Effects of mixing two legume species at seedling stage under different environmental conditions.

While intercropping is known to have positive effects on crop productivity, it is unclear whether the effects of mixing species start at the early plant stage, that is, during germination. We tested whether the germination of two legume species, alsike clover and black medic, characterized by a contrasting response to water availability and temperature is affected by mixing. We set up four experiments in each of which we compared a 1:1 mixture against the two monocultures, and combined this with various other experimental factors. These additional factors were (i) varied seed densities (50%, 100% and 150% of a reference density) in two field trials in 2016 and 2017, (ii) varied seed densities (high and low) and water availability (six levels, between 25% and 100% of water holding capacity (WHC)) in a greenhouse pot trial, (iii) varied seed spacing in a climate chamber, and (iv) varied temperatures (12 °C, 20 °C and 28 °C) and water availability (four levels between 25% and 100% of WHC) in a climate chamber. Across all experiments, the absolute mixture effects (AME) on germination ranged between -9% and +11%, with a median of +1.3%. Within experiments, significant mixture effects were observed, but the direction of these effects was inconsistent. In the field, AME on germination was significantly negative at some of the tested seed densities. A positive AME was observed in the climate chamber at 12 °C, and the mean AME decreased with increasing temperature. Higher density was associated with decreased germination in the field, indicating negative interaction through competition or allelopathy, among seedlings. Our findings indicate that interaction among seeds in species mixtures may be ongoing during germination, but that the direction of the mixture effect is affected by complex interactions with abiotic and biotic factors.