) model. The soil clay content was derived from the electrical resistivity (ER) and the collected soil samples using a simple linear regression approach (the mean R-2 of clay = 0.39). The additional required structural and hydrological properties were derived from pedotransfer functions. The modelling results, derived soil texture distributions and original ER data were compared with the spatial winter wheat yield distribution in a relatively dry year using regression and boundary line analysis. The yield variation was best explained by the simulated soil water content (R-2 = 0.18) during the grain filling and was additionally validated by the measured soil water content with a root mean square error (RMSE) of 7.5 Vol%.

The particle density of soil (ρS) represents one of the basic physical properties of soil. However, measurement of this parameter is not part of common routine soil inventories in most countries. Therefore, pedotransfer functions (PTFs) were developed to calculate ρS. Here, we used a complex, hierarchically structured PTF to calculate ρS, separating the soil mineral substance (SMS) into clay, silt and sand fractions as well as separating the soil organic matter (SOM) into heavy-density and low-density fractions. This PTF was recently published, and here, we introduced an additional hierarchical level to consider the particle size fraction-dependent effect of the mineral composition on ρS. This extended PTF was calibrated and validated using data from soils of 16 German long-term experiments contrasting in soil texture and in soil mineral composition. Χ-ray diffraction analysis was applied to identify the mineralogical composition of the clay, silt and sand fractions. We fitted the particle densities of the identified minerals occurring in the respective particle size fractions by minimising the squared differences between the measured and predicted ρS. The model performed very well (RMSE = 0.011 Mg m−3 soil). According to the mechanistic base of the model and its hierarchical structure, it is very easy to include available information about the composition of any fraction or subfraction of soil mineral substances and to use the model to calculate the ρS corresponding to the specific site conditions.

The Box Plot Experiment in Grossbeeren was set up in 1972 to investigate the efficiency of diverse fertilization strategies within an irrigated vegetable crop rotation system for three different soils. Here, we report on the long-term effects on nitrogen, carbon and energy balances of applying various organic amendments and different amounts of mineral N fertilizer to soils. Fertile soils (sandy loam and clayey silt) were characterized by higher yield and N removal potential by plants, higher acceptable N input amounts, and a higher proportion of total N input recovered by soil N storage and N uptake by plants. After 40 years, organic carbon equilibrium in the soil has not been reached. Rates of change of organic carbon stocks are still affected by the different organic matter levels at the beginning of the experiment. An increasing net N supply increased and organic C input decreased net C gains. Because of the manifold possibilities of producing energy from current or former agricultural land, energetic...

GEOPHILUS ELECTRICUS (nickname GEOPHILUS) is a novel system for mapping the complex electrical bulk resistivity of soils. Rolling electrodes simultaneously measure amplitude and phase data at frequencies ranging from 1 mHz to 1 kHz. The sensor's design and technical specifications allow for measuring these parameters at five depths of up to ca. 1.5 m. Data inversion techniques can be employed to determine resistivity models instead of apparent values and to image soil layers and their geometry with depth. When used in combination with a global positioning system (GPS) and a suitable cross-country vehicle, it is possible to map about 100 ha/day (assuming 1 data point is recorded per second and the line spacing is 18 m). The applicability of the GEOPHILUS system has been demonstrated on several sites, where soils show variations in texture, stratification, and thus electrical characteristics. The data quality has been studied by comparison with ‘static’ electrodes, by repeated measurements, and by comparison with other mobile conductivity mapping devices (VERIS3100 and EM38). The high quality of the conductivity data produced by the GEOPHILUS system is evident and demonstrated by the overall consistency of the individual maps, and in the clear stratification also confirmed by independent data. The GEOPHILUS system measures complex values of electrical resistivity in terms of amplitude and phase. Whereas electrical conductivity data (amplitude) are well established in soil science, the interpretation of phase data is a topic of current research. Whether phase data are able to provide additional information depends on the site-specific settings. Here, we present examples, where phase data provide complementary information on man-made structures such as metal pipes and soil compaction.

Soil bulk density (ρ b ) is required to estimate, evaluate, and calculate many physical soil properties and processes and is essential to convert data from weight-based to volume- and area-related data. One of the dominating factors changing ρ b is the soil's organic matter (SOM) concentration that alters the soil's compressibility; ρ b is an important soil structure attribute. Currently, no parameter for characterizing soil compactness giving directly comparable values for all soils is available. Therefore, our aim was to develop a general approach to calculate the effect of SOM concentration on ρ b that would be universally valid for soils different in their genesis, compaction, and type of land use. To describe the effect of SOM on ρ b mathematically, we used a nonlinear regression model that was parameterized and validated using published data from experiments where SOM concentration was the main ρ b -affecting factor (long-term fertilization and proctor experiments, wetlands, reclaimed soils, and volcanic soils). To obtain a standardized parameter describing the present compaction status of a site, we introduced the standardized bulk density sρ b . Mathematically, sρ b is the intercept parameter of the used nonlinear regression model, and ranged between 0.7 and 2.1 Mg m -3 and was very simple to estimate. Another distinct advantage of this novel concept is that only one representative pair of ρ b and SOM has to be known to calculate sρ b as well as the bulk densities corresponding to other SOM concentrations measured on the site. This concept might also be helpful for identifying similar universal approaches to standardize the effect of other ρ b affecting parameters (e.g., texture, soil depth, tillage regime), however, reassessed from the SOM effect.

Swedish long-term soil fertility experiments were used to investigate the effect of texture and fertilization regime on soil electrical conductivity. In one geophysical approach, fields were mapped to characterize the horizontal variability in apparent electrical conductivity down to 1.5 m soil depth using an electromagnetic induction meter (EM38 device). The data obtained were geo-referenced by dGPS. The other approach consisted of measuring the vertical variability in electrical conductivity along transects using a multi-electrode apparatus for electrical resistivity tomography (GeoTom RES/IP device) down to 2 m depth. Geophysical field work was complemented by soil analyses. The results showed that despite 40 years of different fertilization regimes, treatments had no significant effects on the apparent electrical conductivity. Instead, the comparison of sites revealed high and low conductivity soils, with gradual differences explained by soil texture. A significant, linear relationship found between apparent electrical conductivity and soil clay content explained 80% of the variability measured. In terms of soil depth, both low and high electrical conductivity values were measured. Abrupt changes in electrical conductivity within a field revealed the presence of 'deviating areas'. Higher values corresponded well with layers with a high clay content, while local inclusions of coarse-textured materials caused a high variability in conductivity in some fields. The geophysical methods tested provided useful information on the variability in soil texture at the experimental sites. The use of spatial EC variability as a co-variable in statistical analysis could be a complementary tool in the evaluation of experimental results.

Wettability affects water configuration and thereby transport processes and microbial activity in soil. Approaches to visualize the effect of porous media wettability on water films surrounding particles are rarely available in the literature. The aim of this study is therefore (1) to visualize the effect of wettability on area and connectivity of the water phase and (2) to develop a segmentation strategy to enable water films and bulk water to be differentiated. Wettability of silica sand was rendered by silanization using dichlorodimethylsilane. The resulting contact angle was measured using the sessile drop method. Furthermore, wettability was characterized by the water penetration time test in air-dry samples and at a volumetric water content of 8 vol.%. Sulforhodamine B was used to stain distilled water at a concentration of 40 mg/l. By means of the Wilhelmy plate method, the influence of the dye on the liquid surface tension was tested. Confocal laser scanning microscopy (CLSM) was used to visualize the area and connectivity of the fluorescent-dyed water phase and the thickness of water films in untreated and silanized samples at water contents of 8, 16, and 32 vol.%. The silanization significantly increased the contact angle of silica sand, whereas the surface tension of the dye solution did not differ significantly from that of undyed water. CLSM visualized the distribution of water, focused across the surface grain layer. Thresholding of fluorescence signal in two-dimensional intensity projections enables the discrimination between film and bulk water. It was shown that even subcritical water repellency (contact angle