Plant phenology and soil fertility effects on below-ground carbon allocation for an annual (Bromus madritensis) and a perennial (Bromus erectus) grass species

Plant strategies for using available resources were explored through a comparative analysis of C allocation and use in the rhizosphere. The seasonal dynamics of plant C partitioning was compared for annual and perennial species of bromegrass (Bromus madritensis and B. erectus) in soils of different fertility using successive 2-week labelling with 14CO2. Measurements of labelled soil CO2 indicated that current assimilates were readily used by roots and rhizosphere microflora. For vegetative stages of plant growth, there was a regular decrease during the year of carbon translocation from above to below-ground and 10–20% more of net C assimilation was directed toward the roots in the poor soil than in the fertile soil. During reproductive stages, there were drastic decreases to as low as 10 and 25% of net C assimilation reaching below-ground in the annual and perennial species, respectively. Over an entire year, below-ground C allocation in B. madritensis was 37% of net assimilation in the fertile soil and 44% in the poor soil. In B. erectus the figures varied from 42-49 to 60% in the fertile and poor soil, respectively. The proportion of current below-ground C respired and exuded in the rhizosphere was more than 15% higher in the fertile soil than in the poor soil. During reproductive stages, C losses exceeded gain by the roots and the variation from vegetative to reproductive stages reached 30% in B. madritensis (25–57% in poor soil, 45–73% in fertile soil), 20% in B. erectus (39–59% for plants developed from cuttings in the rich soil). In B. erectus, the development of reproductive structures was strongly correlated with soil fertility and a determinant in rhizosphere C budgets. This feature characterised the difference between the annual and perennial species. When expressed in units of C translocated below-ground per unit of root C, the highest C use efficiency was in the poor soil. In B. madritensis, 64% of annual below-ground C allocation remained in the roots in the poor soil, 44% in the fertile soil. In B. erectus, this percentage varied from 62 to 50% between low and high fertility soils. The trade-off between rate of growth and resource use efficiency defined in the literature for above ground plant traits was confirmed below-ground: faster growth rate, higher losses and, therefore, lower resource use efficiency in the rich soil than in the poor soil.