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Converting swine manure to biochar is an effective way to recycle valuable nutrients, but there are few reports on its feasibility as a phosphorus (P) source. The objective of this study was to clarify the unique nature, including P speciation, of manure biochar products under various pyrolysis temperatures. We used solution P nuclear magnetic resonance and P K-edge X-ray adsorption near-edge spectroscopy (P XANES) to characterize P species in swine manure biochar. For every 100 °C increment starting from 300 °C, the P content in manure biochar increased by 2.16 to 3.37 g kg. However, above 400 °C, organic P species did not appear anymore, and only inorganic P, including orthophosphate and pyrophosphate, existed. P K-edge XANES spectra further showed all biochar samples had higher percentages of Ca (PO) and NaPO, and lower percentages of FePO, AlPO, and inositol hexaphosphate compared to manure. Interestingly, percentages of Ca(PO), FePO, and AlPO in MB400 (indicating manure pyrolysed at 400 °C) were comparable with those in MB700 while the percentage of NaPO was higher in MB400. Phosphorus release from MB400 maintained a relatively high level at 0.33 g kg during the whole 300-h observation period. These results suggest that with a suitable pyrolysis temperature, it was feasible for manure biochar to be a P source alternative. Graphical abstract ᅟ.

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Manure biochar (MB) is recognized to be a beneficial material for absorbing heavy metals from soil and alleviating soil degradation. However, the effects of the addition of MB on the phospholipid fatty acid concentrations and their linkage to heavy metals accumulation in rice are poorly understood. A microcosm incubation experiment was conducted to study the effects of a swine MB amendment on (a) the composition of the soil microbial community in 2 soils (clay‐loam vs. silt‐loam) and (b) heavy metals accumulation in rice grains and straws. MB amendment increased microbial diversity, and bacteria had greater magnitude of increase than did fungi in 2 soils after 98 days of incubation. The G− rather than G+ bacteria phospholipid fatty acid concentrations were significantly increased with MB amendment rates for both soils. The higher MB addition rate (1.5%) did not get more benefit for aerobic bacteria but significantly led to anaerobic bacteria proliferation as compared with the 0.5% MB treatment. The 1.5% MB addition suppressed grain and straw Pb, Cu, and As, while it increased grain and straw Cd and Zn from two soils. Significant soil type × MB rate interactions were observed in most microbial indicators (except F/B and G+) and grain Pb, grain As, and straw Zn. Linkages of bacteria (mainly as G− and/or An), actinobacteria, fungi, and protozoa (in silt‐loam soil) to the MB rate and heavy metals in rice were identified.