Sustainable Land Management for Mitigating Climate Change

C. Carbon Budget of Savanna Ecosystems

54. It is vital to note that native savannas, under undisturbed and natural conditions, are a relatively small sink of atmospheric CO₂. Grace et al. (2006) reported that NPP of 20 Gt/yr, supports a SOC pool of about 480 Gt out of the global SOC pool (1-m depth) of 1550 Gt (~31%). Whether managed savannas are a source or sink for CO₂ and other GHGs depends on the degree and scale of adoption of SLM technologies. It is estimated that savanna biomes sequester as much as 0.5 Gt C/yr, which may contribute to the so-called missing sink (Scurlock and Hall, 1998). Lal (2008) estimated that grasslands and rangelands together have a potential SOC sink capacity of 0.5 to 1.7 Gt C/yr. But as has been demonstrated, the actual and attainable C sink capacity can be enhanced through adoption of selected SLM technologies and practices.

55. Realization of this vast potential, however, necessitates detailed life cycle analysis of pool and fluxes under principal land use systems. It is widely recognized that the ecosystem C pool declines with conversion from native to agricultural ecosystems with drastic loss of biomass C (both above and below ground) and also of the SOC pool. Such ecosystem C pool can be restored through conversion to planted forests (Eucalyptus, Pinus, etc.). Soil and vegetation degradation, such as is the case with degraded pastures, make these ecosystems a source of CO₂ and other GHGs.

56. Conversion of PT to NT can also lead to SCS at the rate of 0.3 to 1.0 t C/yr. There is also a saving in fossil fuel because of elimination of primary and secondary tillage operations. It is possible that adoption of NT system on the 18 Mha of croplands in the Brazilian Cerrados can lead to sequestration of about 15 Mt C/yr in the SOC pool. However, the net C sequestration must be assessed with due consideration of the hidden C costs and increase in N₂O emission as described earlier. In addition to soybean, cultivation of upland rice (covering 2 Mha) is another option that needs a careful evaluation (Pinheiro et al., 2006). Aerobic rice has lower CH₄ emission and lesser water requirements than continuously flooded rice paddies.

Table 13. Hidden carbon costs of farming practices (Lal, 2004a).
Source/ Practice	Equivalent carbon emission (kg C E)
I. Fuel (kg of fuel)
1. Diesel	0.94
2. Gasoline	0.59
3. Oil	1.01
4. Natural gas	0.85
II. Tillage (per ha)
1. Moldboard plowing	15.2
2. Chisel plowing	7.9
3. Disking	8.3
4. Cultivation	4.0
III. Fertilizers (Per kg)
1. Nitrogen	1.3
2. Phosphorus	0.2
3. Potash	0.15
4. Lime	0.16
IV. Pesticides
1. Herbicides	6.3
2. Insecticides	5.1
3. Fungicides	3.9

57. Recommended SLM options for managed TSREs that have direct implications for CC include: (i) afforestation and reforestation of degraded ecosystems, (ii) restoration of degraded pastures and judicious management with controlled/rotational grazing, (iii) conversion of PT to NT farming with mulch, cover crops, integrated nutrients and pest management, and (iv) increasing productivity per unit input of C-based input (e.g., diesel, fertilizers, pesticides, irrigation). The overriding strategy is to (a) scale-up these SLM practices in managed TSREs, and (b) minimize or avoid further conversion of TSREs and adopt land saving options for nature conservancy.

XII. Cropland Management

58. The key SLM options for croplands outlined in Figure 9 are: (i) conversion of degraded/desertified and agriculturally marginal soils to a restorative/perennial land use (e.g., tree crops, afforestation), (ii) adoption of conservation-effective measures to control erosion and conserve water and nutrients in the root zone, (iii) conversion of PT to NT farming with crop residue mulch and incorporation of cover crops in the rotation cycle, (iv) creation of a positive nutrient budget in the soil through INM techniques, and (v) adoption of complex cropping/farming systems including agroforestry (Figure 12).