Above and below Ground Indicators for Stress Induced By Compacted Soils of Small Grain Cereals and Soybean.

Soil compaction due to heavy agricultural machinery is a major environmental threat to arable soils, which is still difficult to detect and to quantify at the crop level. Increased mechanical impedance and disturbed water, solute and gas flows in compacted soils reduce root growth and crop productivity. A precise quantification of the above- and below-ground reaction of crops to this stress will help to identify indicators for compaction and suitable species, genotypes and management opportunities to mitigate compaction.

Soybean and small grain cereals were chosen to cover the variation between some of the globally most relevant crops in terms of plant development and root system architecture. To account for different plant developmental stages experiments were conducted under controlled conditions and in the field with the same soil.

In growth chamber experiments early responses of roots to two different soil bulk densities (1.34 and 1.60 g cm^-3) were evaluated using X-ray computed tomography periodically throughout 14 days. Five days after sowing soil compaction caused increased root diameters and decreased lateral root numbers in both species. Plant height and shoot dry weight decreased due to compaction in wheat, whereas no such responses occurred in soybean.

In the field the soil was compacted by multiple track-by-track passing with a heavy vehicle. In the case of triticale the topsoil was either ploughed or remained undisturbed, whereas for soybean the top five centimeters were tilled after compaction. Leaf growth rates and shoot biomass of both species decreased due to compaction. For triticale very similar root architectural responses to compaction were observed as under controlled conditions. Even if the topsoil was ploughed, numbers of nodal and lateral roots decreased, while their diameters increased under compaction. In soybean the lateral root number also decreased, while root diameters also decreased as a result of compaction.