Water Limitation and Plant Inter-specific Competition Reduce Rhizosphere-Induced C Decomposition and Plant N Uptake

Authors: Dijkstra, Feike; Morgan, Jack; Blumenthal, Dana; Follett, Ronald

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2010
Publication Date: 5/10/2010
Citation: Dijkstra, F.A., Morgan, J.A., Blumenthal, D.M., Follett, R.F. 2010. Water Limitation and Plant Inter-specific Competition Reduce Rhizosphere-Induced C Decomposition and Plant N Uptake. Soil Biology and Biochemistry. 42:1073-1082.

Interpretive Summary: Plants interact with soil microorganisms by competing for water and nutrients. Plants can also stimulate soil microbial activity by root exudation of energy rich compounds. In return, plants depend on soil microorganisms because they are responsible for soil organic matter decomposition and nutrient release. Although little is known, there are reasons to believe that plant-soil microbial interactions affecting soil organic matter decomposition and nutrient release (rhizosphere effects) in mixtures of different plant species can be different from rhizosphere effects in monocultures of the component plant species. Differences in rhizosphere effects among plant species and plant species combinations can have profound impacts on soil carbon sequestration and how plant communities respond to global change. We used a novel technique (using the stable isotope 13C as a tracer) to study rhizosphere effects on decomposition and plant nitrogen uptake in plant mixtures and monocultures of semi-arid grassland species in a greenhouse experiment. We observed that rhizosphere effects reduced soil organic matter decomposition and plant N uptake in mixtures compared to component monocultures, particularly when soils were dry. Most likely, competition for water and nutrients was greater in the mixtures than in the monocultures, which resulted in decreased microbial activity in the mixtures. Our results suggest that when plant-microbial competition for water and nutrients is high (i.e., high plant diversity and low water availability), negative rhizosphere effects on soil organic matter decomposition could ultimately increase soil C sequestration, and possibly influence how plant communities in semi-arid grasslands respond to global change.

Technical Abstract: 1. 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 interspecific interactions among plants may influence rhizosphere effects on soil C decomposition and plant N uptake. 2. 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 increased plant-microbial competition for belowground resources in the mixtures and the low-water treatments would reduce microbial decomposition and N release relative to the monocultures and the high water treatments. 3. 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. 4. Our results suggest that when water availability is low, plant species complementarity and selection effects on water and N use can cause lower than expected rhizosphere effects on soil C decomposition in mixtures compared to monocultures. Despite possible complementarity and selection effects on N utilization, plant N uptake in the mixtures was still lower than expected, because rhizosphere effects reduced N supply in the mixtures more than in the monocultures. 5. Synthesis. 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 change.