D. S. Jenkinson

Modelling the fate of nitrogen in crop and soil in the years following application of 15N-labelled fertilizer to winter wheat

A computer model is presented that describes the flow of nitrogen between crop and soil on the field scale. The model has a compartmental structure and runs on a weekly time-step. Nitrogen enters via atmospheric deposition and by application of fertilizer or organic manures, and is lost through denitrification, leaching, volatilization and removal in the crop at harvest. Organic nitrogen is contained within three of the model compartments – crop residues (including plant material dying off through the growing season), soil microbial biomass and humus. Inorganic nitrogen is held in two pools as NH 4 + or NO 3 - . Nitrogen flows in and out of these inorganic pools as a result of mineralization, immobilization, nitrification, leaching, denitrification and plant uptake. The model requires a description of the soil and the meteorological records for the site – mean weekly air temperature, weekly rainfall and weekly evapotranspiration. The model is designed to be used in a ‘carry forward’ mode – one years run providing the input for the next, and so on. The model also allows the addition of 15 N as labelled fertilizer, and follows its progress through crop and soil. Data from a Rothamsted field experiment in which the fate of a single pulse of labelled N was followed over several years were used to set the model parameters. The model, thus tuned, was then tested against other data from this and two contrasting sites in south-east England. Over a period of 4 years, the root mean square (R.M.S.) difference between modelled and measured quantities of labelled N remaining in the soil of all three sites was c . 7·5 kg N/ha, on average. The root mean square error in the measurements was c . 2·5 kg/ha. Similarly, the R.M.S. difference between modelled and measured recovery of labelled N by the crop was 0·6, compared with 0·3 kg/ha in the measurements themselves.

Calculating net primary production and annual input of organic matter to soil from the amount and radiocarbon content of soil organic matter

Soil samples collected from four of the Rothamsted long-term field experiments over the last 100 yr were used to follow the effects of management on soil organic matter content. The experimental sites were:Broadbalk and Geescroft Wildernesses, both on old arable land that had been fenced off in the early 1880s and allowed to revert naturally to deciduous woodland; the unmanured plot in the Rothamsted Park Grass Continuous Hay Experiment, started in 1856; the unmanured and one of the NPK plots in the Broadbalk Continuous Wheat Experiment started in 1843. Total C, radiocarbon and (in some cases) soil microbial biomass C were measured in stored and contemporary soil samples. n nThe current Rothamsted model for the turnover of organic C in soil was then used to calculate how the organic C content of the topsoils from the four sites would change with time for a range of annual inputs. The inputs that generated the best fit to the measured values were: Broadbalk Wilderness 3.5 t C ha−1 yr−1; Geescroft Wilderness 2.5 t C ha−1 yr−1;unmanured plot on Park Grass 3.01 t C ha−1 yr−1; unmanured plot on Broadbalk Wheat 1.3 t C ha−1 yr−1; NPK plot on Broadbalk Wheat 1.71 t C ha−1 yr−1. The model also generated the radiocarbon content of soil organic C for these inputs of C, knowing the 14C content of the atmosphere over the period 1840–1985. The marked increase in the 14C content of soil organic C brought about by thermonuclear testing during the 1960s was accurately represented by the model. The quantities of soil microbial biomass (measured by fumigation-incubation) in the top 23 cm of soil from the four sites were: Broadbalk Wilderness,1.571 t C ha−1; Geescroft Wilderness, 0.58 t C ha−1; unmanured plot on Park Grass,1.621C ha−1; unmanured plot on Broadbalk Wheat, 0.47 t C ha−1; NPK plot on Broadbalk Wheat, 0.76 t C−1.The values for soil microbial biomass C generated by the model for the above annual inputs agreed closely (with one exception) with the measured values. n nFor a site under steady-state conditions, the annual input of organic matter to the soil plus the annual removal of organic matter from the site (if any) gives the Net Primary Production (NPP). NPP thus calculated was 4.0 t C ha−1 yr−1 for the unmanured plot on Park Grass, 2.2 for the unmanured plot on Broadbalk and 5.2 for the NPK plot on Broadbalk. The two Wilderness sites are still accumulating C in both soil and vegetation and here NPP is given by annual input to the soil, plus the annual increment of C in the trees. NPP calculated in this way was 4.8 t C ha−1 yr−1 for Broadbalk Wilderness and 3.3 for Geescroft Wilderness. n nThis new way of estimating NPP, from measurements made on soil organic matter, needs to be tested on a wider range of soils, climates and vegetation types before it can be generally recommended. However, it has many potential advantages, not least that it can give a value for NPP that is integrated over many years from a single sampling. For sites under steady-state conditions it is not essential to have stored soil samples—the necessary measurements can be made on contemporary samples alone.

The nitrogen cycle in the Broadbalk Wheat Experiment: recovery and losses of 15N-labelled fertilizer applied in spring and inputs of nitrogen from the atmosphere

15N-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.

Influence of soil type, crop management and weather on the recovery of 15N-labelled fertilizer applied to winter wheat in spring

15 N-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 15 NH 4 15 NO 3 , but less (7–14%) where K 16 NO 3 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) R 3 accounted for 73% of the variation in the data where: L 70 = 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; R 3 = 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 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.

Effects of season, soil type and cropping on recoveries, residues and losses of 15N-labelled fertilizer applied to arable crops in spring

15 N-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 heavyn clay. The 15 N-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%. Removalsn of 15 N-labelled fertilizer N in the harvested products were rathern 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 rapen 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,n 29 and 25% of that applied to winter wheat, oilseed rape and sugarbeet respectively. Less remainedn 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. 15 N-labelled fertilizer unaccounted for in crop and soiln (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 denitrificationn were probably greater following applications to winter wheat and oilseed rape, where the N was appliedn earlier (and the soils were wetter) than with potatoes and sugarbeet. Consequently, it may well ben advantageous to delay the application of fertilizer N to winter wheat and oilseed rape if the soiln is wet. Total inorganic N (labelled and unlabelled) in soils (0–100 cm) following harvest of potatoes given 15 N-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 53n 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, averagingn 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=wintern 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;n 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,n 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.n A decrease in the amount of nitrogen applied to winter wheat and sugarbeet in spring would thereforen not significantly decrease the quantity of nitrate at risk to leaching during the following autumnn and winter, but may be more effective with rape and potatoes. However, if wheat growth is severelyn impaired by take-all, significant amounts of fertilizer-derived nitrate will remain in the soil at harvest, at risk to leaching.

The nitrogen cycle in the Broadbalk Wheat Experiment: 15N-labelled fertilizer residues in the soil and in the soil microbial biomass

Abstract A single pulse of 15 N-labelled fertilizer was applied in spring as NH 4 NO 3 to each of four plots on the Broadbalk Continuous Wheat Experiment. Wheat has been grown on this experiment for more than 140 yr. The labelled N was given at the customary rates for the four plots, nominally 48, 96, 144 and 192 kg N ha −1 yr −1 . In subsequent years the plots reverted to unlabelled N, again given at the customary rates for the main Broadbalk experiment. Soils receiving inorganic fertilizer contained more biomass than soil from the corresponding plot that has never received inorganic N, but there was little difference in the microbial biomass N content of soils that had had 48, 96, 144 or 192 kg N ha −1 for many years. There were no consistent changes during a 4-yr period in total microbial biomass N, which averaged 190 kg N ha −1 in the plot receiving 192kg fertilizer N ha −1 yr −1 . Of the labelled fertilizer applied, 3–8% was present in the soil microbial biomass at the first harvest after application of labelled fertilizer. Expressed as a percentage of the total labelled N remaining in the soil at the first harvest, 19–27% was present in the microbial biomass. In the plot receiving 192 kg N ha −1 yr −1 , labelled biomass N declined from 5.69 kg ha −1 at the first harvest, to 4.50 at the second, to 3.35 at the third and to 2.35 at the fourth. In a subsidiary experiment, more N was retained in the soil at harvest when the fertilizer was added in the ammonium form than as nitrate; 32.6 and 19.5 kg ha −1 , respectively, for additions of 147 kg fertilizer N ha −1 . However, of the labelled N retained in the soil, 34% was present in the microbial biomass, whether the labelled N had originally been added in the ammonium form or in the nitrate form.

The measurement of15N in soil and plant material

A complete procedure for analysing soil and plant samples for total N and atom % excess15N is described. The salicylic acid version of the Kjeldahl method for measuring total N was modified for use in a digestion block, giving quantitative reduction of nitrate in both soil and plant material. Procedures for minimising cross-contamination between samples are specified, including a double-distillation procedure that eliminates ‘memory effects’ when distilling NH3 from Kjeldahl digests. A simple and robust apparatus for converting (NH4)2SO4 to N2 gas for mass spectrometric determination of atom % excess15N is described. The coefficient of variation for replicate measurements of total N in soil and plant material over the range 0.1–2.2% N was 1.0%. The coefficient of variation for measurements of15N in plant material over the range 0.4–2.9 atom % excess15N was 0.2%.

Fate of 15N-labelled fertilizer applied to spring barley grown on soils of contrasting nutrient status

An experiment with 15N-labelled fertilizer was superimposed on the Rothamsted Hoosfield Spring Barley Experiment, started in 1852. Labelled 15NH415NO3 was applied in spring at (nominal) rates of 0, 48, 96 and 144 kg N ha-1. The labelled fertilizer was applied to microplots located within four treatments of the original experiment: that receiving farmyard manure (FYM) annually, that receiving inorganic nutrients (PK) annually and to two that were deficient in nutrients: applications were made in two successive years, but to different areas within these original treatments. Maximum yields in 1986 (7.1 t grain ha-1) were a little greater than in 1987. In 1987, microplots on the FYM and PK treatments gave similar yields, provided enough fertilizer N was applied, but in 1986 yields on the PK treatment were always less than those on the FYM treatment, no matter how much fertilizer N was applied. In plots with adequate crop nutrients, about 51% of the labelled N was present in above-ground crop and weed at harvest, about 30% remained in the top 70 cm of soil (mostly in the 0–23 cm layer) and about 19% was unaccounted for, all irrespective of the rate of N application and of the quantity of inorganic N in the soil at the time of application. Less than 4% of the added fertilizer N was present in inorganic form in the soil at harvest, confirming results from comparable experiments with autumn-sown cereals in south-east England. Thus, in this experiment there is no evidence that a spring-sown cereal is more likely to leave unused fertilizer in the soil than an autumn-sown one. With trace applications (ca. 2 kg N ha-1) more labelled N was retained in the soil and less was in the above-ground crop. Where P and K were deficient, yields were depressed, a smaller proportion of the labelled fertilizer N was present in the above-ground crop at harvest and more remained in the soil.Although the percentage uptake of labelled N was similar across the range of fertilizer N applications, the uptake of total N fell off at the higher N rates, particularly on the FYM treatment. This was reflected in the appearance of a negative Added Nitrogen Interaction (ANI) at the highest rate of application. Fertilizer N blocked the uptake of soil N, particularly from below 23 cm, once the capacity of the crop to take up N was exceeded. Denitrification and leaching were almost certainly insufficient to account for the 19% loss of spring-added N across the whole range of N applications and other loss processes must also have contributed.

The fate of residual 15N-labelled fertilizer in arable soils: its availability to subsequent crops and retention in soil

An earlier paper (Macdonald et al., 1997; J. Agric. Sci. (Cambridge) 129, 125) presented data from a series of field experiments in which 15N-labelled fertilizers were applied in spring to winter wheat, winter oilseed rape, potatoes, sugar beet and spring beans grown on four different soils in SE England. Part of this N was retained in the soil and some remained in crop residues on the soil surface when the crop was harvested. In all cases the majority of this labelled N remained in organic form. In the present paper we describe experiments designed to follow the fate of this `residual 15N over the next 2 years (termed the first and second residual years) and measure its value to subsequent cereal crops. Averaging over all of the initial crops and soils, 6.3% of this `residual 15N was taken up during the first residual year when the following crop was winter wheat and significantly less (5.5%) if it was spring barley. In the second year after the original application, a further 2.1% was recovered, this time by winter barley. Labelled N remaining after potatoes and sugar beet was more available to the first residual crop than that remaining after oilseed rape or winter wheat. By the second residual year, this difference had almost disappeared. The availability to subsequent crops of the labelled N remaining in or on the soil at harvest of the application year decreased in the order: silty clay loam>sandy loam>chalky loam>heavy clay. In most cases, only a small proportion of the residual fertilizer N available for plant uptake was recovered by the subsequent crop, indicating poor synchrony between the mineralization of 15N-labelled organic residues and crop N uptake. Averaging over all soils and crops, 22% of the labelled N applied as fertilizer was lost (i.e., unaccounted for in harvested crop and soil to a depth of 100 cm) by harvest in the year of application, rising to 34% at harvest of the first residual year and to 35% in the second residual year. In the first residual year, losses of labelled N were much greater after spring beans than after any of the other crops.

The availability of the nitrogen in the crop residues of winter wheat to subsequent crops

Three field experiments in Eastern England, in which 15 N-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.