Water limitation and plant inter-specific competition reduce rhizosphere-induced C decomposition and plant N uptake

Plants can affect soil organic matter decomposition and mineralization through litter inputs, but also more directly through root-microbial interactions (rhizosphere effects). Depending on resource availability and plant species identity, these rhizosphere effects can be positive or negative. To date, studies of rhizosphere effects have been limited to plant species grown individually. It is unclear how belowground resources and inter-specific interactions among plants may influence rhizosphere effects on soil C decomposition and plant N uptake. In this study, we tested the simple and interactive effects of plant diversity and water availability on rhizosphere-mediated soil C decomposition and plant N uptake. The study was conducted in the greenhouse with five semi-arid grassland species (monocultures and mixtures of all five species) and two water levels (15 and 20% gravimetric soil moisture content). We hypothesized that microbial decomposition and N release would be less in the low compared to high water treatment and less in mixtures compared to monocultures. Rhizosphere effects on soil C decomposition were both positive and negative among the five species when grown in monoculture, although negative effects prevailed by the end of the experiment. When grown in mixture, rhizosphere effects reduced soil C decomposition and plant N uptake compared to monocultures, but only at the low-water level. Our results suggest that when water availability is low, plant species complementarity and selection effects on water and N use can decrease soil C decomposition through rhizosphere effects. Although complementarity and selection effects can increase plant N uptake efficiency, plant N uptake in the mixtures was still lower than expected, most likely because rhizosphere effects reduced N supply in the mixtures more than in the monocultures. Our results indicate that rhizosphere effects on C and N cycling depend on water availability and inter-specific plant interactions. Negative rhizosphere effects on soil C decomposition and N supply in mixtures relative to monocultures of the component species could ultimately increase soil C storage and possibly influence how plant communities in semi-arid grasslands respond to global climate change.