Net soil carbon input under ambient and elevated CO2 concentrations: isotopic evidence after 4 years

Elevation of atmospheric CO2 concentration is predicted to increase net pri mary production, which could lead to additional C sequestration in terrestr ial ecosystems. Soil C input was determined under ambient and Free Atmosphe ric Carbon dioxide Enrichment (FACE) conditions for Lolium perenne L. and T rifolium repens L. grown for four years in a sandy-loam soil. The C-13 cont ent of the soil organic matter C had been increased by 5 parts per thousand compared to the native soil by prior cropping to corn (Zea mays) for >20 y ears. Both species received low or high amounts of N fertilizer in separate plots. The total accumulated above-ground biomass produced by L. perenne d uring the 4-year period was strongly dependent on the amount of N fertilize r applied but did not respond to increased CO2. in contrast, the total accu mulated above-ground biomass of T. repens doubled under elevated CO2 but re mained independent of N fertilizer rate. The C:N ratio of above-ground biom ass for both species increased under elevated CO2 whereas only the C:N rati o of L. perenne roots increased under elevated CO2. Root biomass of L. pere nne doubled under elevated CO2 and again under high N fertilization. Total soil C was unaffected by CO2 treatment but dependent on species. After 4 ye ars and for both crops, the fraction of new C (F-value) under ambient condi tions was higher (P=0.076) than under FACE conditions: 0.43 vs. 0.38. Soil under L. perenne showed an increase in total soil organic matter whereas N fertilization or elevated CO2 had no effect on total soil organic matter co ntent for both systems. The net amount of C sequestered in 4 years was unaf fected by the CO2 concentration (overall average of 8.5 g C kg(-1) soil). T here was a significant species effect and more new C was sequestered under highly fertilized L. perenne. The amount of new C sequestered in the soil w as primarily dependent on plant species and the response of root biomass to CO2 and N fertilization. Therefore, in this FACE study net soil C sequestr ation was largely depended on how the species responded to N rather than to elevated CO2.