Describe how the process of nitrification affects the chemical form and availability of nitrogen compounds in the soil.

An agroforestry system integrates alder trees (Alnus glutinosa), which form a symbiotic relationship with nitrogen-fixing bacteria in their root nodules, alongside a companion crop of winter rye. After leaf fall in autumn, the fallen leaves and pruning debris from the alder trees are incorporated into the soil. Suggest how this management practice increases the growth and yield of the winter rye.

Explain why only a small proportion of the chemical energy stored in the biomass of the winter rye is transferred to primary consumers, such as field voles, that feed on the crop.