Changes in total nitrogen and total carbon contents in soil and the leaf yield of a long-term unfertilized mulberry under field condition

Total nitrogen and total carbon contents of the soil and the leaf yield in a long-term unfertilized mulberry field (NF) were compared with those in a chemically fertilized field (F) for 19 years from 1985 to 2003. Each year, 30, 20 and 20g m-2 of nitrogen, phosphorus and potassium, respectively, were applied to the F field; a half in spring and the remaining half in summer. Weeds and litter fall were all removed from both F and NF fields. Originally, the mulberry cultivar Kairyonezumigaeshi had been grown in both fields for more than 25 years. In the spring of 1991 (the 7th year of the experiment), mulberry (cv. Ichinose) was replanted to both fields. The mulberry leaves were harvested twice a year in spring and in late-autumn, by cutting and leaf plucking methods, respectively Total soil nitrogen and carbon contents of the soil in the NF field reached equilibrium after 15 years of treatment, and the yearly yield of fresh leaves from NF was stable at 1800 to 2000g m-2 for 5 years after replanting. The changes in nitrogen content of the plant and that of soil nitrogen suggested 17.5g m-2 of nitrogen was annually input to the NF field from the natural environment. The source of nitrogen input remains unknown, but a major part of nitrogen was supposed to come from the nitrogen pool in the subsoil layers, from adjacent environment and/or through atmospheric nitrogen fixation. In the spring rearing season, during which the time interval between successive harvesting was long and fertilizer application was less effective, the leaf yield was higher in the NF field than in the F field. On the other hand, in the late-autumn rearing season, during which intervals between successive harvesting was short and applied fertilizer was effective, the leaf yield was higher in the F field than in the NF field. The rate of nitrogen content allocation to leaf blades was higher in the NF than in the F field, even though the nitrogen content of the whole plant was lower in the NF field. The characteristic nutrient distribution pattern in the plant may also be a reason for the higher leaf yield in the unfertilized field.