Agroforestry system reduces subsurface lateral flow and nitrate loss in Jiangxi Province, China

Subsurface lateral flow is an important pathway responsible for agricultural non-point source pollution and may be affected by land use. An agroforestry system, citrus (Citrus reticulata) tree intercropped with peanut (Arachis hypogaea) crop and a mono peanut cropping system were compared over the period from 2003 to 2005 in Jiangxi Province, China. The objectives of this study were (i) to identify subsurface lateral flow by monitoring soil matric potential and NO3-N concentration in soil water, and (ii) to estimate subsurface lateral flow and associated NO3-N loss by modelling the water budget using Hydrus-2D. The dynamics of soil water, either during a particular storm or on an annual basis, demonstrated that subsurface lateral flow generated along the slopes under the two systems. The agroforestry system had a larger domain and a longer resident time of water saturation in the deeper soil layers than the mono cropping system, suggesting that the agroforestry system may have increased water retention capacity of subsurface soil due to its deeper root system. The simulated annual water budget showed that, compared with the mono cropping system, the agroforestry system reduced subsurface lateral flow by 9.2% of annual rainfall, which was equivalent to the amount of precipitation predicted to be reduced by interception. The two cropping systems received the similar amount of organic and inorganic N fertilizers (160.0–170.0 kg N ha−1 a−1), the total amount of N inputs in agroforestry system was smaller as it may have received less biological fixed N by about 0 to 70 kg N ha−1 a−1 due to the smaller effective area occupied by peanut crops. NO3-N concentration measured in soil water in the agroforestry system was low within the soil profile (0.6–7.6 mg L−1) and had little seasonal variation. However, in the mono cropping system the NO3-N concentration had two peaks in a year, which ranged from 14 to 52 mg L−1 at all soil depths between 0.20 and 0.85 m, and was higher on the lower slope than on the upper slope position. The estimated NO3-N loss associated with subsurface lateral flow ranged from 45 to 64 kg ha−1 a−1 in the mono cropping system and from 16 to 48 kg ha−1 a−1 in the agroforestry system. The smaller NO3-N loss in the agroforestry system was probably attributed to the smaller total N inputs and/or to the reduced subsurface lateral flow. However, the underlying mechanisms and the effectiveness of agroforestry systems need further study.