Schneider, Florian: Root-restricting layers in German agricultural soils : Extent, cause and management strategies. - Bonn, 2020. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Florian Schneider}},
title = {Root-restricting layers in German agricultural soils : Extent, cause and management strategies},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2020,
month = oct,

note = {Subsoil, i.e., soil below 30 cm depth, harbours great amounts of water and nutrients. However, physical and chemical barriers for vertical root elongation may restrict plants from accessing subsoil resources. Such root-restricting soil layers (RRLs) were subject of the present thesis. Focusing on agricultural soils in Germany, the major aims of this thesis were to (i) identify the main soil-borne causes for restricted vertical root elongation, (ii) examine the effects of RRLs on the productivity function and carbon (C) storage function of soil, (iii) quantify the contribution of anthropogenically compacted soil to the spatial extent of RRLs, and (iv) evaluate management strategies for agricultural soil with RRLs.
Most of this thesis has evolved from data of the first German Agricultural Soil Inventory (2011-2018), which comprises information on the soil and management at 3104 sites covering all agricultural land in Germany in a regular 8 km x 8 km grid. For describing the cause and extent of root restriction, literature was reviewed for threshold values, which were subsequently validated with root count data from the inventory. Effects of RRLs on the productivity function and C storage function of soil were examined by comparing yield of winter wheat and depth gradients of organic C densities at different magnitudes of root restriction (inventory data). Additionally, a global meta-analysis of long-term field experiments was carried out to compare yield of annual crops growing on sites with RRLs to adjacent sites where RRLs were previously meliorated by deep tillage. Anthropogenic soil compaction in German agricultural soils was quantified with a novel, data-driven approach. Management options were examined in a literature review and the popularity of these options was assessed based on the inventory.
The dominant soil-borne cause for restricted elongation of deep roots was high compactness - almost half (46 %) of German agricultural land was compacted to an extent that restricted root growth. Other causes included groundwater-induced anoxia (14 %), sandy subsoil texture (12%), acidity (10%), large rock fragment content (8%), shallow bedrock (6%), and cementation (2%). These RRLs significantly decreased the productivity function of soils. In the meta-analysis of long-term field experiments, RRLs decreased yield of annual crops on average by about 20 %. On German farms, grain yield of winter wheat was on average 0.5 Mg ha-1 (6%) lower in the presence of severe RRLs compared to reference sites without RRLs. Anthropogenic compaction has significantly increased the spatial extent of RRLs in German agricultural soils. About 10% of cropland was estimated to be compacted due to traffic, 1 % due to organic C loss-induced collapse of soil structure, and further 2 % due to a combination of both factors. German farmers either accepted the presence of RRLs and adapted land use and management accordingly or they meliorated affected sites. Melioration measures included drainage (45%), deep loosening (6% at least once within ten years prior to sampling), deep ploughing (5% at least once in history), and liming to correct soil acidity (54%).
In the upper metre of German agricultural soils, about 30 % of the available water capacity, 30 % of total phosphorus and 20 % of total nitrogen resources were hidden below RRLs. Thus, the melioration of RRLs could significantly improve plant nutrition. Furthermore, the melioration of RRLs could increase the transfer of atmospheric C into the subsoil via deep roots and increase soil C storage. It was estimated that compacted cropland with packing densities >1.75 g cm-3 could store up to 2.3 Mg ha-1 more organic C in 30-100cm if sustainably meliorated (loosened). However, not every soil is capable of being meliorated. In terms of a sustainable bioeconomy, it is therefore of central importance to stop the further spread of RRLs and prevent traffic-induced soil compaction.},

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