Soil Formation - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 90

conventional tillage agriculture of ~1 mm
year − 1 (Montgomery, 2007) and with around
70% of Earth's land surface eroding at less
than 0.05 mm year − 1 (Hilley et al ., 2010).
Rasmussen et al . (2011) were able to
disentangle climatic and tectonic (i.e. relief)
factors, utilizing as a weathering proxy the
Na depletion relative to Zr (i.e. application
of Eqn 6.2) throughout the full depth of
regolith within weathering granite terrain.
The data were constrained to moderately
eroding terrain (0.005-0.072 mm year −1 )
and the relative degree of water availabil-
ity for the range of sites was estimated by
defining the humidity index, (HI, dimen-
sionless) as the mean annual precipitation
(MAP, mm year −1 ) divided by the annual po-
tential evapotranspiration (PET, mm year −1 )
(Rasmussen et al ., 2011).
Sites with HI < 1 were defined as water
limited, with the subsurface water availabil-
ity determined primarily by the availability
of precipitation. Sites with HI > 1 were defined
as energy limited where precipitation ex-
ceeded the energy available for evapotrans-
piration. Separating the data sets for weathering

rates into those for sites with HI greater and
less than 1, respectively, yielded linear cor-
relation of the natural logarithm of the
weathering rates against temperature ( Fig. 6.3 ;
Rasmussen et al ., 2011). The correlations
allow apparent activation energies (Ea, kJ mol −1 )
to be estimated, yielding a value of 69 kJ mol − 1
and 136 kJ mol − 1 , respectively, for conditions
of HI > 1 (wet conditions) and HI < 1 (dry
conditions) (Rasmussen et al ., 2011). Further-
more, the sites with HI < 1 (dry conditions)
exhibited weathering rates that were inde-
pendent of total denudation rates, suggesting
strongly weathering-limited conditions. The
sites with HI > 1 (wetter conditions) exhibited
a dependence of weathering rate with total
denudation rate, but substantially less than
a 1:1 relation, which would be indicative of
strictly erosion-limited conditions ( Fig. 6.4 ;
Rasmussen et al ., 2011).
Although other studies have demon-
strated the effect of climate-related factors
such as hydrological conditions and aver-
age temperature on soil pedogenesis, the
above study is one of the first to demon-
strate a methodology to separate the effects

-1.5

-2.0

Ea = 69 kJ mol - 1

-2.5

r 2 = 0.79; P < 0.01

-3.0

-3.5

-4.0

Ea = 136 kJ mol - 1

r 2 = 0.40; P = 0.07

-4.5

-5.0
3.35 3.40 3.45 3.50

1000/MAT (Kelvin)

3.55 3.60 3.65 3.70

Fig. 6.3. Arrhenius plot of weathering rate (total Na weathering rate W Na-total ) as a function of mean annual
temperature (MAT), illustrating the role of climate regime. Climate is represented by the humidity index
(HI) for relatively wetter (HI > 1, filled square) and dryer (HI < 1, open triangles) locations (Rasmussen et al .,
2011). The linear plot demonstrates that when hydrologic conditions are accounted for, the effect of
temperature on chemical weathering and soil formation rates is discernible and significant.