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

J Ben Keane,Iain P Hartley,Gareth K Phoenix,Franco Miglietta,Christopher R Taylor,Marcel R Hoosbeek

doi:10.1007/s10533-020-00723-1

Abstract

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.Graphic abstract

Highlights

Terrestrial ecosystems sequester nearly one-third of anthropogenic CO2 emissions (IPCC, 2013), with this uptake largely caused by elevated atmospheric CO2 concentrations increasing photosynthesis and carbon (C) storage in plant biomass and soils (IPCC, 2013)
There was a near-significant change (F[3,12] = 3.31, p \ 0.058) in LAP activity, with activity increasing with P limitation by nearly 30% from 29 nmol g-1 h-1 in the P35 treatment to 21 nmol g-1 h-1 in the N140 treatment (Fig. 1j–l)
There was, a significant interaction between nutrient and grassland in enzyme activity (EEA) of both NAG (F[3,80] = 4.28, p \ 0.01) and LAP (F[3,80] = 4.42, p \ 0.01), where EEA of LAP declined with increasing P limitation, and NAG remained fairly constant in the limestone grassland, but less clear, if not opposing effects for both enzymes was seen in the acidic grassland (Fig. 1g–i)

Summary

Introduction

Terrestrial ecosystems sequester nearly one-third of anthropogenic CO2 emissions (IPCC, 2013), with this uptake largely caused by elevated atmospheric CO2 concentrations (eCO2) increasing photosynthesis and carbon (C) storage in plant biomass and soils (IPCC, 2013). Despite the spatial extent and increasing importance of P limitation, we know very little about how P limitation affects ecosystem responses to eCO2, limiting our ability to predict future rates of C uptake by the terrestrial biosphere (Zhang et al 2014). In this context, grasslands are important, representing 20% of global terrestrial net primary productivity (NPP) (Chapin et al 2011) and being the most spatially extensive P-limited ecosystems in temperate regions (Jackson et al 2002; Watson et al 2011)

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Conclusion