Glomalin-related soil protein enriched in δ13C and δ15N excels at storing blue carbon in mangrove wetlands

Abstract

Glomalin-related soil protein (GRSP) derived from arbuscular mycorrhizal fungi can be transported from land to sea and captured in mangrove wetlands, thereby contributing to soil C and N pools. However, the stable isotope signatures of GRSP and the key influencing factors that affect its isotope values in coastal wetlands remain unknown. In this study, the results showed that total-GRSP (T-GRSP) was a significant contributor of C and N content to mangrove soil. We first compared stable isotope (δ¹³C and δ¹⁵N) values and C/N ratios of GRSP with those of other blue carbon sources in a typical mangrove wetland. The isotope fingerprints of T-GRSP, mangrove soils, mangrove plants, and tidal waters were identified. Unlike those of the conventional sources, the δ¹³C and δ¹⁵N values of T-GRSP were -25.04‰ to -22.83‰ and 3.22‰ to 7.24‰, respectively, and the mean C/N ratio was 12.95 in the mangrove cover sites. These findings indicated that T-GRSP is a novel blue carbon source mainly originating from terrestrial ecosystems. Moreover, the δ¹³C and δ¹⁵N values of T-GRSP in mangrove wetlands were affected by vegetation interception and soil properties. Redundancy analysis results indicated that pH, moisture, depth, and salinity were key factors influencing the T-GRSP isotope fingerprints in mangrove wetlands. Additionally, the simultaneous changes in T-GRSP content, isotope values, and C/N ratios among mangrove cover sites, a mudflat, and tidal waters suggested that this protein is a sensitive tracer between land and sea. Overall, the isotope signatures of GRSP captured by mangroves were identified for the first time, which will have important implications for the estimation of the blue carbon budget and identification of the blue carbon sources in global coastal regions.

Keywords: Blue carbon; Glomalin-related soil protein; Isotope; Mangrove wetland; Soil organic matter.