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5.4 Discussion
5.4.1 Maize and Groundnut Response to CO 2 , Temperature,
Rainfall and Fertilizer
Temperature increases in areas such as Zimbabwe where crops are grown near
thresholds can be detrimental to rain-fed crop production. Increased temperatures
can negatively affect crop yields by accelerating crop phonological stages hence
less time for biomass accumulation (Asseng et al. 2015). In this study, increased
temperatures show negative effects on both maize and groundnut yields across soil
types with higher yield reductions simulated on the better soils. Although simulated
yield reductions were higher on better soils, average grain and stover yields were
always higher than those for poor soils.
Responses to CO 2 can vary by crop species (Asseng et al. 2015). In our study,
maize showed minimal increases of about 5% for both the APSIM and DSSAT
model with fertilizer, however, groundnuts showed average increases of about 23%.
Asseng et al. (2015) reported that C4 (e.g., maize, sorghum, millet) and C3 (e.g.,
wheat, groundnuts, potatoes) plants when CO 2 is increased to 500–550 (ppm), grain
yield can be increased by 10–20% and by <13% for C3 and C4, respectively.
Responses to CO 2 also depend on soil water and nutrient availability with highest
responses being reported under soil water limiting conditions (Kang et al. 2002).
However low soil fertility can reduce the possible positive effects of elevated CO 2
on yields (Yang et al. 2006). This was also simulated in the current study where
there were minimal to no benefits at all with increases in CO 2 across all soil types
when no fertilizer was added. However positive responses were simulated with
application of 60 kg N/ha with higher increases simulated on better soils. Both stover and grain yields increased as CO 2 concentrations increased up to the 720 ppm
level. Increases of CO 2 in the atmosphere is one of the most certain aspects of climate over the coming decades and leguminous crops such as groundnuts have the
potential to benefit from this. Leguminous crops fix the atmospheric nitrogen,
release high-quality organic matter in the soil and allow sequestration of carbon in
soil. If used as feed (provided the quality is not affected), leguminous crops could
reduce methane emissions from livestock. These multiple benefits provide both
mitigation and adaptation benefits to farmers.
Rainfall variability can have both positive and negative impacts on agriculture
depending on the environment. Reduced rainfall by about 25% can be detrimental
to crop yields while increases by similar magnitude would not be as beneficial in
low input systems and more importantly on poor soils. Rainfall distribution also
plays an important role, as lack of rainfall at crop critical growth stages such as
anthesis can substantially reduce grain yield.
Smallholder farming systems are low input systems with an average nitrogen
application rate of 3 kg/ha and zero fertilizer application for legumes such as
groundnuts. General fertilizer recommendations for different soil types are up to
110, 110–140 and 140–180 kg N/ha for better, average and poor soils, respectively,
5 Understanding the Role of Soils and Management on Crops in the Face of Climate…
5.4 Discussion
5.4.1 Maize and Groundnut Response to CO 2 , Temperature,
Rainfall and Fertilizer
Temperature increases in areas such as Zimbabwe where crops are grown near
thresholds can be detrimental to rain-fed crop production. Increased temperatures
can negatively affect crop yields by accelerating crop phonological stages hence
less time for biomass accumulation (Asseng et al. 2015). In this study, increased
temperatures show negative effects on both maize and groundnut yields across soil
types with higher yield reductions simulated on the better soils. Although simulated
yield reductions were higher on better soils, average grain and stover yields were
always higher than those for poor soils.
Responses to CO 2 can vary by crop species (Asseng et al. 2015). In our study,
maize showed minimal increases of about 5% for both the APSIM and DSSAT
model with fertilizer, however, groundnuts showed average increases of about 23%.
Asseng et al. (2015) reported that C4 (e.g., maize, sorghum, millet) and C3 (e.g.,
wheat, groundnuts, potatoes) plants when CO 2 is increased to 500–550 (ppm), grain
yield can be increased by 10–20% and by <13% for C3 and C4, respectively.
Responses to CO 2 also depend on soil water and nutrient availability with highest
responses being reported under soil water limiting conditions (Kang et al. 2002).
However low soil fertility can reduce the possible positive effects of elevated CO 2
on yields (Yang et al. 2006). This was also simulated in the current study where
there were minimal to no benefits at all with increases in CO 2 across all soil types
when no fertilizer was added. However positive responses were simulated with
application of 60 kg N/ha with higher increases simulated on better soils. Both stover and grain yields increased as CO 2 concentrations increased up to the 720 ppm
level. Increases of CO 2 in the atmosphere is one of the most certain aspects of climate over the coming decades and leguminous crops such as groundnuts have the
potential to benefit from this. Leguminous crops fix the atmospheric nitrogen,
release high-quality organic matter in the soil and allow sequestration of carbon in
soil. If used as feed (provided the quality is not affected), leguminous crops could
reduce methane emissions from livestock. These multiple benefits provide both
mitigation and adaptation benefits to farmers.
Rainfall variability can have both positive and negative impacts on agriculture
depending on the environment. Reduced rainfall by about 25% can be detrimental
to crop yields while increases by similar magnitude would not be as beneficial in
low input systems and more importantly on poor soils. Rainfall distribution also
plays an important role, as lack of rainfall at crop critical growth stages such as
anthesis can substantially reduce grain yield.
Smallholder farming systems are low input systems with an average nitrogen
application rate of 3 kg/ha and zero fertilizer application for legumes such as
groundnuts. General fertilizer recommendations for different soil types are up to
110, 110–140 and 140–180 kg N/ha for better, average and poor soils, respectively,
5 Understanding the Role of Soils and Management on Crops in the Face of Climate…