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9 pages; International audience; It has been repeatedly argued that mineral fertiliser application combined with in situ retention of crop residue biomass can sustain long-term productivity of West African soils. Using 20-year experimental data from southern Togo, a biannual rainfall area, we analysed the effect of two rates of mineral NPK fertiliser application to maize–cotton rotation on the long-term dynamics of soil C and nutrient contents, as compared with two control treatments. Mineral fertiliser treatments consisted of application to both maize (first season) and cotton (second season) the research-recommended NPK rates (Fertiliser-RR) and 1.5 times these rates (Fertiliser-1.5 RR). Control treatments consisted of cropping maize and cotton without fertiliser use (No-Fertiliser) and of double annual soil tillage (as done for planted treatments) without planting a crop (Tillage-NoCrop). Maize residue biomass was every year returned to the soil of crops planted treatments, whereas cotton stems were uprooted, piled and burnt on the experimental plots as done locally for phyto-sanitary reasons. Treatment effects were analysed through a long-term change in crop productivity, in soil C and nutrient contents. Our results indicate that productivity of maize and notably of cotton cannot be sustained in this Ferralsol without nutrient inputs. On average, maize yields without fertilisers decreased from 2 t ha−1 after woodland clearing to 0.5 t ha−1 after 10 years of cultivation, while cotton yields decreased from 1.5 to 0.5 t ha−1 only after 5 years. In spite of the need of mineral fertiliser use to sustain productivity of this soil, there was little justification to increase inputs of mineral fertiliser over the research recommended rate. Over 20-year experiment, both maize and cotton while received N, P and K inputs at the research-recommended rates produced virtually the same yields as when these rates were increased by 50%. Although C inputs to soil under RR and 1.5 RR were greater than in the No-Fertiliser control (nil for Tillage-NoCrop), and the N input was more favourable for 1.5RR, the rates in which contents of soil C and N decreased over time did not differ substantially between treatments. Soil available P decreased for all treatments, while exchangeable K concentration increased under RR and 1.5 RR and decreased in unfertilised treatments (No-Fertiliser and Tillage-NoCrop). In fertilised plots and in tillage no-planted plots, soil pH decreased more than in No-Fertiliser plots. A decline of soil pH was associated with a decline of exchangeable Ca and Mg, which were on average 20 and 40% higher in fertilised plots than in No-Fertiliser plots. We conclude that soil C and N decline in this Ferralsol was more determined by a change in soil conditions due to woodland clearance and continuous tillage than by the quantities of C or N inputs added annually.

16 pages; International audience; Soil degradation in the savannah-derived agroecosystems of West Africa is often associated with rapid depletion of organic carbon stocks in soils of coarse texture. Field experiments were conducted over a period of more than 30 years at two sites in semiarid Togo to test the impact of agricultural management practices on soil C stocks and crop productivity. The resulting datasets were analysed using dynamic simulation models of varying complexity, to study the impact of crop rotation, fertiliser use and crop residue management on soil C dynamics. The models were then used to calculate the size of the annual C inputs necessary to restore C stocks to thresholds that would allow positive crop responses to fertilisers under continuous cultivation. Yields of all crops declined over the 30 years irrespective of crop rotation, fertiliser use or crop residue management. Both seed-cotton and cereal grain yields with fertiliser fluctuated around 1 t ha−1 after 20 years. Rotations that included early maturing sorghum varieties provided larger C inputs to the soil through residue biomass; around 2.5 t C ha−1year−1. Soil C stocks, originally of 15 t ha−1 after woodland clearance, decreased by around 3 t ha−1 at both sites and for virtually all treatments, reaching lower equilibrium levels after 5–10 years of cultivation. Soil C dynamics were well described with a two-pool SOM model running on an annual time step, with parameter values of 0.25 for the fraction of resistant plant material (K1), 0.15–0.20 for the decomposition rate of labile soil C (K2) and 8–10 t C ha−1 for the fraction of stable C in the soil. Simulated addition of organic matter to the soil 30 years after woodland clearance indicated that additions of 3 t C ha−1year−1 for 15–20 years would be necessary to build ‘threshold' soil C stocks of around 13 t ha−1, compatible with positive crop response to fertiliser. The simulated soil C increases of 0.5 to 1.6% per year are comparable with results from long-term experiments in the region. However, the amounts of organic matter necessary to build these soil C stocks are not readily available to resourcepoor farmers. These experimental results question the assumption that crop residue removal and lack of fertiliser input are responsible for soil C decline in these soils. Even when residues were incorporated and fertilisers used at high rates, crop C inputs were insufficient to compensate for C losses from these sandy soils under continuous cultivation.