Effects of nitrogen application on alfalfa plant growth and soil nitrogen content

A pot experiment was conducted to examine the effects of variable rates of nitrogen application (0, 30, 60 and 90 kg N·ha-1, hereafter referred as N0, N30, N60 and N90 respectively) on growth of alfalfa (Medicago sativa), plant N content and soil nitrate N and ammonium N levels. The result showed that the dry weight per plant increased with N application rate at all the 4 cuts while this effect was weakened at the later cuts. The total accumulated growth (TAG) of both above-and under-ground parts increased with N application rate. The TAG of N30, N60 and N90 was higher than control (N0) by 1.88%, 13.88% and 21.99%, respectively. No significant effect of N rate on plant tissue N level was detected. The soil nitrate N level decreased first and then increased to a stable level, while the soil ammonium N level increased first and then decreased to a stable level. In all treatments, both nitrate and ammonium N levels in the soil were significantly lower than the level before planting. There was a 56.6%, 53.2%, 32.7% and 47.6% decrease in nitrate N and 38.8%, 27.1%, 35.3% and 28.1% decrease in ammonium N for N0, N30, N60 and N90 respectively.