A decline in soil organic matter (SOM) levels in the KwaZulu-Natal region was predicted with the cultivation of natural veld (Figure 1). Predicted decline in SOM in the other summer rainfall grain production regions of this study was confirmed by a study conducted by du Toit et al., (1994).
Improved cropland management can increase SOM 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, crop yields and ecosystem services in an environmentally friendly way. This creates a win–win situation for both farmers and the environment.
This page contains information on the potential of conservation agriculture (CA) to sequester soil organic carbon (SOC) for the grain production region of KwaZulu-Natal. The information is based on predictions obtained from numerical modelling. The range in climate and soil, with tillage, agronomic and grazing practices that were used to represent this region are also summarised, followed by notes on model calibration for the numerical modelling conducted.
For this study, the grain production region of KwaZulu-Natal includes the magisterial districts of of Bergville, Dannhauser, Dundee, Escort, Glencoe, Kliprivier, Lions river, Mooirivier, Newcastle, Paulpietersburg, Umvoti and Winterton.
Carbon sequestration modelling
A detailed numerical modelling approach was followed that can simulate the effects of crop rotations and tillage with planned agronomic, forage and grazing practices to a level of detail that can predict how the SOC contents (and stock) change over time for region-specific climate, soil properties and farming systems.
Simulation of SOC sequestration was conducted for two periods, namely:
- SOC sequestration modelling assumed a baseline of natural veld conditions followed by 50 years simulation of conventional tillage (which includes mouldboard ploughing and discing). A decline in SOC levels was predicted due to the high disturbance and mixing of soil that occurred with mould board ploughing and deep discing.
- A 50-year simulation of CA farming systems was modelled with a baseline of the conventional tillage (with degraded soil carbon stocks). SOC sequestration simulation results for 50 years of CA farming, per farming system, are summarised below.
Modelling scenarios of the simulated farming systems are summarised in Table 1.
Conservation Agriculture (CA) practices involve:
- Diversification of crop rotations and sequence that include soybeans as a legume.
- 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 untilled soil.
- Controlled grazing of maize residues, leaving enough residues for a mulch cover.
- Increase crop frequency (double cropping) to provide higher annual biomass production, root mass and carbon input for SOC sequestration (future CA).
- Include cover crops into the rotation cycle that provide high biomass production and root mass (carbon input) for SOC sequestration (future CA).
- Soil fertility management that includes lime and fertiliser applications to prevent deficits of vital plant nutrients, sustain biomass, forage, and grain production, and optimise SOC sequestration.
- Improved forage grazing practices of cover crops for future CA, such as high-density grazing approach, to provide sufficient time for the cover crops to recover.
Predicted soil carbon sequestration
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). The IPCC soil carbon analysis method specifies that a 30 cm depth must be used to report soil carbon sequestration. The predicted change in SOC stock and SOC sequestration potential following implementation of CA farming systems are shown in Figures 2, 3 and 4.
Historic conventional till. Predicted changes in SOC and research results from du Toit et al., (1994) indicate initially high rates of decline in SOC stock during the first 5-6 years after cultivation of natural veld, followed by lower rates of decline (Figure 1). The high rates of SOC decline are the result of high rates of SOM mineralisation and crop residue decomposition due to the extensive soil disturbance and mixing by fully inverting soil layers and related elevated oxygen levels in the plough layer, and the incorporation of almost all crop residues into the plough layer.
Conventional till (CT). Continuation of CT caused further deceases in SOC stocks (Figure 2), but at a lower rate than the preceding CT that used mouldboard ploughing. Reduction in SOC stock with CT, but without mouldboard ploughing, is supported by numerous studies, including results from meta-analysis conducted by Nicoloso and Rice (2021), Corbeels et al., (2019),
Powlson et al. (2016), World Bank (2012) and West and Post (2002).
Current CA. SOC stocks increase slightly with a change from CT to current CA. The increase can be ascribed to a reduction in SOM mineralisation and residue decomposition due to the effect of less soil disturbance and mixing, less residue being mixed into the soil with reduced till, and with more residues left as a soil mulch layer. Predicted SOC sequestration rates were very low, even during the initial 6 years.
The main advantage of current CA is that further reductions in SOC stock that would have occurred with continued conventional tillage are avoided. The carbon input from a grain cropping system (maize with grazing of rests and soybean) is too low to significantly increase SOC stock over time. Results from meta-analysis conducted by Corbeels et al., (2019), Powlson et al., (2016) and Luo et al., (2010) indicated that reduced- or no tillage alone did not increase SOC stocks in most of the studies, except when crop residue is retained for mulch and annual biomass production is increased with double cropping (two crops per year) and/or inclusion of cover crops to provide the needed carbon input to facilitate SOC sequestration.
Future CA. Considerably higher SOC sequestration was predicted for future CA, which includes the added cover crops (3 cover crops over 3 years) and double cropping (6 crops over 3 years). Forage sorghum and rye as summer and winter cover crops have high biomass yield and root mass, providing the needed high carbon input for SOC. Similar results, where SOC sequestration more than doubled with double cropping systems, the inclusion of cover crops, and no tillage, were determined from meta-analyses conducted by Nicoloso and Rice (2021), Corbeels et al., (2019), Powlson et al., (2016) and West and Post (2002), and reported by Gustavo et al., (2015) and Causarano et al., (2008).
SOC stabilised (reached equilibrium) and declined steadily soon after the initial 5 years for the future CA scenario that is based on the proposed fertiliser applications only during planting. SOC declined steadily after the initial 5 years when SOC sequestration has occurred. The sequestered SOC was lost because of nitrogen (N), and (to a lesser extent) phosphorus (P), deficits experienced by forage sorghum and rye.
Significantly higher cover crop biomass production and SOC sequestration rates were predicted if the cover crops received nitrogen (N) top dressing(s) in addition to the fertiliser rates proposed for planting. This indicates that cover crop biomass production and SOC sequestration are largely determined by the amounts of available N in the soil for the future CA systems.
SOC sequestration rates reported by various meta-analysis studies are summarised in Table 2.
According to Gonzalez-Sanchez, et al., (2019), rates were initially high during the first 5 years, which is followed by a second period with lower rates. Predicted SOC sequestration rates for future CA that received N top dressing(s) are comparable to the average to high rates listed in Table 2 for the period 6 to 12 years.
Most of the changes of SOC stocks occurred in the first 10-12 years for the future CA scenarios with N top dressing of the cover crops. SOC sequestration occurs still occurs at low rates after 12 years for the scenario with two N top dressings (85 kgN/ha). SOC stocks did reach equilibrium after 30 years of simulation for this scenario, which is within the period of 20-30 determined for sub-Sahara Africa from a meta-analysis conducted by the World Bank (2012).
Background values. One of the aims of proposed future CA practices is to allow SOC levels to recover to near the background values (healthy natural veld). The SOC stock predicted for the future CA scenario with two N top dressings are lower than the background value at 50 years. This is an indication that higher N application rates than the 85 kgN/ha rate used for the future CA scenario with two top dressings is required to achieve the background value.
Lessons learned from modelling results and practical implications
The cultivated soils of the region have a high potential for SOC sequestration as a result of previous SOC depletion (historic scenario).
Effect of tillage system. Continuation of conventional till caused further deceases in the already low SOC stock. A change from conventional till to reduced- or no tillage prevents further reductions in SOC stock that would have occurred with continued conventional tillage. This effect can be ascribed to reduced soil disturbance and less mixing of residues into the soil. More crop residues are left as a soil mulch layer with reduced- and no tillage.
Effect of intensified cropping system. The carbon input from a single cropping system that does not include cover crops is too low to significantly increase SOC stock over time (current CA). Significantly higher SOC sequestration was predicted for future CA with double cropping (6 crops over 3 years) and inclusion of cover crops (3 cover crops over 3 years) if sufficient N and P are available in the soil. Significant increases in SOC stock depend on higher annual biomass production to provide the higher carbon inputs needed for SOC sequestration.
Effect of cover crops. The combination of cover crops and soil fertility management clearly emerged as the most important element in facilitating SOC sequestration. Cover crops have high biomass production and root mass that provides the high carbon input required to effectively increase SOC stocks over time. SOC sequestration rates increased significantly with the inclusion of cover crops in the crop rotation.
Effect of soil fertility management. The potential benefits obtained from cover crops and double cropping systems are determined by soil fertility and nutrient management, where healthy root development and high biomass yield are a function of nutrients that are readily available in soil. N and P are also building blocks of soil organic matter, and SOC sequestration can be constrained when carbon and nitrogen levels are not adequately balanced (Lal, 2004).
Modelling results of future CA systems demonstrated a significant increase in SOC sequestration rates when cover crops received N top dressing(s) based on the guide by Grain SA (2021) for forage sorghum and rye. This confirms that carbon input from the cover crops and SOC sequestration is limited by available nitrogen in the soil based for the proposed fertiliser application rates at planting. The rate and extent of SOC sequestration (or increase in SOC stocks) is largely determined by levels of available nitrogen in the soil.
It is vitally important that soil fertility is assessed regularly, and managed, to maintain high sequestration rates. The elemental requirements for SOC sequestration, and alleviation of elemental deficiencies, need to be resolved for soil type, crop sequence, required grain and forage yield and desired rates of SOC sequestration. Soil phosphate levels should be monitored and managed to ensure that optimum soil P levels are maintained to allow an increase in SOC stock since high SOC sequestration rates can require additional P as a building block of SOM to that required by crops.
Effect of grazing. Overgrazing of maize residue can reduce the SOC sequestration significantly, especially for farming systems that do not include a cover crop. Modelling results indicate that SOC sequestration is significantly affected when more than about 50% (live weight) of the residue is removed, since as much as 20% of the residue can be trampled into the soil and a further 10% (which can be as high as 20%) of the residue is incorporated into the soil during planting.
The benefit of high density grazing of the cover crops on SOC sequestration is clear in model simulations. High density grazing with limited duration provides the cover crop sufficient time to recover without losing root mass. Continuous (over)grazing can result in significant losses in root mass and in forage production if the cover crop is not allowed sufficient time to recover between grazing activities. Overgrazing will result in a reduced SOC sequestration potential.
Wide scale application of the proposed future CA practices should have an immediately implementable and measurable impact to sequester atmospheric carbon as SOC, mitigating the impacts of climate change. This is a win-win situation for farmers and the environment.
Model input data
Climate
The region is characterised by warm summers and moderate to cold winters with severe frost in the winter(Figure 5). Most of the rain (about 93% of annual rainfall) occurs between September and April as thunderstorms late afternoon. Rainfall usually exceeds 100 mm/month during the months of November to January. A mean annual precipitation of 786 mm/yr was used for the region.
Soil
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 region. 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 soils for the region, based on soil texture and clay content (discussed in Section 4.3 of Numerical modelling webpage). The median (50th percentile) clay content determined from 263 soil profiles is 32, 34 and 37% for the A-, B1 and B2 soil horizons respectively. The soil texture of the horizons is a sandy clay loam A- and B1 soil horizons and sandy clay B2 soil horizon. Soil phosphorus- and exchangeable potassium levels of 25 and 120 mg/kg were used for the A-soil horizon respectively for the initial conditions (after 50 years of historical conventional tillage) of the implementation of CA farming systems scenarios. The levels are based on several decades of historic fertiliser application to increase soil fertility to these optimum levels for crop production.
Tillage-, agronomic- and grazing practices
The simulation of cropping systems required information related to the following activities:
- Planting.
- Tillage before, during and after planting.
- Fertiliser and lime application.
- Harvesting.
- Grazing of forage sorghum and maize residue after harvesting.
Activities summarised in Table 3 and the scheduling thereof, are based on data provided by Grain SA and ASSET Research.
Data on suggested nitrogen, phosphorus, potassium and lime application rates were provided by Grain SA for each crop system scenario (Table 4). The rates were increased if the modelling indicated that a crop experience a nutrient deficit that limits the potential to sequester SOC. Modelling also indicated that forage sorghum and rye went into a nitrogen deficit to achieve the high biomass production indicated by ASSET research, resulting in that SOC contents stabilised at reduced levels determined by the extent of nitrogen deficit. Proposed nitrogen application rates were adjusted in the model to both sustain high (forage) biomass production rates and to limit the effect of nitrogen deficiency stress on the crops that sequester SOC.
Model calibration
Model calibration is discussed in Section 5 of the numerical modelling webpage for this study. This section focusses on the results from the calibration.
Crop growth model component
Selected parameter values in the WinEPIC plant growth and development module were refined until the predicted above ground dry matter and grain yield corresponded within 5% of the data provided by ASSET Research (Table 5). The dry matter and grain yields predicted corresponded with the data provided by ASSET Research for maize and sunflower. Predicted dry matter for forage sorghum and rye was 7.5, 12.7, 15.7 and 4.1, 6.5, 6.5 t/ha for the respective Future CA scenarios, which indicates the effect of nitrogen deficiencies on the above-ground biomass production.
Soil organic matter model component
Model calibration could not be conducted for KwaZulu Natal region, such as at the other regions, since the data used for model calibration on the effect of cultivation on soil organic carbon was not included in the study by du Toit (du Toit et al., 1994) for the KwaZulu Natal region. However, model calibration results for the Eastern Highveld production area, that includes the same farming systems, showed that WinEPIC could predict the non-linear change in SOC contents that were determined by du Toit et al., (1994) for the Eastern Highveld.
References
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