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Soil nematode community is an important component of the soil food web, which has been widely recognized as a key bio-indicator for assessing the influence of nature restoration on ecological functions. However, the dynamics of the abundance, structure of soil nematode community remain unclear under nature restoration. The soil nematode community of natural secondary forests was investigated using a chronosequence approach in subtropical forests in China. Six succession stages of nature restoration were sampled to represent forest stand age groups with 4-5, 8-12, 18-22, 25-30, 35-40 and over 100 years. To enhance our understanding of the factors influencing soil nematode communities, we also examined the relationships between plant community, soil microbial community, and soil properties by structural equation modeling. Soil nematode abundance gradually increased with forest stand ages, which reached a peak value (574 individuals 100 g−1 dry soil) in the oldest stands. Soil available nitrogen and phosphorus were key factors influencing soil nematode abundance and diversity during secondary forest succession. The plant parasite index decreased with forest stand ages, which indicated that ecosystem function and health would be improved as nature restoration. The structure of soil nematode community was more sensitive to microbial community compared to plant community. The bottom-up effects of microbial communities on soil nematode communities were important drivers of nematode communities in subtropical forests. This study demonstrates the active responses of soil nematode community to nature restoration and highlights the importance of the above-ground and below-ground interactions to the soil food web.

This study aimed to explore the effects of biochar on pulse CO2 and N2O emissions and N cycling microbial functional genes after a short-term drought through a soil incubation experiment. Soil samples were collected in a macadamia orchard where biochar was applied 5 years prior to the incubation. Samples were wetted after being subjected to short-term (2-month) drought conditions. Samples were analysed for gas emissions (N2O and CO2), available NH4+-N, and NO3−-N, water soluble organic carbon (WSOC), water soluble total N (WSTN), and N cycling microbial gene abundance for a period of 21 days post-drought. Soil CO2 emissions were significantly higher in the drought-affected soil with no biochar than in the control soil with no biochar. No effect of biochar was detected on CO2 emissions for drought-affected soil. Available labile C (WSOC) in drought-affected soil was higher than in soils not subjected to drought, regardless of the presence of biochar. Therefore, C loss after adding water could be explained by the release of labile C accumulated during drought. Drought-affected soil with biochar did not influence N2O emissions compared with control soil subject to drought. In soils not subjected to drought, biochar had higher NO3−-N than the soil without biochar at day 7 post-drought, which could partly be explained by increased soil ammonia-oxidising bacteria (AOB) gene abundance. Our study suggested that a pulse C loss was more likely to occur post-drought whereas pulse N loss through N2O emission was not evident regardless of biochar application particularly within first day after being rewetted. Our study highlights the pulse effects of drought on GHG emissions from the soil after being wetted.

The abundance, community composition and activity of key nitrogen (N)-cycling functional guilds were monitored at a site where coniferous Cunninghamia lanceolate and broadleaved Mytilaria laosensis were planted in 1993. Leaf cellulose, litter C/N ratio, leaf dry matter content, soil inorganic N content were significantly higher under C. lanceolata, while specific leaf area, litter production, litter magnesium, soil C and soil C/NO3− ratio were higher under M. laosensis. The 15N tracing experiment together with quantitative PCR revealed that autotrophic nitrification rates, as well as the abundances of ammonia-oxidizing archaea and comammox Nitrospira were significantly higher under C. lanceolate than M. laosensis. However, M. laosensis exhibited substantially higher nitrate retention capacity via dissimilatory nitrate reduction to ammonium (DNRA), accompanied by a significantly higher abundance of nrfA gene than C. lanceolate. The Illumina sequencing indicated that tree species markedly affected soil bacterial community composition regardless of the soil layers. Redundancy analysis suggested that litter C/N ratio was the most influential factor explaining functional gene abundances and bacterial communities. Taken together, our findings showed that M. laosensis improved soil N retention capacity mainly through inhibiting autotrophic nitrification while enhancing DNRA activity. This study highlights the importance of tree species identity in influencing the microbially-mediated N cycling and bacterial community composition.

Global climate change is characterized by enhanced atmospheric carbon dioxide concentration ([CO2]) and temperature, with unknown consequences for soil nematode communities. Soil nematode in response to elevated [CO2], warming and their interaction in paddy field remain largely unknown. Here we aimed to understand how factorial combinations of elevated [CO2] and canopy warming affect soil nematode in a rice paddy field. A rice paddy field was consistently treated with elevated [CO2] (500 ppm), canopy warming (+2 °C) or their combinations. Soil samples after a two-year treatment were collected during the rice growing season and nematode communities were extracted with a modified Baermann funnel extraction to examine the changes in nematode abundance and composition under climate change. Soil nematode communities were altered by elevated [CO2] and warming, but these responses were dependent on rice growing stages. When averaged over the four stages, total nematode abundances were increased by 31.5% under elevated [CO2], and by 25.7% under warming. Elevated [CO2] had no effect on nematode diversity, but slightly altered the composition of different trophic groups. In contrast, warming decreased nematode diversity, but increased plant parasite index, which was negative correlated with crop production. This was attributed to increases in the relative abundance of herbivores under simulated climate change conditions. Elevated [CO2] and warming had a positive effect on nematode abundance, but potentially reduced nematode diversity and soil health. These results suggest that multi-factors interactively affect the responses of soil nematode communities, which is important for food productivity under climate change.

The recent discovery of comammox Nitrospira capable of converting ammonia to nitrate in a single organism radically challenged our century-long perception of the classic two-step nitrification performed by ammonia oxidizers and nitrite oxidizers. However, our understanding of the niche separation of comammox Nitrospira and canonical nitrifiers in forest ecosystems remains limited, especially under a global scenario of elevated nitrogen (N) deposition. Here we evaluated the impacts of six-year N deposition on the dynamics of comammox Nitrospira, ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in a subtropical forest soil. Soil inorganic N concentrations significantly increased under the six-year simulation of N deposition, while soil pH, available phosphorus, total carbon (C), C/N ratio and microbial biomass significantly decreased. Quantitative PCR showed that the amoA gene abundances of comammox Nitrospira clade B and AOA substantially increased under the increasing rates of N deposition. By contrast, the AOB amoA gene abundance significantly decreased with the higher levels of N deposition (100 and 150 kg N ha−1 yr−1). Increased 13CO2 incorporation into the AOA communities, rather than comammox Nitrospira or AOB, was demonstrated in a DNA-stable isotope probing microcosm, indicative of the capacity of AOA to assimilate 13CO2 through autotrophic nitrification in the investigated subtropical forest soil under long-term N deposition. Phylogenetic analysis revealed that the autotrophic AOA assemblages belonged to the Nitrosotalea cluster, and their capacity for assimilating CO2 through autotrophic nitrification was not affected by the long-term N deposition. Taken together, we provided new evidence for the niche separation of comammox Nitrospira and canonical ammonia oxidizers in soil nitrification under the long-term N deposition in the acidic subtropical forest soil.

Soil nematode community is an important component of the soil food web, which has been widely recognized as a key bio-indicator for assessing the influence of climate changes on ecological functions. Climate changes have experienced evident increases in the atmospheric CO2 concentration [CO2] and temperature in the past decades. However, the effects of elevated [CO2] and warming on the abundance, diversity and function of soil nematode community remain unclear. In this study, the soil nematode communities in rhizosphere and non-rhizosphere soil were investigated under factorial combinations of elevated [CO2] (500 ppm) and canopy warming (+2°C) in a winter wheat field. The results showed that canopy warming significantly increased the nematode abundance by 31.3% but decreased its diversity by 14.5%, while elevated [CO2] showed no effect on the nematode abundance whereas significantly increased its diversity by 6.4%. The nematode diversity and genera richness decreased in the rhizosphere soil under combined treatment, indicating that canopy warming negated the positive effect of elevated [CO2] on nematode diversity. Meanwhile, canopy warming alone significantly increased the plant parasite index of nematode trophic communities in rhizosphere soil rather than in non-rhizosphere soil. Overall, the effects of climate changes on soil nematode community are greater in rhizosphere than non-rhizosphere soil, potentially affecting ecosystem functions in agricultural soil, and thus challenging the ecosystem health and food production in the future.