| This paper takes the typical watershed in the gully region of the Loess Plateau as the research area,chooses apple orchards of different tree ages and soil types,and farmlands of different landforms and analyzes the distribution characteristics of nitrate nitrogen and ammonium nitrogen in soil profile and their influencing factors and the law of soil moisture change in orchards of different tree ages.It provides a theoretical basis for scientific fertilization,reduction of nitrate leaching and accumulation,improvement of fertilizer use efficiency and realization of sustainable agricultural development.The results show that:(1)The distribution of mineral nitrogen in the soil profile of Wangdonggou watershed has great variability.The accumulation and distribution characteristics of mineral nitrogen in soil profile are obviously different under different land use types.The content of mineral nitrogen in the soil profile of 0-300cm soil in the survey area ranges from 0.7-587mg/kg,with an average value of 107mg/kg,the standard deviation was 141 and the coefficient of variation was 1.31.The mineral nitrogen content ranges from 0.7-587mg/kg in 0-300cm soil layer of orchard,with an average value of 153mg/kg,a standard deviation of 150.3 and a coefficient of variation of 0.98.The mineral nitrogen content of 0-300cm soil layers ranges from 0.94 to 67mg/kg,with an average value of 9.75mg/kg,a standard deviation of 12 and a coefficient of variation of 1.23.From the point of view of mineral nitrogen content and accumulation in apple orchards,the accumulation of mineral nitrogen in apple orchards is more serious than that in farmlands,and the accumulation depth of nitrate nitrogen in apple orchards is deeper than that in farmlands.Owing to the high level of fertilization in orchards and irrigation and deep application of fertilizers are often used,the content and accumulation of nitrate nitrogen in different soil layers are much higher than that in farmlands,and the accumulation depth is deeper.(2)The mineral nitrogen content and cumulative amount of farmlands in different topographies are significantly different,but the ammonium nitrogen content is relatively stable,and the difference is not obvious.The average nitrate nitrogen content in 60-300 cm of maize plateau is 17.3 mg/kg,which is 6.6 mg/kg higher than that in 60-300 cm of maize slope land(10.7 mg/kg).The nitrate accumulation in the range of 60-300 cm is 540 kg/hm~2(plateau land)and 335 kg/hm~2(slope land),respectively.The nitrate accumulation in plateau land was 1.61 times of that in slope land.For orchards with the same tree age and different terrains,the accumulations of ammonium nitrogen are 23-year-old orchard on sloping land>23-year-old orchard on ridge land>23-year-old orchard on plateau land.However,because of the low content of ammonium nitrogen,it has little effect on the distribution and accumulation of mineral nitrogen in soil profile.There is no significant difference in nitrate nitrogen content between plateau and ridge land,but it is higher than that in slope orchards.The accumulation of nitrate nitrogen in 0-400 cm soil was 5875 kg/hm~2(slope land),13738 kg/hm~2(ridge land)and 14705 kg/hm~2(plateau land),respectively.The trend of nitrate accumulation in 0-400 cm soil was plateau land>ridge land>slope land.(3)Mineral nitrogen content in 0-600 cm soil profile of orchard in Wangdonggou watershed varies greatly,and the mineral nitrogen content varies greatly among different tree ages.Mineral nitrogen accumulation in deep soil is a common phenomenon.In the soil profile of orchards with different tree ages,the content of ammonium nitrogen in each soil layers is small and stable,which has a low impact on the distribution of soil mineral nitrogen.In slope orchards,the accumulation of nitrate nitrogen in orchards with different tree ages is significantly different at 0-200 cm and200-400 cm.The accumulation of nitrate nitrogen in 0-200 cm and 200-400 cm soils of 18-year-old orchards is similar,while the accumulation of nitrate nitrogen in 0-200cm soils of 23-32-year-old orchards accounted for 41%and 38%of the total accumulation of nitrate nitrogen in 0-400 cm soils.This shows that the nitrate accumulation in orchard soil is gradually deep with the increase of tree age.Nitrate accumulation increased with the increase of tree age in orchards.Nitrate accumulation in 32-year-old orchards in slope orchards>23-year-old>18-year-old;28-year-old orchards in ridge orchards>23-year-old orchards;23-year-old orchards in plateau orchards>18-year-old>14-year-old.(4)With the increase of planting years,there is no significant difference in water storage in 0-300 cm soil layer,which may be due to the recharge of precipitation to the soil layer.With the deepening of soil depth,the water storage of orchards aged 14,23 and 32 are significant difference(p<0.05),and the water storage capacity of300-600 cm soil layer is 14-year-old orchard>23-year-old orchard>32-year-old orchard,respectively. |
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