Evidence of carbon stimulated N transformations in grassland soil after slurry application
Introduction
Slurry applied to soil affects the major nitrogen (N) and carbon (C) flows in soil (Stevens and Laughlin, 1997a). Decomposition of the volatile fatty acids (VFA) in the slurry leads to a flush of mineralization (Sørensen, 1998). Another obvious effect of slurry application to soil is the stimulation of the nitrification activity (Flowers and O'Callaghan, 1983). Nitrification can be performed by chemoautotrophic or heterotrophic organisms (de Boer and Kowalchuk, 2001). Autotrophic nitrification is carried out by chemolithotrophic bacteria which are obligate aerobes and gain energy from oxidation of NH4+ to use CO2 as a source of C for biomass synthesis. Heterotrophic nitrification is carried out by bacteria or fungi which can also oxidise reduced N compounds but use organic C for biomass synthesis. Heterotrophic nitrifiers can oxidise either mineral (NH4+) or organic N and apparently do not obtain any energy from this process (Paul and Clark, 1996). The nitrification rate of NH4+ in pig slurry (Cooper, 1975, Flowers and O'Callaghan, 1983) and NH4+ with added peptone (Kreitinger et al., 1985, Adams, 1986) has been shown to be faster than the nitrification rate when using NH4+ salts.
To better understand the mechanisms of N transformations in soil and separate autotrophic from heterotrophic nitrification Barraclough and Puri (1995) developed a labelling technique where either the NH4+ or NO3− pool had to be labelled. The dilution of the labelled NO3− pool was assumed to account for autotrophic plus heterotrophic nitrification and the enrichment of the NO3− pool whenever the NH4+ pool was labelled was considerd to be due to autotrophic nitrification. This method has so far only been carried out in soil supplied with labelled fertilizer and not after treatment with slurry. The organic N contained in the slurry may not only promote higher mineralization rates but also cause enhanced heterotrophic nitrification (Flowers and Arnold, 1983, Flowers and O'Callaghan, 1983, Stevens and Laughlin, 1997a).
The aim of this laboratory study was to determine the effect of cattle slurry on the oxidation of mineral NH4+-N and organic-N by labelling the NH4+ or NO3− pools separately or both together with 15N. The size and enrichment of the mineral N pools were determined at intervals during a laboratory incubation and the data were used to calculate gross nitrification rates. Analytical solutions exist for determining individual N transformation rates from 15N data (Kirkham and Bartholomew, 1954, Kirkham and Bartholomew, 1955, Barraclough, 1991, Davidson, 1991). However, the application of numerical models is necessary to determine simultaneously the rate of more than three transformations (Nishio et al., 1985, Bjarnason, 1988, Nason and Myrold, 1991). Available 15N tracing models base the transformations within the model system on either zero- or first-order kinetics and determine N rates by simultaneous optimization of all model parameters (Mary et al., 1998).
In this study we used a 15N tracing model which allows the simultaneous calculation of nitrification, mineralisation and immobilization (NH4+ and NO3− separately) in soil where fertilizer or slurry had been applied. In the fertilizer and slurry treatments, the pool sizes of NH4+ and NO3− were the same but the slurry supplied extra organic carbon. The NH4+ and NO3− pools were either singly or doubly labelled with 15N. Sub-models for organic matter decomposition of the organic matter pool in the soil and in the slurry were included and were linked to the N transformations.
Section snippets
Soil
The soil, an acid brown earth (Typic Dystrochrept) with a pH of 6.0 and containing 48% sand, 31% silt, 20% clay and 12% organic matter was collected (0–10 cm) from a site at the Agricultural Research Institute of Northern Ireland, Hillsborough, County Down in June 1998. The sward was dominated by perennial ryegrass and had been under a 3-cut silage regime receiving 300 kg N ha−1 yr−1 for the previous 6 years. The soil was sieved (<6 mm), and stored at 4 °C for a week at field moisture before being
Size and enrichment of the ammonium pool
In the control treatment the NH4+ pool size did not change over the 7 day period but in the slurry treatment, the NH4+ pool size decreased significantly (P<0.05) with time. There was no significant difference between any of the labelled treatments with time. In the control treatment the enrichment of the NH4+ pool did not change with time when the pool was labelled, the enrichment stayed at a constant value of 0.52 (Fig. 2). There was no significant enrichment of the NH4+ pool when the NO3−
Nitrification
The application of cattle slurry increased the rate of nitrification from 0.04 μmol N g−1 d−1 in the control treatment to 0.78 μmol N g−1 d−1 in the slurry treatment (Fig. 4). While the control treatments showed almost no change in the pool sizes and enrichment of the NH4+ and NO3− pools, an obvious change occurred in all of them in the slurry treatments (Fig. 2, Fig. 3). This increase in nitrification was obviously associated with the cattle slurry itself and has been reported by other researchers (
Conclusions
The application of cattle slurry to a grassland soil dramatically stimulated the nitrification of NH4+ to NO3− and had no significant effect on the oxidation of organic N. Separation of the nitrification of NH4+ between autotrophs and heterotrophs is impossible without the use of effective selective inhibitors. High rates of nitrification would be ecologically significant as the exchangeable cation, NH4+, is converted to the mobile ions, NO2− and NO3−, which can be rapidly denitrified or easily
Acknowledgements
This project was partly supported by a grant from the Deutsche Forschungsgemeinschaft (German Science foundation).
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