Nitrogen competition in contour hedgerow systems in subtropical China

Contour hedgerow agroforestry has been studied for soil erosion control and soil fertility improvement in subtropical China. However, below-ground competitive and complementary interactions between tree hedges and crops have received relatively little attention in the scientific literature. A field experiment was conducted to explore the effects of a leguminous shrub hedge, false indigo (Amorpha fruticosa) and a non-legume gramineous hedge, vetiver (Vetiveria zizanioides), on the growth of soybean (Glycine max). Pot experiments were also carried out to determine the effect of below-ground interactions on nitrogen uptake between two contour hedgerow agroforestry with a ¹⁵N isotope method and root partition, i.e., a sheet barrier, a mesh barrier and no barrier. The results showed that the relative disadvantage of intercropping, expressed as land equivalent ratio, were 0.96/0.97 for the A. fruticosa–soybean system and 0.99 for the vetiver–soybean system, based on the dry matter (DM) production and N acquisition. Both area-adjusted yield and N content of soybean were significantly decreased in two intercropping treatments compared to those in the sole soybean treatment. The DM production of soybean, for example, was decreased by 10% and 5% under A. fruticosa and vetiver, respectively, when compared to the sole soybean. The intercropping disadvantage was mainly due to interspecific competitive interaction. The result was proved by lower yields and biomasses adjacent to the hedgerows. The ¹⁵N-based estimates of N uptake, in the vetiver–soybean system, of soybean with mesh separation (6.11 mg pot⁻¹) was lower than that (13.85 mg pot⁻¹) with no root separation, for vetiver the higher ¹⁵N uptake was observed in no root separation (13.90 mg pot⁻¹). In the A. fruticosa–soybean system, the lower ¹⁵N uptake of soybean (1.53 mg pot⁻¹) and A. fruticosa (6.42 mg pot⁻¹) were observed in no root separation. It is concluded that the growth of soybean was unexpectedly suppressed in two intercropping systems. The growth of A. fruticosa was clearly suppressed due to below-ground interactions, yet the growth of vetiver was improved to a great extent.