Microbial nutrient cycling

Our aim was to understand if the soil microbial response to eCO₂ is constrained by P limitation, and if P application will shift the system from P to N limitation and how this shifting the limiting nutrient might modify this response of the forest to eCO₂.

I studied the interactive effect of P fertilization and eCO₂ at EucFACE. I followed soil nutrient, microbial pools, N processes and enzymatic activities right before fertilization and two months after. The experiment was conducted from March to May 2023 in the four first FACE rings. Inside each ring, samples were taken in the soil sampling plots and each sampling plot had a subplot that did not receive fertilization that allowed us to compare unfertilized and fertilized plots in each ring. 

The detailed analyses are root biomass, gravimetric moisture, N-NH4+, N-NO3-, free amino acids, Bray-P, Dissolved organic C, Dissolved organic N, Microbial biomass C, N and P, Gross N mineralization, Gross N depolymerization and specific compound N depolymerization, Enzymatic activities: β-1,4-glucosidase (BG), β-d-cellobiosidase (CB), α-glucosidase (AG), and β-xylosidase (Bxyl), β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), Phosphatase (Pho). I also completed this dataset with soil ion exchange membranes measuring available NO3, NH4, PO4 during two years before fertilization (March 2023).

Key findings: We found that P addition decreased extracellular soil enzymatic activities associated with C-N-P-mining (─ 50%), increased microbial NH4+ retention (immobilization: mineralization ratio; + 23%) and microbial C use efficiency (CUE; + 12%), causing a reduction in plant-available N (─ 30%) independently from eCO₂. Under eCO₂, P addition stimulated protein depolymerization and C-P enzyme activities. Compound specific analyses revealed increased microbial biosynthesis with P addition via the assimilation of key amino acids such as alanine, glycine and glutamate. These findings indicate that P limitation constrains microbial C-N cycling under eCO₂ by diverting microbial C investment toward P acquisition rather than growth. While alleviating P limitation can rapidly stimulate microbial cycling and promotes microbial C retention under eCO₂, this response may only be transient, as enhanced microbial growth drives the system towards N limitation.