A function for describing nitrogen uptake, dry matter production and rooting by wheat crops

Abstract

The quantities, Y, of nitrogen taken up, and dry matter produced, at various times during the growth of six winter wheat crops at Rothamsted were shown to be related to thermal time, x, based on soil temperature, by a simple equationwhere A is the ultimate maximum of Y, n a shape factor and k a rate constant that is related to A and n through x', the inflexion point of the function. The value of n was 1.5 for both N uptake and dry matter. The value of A for N uptake, Au, was well described by a multiple regression on sowing data, t,, expressed as the number of days after August 31 st, and rainfall, R~pr, in the April before harvest, but no such regression could be found for the value of A for dry matter, AD. The rate constants, ks and ko, for N-uptake and dry-matter production respectively, could be related to the date of sowing and the weather through the corresponding inflexion points,x'N and x'o. Highly significant regressions were found, for x'u on the time, ts:, between sowing and the return of the soil to field capacity and for x'o on the reciprocal of t~. The function was used to generate N uptake curves from values of Au and ku (obtained from x'u) given by inserting the appropriate values of RApt, t~ and t,:in the regressions. These fitted measured N uptakes satisfactorily for the six crops used to obtain the regressions, and four grown subsequently, at Rothamsted, and also for six crops at Woburn. Values of Ao had to be set arbitrarily because no regression had been found to predict them, but using these arbitrary values in the function gave dry matter curves that fitted the measurements satisfactorily for all ten Rothamsted crops and two of the Woburn crops. Tests for seasonal and site effects showed that Au was influenced more by differences between the two sites than by seasonal differences other than those in RApt, whereas Ao was strongly influenced by these seasonal differencess and very little by those between the sites.

Estimating soil strength in the rooting zone of wheat

“…The characteristic rooting depth, d 63 , is that which contains the fraction (e-1)/e, where e is the base of Naperian logarithms, (i.e. 63% of the roots) and increases with thermal time as described by Whitmore and Addiscott (1987). The maximum value that this parameter can take, Md 63 was set at 43 cm depth for winter wheat.…”

Simulating nitrate concentrations in water draining from a small arable catchment in England

“…The SLIMMAC model provided simulations of processes in the soil profile. This model combines the SLIM leaching model of Addiscott & Whitmore (1991) with a simple model for mineralization and the equation for uptake of nitrogen by the crop given by Whitmore & Addiscott (1987). The SLIM leaching model SLIM is a layer model that allows for three categories of water in the soil.…”

“…The model needs soil temperature data, but the soil water content information is supplied by the leaching model. Nitrogen uptake by the crop The uptake of nitrogen is simulated using the equation of Whitmore & Addiscott (1987), Y A Àlan e Àfx Àn 1 where x is time or thermal time, A the ultimate maximum of Y, n a shape factor and f a rate constant that is related to A and n through x H , the point of inflexion of the function.Yand A can represent dry matter production or nitrogen uptake, and we are concerned with the latter here. In the model, nitrogen uptake can be restricted by a shortage of crop-available nitrogen or water.…”

“…In the model, nitrogen uptake can be restricted by a shortage of crop-available nitrogen or water. For winter wheat, A (kg ha 71 ) can be estimated from the yield of grain on the basis of the widely-used observation that 23 kg of N are taken up for each tonne of grain produced, n is 1.5, and the point of inflexion, from which f can be estimated, depends on the number of days between sowing and the return of the soil to field capacity (Whitmore & Addiscott, 1987). Whitmore (1988) used a slightly different approach for winter barley.…”

Modelling the change in soil organic C and N and the mineralization of N from soil in response to applications of slurry manure

“…Addiscott and Whitmore (1987) assessed the goodness of fit of their model with reference to the mean difference between simulation and measurement and whether or not this was significantly different from zero, the correlation between measurement and simulation and the number of simulations lying within a specified range greater than or less than the observations: for example 5 tons C ha -1. The mean difference assesses whether or not the simulations differ systematically from the measurements and the correlation coefficient estimates how well measurement and simulation are associated with one another throughout the full range of the measurements.…”