Effects of CO2 enrichment on plant-soil relationships of Lepidium latifolium

The exotic crucifer Lepidium latifolium L. (perennial pepperweed) is invading wetland and riparian habitats throughout the western United States. Based on previous field studies, our working hypothesis proposed that L. latifolium elevates soil nutrient acquisition ability in response to CO2 enrichment. Replicates of L latifolium were grown in a high fertility and low fertility soil (along with unplanted controls) in a glasshouse at ambient and elevated CO2 concentrations (360 and 699 mumol mol(-1), respectively). Plants were harvested after 81 days and numerous plant and soil attributes measured. Above-ground plant mass was influenced by a significant CO2 treatment x soil interaction (P < 0.001) with CO2 enrichment inducing a greater proportional increase in mass for the low fertility soil. Root concentrations of citrate, malate, and ortho-phosphate and enzyme activities of amidase and asparaginase did not differ between the CO2 treatments across soils. Above-ground tissue concentrations of N, S, P, Mg, K, Fe, and Zn consistently decreased for both soils with CO2 enrichment, corresponding with higher biomass per unit nutrient. Plants grown in the low fertility soil had higher concentrations of N, S, P, Ca, and Mg in above-ground tissue than plants grown in the high fertility soil. Carbon dioxide enrichment decreased tissue N:S ratios by > 20% and increased, though not significant, tissue C:N ratio by 38% in high fertility soil and by 51% in low fertility soil. For most soil attributes measured, there was a main effect or interaction with soil fertility level. Soil attributes differed between soil fertility levels and, with the exception of SO42-, were not influenced by the presence of L. latifolium. Soil attributes increased by CO2 enrichment included acetate extractable Mg2+ (high fertility soil only), net 30 day N mineralization potential (unplanted control soils only), available N (high fertility soil), bicarbonate extractable P, soil-solution SO42- (L. latifolium planted pots only), and soil-solution Mg2+ (high fertility control soil only). Collectively, these data tangentially support our working hypothesis that CO2 enrichment increases nutrient availability. That availability of some nutrients increases without plant growth (control soils), however, suggests an interaction of elevated CO2 with soil microflora.