4. Methods of Estimating Belowground Net Primary Production
59
fine root production (NFRP) (forests and shrublands). In ecosystems dominated by herbaceous
plants, it is possible to assess BNPP (all belowground plant organs) because of the lack of large
woody roots. In ecosystems dominated by woody
plants, effectively all of the data are for NFRP
because of the difficulties associated with estimating the increment to large woody roots. Fine roots
«3 mm in diameter) represent the most dynamic
component of the root systems of woody plants
(Hendrick and Pregitzer 1992) and a large fraction
of the annual carbon balance of ecosystems dominated by woody plants (Nadelhoffer and Raich
1992). The relationship between NFRP and BNPP
is unknown. Unless stated otherwise, all further
references in this chapter to either BNPP or NFRP
will assume an annual time scale. Furthermore, I
will use BNPP as a generic term to refer to both
belowground net primary production and net fine
root production. I will use NFRP when I specifically mean net fine root production.
Estimating BNPP requires, at the minimum, an
estimate of M3 from Equation 4.1. Many of the
differences among methods are related to how
M3 is determined. None of the methods deals
explicitly with herbivory (If) and therefore it
represents a source of underestimation except for
the carbon isotope turnover method in which it
is lumped with decomposition as a portion of
total loss. Although there is evidence that belowground herbivory is large and on the same order
as aboveground herbivory, very few data exist
(Lauenroth and Milchunas 1992). Exudation and
sloughing (E) are likewise ignored by all methods except carbon isotope turnover (Milchunas
and Lauenroth 1992). Estimates from field and
laboratory experiments suggest that, at least in
grasslands, E can account for a measurable loss
from BNPP (Milchunas et al. 1985; Milchunas
and Lauenroth 1992). Death and detachment (D)
have received considerable attention in methods
for aboveground NPP (ANPP) (Wiegert and
Evans 1964; Long et al. 1989), but they are ignored in methods for BNPP, except those using
minirhizotrons, and represent a loss of unknown
magnitude. Death and detachment losses are
thought to be large for estimates of ANPP from
ecosystems dominated by herbaceous plants, especially those in tropical climates (Long et al.
1989, 1992).
Methods
Biomass
The majority of the estimates of BNPP in the literature have been calculated from data that were
collected by directly sampling belowground biomass (Milchunas and Lauenroth 1992; Nadelhoffer
and Raich 1992; Publicover and Vogt 1993). This
process involves two steps at which the decisions
that are made have a large influence on the resulting
estimate of BNPP. The first step involves a decision
about when to sample and how to collect and process the samples. The decision about when and how
often to sample will be influenced by the specific
characteristics of the ecosystem and project objectives. Some of the complications involved with
sampling frequency are discussed in Chapter 2 with
respect to ANPP. The issues are directly applicable
to BNPP as well and therefore do not need to be
repeated here. The most common sampling procedures involve collecting volumetric soil samples
using either a coring device or an auger (Bohm
1979 1 ). The samples are then processes by sieving
and flotation after which the resulting organic material is dried and weighed (Lauenroth and Whitman 1971; Bohm 1979; Smucker et al. 1982). In
some cases, the organic material is sorted into categories such as live roots and dead roots (e.g., Aerts
et al. 1989; McClaugherty et al. 1982; Hook et al.
1994), although there is considerable uncertainty
associated with such separations (Bohm 1979). Another controversial topic has been whether there is
a need to correct the organic matter values for ash
content. High ash contents (;:::50%) and high
sample-to-sample variability argue for the need for
such corrections (Bohm 1979).
The second step involves a decision about how
the biomass data will be used to calculate BNPP
(Singh et al. 1984). While there are a large number
of possibilities (Singh et al. 1984), there are two
common methods of calculation. The first is to subtract the annual minimum in root biomass from the
annual maximum (max-min method); the second is
to calculate differences in biomass between each
pair of sample dates and sum the positive changes
(Dahlman and Kucera 1965; Lauenroth and WhitITbis small book is the authoritative reference on collection and processing root biomass samples.
59
fine root production (NFRP) (forests and shrublands). In ecosystems dominated by herbaceous
plants, it is possible to assess BNPP (all belowground plant organs) because of the lack of large
woody roots. In ecosystems dominated by woody
plants, effectively all of the data are for NFRP
because of the difficulties associated with estimating the increment to large woody roots. Fine roots
«3 mm in diameter) represent the most dynamic
component of the root systems of woody plants
(Hendrick and Pregitzer 1992) and a large fraction
of the annual carbon balance of ecosystems dominated by woody plants (Nadelhoffer and Raich
1992). The relationship between NFRP and BNPP
is unknown. Unless stated otherwise, all further
references in this chapter to either BNPP or NFRP
will assume an annual time scale. Furthermore, I
will use BNPP as a generic term to refer to both
belowground net primary production and net fine
root production. I will use NFRP when I specifically mean net fine root production.
Estimating BNPP requires, at the minimum, an
estimate of M3 from Equation 4.1. Many of the
differences among methods are related to how
M3 is determined. None of the methods deals
explicitly with herbivory (If) and therefore it
represents a source of underestimation except for
the carbon isotope turnover method in which it
is lumped with decomposition as a portion of
total loss. Although there is evidence that belowground herbivory is large and on the same order
as aboveground herbivory, very few data exist
(Lauenroth and Milchunas 1992). Exudation and
sloughing (E) are likewise ignored by all methods except carbon isotope turnover (Milchunas
and Lauenroth 1992). Estimates from field and
laboratory experiments suggest that, at least in
grasslands, E can account for a measurable loss
from BNPP (Milchunas et al. 1985; Milchunas
and Lauenroth 1992). Death and detachment (D)
have received considerable attention in methods
for aboveground NPP (ANPP) (Wiegert and
Evans 1964; Long et al. 1989), but they are ignored in methods for BNPP, except those using
minirhizotrons, and represent a loss of unknown
magnitude. Death and detachment losses are
thought to be large for estimates of ANPP from
ecosystems dominated by herbaceous plants, especially those in tropical climates (Long et al.
1989, 1992).
Methods
Biomass
The majority of the estimates of BNPP in the literature have been calculated from data that were
collected by directly sampling belowground biomass (Milchunas and Lauenroth 1992; Nadelhoffer
and Raich 1992; Publicover and Vogt 1993). This
process involves two steps at which the decisions
that are made have a large influence on the resulting
estimate of BNPP. The first step involves a decision
about when to sample and how to collect and process the samples. The decision about when and how
often to sample will be influenced by the specific
characteristics of the ecosystem and project objectives. Some of the complications involved with
sampling frequency are discussed in Chapter 2 with
respect to ANPP. The issues are directly applicable
to BNPP as well and therefore do not need to be
repeated here. The most common sampling procedures involve collecting volumetric soil samples
using either a coring device or an auger (Bohm
1979 1 ). The samples are then processes by sieving
and flotation after which the resulting organic material is dried and weighed (Lauenroth and Whitman 1971; Bohm 1979; Smucker et al. 1982). In
some cases, the organic material is sorted into categories such as live roots and dead roots (e.g., Aerts
et al. 1989; McClaugherty et al. 1982; Hook et al.
1994), although there is considerable uncertainty
associated with such separations (Bohm 1979). Another controversial topic has been whether there is
a need to correct the organic matter values for ash
content. High ash contents (;:::50%) and high
sample-to-sample variability argue for the need for
such corrections (Bohm 1979).
The second step involves a decision about how
the biomass data will be used to calculate BNPP
(Singh et al. 1984). While there are a large number
of possibilities (Singh et al. 1984), there are two
common methods of calculation. The first is to subtract the annual minimum in root biomass from the
annual maximum (max-min method); the second is
to calculate differences in biomass between each
pair of sample dates and sum the positive changes
(Dahlman and Kucera 1965; Lauenroth and WhitITbis small book is the authoritative reference on collection and processing root biomass samples.