Effects of poplar-wheat intercropping system on soil nitrogen loss in Taihu Basin

Agricultural non-point pollution caused by the overuse of fertilizer in farmland has become one of the main factors for water quality deterioration and eutrophication in Taihu Lake. In order to select suitable agroforestry intercropping system to control soil nitrogen loss in Taihu Lake basin, a study was conducted at a distance of 1 km away from the lakeshore to find out how poplar-wheat intercropping system with 2 kinds of densities (AS1: poplar spacing 2 m×5 m; and AS2: poplar spacing 2 m×15 m) affected the amount of surface runoff, leaching and soil nitrogen loss. The interception of rainfall by the poplar canopy was mainly concentrated from April to October and ranged from 8.6% to 44.5%. The control effects of canopy layer on sprinkle and moderate rain (<1.0 mm/h) were obvious, which were reduced by 47.7% on average. Rainfall interception by canopy was mainly related to the LAI (leaf area index). The volume of rainfall difference between AS1 and MS (monoculture system) was only 0.8% during the defoliated period while the control effect of AS1 was raised to 6.4% during the growing period. Compared to AS2R (means removing litter) and MSL0 (monoculture of wheat without litter coverage), the runoff amount of AS1R decreased by 9.1% and 11.2%, respectively. There was a significant difference between AS1R and AS2R/MSL0 (P<0.05), while the difference between AS2R and MSL0 was not significant (P>0.05). Compared to MS, the volume of surface runoff, leaching and soil nitrogen loss were all effectively controlled by the 2 poplar-wheat intercropping systems. The control effects were more significant under the higher intercropping density. There was no significant difference between the average runoff volume of AS1S (means saving litter) and AS2S/MSL0 during the whole growing period (P>0.05), while their average concentration and leaching amount of total nitrogen (TN) and NO3--N were both significantly lower than MSL0 (P<0.05). The average leaching water amount of AS1S in the depth of 60 cm was significantly different from that of AS2S and MSL0 (P<0.05), while the difference between AS2S and MSL0 was not significant (P>0.05). The difference of average concentration and leaching amount of TN and NO3--N in 20 cm was not significant among AS1S, AS2S and MSL0 (P>0.05) while that in 60 cm was significant (P<0.05). The average runoff amounts of 2 intercropping systems without litter coverage were both higher than those with litter coverage during the poplar defoliated period. It was obvious that the runoff amount, average concentration and leaching amount of TN and NO3--N with litter coverage were lower than those without litter coverage in AS1 (P<0.05), while there was no significant difference between AS2 with litter coverage and without (P>0.05). The litter volume was the major factor to decide whether there was significant difference between the surface runoff and leaching loss. The average surface runoff, concentration and leaching amount of TN and NO3--N with the litter coverage of 0.4 t/hm2 were significantly lower than those of 0.2, 0.1 and 0 t/hm2. The average leaching water volumes of AS1R in 20 and 40 cm soil depth were higher than those of MSL0 while the result in 60 cm was opposite. The decreasing range of leaching volume of AS1R in 60 cm depth was 1.3 times higher than that of MSL0, and the leaching volume of AS1R were different significantly in 20 and 60 cm (P<0.05), while there was no significant difference among 20, 40 and 60 cm for leaching volume of MSL0 (P>0.05). There was significant difference between average concentration of TN in 20 and 60 cm for AS1R, which were respectively 1.8 and 1.5 times lower than those for MSL0. The leaching loss of TN and NO3--N in 60 cm for AS1R was significantly lower than those in 20 cm. Thus it can be concluded that the effects of surface runoff, leaching and nitrogen loss reduction are more significant under the poplar-wheat intercropping system with the spacing of 2 m×5 m.