Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 231

The additional source of CO 2 implies that the total net exchange of CO 2 between
an ecosystem and the atmosphere consists of the following terms:

NEE =−=−− ARARR

(6.32) n s g d s

where NEE is the net ecosystem exchange, A n is the net assimilation rate (often
denoted as net primary production NPP), R s is the soil respiration, R d is the dark res-
piration and A g is the gross assimilation (often denoted as gross primary production
[GPP]). All luxes have units of mass CO 2 per unit time per unit surface area. The
NPP is comparable to the net photosynthesis discussed in Section 6.4.2 , whereas NEE
is the net total exchange, including soil respiration, as it would for instance be mea-
sured by an eddy-covariance system installed over that ecosystem (see Chapter 3 ).
The NPP leads to the buildup of biomass: both above ground and below ground.
Because GPP is close to zero when there is no radiation input, and the dark res-
piration also decays to zero after sunset. Hence the soil respiration can be estimated
from the night time NEE observations. For a given ecosystem, under given conditions
of soil moisture and nutrition, the soil respiration is mainly a function of temperature.
Hence, the diurnal cycle of temperature needs to be taken into account to translate
night time NEE observations to day time estimates of R s . A commonly used method
is to take the respiration rate at 10 °C ( R s,10 ) as a reference and use an Arrhenius type
of temperature dependence. Lloyd and Jackson ( 1994 ) suggest the following formu-
lation, based on a large number of data sets:







1 1

E

− − −

    0

TTTT

RR =

s ref e

(6.33)

ref 0 0

s ,

where E 0 is a temperature sensitivity factor (in K), T 0 equals 227.13 K and T ref is the
reference temperature at which R s,ref is determined (e.g., 283.15 K or 10°C). Reichstein
et al. ( 2005 ) show that E 0 is not a universal constant, but may be site speciic. More
importantly, they show that it strongly depends on the time period over which the tem-
perature dependence is determined (if the period is too long, seasonal trends in soil
conditions may erroneously be taken into account). They suggest a time window of the
order of 15 days yielding values for E 0 that vary roughly between 100 and 250 K.
Because during nighttime no photosynthesis takes place, the surface is a source of
CO 2 . When conditions are very stable, vertical turbulent transport of CO 2 away from
the surface may be absent. This will lead to a buildup of high CO 2 concentrations
close to the surface. This effect can be enhanced when small-scale topography gives
rise to density currents that horizontally transport CO 2 to the lowest points in the land-
scape (de Araújo, 2008 ).

Question 6.6: Given are soil respiration data from two nights: 4.0·10 -7 kg m -2 s -1 and
4.7·10 -7 kg m -2 s -1 . The soil temperatures were 15 and 20 °C during those nights.