The low-nitrogen status of highly weathered soils may offer a potential alternative for weed suppression in agricultural systems with N2-fixing crops. In this study, we used sicklepod as a model to evaluate weed response that might occur with managed reductions in nitrogen-soil fertility. A field study was conducted with the parental generation supplied 0, 112, 224, or 448 kg N ha−1. Decreased nitrogen fertility led to reduced shoot biomass, seed number, and total seed mass. Individual seed mass was lower, but seed % nitrogen was not affected. Analysis of seed-mass distribution confirmed that low parental fertility was associated with more small seeds as a proportion of total seeds produced. Additional experiments in hydroponics culture revealed slower growth rates of seedlings produced from small seeds when grown under low-nitrogen conditions. Competitiveness of plants from small (low nitrogen) and large (high nitrogen) seed classes was determined in a replacement-series experiment conducted in sand culture in a controlled environment at two densities and two levels of nitrogen nutrition. Plants produced from smaller seeds were less competitive in low-nitrogen fertility conditions, but plants from small and large seeds competed similarly when grown under high-nitrogen fertility. The results support the hypothesis that comprehensive management strategies to reduce nitrogen availability for weed growth in low-fertility conditions could decrease weed interference by decreasing growth and seed production of parental plants and through maternal effects that lower competitiveness of offspring.
Nomenclature: Sicklepod, Senna obtusifolia (L.) H.S. Irwin & Barneby CASOB.