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Soil Physics

Determining Vertical Root and Microbial Biomass Distributions from Soil Samples

^a CSIRO Land and Water, 120 Meiers Road, Indooroopilly, QLD 4068, Australia, and The Univ. of Queensland, St. Lucia, QLD 4067, Australia
^b Manaaki Whenua–Landcare Research, P.O. Box 69, Lincoln, New Zealand

^* Corresponding author (freeman.cook{at}csiro.au)

When vertical density distributions of root or microbial biomass are calculated using each sampling interval's midpoint as the depth coordinate, the calculated distribution is biased if it is a nonlinear function with depth. In the root biomass literature, distributions are often described by a power function R ß^z, where ß is a decay coefficient and z is depth. A common alternative formulation is an exponential function, R e^–z/Z_r, where Z_r is a characteristic length scale. These functions are equivalent when Z_r = –1/ln ß, so the data according to either function may be unified. The bias can be eliminated by representing the vertical distribution with a continuous function, integrating it over the sampling interval, and using a least squares method to determine the function's parameters. The bias increased by nearly threefold when the sampling interval increased from 0.01 to 1 m. As the sampling interval increases, the bias shifts the function down the z axis. This results in the intercept increasing with increasing sampling interval. When a single profile was sampled at different intervals, the function's intercept and Z_r changed. The parameter Z_r changed fivefold when the sampling interval increased from 0.1 to 0.5 m, while the calculated fraction of roots above a depth of 0.1 m decreased threefold for the same change in sampling interval. Beneath a tropical forest where root biomass and microbial respiration were sampled throughout the same soil profile, the corresponding microbial and root biomass length scales averaged 0.17 m and differed by only 11%.