Heterotrophs release carbon dioxide by respiration. Carbon is stored in reserves as petroleum, coal, peat, etc. This effect also is due to increased fuel consumption. 3 - Bicarbonate is stored in large quantities in the ocean. 3 - Denitrifying bacteria convert nitrite and nitrate back to nitrogen.
Plants mostly take up nitrogen as nitrate.
Animals can only absorb organic nitrogen, mostly as amino acids and protein. Phosphorus is a limiting nutrient in many situations. The only important form for plants is PO 4 3- . This mostly comes from weathering of rocks. Plants convert phosphate into organic forms.