Salt stress, and particularly short-term salinity stress, is one of the most serious abiotic factors limiting plant survival and growth in China. It has been established that exogenous spermidine (Spd) stimulats tolerance to salt stress in plants. In the present study, two cultivars that are typically grown in China were used. The two zoysiagrass cultivars, exhibiting a sensitive ( cv. Z081) or tolerant ( cv. Z057) salt stress adaptation ability, were subjected to 200 mM salt stress and treated with different exogenous Spd concentrations for 8 days. Polyamine (Put, Spd and Spm) contents and polyamine metabolic enzyme (ADC, ODC, SAMDC, PAO and DAO), malondialdehyde (MDA), H2O2 and antioxidant enzyme activities (superoxide dismutase, catalase, and peroxidase) were measured. The results showed that salt stress induced increases in Spd and Spm contents and the activity of ornithine decarboxylase (ODC), S-adenosylmethionine decarboxylase (SAMDC) and diamine oxidase (DAO) in both cultivars. Exogenous Spd application did not compromise polyamine contents through the regulation of polyamine-degrading enzymes, and an increase in PA synthesis enzymes was observed during the experiment. The application resulted in a tendency for the Spd and Spm contents and the activities of ODC, S-adenosylmethionine decarboxylase (SAMDC), DAO, and antioxidant enzymes to first increase and then decrease in both cultivars with an increase in the exogenous Spd concentration. H2O2 and MDA significantly decreased in both cultivars treated with Spd. With an increase in the exogenous Spd concentration, the Spd + Spm level scores showed positive correlations with polyamine synthesis enzymes (ADC, SAMDC), DAO, antioxidant enzymes (SOD, POD, CAT), while showing negative correlations with H2O2 and MDA in both cultivars.

Pecan forests (Carya illinoinensis) are significant contributors to both food and oil production, and thrive in diverse soil environments, including coastal regions. However, the interplay between soil microbes and pecan forest health in coastal environments remains understudied. Therefore, we investigated soil bacterial and fungal diversity in coastal (Dafeng, DF) and inland (Guomei, GM) pecan plantations using high-throughput sequencing. The results revealed a higher microbial diversity in the DF plantation than in the GM plantation, significantly influenced by pH and edaphic factors. The dominant bacterial phyla were Proteobacteria, Acidobacteriota and Bacteroidota in the DF plantation, and Acidobacteriota, Proteobacteria, and Verrucomicrobiota in the GM plantation. Bacillus, Nitrospira and UTCFX1 were significantly more abundant bacterial genera in DF soil, whereas Candidatus Udaeobacter, HSB_OF53-F07 and ADurbBin063-1 were more prevalent in GM soil. Basidiomycota dominated fungal sequences in the GM plantation, with a higher relative abundance of Ascomycota in the DF plantation. Significant differences in fungal genus composition were observed between plantations, with Scleroderma, Hebeloma, and Naucoria being more abundant in DF soil, and Clavulina, Russula, and Inocybe in GM soil. A functional analysis revealed greater carbohydrate metabolism potential in GM plantation bacteria and a higher ectomycorrhizal fungi abundance in DF soil. Significantly positive correlations were detected between certain bacterial and fungal genera and pH and total soluble salt content, suggesting their role in pecan adaptation to coastal environments and saline–alkali stress mitigation. These findings enhance our understanding of soil microbiomes in coastal pecan plantations, and are anticipated to foster ecologically sustainable agroforestry practices and contribute to coastal marshland ecosystem management.

Background and aims Nitrogen (N) deposition is a global driver of change that affects microbial biomass and ecosystem processes. However, it remains unclear whether N enrichment impacts microbial biomass in soil across ecosystems. Methods In this study, we synthesized 1,385 paired observations from 125 studies worldwide to evaluate the effects of N addition on soil microbial biomass. Results We found that N enrichment substantially suppresses total microbial biomass, including bacterial, fungal, and arbuscular mycorrhizal fungi biomass. Importantly, we found strong negative effects on microbial biomass globally caused by N-enrichment-induced soil acidification. Moreover, N enrichment mainly shapes the acidification of soil through the increased concentration of ammonium and by changing exchangeable cations in soil, including magnesium, calcium, sodium, manganese, and aluminum ions. Conclusions Overall, our findings demonstrate that N-enrichment-induced soil acidification is a crucial factor to consider for preserving microbial communities and ecosystem function under future trends of increasing N enrichment.