Improved cropland management can increase soil organic carbon (SOC) levels by sequestering atmospheric carbon into soil (SOC sequestration). This provides a practical and cost-effective means to reduce atmospheric CO2 levels that can be implemented immediately and on a wide scale to mitigate the impacts of climate change. These impacts can be mitigated while improving soil quality and health and ecosystem services in an environmentally friendly way. This provides a win–win situation for both farmers and the environment.

This page contains information on predicted potential of Conservation Agriculture (CA) to sequester SOC for smallholder farmers in the Bergville district, KwaZulu Natal.

Key study findings

  • Loss in SOC (of grassland) occurred historically with conventional till that included oxen ploughing.
  • SOC levels decline at 0-30 cm when more than 40% of crop residue is removed (by grazing), except when a cover crop is introduced.
  • Decline in SOC with conventional till is less for a high energy input system (with higher fertiliser application rates) than for a low input system.
  • A change from conventional to minimum or no till alone does not sequester SOC at
    0-30 cm since the required increase in soil carbon input for sequestration is not provided. However, SOC decline with minimum or no till is less than conventional till.
  • SOC levels do not increase for maize and cowpeas intercropping at 0-30 cm with 50% residue removal, even though an increase in SOC occurs at 0-15 cm. This is an indication that SOC sequestration occurs mainly at a shallower depth (e.g. 0-20 cm), and that soil carbon input is just too low for SOC sequestration at 0-30 cm for the biomass yield and root mass for the maize and cow peas intercropping.
  • SOC levels increases considerably at 0-30 cm when a winter cover crop is introduced to the intercropping due to sufficiently high soil carbon input for SOC sequestration.

Modelling of SOC sequestration potential

A modelling approach was followed to predict the effects of smallholder farming systems
since long-term data on SOC sequestration (>5-10 years) is not (readily) available. A soil biochemical model was applied to predict the effects of crop rotations and tillage with planned agronomic, forage and grazing practices on the potential to sequester SOC. The sequestration potential was predicted to a level of detail that can predict how the SOC contents (and stock) change over time for smallholder farming systems for the climate and soil properties at the Bergville district.

Prediction of SOC sequestration/loss was conducted for two periods, namely:

  • SOC modelling assumed a baseline of natural veld conditions followed by 50 years prediction of conventional till which includes ploughing with oxen.
  • A 50-year prediction of CA farming systems was modelled with a baseline of historic conventional till (with degraded soil carbon stocks).

The modelling scenarios of the various farming systems are summarised in Table 1.

Conservation Agriculture (CA) practices that was modelled involves:

  • Avoid or minimise soil disturbance and mixing of crop residues into the soil with reduced till (current CA) and no till (future CA) by planting directly into the soil.
  • Diversification of crops that include a legume and a cover crop.
  • Intercropping of maize and cowpeas as a legume (future CA).
  • Include a cover crop into the rotation cycle that provide high biomass production and root mass (carbon input) for SOC sequestration (future CA).
  • Controlled grazing of residues (50% residue removal), leaving enough residues for a mulch cover (future CA).

Predicted SOC sequestration potential

For the purposes of this study, carbon stocks are defined as the soil organic carbon (SOC) contained in the upper 30 cm of soil and are expressed as tonnes carbon per hectare (tC/ha/30cm). The IPCC soil carbon analysis method specifies that a 30 cm depth must be used to report soil carbon sequestration.

Historic conventional till. Predicted changes in SOC indicate initially high rates of decline in SOC stock during the first 6-8 years after cultivation of natural veld, followed by lower rates of decline (Figure 1). The decline in SOC is the result of high mineralisation rates of soil organic matter (SOM) due to high soil disturbance and mixing by inverting soil, and the incorporation of most of the crop residues into the tillage layer.

Figure 1: Decline in soil organic carbon since natural veld was first cultivated.

Conventional till (CT). Continuation of conventional till results in further deceases in predicted SOC stocks due to the high fraction (80%) of crop residue removal with grazing (Figure 2). The rate of SOC decline is less for the high energy input system with over two time higher biomass that is produced than the low input system.

Figure 2: Predicted change in SOC levels and stock for maize mono-cropping systems.

Current CA. Predicted SOC stocks declines at 0-30 cm with a change from historic conventional till to minimum till, but at a lower rate than conventional till (Figure 2). The lower rate in SOC decline can be ascribed to the reduction in residue decomposition and SOM mineralisation associated with intensive conventional till. The main advantage of current CA is that reductions in SOC stock are lower than that would have occurred with conventional till. However, the high loss of crop residue (80%) is too high to provide sufficient soil carbon input for SOC sequestration.

Future CA intercropping. Predicted SOC stocks declines at 0-30 cm with a change from historic conventional till to intercropping with maize and cow peas under no till (Figure 3). The high loss of crop residue of 80% is too high to provide sufficient soil carbon input for SOC sequestration. SOC stocks do also not increase with 50% residue removal, even though an increase at 0-15 cm occurs. This is an indication that SOC sequestration occurs mainly at a shallower depth (e.g. 15 or 20 cm), and that soil carbon input is just too low for SOC sequestration at 0-30 cm for the biomass yield and root mass provided by ASSET Research for the intercropping. Residue removal has to be reduced to at least 40% (60% crop residue retention as mulch) to provide the required soil carbon input for SOC sequestration at 0-30 cm, or the biomass production has to be increased. The main advantage of the intercropping under no till is that reductions in SOC stocks are considerably lower than that would have occurred with conventional till.

Figure 3: Predicted change in SOC levels and stock for conservation agriculture systems.

Future CA intercropping with winter cover crop. SOC sequestration increases considerably with the inclusion of a winter cover crop (rye) to the intercropping (Figure 3). Rye has relatively high vegetation and root mass that provides the needed additional high carbon input for SOC sequestration. Similar results, where SOC sequestration more than doubled with the inclusion of a cover crop are confirmed by several studies when no till is implement and crop residue as mulch is retained (Nicoloso and Rice, 2021, Corbeels et al., 2019; Powlson et al., 2016; West and Post, 2002; Gustavo et al., 2015 and Causarano et al., 2008). The intensification of farming system with summer and winter crops also increases biomass production and SOC inputs to the soil sustaining increased SOC stocks. Significant increase in SOC sequestration demonstrates the importance of cover crops to effectively sequester SOC.

According to Gonzalez-Sanchez et al. (2019), SOC sequestration rates are initially high during the first 5 years, which is followed by a second period with lower rates. SOC sequestration reached equilibrium soon after the initial 5 years if 50% of the crop residue and rye is removed with grazing (Table 1), whereas equilibrium is reached after 20 years if only 40% of the crop residue is removed (Figure 4). West and Post (2015) reported that equilibrium conditions in SOC can be expected to occur from 15- 20 years for systems with SOC sequestration potential. According to Gonzalez-Sanchez et al. (2019), SOC sequestration can still occurs even if sequestration rates are lower after 10-15 years, supports the value of a long-term and continuing engagement with CA land management.

Figure 4: Predicted SOC sequestration potential of CA farming systems.

Predicted SOC sequestration rates with 50% loss in crop residue are comparable to the lower range of rates listed in Table 2 for the initial 5 years. Predicted rates with 40% loss in crop residue are comparable to the range reported by Powlson et al. (2016) for a 30 cm soil depth, and to the rates reported by Lemma et al., (2021) and World Bank (2012) for the period
5-10 years. However, the rates are considerably lower than the rates calculated from the change in SOM levels with intercropping that was reported by Kruger and Ngcobo (2019). 

Model input data

Climate. The study area is characterised by warm summers and moderate to cold winters with severe frost in the winter. A mean annual precipitation of 786 mm/yr was used for the modelling exercise with rainfall that exceeds 100 mm/month during November to January.

Soils

Soils of the Avalon, Bainsvlei, Bloemdal, Clovelly, Hutton, Glencoe, Inanda, Kranskop, Magwa and Pinedene soil forms were identified as suitable soils for crop production for the study area. Soil profiles of these soil forms that are included in the Digital National Soil Profile Database (Soil Survey Staff, 1972-2010) were used to determine representative soil for the study, based on soil texture and clay content. The median clay content determined from 63 soil profiles is 35% and 38% for the A and B soil horizons respectively. The soil texture is a sandy clay loam A and clay loam to clay B horizons.

Tillage-, agronomic- and grazing practices:

The modelling of the cropping systems is based on information related to the following activities:

  • Planting.
  • Tillage before and after planting.
  • Fertiliser and lime application.
  • Harvesting.
  • Grazing of maize residue after harvesting and of rye as a winter cover crop.

Model input on these activities and the scheduling thereof are based on data provided by GrainSA and ASSET Research. Data on nitrogen, phosphorus, potassium and lime application rates were also provided by GrainSA

Model calibration

Plant growth module. Selected parameter values on plant growth and development were refined until predicted above ground dry matter and grain yield corresponded with the data provided by ASSET Research (Table 3). Soil organic matter module. Model calibration could not be conducted for the study area since (long-term) data on the effect of farming systems on SOC sequestration are not (readily) available. SOC sequestration rates that were calculated from changes in SOM levels reported by Kruger and Ngcobo (2019) at a 15 cm depth could also not be used, since these rates are considerably (about order of magnitude) higher than the rates reported in literature for sub-Sahara Africa and smallholder croplands, and could likewise not be predicted (Table 3). 

References

Corbeels, M, Cardinael,R, Naudin,K, Guibert, H and Torquebiau, E, 2019. The 4 per 1000 goal and soil carbon storage under agroforestry and conservation agriculture systems in sub-  Saharan Africa. Soil Tillage Res. 188: 16-26.

Kruger, E and Ngcobo, P, 2019. Farmer centred innovation in conservation agriculture in upper catchment areas of the Drakensberg in the Bergville region of KwaZulu-Natal.          Appendix 2:    Bergville annual progress report – CA Farmer Innovation Programme for smallholders       in       Bergville Period: October 2018 – September 2019. Report to Maize trust.

Nicoloso, RS and Rice, CW, 2021. Intensification of no-till agricultural systems: An opportunity   for carbon sequestration. Soil Sci. Soc. Am. J. 1: 1–15.

Powlson, DS, Stirling, CM, Thierfelder, C, White, RP and Jat, ML, 2016. Does conservation        agriculture deliver climate change mitigation through soil carbon sequestration in tropical     agro-ecosystems? Agric. Ecosyst. Environ. 220: 164–174.

Soil Survey Staff, 1972-2010. Soil profile descriptions and soil analyses data. In: ARC-ISCW Soil Profile Information System. ARC-Institute for Soil, Climate and Water, Pretoria.

West, TO and Post, WM, 2015. Soil Organic Carbon Sequestration Rates by Tillage and Crop     Rotation: A Global Data Analysis. Soil Sci. Soc. Am. J. 66: 1930–1946.

World Bank (2012). Carbon sequestration in agricultural soils. Report No. 67395-GLB.

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