Movement of early plants fromaquatic to terrestrial ecosystems in the early Paleozoictime involved adaptation to available nitrogen resources,ammonia, nitrate and organic nitrogen (e.g., free aminoacids). Ammonia and organic nitrogen are vital for microbial metabolism and energy production. Plants havetherefore evolved multiple strategies for acquiring sufficient nitrogen to support their photosynthetic activitiesand reproduction. Plants develop large root systemsincreasing the explored soil area, from where they useprimarily nitrate, which even if directly uptaken from soilis energetically costly compared to ammonium. An alternative strategy developed by plants to improve theirnitrogen supply, was the evolution of trophic allianceswith specially adapted soil microbes, and by this, theygain access to alternative forms of nitrogen