Why should ferrous sulfate be added in the determination of hydrolyzed nitrogen in dry land soil?

Nitrate nitrogen content is high. Ferrous sulfate must be added to determine the content of alkali-hydrolyzable nitrogen in dry land soil. The content of nitrate nitrogen in dryland soil is high, so zinc powder and ferrous sulfate should be added to reduce it to ammonium nitrogen. The advantage of this method is that it is simple to operate and suitable for mass detection. It is necessary to increase the alkali concentration (1.8mol/L) to keep the alkali concentration at 1.2mol/L, and the nitrate content in paddy soil is very small, so it can be directly hydrolyzed with 1.2mol/L sodium hydroxide.