$9.$ Investigate the global trade dynamics of phosphate rock and phosphorus fertilizers, including the geopolitical implications of phosphorus supply security and the role of phosphorus recycling in mitigating supply risks.

$10.$ Evaluate the environmental risks and benefits of using calcium ammonium nitrate $($CAN$)$ as a nitrogen source in agriculture, focusing on its lower volatilization risk compared to urea and its impact on soil acidification and water quality.