Multi-Year Simulations Reveal the Challenges of Generalizing the Impact of Soil Nitrogen On Crop-Weed Competition.

With the significant year-to-year climate variability that characterizes many cropping systems and the dynamic interactions that can arise among weather factors, labile nitrogen (N) stocks, and plant growth, the impact of soil N status on crop-weed competition can be extremely difficult to distinguish through short-term field experimentation. We developed a mechanistic competition model of individual plant growth, COMPETE, to simulate crop-weed competition. This model builds on the approaches of several other models, including MAESTRA (for light interception), GECROS (for photosynthesis and respiration), and PNM (for soil processes including heat, water and N fluxes) within a spatially-explicit framework that allows individual plants to compete for solar radiation, soil water and soil N.  Maize growth in competition with A. theorphrasti and in monoculture was simulated for 37 years of historical weather data at three levels of N fertilizer (90, 198, and 347 kg ha-1).  Results indicate that there is considerable year-to-year variability in weed-free maize yield and that weed-induced maize yield losses also vary across years at all N levels. Relative maize yield losses from competition were frequently greater under low compared to high soil N conditions, but the magnitude of this difference was strongly dependent on weather.  In years with higher yield potential, relative yield losses due to weed competition generally increased at low N levels. In contrast, soil N had little effect on relative yield losses in years with low yield potential. In addition to N fertility and growing season factors, early season structure of competition also had a marked influence on maize yield losses at all N levels. These findings illustrate the dynamic and complex nature of competitive interactions and indicate why it is difficult to draw definitive conclusions about the importance of factors like soil N to crop yield loss from short-term field experimentation.