SUMMARYMeasurements of profiles of ferrous and ferric iron and pH in blocks of reduced soil in contact with planar layers of rice {Oryza sativa L.) roots are reported. Initially 11-d-oId plants were kept in contact with the soil for up to 12 d. Over this period, substantial quantities of iron were transferred towards the root plane, producing a welldefined zone of ferric hydroxide accumulation. The pH in this zone fell by more than two units. The profiles changed with time. The decrease in pH was in part due to protons generated in ferrous iron oxidation, and in part due to protons released from the roots to balance excess intake of cations over anions, N being taken up chiefly as NH/. But the decrease in pH was less than expected from the net acid production in these two processes, possibly because of proton consumption in CO., uptake by the roots. Because of the pH-dependence of soil acidity diffusion, the two sources of acidity greatly reinforce each other. Some implications for nutrient and toxin dynamics are discussed.

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Nitrous oxide (N2O) produced from agricultural activities must be determined if management procedures to reduce emissions are to be established. From 1994 to 1996, N2O emissions were determined using a closed chamber technique. Continuous corn (Zea mays L.) at four N rates of 0, 170, 285, and 400 kg of N ha−1 was used on a Ste. Rosalie heavy clay (a very‐fine‐silty, mixed, nonacid, frigid Typic Humaquept) and a Chicot sandy loam (a fine‐loamy, frigid, Typic Hapludalf). On two additional sites, a Ste. Rosalie clay and an Ormstown silty clay loam (a fine‐silty, mixed, nonacid, frigid Humaquept) no‐till (NT) and conventional tillage (CT); monocultural corn (CCC), monocultural soybean (Glycine max L.) (SSS); corn‐soybean (SSC, CCS); and soybean‐corn‐alfalfa (Medicago sativa L.) phased rotations (SAC, CSA, and ACS) were used. Nitrogen rates of 0, 90, and 180 kg of N ha−1 for corn and 0, 20, and 40 kg of N ha−1 for SSS were used. Rates of N2O emission were measured from April to November in 1994 and 1995, and from mid‐March to mid‐November in 1996. Maximum N2O emissions reached from 120 to 450 ng of N m−2 s−1 at the Ormstown site to 50 to 240 ng of N m−2 s−1 at the Ste. Rosalie soil. Generally, N2O emissions were higher in the NT systems, with corn, and increased linearly with increasing N rates, and amounted to 1.0 to 1.6% of fertilizer N applied. The N2O emission rates were significantly related to soil denitrification rates, water‐filled pore space, and soil NH4 and NO3 concentrations. A corn system using conventional tillage, legumes in rotation, and reduced N fertilizer would decrease N2O emission from agricultural fields.

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clay-(Humic Gleysol). Under fivourable heat units and rainfall, yields of grain corn were quadratic functions of fertilizer N rates on both soili. Maximum grain yields of corn were calculated to requiri from 300 to 350 kg N ha-r fertilizer application. However, large amounts of N fertilizer at both 285 kg ha -r and 400 kg ha-t were not recovered by the crop. Optimum economic grain yields at a price ratio of 10:1 N:grain were found at273 kg N ha t on the Ste. Rosalie site in 1990, and 286 and t;9'i.;"N il;ti-ioi iSsg uno 1990, respectiveiy, on the Chicot site. A normal fertllizer rute (170-44-141 kg N-P-K ha-r) and a high-fertilizer rate (400-132-332kgN-f-f ni-t) were used in 1989

. 1997. Nitrous oxide emission as affected by tillage, corn-soybean-alfalfa rotations and nitrogen fertilization. Can. J. Soil Sci. 77: 145-152. Nitrous oxide (N 2 O) produced from agricultural activities represents a threat to the ozone layer and economic losses. Rates and magnitudes of N 2 O emissions of cropping systems must be determined to establish corrective management procedures. In 1994, N 2 O emissions were determined with corn (Zea Mays L.) and corn-legume rotations. Continuous corn was studied on four soils, two from a long-term experiment, a Ste. Rosalie heavy clay (Humic Gleysol) and a Chicot sandy loam (Grey-Brown Podzol), at 0, 170, 285 or 400 kg N ha -1 , and two from a corn rotation study, a Ste. Rosalie clay (Humic Gleysol) and an Ormstown silty clay loam (Humic Gleysol). Treatments included no-till (NT) and conventional tillage (CT), monoculture corn (CCCC), monoculture soybean; corn-soybean; and soybean-corn-alfalfa phased rotations. Nitrogen rates of 0, 90, or 180 kg N ha -1 for corn and 0, 20, or 40 kg N ha -1 for continuous soybean were used, and soybean/alfalfa following corn no fertilizer N. Rates of N 2 O emission were measured from closed chambers through the growing season. About 0.99 to 2.1% of N added was lost as N 2 O. Nitrous oxide emission increased with increased soil water content, NO 3 concentration and fertilizer N rates. Emission of N 2 O was higher with NT than with CT, and with corn than with soybean or alfalfa. A corn system using CT, legumes in rotation and moderate fertilizer N would reduce N 2 O emission.

Corn (Zea mays L.) monoculture is a common management practice due to high production levels with economic and agronomic advantages. Phosphorus reactions in soil affect sustainable, economic corn management and environmental quality. Our objectives were to quantify changes in soil P fractions as related to additions of manure and inorganic P with time using a modified Hedley fractionation scheme. A field experiment was conducted on a Chicot sandy clay loam soil (fine‐loamy, mixed, frigid Typic Hapludalf) from 1984 to 1993. With an annual rate of 60 kg P ha−1 manure P plus 44 kg P ha−1 inorganic P, labile inorganic P (Pi; Bicarb‐Pi) increased and moderately labile Pi (NaOH‐Pi) decreased. A higher annual P rate of 132 kg P ha−1 plus manure P increased soil Pi through Bicarb‐Pi, NaOH‐Pi, and stable Ca‐bound Pi (HCl‐Pi). Inorganic P at either rate combined with manure P decreased labile Bicarb‐Po fractions and increased moderately labile NaOH‐Po, with the result that total extractable soil organic P (Po) increased. Without manure, 44 kg P ha−1, approximately equal to P removal by the crop, resulted in no change in soil P fractions except for residual P (Res‐P), which decreased. Soil Po and Pi levels increased continuously in soil fertilized with 132 kg P ha−1 yr−1 inorganic P only. With the manure‐inorganic system, 86% of the Bicarb‐Pi was supplied from added inorganic P, indicating reduced P fixation compared with the inorganic P system. The NaOH‐Pi and Res‐P were soil P sinks, with Res‐P being supplied from both the NaOH‐Po and HCl‐Pi fractions. In contrast, NaOH‐Pi in the inorganic P system was the major source for Bicarb‐Pi.