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Experiments with ' 5N labelled fertilizers often show that plants given fertilizer N take up more N from the soil than plants not given N-the priming effect or 'added nitrogen interaction' (ANI). This paper is a theoretical study of ANIs and how they can affect the interpretation of experiments with "N labelled fertilizers. ANIs can be 'real', if for example, fertilizer N increases the volume of soil explored by roots, or 'apparent', caused by pool substitution or by isotope displacement reactions. Pool substitution is the process by which added labelled N stands proxy for native unlabelled N that would otherwise have been removed from that pool. Microbial immobilization of N, whether driven by the decomposition of soil organic matter or by the decomposition of plant roots, can lead to pool substitution and is the dominant cause of apparent ANIs. Denitrification and plant uptake of N can also, under special circumstances, lead to pool substitution and thus give rise to apparent ANIs. Isotope displacement reactions, in which the added labelled N displaces native unlabelled N from a 'bound' pool, can lead to apparent ANIs but are only likely to be of significance in exceptional circumstances.The relationship between ANIs, 'A' values and N fertilizer uptake efficiencies are examined by means of a simple model for uptake of "N-labelled fertilizer by a cnp. A positive 'apparent' AN1 is accompanied by an 'A' value that changes as fertilizer applications increase. Likewise, a positive 'apparent' AN1 also causes fertilizer uptake efficiency to appear lower when measured by the uptake of I5N than when measured by the non-isotopic 'difference' method. I N T R O D U C T I O NNitrogen-15 labelled fertilizer is often used in experiments on the fate of fertilizer nitrogen, one of the aims being to distinguish between soil-derived (unlabelled) nitrogen and fertilizer-derived nitrogen. It is commonly (but not always, e.g. Leitch & Vaidyanathan, i983) observed in such experiments that plants given fertilizer N take up more unlabelled N from the soil than plants receiving no fertilizer N. Such an increase in N derived from soil following fertilizer additions is sometimes referred to as a 'priming' effect (Hauck & Bremner, 1976), a term introduced by Bingeman et al. (1953) in a paper about the effects of the addition of fresh organic material on the decomposition of organic matter already in the soil. The presence of plants is not essential for a priming effect; more unlabelled inorganic N is often observed in soil incubated with labelled inorganic N than in the corresponding soil incubated without labelled N. There has been considerable controversy over the cause and interpretation of this phenomenon (Broadbent

A whole crop computer simulation model of winter wheat has been written in FORTRAN and used to simulate the growth of September-and October-sown crops of Hustler wheat at Rothamsted for the years 1978-9, 1979-80 and 1980-1. Results of the simulations, which are for crops with adequate water and nutrients, are compared with observations from experiments at Rothamsted. The model uses daily maximum and minimum temperatures and daylength to calculate the dates of emergence, double ridge, anthesis and maturity of the crops and the growth and senescence of tillers and leaves. In the simulations, the canopy intercepts daily radiation and produces dry matter that is partitioned between roots, shoots, leaves, ears and grain. Partial simulations, using observed LAI values, produced dry matter in close agreement with observations of late-sown crops, but consistently overestimated the total dry-matter production of the early-sown crops. Full simulation described satisfactorily the average difference in dry-matter production to be expected with changes in time of sowing, but did not give as close correspondence for individual crops. A grain growth submodel, that linked maximum grain weight to average temperatures during the grain growth period, correctly simulated the observed growth of individual grains in the 1981 crop. The benefits to be obtained by combining whole crop modelling with detailed crop observations are discussed.

The uptake of labelled and unlabelled N by wheat was measured in pot and field experiments with "N-labelled fertilizer. Soils from two sites on the same series were used in the pot experiment; one had been bare-fallowed for 22 years and contained 1.6% organic C, the other had been under grass for many years and contained 3.8% organic C.Fertilizer N increased the uptake of unlabelled soil N in both soils, i.e. there was a positive 'added nitrogen interaction' (ANI). There was no AN1 in the field experiment. A simulation model is used to show how positive ANIs can arise as a result of 'pool substitution'-labelled inorganic fertilizer N standing proxy for unlabelled inorganic soil N that would otherwise have been immobilized. In the low-organic fallow soil, pool substitution accounted for the whole of the observed AN1 and fertilizer N did not enhance either gross or net mineralization of soil N. Pool substitution also operated in the high organic grassland soil, but here net mineralization of soil N increased with increasing additions of fertilizer, giving rise to a 'real' AN1 in addition to the larger 'apparent' AN1 caused by pool substitution. This increase in net mineralization is probably caused by a decrease in immobilization of N as fertilizer N additions increase, not by an increase in gross mineralization of soil N. For pool substitution to operate, fertilizer N and soil inorganic N must occupy the same pool. This occurred in the pot experiment but not in the field experiment, where fertilizer and soil inorganic N remained separate and there was no ANI. When pool substitution occurs, fertilizer use efficiency is predictably lower as measured by the isotopic method than as measured by the conventional non-isotopic procedure.

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