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Potentially toxic metals (PTEs) and antibiotic resistance genes (ARGs) present in bio-wastes were the major environmental and health risks for soil use. If pyrolyzing bio-wastes into biochar could minimize such risks had not been elucidated. This study evaluated PTE pools, microbial and ARGs abundances of wheat straw (WS), swine manure (SM) and sewage sludge (SS) before and after pyrolysis, which were again tested for soil amendment at a 2% dosage in a pot experiment with a vegetable crop of pak choi (Brassica campestris L.). Pyrolysis led to PTEs concentration in biochars but reduced greatly their mobility, availability and migration potential, as revealed respectively by leaching, CaCl2 extraction and risk assessment coding. In SM and SS after pyrolysis, gene abundance was removed by 4–5 orders for bacterial, by 2–3 orders for fungi and by 3–5 orders for total ARGs. With these material amended, PTEs available pool decreased by 25%–85% while all ARGs eliminated to background in the pot soil. Unlike a >50% yield decrease and a >30% quality decline with unpyrolyzed SM and SS, their biochars significantly increased biomass production and overall quality of pak choi grown in the amended soil. Comparatively, amendment of the biochars decreased plant PTEs content by 23–57% and greatly reduced health risk of pak choi, with total target hazard quotient values well below the guideline limit for subsistence diet by adult. Furthermore, biochar soil amendment enabled a synergic improvement on soil fertility, product quality, and biomass production as well as metal stabilization in the soil-plant system. Thus, biowastes pyrolysis and reuse in vegetable production could help build up a closed loop of production-waste-biochar-production, addressing not only circular economy but healthy food and climate nexus also and contributing to achieving the United Nations sustainable development goals., Graphical abstract Image 1

Approaches to reduce N 2 O emission from crop ecosystems deserves urgent need for climate change mitigation in world agriculture. Yet, unique ecological measures to depress N emission while conserving crop productivity have not yet been well developed for wide ranges of crop ecosystems. In order to establish an ecological engineering option to mitigate N 2 O emission in rice ecosystems, we conducted a field experiment with biochar amendment on N 2 O emission from rice paddies in three sites across South China in 2010. This experiment was performed with 6 treatments of biochar rates of 0, 20, and 40 t ha −1 with and without N fertilization respectively. The rice ecosystem was managed with conventional crop production practices as seasonally man-managed wetlands, which were under flooding after seedling transplantation till panicling and drainage during spiking followed by a subsequent moist condition (F-D-M) till harvest across sites. Emission of N 2 O from rice soil was monitored with closed chambers at 7 days interval throughout the whole rice growing season (WRGS) and the gas samples analyzed with a gas chromatograph (Agilent 7890D) equipped with an electron capture detector (ECD). Total emission of N 2 O-N ranged from 1.5 kg N 2 O-N ha −1 to 1.9 kg N 2 O-N ha −1 without biochar, and from 0.8 kg N 2 O-N ha −1 to 1.3 kg N 2 O-N ha −1 and from 0.7 kg N 2 O-N ha −1 to 0.9 kg N 2 O-N ha −1 with biochar amendment at 20 t ha −1 and 40 t ha −1 , respectively. Thus, biochar amendment depressed total N 2 O emission from chemical N fertilizer, as the calculated EF of N 2 O-N emission was reduced from 0.57 ± 0.15% under chemical N fertilizer only to 0.36 ± 0.08% and 0.22 ± 0.04% under biochar amendment at 20 t ha −1 and 40 t ha −1 respectively. The value under biochar amendment at 40 t ha −1 was found even much smaller than that of a continuously flooding rice ecosystem. As soil pH (H 2 O), content of soil organic carbon and total N were all upraised significantly, biochar amendment improved rice ecosystem functioning by decreasing N 2 O-N emission per metric ton of rice production from 0.17 ± 0.02 kg N 2 O-N without biochar to 0.10 ± 0.02 and 0.07 ± 0.03 kg N 2 O-N under biochar respectively at 20 t ha −1 and 40 t ha −1 . Thus, soil amendment of biochar from crop straw could be adopted as a unique ecological engineering measure to reduce N 2 O emission while enhancing soil fertility and sustaining rice productivity in rice ecosystems.