Plant species,atmospheric CO2 and soil N interactively or additively control C allocation within plant-soil systems

Two plant species,Medicago truncatula (legume) and Avena sativa (non-legume),were grown in low-or high-N soils under two CO2 concentrations to test the hypothesis whether C allocation within plant-soil system is interactively or additively controlled by soil N and atmospheric CO2 is dependent upon plant species. The results showed the interaction between plant species and soil N had a significant impact on microbial activity and plant growth. The interaction between CO2 and soil N had a significant impact on soil soluble C and soil microbial biomass C under Madicago but not under Avena. Although both CO2 and soil N affected plant growth significantly,there was no interaction between CO2 and soil N on plant growth. In other words,the effects of CO2 and soil N on plant growth were additive. We considered that the interaction between N2 fixation trait of legume plant and elevated CO2 might have obscured the interaction between soil N and elevated CO2 on the growth of legume plant. In low-N soil,the shoot-to-root ratio of Avena dropped from 2.63±0.20 in the early growth stage to 1.47±0.03 in the late growth stage,indicating that Avena plant allocated more energy to roots to optimize nutrient uptake (i.e. N) when soil N was limiting. In high-N soil,the shoot-to-root ratio of Medicago increased significantly over time (from 2.45±0.30 to 5.43±0.10),suggesting that Medicago plants allocated more energy to shoots to optimize photosynthesis when N was not limiting. The shoot-to-root ratios were not significantly different between two CO2 levels.