Soil nitrate leaching and control methods in the piedmont of North China Plain

Soil core and soil water samples were collected in a long-term field experiments to study soil nitrate nitrogen (NO₃^--N) accumulation and leaching in winter wheat/summer maize double-cropping system under different agricultural management practices in the North China Plain (NCP). The results showed that NO₃^--N accumulation in the soil profile and NO₃^--N leaching through the root zone increased with increasing N fertilizer application (P<0.05). Application of P and K fertilizers improved the grain yield and harvested more N in grains. P and K inputs increased the amounts of harvested N in grains by 123 kg·hm^-2·a^-1 and 31 kg·hm^-2·a^-1, respectively. Based on the experiment, the amount of irrigation also affected NO₃^--N accumulation and distribution in the soil profile. The amounts of accumulated NO₃^--N in the 0~400 cm soil profile significantly decreased with increasing irrigation frequency. Arid irrigation treatment (i.e., no irrigation and one irrigation during winter wheat and summer maize seasons, respectively) produced 1 698 kg(N)·hm^-2 of accumulated NO₃^--N in the 0~400 cm soil profile. This was significantly higher (P < 0.05) than those of deficient irrigation (i.e., 2~3 irrigations during winter wheat season, irrigation when needed during summer maize season) and sufficient irrigation (i.e., 4~5 irrigations during winter wheat season, irrigation when needed during summer maize season) with accumulated NO₃^--N in the 0~400 cm soil profile of 1 148 kg(N)·hm^-2 and 961 kg(N)·hm^-2, respectively. Compared with deficient and sufficient irrigation treatments, accumulated NO₃^--N in the 100~200 cm soil layer was higher than in the other soil layers under arid irrigation treatment. From 2003~2005, increases in NO₃^--N in the 0~400 cm soil profile were different among different irrigation treatments. The amounts of fertilizer N left in the soil under arid irrigation, deficient irrigation and sufficient irrigation were 23%, 22% and 47%, respectively. No-tillage decreased grain yield, changed soil water movement and increased water storage in deep soils, which in turn increased the risk of NO₃^--N leaching. Based on the results, 200 kg·hm^-2·a^-1 N input with less irrigation and balanced fertilization were the most effective mode that protected groundwater from nitrate pollution in NCP.