1. The Nitrogen Cycle
In the nitrogen (N) cycle, organic nitrogen exists in materials formed from animal,
human, and plant activities that produce manures, sewage waste, compost, and
decomposing roots or leaves. These organic products transform into organic soil
material called humus. Inorganic nitrogen comes from minerals, and is added to soil
from precipitation, or as fertilizers. Adding N to the soil is done to help living plants
growa and remain healthy. However, plants cannot use organic forms of nitrogen so
certain microbes living in the soil come to the rescue and convert organic forms of N
into inorganic forms that plants can then use.
There are different forms of inorganic nitrogen that are available to plants. Some of
these can be stored in the soil (such as ammonium, NH4
+). Other forms of nitrogen
that are not held by the soil particles (such as nitrate NO3
-, and nitrite NO2
-) can
leach out of the soil and into the groundwater, or can be transformed into nitrogen
gases (N2, NO, or N2O) and ammonia gas (NH3), escape out of the soil (volatilize),
and into the Earth's atmosphere.
Luckily there are other kinds of microbes that live in the soil, and close to Earth's
surface, that can convert Nitrogen gas into inorganic forms of N that plants can
then use. When plants die they contribute organic N to the soil and certain
microbes, yet again, do their job of converting organic N into inorganic N that living
plants can use! Below, see an illustration of the N cycle, how N changes when it is in
soil, water and air, and find out what those interesting nitrogen cycle terms mean!