Abstract
Green manure can be a source of nutrients for crops to spare some of the mineral fertilizer required for sus-tainable intensification of agriculture in smallholder context. We explored with a calibrated model the potential of legume green manure to help spare mineral fertilizer for sustainable intensification of rainfed rice production in Madagascar highlands. This study builds on a two-year rice experiment with rice and green manure as pre-vious crop. The experiment investigated the response of rainfed rice growth to the incorporation into the soil of i) rice residue only (C:N ratio of 44.9) and ii) mucuna + crotalaria green manures (C:N ratio of 18.7). Two levels of fertilizer were investigated: manure only (5 t DM ha-1) (F1) and manure (5 t DM ha-1) in combination with mineral fertilizer (136 kg N ha-1) (F2). The crop model STICS was calibrated against observed soil water and soil mineral N, plant N uptake, aboveground biomass and grain yield to reproduce the impact of residue type and fertilizer input on mineralization of organic N in soil and in crop residue, crop growth and grain yield. The calibrated crop model was then used to explore rice response to mineral N fertilizer, with or without green manure, for the historical climate (1994-2018). Relative Root Mean Square Error (rRMSE) of model simulations fell within the observed relative variation in grain yield due to treatment effect. With green manure, mineral N available for rice increased compared with the situation without green manure, leading to greater agronomic use efficiency of N fertilizer. Green manure helped spare mineral fertilizer to achieve a reference yield set at 75 % of potential yield. This reference yield (3.67 t ha- 1 on average across years) was reached with 40 kg N ha-1 with 6 t ha-1 of green manure, and 100 kg N ha-1 without green manure. The analysis of rice yield inter-annual vari-ability indicated that (i) N leaching increased with total seasonal rainfall, which undermined the benefits of green manure on rice productivity in wetter years, and (ii) the increase in rice productivity allowed by the green manure did not fully compensate the rice yield foregone with the cultivation of the green manure. The calibrated model can be a useful tool to explore additional management strategies that will help maximize the benefit of green manure for sustainable intensification of rice-based cropping systems, especially in low-input cropping systems.