Edaphic factors mediate the responses of forest soil respiration and its components to nitrogen deposition along an urban-rural gradient.

Exploring the influences of nitrogen deposition on soil carbon (C) flux is necessary for predicting C cycling processes; however, few studies have investigated the effects of nitrogen deposition on soil respiration (Rs), autotrophic respiration (Ra) and heterotrophic respiration (Rh) across urban-rural forests. In this study, a 4-year simulated nitrogen deposition experiment was conducted by treating the experimental plots with 0, 50, or 100 kg·ha-1·year-1 of nitrogen to check out the mechanisms of nitrogen deposition on Rs, Ra, and Rh in urban-rural forests. Our finding indicated a positive association between soil temperature and Rs. Soil temperature sensitivity was significantly suppressed in the experimental plots treated with 100 kg·ha-1·year-1 of nitrogen only in terms of the urban forest Rs and Ra and the rural forest Ra. Nitrogen treatment did not significantly increase Rs and had different influencing mechanisms. In urban forests, nitrogen addition contributed to Rh by increasing soil microbial biomass nitrogen and inhibited Ra by increasing soil ammonium­nitrogen concentration. In suburban forests, the lack of response of Rh under nitrogen addition was due to the combined effects of soil ammonium­nitrogen and microbial biomass nitrogen; the indirect effects from nitrate­nitrogen also contributed to a divergent effect on Ra. In rural forests, the soil pH, dissolved organic C, fine root biomass, and microbial biomass C concentration were the main factors mediating Rs and its components. In summary, the current rate of nitrogen deposition is unlikely to result in significant increases in soil C release in urban-rural forests, high nitrogen deposition is beneficial for reducing the temperature sensitivity of Rs in urban forests. The findings grant a groundwork for predicting responses of forest soil C cycling to global change in the context of urban expansion.

Exogenous spermidine regulates the anaerobic enzyme system through hormone concentrations and related-gene expression in Phyllostachys praecox roots under flooding stress.

PURPOSE: Acclimation to hypoxia and anoxia is important in various ecological systems, especially flooded soil. Phyllostachys pracecox is sensitive to flooding, and therefore, it is important to explore ways of alleviating hypoxia stress in the roots. In this study, we investigated the regulatory effect of spermidine (Spd) on flooded P. praecox seedlings. METHODS: A batch experiment was carried out in roots treated with Spd under flooding for eight days. The following factors were subsequently measured: growth, survival rate, root respiratory activity, soluble protein and anaerobic respiration enzyme contents (pyruvate decarboxylase, PDC; alcohol dehydrogenase, ADH; lactate dehydrogenase, LDH; alanine aminotransferase, AlaAT), S-adenosylmethionine decarboxylase (SAMDC), nitrate reductase (NR), ACC oxidase (ACO) and ACC synthetase (ACS) activities, free Spd, spermine (Spm) and the diamine precursor putrescine (Put) content, indole-3-acetic acid (IAA) and abscisic acid (ABA) content, ethylene emissions and expression of hormone-related genes. RESULTS: Application of Spd promoted root growth (root length, volume, surface and dry weight) and root respiratory inhibition, improved the soluble protein content, and reduced the O2·- production rate, H2O2 and MDA content to alleviate the damage of roots under flooding. A significant increase in SAMDC activity, and ABA and IAA contents were also observed, along with a reduction in ethylene emissions, NR, ACO and ACS activities (p < 0.05). Exogenous Spd increased the free Spd and Spm contents in the P. praecox roots, but decreased the free Put content. Taken together, these findings suggest that hypoxia stress was alleviated. Moreover, exogenous Spd up-regulated the expression of auxin-related genes ARF1, AUX1, AUX2, AUX3 and AUX4, and down-regulated the expression of ethylene-related ACO and ACS genes during flooding. In addition, correlation and RDA analysis showed that ARF1, ACO and ACS significantly promoted the expression of auxin, ACO and ACS enzyme activities, respectively (p < 0.05), while ADH, NR, AlaAT, ethylene emissions, Put, Spd, ACS and ACO were significantly correlated with ACS, ACO, and auxin-related gene expression (p < 0.05). Overall, ethylene emissions, ACS and ACO were identified as the main drivers of ethylene and auxin-related gene structure. CONCLUSIONS: These results suggest that Spd regulated hormone concentrations, the content of Spd, Spm and Put, and related gene expression, in turn regulating physiological changes such as anaerobic enzyme activity, mitigating flooding stress in the roots and improving overall growth. Spd therefore has the potential to improve the adaptability of P. praecox to flooding stress.

Exogenously applied spermidine alleviates hypoxia stress in Phyllostachys praecox seedlings via changes in endogenous hormones and gene expression.

BACKGROUND: Hypoxia stress is thought to be one of the major abiotic stresses that inhibits the growth and development of higher plants. Phyllostachys pracecox is sensitive to oxygen and suffers soil hypoxia during cultivation; however, the corresponding solutions to mitigate this stress are still limited in practice. In this study, Spermidine (Spd) was tested for regulating the growth of P. praecox seedlings under the hypoxia stress with flooding. RESULTS: A batch experiment was carried out in seedlings treated with 1 mM and 2 mM Spd under flooding for eight days. Application of 1 mM and 2 mM Spd could alleviate plant growth inhibition and reduce oxidative damage from hypoxia stress. Exogenous Spd significantly (P < 0.05) increased proline, soluble protein content, catalase (CAT), superoxide dismutase (SOD), and S-adenosylmethionine decarboxylase (SAMDC) activity, enhanced abscisic acid (ABA) and indole-3-acetic acid (IAA) content, and reduced ethylene emission, hydrogen peroxide (H2O2), superoxide radical (O2·-) production rate, ACC oxidase (ACO) and ACC synthase (ACS) to protect membranes from lipid peroxidation under flooding. Moreover, exogenous Spd up-regulated the expression of auxin-related genes auxin responsive factor1 (ARF1), auxin1 protein (AUX1), auxin2 protein (AUX2), auxin3 protein (AUX3) and auxin4 protein (AUX4), and down-regulated the expression of ethylene-related ACO and ACS genes during flooding. CONCLUSION: The results indicated that exogenous Spd altered hormone concentrations and the expression of hormone-related genes, thereby protecting the bamboo growth under flooding. Our data suggest that Spd can be used to reduce hypoxia-induced cell damage and improve the adaptability of P. praecox to flooding stress.

Effects of Flooding and Endogenous Hormone on the Formation of Knee Roots in Taxodium ascendens.

Taxodium ascendens is a typical tree species with high flood tolerance, and it can generate knee roots in the wetlands. This study investigated the number and size of knee roots and the soil flooding conditions. Furthermore, we also measured physiology, biochemical responses, and the anatomical structure of knee roots and underground roots at different developmental stages. This study aimed to understand the adaptation mechanism of T. ascendens to flooding stress and the formation mechanism of the knee roots. The results showed that the formation of knee roots was significantly affected by the soil water table (P < 0.05). The middle water table was more conducive to the formation of knee roots. In the middle water table, the 1-aminocyclopropane-1-carboxylic acid (ACC) content and ACC synthase activity were significantly lower in the knee roots than in the underground roots. The knee roots at the young-aged stage showed the highest ACC oxidase activity among the development stages of the knee roots. The ethylene release rate was significantly higher in the knee roots than in the underground roots (P < 0.05). Indole-3-acetic acid (IAA) content first increased, then decreased with knee root development. The periderm cells at the apex of the knee roots were dead and had many intercellular spaces, which was beneficial for the growth of T. ascendens. In conclusion, the middle water table induced the ethylene and IAA production, which promoted the formation of knee roots, which improved roots ventilation and flooding tolerance of T. ascendens. The results obtained can provide information about mechanisms of knee roots formation and provide scientific evidence for the afforestation and management under wetland conditions.

Aeration increases soil bacterial diversity and nutrient transformation under mulching-induced hypoxic conditions.

Soil oxygen (O2) deficiency induced by organic mulching is easy to overlook. Aeration has been shown to potentially alleviate soil hypoxia stress. However, the responses of soil bacterial communities to such mulching-induced hypoxic conditions and aeration remain elusive. Therefore, a three-year field experiment, consisting of mulching (T1), mulching with aeration (TA1, poor aeration; TA2, strong aeration), and no-mulching (CK) treatments, was conducted in bamboo (Phyllostachys praecox) plantations. According to our results, the strong aeration treatment (TA2) alleviated soil acidification, increased soil nutrient availability, and significantly increased soil O2 content by 18.44% (P < 0.05) when compared with T1. In addition, TA2 significantly increased soil ß-glucosidase, invertase, urease, and acid phosphatase activities compared with CK and T1 (P < 0.05). The alpha diversity indices with TA2 treatment were the highest, indicating that aeration increased the species richness and diversity of bacteria. The changes in bacterial community composition associated with TA2 treatment (i.e., an increase in Firmicutes, Verrucomicrobia, and Faecalibacterium abundance and a decrease in Chloroflexi and Bradyrhizobium abundance) were mainly related to nutrient and O2 content. Mantel Test results suggested that soil O2 content and temperature were the key factors shaping bacterial community composition. Structural equation modeling revealed that soil O2 content had a positive and direct influence on bacterial community diversity. Functional annotation of prokaryotic taxa predicted that TA2 significantly increased the relative abundance of bacterial communities associated with nitrification, nitrogen fixation, and ureolysis. Our results demonstrated that optimal soil aeration conditions (17.60% of O2 content) could enhance the diversity and function of soil bacterial communities. Overall, the findings of this study could serve as a benchmark for alleviating soil hypoxia caused by organic mulches, which is important for increasing the functionality of nutrient cycling bacterial communities in the soil.