A. F. Mackenzie

Elucidation of the source and turnover of water soluble and microbial biomass carbon in agricultural soils

Understanding the dynamics of soil C is key to managing soil organic matter to enhance soil quality and ecosystem functioning, and reduce trace gas emissions from soils. Our objective was to determine the source and turnover of C pools in some agricultural soils in eastern Canada. Soils from five field experiments under continuous maize cropping for 4–37 yr were sampled, and the organic C content and stable C isotope (13C) composition of whole soil and water soluble and microbial biomass fractions determined. The 13C results showed a clear distinction between the water soluble organic C and microbial biomass C, with the water soluble organic C more like the whole soil and the microbial biomass more like the maize residues. A simple linear model was used to explore the relationship among the soil organic constituents and evaluate the turnover of these carbon pools. Even though the water soluble organic C had a higher turnover rate than the microbial biomass C, the proportion of C4-derived C in the biomass was about 2.5 times greater than that in water soluble organic C. Apparently the large amount of native soil C, the small amount of water soluble organic C, and its equilibrium with the native soil C, cause humus to dominate the isotopic composition of water soluble organic C even though the water soluble C is very active. Our results suggest that the quantity, as well as the turnover rate, of soil organic matter constituents that are in equilibrium influence the isotopic composition of such constituents.

Effects of legumes on soil physical quality in a maize crop

The effect of intercropped legumes and three N fertilizer rates in a continuous maize (Zea mays L.) cropping system on the physical properties of two soils were investigated for three years. The legumes, being a mixture of alfalfa, clover and hairy vetch, had a significant cumulative effect on some physical properties of both soil. The lowest stability and smallest mean weight diameter of soil aggregates were associated with monoculture maize plots. Aggregate size and stability were not affected by N fertilization at any of the rates of 0, 70, and 140 kg ha-1 in intercropped plots, except that aggregate stability was actually reduced by N fertilization in one soil, the Ste. Rosalie clay. In maize plots in both soils, stability and size of soil aggregates were significantly increased with increased added N. Intercropped legumes significantly decreased dry bulk density and soil penetration resistance. Added N had no measurable influence on these compaction factors. Soil water properties were not significantly affected by either intercropping or N fertilization. Positive effects noted on soil aggregation and other physical properties in intercropped plots are the result of enhanced root activity, or incorporation of legumes as green manure, or both. Improvement of soil structure in maize plots associated with increasing N application was the result of increased maize-root residues.

Changes of phosphorous fractions under continuous corn production in a temperate clay soil

Limited efficiency of fertilizer P may be improved through an understanding of soil P fraction changes with time. This study examined sequential changes in soil organic P (Po) and inorganic P (Pi) in a Ste. Rosalie clay (Humic Gleysol; fine, mixed, frigid, Typic Humaquept) under continuous corn with and without P fertilization. Soil P was fractionated into Bicarb-Pi and Po, NaOH-1-Pi and Po, HCl-Pi, NaOH-Pi and Po, and Residue-P. In the non-P fertilized plots, soil total extractable Po declined by 14% of the initial value over five years of corn production, whereas soil Pi fractions were unchanged. The losses of soil Po were mainly from NaOH-1-Po. Added fertilizer P increased NaHCO3-Pi and NaOH-1-Pi in plots receiving 44 and 132 kg P ha-1 yr-1 and increased Residue-P in plots receiving 132 kg P ha-1 yr-1. Although NaOH-1-Po decreased slightly in the plots receiving 44 kg ha-1 yr-1 P fertilizer, total soil extractable Po was maintained in P fertilized plots. Mineralization of from 16 to 29 kg P ha-1 yr-1 Po was needed to account for soil Po losses. Bicarb-Pi and NaOH-1-Pi appeared to be most important for assessment of soil P fertility changes in long-term fertilized soils.

Short-term mineralization of maize residues in soils as determined by carbon-13 natural abundance

Studies of soil organic matter equilibria must include estimates of C turnover. The objective of this study was to provide data on how the natural 13C abundance method can be used to determine the flow of C from C4 residues and soil organic matter (C3-source) in a short-term incubation. Corn residue was added at a rate of 5.7 mg C g−1 soil to two soils, a clay and a sandy clay loam. During the course of a 35-day incubation in a CO2-free system, CO2-C and 13C natural abundance of the respired CO2 were measured. About 20% of the corn residue-C added was mineralized in both soils as determined from the CO2 respired and the 13C natural abundance of the respired CO2. Mineralization of the added residues was also calculated as the difference of the total amount of the respired CO2-C between the control and the corn residue-treated soils divided by the total amount of corn residue-C. Values were 35% for the clay soil, and 30% for the sandy clay loam soil. The difference in values calculated from the 13 C natural abundance and the difference method was due to mineralization of the indigenous soil organic C resulting from the addition of corn residues. Use of the natural 13C abundance method could determine the degree of ‘priming effect’ in soils amended with C4-C residues.

Fertilization rates and clay fixed ammonium in two Quebec soils

Clay fixed NH4+ can provide a significant sink for fertilizer N, as well as a source of N for plant uptake. Knowledge or soil NH4+ fixing capacity and release for crops is necessary to develop long-term fertilizer programs. Field experiments with corn (Zea mays L.) were carried out to investigate soil NH4+ fixing capacity and subsequent release as influenced by fertilizer rates using 15N in a Ste. Rosalie clay (fine, mixed, frigid, Typic Humaquept) and a Chicot sandy clay loam (fine-loamy, mixed, frigid, Typic Hapludalf). With high N rates increased NH4+ fixation occurred only in the Ste. Rosalie soil. At the end of the first growing season, fertilizer N recovery as clay fixed NH4+ for high and normal rates of fertilizer in the Ste. Rosalie soil was 17.8% and 28.7%, respectively and the recovery for the high and normal rates in the Chicot soil was 4.6 and 10.5%, respectively. Significant amounts of clay fixed NH4+-N were released in the soil profile in the second year after 15N application on the Chicot soil. Recently clay fixed fertilizer NH4+N was released more rapidly than that of the native fixed NH4+, from the surface layer of the Ste. Rosalie soil. The fertilizer fixed NH4+ seems to be in a more labile N pool than the native fixed NH4+-N in the Chicot soil.

Effects of shading and nitrogen on growth of corn (Zea mays L.) under field conditions

SummaryField studies were performed in the growing seasons of 1967 and 1968 to determine the differential response of vegetative corn growth to three levels of shading (27, 51, and 100% sunlight) and three levels of nitrogen (0, 222, and 444 kg/ha). Ammonium nitrate was used as the N fertilizer. Shades were removed when the unshaded plants started tasselling. Subsequent stover and grain production was determined.During the vegetative period, N increased dry matter yield per plant, and the rate of increase was enhanced at higher light levels. This was consistent in both years.The effect of shading during vegetative period on N use appeared to depend on season, or perhaps on planting date. In 1967, with a late planting date, the N-shading interaction was found in final stover and ear yields. In 1968, with a planting date in early spring shading-N interactions were noted at 8 1/2 weeks of growth but not at maturity.

Utilization of15N-urea fertilizer by irrigated wheat in Zambia

A two year field study was conducted to assess the efficiency of urea-N use under various irrigation schedules and fertilizer N placement methods and application times. The field studies were conducted during the cool and dry season (May–October) at the National Irrigation Research Centre, Nanga, Zambia on a typic Haplustalf. Broadcast applications of 150 kg N ha−1 resulted in higher utilization and efficiency of fertilizer N use compared to placement of fertilizer in a narrow band 2.5 cm to the side of the seeding row. Maximum fertilizer utilization and grain dry matter yields, however, were obtained with split application of urea-N of which the initial portion was broadcast and incorporated prior to sowing or broadcast two weeks after sowing and topdressing was applied at tillering under a weekly irrigation schedule at which irrigation was applied at 70 percent of the total class A pan evaporation during the whole irrigation interval. The percent N derived from fertilizer was independent of fertilizer placement at various water regimes, thus leading to the conclusion that under the experimental, conditions N utilization was primarily a function of water availability.

Effects of added organic residues and calcium carbonate on polyphosphate hydrolysis in four Quebec soils

Polyphosphate hydrolysis was studied in three surface samples and one subsurface sample of Quebec soil treated with alfalfa residues (44.8 t ha−1) and farmyard manure (FYM; 44.8 t ha−1); and in two acid soil samples treated with CaCO3 (12.5 t ha−1). The polyphosphates used were Na4P2O7. 10H2O (NaPP) and PolyN (a triammonium pyrophosphate-orthophosphate mixture). Each polyphosphate was added at a rate of 1.00g P kg−1 soil.Addition of alfalfa residues promoted polyphosphate hyrolysis of both NaPP and PolyN through reduced polyphosphate sorption and increasing pyrophosphatase activity. Application of FYM increased polyphosphate hydrolysis in Uplands topsoil and retarded hydrolysis in the other soils. Hydrolysis was probably reduced because of orthophosphate (OP) introduced with the FYM. Added CaCO3 accelerated polyphosphate hydrolysis in an acid topsoil sample through reduced sorption, but slowed hydrolysis in the subsoil sample, due to a reduction in enzyme activity.

Effects of light intensity and nitrogen level and source on growth of corn (Zea mays L.) in a controlled environment

SummaryThe growth response of corn (Zea mays L.) to two light intensities and two levels of nitrogen from ammonium and nitrate sources in nutrient solution was studied in a growth chamber under controlled humidity, photoperiod and temperatures.Light intensity and NO3-N increased the first 4-week vegetative growth of leaf, stalk and root, resulting in each case in a positive significant light intensity × NO3-N interaction.While light intensity enhanced the vegetative growth of the NH4-treated plants, NH4-N itself had no effect. No interaction was noted.