Chemical fertilizers and manures with high N contents are commonly added to agricultural soils. Yet these soils are often lower in total N than are nearby soils under natural forest or grassland...

Chemical fertilizers and manures with high N contents are commonly added to agricultural soils. Yet these soils are often lower in total N than are nearby soils under natural forest or grassland vegetation. Explain why this is the case.

In some tropical regions, agroforestry systems that involve mixed cropping of deep rooted trees and shallow-rooted food crops are used. What advantages in nitrogen management do such systems have over monocropping systems that do not involve trees?