Evaluation of an undisturbed-soil incubation method as an index of soil nitrogen availability in a limed and non-limed soil
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Evaluation of an undisturbed-soil incubation methodas an index of soil nitrogen availability in a limedand non-limed soilP. J. Raath a & D. Saayman ba Nietvoorbij Institute for Viticulture and Oenology, Agricultural Research Council ,Private Bag X5026, Stellenbosch , 7599 , Republic of South Africab Department of Soil and Agricultural Water Science , University of Stellenbosch ,Stellenbosch , 7600 , Republic of South AfricaPublished online: 15 Jan 2013.
To cite this article: P. J. Raath & D. Saayman (1996) Evaluation of an undisturbed-soil incubation method as an indexof soil nitrogen availability in a limed and non-limed soil, South African Journal of Plant and Soil, 13:2, 35-41, DOI:10.1080/02571862.1996.10634372
To link to this article: http://dx.doi.org/10.1080/02571862.1996.10634372
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S. Afr. J. Plant Soil 1996, 13(2)
Evaluation of an undisturbed-soil incubation method as an index of soil nitrogen availability in a limed and non-limed soif
P.J. Raath* Nietvoorbij Institute for Viticulture and Oenology, Agricultural Research Council, Private Bag X5026, Stellenbosch 7599,
Republic of South Africa
D. Saayman Department of Soil and Agricultural Water Science, University of Stellenbosch, Stellenbosch 7600, Republic of South Africa
Accepted 4 December 1995
Nitrogen mineralization potentials were determined using an undisturbed-soil incubation procedure. The pur-pose of this study was to evaluate the above-mentioned method as an index of soil nitrogen availability. Undis-turbed soil samples (0-150 and 150-300 mm) of a limed (pH 7.3) and an. equivalent non-limed (pH 4.9) soil were incubated at 30C and field water capacity. After every consecutive 10-day period, four samples of each treatment/depth combination were removed, dried and analysed for mineral N content. This was conducted over a period of 60 days. Distinction was made between changes in NH: and NO; contents of the incubated soil, assuming the NO; release represented N release from the soil due to high nitrification rates. The release of NO; was depicted by a straight-line regression, where the slope was equal to the rate constant of mineralization (k). The mineralization potential (No) was assumed to be the maximum mineral N content obtained in the soil during incubation. Comparing the amount of CO2 that evolved per unit NO; released, it was found that the min-eralizing microbial activity was lower, but more efficient in releasing N at pH 7.3 than at pH 4.9. Both ammonifi-cation and nitrification proved to be pH-sensitive processes. Although unsatisfactory for making fertilizer recommendations, the total N content of the soil was found to be a good indication of its N mineralization potn-tial. The method used in this study gave no indication of the fraction of No that is released during the different seasons. The use of undisturbed samples makes this method cumbersome because of the large number of samples needed to accommodate a large spatial variability component.
Stikstofmineralisasiepotensiaal is ondersoek deur gebruik te maak van 'n onversteurde grondinkubasietegniek. Die doel van hierdie studie was om laasgenoemde tegniek te evalueer as 'n indeks van die stikstoflewerings-vermoe van grond. Onversteurde grondmonsters (0-150 en 150-300 mm) van 'n bekalkte (pH 7.3) en ooreen-stemmende onbekalkte (pH 4.9) kontrole-grond is by 30C en veldwaterkapasiteit ge'inkubeer. Na elke opeenvolgende peri ode van 10 dae is vier monsters, verteenwoordigend van elke behandelinglgronddiepte-kombinasie, verwyder, gedroog en geanaliseer vir minerale N-inhoud. Dit is oor 'n periode van 60 dae gedoen. Onderskeid is tussen die veranderinge van die ge'inkubeerde grond se NH: - en NO; - inhoude gemaak. Die NO; - vrystelling is as verteenwoordigend van die N-vrystelling in die grond aanvaar weens die hoe nitrifikasie-tempo's wat verkry is. Dit is deur 'n reguitlynregressie voorgestel waarvan die helling gelyk was aan die miner-alisasiekonstante (k). Die maksimum minerale N-inhoud wat in die grond tydens inkubasie waargeneem is, is aanvaar om gelyk aan die mineralisasie-potensiaal (No) te wees. Waar die hoeveelheid CO2-vrystelling per een-heid vrygestelde NO; in ag geneem is, is gevind dat die mineraliserende mikrobe-aktiwiteit laer, maar meer effektief ten opsipte van N-vrystelling was by pH 7.3 as by pH 4.9 grond. Ammonifikasie blyk ook, tesame met nitrifikasie, 'n pH-sensitiewe proses te wees. Hoewel totale N-inhoude van die grond 'n onbetroubare riglyn vir N-vrystelling in die grond was, om as riglyn vir bemestingsaanbevelings te dien, gee dit 'n goeie indikasie van die grond se N-mineralisasiepotensiaal. Die inkubasiemetode wat in hierdie studie gevolg is, gee geen aandui-ding van die fraksie van No wat tydens die verskillende seisoene vrygestel word nie. Die gebruik van onver-steurde monsters maak hierdie metode omslagtig as gevolg van die groot hoeveelheid monsters wat nodig is om die groot ruimtelike variasie te akkommodeer.
Keywords: Incubation, limed soil, mineral nitrogen, nitrogen mineralization potential, nitrogen release rate
*To whom correspondence should be addressed tpart of an M.Sc. (Agric.) dissertation to be submitted by the senior author to the University of Stellenbosch
Soil acidity is often regarded as a factor which restricts the rate at which organic N is converted to mineral N (Harmsen & van Schreven, 1955). Other evidence, however, indicates that soil acidity has no negative effect on the mineralization of organic N (Thompson, Black & Zoellner, 1954). According to Nyborg & Hoyt (1978), Black (1956) suggested that when acid soils are limed, a portion of the soil organic matter
becomes more susceptible to mineralization, but after this portion has been decomposed, the rate of mineralization returns to near its original level, despite an altered composi-tion of the soil microbial population.
Nitrification, in particular, is sensitive to soil acidity (Harmsen & van Schreven, 1955). Nyborg & Hoyt (1978) found that liming of soil stimulates the activity of nitrifying bacteria. With time, however, bacterial adaptation occurred,
and no difference was obtained between the activities of nitri-fying bacteria in soils limed to different pH values. During incubation of disturbed soil, liming increased nitrification such that practically all NH: - N in the limed samples was nitrified during the incubation period (Lyngstad, 1992). The effect of liming also decreased from the first year until it finally disappeared after the sixth year. In a loam soil, liming hardly had any effect, while in a -clay loam soil, rich in organic matter, liming significantly increased the mineraliza-tion rate and the effect lasted for several years. Further liming of acid, organic soils may result in large amounts of N being mineralized, causing luxurious consumption of N by crops, while in mineral soils with little organic matter, liming can seldom replace fertilizer application. It is therefore difficult to predict the effect of liming on N mineralization since many divergent results for different soils have been obtained (Lyngstad, 1992).
Several studies to evaluate N mineralization were based on laboratory or field incubation of disturbed soil samples (Stan-ford & Smith, 1972; Smith, Young & Miller, 1977; Campbell, lame & Winkleman, 1984). Raison, Connel & Khanna (1987), Cabrera & Kissel (1988), Hadas et al. (1989) and Sierra (1992) found that N mineralization of undisturbed and disturbed samples were not in good agreement. N mineraliza-tion was h