Besides being the most important factor for amelioration of soil physical and biological characteristics, green manure and crop residues, after degradation to soil organic matter, play important role as nutrients supplier for subsequently grown crop. But, how much do they contribute?

Isotopes is the unique tool for quantitatively and precisely measuring the amount of nutrients a certain crop utilizes from green manure or  crop residue, labeling isotopicaly plants used as green manure or plant of crop of which its residue is going to considered as certain nutrient source, e.g.,  nitrogen, using ¹⁵N. Several researches have been carried in Brazil, most of them under IAEA contract, using this technique. Among the nutrients, nitrogen is certainly the most studied element, due not only  by  its importance for most of crop, due to  its frequent  deficiency in tropical soils, but also because there is a very adequate isotope, the ¹⁵N, by its very convenient characteristics: stable and  excellent isotopic ratio (¹⁵N/¹⁴N): 0.366% ¹⁵N and 99.634% ¹⁴N. Different green manure crops (crotalaria, mucuna, pueraria, millet and sesbania) and crop residues (rice, corn, bean, soy bean) have been studied as nitrogen source, labeling them with ¹⁵N. Crotalaria and mucuna have the potential to furnish more than  150 and 360 kg/ha of N, respectively, although  the contributions of these green manures to rice crop grown subsequently as nitrogen supplier was between 25 to 38%. Combining green manure with fertilizer increased considerably both green manure and fertilizer nitrogen utilization by rice crop. Millet, although it is largely recommended in Cerrado (Savannah) as soil cover crop, studies carried during two consecutive year with corn, showed no advantage over fallow system, unlike crotalaria, which supplied more nitrogen to corn.  Studies using ³⁵S and ³⁴S have shown that green manure is also an important sulfur supplier for corn crop.  Among the crop residues, rice and bean (Phaseolus vulgaris L.) residues resulted in net mineralization, while corn residues caused immobilization of native soil sulfate.

Keywords: isotopes, plant nutrients, crop residues, green manure.