The efficient use of fertilizer nitrogen (N) is crucial to sustainable human nutrition. All crops receive significant amounts of additional N in temperate environments, through fixation or fertilizer use. This paper reviews progress towards the efficient use of fertilizer N by winter wheat (Triticum aesitivum L.) and spring barley (Hordeum vulgare L.) in the UK, acknowledging that on-farm this is governed by economics. Recent multi-site N response experiments on old and modern varieties show that yield improvements since the 1980s have been accompanied by increases in economic optimum N amounts for wheat but not for spring barley. On-farm N use efficiency (NUE) has increased for barley because increased yields with optimum N were associated with compensatory decreases in grain N concentration, whereas on-farm NUE has not increased for wheat because grain N concentration has not changed and improvements in N capture were insufficient to make up for the increased yield. Genetic effects on NUE are shown to differ markedly depending on whether they are determined at a single N rate, as in variety trials, or with optimum N amounts. It is suggested that, in order to elicit faster improvement in NUE on farms, breeding and variety testing should be conducted at some sites with more than one level of applied N, and that grain N%, N harvest index, and perhaps canopy N ratio (kg N ha⁻¹ green area) should be measured more widely. It is also suggested that, instead of using empirical functions, N responses might be analysed more effectively using functions based on explanations of yield determination for which the parameters have some physiological meaning.

The notion of a phosphorus (P) circular economy provides the philosophy, framework, and opportunity to enable food production systems to become more efficient, sustainable, and resilient to a future P scarcity or sudden price shock. Whilst P recovery and recycling are central strategies for closing the P cycle, additional gains in environmental performance of food systems can be obtained by further minimising the amounts of P (a) introduced into the food system by lowering system P demand and (b) lost from the system by utilising legacy P stores in the landscape. This minimisation is an important cascading component of circularisation because it reduces the amounts of P circulating in the system, the amounts of P required to be recycled/recovered and the storage of unused P in the landscape, whilst maintaining agricultural output. The potential for circularisation and minimisation depends on regional differences in these P flow dynamics. We consider incremental and transformative management interventions towards P minimisation within circular economies, and how these might be tempered by the need to deliver a range of ecosystem services. These interventions move away from current production philosophies based on risk-averse, insurance-based farming, and current consumption patterns which have little regard for their environmental impact. We argue that a greater focus on P minimisation and circularisation should catalyse different actors and sectors in the food chain to embrace P sustainability and should empower future research needs to provide the confidence for them to do so without sacrificing future regional food security.

In seeking better predictions of grain yield under light-limited conditions, shading was applied to field-grown winter wheat cv. Slejpner during each of five consecutive phases (canopy expansion, ear expansion, pre-flowering, grain expansion and grain filling). Absolute measures were taken of solar radiation and its effects on growth in three seasons, at a site where water and nutrient supplies were not limiting. Replicate mobile shades automatically occluded 0·80 of incident light when mean total solar radiation exceeded 250 J/m² per s. Mean effects over seasons of shading on incident total solar radiation were −296, −139, −78, −157 and −357 MJ/m² for the five phases respectively, and corresponding effects on shoot dry weight were −236, −184, −58, −122 and −105 g/m². Estimated efficiency of radiation use after flowering was 1·2 g/MJ unshaded, tending to increase with shading. Shading in all phases reduced grain dry matter yield: mean effects over seasons were −106, −64, −61, −93 and −281 g/m² for the five consecutive shading periods. Shading from GS31–39 increased mean maximum area of the two top leaves from 2700 to 3100 mm² per leaf but, with fewer stems, canopy size remained unaffected. This and the next shading, from GS39–55, reduced specific leaf weight from 42 g/m² by 4 and 3 g/m² respectively, but effects on shoot dry weight were largely due to stem and ear. By flowering, stem weights, and especially their reserves of water-soluble carbohydrates, had partially recovered. Effects on yield of shading from GS31–39 were explained by a reduction in grains/m² of 3070 from 26109. Shading from GS39–55 reduced grains/m² by 4211 due to fewer grains per ear, whilst mean weight per grain increased in compensation. Shading from GS55–61 decreased grains/ear by 2·5. Shading from GS61–71 decreased ear growth and reduced stem weight, and at harvest resulted in 4·3 less grains/ear. Effects of the final shading from GS71–87 were fully explained by a reduction in mean dry weight/grain of 10·3 mg. Except for shading from GS71–87, source- and sink-based explanations of grain yield both proved feasible, within the precision of the measurements. Constraints to accurate comparison of source- and sink-based approaches are identified, and the implications for yield forecasting are discussed.

Experiments at three sites in 1993, six sites in 1994 and eight sites in 1995, mostly after oilseed rape, tested effects of previous fertilizer N (differing by 200 kg/ha for 1993 and 1994 and 300 kg/ha for 1995) and date of sowing (differing by about 2 months) on soil mineral N and N uptake by winter wheat cv. Mercia which received no fertilizer N. Soil mineral N to 90 cm plus crop N (‘soil N supply’; SNS) in February was 103 and 76 kg/ha after large and small amounts of previous fertilizer N respectively but was not affected by date of sowing. Previous fertilizer N seldom affected crop N in spring because sowing was too late for N capture during autumn, but it did affect soil mineral N, particularly in the 60–90 cm soil horizon, presumably due to over-winter leaching. Tillering generally occurred in spring, and was delayed but not diminished by later sowing. Previous fertilizer N increased shoot survival more than it increased shoot production. Final shoot number was affected by previous fertilizer N, but not by date of sowing. Overall, there were 29 surviving tillers/g SNS.N uptakes at fortnightly intervals from spring to harvest at two core sites were described well by linear rates. The difference between sowings in the fitted date with 10 kg/ha crop N was 1 month; these dates were not significantly affected by previous fertilizer. N uptake rates were increased by both previous fertilizer N and late sowing. Rates of N uptake related closely to soil mineral N in February such that ‘equivalent recovery’ was achieved in late May or early June. At one site there was evidence that most of the residue from previous fertilizer N had moved below 90 cm by February, but N uptake was nevertheless increased. Two further ‘satellite’ sites behaved similarly. Thus at 14 out of 17 sites, N uptake until harvest related directly and with approximate parity to soil mineral N in February (R² = 0·79), a significant intercept being in keeping with an atmospheric contribution of 20–40 kg/ha N at all sites.It is concluded that, on retentive soils in the UK, SNS in early spring was a good indicator of N availability throughout growth of unfertilized wheat, because the N residues arising from previous fertilizer mineralized before analysis, yet remained largely within root range. The steady rates of soil mineral N recovery were taken as being dependent on progressively deeper root development. Thus, even if soil mineral N equated with a crop's N requirement, fresh fertilizer applications might be needed before ‘equivalent recovery’ of soil N, to encourage the earlier processes of tiller production and canopy expansion. The later process of grain filling was sustained by continued N uptake (mean 41 kg/ha) coming apparently from N leached to the subsoil (relating to previous fertilizer use) as well as from sources not related to previous fertilizer use; significant net mineralization was apparent in some subsoils.

SUMMARY: Fertilizer nitrogen (N) accounts for the majority of the greenhouse gas (GHG) emissions associated with intensive wheat production, and the form of fertilizer N affects these emissions. Differences in manufacturing emissions (as represented in the current carbon accounting methodologies) tend to favour urea, even when using the best available manufacturing technologies (BAT), whereas differences in fertilizer N efficiency and emissions of ammonia tend to favour ammonium nitrate (AN). To resolve these differences, data from 47 experiments in two large UK studies conducted from 1982 to 1987 and from 2003 to 2005 were reanalysed, showing that on average urea efficiency was 0·9 of AN (although mean ammonia emissions in 10 subsidiary experiments indicated an efficiency difference of 0·2); treating urea with a urease inhibitor (TU; AGROTAIN^®, active ingredient N-(n-butyl) thiophosphoric triamide (n-BTPT)) brought its efficiency almost in line with AN; however, a significantly greater mean optimum N amount for TU (+0·1 of AN) was not fully explained. A standard response function relating wheat yield to applied AN was modified for degrees of relative inefficiency, thus enabling yields and GHG intensities (kg CO₂e/tonne (t) grain) to be calculated using a PAS2050 compatible model for GHG emissions for any N amount of any N form. With AN manufactured by average European technology (AET), the estimated GHG intensity of wheat producing 8 t/ha was 451 kg/t; whereas with urea or TU made by AET it was 0.87–0.99 or 0.84–0.86 of this respectively. Using BAT for fertilizer manufacture, the grain's GHG intensity with AN and TU was 368 kg/t, but was 1·03–1·17 of this with untreated urea. The range of effects on GHG intensities arose mainly from remaining uncertainties in the inefficiencies of the N forms. Generally, economic margins and GHG intensities were not much affected by adjustments in N use for relative inefficiencies or different prices of urea-based fertilizers compared with AN. Overall, TU appeared to provide the best combination of economic performance and GHG intensity, unless the price for N as TU exceeded that for N as AN.

An experiment was conducted over seven sites in eastern England sown to winter cereals, whereby the soil mineral nitrogen (N) and N recovery in the crop were measured frequently during the active growth period. Plots received 100 kg/ha fertilizer N on one of three dates in spring (28 March, 11 April and 25 April) on each site in each of the three years (1994 to 1996), and these were compared with controls with no applied N. At one site, uncropped plots were included, and measurements of gaseous N emission, soil respiration, soil microbial biomass and root tissue N concentration were also made. The fertilizer applications boosted yields by at least 3 t/ha at all sites, but apparent recovery of fertilizer N varied throughout the previously known range of 45–85%. Soil type and timing of application had no effect on N recovery, or on final yields. N was mineralized from soil sources throughout the growth period, but mineralization was outweighed by ‘‘immobilization’’ of large amounts of N (around 30 kg/ha) in the soil, chiefly during May when crop growth was most rapid. This occurred on all sites with a crop, but not where the crop was absent. Measured losses of N₂ and N₂O were very small (<70 g/ha/dayN) on the site where they were measured, and no other evidence of loss or storage of N was found. The apparent recovery of fertilizer N at each site, was almost exactly explained by the amount immobilized during May. We conclude that the poor recovery of spring-applied N was due, not to losses as previously assumed but to temporary immobilization during the period of most active crop uptake. Immobilization was caused primarily by the presence of the crop.