Novel Straw-biochar Based Fertilizers:Preparation And Study On The Slow-release Performance

The widely application of chemical fertilizer（CF）has become the main way to promote agricultural production.However,the high solubility of nitrogen and phosphorus in traditional chemical fertilizers not only reduced the nutrient absorption rate of crops,but also caused serious environmental problems.Therefore,it is crucial to find an effective technique to recover nitrogen（N）and phosphorus（P）organic matter from water,and develop alternative fertilizers to promote the sustainable development of agricultural production.In this work,the agricultural wastes,such as corn straw and egg shell,were employed to prepare the Ca/Mg-enriched biochar（Ca/Mg-BC）and Mg-enriched biochar（MB）via magnesium salt impregnation and pyrolysis transformation process.The simultaneous recovery performance of nitrogen and phosphorus using Ca/Mg biochar were systematically studied under different preparation conditions.The N,P recovery mechanism were investigated by characterizations.Moreover,the fertilizer efficiency of Ca/Mg biochar after N,P recovery was observed by pot experiment.On the other hand,based on our previous work,the Mg-enrich biochar fertilizer（MBF）with high nitrogen and phosphorus contents was selected as the representative（TN:200 mg/g;TP:381.1 mg/g）.The N and P slow-release performances of MBF were investigated by both kinetic study and leaching experiment.Meanwhile,the effects of environmental factors（p H and temperature）on N,P slow-release performance were also studied.In addition,the effects of MBF on seed germination and plant growth were determined and evaluated by comparison with CF.Finally,the associated mechanism of N,P slow-release by MBF was discussed and proposed.The results showed that Ca/Mg biochar prepared under the optimum condition（microwave-assisted impregnation,1:1 of Ca and Mg mass ratio and 700℃of pyrolysis temperature）had the highest N,P recovery rate.The resulting Ca/Mg biochar material after nutrient recovery possessed 198.66 mg/g of TN and 220 mg/g of TP,which has potential to be used as a new type of fertilizer.Mechanism study revealed that the N,P simultaneous recovery performance of Ca/Mg biochar was mainly controlled by multi-effects,including the confinement effect,electrostatic attraction and precipitation reaction.Pot experiment showed that Ca/Mg biochar fertilizer（Ca/Mg-BCF）had significant promoting effect on seed germination and corn growth.For MBF,Mathematical modeling and column leaching method demonstrated that the MBF exhibited excellent slow-release performances of total P and N with sustainable release rates.Leaching experiment indicated that the final accumulative release ratios of N and P from MBF were 7 times and 6 times lower than those of chemical fertilizer（CF）,respectively.The mechanism study reveals that the P-release performance of MBF was not only controlled by the low solubility of Mg-P precipitates formed on the biochar surface,but also enhanced by the‘P-trap’effect of Mg O through re-precipitation process of PO₄^3-.Meanwhile,the N-release behavior of MBF was dominated by the multi-effects of biochar carrier,including the confinement effect and electrostatic attraction for NH₄⁺,as well as the hydrogen bonds and pore-filling effect for N-containing organic matter.In addition,MBF significantly promoted the corn growth and enhanced the nutrient uptake efficiency of corn.These results suggested that MBF may therefore have promising potential in sustainable agriculture application with multiple environmental benefits.