Organic Matter Inputs to Soil After Growth of Carbon‐14‐Nitrogen‐15 Labeled Maize

After growth of doubly labeled ( 14 C and 15 N) maize ( Zea mays L.), two loamy soils were labeled by root exudation and rhizodeposition, and by direct microbial immobilization of N. Fresh roots were then carefully separated and washed, eliminating organic and organomineral cementing agents by acid and alkaline solubilizing reagents, and the remaining insoluble humin was water dispersed in order to separate coarse, medium, and fine fractions. At harvest time, fresh roots represented 85% of the total C input, and rhizodeposition 15%. Sixty to 70% of the N input was still in living roots at this time, and other organic forms of N were more a result of microbial activity than of rhizodeposition. The largest and most homogeneous organic fraction was the finest insoluble fraction, in which about half of the label for both C and N was found.