Thinning enhances forest soil C storage by shifting the soil toward an oligotrophic condition.

Forests are significant carbon reservoirs, with approximately one-third of this carbon stored in the soil. Forest thinning, a prevalent management technique, is designed to enhance timber production, preserve biodiversity, and maintain ecosystem functions. Through its influence on biotic and abiotic factors, thinning can profoundly alter soil carbon storage. Yet, the full implications of thinning on forest soil carbon reservoirs and the mechanisms underpinning these changes remain elusive. In this study, we undertook a two-year monitoring initiative, tracking changes in soil extracellular enzyme activities (EEAs), microbial communities, and other abiotic parameters across four thinning intensities within a temperate pine forest. Our results show a marked increase in soil carbon stock following thinning. However, thinning also led to decreased dissolved organic carbon (DOC) content and a reduced DOC to soil organic carbon (SOC) ratio, pointing toward a decline in soil carbon lability. Additionally, fourier transform infrared spectroscopy (FTIR) analysis revealed an augmented relative abundance of aromatic compounds after thinning. There was also a pronounced increase in absolute EEAs (per gram of dry soil) post-thinning, implying nutrient limitations for soil microbes. Concurrently, the composition of bacterial and fungal communities shifted toward oligotrophic dominance post thinning. Specific EEAs (per gram of soil organic matter) exhibit a significant reduction following thinning, indicating a deceleration in organic matter decomposition rates. In essence, our findings reveal that thinning transitions soil toward an oligotrophic state, dampening organic matter decomposition, and thus bolstering the soil carbon storage potential of forest. This study provides enhanced insights into the nuanced relationship between thinning practices and forest soil carbon dynamics, serving as a robust foundation for enlightened forest management strategies.

Calcium-enriched biochar shifts negative effects of fluoride on the properties of arid sandy soil.

This study sheds light on the influence of fluoride on the changes in the properties of alkaline sandy soils and the efficiency of calcium-enriched biochar application. The investigation involved an incubation experiment with soil contaminated with varying NaF concentrations (0, 400, 800, and 1200 mg NaF kg-1 soil) and biochar (1% w/w). The data revealed that adding NaF to the soil resulted in significant increases in soil pH and decreases in total nitrogen (TN) content. Short-term fluoride pollution did not affect the microbial abundance due to certain factors such as increased soil pH and decreased microbial metabolism promoting the survival of cells under fluoride stress. However, a shift from bacterial to fungal-dominated microbial communities was observed at the highest NaF concentration. The nitrogen functional gene amoA was found to be highly sensitive to fluoride toxicity. The decrease in the abundance of amoA gene and the increase in soil pH can explain reduced nitrogen concentration. On the other hand, our findings indicated a significant decrease in enzyme activity in soil contaminated with mild to severe levels of NaF. This reduction in enzyme activity can be attributed to increased soil pH, decreased TN content, and the inhibition of microbial metabolism due to fluoride toxicity. Furthermore, the addition of calcium-rich biochar reduced fluoride solubility and adjusted pH, mitigating the negative effects of fluoride toxicity on soil properties. The use of biochar was also found to inhibit the accumulation of soil fluoride-resistant microbial genes.

Soil organic carbon is a key determinant of CH4 sink in global forest soils.

Soil organic carbon (SOC) is a primary regulator of the forest-climate feedback. However, its indicative capability for the soil CH4 sink is poorly understood due to the incomplete knowledge of the underlying mechanisms. Therefore, SOC is not explicitly included in the current model estimation of the global forest CH4 sink. Here, using in-situ observations, global meta-analysis, and process-based modeling, we provide evidence that SOC constitutes an important variable that governs the forest CH4 sink. We find that a CH4 sink is enhanced with increasing SOC content on regional and global scales. The revised model with SOC function better reproduces the field observation and estimates a 39% larger global forest CH4 sink (24.27 Tg CH4 yr-1) than the model without considering SOC effects (17.46 Tg CH4 yr-1). This study highlights the role of SOC in the forest CH4 sink, which shall be factored into future global CH4 budget quantification.

Attenuation of Methane Oxidation by Nitrogen Availability in Arctic Tundra Soils.

CH4 emission in the Arctic has large uncertainty due to the lack of mechanistic understanding of the processes. CH4 oxidation in Arctic soil plays a critical role in the process, whereby removal of up to 90% of CH4 produced in soils by methanotrophs can occur before it reaches the atmosphere. Previous studies have reported on the importance of rising temperatures in CH4 oxidation, but because the Arctic is typically an N-limited system, fewer studies on the effects of inorganic nitrogen (N) have been reported. However, climate change and an increase of available N caused by anthropogenic activities have recently been reported, which may cause a drastic change in CH4 oxidation in Arctic soils. In this study, we demonstrate that excessive levels of available N in soil cause an increase in net CH4 emissions via the reduction of CH4 oxidation in surface soil in the Arctic tundra. In vitro experiments suggested that N in the form of NO3- is responsible for the decrease in CH4 oxidation via influencing soil bacterial and methanotrophic communities. The findings of our meta-analysis suggest that CH4 oxidation in the boreal biome is more susceptible to the addition of N than in other biomes. We provide evidence that CH4 emissions in Arctic tundra can be enhanced by an increase of available N, with profound implications for modeling CH4 dynamics in Arctic regions.

Distinct Nitrification Rates and Nitrifiers in Needleleaf and Evergreen Broadleaf Forest Soils.

Research on niche specialization in the microbial communities of ammonia oxidizers is important for assessing the consequences of vegetation shift on nitrogen (N) cycling. In this study, soils were sampled from three tree stands (needleleaf, mixed, and evergreen broadleaf) from the Hannam experimental forest in South Korea in spring (May 2019), summer (August 2019), autumn (November 2019), and winter (January 2020). Quantitative polymerase chain reaction (qPCR) and high-throughput sequencing were used to measure the abundance and community structure of various nitrifiers: ammonia-oxidizing archaea and bacteria (AOA and AOB, respectively) as well as complete ammonia oxidizers (comammox). Nitrification rates and total ammonia oxidizer abundance were significantly higher in needleleaf forest soil than those in other forest stands, and they were lowest in evergreen broadleaf forest soil. Comammox clade B was most abundant in needleleaf and evergreen broadleaf forest soils, while AOA were significantly more abundant in mixed forest soil. The abundances of comammox clade B and AOA were negatively correlated with dissolved organic carbon. Phylogenetic analysis showed that NT-alpha and NS-gamma-2.3.2 were the most abundant AOA lineages in all the samples. The seasonal of AOA, AOB, and comammox varied with the sites, suggesting the need to examine the combinations of environmental factors when considering the effects of seasonal changes in the environment. Overall, the results suggest that potential vegetation shifts in forest ecosystems might affect nitrification activities by regulating the abundance and community structure of ammonia oxidizers.

Effects of vegetation shift from needleleaf to broadleaf species on forest soil CO2 emission.

Forest soil harbors diverse microbial communities with decisive roles in ecosystem processes. Vegetation shift from needleleaf to broadleaf species is occurring across the globe due to climate change and anthropogenic activities, potentially change forest soil microbial communities and C cycle. However, our knowledge on the impact of such vegetation shift on soil microbial community and activities, and its consequences on forest soil C dynamics are still not well established. Here, we examined the seasonal variation of soil CO2 emission, soil extracellular enzyme activities (EEAs), and soil bacterial, fungal communities in subtropical forest from broadleaf, needleleaf, and mixed stands. In addition, soil CO2 emission and soil EEAs were measured in temperate forest during the growing season. Soil organic matter (SOM) content significantly differs between broadleaf and needleleaf forests and primarily distinguish various soil chemical and microbial characteristics. Significantly higher EEAs and soil CO2 emission in broadleaf forest compared to needleleaf forest were observed both in subtropical and temperate forests. The relative abundance of Basidiomycota positively correlated with SOM and EEAs and indirectly increase soil CO2 emission whereas the relative abundance of Ascomycota exhibits opposite trend, suggesting that soil fungal communities play a key role in determining the different microbial activities between broadleaf and needleleaf stands. The temperature sensitivity of soil CO2 emission was significantly higher in broadleaf forest compared to needleleaf forest, further suggesting that the soil organic carbon in broadleaf forests is more vulnerable to warming.

Soil salinity and its associated effects on soil microorganisms, greenhouse gas emissions, crop yield, biodiversity and desertification: A review.

Significant research has been conducted on the effects of soil salinity issue on agricultural productivity. However, limited consideration has been given to its critical effects on soil biogeochemistry (e.g., soil microorganisms, soil organic carbon and greenhouse gas (GHG) emissions), land desertification, and biodiversity loss. This article is based on synthesis of information in 238 articles published between 1989 and 2022 on these effects of soil salinity. Principal findings are as follows: (1) salinity affects microbial community composition and soil enzyme activities due to changes in osmotic pressure and ion effects; (2) soil salinity reduces soil organic carbon (SOC) content and alters GHG emissions, which is a serious issue under intensifying agriculture and global warming scenarios; (3) soil salinity can reduce crop yield up to 58 %; (4) soil salinity, even at low levels, can cause profound alteration in soil biodiversity; (5) due to severe soil salinity, some soils are reaching critical desertification status; (6) innovate mitigation strategies of soil salinity need to be approached in a way that should support the United Nations Sustainable Development Goals (UN-SDGs). Knowledge gaps still exist mainly in the effects of salinity especially, responses of GHG emissions and biodiversity. Previous experiences quantifying soil salinity effects remained small-scale, and inappropriate research methods were sometimes applied for investigating soil salinity effects. Therefore, further studies are urgently required to improve our understanding on the effects of salinity, address salinity effects in larger-scale, and develop innovative research methods.

The Effects of N Enrichment on Microbial Cycling of Non-CO2 Greenhouse Gases in Soils-a Review and a Meta-analysis.

Terrestrial ecosystems are typically nitrogen (N) limited, but recent years have witnessed N enrichment in various soil ecosystems caused by human activities such as fossil fuel combustion and fertilizer application. This enrichment may alter microbial processes in soils in a way that would increase the emissions of methane (CH4) and nitrous oxide (N2O), thereby aggravating global climate change. This review focuses on the effects of N enrichment on methanogens and methanotrophs, which play a central role in the dynamics of CH4 at the global scale. We also address the effects of N enrichment on N2O, which is produced in soils mainly by nitrification and denitrification. Overall, N enrichment inhibits methanogenesis in pure culture experiments, while its effects on CH4 oxidation are more complicated. The majority of previous studies reported that N enrichment, especially NH4+ enrichment, inhibits CH4 oxidation, resulting in higher CH4 emissions from soils. However, both activation and neutral responses have also been reported, particularly in rice paddies and landfill sites, which is well reflected in our meta-analysis. In contrast, N enrichment substantially increases N2O emission by both nitrification and denitrification, which increases proportionally to the amount of N amended. Future studies should address the effects of N enrichment on the active microbes of those functional groups at multiple scales along with parameterization of microbial communities for the application to climate models at the global scale.

Microbial decomposition of soil organic matter determined by edaphic characteristics of mangrove forests in East Asia.

Mangrove forests cover only 0.1% of the world's continental area; however, these are a substantial carbon sink owing to the high primary production and low rate of decomposition of soil organic matter (SOM). The extremely low decomposition rate of SOM in mangrove forests is believed to be caused by low oxygen and nutrient availability as well as recalcitrant biomass from mangrove. However, only a few studies have addressed the microbial mechanism that plays a key role in the decomposition of SOM. In this study, the decomposition of SOM were determined by conducting a field survey and an lab incubation experiment using soil samples from mangrove forests in three regions; Okinawa, Shenzhen, and Hong Kong. In particular, we examined the occurrence of the enzymic latch mechanism, which involves phenolic inhibition of enzymic decomposition, in mangrove forest soils that highlights the importance of phenol oxidase as a key controlling factor. The results clearly showed that enzymic latch involved in the accumulation of SOM in the mangroves of Shenzhen and Hong Kong, whereas the accumulation of SOM in Okinawa was controlled by other mechanisms, such as the iron gate mechanism, which involves stabilization of soil carbon in iron-SOM complexes. The characteristics of mangrove forests, such as iron concentration, were shown as substantial determination factors in the dynamics of SOM. We concluded that the decomposition of SOM were strongly affected by the characteristics of mangrove forests, and the occurrence of enzymic latch in mangrove forests has a potential application in geoengineering technology to enhance the carbon sequestration capacity of mangrove forests.