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. 1999. Tillage effects on the dynamics of total and corn-residue-derived soil organic matter in two southern Ontario soils. Can. J. Soil Sci. 79: 473-480. Agricultural management practices affect the dynamics of soil organic matter (SOM) by influencing the amount of plant residues returned to the soil and rate of residue and SOM decomposition. Total organic C and δ 13 C of soil were measured in two field experiments involving corn cropping to determine the effect of tillage practices on SOM dynamics. Minimum tillage (MT) and no tillage (NT) had no significant impact on the soil C compared with conventional tillage (CT) in the 0-to 50-cm soil depth sampled at both sites. Continuous corn under MT and CT for 29 yr in a silt loam soil sequestered 61-65 g m -2 yr -1 of corn-derived C (C 4 -C), and it accounted for 25-26% of the total C in the 0-to 50-cm depth. In a sandy loam soil cropped to corn for 6 yr, SOM contained 10 and 8.4% C 4 -C under CT and NT, respectively. Reduced tillage practices altered the distribution of C 4 -C in soil, causing the surface (0-5 cm) soil of reduced tillage (MT and NT) plots to have higher amounts of C 4 -C compared to CT. Tillage practices did not affect the turnover of C 3 -C in soil.Key words: Soil organic matter, 13 C natural abundance, tillage practices Wanniarachchi, S. D., Voroney, R. P., Vyn, T. J., Beyaert, R. P. et MacKenzie, A. F. 1999. Effets du travail du sol sur la ciné-tique de la matière organique totale et de la fraction dérivée des restes de culture du maïs dans deux sols du sud de l'Ontario. Can. J. Soil Sci. 79: 473-480. Les pratiques agronomiques se répercutent sur la cinétique de la matière organique du sol (MOS) en jouant sur la quantité de restes de culture restitués au sol ainsi que sur le taux de décomposition de ces résidus et de la MOS. Nous avons mesuré le C organique total et le δ 13 C du sol dans deux expériences au champ sous culture de maïs, pour établir l'effet des pratiques de travail du sol sur la cinétique de la MOS. Le travail minimum (TM) et le semis direct (SD) ne donnaient lieu à aucun effet significatif par comparaison avec le travail classique (TC), sur le C du sol mesuré à partir d'échantillons de sol prélevés aux deux emplacements, dans la tranche de 0 à 50 cm de profondeur. Le maïs cultivé en continu en régimesTM et SD pendant 29 ans dans le loam limoneux (Elora) retenait 61-65 g m -2 an -1 de carbone (type C 4 ) et il comptait pour 25-26 % du carbone total récupéré dans la tranche de 0 à 50 cm. Dans le loam sableux (Delhi) cultivé en maïs depuis 6 ans, la MOS contenait, respectivement, 10 et 8,4 % C de type C 4 en régimes TC et SD. Les régimes de travail réduit (TM et SD) modifiaient la réparti-tion de C de type C 4 dans le sol, les 5 cm supérieurs des sols ainsi conduits produisant une plus forte proportion de C de type C 4 par rapport au carbone total que les sols conduits en TC. Les pratiques de travail du sol n'ont pas eu d'effet sur le renouvellement du C de type C 3 dans le sol.

The contents of sulfur amino acids in seeds of common bean ( Phaseolus vulgaris L.) are suboptimal for nutrition. They accumulate large amounts of a gamma-glutamyl dipeptide of S-methyl-cysteine, a nonprotein amino acid that cannot substitute for methionine or cysteine in the diet. Protein accumulation and amino acid composition were characterized in three genetically related lines integrating a progressive deficiency in major seed storage proteins, phaseolin, phytohemagglutinin, and arcelin. Nitrogen, carbon, and sulfur contents were comparable among the three lines. The contents of S-methyl-cysteine and gamma-glutamyl-S-methyl-cysteine were progressively reduced in the mutants. Sulfur was shifted predominantly to the protein cysteine pool, while total methionine was only slightly elevated. Methionine and cystine contents (mg per g protein) were increased by up to ca. 40%, to levels slightly above FAO guidelines on amino acid requirements for human nutrition. These findings may be useful to improve the nutritional quality of common bean.

yield response to N application (Patel et al.Forage sorghum-sudangrass [Sorghum bicolor (L.) Moench] is a and Singh, 2001). Results obtained in New York state relatively new crop to eastern Canada and the effects of additions of fertilizer N on yield, N accumulation, and N use efficiency are not suggest that brown midrib (BMR) sorghum-sudangrass available for this region. In 1998, 1999, and 2000 the response of hybrids should be fertilized more like an intensively forage sorghum-sudangrass to additions of fertilizer N rates (0, 50,

Widespread global changes, including rising atmospheric CO 2 concentrations, climate warming and loss of biodiversity, are predicted for this century; all of these will affect terrestrial ecosystem processes like plant litter decomposition. Conversely, increased plant litter decomposition can have potential carbon-cycle feedbacks on atmospheric CO 2 levels, climate warming and biodiversity. But predicting litter decomposition is difficult because of many interacting factors related to the chemical, physical and biological properties of soil, as well as to climate and agricultural management practices. We applied 13 C-labelled plant litter to soil at ten sites spanning a 3500-km transect across the agricultural regions of Canada and measured its decomposition over five years. Despite large differences in soil type and climatic conditions, we found that the kinetics of litter decomposition were similar once the effect of temperature had been removed, indicating no measurable effect of soil properties. A two-pool exponential decay model expressing undecomposed carbon simply as a function of thermal time accurately described kinetics of decomposition. (R 2 = 0.94; RMSE = 0.0508). Soil properties such as texture, cation exchange capacity, pH and moisture, although very different among sites, had minimal discernible influence on decomposition kinetics. Using this kinetic model under different climate change scenarios, we projected that the time required to decompose 50% of the litter (i.e. the labile fractions) would be reduced by 1-4 months, whereas time required to decompose 90% of the litter (including recalcitrant fractions) would be reduced by 1 year in cooler sites to as much as 2 years in warmer sites. These findings confirm quantitatively the sensitivity of litter decomposition to temperature increases and demonstrate how climate change may constrain future soil carbon storage, an effect apparently not influenced by soil properties.

Agricultural practices determine the level of soil organic C by influencing the amount of residue returned to, and retained by, soil. We conducted field experiments on four soils in Ontario and Quebec to test the hypothesis that short‐term changes in soil organic C and the proportion of corn (Zea mays L.)‐residue C retained in soils are affected by soil texture, fertility, and tillage management. We measured the amount of stover to estimate C input into the soils, and total soil C and 13C natural abundance to determine the soil C derived from corn residues. Only the sandy clay loam exhibited changes in total soil C, related to levels of fertilizer N and quantity of residues returned to the soil. The proportion of soil C derived from corn residues varied from 4 to 19%, depending on the time under continuous corn. Our data indicate a rapid increase in the proportion of cornderived C in the early stage of cropping to corn, but a markedly reduced rate of increase beyond 12 yr. Measured δ13C of archived samples of two soils with different clay contents (71 and 28% clay) but similar residue inputs (11 Mg stover ha‐1 yr‐1) indicated that the rate of increase in the proportion of corn‐derived C was higher in the soil with higher clay content (2.4 vs. 1.9% yr‐1). The results of this study are consistent with the hypothesis that the quantity of crop residue inputs and soil texture greatly influence the retention and turnover of crop residue C.

Asparaginase catalyzes the degradation of L-asparagine to L-aspartic acid and ammonia, and is implicated in the catabolism of transported asparagine in sink tissues of higher plants. The Arabidopsis genome includes two genes, ASPGA1 and ASPGB1, belonging to distinct asparaginase subfamilies. Conditions of severe nitrogen limitation resulted in a slight decrease in seed size in wild-type Arabidopsis. However, this response was not observed in a homozygous T-DNA insertion mutant where ASPG genes had been inactivated. Under nitrogen-sufficient conditions, the ASPG mutant had elevated levels of free asparagine in mature seed. This phenotype was observed exclusively under conditions of low illumination, when a low ratio of carbon to nitrogen was translocated to the seed. Mutants deficient in one or both asparaginases were more sensitive than wild-type to inhibition of primary root elongation and root hair emergence by L-asparagine as a single nitrogen source. This enhanced inhibition was associated with increased accumulation of asparagine in the root of the double aspga1-1/-b1-1 mutant. This indicates that inhibition of root growth is likely elicited by asparagine itself or an asparagine-derived metabolite, other than the products of asparaginase, aspartic acid or ammonia. During germination, a fusion between the ASPGA1 promoter and beta-glucuronidase was expressed in endosperm cells starting at the micropylar end. Expression was initially high throughout the root and hypocotyl, but became restricted to the root tip after three days, which may indicate a transition to nitrogen-heterotrophic growth.