Growing a grass and forb mixture in drying soil – root systems do not show spatial niche complementarity

Differences in root system architecture of species grown in mixtures may contribute to overyielding through spatial niche complementarity, depending on soil conditions. We investigated if differences in root system architecture in a grass-forb combination under drying soil conditions contributed to complementarity in root distribution.

Bristle oat (Avena strigosa Schreb.) and fodder radish (Raphanus sativus L.) were grown in containers (35 cm soil depth) in a greenhouse as sole stands and alternate row mixtures with continuous rewetting or in drying soil profiles. Drying soils reached 7.5 and 9.5% cm³/cm³ moisture at 0–10 and 10–20 cm depth respectively. Aboveground biomass and root system traits were quantified.

Mixtures produced 60 and 34% more root length and 11 and 24% higher biomass than sole stands in moist and drying soil respectively. In drying soil bristle oat and fodder radish had respectively 58% and 13% higher root length density in mixtures than in sole stands, whereas fodder radish dominated the aboveground biomass. In drying soil, sole-growing bristle oat produced more root length in the bottom layer than in other layers, while sole-growing fodder radish produced more root length in the upper layer. In mixtures both species produced more root length in the upper layer.

Both species over-yielded when grown as a mixture either in moist or drying topsoil; however, they did not exhibit complementary root distribution. Contrary to the expected niche complementarity, plants of both species enhanced root placement in the top layer, whether dry or moist.