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L’estimation précise des biomasses de résidus de culture présents sur un sol après une récolte est importante vis-à-vis des enjeux agronomiques et environnementaux de la gestion des matières organiques dans les systèmes cultivés. Elle n’est pas aisée dans les parcelles agricoles en raison d’une forte variabilité de leur distribution sur les sols, liée aux opérations de récolte. L’objectif de cette étude est de proposer une méthode de mesure des biomasses de résidus de culture au champ précise, robuste et générique pour une diversité de cultures. Cette méthode repose sur l’utilisation de placettes de prélèvement (0,5 0,5 m) localisées selon un transect en tenant compte des opérations de récolte (sens de la récolte, largeur de la récolteuse) de façon à capter cette variabilité. Cette méthode a été testée après les récoltes de 2009 à 2011, en région de « grandes cultures », sur des exploitations pratiquant les techniques culturales simplifiées (TCS) et de semis direct (SD), avec les cultures de blé, maïs grain, orge de printemps, pois protéagineux, colza, féverole et tournesol. La comparaison des résultats obtenus avec un ou deux transects montre que la précision des estimations est suffisante en réalisant un seul transect, ce qui diminue significativement le temps de travail. La précision des estimations de biomasse par cette méthode de prélèvement n’est pas affectée par les modes de gestion (TCS versus SD)., The accurate quantification of crop residue biomass left on the soil after harvesting is important, due to the agricultural and environmental issues associated with the management of soil organic matter in croplands. This quantification is difficult in farmers’ fields due to a high variability of crop residue distribution on soil surface, because of harvesting operations. The objective of this study was to provide, for measuring crop residue biomass in the field, an accurate, robust and generic method for a large range of crops. This method relies on the use of sampling microplots (0.5 0.5 m), located along a transect and taking into account harvesting operations (harvesting direction and width) to capture this variability. This method was tested from 2009 to 2011, in farmers’ plots after harvesting, on wheat, grain maize, spring barley, field pea, rapeseed, fababean and sunflower crops. The comparison of results obtained with one or two transects (one perpendicular and the other diagonally across harvesting direction) shows that the accuracy of the estimates is sufficient using a single transect, allowing for a reduction of the working time. The accuracy of this method of collection is not affected by soil management practices (shallow till versus no-till). Keyword

The availability of soil nutrients, which are recycled through the decomposition of crop residues, is important for the management of cropped soils. However, knowledge regarding the influence of contact between crop residues and soil on the dynamics of carbon (C) and nitrogen (N) in soil is limited. In particular, the effect of particle size on decomposition is not well-known, and conceptual approaches for modelling the soil-residue contact in a decomposition model remain scarce. Therefore, we analysed and modelled the effect of maize stem particle length on decomposition. We incubated maize stem residues with particle sizes of 0.02, 0.5, 2 and 5 cm in length in a loamy soil at 25 °C over 301 days. We continuously measured the mineralisation of C and N and determined the chemical evolution of the remaining particles. We used the decomposition model CANTIS which takes into account the soil-residue contact, using a contact factor, KMZ. The decomposition rates of smaller maize particles were higher than those of larger particles during the early phases of decomposition. However, these differences were not maintained after 301 days. These results suggest that a larger size of the maize particles only slowed the rate of mineralisation in the short term but did not modify decomposition in the medium term. We proposed a new formalism for expressing the changes in soil-residue contact with different particle sizes. The contact factor KMZ was calculated using the standardised specific surface area and can be applied more widely to residues that differ in morphology and density.