Effects of bio-subsoilers on subsoil pore-system functionality: Case study with intact soil columns

Abstract

Field traffic-induced subsoil compaction poses an escalating problem in relation to key soil functions because of the use of increasingly heavier machinery. Natural recovery from subsoil compaction is slow and therefore there is a need for measures such as bio-subsoilers to stimulate the recovery. This study thus investigated the influence of potential bio-subsoilers on pore functionality and morphology of a sandy loam soil, with an emphasis on the transition between the topsoil and compacted subsoil and on the compacted subsoil per se, in an intact soil column experiment. The control crop was spring barley, and potential bio-subsoilers included chicory, lucerne, fodder radish, tall fescue and kernza. The intact soil columns (Ø = 0.20 m, h = 0.50 m) were X-ray CT-scanned prior to and two years after crop establishment. Undisturbed subsamples (246 cm3) were collected one and two years after crop establishment in five replicates at 0.10, 0.25 and 0.35-m depth from the soil columns. The laboratory measurements and derived indices showed that after one-year non-restrictive air-filled porosity (εa = 0.13 m3 m−3) and relative gas diffusivity (Ds/Do = 0.0059) was reached by lucerne in the Transition layer (0.25 m). In the second year, lucerne also showed a significantly higher εa (0.09 m3 m−3), and a non-significantly higher Ds/Do (0.0029) compared to barley in the Compacted layer (0.30 m). Pore geometry indices derived from the relationship between εa, Ds/Do and air permeability indicated the formation of less tortuous and more connected small pores (at −100 hPa matric potential) throughout the Transition and Compacted layers by chicory, lucerne and tall fescue. A significantly higher CT-number of pores < 1 mm3 was found for lucerne and a higher CT-volume of pores < 1.5 mm in diameter for lucerne and chicory in the Compacted layer. Root number density was significantly higher for chicory, lucerne and tall fescue compared to barley from the first year, and also for kernza from the second year. Lucerne had the greatest subsoil compaction mitigation potential in our experiment in intact columns. Further studies in real field situations are needed.