Simulation of Soil C Sequestration Potential RT vs CT

After soil and crop parameters were adjusted to mimic CT vs. RT as described above, simulated changes in SOC were compared over the 10 simulated years (Figure 16.1). SOC for CT

Table 16.3 Simulated Maize Grain and Biomass Production Under CT and RT Plus Amendments and Percent Increase, Compared with Similar Results in 2000 Study

Maize Grain (kg ha-1) Maize Biomass (kg ha-1) ~CT RT % Increase ~CT RT % Increase

Simulated 10-year 2651 3565 34 6007 7750 29 average

Gigou et al. (2000) 2603 3599 38 6339 8704 37 Notes: CT = conventional tillage; RT = ridge tillage.

remained nearly constant for the initial C fractions used in the simulations but the RT-all treatment increased by 54%, from 0.24% to 0.37% in the top 20 cm of soil over the 10 years (10,527 to 7016 kg C ha- 1; bulk density was 1.44 g cm3). This increase amounted to 3511 kg ha-1, or about 351 kg ha-1 year-1, and was about 10% of the total carbon added to the soil from residues and manure during the 10 years. This result indicates that the potential for SOC sequestration for the conditions studied may be less than the 0.20% increase that was hypothesized, and less than the average rates for no till vs. CT reported by West and Post (2002), but similar to values obtained by Pichot et al. (1981) in Burkina Faso and by Lal (2000) in the no-till treatment in Nigeria.

Two other treatments were simulated in the computer experiment to estimate how much SOC sequestration would occur under RT only (no amendments added), and under RT with 40 kg ha-1 N and return of 90% of crop residue (RT, Fl, R, no manure addition). Data in Table 16.4 indicate that yield increased under the RT only treatment, but that SOC did not increase. Although roots added C to the soil in all treatments, simulated results show that roots of maize alone would not be enough to increase SOC in this environment. For RT plus N fertilizer and residue return to the field, yield increased about the same as for RT plus all amendments, but the SOC increase was only 2058 kg ha-1, or about 60% of the amount sequestered when manure was included.

Table 16.4 Grain Yield, Crop Residue, and Change in SOC After 10 Years Under Each Management System3

Treatment CT RT Only RT, F, R RT All

Table 16.4 Grain Yield, Crop Residue, and Change in SOC After 10 Years Under Each Management System3

Treatment CT RT Only RT, F, R RT All

Mean grain yield (kg ha-1)

2651

3GG6

3592

3565

Mean harvest residue (kg ha-1)

6GG7

6875

775G

7751

Change in SOC (kg ha-1 [10 year)-1])

-46

-355

2G58

3511

Ending SOC (%)

G.24

G.23

G.32

G.37

a For top 20 cm of soil, bulk density = 1.44 g cm-3.

Notes: CT = conventional tillage; F = 40 kg ha-1 of N fertilizer added; R = 90% of crop residue left on the field each year; RT = ridge tillage; SOC = soil organic carbon.

a For top 20 cm of soil, bulk density = 1.44 g cm-3.

Notes: CT = conventional tillage; F = 40 kg ha-1 of N fertilizer added; R = 90% of crop residue left on the field each year; RT = ridge tillage; SOC = soil organic carbon.

These preliminary results agree with the trend observed by Pieri (1992) in long-term studies in West Africa that, in treatments without manure, soil C remained constant or declined, and that fertilizer alone aggravated the condition. This also agrees with the study in Western Nigeria (Lal, 1997a, 1997b), which illustrated the importance of residue and tillage operations on SOC and maize grain yield. Under those conditions, the no-till plus mulch treatment had the greatest effect with a doubling of SOC during the first 4 years of the study. However, it is notable that all treatments showed initial increases followed by subsequent declines in SOC within 8 years. Several important differences existed in his study, however, namely, that average rainfall was higher (1200 mm), two crops were planted each year and more fertilizers were applied.

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