Introduction de jachères florales en zones de grandes cultures : comment mieux concilier agriculture, biodiversité et apiculture ?

Les abeilles au sens large représentent plus de 20 000 espèces dans le monde. Or, un déclin des populations d'abeilles a été récemment observé en Europe (Biesmeijer et al., 2006 ; Rasmont et al., 2006). Il pose le problème du risque de disparition de ces insectes auxiliaires et de sa répercussion sur les activités humaines qui leur sont liées comme l’apiculture, la production de fruits, de légumes, de semences. La FAO (Nations-Unies) a lancé, en 1996, un cri d’alarme à l’attention de tous les gouvernements pour sauvegarder cette faune d’auxiliaires. Les causes possibles de ce déclin sont multiples (Kearns et al., 1998 ; Ghazoul, 2005). Les plus citées concernent la destruction et la fragmentation de l’habitat des abeilles (Richards, 2001 ; Steffan-Dewenter et al., 2006) et l'impact des produits phytopharmaceutiques (Kevan, 1975 ; Johansen et al., 1983 ; Taséi, 1996 ; Haskell et McEwen, 1998)

Predicting field N2_{2}O emissions from crop residues based on their biochemical composition: A meta-analytical approach

Crop residue incorporation is a common practice to increase or restore organic matter stocks in agricultural soils. However, this practice often increases emissions of the powerful greenhouse gas nitrous oxide (N$_{2}$O). Previous meta-analyses have linked various biochemical properties of crop residues to N$_{2}$O emissions, but the relationships between these properties have been overlooked, hampering our ability to predict N$_{2}$O emissions from specific residues. Here we combine comprehensive databases for N$_{2}$O emissions from crop residues and crop residue biochemical characteristics with a random-meta-forest approach, to develop a predictive framework of crop residue effects on N$_{2}$O emissions. On average, crop residue incorporation increased soil N$_{2}$O emissions by 43% compared to residue removal, however crop residues led to both increases and reductions in N$_{2}$O emissions. Crop residue effects on N$_{2}$O emissions were best predicted by easily degradable fractions (i.e. water soluble carbon, soluble Van Soest fraction (NDS)), structural fractions and N returned with crop residues. The relationship between these biochemical properties and N$_{2}$O emissions differed widely in terms of form and direction. However, due to the strong correlations among these properties, we were able to develop a simplified classification for crop residues based on the stage of physiological maturity of the plant at which the residue was generated. This maturity criteria provided the most robust and yet simple approach to categorize crop residues according to their potential to regulate N$_{2}$O emissions. Immature residues (high water soluble carbon, soluble NDS and total N concentration, low relative cellulose, hemicellulose, lignin fractions, and low C:N ratio) strongly stimulated N$_{2}$O emissions, whereas mature residues with opposite characteristics had marginal effects on N$_{2}$O. The most important crop types belonging to the immature residue group – cover crops, grasslands and vegetables – are important for the delivery of multiple ecosystem services. Thus, these residues should be managed properly to avoid their potentially high N$_{2}$O emissions

Challenges of accounting nitrous oxide emissions from agricultural crop residues

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N2O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N2O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N2O emissions

Challenges of accounting nitrous oxide emissions from agricultural crop residues

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N2O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N2O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N2O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N2O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N2O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N2O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N2O emissions

Challenges of accounting nitrous oxide emissions from agricultural crop residues

Crop residues are important inputs of carbon (C) and nitrogen (N) to soils and thus directly and indirectly affect nitrous oxide (N$_2$O) emissions. As the current inventory methodology considers N inputs by crop residues as the sole determining factor for N$_2$O emissions, it fails to consider other underlying factors and processes. There is compelling evidence that emissions vary greatly between residues with different biochemical and physical characteristics, with the concentrations of mineralizable N and decomposable C in the residue biomass both enhancing the soil N$_2$O production potential. High concentrations of these components are associated with immature residues (e.g., cover crops, grass, legumes, and vegetables) as opposed to mature residues (e.g., straw). A more accurate estimation of the short-term (months) effects of the crop residues on N$_2$O could involve distinguishing mature and immature crop residues with distinctly different emission factors. The medium-term (years) and long-term (decades) effects relate to the effects of residue management on soil N fertility and soil physical and chemical properties, considering that these are affected by local climatic and soil conditions as well as land use and management. More targeted mitigation efforts for N$_2$O emissions, after addition of crop residues to the soil, are urgently needed and require an improved methodology for emission accounting. This work needs to be underpinned by research to (1) develop and validate N$_2$O emission factors for mature and immature crop residues, (2) assess emissions from belowground residues of terminated crops, (3) improve activity data on management of different residue types, in particular immature residues, and (4) evaluate long-term effects of residue addition on N$_2$O emissions

Mesure de l’humidité du sol en continu en sols caillouteux à l’aide de capteurs capacitifs

La mesure de l’humidité du sol est une composante agronomique indispensable au suivi de lacroissance des cultures, très utilisée en agriculture de précision (Camilli et al., 2007 ; Garcia-Sanchezet al., 2011). C’est également une variable indispensable au fonctionnement de nombreux modèlesde simulation de la décomposition de la matière organique, du suivi de la dynamique du stock d’eaudu sol (Gabriel et al., 2010). Plusieurs matériels permettant le suivi permanent de la mesure de lateneur en eau du sol existent sur le marché ; mais peu d’entre-eux sont adaptés à une utilisation ensols caillouteux. Or, dans le cadre de la production de biomasse destinée à la fabrication decarburants de seconde génération, les cultures destinées à cette production sont avant toutimplantées sur des parcelles intéressant peu ou plus le monde agricole. Ces sols de faible qualitéagronomique sont parfois très caillouteux. Dans ces conditions, la majorité des matériels de mesuresde l’humidité du sol en continu présents sur le marché ne peuvent être utilisés. Dans le cadre d’untravail sur le suivi de croissance d’arbres cultivés en taillis à courte ou très courte rotation (Thiébeauet al., 2013), nous avons utilisé des capteurs capacitifs en sols possédant une forte proportion degraviers (2 à 13%) et de cailloux (18 à 44%), afin de disposer de mesures d’humidité du sol en continuen nous équipant de sondes ECH2O (Decagon Device Inc.) de deux types : EC5 de 5 cm et 10HS de 10cm de longueur.Les deux types de matériels testés ont été mis en oeuvre sur deux types de sols contrastés : sablolimoneuxen région Centre (Loiret) et argilo-limoneux en région Champagne-Ardenne (Aube). Lesrésultats d’étalonnage obtenus au cours de quatre années d’utilisation au champ montrent de bonsrésultats par rapport à l’humidité de référence : l’humidité massique du sol. Au regard des capteursutilisant le principe de la réflectance temporelle (TDR), ces résultats sont obtenus en faisantabstraction de la mesure de la densité apparente du sol et de sa température, ce qui simplifiebeaucoup les conditions d’emploi au champ. Dans nos conditions de travail, nous avons pu utiliserune seule et même équation pour l’obtention de l’humidité massique à partir de données initialesdélivrées en millivolt

Production de biomasse et immobilisation de carbone et d’azote sur des sols marginaux : cas de taillis à très courte rotation conduits sans fertilisation

National audienceDescription of the subject. This article presents the aboveground and belowground biomass balances of young non-fertilized short-rotation coppices, planted on marginal soils. Objectives. On two contrasting soils, the growth of willow, poplar and black locust was monitored firstly, to estimate their harvested biomass production, and the immobilization of root-derived carbon and nitrogen, and secondly, to quantify the restitutions occurring at the soil surface through leaf senescence. A comparison of two densities of black locust (2,500 vs 5,000 feet·ha-1) was carried out. Method. The devices used were instrumented to quantify the levels of carbon (C) and nitrogen (N) involved in: i) leaf senescence returning to the soil, ii) exports by the two harvests. Results. The total C immobilized in the second harvest oscillated from 7.8 to 16.1 t·ha-1, and the total N from 125 to 393 kg·ha-1. Leaf senescence corresponded to 15 to 22% of the total C on the soil surface, while 17 to 31% was immobilized in the roots. The N released into the soil by senescence varied from 48 to 79 kg·ha-1, i.e. 38 to 20% of the total respectively, and ranged and the one in the roots ranged from 22% to 48% of the total. We did not find an effect of planting density on different compartments of black locust. Conclusions. Willow and black locust have adapted better to the difficult conditions encountered than poplar. The symbiotic nitrogen fixation of black locust should be better exploited to increase the biomass production of other species, in mixed crops for example.Description du sujet. Cet article présente les productions de biomasses aériennes et souterraines de jeunes taillis à très courte rotation, non fertilisés, plantés sur des sols marginaux. Objectifs. Deux sols contrastés ont permis de suivre la croissance de saule, peuplier et robinier pour estimer leur production de biomasse exportable, l’immobilisation de carbone et d’azote dans les racines, et quantifier les restitutions au sol par la sénescence foliaire. Une comparaison de deux densités de robinier (2 500 vs 5 000 pieds·ha-1) est également réalisée. Méthode. Les dispositifs sont instrumentés afin de quantifier le carbone (C) et l’azote (N) impliqués dans i) la sénescence foliaire retournant au sol, ii) les exportations et immobilisations de deux récoltes. Résultats. Le C total immobilisé en seconde récolte varie de 7,8 à 16,1 t·ha-1 et l’azote de 125 à 393 kg·ha-1. La sénescence foliaire restitue au sol 15 à 22 % du C total fixé, tandis que 17 à 31 % est immobilisé dans les racines. L’azote restitué au sol par la sénescence a varié de 48 à 79 kg·ha-1, soit respectivement 38 à 20 % du total, et celui retrouvé dans les racines a varié de 22 à 48 % du total. Nous n’avons pas constaté d’effet de la densité de plantation sur les différents compartiments des robiniers. Conclusions. Saule et robinier ont su mieux s’adapter que le peuplier aux conditions difficiles rencontrées. La fixation d’azote symbiotique du robinier pourrait être mieux exploitée pour accroitre la production de biomasse d’autres espèces, en cultures mixtes par exemple

Luzerne sous couvert de pois protéagineux. Un seul travail du sol pour implanter deux cultures principales

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Une méthode pour estimer l'interception du rayonnement par un couvert bas: application au colza avant montaison

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Effet précédent de cultures intermédiaires, seigle et ray-grass, sur l'implantation et la production d'une luzerne semée au printemps

National audienceSowing a catch crop between two main crops at the end of summer, in this case between a cereal crop and a spring-sown lucerne (Medicago sativa L.), gives the possibility of putting to good use the soil, which otherwise would generally remain bare, and to reduce the leaching of nitrate. In addition, the catch crop can be used on the farm itself as green manure or silage. The object of this work was to compare the effects of two catch crops, rye (Secale cereale L.) and Italian ryegrass (Lolium multiflorum Lam.), and of the bare soil, on the establishment and growth of spring-sown lucerne. The two catch crops tested were harvested and/or incorporated into the soil at three different dates during March and April (D1, D2, and D3). These dates were in agreement with those usually practiced for the sowing of lucerne in spring. They had an effect on the yield of the lucerne crop in the sowing year, which was depressed for the two later dates (D2, D3), as compared to the first date. However, for these dates D2 and D3, the total dry matter yield of the catch crop and the lucerne crop was equivalent to the dry matter yield of the lucerne crop alone, sown at D1. In the first year after sowing, there was no effect of the catch crops on the yield of lucerne, as compared to the bare soil.Une culture intermédiaire, seigle ou ray-grass, entre la récolte d'une céréale et le semis d'une luzerne au printemps, a un rôle de piège à nitrates. Elle peut aussi être valorisée sous forme d'engrais vert ou d'ensilage. Mais quel est son impact sur la production de la luzerne semée au printemps ? On a étudié l'effet de 2 cultures intermédiaires (seigle et ray-grass d'Italie) sur l'implantation de la luzerne. Les cultures intermédiaires ont été détruites à 3 dates différentes au printemps suivant, correspondant à 3 dates de semis de luzerne. En conditions climatiques normales, la réserve hydrique du sol n'est pas affectée par leur présence. Une date tardive de semis de la luzerne a un impact négatif significatif sur sa production l'année d'implantation mais cet impact est compensé par la production de la culture intermédiaire, laquelle contribue à faire diminuer la teneur en nitrate de l'eau drainée pendant l'interculture. La production de matière sèche de la luzerne au cours de l'année suivante n'est affectée ni par la date de semis, ni par le précédent