The Role of Fine Roots in the Organic Matter and Nitrogen Budgets of Two Forested Ecosystems

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Abstract
Standing crop, rates of production, mortality, decomposition, and nitrogen dynamics of two size classes of fine roots (0—05 mm and 0.5—3.0 mm diameter) were estimated for 1 yr in a 53—yr—old red pine (Pinus resinosa Ait.) plantation and in an adjacent 80—yr—old mixed hardwood stand in north—central Massachusetts. Dry matter of live fine roots was higher in the hardwoods (mean = 6.1 Mg/ha; annual range 3.6—8.6 Mg/ha) than in the plantation (mean = 5.1 Mg/ha; annual range 2.5—7.8 Mg/ha.) Dead root mass was similar in the hardwoods (mean = 4.4 Mg/ha) and the plantation (mean = 4.0 Mg/ha). Nitrogen standing crop of live roots in the hardwoods was higher than in the plantation (mean = 65 kg/ha and 42 kg/ha, respectively). Net fine root production was estimated from changes in standing crop. Production estimates ranged from 4.1 to 11.4 Mg°ha—1°yr—1 in the hardwoods and from 3.2 to 10.9 Mg°ha—1°yr—1 in the plantation, depending on the assumptions made in the calculations. Concurrent estimates of total nitrogen requirement for this production ranged from 73 to 184 kg°ha—1°yr—1 in the hardwoods and from 44 to 122 kg°ha—1°yr—1 in the plantation. Decomposition, measured as mass loss from buried cloth bags, was °20% in 0.4—mm mesh bags and as high as 47% in 3—mm mesh bags after 1 yr. Integrating production and nitrogen requirements with estimates of decomposition rates and nitrogen mineralization for these ecosystems suggested that the lower estimates of production are more accurate.