Knowledge of how plants will partition their new biomass will aid in understanding competition between crops and weeds. This study determined if the amount of biomass partitioned to the root versus the shoot can be predicted from tissue carbon [C] and nitrogen [N] concentrations and the daily gain in C (GC) and N (GN) for each unit shoot and root biomass, respectively. Pots measuring 28 cm diameter and 60 cm deep were embedded in the ground, and each contained one plant of either corn or velvetleaf. Each plant received one of three nitrogen treatments: 0, 1, or 3 g of nitrogen applied as ammonium nitrate in 2001 and 0, 2, or 6 g of nitrogen in 2002. Measurements of total above- and belowground biomass and tissue [C] and [N] were made at 10 different sample dates during the growing season. Fraction of biomass partitioned to roots (Pr) was predicted from [C], [N], GC, and GN. Accurate prediction of the fraction of biomass partitioned to roots versus shoots was evaluated by comparing observed and predicted Pr across all treatments. The coordination model has potential as a reliable tool for predicting plant biomass partitioning. Normalized error values were close to zero for corn in 2001 and 2002 and for velvetleaf in 2001, indicating that biomass partitioning was correctly predicted.

Competitive outcome between crops and weeds is affected by partitioning of new biomass to above- and belowground plant organs in response to nutrient supply. This study determined the fraction of biomass partitioned to roots vs. shoots in corn and velvetleaf in response to nitrogen (N) supply. Pots measuring 28 cm in diam and 60 cm deep were embedded in the ground and each contained one plant of either corn or velvetleaf. Each plant received one of three N treatments: 0, 1, or 3 g N applied as ammonium nitrate in 2001, and 0, 2, or 6 g N in 2002. Measurements of total above- and belowground biomass were made at 10 sampling dates during each growing season. The root:shoot ratio decreased over time for both corn and velvetleaf as a result of normal plant growth and as N supply increased. Root:shoot ratio was greater for corn than for velvetleaf at comparable stages of development and at all levels of N supply. Both corn and velvetleaf display true plasticity in biomass partitioning patterns in response to N supply. Velvetleaf root:shoot ratio increased by 46 to 82% when N was limiting in 2001 and 2002, respectively, whereas corn root:shoot ratio increased by only 29 to 45%. The greater increase in biomass partitioned to roots by velvetleaf might negatively impact its ability to compete with corn for light when N supply is limited.