Impacts des systèmes de culture en semis direct avec couverture végétale (SCV) sur la dynamique de l'eau, de l'azote minéral et du carbone du sol dans les cerrados brésiliens

Dans les conditions tropicales humides des cerrados brésiliens, de nouveaux systèmes de culture étaient nécessaires pour assurer une production agricole durable. Des systèmes en semis direct avec couverture végétale (SCV) ont été mis au point en incorporant des plantes de couverture avant ou après la culture commerciale. Depuis 1999, différentes études ont été réalisées dans le cadre d'un projet de recherche franco-brésilien. Elles ont permis de quantifier les effets de ces SCV sur les dynamiques de l'eau, de l'azote et du carbone en particulier. Il a été ainsi confirmé que les SCV permettent de diminuer le ruissellement et donc l'érosion qui lui est liée. De même, les SCV permettent une fourniture en azote supérieure pour la plante commerciale. L'eau et l'azote non valorisées par cette culture peuvent l'être par la plante de couverture additionnelle, de même que l'eau et l'azote mis à disposition en dehors de ce cycle commercial. Les importantes restitutions de phytomasse de ces nouveaux systèmes à plusieurs plantes dans l'année aboutissent à un bilan de carbone positif et une augmentation des taux de matière organique des sols. (Résumé d'auteur

Impacts des systèmes de culture en semis direct avec couverture végétale (SCV) sur la dynamique de l'eau, de l'azote minéral et du carbone du sol dans les cerrados brésiliens.

Under the humid tropical climate of the Brazilian cerrados, new cropping systems have become necessary to ensure sustainable agricultural production. Direct-sowing mulch-based cropping systems (DMC) introduce a cover crop before or just after the commercial crop. They help farmers enhance natural resources and produce more total biomass during the year. Crop residues protect the soil and increase organic restitution to the system. In 1999, a Franco-Brazilian collaborative project began scientific studies aimed at quantifying the impact of DMC, especially on the dynamics of water, carbon and nitrogen. DMC systems significantly decrease water runoff. In this way they effectively fight against soil erosion, even if additional infiltrated water can sometimes saturate the system and escape by drainage. The cover crop can then recycle some of that drainage loss and protect and use water resources better throughout the year. DMC systems also provide more favourable conditions that ensure the more intensive and more regular mineralisation of soil organic matter during the rainy season. Some of the organic mulch mineralises at the same time, providing the commercial crop with the benefit of a higher pool of inorganic nitrogen. The additional cover crop can recycle part of the nitrogen not used by the commercial crop and some of the nitrogen mineralised before or after this commercial cycle. Under DMC high microbial activity is responsible for higher CO2 emissions to the atmosphere, but the soil carbon balance is nonetheless positive because of the large biomass production and organic input from the introduction of the cover crop. Even under the hot and humid conditions of the cerrados, soil carbon content increases under the more productive DMC systems

Impactos do sistema de plantio direto com cobertura vegetal (SPDCV) sobre a dinâmica da água, do nitrogênio mineral e do carbono do solo do cerrado brasileiro.

Nas regiões tropicais, principalmente no cerrado brasileiro, para assegurar uma produção agrícola sustentável, é preciso recorrer a novos sistemas de cultura. Por isso, foi desenvolvido o Sistema de Plantio Direto com Cobertura vegetal, que preconiza o não-revolvimento do solo e o cultivo de plantas de cobertura, antes ou depois da cultura comercial, para produzir mais biomassa e proteger permanentemente o solo

A meta-analysis of long-term effects of conservation agriculture on maize grain yield under rain-fed conditions

Conservation agriculture involves reduced tillage, permanent soil cover and crop rotations to enhance soil fertility and to supply food from a dwindling land resource. Recently, conservation agriculture has been promoted in Southern Africa, mainly for maize-based farming systems. However, maize yields under rain-fed conditions are often variable. There is therefore a need to identify factors that influence crop yield under conservation agriculture and rain-fed conditions. Here, we studied maize grain yield data from experiments lasting 5 years and more under rain-fed conditions. We assessed the effect of long-term tillage and residue retention on maize grain yield under contrasting soil textures, nitrogen input and climate. Yield variability was measured by stability analysis. Our results show an increase in maize yield over time with conservation agriculture practices that include rotation and high input use in low rainfall areas. But we observed no difference in system stability under those conditions. We observed a strong relationship between maize grain yield and annual rainfall. Our meta-analysis gave the following findings: (1) 92% of the data show that mulch cover in high rainfall areas leads to lower yields due to waterlogging; (2) 85% of data show that soil texture is important in the temporal development of conservation agriculture effects, improved yields are likely on well-drained soils; (3) 73% of the data show that conservation agriculture practices require high inputs especially N for improved yield; (4) 63% of data show that increased yields are obtained with rotation but calculations often do not include the variations in rainfall within and between seasons; (5) 56% of the data show that reduced tillage with no mulch cover leads to lower yields in semi-arid areas; and (6) when adequate fertiliser is available, rainfall is the most important determinant of yield in southern Africa. It is clear from our results that conservation agriculture needs to be targeted and adapted to specific biophysical conditions for improved impact