EFFECTS OF AN ORGANIC MANURE ON THE TRANSFORMATIONS OF AMMONIUM NITROGEN IN PLANTED AND UNPLANTED SOIL

Cattle slurry supplemented with N-15 labelled ammonium sulphate was ap plied to unplanted soil and to soil planted with sprouted potato tuber s. For comparison, there was a similar treatment with N-15 labelled am monium sulphate alone. The pots of soil were kept at 20-degrees-C and the plants were harvested after 21, 42, 70 and 98 days. Labelled and u nlabelled nitrogen were measured in the plants and, after the same int ervals, in the soil as mineral, organic and clay-fixed nitrogen, The r ecovery of labelled nitrogen in plants plus soil by the end of the exp eriment was 90% with ammonium sulphate alone and 77% with cattle slurr y; the corresponding recoveries in unplanted soil were only 65% and 48 %. The greater recoveries of the labelled nitrogen in the planted soil are attributed to its greater protection against gaseous loss when wi thin the plants. Another effect of the plants was to decrease the amou nt of labelled nitrogen that had been initially fixed by the clay. Dur ing the first 21 days with cattle slurry almost half of the labelled n itrogen became immobilized in organic matter. In the same period there was mineralization of unlabelled nitrogen, but the overall reaction w as net immobilization. In later periods, immobilized labelled nitrogen in the unplanted soil decreased indicating remineralization. Estimate s are given of the rates of gross mineralization, but the periods betw een sampling occasions were too long to yield reliable values.