[Effects of Fertilization on the Nitrogen Residual Amounts and Leaching from Citrus Orchard Soil in the Three Gorges Reservoir Area].

This experiment has the citrus orchard soil in the basin of the Zigui County Ecological Station in the Three Gorges Reservoir Area as its research object. Leaching tests of an undisturbed soil column at 0-20, 0-40, and 0-60 cm depth were performed in a citrus orchard. The effects of nitrogen leaching and residual amounts in the soil were studied. Four kinds of fertilization treatments were set up in the experiment:no fertilization treatment (CK), low nitrogen fertilization treatment (T1:250kg·hm-2), medium nitrogen fertilization treatment (T2:500kg·hm-2), and high nitrogen fertilization treatment (T3:750kg·hm-2). The results showed that:① The main form of nitrogen leaching in citrus orchard soil was nitrate nitrogen (NO3--N), accounting for 36.93%-60.07% of total nitrogen (TN) leaching. The ratio of TN to ammonium nitrogen (NH4+-N) was 4.40% to 5.79%. The ratio of NO3--N residues in soil to TN residues was 11.31‰-45.66‰, and the ratio of NH4+-N residues to TN residues was 1.051‰-2.07‰. ② Nitrogen leaching and residual amounts in citrus orchards at the same depth showed significant positive correlation with the amount of fertilizer applied. The leaching losses and residual amounts of TN in soil at different fertilization rates were 11.35-30.11 kg·hm-2and 0.30-1.86 g·kg-1, respectively. Among them, the ratio of leaching losses of NO3--N and NH4+-N to TN leaching amounts peaked under T2 treatment, and the peaks of NO3--N and NH4+-N residues in TN residual amounts appeared in T1 and T2 treatment, respectively. ③ Given the same fertilization amounts, the leaching amounts and residual amounts of different forms of nitrogen in the soil were greatly affected by soil depth. After fertilization, the peaks of NO3--N leaching and residual amounts appeared at 20 cm and 40 cm, respectively, and the peaks of NH4+-N leaching and residual amounts mainly appeared at 20 cm depth. Inferred from the results of the experiment, the neutral nitrogen treatment in the 0-40 cm soil column is more conducive to the conversion of fertilizer nitrogen to inorganic nitrogen for plant absorption, and reduces the risk of nitrogen leaching after fertilization.

[Characteristics of Nitrogen and Phosphorus Output in Runoff and Rainfall Runoff in Lanlingxi Watershed, Three Gorges Reservoir Area].

The small watershed of an agroforestry system in the Lanlingxi watershed in the Three Gorges Reservoir area was studied by performing continuous monitoring of nitrogen and phosphorus output and concentrations and analyzing the changes in the output of nitrogen and phosphorus in runoff and its response to different rainfall conditions. The results showed that:①The total runoff loss was 50.92×104 m3 in this watershed during the rainy season, with 52.43 kg·hm-2 lost by total nitrogen, including nitrate nitrogen (30.26 kg·hm-2) and particulate nitrogen (21.61 kg·hm-2), and 0.06 kg·hm-2 and 0.10 kg·hm-2 lost by ammonium nitrogen and total phosphorus; ②The distribution of rainfall has the characteristics of stage and strong rainfall during the wet season was the main driving force of soil nutrient output. Rainfall runoff contributed to 88% of total nitrogen loss and 90% of total phosphorus loss in the rainy season; ③The main path of nitrogen and phosphorus loss in the rainy season was the surface runoff caused by rainfall, accounting for 68% of total nitrogen loss and 74% of total phosphorus loss; ④Nitrate nitrogen concentrations showed a negative correlation with rainfall runoff and the output was mainly distributed in the later runoff process. Ammonium nitrogen and total phosphorus concentrations were positively correlated with rainfall and both were mainly distributed in the early stage of the runoff process; ⑤The concentrations of total nitrogen in this watershed exceeded the standard values. The water quality in the rainfall and the non-rainfall period was worse than Grade Ⅴ.