Dynamics of nitrogen and phosphorus under the impact of climate change and agricultural land use in the West African Sudan Savannah

Abstract

Changing climate and agricultural land-use dynamics seriously challenge the future of cropping in the West African Dry Savannah, and, in turn, the livelihoods and food security of rural populations. Current production systems, already vulnerable to soil fertility depletion, are increasingly exposed to rainfall variability and generally to climate change, which, reportedly is expected to increase. Although consent exist that under the “business-as-usual-scenarios” these challenges will exacerbate resource use efficiency and jeopardize the sustainability of the agro-ecosystems, little is predicted about the magnitude of the adverse effects of changing climate on crop responses and hence land use. This obviously hampers the essential development and implementation of both appropriate adaptation measures and policies to increase the resilience of production systems. This study therefore aimed at quantifying and assessing the impact of predicted climate change on growth, yields, and water- and nutrient- use efficiencies of maize-, sorghum-, and cotton-based production systems in the dry savannah of northern Benin. Through a series of farmer- and researcher-managed on-farm trials, data were collected on crop responses to an un-amended soil (no fertilizer application), an integrated soil-crop management practice (recommended fertilizer rates and crop residues retention), a low use of external inputs (i.e. farmers determined the mineral fertilizer rate), and a high rate of mineral fertilizer use. The datasets, collected in 2014 and 2015 at Ouri Yori village in northern Benin, were used to investigate productivity and nutrient use efficiency of three target crops, and to parameterize and evaluate the CERES-Maize, CERES-Sorghum, and CROPGRO-Cotton Cropping System Models. The three crop models were subsequently applied to assess the impact of climate change on responses of maize, sorghum, and cotton to the different soil fertility management practices tested, considering the historical climate (1986-2005) and the ensemble mean of bias-corrected projected climate (2080-2099) from three Global Climate Models for three Representative Concentration Pathways (RCPs 2.6, 4.5, and 8.5). Biomass and economic yields of all three crops responded to both the high use of mineral fertilizer and the integrated soil-crop management practice, but the extent of this response was crop-specific. The highest agronomic efficiencies of nitrogen (N) and phosphorus (P), their apparent recovery as well as the positive partial N and P balances were recorded with the integrated soil-crop management practice, irrespective of the crops. The CERES-Maize model satisfactorily simulated in-season soil moisture and nitrate dynamics, N and P uptake, biomass accumulation, and grain yield. CERES-Maize predicted furthermore a more vigorous crop growth in the projected than in the historical runs, albeit only during the vegetative growth phase. Under the projected climate change, CERES-Maize predicted decreases in water- and N-use efficiencies, N and P uptake, and grain yield, irrespective of the soil fertility management strategies assumed. Similarly, CERES-Sorghum adequately simulated the observed soil water and N dynamics, biomass accumulation, N and P uptake, and the yield of sorghum. It predicted reductions in water- and N- use efficiencies, N and P uptake, and yield across all climate change scenarios and soil fertility management options. CROPGRO-Cotton simulated well soil water dynamics and N uptake during cotton growth, and seed cotton yield. Under the projected climate scenarios, CROPGRO-Cotton predicted increases in water- and N- use efficiencies and yield with the high use of mineral fertilizer or the integrated soil-crop management practice. Cotton responded more efficiently to N applied with integrated soil-crop management practice under future climate scenarios. The increases in productivity will occur, however, at the expense of soil fertility, unless targeted fertilizer management practices are introduced. The overall increase in understanding water- and nutrient- use efficiencies and yields of maize, sorghum, and cotton under both historical and future climate conditions can contribute to updating soil fertility management recommendations for reaching sustainable agricultural production in the Dry Savannah region of West Africa.

<strong>Einfluss von Klimawandel und Landnutzungsänderungen auf die Stickstoff und Phosphor Dynamik in Anbausystemen der westafrikanischen Trockensavanne </strong><br /> Changing climate and agricultural land-use dynamics seriously challenge the future of cropping in the West African Dry Savannah, and, in turn, the livelihoods and food security of rural populations. Current production systems, already vulnerable to soil fertility depletion, are increasingly exposed to rainfall variability and generally to climate change, which, reportedly is expected to increase. Although consent exist that under the “business-as-usual-scenarios” these challenges will exacerbate resource use efficiency and jeopardize the sustainability of the agro-ecosystems, little is predicted about the magnitude of the adverse effects of changing climate on crop responses and hence land use. This obviously hampers the essential development and implementation of both appropriate adaptation measures and policies to increase the resilience of production systems. This study therefore aimed at quantifying and assessing the impact of predicted climate change on growth, yields, and water- and nutrient- use efficiencies of maize-, sorghum-, and cotton-based production systems in the dry savannah of northern Benin. Through a series of farmer- and researcher-managed on-farm trials, data were collected on crop responses to an un-amended soil (no fertilizer application), an integrated soil-crop management practice (recommended fertilizer rates and crop residues retention), a low use of external inputs (i.e. farmers determined the mineral fertilizer rate), and a high rate of mineral fertilizer use. The datasets, collected in 2014 and 2015 at Ouri Yori village in northern Benin, were used to investigate productivity and nutrient use efficiency of three target crops, and to parameterize and evaluate the CERES-Maize, CERES-Sorghum, and CROPGRO-Cotton Cropping System Models. The three crop models were subsequently applied to assess the impact of climate change on responses of maize, sorghum, and cotton to the different soil fertility management practices tested, considering the historical climate (1986-2005) and the ensemble mean of bias-corrected projected climate (2080-2099) from three Global Climate Models for three Representative Concentration Pathways (RCPs 2.6, 4.5, and 8.5). Biomass and economic yields of all three crops responded to both the high use of mineral fertilizer and the integrated soil-crop management practice, but the extent of this response was crop-specific. The highest agronomic efficiencies of nitrogen (N) and phosphorus (P), their apparent recovery as well as the positive partial N and P balances were recorded with the integrated soil-crop management practice, irrespective of the crops. The CERES-Maize model satisfactorily simulated in-season soil moisture and nitrate dynamics, N and P uptake, biomass accumulation, and grain yield. CERES-Maize predicted furthermore a more vigorous crop growth in the projected than in the historical runs, albeit only during the vegetative growth phase. Under the projected climate change, CERES-Maize predicted decreases in water- and N-use efficiencies, N and P uptake, and grain yield, irrespective of the soil fertility management strategies assumed. Similarly, CERES-Sorghum adequately simulated the observed soil water and N dynamics, biomass accumulation, N and P uptake, and the yield of sorghum. It predicted reductions in water- and N- use efficiencies, N and P uptake, and yield across all climate change scenarios and soil fertility management options. CROPGRO-Cotton simulated well soil water dynamics and N uptake during cotton growth, and seed cotton yield. Under the projected climate scenarios, CROPGRO-Cotton predicted increases in water- and N- use efficiencies and yield with the high use of mineral fertilizer or the integrated soil-crop management practice. Cotton responded more efficiently to N applied with integrated soil-crop management practice under future climate scenarios. The increases in productivity will occur, however, at the expense of soil fertility, unless targeted fertilizer management practices are introduced. The overall increase in understanding water- and nutrient- use efficiencies and yields of maize, sorghum, and cotton under both historical and future climate conditions can contribute to updating soil fertility management recommendations for reaching sustainable agricultural production in the Dry Savannah region of West Africa.