Describe how the process of denitrification affects the availability of nitrates to plants.

A farmer grows a cover crop of vetch (a legume plant) in a field during winter. In spring, the farmer ploughs the vetch back into the soil before sowing a crop of barley. Suggest how this practice increases the barley yield.

Explain why only about 10% of the energy stored in the biomass of the barley is transferred to the herbivores that feed on it.