Urbanization-induced soil organic carbon loss and microbial-enzymatic drivers: insights from aggregate size classes in Nanchang city, China.

Soil microorganisms and enzymes play crucial roles in soil organic carbon (SOC) sequestration by promoting soil aggregate formation and stability and by participating in SOC cycling and accumulation. However, the effects by which soil microorganisms and enzymes act as mediators driving dynamic changes in SOC during rapid urbanization remain unclear. Therefore, this study selected the built-up area of Nanchang City, China (505 km2), as the study area. Sampling surveys were conducted using 184 sample plots stratified based on the proportion of impermeable surface area to distinguish different urbanization levels. The driving factors of dynamic changes in SOC of different aggregates during the process of urbanization were analyzed using the soil microbial community and enzyme activities. The results demonstrated that with an increase in urbanization intensity, both SOC content and stock exhibited a significant decline (p < 0.05). The highest SOC stock and contribution rate were observed in the 0.25-1 mm aggregates, and they were significantly influenced by urbanization (p < 0.05). In addition, the biomass of gram-positive bacteria (G+) and actinomycetota, and the activities of N-acetylglucosaminidase and acid phosphatase (AP) were significantly higher in low-urbanization areas than in high-urbanization areas (p < 0.05). SOC of each aggregate was positively correlated with fungi, arbuscular mycorrhizal fungi, G+, gram-negative bacteria, actinomycetota, protozoa, ß-1,4-glucosidase, N-acetylglucosaminidase, AP, urease, and catalase. Compared to soil enzymes, soil microorganisms exhibited a greater role in SOC sequestration (22.7%). Additionally, a structural equation model indicated that urbanization can directly or indirectly lead to a decrease in SOC of aggregates by altering soil physicochemical properties and affecting microbial and enzyme dynamics. However, the larger vegetation characteristics index mitigate the negative impacts of urbanization on SOC. Overall, urbanization had a negative impact on soil carbon storage. In the future, it is important to consider strategies that focus on improving soil nutrients, maintaining soil structure, protecting existing urban trees, and enhancing plant diversity during the urbanization process. These measures can help increase soil microbial biomass and enzyme activity, thereby improving soil and aggregate-related SOC content. The study could contribute to enhancing carbon sequestration in urban greenspaces.

Reforestation Increases the Aggregate Organic Carbon Concentration Induced by Soil Microorganisms in a Degraded Red Soil, Subtropical China.

In the process of biological carbon (C) sequestration during reforestation in degraded red soil, due to the decomposition of soil microorganisms, the interaction between soil organic carbon (SOC) and aggregates has an important effect on soil C sequestration. In this study, six common reforestation models and three soil layers were selected in a degraded red soil area of the central subtropical region to determine the composition of soil aggregates and the distribution of SOC in soil aggregates. Based on the results of the soil physicochemical properties and microbial community composition biomass, we assessed the changes in aggregate-associated organic C storage during fluctuations in the stability of the aggregates. After reforestation, the SOC stock increased by 131.28-140.00%. Compared with the three pure forests and broad-leaved mixed forests, coniferous and broad-leaved mixed forests showed the largest proportion of macroaggregates (85.48-89.37%) and higher SOC accumulation. Soil microbial biomass mainly affected the decomposition process of SOC by affecting the stability of the soil aggregates, and the effect of bacteria was more significant. Coniferous and broad-leaved mixed forests can provide more soil microorganisms and C sources than pure forest, thus promoting macroaggregate formation and stability and related organic C storage. This reforestation model has greater C sequestration potential.

Ca2+ participates in programmed cell death by modulating ROS during pollen cryopreservation.

KEY MESSAGE: After cryopreservation, the Ca2+ content increased, which affected the intracellular ROS content, then participated in the occurrence of programmed cell death in pollen. Programmed cell death (PCD) is one of the reasons for the decline in pollen viability after cryopreservation. However, the role of calcium ions (Ca2+) in PCD during pollen cryopreservation has not been revealed in the existing studies. In this study, Paeonia lactiflora 'Fen Yu Nu' pollen was used as the research material for investigating the effects of Ca2+ changes on PCD indices and reactive oxygen species (ROS) during pollen cryopreservation. The results showed that after cryopreservation, with the decrease of pollen viability, the Ca2+ content significantly increased. The regulation of Ca2+ content had a significant effect on PCD indices, which showed that the Ca2+ carrier A23187 accelerated the decrease of mitochondrial membrane potential level and increased the activity of caspase-3-like and caspase-9-like proteases and the apoptosis rate. The expression levels of partial pro-PCD genes were upregulated, the anti-PCD gene BI-1 was downregulated, and the addition of Ca2+-chelating agent EGTA had the opposite effect. The addition of the Ca2+ carrier A23187 after cryopreservation significantly increased the ROS content of pollen, the addition of the Ca2+-chelating agent EGTA had the opposite effect, and Ca2+ regulators also had significant effects on the contents of ROS production and clearance-related substances. Ca2+ affected intracellular ROS content by acting on the ROS production and clearance system during the cryopreservation of pollen and is thus involved in the occurrence of PCD.

Cryopreserved-pollen viability is regulated by NO-induced programmed cell death.

KEY MESSAGE: After cryopreservation, the NO content in pollen increased, inducing programmed cell death as a key reason for reduced viability. Low recovery of biomaterials after cryopreservation is a bottleneck that limits the application of this technology. At present, the mechanism of viability decline after cryopreservation is not fully understood. In this study, the effects of nitric oxide (NO) on programmed cell death (PCD) and its relationship with viability were investigated, using Paeonia lactiflora 'Fen Yu Nu' pollen with significantly decreased viability after cryopreservation. The results showed that: the activity of caspase-3-like and caspase-9-like protease and the apoptosis rate of pollen cells were significantly increased, the expression level of the promoting PCD (pro-PCD) genes was up-regulated, while the expression level of the inhibiting PCD (anti-PCD) genes was down-regulated after preservation in liquid nitrogen (LN); the NO content in pollen cells increased significantly after LN exposure. The correlation analysis showed that NO was significantly correlated with pollen viability and all indicators of PCD. The addition of a NO carrier SNP after LN storage reduced pollen viability, increased endogenous NO content, decreased mitochondrial membrane potential level, activated caspase-3-like and caspase-9-like protease in pollen cells, and increased cell apoptosis rate. The expression levels of pro-PCD genes PDCD2 and ATG8CL were significantly up-regulated, while the expression levels of anti-PCD genes DAD1, BI-1 and LSD1 were significantly down-regulated. The addition of NO scavenger c-PTIO improved pollen viability, and produced the opposite effect of sodium nitroferricyanide (III) dihydrate (SNP), but did not change the mitochondrial membrane potential. These results suggest that NO induced PCD during the cryopreservation of pollen, which was one of the reasons for the significant decrease of pollen viability after cryopreservation.

Local patterns of arbuscular mycorrhizal fungal diversity and community structure in a natural Toona ciliata var. pubescens forest in South Central China.

Toona ciliata var. pubescens (Toona in Meliaceae) (Tc) is listed as an endangered species, and there are natural regeneration obstacles due to its long-term excessive exploitation and utilization. Arbuscular mycorrhizal fungi (AMF) can produce beneficial effects for plant growth and natural regeneration. However, the characteristics of the AMF community in natural Tc forests are poorly understood. The Illumina PE250 high-throughput sequencing method was used to study the characteristics of the AMF community in the rhizosphere soil and roots associated with three dominant tree species (Tc; Padus buergeriana, Pb; and Maesa japonica, Mj) in a natural Tc forest in Guanshan National Natural Reserve, South Central China. The results found that Glomeraceae was the most abundant AMF family in the rhizosphere soil and roots. Moreover, the relative abundance of Archaeosporaceae in rhizosphere soil was significantly larger than that in the roots; in contrast, the relative abundance of Glomeraceae in rhizosphere soil was significantly lower than that in the roots (p < 0.05). Regarding different tree species, the relative abundances of Acaulosporaceae and Geosiphonaceae were larger in Mj and Tc than in Pb. AMF operational taxonomic units (OTUs) were 1.30-, 1.43-, and 1.71-fold higher in the Tc, Pb, and Mj rhizosphere soil, respectively, than in the corresponding roots. Nevertheless, higher AMF community richness was found in the roots compared to that in the rhizosphere soil based on the Chao index. This finding indicated that AMF of a relatively high aggregation degree were in roots, and more AMF groups with relatively low abundance occurred in the rhizosphere soil, which correspondingly lowered the calculated richness index of the AMF community. A redundancy analysis showed that different soil chemical properties impacted variations in the AMF community characteristics differently. This study has great significance for the interpretation of AMF diversity survey and the application design of AMF in vegetation restoration.

Depression of the soil arbuscular mycorrhizal fungal community by the canopy gaps in a Japanese cedar (Cryptomeria japonica) plantation on Lushan Mountain, subtropical China.

Both canopy gaps (CG) and arbuscular mycorrhizal fungi (AMF) play key roles in seedling establishment and increasing species diversity in forests. The response of AMF to canopy gaps is poorly understood. To assess the long-term effects of canopy gaps on soil AMF community, we sampled soil from plots in a 50-year Cryptomeria japonica (L.f.) D. Don. plantation, located in Lushan Mountain, subtropical China. We analyzed the AMF community, identified through 454 pyrosequencing, in soil and edaphic characteristics. Both richness and diversity of AMF in CG decreased significantly compared to the closed canopy (CC). The differences of the AMF community composition between CG and CC was also significant. The sharp response of the AMF community appears to be largely driven by vegetation transformation. Soil nutrient content also influenced some taxa, e.g., the low availability of phosphorus increased the abundance of Acaulospora. These results demonstrated that the formation of canopy gaps can depress AMF richness and alter the AMF community, which supported the plant investment hypothesis and accentuated the vital role of AMF-plant symbioses in forest management.

The ROS-associated programmed cell death causes the decline of pollen viability recovered from cryopreservation in Paeonia lactiflora.

KEY MESSAGE: After cryopreservation, the occurrence of apoptosis-like programmed cell death events induced by the accumulation of ROS reduces pollen viability. Cryopreservation, as a biotechnological means for long-term preservation of pollen, has been applied to many species. However, after cryopreservation, the viability of pollen significantly decreases via a mechanism that is not completely clear. In this study, the pollen of Paeonia lactiflora 'Zi Feng Chao Yang', which exhibits significantly reduced viability after liquid nitrogen (LN2) storage, was used to study the relationship among pollen viability, programmed cell death (PCD) and reactive oxygen species (ROS). The apoptosis rate was increased significantly in pollen with decreased viability after cryopreservation, and the changes in ROS generation and hydrogen peroxide (H2O2) were consistent with the apoptosis rate. Correlation analysis results showed that the apoptosis rate is positively correlated with ROS generation and H2O2 content. In addition, ascorbic acid (AsA), glutathione (GSH) and ascorbic acid reductase (APX) levels were significantly correlated with ROS and H2O2. After LN2 preservation for 8 months, the exogenous antioxidants AsA and GSH at appropriate concentrations significantly decreased H2O2 content, inhibited PCD indicator levels, and increased cryopreserved pollen viability. These observations suggest that PCD occurred in pollen during LN2 preservation for 1-8 months and was induced by the accumulation of ROS in pollen after cryopreservation, thus explaining the main reasons for the reduction in pollen viability after cryopreservation in LN2.

Changes of pollen viability of ornamental plants after long-term preservation in a cryopreservation pollen bank.

This study determined the changes in pollen viability of 102 species/cultivars of ornamental plants (affiliated to 32 genera of 14 families) following long-term liquid nitrogen storage in a cryopreservation pollen bank. The goal was to provide information on the safety and stability of pollen cryopreservation technology. Fresh pollen at the time of storage was used as the control, and the study examined the pollen viability of ornamental plants cryopreserved for 8, 9, or 10 years. The results show that pollen of the 102 species/cultivars in the cryopreservation pollen bank retained viability ranging from 1% to 58%, After long-term storage there were changes in viability: 11.76% (12 species/cultivars) had increased viability, 16.67% (17 species/cultivars) had stable viability, and the viability of 71.57% (73 species/cultivars) showed a decreasing trend.