François-P. Chalifour

Dynamics of soluble organic C and C mineralization in cultivated soils with varying N fertilization

Laboratory studies have shown that N availability may affect C decomposition in soils. A field study was undertaken to determine to what extent water-soluble organic C (WSOC) and C mineralization could be affected by mineral N fertilizers. Soil cores were collected in the 1993 and 1994 growing seasons, from a sandy loam (loamy, mixed, frigid, Typic Haplorthod) and a sandy clay (fine, mixed, frigid, Typic Humaquept) under early-maturing maize (Zea mays L.). Soils were fertilized with NH4NO3 at reduced (10, 60, 120 kg N ha−1) or recommended (180 kg N ha−1) rates. For both soils, C mineralization rate increased significantly (P<0.05) with amount of N fertilizer only at two of the nine sampling dates. However, WSOC contents generally decreased (P<0.05) with increasing N fertilizer rate. Overall, a decrease in soil mineral N content was consistently associated with an increase in WSOC content. The relationship between soil WSOC and mineral N contents was logarithmic for both soils (r2=0.70 in the sandy loam and r2=0.89 in the sandy clay). For both soils, we estimated a critical level of N of about 60 mg kg−1, below which small decreases in mineral N were associated with large increases in WSOC contents.

Winter damage to perennial forage crops in eastern Canada : Causes, mitigation, and prediction

Harsh winter climate results in frequent losses of stands and yield reduction in many forage-growing areas of Canada and other parts of the world. Climatic conditions and crop management both affect the winter survival of perennial forage crops. In this review, we present the main causes of winter damage in eastern Canada and we discuss crop management practices that help mitigate the risks of losses. Predictive tools available to assess the risks of winter damage both spatially and temporally are also presented. Our understanding of the causes of winter damage and of the plant adaptation mechanisms to winter stresses, particularly the role of N and C organic reserves, has improved. Forage species commonly grown in eastern Canada differ in their tolerance to subfreezing temperatures and to anoxia caused by the presence of ice on fields. Some improvement in winter hardiness of forage legume species has been achieved through breeding in eastern Canada but new technologies based on laboratory freezing tests ...

Nitrous oxide production in soils cropped to corn with varying N fertilization

Mineral N fertilizers may contribute to N gas emissions to the atmosphere. Soil cores were collected in 1993 and 1994, in a sandy loam and a sandy clay cropped with an early-maturing corn (Zea mays L.) hybrid and fertilized with ammonium nitrate at rates of 10, 60, 120 or 180 kg N ha−1. Denitrification and N1O production rates, air-filled porosity (AFP), water-soluble mineral N (WSMN) and water-soluble organic C (WSOC) were measured. Denitrification and N2O production rates were generally small, but values >2 µg N2O-N kg−1 h−1 were measured (i) when WSMN contents exceeded 5 mg kg−1, and (ii) when AFP was <50 to 55% in the sandy loam, and <40 to 45% in the sandy clay. For most sampling dates, N2O production and denitrification rates increased with N fertilizer level. In 1993, AFP was relatively high and variable in soil cores, and regression analyses revealed that denitrification rates were closely related to AFP. In 1994, AFP was relatively low in soil cores, and regression analyses showed that denitrific...

Construction of a Tn5 derivative encoding bioluminescence and its introduction in Pseudomonas, Agrobacterium and Rhizobium

SummaryA simple method based upon the use of a Tn5 derivative, Tn5-Lux, has been devised for the introduction and stable expression of the character of bioluminescence in a variety of gram-negative bacteria. In Tn5-Lux, the luxAB genes of Vibrio harveyi encoding luciferase are inserted on a SalI-BglII fragment between the kanamycin resistance (Kmr) gene and the right insertion sequence. The transposon derivative was placed on a transposition suicide vehicle by in situ recombination with the Tn5 suicide vector pGS9, to yield pDB30. Mating between Escherichia coli WA803 (pDB30) and a strain from our laboratory, Pseudomonas sp. RB100C, gave a Kmr transfer frequency of 10-6 per recipient, a value 10 times lower than that obtained with the original suicide vehicle pGS9. Tn5-Lux was also introduced by insertion mutagenesis in other strains of gram-negative soil bacteria. The bioluminescence marker was expressed in the presence of n-decanal, and was monitored as chemiluminescence in a liquid scintillation counter. The recorded light intensities were fairly comparable among the strains, and ranged between 0.2 to 1.8x106 cpm for a cell density of 103 colony forming units/ml. Nodules initiated by bioluminescent strains of Rhizobium leguminosarum on two different hosts were compared for intensity of the bioluminescence they produced.

Microbial biomass and N transformations in two soils cropped with annual and perennial species

A field study was undertaken to determine the effects of different plant species on soil microbial biomass and N transformations in a well drained silty clay loam (Typic Dystrochrept) and a poorly drained clay loam (Typic Humaquept). The crop treatments were faba bean (Vicia faba L.), alfalfa (Medicago sativa L.), timothy (Phleum pratense L.), bromegrass (Bromus inermis L.), reed canarygrass (Phalaris arundinacea L.), and wheat (Triticum aestivum L.). Measurements of microbial biomass C, denitrification capacity, and nitrification capacity were performed periodically in the top 2–10 cm of soil. On most sampling dates, all three parameters were higher under perennial than under annual species. The nitrification capacity was positively affected by the level of N applied to each species (r=0.65** for the silty clay loam and 0.84*** for the clay loam) and not directly by the plant. The differences found in microbial biomass C were significantly correlated with the water-soluble organic C present under each plant species (r=0.74*** for the silty clay loam and 0.90*** for the clay loam), suggesting differences in C deposition in the soil among plant species. In the silty clay loam, the denitrification capacity was positively related to the amount of organic C found under each plant species, while in the clay loam, it was dependent on the amount of N applied to each species. There was less denitrification activity per unit biomass under legume species than under graminease, suggesting that, depending on their composition, root-derived materials may be used differently by soil microbes.

Fusarium root and crown rot in alfalfa subjected to autumn harvests

Fusarium root and crown rot in alfalfa subjected to an autumn harvest was studied in plots established in the spring of 1996 or 1997, at three sites in Quebec. Autumn harvest treatments were started the year following establishment at each site. They were made at either 400, 500 or 600 growing degree-days (GDD; 5°C) after the last summer harvest. Samples were assessed for fusarium root and crown rot severity in autumn 1998 and spring 1999. Disease severity over all sites, cultivars, and sampling periods was ranked in the following order: 400 GDD > 500 GDD > 600 GDD > control (no fall harvest). Key words: Fusarium, root and crown rot, alfalfa, harvest management

Optimization of Biological Dinitrogen Fixation in Compost of Deinking Paper

The deinking process of used paper produces a waste by-product called deinking sludge. Composting is one of the promising avenues to recycle deinking paper sludge. The high C/N ratio, a limiting factor for composting, can be reduced by the addition of manure, but N content decreases over time. N2-fixing bacteria can increase nitrogen levels in soils (Patriquin, 1982) and inoculation of composts with bacteria increased N2-fixing activity (Kostov et al., 1991). Therefore, inoculating composts with N2-fixing bacteria should be a valuable approach to maintain an optimal C/N ratio of about 25. The aim of the present study was to isolate N2-fixing bacteria from composts, characterize their N2-fixing activity and evaluate the effect of C source on N2-frxing activity in composts.