Effects of season, soil type and cropping on recoveries,

residues and losses of 15N-labelled fertilizer applied to

arable crops in spring
Authors: MACDONALD A. J. , POULTON P. R. , POWLSON D. S. , JENKINSON D. S. 
Published: July 2002

15N-labelled fertilizer was applied in spring

to winter wheat, winter oilseed rape, potatoes, sugarbeet

and spring beans in field experiments done in 1987 and 1988 in SE

England on four contrasting soil

types – a silty clay loam, a chalky loam, a sandy loam and a heavy
clay. The

15N-labelled fertilizers

were applied at recommended rates; for oilseed rape, a two-thirds

rate was also tested. Whole-crop

recoveries of labelled nitrogen averaged 52% for winter wheat, 45%

for oilseed rape, 61% for

potatoes and 61% for sugarbeet. Spring beans, which received only

2·5 kg ha−1 of labelled N, recovered 26%. Removals
of

15N-labelled fertilizer N in the harvested products were rather
less,

averaging 32, 25, 49, 27 and 13% in wheat grain, rape seed, potato

tubers, beet root and bean grain, respectively.

Crop residues were either baled and removed, as with wheat and rape

straw, or were flailed or

‘topped’ and left on the soil surface, as was the case

with potato tops and sugarbeet tops. Wheat

stubble and rape stubble, together with leaf litter and weeds, were

incorporated after harvest. The

ploughing in of crop residues returned 4–35% of the original

nitrogen fertilizer application to the soil,

in addition to that which already remained at harvest, which averaged 24,
29 and 25%

of that applied

to winter wheat, oilseed rape and sugarbeet respectively. Less remained

at harvest after potatoes

(c. 21%) and more after spring beans (c. 49%).

Most of the labelled residue remained in the top-soil (0–23cm) layer.

15N-labelled fertilizer unaccounted for in crop and soil

(0–100 cm) at harvest of winter

wheat, oilseed rape, potatoes, sugarbeet and spring beans averaged

23, 25, 19, 14 and 26% of that

applied, respectively. Gaseous losses of fertilizer N by denitrification

were probably greater following

applications to winter wheat and oilseed rape, where the N was applied

earlier (and the soils were

wetter) than with potatoes and sugarbeet. Consequently, it may well be

advantageous to delay the

application of fertilizer N to winter wheat and oilseed rape if the soil
is wet.

Total inorganic N (labelled and unlabelled) in soils

(0–100 cm) following harvest of potatoes given

15N-labelled fertilizer in spring averaged 70 kg N ha−1

and was often greater than after the

corresponding crops of winter wheat and oilseed rape, which averaged 53
kg

N ha−1 and 49 kg N ha−1,

respectively. On average, 91 kg ha−1 of inorganic N

was found in soil (0–100 cm) following spring

beans. Least inorganic N remained in the soil following sugarbeet, averaging

only 19 kg N ha−1. The

risk of nitrate leaching in the following winter, based on that which

remained in the soil at harvest,

ranked in decreasing order, was: spring beans=potatoes>oilseed rape=winter

wheat>sugarbeet.

On average, only 2·9% of the labelled fertilizer applied to

winter wheat and oilseed rape remained

in the soil (0–100 cm) as inorganic N

(NO−3+NH+4) at harvest;

with sugarbeet only 1·1% remained.

In most cases c. 10% of the mineral N present in the soil at

this time was derived from the nitrogen

fertilizer applied to arable crops in spring. However, substantially

more (c. 21%) was derived from

fertilizer following harvest of winter wheat infected with take-all

(Gaeumannomyces graminis var.

tritici) and after potatoes. With winter wheat and sugarbeet,

withholding fertilizer N had little effect

on the total quantity of inorganic N present in the soil profile at

harvest, but with oilseed rape and

potatoes there was a decrease of, on average, 38 and 50%, respectively.

A decrease in the amount of

nitrogen applied to winter wheat and sugarbeet in spring would therefore

not significantly decrease

the quantity of nitrate at risk to leaching during the following autumn

and winter, but may be more

effective with rape and potatoes. However, if wheat growth is severely

impaired by take-all, significant

amounts of fertilizer-derived nitrate will remain in the soil at

harvest, at risk to leaching.