Relaxing P limitation in a P-limited Eucalyptus forest rapidly enhances microbial C-N cycling under elevated CO2
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Phosphorus (P) can restrict the capacity of forests to store additional carbon (C) with increasing carbon dioxide (CO 2 ) concentration. Although P limitation is widespread, P addition experiments in mature forests are rare, leaving large uncertainties about whether alleviating P limitation under elevated CO 2 (eCO 2 ) will enhance C storage or instead shift limitation toward nitrogen (N). Here, we used a parallel P-fertilization x eCO 2 manipulation in a mature forest to investigate the acute nutrient cycling response to P fertilization under eCO 2 with a particular focus on N cycling. In April 2023, a mature P-limited Eucalyptus Forest at the Euc-Free Air CO 2 Enrichment (Euc-FACE) experiment in Australia, was fertilized with 1.5 g P m -2 following 10 years of CO 2 enrichment. We measured soil gross N mineralization and compound-specific depolymerization rates – offering novel insights into microbial metabolic pathways – alongside extracellular enzymatic activities, and nutrient pools in the top 10 cm of soil before P addition, 10 days and two months afterwards. We found that P addition decreased extracellular soil enzymatic activities associated with C-N-P-mining ( ─ 50%), increased microbial NH 4 + retention (immobilization: mineralization ratio; + 23%) and microbial C use efficiency (CUE; + 12%), causing a reduction in plant-available N (─ 30%) independently from eCO 2 . Under eCO 2 , 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 eCO 2 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 eCO 2 , this response may only be transient, as enhanced microbial growth drives the system towards N limitation.
