Maize functional requirements drive the selection of rhizobacteria under long-term fertilization practices.

Rhizosphere microbiomes are pivotal for crop fitness, but the principles underlying microbial assembly during root-soil interactions across soils with different nutrient statuses remain elusive. We examined the microbiomes in the rhizosphere and bulk soils of maize plants grown under six long-term (≥ 29 yr) fertilization experiments in three soil types across middle temperate to subtropical zones. The assembly of rhizosphere microbial communities was primarily driven by deterministic processes. Plant selection interacted with soil types and fertilization regimes to shape the structure and function of rhizosphere microbiomes. Predictive functional profiling showed that, to adapt to nutrient-deficient conditions, maize recruited more rhizobacteria involved in nutrient availability from bulk soil, although these functions were performed by different species. Metagenomic analyses confirmed that the number of significantly enriched Kyoto Encyclopedia of Genes and Genomes Orthology functional categories in the rhizosphere microbial community was significantly higher without fertilization than with fertilization. Notably, some key genes involved in carbon, nitrogen, and phosphorus cycling and purine metabolism were dominantly enriched in the rhizosphere soil without fertilizer input. In conclusion, our results show that maize selects microbes at the root-soil interface based on microbial functional traits beneficial to its own performance, rather than selecting particular species.

nirS and nosZII bacterial denitrifiers as well as fungal denitrifiers are coupled with N2O emissions in long-term fertilized soils.

Fertilizer application plays a critical role in soil fertility and crop yield and has been reported to significantly affect soil denitrification. However, the mechanisms by which denitrifying bacteria (nirK, nirS, nosZI, and nosZII) and fungi (nirK and p450nor) affect soil denitrification are poorly understood. Therefore, in this study, we investigated the effect of different fertilization treatments on the abundance, community structure, and function of soil denitrifying microorganisms in an agricultural ecosystem with long-term fertilization using mineral fertilizer or manure and their combination. The results showed that the application of organic fertilizer significantly increased the abundance of nirK-, nirS-, nosZI-, and nosZII-type denitrifying bacteria as the soil pH and phosphorus content increased. However, only the community structure of nirS- and nosZII-type denitrifying bacteria was influenced by the application of organic fertilizer, which led to a higher contribution of bacteria to nitrous oxide (N2O) emissions than that observed after inorganic fertilizer application. The increase in soil pH reduced the abundance of nirK-type denitrifying fungi, which may have presented a competitive disadvantage relative to bacteria, resulting in a lower contribution of fungi to N2O emissions than that observed after inorganic fertilizer application. The results demonstrated that organic fertilization had a significant impact on the community structure and activity of soil denitrifying bacteria and fungi. Our results also highlighted that after organic fertilizer application, nirS- and nosZII-denitrifying bacteria communities represent likely hot spots of bacterial soil N2O emissions while nirK-type denitrifying fungi represent hot spots for fungal soil N2O emissions.

Differences in total stored C and N in dryland red soil caused by different long-term fertilization practices.

Fertilizer application is important to achieve sustainable agriculture. However, it remains unclear about the effects of long term fertilization on C and N immobilization as well as C/N ratios in soil aggregates at different depths. Samples taken at depths of 0 to 40 cm from dryland red soil subjected to long-term fertilization were analyzed. Four treatments were involved in the long term fertilization including no fertilizer (control), chemical fertilizer applied at two different rates, and manure combined with chemical fertilizers (MNPK). The C and N concentrations in the soil aggregates of different sizes were significantly higher (P < 0.05) and the C/N ratios in the particulate organic matter were significantly lower (P < 0.05) for soil to 20 cm deep for the MNPK treatment than for the other treatments. ANOVA indicated that the C and N concentration and C/N ratios in different sizes of aggregates significantly varied with soil depth (P < 0.05). Microaggregates contained most of the C and N, and the C/N ratios for silt-clay particles in macroaggregates were 1.37 unit (ranging - 0.25 to 2.44) lower than for other soil particles with diameters < 53 µm. The C and N contents in aggregates of different sizes increased as the C input rate increased to a depth of 40 cm because of the fertilization practices. Overall, both increased C input and deep application of C sources promoted the storage of C and N in microaggregates, which in turn increased C and N sequestration in dryland red soils.

A highly conserved core bacterial microbiota with nitrogen-fixation capacity inhabits the xylem sap in maize plants.

Microbiomes are important for crop performance. However, a deeper knowledge of crop-associated microbial communities is needed to harness beneficial host-microbe interactions. Here, by assessing the assembly and functions of maize microbiomes across soil types, climate zones, and genotypes, we found that the stem xylem selectively recruits highly conserved microbes dominated by Gammaproteobacteria. We showed that the proportion of bacterial taxa carrying the nitrogenase gene (nifH) was larger in stem xylem than in other organs such as root and leaf endosphere. Of the 25 core bacterial taxa identified in xylem sap, several isolated strains were confirmed to be active nitrogen-fixers or to assist with biological nitrogen fixation. On this basis, we established synthetic communities (SynComs) consisting of two core diazotrophs and two helpers. GFP-tagged strains and 15N isotopic dilution method demonstrated that these SynComs do thrive and contribute, through biological nitrogen fixation, 11.8% of the total N accumulated in maize stems. These core taxa in xylem sap represent an untapped resource that can be exploited to increase crop productivity.

The response of potato tuber yield, nitrogen uptake, soil nitrate nitrogen to different nitrogen rates in red soil.

Nutrient-deficient red soil found in the southern region of China is increasingly being used for potato crops to meet the demand for this staple food. The application of nitrogen fertilizer is necessary to support the production of higher tuber yields; however, the links between nitrate nitrogen and the nitrogen balance in red soil are unknown. A field experiment was conducted in Jiangxi Province in 2017 and 2018 to determine the effects of different nitrogen application rates, 0 kg ha-1 (N0), 60 kg ha-1 (N60), 120 kg ha-1 (N120), 150 kg ha-1 (N150), 180 kg ha-1 (N180), 210 kg ha-1 (N210), and 240 kg ha-1 (N240, the highest rate used by local farmers), on potatoes growing in red soil. Data on tuber yield, crop nitrogen uptake, and the apparent nitrogen balance from the different treatments were collected when potatoes were harvested. Additionally, the content and stock of nitrate nitrogen at different soil depths were also measured. Nitrogen fertilization increased tuber yield but not significantly at application rates higher than 150 kg ha-1. We estimated that the threshold rates of nitrogen fertilizer application were 191 kg ha-1 in 2017 and 227 kg ha-1 in 2018, where the respective tuber yields were 19.7 and 20.4 t ha-1. Nitrogen uptake in potato in all nitrogen fertilization treatments was greater than that in N0 by 61.2-237% and 76.4-284% in 2017 and 2018, respectively. The apparent nitrogen surplus (the amount of nitrogen remaining from any nitrogen input minus nitrogen uptake) increased with increasing nitrogen application rates. The nitrate nitrogen stock at a soil depth of 0-60 cm was higher in the 210 and 240 kg ha-1 nitrogen rate treatments than in the other treatments. Moreover, double linear equations indicated that greater levels of nitrogen surplus increased the nitrate nitrogen content and stock in soils at 0-60 cm depths. Therefore, we estimate that the highest tuber yields of potato can be attained when 191-227 kg ha-1 nitrogen fertilizer is applied to red soil. Thus, the risk of nitrate nitrogen leaching from red soil increases exponentially when the apparent nitrogen balance rises above 94.3-100 kg ha-1.

Evaluation of grain yield based on digital images of rice canopy.

BACKGROUND: Rice canopy changes are associated with changes in the red light (R), green light (G), and blue light (B) value parameters of digital images. To rapidly diagnose the responses of rice to nitrogen (N) fertilizer application and planting density, a simple model based on digital images was developed for predicting and evaluating rice yield. RESULTS: N application rate and planting density had significant effects on rice yield. Rice yield first increased and then decreased with increasing of N rates, while the rice yield always increased significantly with increasing planting density. The normalized redness intensity (NRI), normalized greenness intensity (NGI), and normalized blueness intensity (NBI) values of the rice canopy varied among stages; however, they were primarily affected by N fertilizer rates, while planting density had no significant effects. Furthermore, the significant relationships of grain yield with NRI and NBI at the late filling stage could be fitted by quadratic equations, but there was no significant relationship observed between grain yield and NGI across all stages. In addition, a field validation experiment showed that the predicted yield based on the fitted quadratic equations was consistent with the measured yield. CONCLUSION: The NRI, NGI, and NBI values of rice canopy were mainly affected by N fertilizer rates, while the planting density had no significant effect. The significant relationships between grain yield with NRI and NBI at the late filling stage could be fitted by quadratic equations. Therefore, the canopy NRI and NBI at the late filling stage as measured by digital photography could be used to predict grain yield in southern China.