Evolution of carbon and nitrogen biomass of vertisol and fersiallitic soil after previous cultivation of wheat and sugar beet in the irrigated perimeter of the Doukkala in Morocco

The restoration of crop residues on the ground has a predominant role in soil fertilization. Their decomposition allows an evolution of the carbon biomass and the nitrogen biomass of the soil. In this study, we investigated the impact of residues from previous wheat and sugar beet crops on the evolution of the microbial biomass and mineral nitrogen at the harvest and after the first fall rains on two types of soils. These are vertisol and fersiallitic soil of the Doukkala irrigated perimeter in Morocco. The effect of wheat and sugar beet residues on the change in microbial biomass and soil mineral nitrogen shows that: (i)Between harvest and end of summer, the size of the microbial biomass of The 0-20 cm soil layer decreases in both soil types. It decreases on average by 33% and 46% for vertisol and fersiallitic soil, respectively. ii)After the first fall rains, the carbon biomass of the two soil types increases. In vertisol, it increases by 55% for the previous wheat and 125% for the previous beet. In fersiallitic soil, it increases by 48% for the previous wheat and 105% for the previous sugar beet. (iii)The contribution of biomass nitrogen to total soil nitrogen between end of summer and first fall rains is of the order of 29.6 kg N/ha and 70.4 kg N/ha in the vertisol and 12.3 kg N/ha and 29.6 kg N/ha in the fersiallitic soil, respectively, after the previous crops of wheat and sugar beet. iv) A high mineralization of the organic matter in the 2 layers 0-20 cm and 20-40 cm combined after the first fall rains. For the previous sugar beet, the amount of mineral nitrogen released is 75 kg/ha and 41 kg/ha for vertisol and fersiallitic soil, respectively. This form of nitrogen is likely to be leached. For the pre-crop wheat, mineral nitrogen is immobilized by microorganisms in the soil and therefore the risk of pollution of the water table by nitrates is attenuated. All the results obtained show that the restitution of crop residues constitutes a source of organic matter which could be used as fertilizers of the degraded soils of Morocco.