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.
Modern intensive agricultural practices, particularly the use of nitrogen fertilizers, have accelerated soil acidification on a global scale. The soil pH buffering capacity (pHBC) is often used to quantify the soil acidification rate. Calcareous soils have relatively higher pH and pHBC, reflecting the presence of carbonate minerals; however, the impact of long-term fertilization treatment on pH and pHBC is poorly understood for calcareous soils. Here, calcareous soil samples (0-20 cm) were collected from fields receiving six different fertilization treatments for 22 years: control (CK, unfertilized but planted); nitrogen (N); nitrogen and phosphorus (NP); nitrogen, phosphorus and potassium (NPK); combined manure and NPK (NPKM); and combined corn-stover and NPK (NPKS). Both pH and pHBC significantly decreased for all treatments relative to CK. NPKS treatment had the lowest soil pH. Compared with CK, the soil pHBC decreased 5.7 to 17.3% under different treatments. The calcium carbonate (CaCO 3) content was significantly reduced by fertilization treatments, with a maximum decrease under the NPKS treatment. Structural equation model (SEM) analysis revealed that calcium carbonate and soil organic matter (SOM) made important contributions to effective cation exchangeable capacity (ECEC). Soil pHBC was directly controlled by ECEC, while CaCO 3 and SOM indirectly contributed to the pHBC through ECEC. These results indicated that NPKS treatment induces more severe soil acidification, reflecting the higher H + input and lower pHBC under this treatment.
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