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The identification of phosphorus (P) fractions is essential for understanding the transformation and availability of P in paddy soils. To investigate the soil P fractions associated with soil properties under long-term fertilization, we selected three fertilization treatments, including no fertilization (CK), chemical fertilizers (NPK) and chemical fertilizers combined with manure (NPKM), from three long-term experiments located in Nanchang (NC), Jinxian (JX) and Ningxiang (NX). The results showed that chemical fertilizers combined with manure (NPKM) significantly ( P ≤ 0.05) increased the soil total phosphorus, Olsen P and soil organic matter (SOM) by 2, 3 and 1 times, respectively, compared with the NPK treatment, and by 4, 17 and 2 times, respectively, compared with the CK treatment. NPKM significantly increased the grain yield compared with CK and NPK at all sites. The apparent P balance with NPK was higher in NC and NX but lower in JX compared with NPKM. Hedley fractionation revealed the predominance of most of the organic and inorganic phosphorus (Po and Pi) fractions with long-term fertilization, especially with the NPKM treatment, at all sites. The nonlabile P pool decreased by 14% and 18% whereas the moderately labile P pool proportions increased by 3 and 6 times with the NPK and NPKM treatments, respectively, compared to the CK treatment. The labile P pool showed a significant positive relationship with the SOM, total P and Olsen P contents. The moderately labile P was positively correlated with the total P and Olsen P. A significant positive correlation was observed between soil pH and the nonlabile P pool. Redundancy analysis revealed that the moderately labile P fraction (HCl dil. Pi fraction) was remarkably increased by the NPKM treatment and significantly correlated with the soil pH and total P concentration. The labile P fraction (NaHCO 3 -Pi) showed a strong relationship with the Olsen P and total P. However, the residual P fraction was negatively correlated with the HCl. dil. Pi fraction. We concluded that NPKM application improved P availability by many folds compared to NPK, which could lead to environmental pollution; therefore, the rate of combined application of manure and chemical fertilizer should be reduced compared to chemical fertilizer inputs to minimize the wastage of resources and environmental P losses.

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 effects of soil type and organic material quality on the microbial biomass and functional diversity of cropland soils were studied in a transplant experiment in the same climate during a 1-year field experiment. Six organic materials (WS: wheat straw, CS: corn straw, WR: wheat root, CR: corn root, PM: pig manure, CM: cattle manure), and three contrasting soils (Ferralic Cambisol, Calcaric Cambisol and Luvic Phaeozem) were chosen. At two time points (at the end of the 1st and 12th months), soil microbial biomass carbon (C) and nitrogen (N) (MBC and MBN) and Biolog Ecoplate substrate use patterns were determined, and the average well color development and the microbial functional diversity indices (Shannon, Simpson and McIntosh indices) were calculated. Organic material quality explained 29.5–50.9% of the variance in MBC and MBN when compared with the minor role of soil type (1.4–9.3%) at the end of the 1st and 12th months, and C/N ratio and total N of organic material were the main parameters. Soil properties, e.g., organic C and clay content were the predominant influence on microbial functional diversity in particular at the end of the 12th month (61.8–82.8% of the variance explained). The treatments of WS and CS significantly improved the MBC and microbial functional diversity indices over the control in the three soils in both sampling periods (P < 0.05). These results suggest that the application of crop straw is a long-term effective measure to increase microbial biomass, and can further induce the changes of soil properties to regulate soil microbial community.

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Low phosphorus use efficiency (PUE) is one of the main problems of acidic soil that limit the crop growth. Therefore, in the present study, we investigated the response of crop yield and PUE to the long-term application of fertilizers and quicklime (CaO) in the acidic soil under wheat–maize rotation system. Treatments included, CK (no fertilization), NP (inorganic nitrogen and P fertilization), NPK (inorganic N, P and potassium fertilization), NPKS (NPK + straw return), NPCa (NP + lime), NPKCa (NPK + lime) and NPKSCa (NPKS + lime). Results showed that, fertilizer without lime treatments, significantly (p ≤ 0.05) decreased soil pH and crop yield, compared to the fertilizer with lime treatments during the period of 2012–2018. Average among years, compared to the CK treatment, wheat grain yield increased by 138%, 213%, 198%, 547%, 688% and 626%, respectively and maize yield increased by 687%, 1887%, 1651%, 2605%, 5047% and 5077%, respectively, under the NP, NPK, NPKS, NPCa, NPKCa and NPKSCa treatments. Lime application significantly increased soil exchangeable base cations (Ca2+ and Mg2+) and decreased Al3+ cation. Compared to the NP treatment, phosphorus use efficiency (PUE) increased by 220%, 212%, 409%, 807% and 795%, respectively, under the NPK, NPKS, NPCa, NPKCa and NPKSCa treatments. Soil pH showed significant negative relationship with exchangeable Al3+ and soil total N. While, soil pH showed significant (p ≤ 0.05) positive relationship with exchangeable Ca2+, PUE and annual crop yield. PUE was highly negatively correlated with soil exchangeable Al3+. In addition, soil exchangeable Ca2+, pH, exchangeable Al3+ and available N were the most influencing factors of crop yield. Therefore, we concluded that lime application is an effective strategy to mitigate soil acidification and to increase PUE through increasing exchangeable base cations and reducing the acidic cations for high crop yield in acidic soil.