Explanation:

The usage of biochar can be also interesting for the urban farming industry. In fact, green roofs are seen as “privileged areas” for retaining rainwater and reducing the effects of urban heat islands. Becket al. 2011 evaluated the quantity and quality of water passing over a vegetated roof extensively grown after adding biochar.

• Adding biochar to the substrate at 7% (by weight) can be a way to improve the quality of water downstream water by reducing nitrogen, phosphorus, organic carbon concentrations and thus reducing turbidity in runoff… A promising solution for the urban green rooftop.
• Soil Amendment: Biochar is recognized as offering a number of soil health benefits. The extremely porous nature of biochar is found to be effective at retaining both water and water-soluble nutrients. Its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity, and reduce pressure on old-growth forests.
• Soil amendment with biochar can increase biological nitrogen fixation (BNF) and improve growing conditions for legumes, while such effects may alter when biochar properties change with time(biochar aging). 
• Water retention: Biochar is hygroscopic. Thus, it is a desirable soil material in many locations due to its ability to attract and retain water.

Hence all statements are correct.