The influence of elevated CO₂ and soil depth on rhizosphere activity and nutrient availability in a mature Eucalyptus woodland

Abstract

Abstract. Elevated carbon dioxide (eCO2) in the atmosphere increases forest biomass productivity but only where soil nutrients, particularly nitrogen (N) and phosphorus (P), are not limiting growth. eCO2, in turn, can impact rhizosphere nutrient availability. Our current understanding of nutrient cycling under eCO2 is mainly derived from surface soil, leaving mechanisms of the impact of eCO2 on rhizosphere nutrient availability at deeper depths unexplored. To investigate the influence of eCO2 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 CO2 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 eCO2, which we suggest is due to enhanced root influence. Increases in available PO43-, 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 eCO2 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 eCO2 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 eCO2. 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 eCO2.

Microbial competition for phosphorus limits the CO2 response of a mature forest

“…It remains to be seen whether the eCO 2 -induced increase in plant belowground C allocation leads to a more detectable response of P availability to eCO 2 being realized over longer time frames. The observed reduction in soil pH at depth is consistent with enhanced plant exudates and provides an indication that this may occur 33 —it reflects an additional pathway through which soil P can be made available to plants under eCO 2 (ref. 39 ).…”

“…The extractable inorganic P pool (that is, labile P i ) was determined quarterly between 2013 and 2015 on 0–10 cm layer soils using the Bray-1 P extraction 30 , 73 method, and once in 2017 (0–10 cm, 10–30 cm and 30–60 cm) 33 . Phosphate concentrations in soil extracts were determined colorimetrically using the molybdate blue assay (AQ2 Discrete Analyzer SEAL Analytical) using an established method for available P (EPA-118-A rev.5).…”

“…3c ), suggesting that the soil P supply was sufficient to meet the annual plant P demand. Nevertheless, given that 92% of the fine-root and similar fractions of microbial biomass and microbial P content were found in the top 30 cm of the soil 33 , it is probable that plant P uptake occurred predominantly in the shallower soil layers. Fluxes for soil P leaching and atmospheric P deposition were negligible at the ecosystem scale (Fig.…”

Synergistic Effect of Different Soilless Substrates on the Hardening of In- vitro Raised Banana (Musa sp.) Saplings of Grand Naine

“…The maximum number of roots (5.9) was observed in PHT7 treatment which was 7.27% higher as compared to control (5.5), followed by PHT4 (5.6) which was 1.81% higher than the control as shown in Table 3. More number of roots brings adequate amount of nutrients which are needed for better growth and development to the plants due to the nutrients uptake from soil [15,16].…”