Nutrients – the real constraint to sequestering carbon in soil organic
matter?
Kirkby C1, Kirkegaard J1, Richardson A1, Wade L2, Blanchard C2, Batten G2
1
CSIRO Plant Industry, Canberra, ACT 2601, Australia; [email protected]
2
Charles Sturt University, E H Graham Centre for Agricultural Innovation,
Wagga Wagga, NSW 2678, Australia
Keywords: soil organic matter, humus, C:N:P:S ratios, carbon sequestration
Introduction
Soil organic carbon (SOC) levels are a balance between carbon (C) inputs and outputs.
Despite some claims that SOC levels are directly related to C inputs only (e.g. Christopher
and Lal 2007 and references therein), many studies have shown surprisingly little response in
SOC due to large differences in residue input. For example Soon (1998) found no difference
in SOC levels between complete residue removal and residue incorporated over a ten year
period and SOC levels decreased in both treatments. A similar outcome was reported by
Rumpel (2008) where 31 years of stubble burning compared to residue retention had no
impact on SOC stocks or composition. Campbell et al (1991) found various rotation and
fertiliser treatments differing by up to 50% in C return to the soil showed no difference in
SOC after a similar period of 31 years. While Walker and Adams (1958) hypothesised that
soil organic matter (SOM), presumably as a whole, has constant proportions of C:N:P:S,
Himes (1998) hypothesised that only the stable portion of the SOM, or humus, has constant
proportions of these elements. This prompts a further hypothesis - that the availability of N,
P and S may limit the formation of humus, not only by limiting primary production (and
thereby organic C inputs) but also by limiting the conversion of C inputs to humus
(humification efficiency).
We tested these two hypotheses by (i) comparing the C:N:P:S ratios in the humus fraction for
a wide range of soils and (ii) measuring changes in SOC in soils incubated with a standard
amount of wheat straw with and without the addition of supplementary N, P and S.
Materials and Methods
Freshly-collected Australian soils were analysed for total C, N, P, organic P (OP) and S, and
the ratios were compared with values for soils from numerous locations around the world,
hereafter known as the International soils. The main Australian soils were chosen from four
agricultural areas in Australia with varying rainfall and soil type. Farmers in the region near
each experimental site were also invited to provide soils for evaluation with no restriction on
the type of soil to be submitted. Soils included cropped, pasture and virgin soils. The cropped
soils were generally regularly fertilised while the pastures soils ranged from irregularly to
rarely fertilised. A total of 59 Australian soils (4 main and 55 farmer) were analysed and data
from a total of 976 International soils were examined - 761 for C:N; 478 for C:P and 531 for
C:S.
Total C and N were determined using a dry combustion, total acid extractable P and S were
determined by inductively-coupled plasma optical-emission spectroscopy following
microwave-assisted acid digestion using reverse-aqua regia according to method 3051A of
the USEPA (1998). Organic P was determined by the ignition-extraction procedure of Olsen
and Sommers (1982).
A further experiment involved incubating the four contrasting Australian soils with wheat
straw with and without supplementary nutrients. The experiment involved repeated additions
of the equivalent of 10t/ha wheat straw to soil in large tubs over multiple cycles, with and
without multiple supplementary nutrients (52, 20 and 13 kg/ha N, P and S equivalent). Each
cycle involved incubating the soils for three months at optimum moisture and temperature to
facilitate the wheat straw decomposition. After each three month cycle total C, N, P and S in
the humus fraction were measured (as above) and further straw (and nutrients if necessary)
was added. A total of 7 incubation cycles were completed over 21 months.
Results and Discussion
There was a constant ratio between C:N and C:S in the soils and a similar but more variable
relationship between C:OP (Figure 1). On average, it took 833 units of N and 143 units of S
to sequester 10,000 units of C as humus. Due to methodological and theoretical
considerations we were unable to place such a predictable estimate for the amount of P
required, but the range was from between 53 to 188 units of P. There appears to be a
relationship between C and P but more research is needed to provide a more definitive
estimate on the amount of P required per 10,000 units of humus carbon.
16
14
Total soil C (%)
12
r2=0.89
10
r2=0.37
r2=0.89
8
6
4
2
0
0.0
A
0.2
0.4
0.6
0.8
Total soil N (%)
1.0
1.2
B
0.00
0.05
0.10
0.15
Soil organic P (%)
0.20
C
0.00
0.05
0.10
0.15
0.20
Total soil S (%)
Figure 1. (A) Total C:N; (B) C:organic P (C) C:S for Australian and International soils
All soils incubated with supplementary nutrients sequestered more carbon into the humus
pool than soils without supplementary nutrient addition. Soil N, P and S increased in unison
with humus C increases (e.g. Figure 2 – Leeton soil).
0.25
0.26
+ nutrients
- nutrients
3.0
Carbon
Nitrogen
0.24
0.20
0.18
2.0
0.16
Nitrogen (%)
Carbon (%)
0.22
2.5
0.14
1.5
0.12
0.045
0.09
Phosphorus (%)
Sulphur
0.040
0.07
0.035
0.06
0.030
0.05
0.025
0.04
0.020
0.015
0.03
0
1
2
3
4
5
6
7
0
1
Incubation cycle
2
3
4
5
6
7
Incubation cycle
Figure 2. Change in soil C, N, P and S over 7 incubations cycles for Leeton soil. During
each incubation phase, wheat straw equivalent to 10t/ha DW was added with or without
supplementary nutrients equivalent to 52, 20 and 13 kg/ha N:P:S.
Together these data demonstrate that sequestering carbon into the stable SOC pool requires
predictable amounts of N, P and S and that carbon sequestration will be limited where these
nutrients are insufficient despite large amounts of carbon input. The estimated cost of the
nutrients required to sequester one tonne of humus carbon was $248, (Table 1) if nutrients are
valued at fertiliser equivalents. This “hidden cost” of N, P and S needed to foster “soil carbon
sequestration” needs to be accounted for when considering a carbon sequestration strategy.
Many of the circumstances in which surprisingly little carbon has been sequestered under
conservation agriculture practices such as no-till can be adequately explained by the
stoichiometry of C:N:P:S in stable soil organic matter demonstrated in these experiments.
Table 1: Estimated potential value of N, P and S required to sequester each tonne of humusC
Nutrient
Amount (kg)
Approx price/kg nutrient
Approx Cost ($)
N
P
S
80
20
14
1.50
5.00
2.00
120
100
28
$248
Prices are in Australian dollars and calculated from 2009 fertiliser costs
Sulphur (%)
Phosphorus
0.08
References
Campbell, C., Bowren, K., Schnitzer, M., Zenter, R., Townley-Smith, L. (1991) Effect ofcrop
rotations and fertilization on soil organic matter and some biochemical properties of a
thick Black Chernozem. Can. J. Soil Sci. 71:377-387.
Christopher, S and Lal R (2007) Nitrogen management affects carbon sequestration in North
American cropland soils. Critical reviews in Plant Sciences 26:45-64.
Himes, F. (1998) Nitrogen, sulfur, and phosphorus and the sequestering of carbon. p 315-315,
Soil processes and the carbon cycle. Eds R. Lal et al, CRC Press, Boca Raton, FL
Rumpel, C. (2008) Does burning of harvesting residues increase soil carbon storage. J. Soil
Sci. Plant nutr. 8(2):44-51.
Soon, Y. (1998) Crop residue and fertilizer management effects on some biological and
chemical properties of a Dark Grey Solod. Can. J. Soil Sci. 78:707-713.
Walker, T and Adams, A (1958) Studies on soil organic matter: I Influence of
phosphoruscontent of parent materials on accumulations of carbon, nitrogen, sulfur,
and organic phosphorus in grassland soils. Soil Sci. 85:307-318