Answer:
added to the soil through weathering
Explanation:
1) found free in the atmosphere
: in it is found in the form of phosphates etc.
2 returned to the soil by rhizobium : Rhizobium is a bacteria which is involved in nitrogen fixation.
3 added to the soil through weathering : It is added to soil again by weathering and is absorbed by plants in the form of phosphates.
4 usually very abundant in an ecosystem: it is found in earth nearly 0.1% by weight.
