The influence of elevated CO 2 and soil depth on rhizosphere activity and nutrient availability in a mature Eucalyptus woodland

[摘要] Elevated carbon dioxide ( e CO 2 ) in the atmosphere increases forest biomass productivity but only where soil nutrients, particularly nitrogen (N) and phosphorus (P), are not limiting growth. e CO 2 , in turn, can impact rhizosphere nutrient availability. Our current understanding of nutrient cycling under e CO 2 is mainly derived from surface soil, leaving mechanisms of the impact of e CO 2 on rhizosphere nutrient availability at deeper depths unexplored. To investigate the influence of e CO 2 on nutrient availability in soil at depth, we studied various C, N, and P pools (extractable, microbial biomass, total soil C and N, and mineral-associated P) and nutrient cycling processes (enzyme activity and gross N mineralisation) associated with C, N, and P cycling in both bulk and rhizosphere soil at different depths at the Free Air CO 2 enrichment facility in a native Australian mature Eucalyptus woodland (EucFACE) on a nutrient-poor soil. We found decreasing nutrient availability and gross N mineralisation with depth; however, this depth-associated decrease was reduced under e CO 2 , which we suggest is due to enhanced root influence. Increases in available PO 4 3 - , adsorbed P, and the C : N and C : P ratio of enzyme activity with depth were observed. We conclude that the influences of roots and of e CO 2 can affect available nutrient pools and processes well beyond the surface soil of a mature forest ecosystem. Our findings indicate a faster recycling of nutrients in the rhizosphere, rather than additional nutrients becoming available through soil organic matter (SOM) decomposition. If the plant growth response to e CO 2 is reduced by the constraints of nutrient limitations, then the current results would call to question the potential for mature tree ecosystems to fix more C as biomass in response to e CO 2 . Future studies should address how accessible the available nutrients at depth are to deeply rooted plants and if fast recycling of nutrients is a meaningful contribution to biomass production and the accumulation of soil C in response to e CO 2 .

[学科分类] 大气科学