In order to determine the effect of liming on grass growth, the chemical composition of the soil solution and microbial activities were analyzed. Ground limestone (CaCO.) was topdressed at the rates of 0, 2, 4, and 8 t/ha to a 9 year old orchardgrass sward. The effects of liming on the soil were observed only in the surface layer (0-5 cm) during the 6 month period after lime application. Although the yield of grass was not distinctly affected by Iiming, the amount of phosphorus (P) and calcium (Ca) absorbed by the grass was considerably increased. With increasing rates in lime application, the concentrations of Ca and sulCate (S), and the value of the pH in the soil solution (0-2 cm layer) were increased, while the concentrations of a1uminum (AI), P, and nitrogen (N) were decreased. No change was observed in the chemical composition of the soil solution from the layers below 5 cm. The microbial activities 2,3,5-triphenyltetrazoJium chloride (TTC)-reducing activity. fructose or urea-decomposing activity, and bacterial numbers) were increased by liming.
Nitrogen mineralization properties of major soils in the Tokachi district of Hokkaido in northern Japan were investigated by the kinetic analytical method. Soil samples collected in spring and autumn from plots with tillage experiments consisting of a Brown Andosol (U.S. Soil Taxonomy: Typic Hapludands) and Gleyic Cumulic Andosol (Typic Meranaquands), as well as from a Brown Lowland soil (Typic Udifulvents) were incubated at 20, 25, and 30~ for 196-441 d and the time course of nitrogen mineralization was analyzed.In some soils, the time course of nitrogen mineralization fitted the first order reaction model, while in others it fitted the zero order reaction model. There was no distinct relationship between the nitrogen mineralization parameters (NMPs) and the soil properties. Since the cumulative amounts of mineralized nitrogen of all the studied soils showed a linear relation with the incubation time up to the number of days transformed at standard temperature (DTS) in a crop growing season, the data of the 0, 2, 4, and 8 week incubation experiments were analyzed by the zero order reaction model, which enabled to define the nitrogen mineralization properties of each soil type. The Brown Andosol showed a smaller k value (mineralization rate constant) at 25~ (0.241-0.303 mg kg -1 d -1) and an average Ea value (apparent activation energy) (3,270-4,610 J tool -~) in conventional tillage. The Gleyic Cumulic Andosol exhibited a larger k value at 25"C (0.250-0.379 mg kg -~ d -~) and a larger Ea value (4,350-5,040 J mol -~) in conventional tillage, while the Brown Lowland soil showed a larger k value at 25~ (0.330-0.423 nag kg -~ d -~) and a smaller E, value (3,010-3,300 J mol-1). Also, reduced tillage resulted in a larger k value for the Brown Andosol compared with the conventional tillage. No significant difference in the NMPs between the spring samples and autumn samples was detected.
Summary Plants from agricultural and natural upland ecosystem were investigated for lSN content to evaluate the role of symbiotic N~-fixation in the nitrogen nutrition of soybeans.Increased yields and lower 61SN values of nodulating soybeans vs. non-nodulating isolines gave semi-quantitative estimates of N 2 fixation. A fairly large discrepancy was found between estimations by 6~SN and by N yield at 0kg N/ha of fertilizer. More precise estimates were made by following changes in plant 6~SN when fertilizer 6~5N was varied near ~SN natural abundance level Clearcut linear relationships between ~SN values of whole plants and of fertilizer were obtained at 30kg N/ha of fertilizer for three kinds of soils. In experimental field plots, nodulating soybeans obtained 13 -+ 1% of their nitrogen from fertilizer, 66 +-8% from N 2 fixation and 21 -+ 10% from soil nitrogen in Andosol brown soil; 30%, 16% and 54% in Andosol black soil; 7%, 77% and 16% in Alluvial soil, respectively. These values for N 2 fixation coincided with each corresponding estimation by N yield method.Other results include: 1) ~SN content in upland soils and plants was variable, and may reflect differences in the mode of mineralization of soil organics, and 2) nitrogen isotopic discrimination during fertilizer uptake (6~N of plant minus fertilizer) ranged from --2.2 to + 4.9~ at 0-30 kg N/ha of fertilizer, depending on soil type and plant species. The proposed method can accurately and relatively simply establish the importance of symbiotic nitrogen fixation for soybeans growing in agricultural settings.
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