Biomass Allocation and Competitive Ability of a Semiarid Perennial Grass and a Legume in Mixtures under Periodical Soil Water Decreasing Conditions

Soil moisture is the key factor controlling plant growth in semiarid grasslands. Here, we sought to evaluate the effects of soil moisture decreasing at different growth stages on biomass accumulation, water use efficiency, and plant-plant interaction of a C3 leguminous subshrub Lespedeza davurica (L) and a C4 perennial grass Bothriochloa ischaemum (B) when sown singly and as a mix in five different ratios in a pot experiment. Results showed that soil water decrease significantly reduced total biomass production of the mixtures by 3.7–53.8% compared with well-watered conditions, and plants at the heading and flowering periods were more vulnerable to soil water decline than those at the late stage. The relative yield total (RYT) of the mixtures was mostly greater than those sown singly. Soil water decreasing increased root/shoot ratio and water use efficiency (WUE) of the mixtures, and such effects were mediated by mixture ratio and/or growth stage. In the mixtures, a strong intraspecific competition was observed in B. ischaemum, whereas interspecific competition in L. davurica. The highest overall biomass (86.47 g pot−1) and WUE (6.33 g kg−1) were observed when the mixture ratio was B:L = 10:2 regardless of soil moisture, and thus could be considered an optimal mixture ratio for establishing restored grassland using the two species. Our results suggest that sown seed mixtures of the two species with an appropriate ratio could sustain a relatively high total biomass production and improve WUE under soil water decreasing conditions in the semiarid Loess Plateau.