Sustaining maize yields and soil carbon following land clearing in the forest–savannah transition zone of West Africa: Results from a 20-year experiment

Sustainable alternatives to slash-and-burn shifting cultivation in the (sub)humid tropics rely on the use of external nutrient inputs to address soil fertility decline. The use of organic inputs is widely accepted as a practice to improve soil fertility, in particular soil organic carbon (SOC). On the other hand, its combined use with mineral fertilizer has the potential to maintain or increase crop productivity through positive interactive effects between both resources. Few studies have investigated these effects in the long term. Therefore, the objective of this study was to investigate whether maize productivity and soil SOC can be sustained under permanent cropping with sole and combined use of compost and mineral nitrogen (N) fertilizer. Here, we report results from a long-term experiment carried out in Gagnoa, Ivory Coast, from 1971 to 1990. The experiment followed a randomized block design comprising eight replicates of 12 treatments. The two studied factors were compost (0 or 10 t DM ha⁻¹ yr⁻¹) and mineral N (0, 40, 80, 120, 160 or 200 kg N ha⁻¹ yr⁻¹) additions. Average maize grain yields of the first cropping cycles were significantly lower without compost (5.05 ± 1.57 t ha⁻¹) than with compost addition (6.07 ± 1.31 t ha⁻¹). The annual yield variability as shown by the standard deviation of the mean was reduced by 20% with compost addition. Without compost, 53% of the initial SOC stock in the 0–20 cm soil layer was lost, resulting in a SOC loss rate of − 0.62 t C ha⁻¹ yr⁻¹ compared to 21% with compost (−0.27 t C ha⁻¹ yr⁻¹). Compost addition therefore reduced SOC loss with an apparent SOC storage rate of 0.35 t C ha⁻¹ yr⁻¹. The conversion rate of organic carbon (OC) inputs to SOC was about 12%. The Introductory Carbon Balance Model (ICBM) reproduced well SOC dynamics, especially without compost. Without mineral N and without compost, maize grain yield decreased with decreasing SOC concentration until the introduction of leguminous crops in the second cropping cycle. We conclude that combined application of compost with mineral N fertilizers was effective at maintaining maize productivity but inadequate to prevent the decline of SOC stocks, despite large additions. Leguminous crops in the rotation were key for maize productivity, but probably due to effects non-related to supplementary N supply.