Seagrass beds are considered to be substantial sinks of “blue carbon”. However, differentiation in the carbon sink capacities of seagrass beds in different regions with distinct nutrient conditions remains unclear. In this study, sediment carbon stocks, seagrass biomass, and microbial community structures and potential functions of seagrass beds in eutrophic seawater adjacent to Hainan Island and oligotrophic seawater around the Xisha Islands were compared. Our results showed that sediment mineralizable organic carbon and dry bulk density were substantially higher on Hainan Island than on the Xisha Islands (t-test, p < 0.05), while sediment carbon stocks and the total organic carbon were comparable between the two regions (p > 0.05). Similarly, seagrass biomass was much higher on Hainan Island (p < 0.05). Sediment carbon stocks positively correlated with sediment nitrogen and negatively correlated to sediment pH and grain size (p < 0.05). Bacterial diversities were similar in the two regions, while fungi were more diverse on Hainan Island (p < 0.05). Proteobacteria, Desulfobacterota, Ascomycota and Basidiomycota could account for degrading organic carbon on Hainan Island. Proteobacteria and Bacteroidota may contribute primarily to carbon loss in the seagrass beds of the Xisha Islands. This study strengthens our understanding of the effects of human activities on carbon sequestration in seagrass bed ecosystems.

Effect of Biochar Application on the Efficacy of the Nitrification Inhibitor Dicyandiamide in Soils

Introduction: The transplantation of seagrass fragments with shoots and rhizomes is the most common method for the ecological restoration of damaged seagrass meadows.Methods: The aim of this study was to explore the effects of exogenous addition (10 mg per month for 3 months) of three commonly used plant growth regulators (PGRs), including indoleacetic acid (IAA), gibberellin (GA3), and paclobutrazol (PP333) on the growth and physiology of transplanted Thalassia hemprichii fragments (TTFs), with and without the rhizome apex (RA), using a simulation test. [Results] IAA and GA3 elevated the photosynthetic capacity and growth rate of TTF leaves but did not significantly alter leaf length and width. In contrast, PP333 reduced the leaf photosynthetic capacity and growth rate, while greatly increasing the leaf width and root viability. Additionally, PP333 treatment led to an increase in antioxidant enzyme activities (peroxidase [POD], superoxide dismutase [SOD], and catalase [CAT]), as well as malondialdehyde (MDA) and total phenol contents in TTFs, indicating some stress effects on the seagrass. Furthermore, IAA and GA3 decreased soluble sugar and protein contents and increased starch content in TTF tissues, whereas PP333 treatment elevated both nonstructural carbohydrate and soluble protein contents. The presence of RA positively affected the growth and physiology of T. hemprichii compared with TTFs without RA. There was a significant interaction between the PGRs and RA on leaf growth rate, chlorophyll fluorescence variables, and photosynthetic pigment content of the TTFs (p < 0.05).Discussion: This study provides guidelines for the application of PGRs during the transplantation of T. hemprichii fragments for the restoration of seagrass meadows.

In order to investigate the relationships between the stability of Cu complexes and the nitrification inhibition effects of corresponding ligands, the effects of 22 kinds of Cu (II) ligands and eight kinds of Cu (I) ligands were evaluated in three types of soil on nitrification potentials (NPs) under short-term incubation (5 h) and nitrification inhibitory rates (NIRs) under long-term incubation (14 d). The results showed that the inhibitory effect of Cu (II) or Cu (I) ligands on nitrification exhibited significant linear relationships (P < 0.01) with the logarithms of the first stability constants (lgK₁) and the total stability constants (lgK_f) of corresponding complexes. Furthermore, lgK_f of Cu (II) complexes exhibited a greater linear correlation than the corresponding lgK₁ with the nitrification inhibition effects of the corresponding ligands; lgK_f of Cu (I) complexes exhibited a steeper linear relationship than that of Cu (II) complexes with NPs or NIRs, and hence, nitrification inhibition effects of Cu (I) ligands almost doubled that of Cu (II) ligands when the lgK_f values of Cu (I) and Cu (II) complexes have the same changes.