Nitrate Leaching And Nitrogen Use Under Wheat And Maize Plantation

It is no doubt that nitrogen(N)-fertilizer can increase the grain yield significantly. But in the recent years, the negative effects of over-application of N fertilizer appeared gradually, such as low N-fertilizer use efficiency (NUE), grounder and surface water pollution and the green house effect. The main reason of low NUE is the great N losses. Volatilization, denitrification and nitrate leaching are 3 main N loss ways. Because of close relationship to the environment, more and more attention had been paid to the latter two. In this paper, field experiments were carried out to study the effect of N rate, irrigation rate, N split and soil texture on nitrate movement and accumulation, NUE and apparent budget of soil N. At the same time, to study the effects of nitrate and ammonium fertilizers on N2O emission, an incubation experiment was conducted in laboratory with Ca(NO3)2 and NH4HCO3 under 45%, 65%, 100% field capacity (FC). The results are showed as follows: 1. The results from wheat field experiment with two factors of water and N rate show that the rate of N-fertilizer had a very significant effect on the spatial-temporal variations of soil nitrate-N, especially in 0-200 ㎝soil profile. Nitrate-N accumulation in soil profile increased with increasing of the N rate, but there was no linear relationship between accumulation and N rate, while irrigation rate and the interaction between irrigation and fertilization only affected in deeper layer of soil profiles (200-400 cm) significantly. N application and irrigation rate and their interaction had a significant effect on yield (p<0.01). Thinking about high yield and less nitrate accumulation, the most reasonable ratio of N and irrigation rate was N130I60, it meant 130 kg N hm-2 application and 60 mm irrigation. Under the condition of this experiment, for wheat, 0-200 cm soil N was surplus during the period from elongation to filling stage, and deficit from sowing to elongation and from filling to harvest. At certain rate of irrigation, with the increasing of N application rate, apparent N recovery decreased significantly, while losses increased. At certain N rate, with the increasing of irrigation rate, N accumulation in soil decreased and losses increased. When irrigation rate was 60 mm, the NUE was highest. 2. N rate and application methods used in the maize field experiment and the results show that the N split could significantly increase nitrate accumulation (p<0.01) in soil profile, especially in the depth of 0-100 cm soil. If there was no N split, then the main N surplus in soil was from elongation to before tasseling and main deficit was from before tasseling to harvest stage. N split could decrease N deficit, while increase N surplus in soil. With the increasing of N rate, the amount of apparent N-fertilizer recovery, residual and losses went up, as well as losses rate, but the rate of recovery and residual went down. At the same N rate, N split could increase NUE, decrease the rate of residual and losses, and the less the N rate, the more significant the result was. When 260 kg N hm-2 applied, compared with using one time, N split could increase NUE by about 3%, and decreased the loss rate by 16%, but for the rate of 130 kg N hm-2, it was about 4% and 31%, respectively. 3. Due to less clay content and looseness of Entisol, and a fast speed of nitrification, nitrogen input to the soil mostly presented as nitrate. Therefore, nitrate leaching is the main way of N losses in bottom-land in Entisol area, where the irrigation can be carried out. However, in Anthrosol, Guanzhong plain ,due to a higher clay content relatively and fast speed of nitrification, a clay layer in 80-120 cm depth, which block the nitrate and water leaching through, so most of the nitrate was accumulated in 0-100 cm depth. Since soil water saturated in deep layer, in Gen-Stagnic Anthrosols in southshaanxi, it is very difficult for water and nitrate leaching down, so nitrate was mainly accumulated in top soil layer(0-20 cm). 4. The amount of N2O emission in the soil applied ammonium fertilizer was greater than that applied nitrate fertilizer under water content of 45% and 65% of field capacity (FC). However, N2O flux got to peak in these two N-fertilizer treatments when soil moisture was 100% FC. During the incubation of 10 days, there was no significant difference among the total amount of N2O emission under different soil moisture levels in NH4HCO3 treatment. But there was great difference of N2O emission from nitrification and denitrification. Under 45% FC, the contribution of N2O emission from nitrification and denitrification in NH4HCO3 treatment were 84.77% and15.23%, respectively, and 71.78% and 28.22% for 65% FC, 48.50% and 51.50% for 100% FC. Then we could draw a conclusion that nitrification was the main source of N2O emission in this experiment.