Deciphering the dual role of bacterial communities in stabilizing rhizosphere priming effect under intra-annual change of growing seasons.

The rhizosphere priming effect (RPE) is a widely observed phenomenon affecting carbon (C) turnover in plant-soil systems. While multiple cropping and seasonal changes can have significant impacts on RPE, the mechanisms driving these processes are complex and not yet fully understood. Here, we planted maize in paddy soil during two growing seasons having substantial temperature differences [May-August (warm season, 26.6 °C) and September-November (cool season, 23.1 °C)] within the same calendar year in southern China to examine how seasonal changes affect RPEs and soil C. We identified sources of C emissions by quantifying the natural abundance of 13C and determined microbial metabolic limitations or efficiency and functional genes related to C cycling using an enzyme-based biogeochemical equilibrium model and high-throughput quantitative PCR-based chip technology, respectively. Results showed that microbial metabolism was mainly limited by phosphorus in the warm season, but by C in the cool season, resulting in positive RPEs in both growing seasons, but no significant differences (9.02 vs. 6.27 mg C kg-1 soil day-1). The RPE intensity remained stable as temperature increased (warm season compared to a cool season), which can be largely explained by the simultaneous increase in the abundance of functional genes related to both C degradation and fixation. Our study highlights the simultaneous response and adaptation of microbial communities to seasonal changes and hence contributes to an understanding and prediction of microbially mediated soil C turnover under multiple cropping systems.

Shift of microbial turnover time and metabolic efficiency strongly regulates rhizosphere priming effect under nitrogen fertilization in paddy soil.

Microbial turnover and the decomposition of soil organic matter can be stimulated by living roots in a phenomenon known as the rhizosphere priming effect (RPE). Both the microbial turnover time (MTT) and metabolic efficiency are closely related to RPE. However, changes in MTT, metabolic efficiency and RPE in response to nitrogen (N) fertilization at different levels and the associations between these factors during plant growth are unknown. The effects of N fertilization at different levels (0, 150 and 300 kg N ha-1) on RPE and the underlying mechanisms were investigated in maize (Zea mays L.) grown in paddy soil using a 13Carbon (C) natural abundance method. The RPE varied from -1.49 to 15.93 mg C kg-1 soil day-1, with significant effects at different levels of N fertilization, growth stages and interactions between these factors. Nitrogen fertilization reduced microbial C:N imbalance and soil pH. During the plant growth periods, the RPE was initially low because the microbes preferentially utilized plant-derived C, but later increased due to trade-offs between microbial N acquisition and acidity stress alleviation under N fertilization. The soil microbes altered their MTT and metabolic efficiency with changes in the microbial community structure to maintain stoichiometric homeostasis and adapt to acidity stress. RPE was lowest whereas MTT and metabolic efficiency were highest with N fertilization at 150 kg N ha-1. Changes in MTT and metabolic efficiency explained 84.5% of the variations in the RPE, and the latter had greater impact (55.8%) than the former (28.7%). Changes in MTT and metabolic efficiency to cope with microbial resource acquisition and acidity stress under N fertilization represent an important pathway for RPE regulation in paddy soil. These findings highlight the significance of MTT and metabolic efficiency in RPE regulation for optimization of the N fertilization level to mitigate soil C losses.

Microbial metabolic efficiency functions as a mediator to regulate rhizosphere priming effects.

Microbial metabolic efficiency (MME), a key physiological property that indicates the allocation of carbon (C) to microbial growth, is surely one potential pathway involved in the regulation of priming effect within soil systems. However, the function and mechanism concerning the regulation of the rhizosphere priming effects (RPE) by MME in plant-soil systems remain unclear. In this study, we performed a pot experiment that included two soil types (paddy soil and lou soil), two plant species (sorghum [Sorghum bicolor (L.) Moench] and maize [Zea mays L.]) and three stages of growth (big trumpet, blooming and mature stage) to investigate the MME mechanism of RPE. Both positive (up to 76% at the big trumpet stage) and negative (down to -11% at the mature stage) RPE were observed. A shift in related enzyme activities and microbial biomass indicated that the 'microbial activation' and 'microbial nitrogen (N) mining' hypotheses functioned together at first. The 'preferential substrate utilization' hypothesis then functioned at the latter two stages. After that, according to a correlation analysis method, the MME was introduced to regulate the RPE: the availability of soil C and N and the microbial biomass jointly shaped the microbial C: N imbalance (MIC:N), and the microbes then regulated their MME based on the MIC:N, thus, regulating the RPE. Specifically, the lower MME induced by a higher MIC:N was responsible for a greater RPE at the big trumpet stage across all the planted treatments, while a higher MME induced by a lower MIC:N was responsible for the lower or negative RPE at the blooming and mature stages. Overall, these findings demonstrate that the MME shaped by MIC:N functions as a mediator to regulate the RPE in planted soil.

Microbial mechanism of biochar addition on nitrogen leaching and retention in tea soils from different plantation ages.

The effect of biochar additions on N leaching and retention in tea soils and its microbial mechanism are still unclear. In this study, effects of biochar additions at rates of 0, 3% and 6% on N leaching, N retention and microbial responses in two tea soils with 20- and 60-year plantation ages were investigated under application with 15N-labeled urea. The results showed that cumulative mass of leached NH4+-N, NO3--N and TN was reduced by 20.9%-91.9%, 35.1%-66.9% and 40.0%-72.8% under biochar additions, respectively. The retention of TN in soil was increased by 1.2%-5.8% under biochar amendment. Fertilizer-N in the leachate was reduced by 28.8%-62.1%, while fertilizer-N retention in the soils was enhanced by 3.2%-23.9% with biochar application. Biochar addition of 6% showed the highest mitigation of N leaching and enhancement of TN retention across the two soils. Biochar additions increased soil microbial biomass and enzyme activities and changed the bacterial community composition, indicating that biochar addition increased the microbial N requirement, stimulated soil N cycling, including nitrification and denitrification processes, and enhanced microbial N immobilization in the tea soils. Those microbial responses to biochar addition were higher in 60-year-old soil relative to 20-year-old soil, leading to a higher enhancement of N retention and mitigation of N leaching. Soil pH was the prime factor that influenced soil microbes, and it strongly correlated with microbial biomass, enzyme activity, the relative abundance of dominant phyla and α-diversity indices. Therefore, the enhancement of microbial biomass, activity and shifts of bacterial community composition related to N cycling in response to biochar additions that increased the soil pH could be an important mechanism to better understand the biochar-induced N leaching mitigation and N retention enhancement in tea soils under different plantation ages.

Prediction of future carbon footprint and ecosystem service value of carbon sequestration response to nitrogen fertilizer rates in rice production.

There is concern for variations of the carbon footprint (CF) and ecosystem service value of carbon sequestration (ESVCS) related to nitrogen (N) fertilizer rate in rice production under future climate change. To explore possible future ecological effects of N fertilizer rate, a DeNitrification-DeComposition (DNDC) model combined with Representative Concentration Pathway (RCP) projections (RCP 4.5 and RCP 8.5) were used to predict the CF and ESVCS of rice production. The model was validated by a two-year field experiment, and then seven N fertilizer levels (0, 75, 150, 190, 225, 300, and 375 kg N/ha) were set for prediction from 2015 to 2050. The validation results indicated a good fit between the DNDC-simulated and observed data of GHG emission and rice yield. Under RCP 8.5, the mean CF was 4.5-8.7% higher and the average ESVCS was 3.6-7.4% lower than those under RCP 4.5. The effects of N fertilizer rate on CF and ESVCS were consistent between the two climate change scenarios. In both RCPs, it was found that CF and ESVCS were mainly influenced by N fertilizer rate due to the latter's effect on CH4 emissions and crop carbon fixation. CH4 was the main contributor to CF during 2015-2050, accounting for 43.9-58.3% of the total CF. Agricultural inputs were also large contributors to CF, and N fertilizer increased the indirect GHG emissions by 24.6-122.2% compared with no N fertilization treatment. Among the studied N fertilizer rates, 190 kg N/ha was the optimal rate, obtaining the lowest CF and highest ESVCS. These results indicate that, under future climate change, an N fertilizer rate of 190 kg N/ha could achieve a trade-off between high yield, reduction of CF, and improvement of ESVCS in rice production.