Experiments and sites

Several microcosm experiments were conducted to understand the factors influencing root growth, microbial activity, and nutrient utilization in the subsoil, as well as interactions among these factors:

• subsoil-derived arbuscular mycorrhizal fungi and their influence on iron uptake by spring barley
• biopores generated by earthworms, and their pore lining
• water content or penetration resistance of the soil
• crop genotypes (tested so far for spring barley, winter wheat and oats)
• contact between root (hair) and soil within the biopores
• influence of nutrient availability on isotope fractionation
• composition of the compost incorporated into the subsoil

Dresemann T., Athmann M., Heringer L., Kautz T. 2018. Effects of continuous vertical soil pores on root and shoot growth of winter wheat: a microcosm study. Agricultural Sciences 9, 750-764. Doi: 10.4236/as.2018.96053.

Guigue J., Just C., Luo S., Fogt M., Schloter M., Kögel-Knabner I., Hobley E. 2022. Spatial molecular heterogeneity of POM during decomposition at different soil depths resolved by VNIR hyperspectral imaging. European Journal of Soil Science 73, e13207. Doi: 10.1111/ejss.13207.

Koch M., Boselli R., Hasler M., Zörb C., Athmann M., Kautz T. 2021. Root and shoot growth of spring wheat (Triticum aestivum L.) are differently affected by increasing subsoil biopore density when grown under different subsoil moisture. Biology and Fertility of Soils 57, 1155-1169. Doi: 10.1007/s00374-021-01597-7.

Wang Y., Wu B., Berns A.E., Xing Y., Kuhn A.J., Amelung W. 2020. Magnesium isotope fractionation reflects plant response to magnesium deficiency in magnesium uptake and allocation: a greenhouse study with wheat. Plant and Soil 455, 93-105. Doi: 10.1007/s11104-020-04604-2.

Within the framework of Soil³, long-term field experiments throughout Germany are being used to understand, based on recorded yield, soil and climate data as well as our own investigations, the site-specific interactions of various factors on subsoil processes, as well as the long-term effects (up to > 100 years) of various conventional agricultural methods.

Sites of the long-term field experiments:

Rauischholzhausen (Justus-Liebig-Universität Gießen):

• International Organic Nitrogen Fertilization Experiment (IOSDV)
• Soil Tillage Experiment

Gießen (Justus-Liebig-Universität Gießen):

• Biological Nitrogen Fixation (BSG)
• Depletion Experiment

Freising-Weihenstephan (Technische Universität München):

• Liming Experiment Dürnast II

Müncheberg (Leibniz-Zentrum für Agrarlandschaftsforschung):

• Fertilization and Nutrient Gradient Experiment V140

Thyrow (Humboldt-Universität zu Berlin):

• Static Fertilization and Irrigation Experiment D-I
• Static Nutrient Depletion Experiment D-IV

Berlin-Dahlem (Humboldt-Universität zu Berlin):

• Static Soil Use Experiment D-III

Bonn (Rheinische Friedrich-Wilhelms-Universität Bonn):

• CeFit Klein-Altendorf
• Long-term Fertilization Experiment Dikopshof

Göttingen (Georg-August-Universität Göttingen):

• Soil Management Experiment Garte Süd

Investigated parameters and features:

• yields of field crops
• various parameters of root growth such as root length density
• basic soil properties such as bulk density, carbon and nitrogen stocks, pH, texture, electrical conductivity
• nutrient contents and nutrient use efficiency
• water contents and water dynamics
• subsoil heterogeneity in terms of carbon contents and biopores
• composition and functional properties of the microbial community, root pathogens

Bauke S.L., von Sperber C., Gocke M.I., Sandhage-Hofmann A., Honermeier B., Schweitzer K., Baumecker M., Don A., Tamburini F., Amelung W. 2018. Subsoil phosphorus is affected by fertilization regime in long-term agricultural experimental trials. European Journal of Soil Science 69, 103-112. Doi: 10.1111/ejss.12516.

Gocke M.I., Guigue J., Bauke S.L., Barkusky D., Baumecker M., Berns A.E., Hobley E., Honermeier B., Kögel-Knabner I., Koszinski S., Sandhage-Hofmann A., Schmidhalter U., Schneider F., Schweitzer K., Seidel S., Siebert S., Skadell L.E., Sommer M., von Tucher S., Don A., Amelung W. 2023. Interactive effects of agricultural management on soil organic carbon accrual: A synthesis of long-term field experiments in Germany. Geoderma 438, 116616. Doi: 10.1016/j.geoderma.2023.116616.

Hadir S., Gaiser T., Hüging H., Athmann M., Pfarr D., Kemper R., Ewert F., Seidel S.J. 2021. Sugar beet shoot and root phenotypic plasticity to nitrogen, phosphorus, potassium and lime omission. Agriculture 11, 21. Doi: 10.3390/agriculture11010021.

Hobley E.U., Honermeier B., Don A., Gocke M.I., Amelung W., Kögel-Knabner I. 2018. Decoupling of subsoil carbon and nitrogen dynamics after long-term crop rotation and fertilization. Agriculture, Ecosystems & Environment 265, 363-373. Doi: 10.1016/j.agee.2018.06.021.

Just C., Armbruster M., Barkusky D., Baumecker M., Diepolder M., Döring T.F., Heigl L., Honermeier B., Jate M., Merbach I., Rusch C., Schubert D., Schulz F., Schweitzer K., Seidel S., Sommer M., Spiegel H., Thumm U., Urbatzka P., Zimmer J., Kögel-Knabner I., Wiesmeier M. 2023. Soil organic carbon sequestration in agricultural long-term field experiments as derived from particulate and mineral-associated organic matter. Geoderma 434, 116472. Doi: 10.1016/j.geoderma.2023.116472.

Kaufmann M.S., von Hebel C., Weihermüller L., Baumecker M., Döring T., Schweitzer K., Hobley E., Bauke S.L., Amelung W., Vereecken H., van der Kruk J. 2020. Effect of fertilizers and irrigation on multi‐configuration electromagnetic induction measurements. Soil Use and Management 36, 104-116. Doi: 10.1111/sum.12530.

Kroschewski B., Richter C., Baumecker M., Kautz T. 2022. Effect of crop rotation and straw application in combination with mineral nitrogen fertilization on soil carbon sequestration in the Thyrow long-term experiment Thy_D5. Plant and Soil 488, 121-136. Doi: 10.1007/s11104-022-05459-5.

Rueda-Ayalaa V., Ahrends H., Siebert S., Gaiser T., Hüging H., Ewert F. 2018. Impact of nutrient supply on the expression of genetic improvements of cereals and row crops - A case study using data from a long-term fertilization experiment in Germany. European Journal of Agronomy 96, 34-46. Doi: 10.1016/j.eja.2018.03.002.

Seidel S.J., Gaiser T., Ahrends H.E., Hüging H., Siebert S., Bauke S.L., Gocke M.I., Koch M., Schweitzer K., Schaaf G., Ewert F. 2021. Crop response to P fertilizer omission under a changing climate - Experimental and modeling results over 115 years of a long-term fertilizer experiment. Field Crops Research 268, 108174. Doi: 10.1016/j.fcr.2021.108174.

Skadell L.E., Schneider F., Gocke M.I., Guigue J., Amelung W., Bauke S.L., Hobley E.U., Barkusky D., Honermeier B., Kögel-Knabner I., Schmidhalter U., Schweitzer K., Seidel S.J., Siebert S., Sommer M., Vaziritabar Y., Don A. 2023. Twenty percent of agricultural management effects on organic carbon stocks occur in subsoils – Results of ten long-term experiments. Agriculture, Ecosystems and Environment 356, 108619. Doi: 10.1016/j.agee.2023.108619.

Wang Y., Bauke S.L., von Sperber C., Tamburini F., Guigue J., Winkler P., Kaiser K., Honermeier B., Amelung W. 2021. Soil phosphorus cycling is modified by carbon and nitrogen fertilization in a long-term field experiment. Journal of Plant Nutrition and Soil Science 184, 282-293. Doi: 10.1002/jpln.202000261.

Wang Y., Wu B., Berns A.E., Bol R., Wombacher F., Ellmer F., Amelung W. 2021. A century of liming affects the Mg isotopic composition of the soil and crops in a long-term agricultural field at Berlin-Dahlem, Germany. European Journal of Soil Science 72, 300-312. Doi: 10.1111/ejss.12951.

Wu B., Wang Y., Berns A.E., Schweitzer K., Bauke S.L., Bol R., Amelung W. 2021. Iron isotope fractionation in soil and graminaceous crops after 100 years of liming in the long‐term agricultural experimental site at Berlin‐Dahlem, Germany. European Journal of Soil Science 72, 289-299. Doi: 10.1111/ejss.12944.

Several sites with former subsoil melioration allow us to understand the long-term effects of these tillage techniques on soil and crops. These include, on the one hand, Treposol sites, i.e. soils that were one-time deep plowed (depth 55-90 cm) in the 1960s to introduce organic matter and nutrients from the topsoil into deeper soil layers and thus to enhance root growth in the subsoil. This type of tillage was widespread in northern Germany and was used mainly on sandy sites, but also on loess sites. Also in the 1960s, a subsoil field trial was established near Müncheberg in Brandenburg, where the topsoil was either removed or doubled ("crumb deepening") to increase the fertility of the nutrient-poor sandy soils in northeastern Germany. The trial has now been abandoned for about 30 years.
The long-term effect of mechanical subsoiling was investigated at these sites, focusing on current yields and yield quality, root growth and subsoil properties. The findings from this complement our results from our own subsoil trials (Central Field Experiments).

Burger D.J., Bauke S.L., Amelung W., Sommer M. 2023. Fast agricultural soil re-formation after complete topsoil loss is possible – evidence from a unique historical field experiment. Geoderma 434, 116492. Doi: 10.1016/j.geoderma.2023.116492.

Burger D.J., Bauke S.L., Schneider F., Kappenberg A., Gocke M.I. 2024. Root-derived carbon stocks in formerly deep-ploughed soils – A biomarker-based approach. Organic Geochemistry 190, 104756. Doi: 10.1016/j.orggeochem.2024.104756.

The German Agricultural Soil Inventory is a monitoring program of the Thünen Institute and serves as a uniform inventory of agriculturally used soils in Germany with regard to their carbon stocks down to a depth of 1 meter.
Within the framework of Soil³, the sites of the German Soil Inventory Agriculture are used to record subsoil properties (including nutrient and water contents, rootpenetration) under different soil, climate and tillage conditions. The resulting information on resources in the subsoil of German arable soils as well as their accessibility for crops is used to upscale our findings from the Central Field Experiments and the old melioration sites, i.e. it is used in scenario modeling and finally in the socio-economic assessment.

Gocke M.I., Don A., Heidkamp A., Schneider F., Amelung W. 2021. The phosphorus status of German cropland – an inventory of top- and subsoils. Journal of Plant Nutrition and Soil Science 184, 51-64. Doi: 10.1002/jpln.202000127.

Schneider F., Don A. 2019. Root-restricting layers in German agricultural soils. Part I: extent and cause. Plant and Soil 442, 433-451. Doi: 10.1007/s11104-019-04185-9.

Schneider F., Don A. 2019. Root-restricting layers in German agricultural soils. Part II: Adaptation and melioration strategies. Plant and Soil 442, 419-432. Doi: 10.1007/s11104-019-04186-8.

Within the framework of Soil³, three central field experiments (CF) were set up to

• assess the technical and economic feasibility of specific tillage practices (CF1),
• demonstrate the interactive benefits of legume cultivation and mechanical subsoil melioration (CF 2), and
• validate our concept of subsoil melioration at a sandy site with a dry climate (CF 3).

Field experiment 1 (Klein-Altendorf near Bonn) was established with different techniques of stripwise subsoil manipulation and consists of three sub-trials: In CF 1-1, different materials for subsoil melioration were tested (biocompost, greenwaste compost, straw, sawdust). In CF 1-2, the amounts of compost applied were varied (30, 50, or 70 m3/ha). In CF 1-3, different genotypes were grown.

Field experiment 2 (Klein-Altendorf near Bonn) includes treatments with and without alfalfa as a precrop. After the end of the pre-crop phase in 2018, different methods of mechanical subsoil melioration were applied.

Field experiment 3 (Thyrow near Berlin) follows the experimental design of CF 1 and CF 2 with site-adapted modifications.

To demonstrate the positive effects of our melioration technique, the following parameters and subsoil properties are studied:

• yields of field crops
• various parameters of root growth such as root length density
• basic soil properties such as bulk density, carbon and nitrogen stocks, pH, texture, electrical conductivity
• nutrient contents and nutrient use efficiency
• water contents and water dynamics
• subsoil heterogeneity in terms of carbon contents and biopores
• composition and functional properties of the microbial community, root pathogens

Bauke S.L., Seidel S.J., Athmann M., Berns A.E., Braun M., Gocke M.I., Guigue J., Kautz T., Kögel-Knabner I., Ohan J., Rillig M., Schloter M., Schmittmann O., Schulz S., Uhlig D., Schnepf A., Amelung W. 2024. Short-term effects of subsoil management by strip-wise loosening and incorporation of organic material. Soil & Tillage Research 236, 105936. Doi: 10.1016/j.still.2023.105936.

Ittner S., Gerdes H., Athmann M., Bauke S.L., Gocke M., Guigue J., Jaiswal S., Kautz T., Schmittmann O., Schulz S., Seidel S. 2020. The impact of subsoil management on the delivery of ecosystem services. BonaRes Series 2020/5. Doi: 10.20387/BonaRes-BSZH-QBKN.

Jakobs I., Schmittmann O., Schulze-Lammers P. 2017. Short-term effects of in-row subsoiling and simultaneous admixing of organic material on growth of spring barley (H. vulgare). Soil Use and Management 33, 620-630. Doi: 10.1111/sum.12378.

Jakobs I., Schmittmann O., Athmann M., Kautz T., Schulze Lammers P. 2019. Cereal Response to Deep Tillage and Incorporated Organic Fertilizer. Agronomy 9, 296. Doi: 10.3390/agronomy9060296.

Schmittmann O., Christ A., Schulze Lammers P. 2021. Subsoil Melioration with Organic Material—Principle, Technology and Yield Effects. Agronomy 11, 1970. Doi: 10.3390/agronomy11101970.

Schmittmann O., Schulze Lammers P. 2023. Assessing Subsoil Conditions with an ASABE conform Vertical Penetrometer – Development and Evaluation. Sensors 23, 1306. Doi: 10.3390/s23031306.

In order to test the subsoil melioration technique developed in Soil³ in agricultural practice and for different regions, and to pass on our findings from the central field experiments to the relevant interested parties and stakeholders (farmers, decision-makers, administrations), on-farm demo trials were installed in regions with different precipitation levels and soil qualities (North Rhine-Westphalia and Brandenburg).
The on-farm demo trials were also used to evaluate region-specific process parameters, namely machine specifications, power and time requirements, and yield effects. This is used for scenario analysis.