ALBERT

Description: SUMMARY15N-labelled nitrogen fertilizer (containing equal quantities of ammonium-N and nitrate-N) was applied in 4 consecutive years (1980–3) to different microplots located within the Broadbalk Wheat Experiment at Rothamsted, an experiment which has carried winter wheat continuously since 1843. Plots receiving 48, 96, 144 and 192 kg N/ha every year were given labelled fertilizer in mid-April at (nominally) these rates.Grain yields ranged from 1–2 t/ha on plots given no N fertilizer since 1843 to a maximum of 7·3 t/ha with 196 kg N/ha. On plots given adequate P and K fertilizer, between 51 and 68% of the labelled N was recovered in the above-ground crop; only about 40% was recovered where P deficiency limited crop growth. In 1981 fertilizerderived N retained in soil (0–70 cm) at harvest increased from 16 kg/ha, where 48 kg/ha was applied, to 38 kg/ha, where 192 kg/ha was applied. More than 80% of this retained N was in the plough layer (0–23 cm).Overall recovery of fertilizer N in crop plus soil ranged from 70 % to more than 90 % over the 4 years of the experiments. Losses of N were larger in years when spring rainfall was above average and when soil moisture deficits shortly after application were small.Crop uptake of unlabelled N derived from soil increased from 28 kg N/ha on the plot given no fertilizer N to 67 kg N/ha on the plot given 144 kg N/ha. The extra uptake of unlabelled N was mainly, if not entirely, due to greater mineralization of soil N in the plots that had been given N fertilizer for many years. Presumably fertilizer N increased the annual return of crop residues, which in turn led to an accumulation of mineralizable organic N, although there was only a small increase in total soil N content.Wheat given NH4-N grew less well and took up less N than wheat given N08-N in the relatively dry spring of 1980; there was little difference between the two forms of N in the wetter spring of 1981. In both years more fertilizer N was retained in the soil at harvest when fertilizer was applied as NH4-N than as N03-N.The N content of the soil in several plots of the experiment has been constant for many years, so that the annual removal of N is balanced by the annual input. A nitrogen balance for the plot given 144 kg fertilizer N/ha showed an average annual input of non-fertilizer N of at least 48 kg/ha, of which N in rain and seed accounts for about 14 kg/ha. The remainder may come from biological fixation of atmospheric N2 by blue-green algae, or from dry deposition of oxides of nitrogen and/or NH3 onto crop and soil. The overall annual loss of N from the crop–soil system on this particular plot was 54 kg N/ha per year, 28% of the total annual input from fertilizer and nonfertilizer N.

Description: SUMMARY15N-labelled fertilizer was applied, in spring, to winter wheat crops in nine experiments in eastern England over a period of 4 years. Five were on Batcombe Series silty clay loam, two on Beccles Series sandy clay loam (with a mole-drained clay subsoil) and two on Cottenham Series sandy loam. In three of the experiments, different rates of fertilizer N were applied (up to 234 kg N/ha); in the others, a single rate (between 140 and 230 kg/ha) was used.Recovery of fertilizer N in the above-ground crop (grain, chaff, straw and stubble) ranged from 46 to 87% (mean 68%). The quantity of fertilizer N retained in the soil at harvest was remarkably constant between different experiments, averaging 18% where labelled N was applied as 15NH415NO3, but less (7–14%) where K16NO3 was applied. Of the labelled N present in soil to a depth of 70 cm, 84–88% was within the cultivated layer (0–23 cm).L70 = 5(± 1 63) + 0·264(±00352) R3accounted for 73% of the variation in the data where: L70 = percentage loss of fertilizer N from the crop: soil system, defined as percentage of labelled N not recovered in crop or in soil to a depth of 70 cm at the time of harvest; R3 = rainfall (in mm) in the 3 weeks following application of N fertilizer.There was a tendency for percentage loss of fertilizer N to be greater when a quantity of N in excess of that required for maximum grain yield was applied. However, a multiple regression relating loss both to rainfall and to quantity of N applied accounted for no more variance than the regression involving rainfall alone. In one experiment, early and late sowing were compared on the first wheat crop that followed oats. The loss of N from the early-sown crop, given fertilizer N late in spring, was only 4% compared with 26 % from the later-sown crop given N at the same time, so that sowing date had a marked effect on the loss of spring-applied fertilizer N.Uptake of unlabelled N, derived from mineralization of organic N in soil, autumn-applied N (where given) and from atmospheric inputs, was 〈 30 kg/ha on a low organic matter (0·08% total N) sandy soil but 〉 130 kg/ha when wheat followed potatoes or beans on soil containing c. 0·15 % total N. Unlabelled N accounted for 20–50% of the total N content of fertilized crops at harvest. About 50% of this unlabelled N had already been taken up by the time of fertilizer application in spring and the final quantity was closely correlated with the amount present in the crop at this time. Applications of labelled fertilizer N tended to increase uptake of unlabelled N by 10–20 kg/ha, compared to controls receiving no N fertilizer. This was probably due to pool substitution, i.e. labelled inorganic N standing proxy for unlabelled inorganic N that would otherwise have been immobilized or denitrified.

Description: SUMMARYWhen 15N is used to trace the fate of N fertilizer applied in spring to winter wheat crops, some is not recovered in the crop or the soil and has to be presumed lost. In 13 experiments made from 1980 to 1983 on three widely differing soils, these losses ranged from 1 to 35%. We partitioned them between leaching and denitrification by using models to estimate the loss by leaching, talcing into account the N absorbed by the crops, and subtracting this loss from the total loss to obtain the apparent percentage loss by denitrification, LDN. An analysis of variance showed that LDN increased significantly with the quantity of N applied, so the study considered LDN values for a standard N application of 150 kg/ha subsequently. Regressions showed that LDN was better related to the wetness of the soil during the 3 weeks after fertilizer application than to the corresponding amount of rain, as would be expected for denitrification. Values of LDN could not, however, be satisfactorily related to soil temperature, probably because the range of temperatures was too narrow. The apparent losses by denitrification were, on average, nearly twice as large as those by leaching, but the ratio varied greatly between experiments.

Description: SUMMARYThe Broadbalk Wheat Experiment at Rothamsted (UK) includes plots given the same annual applications of inorganic nitrogen (N) fertilizer each year since 1852 (48, 96 and 144 kg N/ha, termed N1 N2 and N3 respectively). These very long-term N treatments have increased total soil N content, relative to the plot never receiving fertilizer N (N0), due to the greater return of organic N to the soil in roots, root exudates, stubble, etc (the straw is not incorporated). The application of 144 kg N/ha for 135 years has increased total soil N content by 21%, or 570 kg/ha (0–23 cm). Other plots given smaller applications of N for the same time show smaller increases; these differences were established within 30 years. Increases in total soil N content have been detected after 20 years in the plot given 192 kg N/ha since 1968 (N4).There was a proportionally greater increase in N mineralization. Crop uptake of mineralized N was typically 12–30 kg N/ha greater from the N3 and N4 treatments than the uptake of c. 30 kg N/ha from the N0 treatment. Results from laboratory incubations show the importance of recently added residues (roots, stubble, etc) on N mineralization. In short-term (2–3 week) incubations, with soil sampled at harvest, N mineralization was up to 60% greater from the N3 treatment than from N0. In long-term incubations, or in soil without recently added residues, differences between long-term fertilizer treatments were much less marked. Inputs of organic N to the soil from weeds (principally Equisetum arvense L.) to the N0–N2 plots over the last few years may have partially obscured any underlying differences in mineralization.The long-term fertilizer treatments appeared to have had no effect on soil microbial biomass N or carbon (C) content, but have increased the specific mineralization rate of the biomass (defined as N mineralized per unit of biomass).Greater N mineralization will also increase losses of N from the system, via leaching and gaseous emissions. In December 1988 the N3 and N4 plots contained respectively 14 and 23 kg/ha more inorganic N in the profile (0–100 cm) than the N0 plot, due to greater N mineralization. These small differences are important as it only requires 23 kg N/ha to be leached from Broadbalk to increase the nitrate concentration of percolating water above the 1980 EC Drinking Water Quality Directive limit of 11·3mgN/l.The use of fertilizer N has increased N mineralization due to the build-up of soil organic N. In addition, much of the organic N in Broadbalk topsoil is now derived from fertilizer N. A computer model of N mineralization on Broadbalk estimated that after applying 144 kg N/ha for 140 years, up to half of the N mineralized each year was originally derived from fertilizer N.In the short-term, the amount of fertilizer N applied usually has little direct effect on losses of N over winter. In most years little fertilizer-derived N remains in Broadbalk soil in inorganic form at harvest from applications of up to 192 kg N/ha. However, in two very dry years (1989 and 1990) large inorganic N residues remained at harvest where 144 and 192 kg N/ha had been applied, even though the crop continued to respond to fertilizer N, up to at least 240 kg N/ha.

Description: 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.

Description: SUMMARYSoil samples were taken from four field experiments on the growth of cereals in direct-drilled and in mouldboard-ploughed soil. When sampled, one of the experiments had run for 5 years, one for 6, one for 8 and one for 10 years. Sampling was to just below plough depth and was done on an ‘equivalent depth’ basis, i.e. the more compact direct-drilled plots were sampled more shallowly than the ploughed plots in such a way that both samples represented the same weight of soil per unit area. No significant differences in total nitrogen or in total organic carbon were observed between cultivation treatments at any of the four sites.In three of the four sites, there was no significant difference in microbial biomass carbon, adenosine 5'-triphosphate (ATP), or mineralizable nitrogen between directdrilled and ploughed soils. In the fourth, which contained more clay than the others, there was slightly more biomass carbon and ATP in the direct-drilled soil. As microbial biomass carbon (or ATP, which is closely correlated with microbial biomass carbon) responds more rapidly to changes in management than do total carbon and nitrogen, a change in biomass carbon should provide early warning of changes in soil organic matter, long before changes in total carbon and nitrogen become measurable. That no such change was observed, with one partial exception, is evidence that a change from traditional methods of cultivation to direct drilling has little effect on soil organic matter other than altering its distribution in the soil profile.

Description: SUMMARYThe recovery of autumn-applied labelled fertilizer N in winter wheat and in the soil and roots was measured in five experiments on three soil types in eastern England. In four of the experiments, crop recoveries of fertilizer N ranged from 11 to 34 % in years when drainage during winter and early spring was close to, or less than, the long-term average of about 200 mm. Crop recovery was higher (42 %) at a site where the soil was heavier and winter drainage was less. Total recoveries (in crop and soil, 0–50 cm) ranged from 22 to 61 %. Fertilizer N was at least risk to leaching when there was a large soil moisture deficit at the time of application. There was a linear relationship between fertilizer N lost and drainage (but not rainfall) between the time of N application and the end of March of the following year. Autumn-applied fertilizer N increased grain yield slightly in two of the experiments and decreased it in a third.

Description: SUMMARYThree field experiments in Eastern England, in which 15N-labelled fertilizer had been applied to winter wheat, were used to measure the persistence of the labelled N remaining in soil and stubble at harvest and the availability of this N to up to four subsequent wheat crops. A portion of the labelled fertilizer N quickly became stabilized in the soil, with only small and ever-decreasing amounts recovered by subsequent crops. Combining all sites, all years and all applications of fertilizer, 6·6±1·92 (S.D.) % of the labelled fertilizer remaining in soil (0–70 cm) plus stubble in the year of application was taken up by the next wheat crop, i.e. by the first ‘residual year’ crop. A further 3·5±0·39% was taken up in the second residual year, 2·2±0·43% in the third and 2·2% in the fourth. Loss of residual labelled N was more rapid from a sandy soil than from two heavier-textured soils, particularly in the first residual year. After four residual crops on one of the heavier soils (at Rothamsted), 16% of the labelled N remaining in soil (0–70 cm) and stubble in the year of application had been taken up by the crops, c. 29% had been lost from the soil/crop system and 55% remained in the soil.