Soil C, N and P cycling enzyme responses to nutrient limitation under elevated CO₂

Elevated CO₂ (eCO₂) can stimulate plant productivity and increase carbon (C) input to soils, but nutrient limitation restricts productivity. Despite phosphorus (P)-limited ecosystems increasing globally, it is unknown how nutrient cycling, particularly soil microbial extra cellular enzyme activity (EEA), will respond to eCO₂ in such ecosystems. Long-term nutrient manipulation plots from adjacent P-limited acidic and limestone grasslands were exposed to eCO₂ (600 ppm) provided by a mini-Free Air CO₂ Enrichment system. P-limitation was alleviated (35 kg-P ha−1 y−1 (P35)), exacerbated (35 kg-N ha⁻¹ y⁻¹ (N35), 140 kg-N ha⁻¹ y⁻¹ (N140)), or maintained (control (P0N0)) for > 20 years. We measured EEAs of C-, N- and P-cycling enzymes (1,4-β-glucosidase, cellobiohydrolase, N-acetyl β-D-glucosaminidase, leucine aminopeptidase, and acid phosphatase) and compared C:N:P cycling enzyme ratios using a vector analysis. Potential acid phosphatase activity doubled under N additions relative to P0N0 and P35 treatments. Vector analysis revealed reduced C-cycling investment and increased P-cycling investment under eCO₂. Vector angle significantly increased with P-limitation (P35 < P0N0 < N35 < N140) indicating relatively greater investment in P-cycling enzymes. The limestone grassland was more C limited than the acidic grassland, characterised by increased vector length, C:N and C:P enzyme ratios. The absence of interactions between grassland type and eCO₂ or nutrient treatment for all enzyme indicators signaled consistent responses to changing P-limitation and eCO₂ in both grasslands. Our findings suggest that eCO₂ reduces C limitation, allowing increased investment in P- and N-cycle enzymes with implications for rates of nutrient cycling, potentially alleviating nutrient limitation of ecosystem productivity under eCO₂.