Weed competition reduces crop yields and necessitates the use of control practices that have both economic and environmental costs. Improving our understanding of the factors that mediate belowground competition between weeds and crops could help reduce yield loss and the need for weed control practices. We conducted a 24-hour pulse chase experiment to assess uptake of 15N labeled ammonium and nitrate, and 15N- and 13C-labeled amino acids by nine common annual weed and crop species. Individual plants were grown semi-hydroponically in sand using a nitrate based complete nutrient solution for 22 days in a heated greenhouse. Plants were then transitioned to an equimolar mixture of nitrate, ammonium, and amino acids for 24 hours before the pulse chase was applied. After labeling, plants were separated into roots and shoots and biomass was dried, weighed, ground and analyzed using a stable isotope ratio mass spectrometer.
Individual species differed in their uptake and assimilation of the three nitrogen forms. All species were capable of acquiring N derived from all three forms, while 13C patterns from dual labeled amino acids suggest that intact organic N was taken up by most of the species. Relative to the other species, wild oats (Avena sativa) and barnyard grass (Echinochloa crus-galli) demonstrated a preference for amino acids. Lambsquarters (Chenopodium album) and redroot pigweed (Amaranthus retroflexus) had strong affinities for nitrate, and the potential for increased nitrate reductase activity in their shoots. Wild oats, giant foxtail (Setaria faberi), and barnyard grass took up comparatively higher amounts of ammonium. The crop species, sorghum sudangrass (Sorghum bicolor ssp. drummondii) did not exhibit strong affinities for any particular N-form relative to the annual weed species. Our results suggest that organic N may be an important resource for several agriculturally important annual weed species and that both weeds and crops may have the ability to partition the soil N resource niche.