Changes of Soil Microbes Related with Carbon and Nitrogen Cycling after Long-Term CO2 Enrichment in a Typical Chinese Maize Field

Characteristics of soil N2O emission and N2O-producing microbial communities in paddy fields under elevated CO2 concentrations

The effects of elevated carbon dioxide (CO₂) concentration (e[CO₂]) on nitrous oxide (N₂O) emissions from paddy fields and the microbial processes involved in N₂O emissions have recently received much attention. Ammonia-oxidizing microorganisms and denitrifying bacteria dominate the production of N₂O in paddy soils. To better understand the dynamics of N₂O production under e[CO₂], a field experiment was conducted after five years of CO₂ fumigation based on three treatments: CK (ambient atmospheric CO₂), T1 (CK + increase of 40 ppm per year until 200 ppm), and T2 (CK + 200 ppm). N₂O fluxes, soil physicochemical properties, and N₂O production potential were quantified during the rice-growth period. The functional gene abundance and community composition of ammonia-oxidizing archaea (AOA) and bacteria (AOB) were analyzed using quantitative polymerase chain reaction (qPCR) and those of ammonia-denitrifying bacteria (nirS- and nirK-type) were analyzed using Illumina MiSeq sequencing. N₂O emissions decreased by 173% and 41% under the two e[CO₂] treatments during grain filling and milk ripening, respectively (P < 0.05). N₂O emissions increased by 279% and 172% in the T2 treatment compared with T1 during the tillering and milk-ripening stages, respectively (P < 0.05). Furthermore, the N₂O production potential was significantly higher in the CK treatment than in T1 and T2 during the elongation stage. The N₂O production potential and abundance of AOA amoA genes in T1 treatment were significantly lower than those in CK treatment during the high N₂O emission phase caused by mid-season drainage (P < 0.05). Although nirK- and nirS-type denitrifying bacteria community structure and diversity did not respond significantly (P > 0.05) to e[CO₂], the abundance of nirK-type denitrifying bacteria significantly affected the N₂O flux (P < 0.05). Linear regression analysis showed that the N₂O production potential, AOA amoA gene abundance, and nirK gene abundance explained 47.2% of the variation in N₂O emissions. In addition, soil nitrogen (N) significantly affected the nirK- and nirS-type denitrifier communities. Overall, our results revealed that e[CO₂] suppressed N₂O emissions, which was closely associated with the abundance of AOA amoA and nirK genes (P < 0.05).

Differential response of bacterial diversity and community composition to different tree ages of pomelo under red and paddy soils

Rhizosphere soil microbial communities substantially impact plant growth by regulating the nutrient cycle. However, dynamic changes in soil microbiota under different tree ages have received little attention. In this study, changes in soil physicochemical properties, as well as bacterial diversity and community structures (by high-throughput Illumina MiSeq sequencing), were explored in pomelo trees of different ages (i.e., 10, 20, and 30 years) under red and paddy soils cultivated by farmers with high fertilizer input. Moreover, soil factors that shape the bacterial community, such as soil pH, AP (available phosphorous), AK (available potassium), and AN (available nitrogen), were also investigated. Results showed that pH significantly decreased, while AP, AK, and AN increased with increasing tree age under red soil. For paddy soil, pH was not changed, while AP was significantly lower under 10-year-old pomelo trees, and AK and AN contents were minimum under 30-year-old pomelo trees. Both soil types were dominated by Proteobacteria, Acidobacteria, and Actinobacteria and showed contrasting patterns of relative abundance under different tree age groups. Bacterial richness and diversity decreased with increasing tree age in both soil types. Overall, bacterial community composition was different under different tree ages. RDA analysis showed that soil pH, AP, and AN in red soil, and pH and AP in paddy soil showed the most significant effects in changing the bacterial community structure. A random forest model showed Sinomonas and Streptacidiphilus in red soil, while Actinoallomurus and Microbacterium in paddy soil were the most important genera explaining the differences among different age groups. The ternary plot further revealed that genera enrichment for Age_30 was higher than that for Age_10 and Age_20 in red soil, whereas specific genera enrichment decreased with increasing tree age under paddy soil. Co-occurrence network revealed that bacterial species formed a complex network structure with increasing tree age, indicating a more stable microbial association under 20 and 30 years than 10-year-old pomelo trees. Hence, contrasting patterns of changes in soil physicochemical properties and soil microbial communities were recorded under different tree ages, and tree ages significantly affected the bacterial community structure and richness. These findings provide valuable information regarding the importance of microbes for the sustainable management of pomelo orchards by optimizing fertilizer input for different ages of trees.

Yield Response of Spring Maize under Future Climate and the Effects of Adaptation Measures in Northeast China

Agriculture production has been found to be the most sensitive sector to climate change. Northeast China (NEC) is one of the world's major regions for spring maize production and it has been affected by climate change due to increases in temperature and decreases in sunshine hours and precipitation levels over the past few decades. In this study, the CERES-Maize model-v4.7 was adopted to assess the impact of future climatic change on the yield of spring maize in NEC and the effect of adaptation measures in two future periods, the 2030s (2021 to 2040) and the 2050s (2041 to 2060) relative to the baseline (1986 to 2005) under RCP4.5 and RCP8.5 scenarios. The results showed that increased temperatures and the decreases in both the precipitation level and sunshine hours in the NEC at six representative sites in the 2030s and 2050s periods based on RCP4.5 and RCP8.5 climate scenarios would shorten the maize growth durations by (1-38 days) and this would result in a reduction in maize yield by (2.5-26.4%). Adaptation measures, including altered planting date, supplemental irrigation and use of cultivars with longer growth periods could offset some negative impacts of yield decrease in maize. For high-temperature-sensitive cultivars, the adoption of early planting, cultivar change and adding irrigation practices could lead to an increase in maize yield by 23.7-43.6% and these measures were shown to be effective adaptation options towards reducing yield loss from climate change. The simulation results exhibited the effective contribution of appropriate adaptation measures in eliminating the negative impact of future climate change on maize yield.

Fungi Dominated the Incorporation of 13C-CO2 into Microbial Biomass in Tomato Rhizosphere Soil under Different CO2 Concentrations

An elevated CO₂ (eCO₂) fumigation experiment was carried out to study the influence of various CO₂ concentrations on microorganisms involved in the incorporation of root-derived C in greenhouse soil systems. In this study, 400 and 800 µmol·mol⁻¹ CO₂ fumigation treatments were conducted during tomato planting. Phospholipid fatty acid (PLFA) profiling based on the stable isotope probing (SIP) technique was applied to trace active microorganisms. The absolute total abundance of ¹³C-PLFAs was much higher under eCO₂ treatment. Most of the ¹³C-CO₂ was incorporated into the ¹³C-PLFAs 18:2ω6,9 (fungi), 16:0 (general PLFA), 18:1ω9c (Gram-negative bacteria, G⁻) and i17:0 (Gram-positive bacteria, G⁺) via rhizodeposition from tomato under ambient CO₂ (aCO₂) and eCO₂ treatments, suggesting similar responses of active microorganisms to different CO₂ treatments. However, the fungi (characterized by the ¹³C-PLFA 18:2ω6,9) played a much more dominant role in the incorporation of root-derived C under eCO₂. Actinomycetes, marked by the ¹³C-PLFA 10-Me-18:0, occurred only on labeling day 15 under the eCO₂ treatment, indicating that the actinomycetes fed on both soil organic carbon and fresh rhizodeposition. It was indicated that eCO₂ significantly affected microbial biomass and microbial community structures involved in the incorporation of ¹³C-CO₂ via tomato root secretions, as supported by Adonis analysis and the Mantel test.