Soon, Y. K. and Arshad, M. A. 2004. Tillage, crop residue and crop sequence effects on nitrogen availability in a legumebased cropping system. Can. J. Soil Sci. 84: 421-430. A field study was conducted to determine the effects and interactions of crop sequence, tillage and residue management on labile N pools and their availability because such information is sparse. Experimental treatments were no-till (NT) vs. conventional tillage (CT), and removal vs. retention of straw, imposed on a barley (Hordeum vulgare L.)-canola (Brassica rapa L.)-field pea (Pisum sativum L.) rotation. 15 N-labelling was used to quantify N uptake from straw, below-ground N (BGN), and fertilizer N. Straw retention increased soil microbial biomass N (MBN) in 2 of 3 yr at the four-leaf growth stage of barley, consistent with observed decreases in extractable soil inorganic N at seeding. However, crop yield and N uptake at maturity were not different between straw treatments. No tillage increased soil MBN, crop yield and N uptake compared to CT, but had no effect on extractable soil inorganic N. The greater availability of N under NT was probably related to soil moisture conservation. Tillage effects on soil and plant N were mostly independent of straw treatment. Straw and tillage treatments did not influence the uptake of N from its various sources. However, barley following pea (legume/non-legume sequence) derived a greater proportion of its N from BGN (13 to 23% or 9 to 23 kg N ha -1 ) than canola following barley (nonlegumes) (6 to 16% or 3 to 9 kg N ha -1 ). Fertilizer N constituted 8 to 11% of barley N uptake and 23 to 32% of canola N uptake. Straw N contributed only 1 to 3% of plant N uptake. This study showed the dominant influence of tillage on N availability, and of the preceding crop or cropping sequence on N uptake partitioning among available N sources. [421][422][423][424][425][426][427][428][429][430]. Les auteurs ont effectué une étude sur le terrain afin d'établir les effets et les interactions de la succession des cultures, du travail du sol et de la gestion des résidus de récolte sur les réserves et la disponibilité du N labile, car on sait peu de choses en la matière. Les traitements examinés étaient les suivants : non-travail (NT) contre travail ordinaire (TO) du sol et retrait ou rétention de la paille dans un assolement orge (Hordeum vulgare L.)-canola (Brassica rapa L.)-pois de grande culture (Pisum sativum L.). Les auteurs ont recouru au marquage par le 15 N pour quantifier l'absorption du N venant de la paille, du sol (NS) ou de l'engrais. La rétention de la paille a accru le N de la biomasse bactérienne (NBB) deux années sur trois au stade de la quatrième feuille, ce qui concorde avec la diminution du N minéral extractible présent dans le sol aux semis. Toutefois, le rendement et l'absorption de N à maturité ne varient pas avec le traitement de la paille. Comparativement au travail ordinaire du sol, le non-travail accroît le NBB, le rendement et l'absorption du N, sans agir sur la quantité de N minéral extractib...