Impact of Initial SOC

Rate constants and turnover calculations are sensitive to the initial data. For example, sensitivity analysis showed that mixing the NHC into a larger amount of soil impacts the calculated rate constants. For example, if only the SOC in the 0-15 cm zone was considered (SOC = 26,750 kg C ha-1) for data from Larson et al. (1972), then kNHC was 0.14 g (g SOC year)-1. However, if the 0-30 cm soil zone was considered (SOC = 53,500 kg C ha-1), then kNHC was 0.28 g C (g SOC year)-1. In these calculations, increasing the soil depth did not impact kSOC.

Drainage class, tile drainage, soil characteristics, and initial SOC levels can also impact SOC maintenance requirements (Arrouays and Pelissier 1994; Zach et al. 2006; Clay et al. 2007). If the SOC maintenance requirement is related to the SOC level, then the range of values reported by Barber (1978), Wilts et al. (2004), Larson et al. (1972), and Frye and Blevins (1997) may be related to these differences. To assess the impact of SOC level on maintenance requirements, data from Barber (1978), Wilts et al. (2004), Larson et al. (1972), and Frye and Blevins (1997) were analyzed using the Clay et al. (2006) approach (Table 8.1). For these calculations, a common soil depth (0-15 cm) and root to shoot ratios suggested by Johnson et al. (2006) were used. Across the sites, located in the central USA, the analysis suggested that in plowed fields, 15.5% of the SOC contained in the surface 15 cm must be returned annually (Fig. 8.5). The 0-15 cm soil zone was selected because soil data from this zone are available in many studies.

In conservation tillage systems (chisel plow, strip tilled, or no-tillage fields), analysis indicates that the maintenance requirements are less compared with plowed fields (Table 8.1). Differences in C maintenance between the plowed and conservation tillage systems were attributed to the degree of soil disturbance. In Minnesota, the only disturbance was from the planter, whereas in South Dakota, soil disturbance resulted during planting and strip tillage. Differences between the maintenance requirements in the two tillage systems can be used to calculate the impact of tillage on carbon sequestration. For example, based on Eq. 8.1, if NHC is 4,000 kg (ha year)-1, kNHC is 0.20 and kSOC in a tilled and no-tilled system are 0.015 and 0.010 g SOC-C (g SOC year)-1, respectively; then, SOCe will be 53,300 and 80,000 kg C ha-1

Table 8.1 The calculated percentages of SOC, using the non-isotopic approach, that must be returned annually to maintain SOC. Root to shoot ratios for corn, soybean, and wheat were identical to the values reported in Johnson et al. (2006). SOC was from the 0-15 cm soil depth

Location

Tillage

Landscape Position/soil

SOCinitial(kgSOCha-i)

Percentage of SOC returned

Reference and notes

Rosemont, MN

No-till

55,600

5.35

Allmaras et al. (2004)

Chisel

9.41

Plow

23.31

Lafayette

Indiana

Plow

34,900

18.3

Barber (1978)

Morris, MN

Plow

48,400

15.9

Wilts et al. (2004)

Iowa

Plow

26,750

16

Larson et al. (1972)

Kentucky

Plow

28,270

17.7

Frye and Blevins (1997)

Moody, SD

Strip till

Footslope

47,100

9.9

Clay et al. (2005)

Lower backslope

46,700

9.9

Backslope

44,000

9.9

Upper backslope

43,600

9.9

Shoulder/summit

46,700

9.9

Colorado

Sterling

No-tillage

Footslope

14,210

8.72

Peterson and Westfall

(1997)

Backslope

12,980

8.26

Summit

18,530

8.54

SOC. estimated from

Backslope

14,500

9.26

the 0-15 cm depth

Summit

13,570

9.66

were calculated by

Footslope

6,180

8.54

multiplying the 0-5

Summit

16,810

8.04

depth by 2

Brazil

Southern Brazil

No-tillage/

Oxisol

40,180

5.4

Sisti et al. (2004)

plowed

O Huggins et al. (1998) - y=0.158+0.000195x, r2=0.81

Fig. 8.5 A comparison of data collected from multiple sites analyzed using Clay et al. (2005). Tillage was conducted at all sites. In this plot NHC was non-harvested biomass, SOC was soil organic C, and dSOC/dt was the annual change in soil organic matter resulting from the imposed treatments. Root to shoot ratios was assumed to be 0.55 and the soil depth considered was the 0-15 cm zone in a tilled and no-tilled system, respectively. The difference in SOC between the tilled and no-tilled SOC values (26,700 kg C ha-1) represents the amount of sequestered carbon in the surface soil by adopting no-tillage. When making these comparisons it is important to consider the assumptions associated with the calculations.

This analysis is conceptually in agreement with reports from numerous tillage experiments (Rochette et al. 1999). West and Post (2002) reported that: (1) changing from conventional to no-tillage sequestered an additional 570 (±140) kg C (ha year)-1; and (2) enhancing the rotational complexity, excluding a change in rotation from continuous corn to corn/soybean (Glycine max), can also increase carbon sequestration on average 200 (±120) kg C (ha year)-1. Campbell et al. (2005) reported that in unfertilized Great Plains systems, SOC gains were increased with N fertilizer. They also reported that SOC gains were lowest in toe slope areas even though these areas had the highest production. Larson et al. (1972) reported that carbon source (corn versus alfalfa [Medicago sativa]) had a minimal impact on SOC accumulation. Causarano et al. (2006) reported that in southeastern USA, no-tillage with cover crops sequestered 670 kg C (ha year)-1 (±630) while no-tillage without cover crops sequestered 340 kg (hayear)-1 (±470). Data from an Oxisol soils located in southern Brazil did not show tillage differences (Sisti et al. 2004). In this 13-year study, SOC maintenance rates were approximately 5% or 2,170 kg C ha-1 of the 40,180 kg C ha-1 contained in the surface 15 cm of soil. This low maintenance requirement was attributed to the SOC content in the oxisol being highly stable.

The amount of carbon stored or sequestered in the soil is also influenced by the soil carbon content (Fig. 8.6). Sensitivity analysis showed that: (1) the carbon sequestration potential is higher in low than high carbon soils; and (2) the net carbon

SOCinitial (Mt/ha)

Fig. 8.6 The relationship between initial SOC and the net C balance (stover + roots - maintenance requirement), and relative amount of carbon supplied by the roots (maintenance requirement -root biomass carbon) tilled and no-tilled system. Calculations were based on tilled and no-tilled systems with maintenance requirements of 16% and 10% of the SOC, a 11,270 kg grain ha-1 (180 bu acre-1), a harvest index of 0.5, a root to shoot ratio of 0.55, and that non-harvested corn stover contained 0.4 g carbon(g plant)-1

SOCinitial (Mt/ha)

Fig. 8.6 The relationship between initial SOC and the net C balance (stover + roots - maintenance requirement), and relative amount of carbon supplied by the roots (maintenance requirement -root biomass carbon) tilled and no-tilled system. Calculations were based on tilled and no-tilled systems with maintenance requirements of 16% and 10% of the SOC, a 11,270 kg grain ha-1 (180 bu acre-1), a harvest index of 0.5, a root to shoot ratio of 0.55, and that non-harvested corn stover contained 0.4 g carbon(g plant)-1

gain for the maintenance - below-ground biomass value indicates that the amount of non-harvested biomass that can be removed - is influenced by tillage and initial SOC level. At high SOC contents, removing any non-harvested biomass can result in a negative carbon gain (carbon loss).

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