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FIGURE 2.1. Comparison of values for aboveground and belowground biomass (g/m2) predicted by a simulation model with data collected in the field. The curves represent output of the model; vertical bars are means of field data plus and minus one standard error (from Singh et al, 1984).

between net nitrogen mineralization and net nitrogen fluxes into above-ground tissues. FRP is then calculated as the product of annual nitrogen allocation to fine roots and the carbon-nitrogen ratio in fine roots (Nadelhoffer et al., 1985). Interestingly, with this last method, total root allocation (TRA), which equals root respiration plus root production, was predicted from aboveground litterfall carbon with an r2 = 0.36, where p < 0.02. Whereas this method leaves almost 2/3 of the variability in root production unaccounted for, the other methods are even less reliable in predicting FRP.

Isotope-Dilution Method

Another approach, using the 14C-dilution technique, has been used to determine belowground biomass turnover in grasslands. Milchunas et al. (1985) performed a pulse labeling of plants with 14C-CO2 for a few hours, then followed the time-course of new 12C label incorporated into both soluble and structural tissues of the root systems a few weeks to months later. They then calculated the subsequent production, on the assumption that any tissues lost would have a constant ratio of 14C to 12C in the structural tissues. An additional step was to include 14C incorporated into plant cell walls between the first and second sampling times. This greatly reduced the errors of the estimates (Table 2.3). Milchunas et al. (1985) found that the grass roots were continuing to mobilize additional amounts of 14C from storage tissues in the grasses, but then made further measurements of the labeled plants to adequately account for the translocated 14C. Other researchers, notably Caldwell and Camp (1974), have used the isotope-dilution technique with considerable success.

Root-Ingrowth Technique

The root-ingrowth technique (Steen, 1984, 1991) involves removing long cores of soil, sieving the soil free of roots, and then replacing the root-free soil into nylon tubular mesh bags with a mesh size of 5-7mm. The mesh bags are inserted by drawing them over a plastic tube, and the pipe plus mesh bag on the outside is inserted into the hole in the field soil. The soil is tamped down in 5-centimeter (cm) increments as the pipe is gradually withdrawn, leaving the mesh bag in position in the hole. Care must be taken to have a bulk density similar to that in the surrounding matrix. After the soil mesh bags are placed in their respective holes, roots are allowed to regrow into the bags. The bags are then recovered at various intervals, and the living and recent dead root biomass measured (Hansson et al., 1991; Steen, 1991). The principal assumptions are that growth into the root-free soil is the same as the root production would have been in the normal, undisturbed soil. Some concerns one might have about this technique are: Was the bulk density of the soil in the mesh tubes identical to that in the surrounding soil? Were any significant soil aggregates broken in the soil-sieving process, which might alter rates of root growth in the bags? (Larger soil

TABLE 2.3. Comparison of Increments in the Belowground Biomass of Blue Grama and Wheat as Determined by Complete Harvest and 14C/12C Dilution Techniques"

Belowground productionb

Cell-wall carbon

Uncorrected

Corrected for 14C incorporation

Cell-wall carbon

Uncorrected

Corrected for 14C incorporation

Time (days)

12C(g)

14C(g x 10-4)c

R(x10r4)d

Rc(x10-4)'

Harvest

(R1C1IR2C2-1)B{

Error(%)

(RC1lR2C2-1)Bf

Error(%)

Blue grama

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