By 2050, the global human population is expected to rise to 10 billion, and yields of the world’s fourth largest food crop—potato (Solanum tuberosum L.)—need to be increased. However, it is important for the environment that nitrogen fertiliser production and use does not increase in parallel. Present-day methods of supplying plants with nitrogen (N), in the form of ammonium nitrate- or urea-based fertilisers, are inefficient: up to 70% of this N is degraded before plants can acquire it, forming harmful greenhouse gases and leaching polluting nitrate into water systems (Liu et al. 2013); and governments are legislating to control their use (Cantarella et al. 2018). Technologies that stabilise ureic nitrogen in fertiliser, such that it is less easily degraded, can reduce pollution whilst prolonging nitrogen availability to plants. This means that lower fertiliser application rates are viable for attaining the required increases in the production of many crops (Prasad et al. 2016). However, we propose that urea and ureic amine are nitrogen forms that have unique properties that improve plant form and function, such that their stabilisation also increases yield by mechanisms unrelated to prolonging the availability of nitrogen per se.
Urea is an organic amide with the chemical formula CO-(NH2)2. It has two amine (NH2) groups joined by a carbonyl functional group. It is present in natural ecosystems as well as being a constituent of man-made nitrogenous fertiliser. In natural systems, it enters the soil and canopy from urine excreted by animals (Barthelemy et al. 2018); and/or bulky organic matter is broken down by a range of organisms, such as fungi and bacteria, to smaller organic molecules. Plants have evolved sophisticated mechanisms to take up urea from the environment (Neff et al. 2003; Schimel and Bennett 2004); however, bacterial organisms that break it down by secreting the enzyme urease are ubiquitous in soil and on leaf surfaces (Hoult and McGarity 1986; Witte et al. 2002; Dawar et al. 2011) such that urea is only transiently available. Ureases induce urea to hydrolyse within minutes, whether it is naturally present or added as fertiliser, converting it to gaseous ammonia and carbon dioxide (NH2CONH2 + H2O → 2NH3 + CO2), which are then lost via volatilisation to the atmosphere, the extent of which varies with wind speed, temperature, soil pH and water content (Soares et al. 2012; Cantarella et al. 2018). Other soil bacteria induce nitrification of ammonium to leachable nitrate. Despite recognition that crop fertilisation with urea is wasteful and environmentally deleterious, it is currently the primary global fertiliser for crop production (Heffer and Prud’hommer 2014) because it is relatively cheap to manufacture and easily transported and contains 46% N.
Since the advent of large-scale crop fertilisation and the recognition that it is innately inefficient, much effort has been directed towards the development of more sophisticated fertilisers, co-products and application mechanisms. For example, there are several benefits to providing nitrogen to crops via the foliage, usually as solutions which can be applied as a fine spray. These include reductions in leaching and the ability to provide nitrogen when root activity is impaired, e.g. in saline or dry conditions (Gooding and Davies 1992; del Amor et al. 2009). A second example concerns urea itself: methods of stabilising it, and slowing its hydrolysis to NH3 and CO2 by urease, have been developed (e.g. Bhogal et al. 2003; Chalk et al. 2015; Cantarella et al. 2018). Thus far, methods of stabilisation and their effects on yield have been met with mixed success. One such technology provides urease inhibitors as constituents of urea-based fertilisers to prolong the existence of the ureic nitrogen element (Trenkel 2010). The most widely used of these is N-(n-butyl) thiophosphoric triamide (NBPT) (e.g. Watson and Miller 1996). In some cases, nitrogen use efficiency (NUE, the amount of nitrogen taken up by the crop, as a percentage of that applied) has been promoted (Zvomuya et al. 2003; Arkoun et al. 2012); however, the urease inhibitors themselves pass into roots and/or leaves, preventing natural ureic assimilation within plant tissues, allowing internalised urea to build to toxic levels (Krogmeier et al. 1989). Furthermore, alterations of urea assimilation within root or leaf tissue mean that most of the urea provided fails to release the bound nitrogen that it contains; and the transcriptional profiles of genes involved in primary and secondary metabolism are altered, to the detriment of the plant (Zanin et al. 2016). Alternative methods of stabilisation involve the manufacture of urea granules with semi-permeable coatings, such as sulphur and polyurethane. These reduce some of the degradation to other N forms and pollutants by forming a physical barrier to slow urea solubilisation. Yields can be either maintained at lower N input, or, importantly, increased (Wang et al. 2015; Tiana et al. 2018), including in potato (Hutchinson et al. 2002; Hyatt et al. 2009); and ammonia and CO2 volatilisation and nitrate leaching can be reduced, thereby increasing NUE (Zvomuya et al. 2003; Soares et al. 2012).
Despite the fact that urea has long been used as a fertiliser, it is only relatively recently that scientific studies of its uptake, utilisation and effects in plants have begun to proliferate (e.g. Witte 2011). An earlier view was that organic nitrogen needed to be degraded to inorganic forms by microorganisms, before plants could acquire the N it contains (see Paungfoo-Lonhienne et al. 2008; Zanin et al. 2016). As such, more knowledge is available regarding the uptake and effects of inorganic ammonium and nitrate in plants. However, it is now recognised that urea is easily taken up from soil by roots, through specific cell membrane transport proteins and/or aquaporins. It is rapidly assimilated via hydrolysis in the root cytosol by non-bacterial plant-specific urease, releasing ammonium and carbon dioxide internally, or transported to leaves when roots are no longer the dominant sink. Urea-sourced ammonium produced internally is rapidly assimilated by a second cytosolic pathway (Witte 2011; Zanin et al. 2016), to provide proteinaceous substrate for plant growth, photosynthesis and functioning.
In unstressed plants, different N forms are assimilated to protein via different mechanisms, which have different costs in terms of resource use and which are located in different organs and cell types; and this has repercussions for plant functioning and anatomical form (Zerihun et al. 1998; Andrews et al. 2013). A large fraction of nitrate N taken up by roots from soil is, in many cases, delivered to leaves, where the components for its assimilation are predominantly located. Nitrate-fertilised plants thus preferentially use the assimilate for above-ground growth. However, the components for the assimilation of N provided as ammonium are found within roots when these are actively growing; thus, root growth is initially favoured over shoot growth. This biomass partitioning gives rise to plants with differing anatomical appearances, or phenotypes, which are particular to each N form. Organic N forms such as urea are also assimilated in roots as described above, and organically fertilised plants are also characterised by increased biomass partitioning to roots, forming phenotypes with increased root to shoot weight ratios under experimental conditions (Franklin et al. 2017). The enhanced root to shoot weight ratio of urea-fertilised plants, in comparison to nitrate-fertilised plants, allows amplification of their capacity to scavenge soil for the water and nutrients required for enhanced above-ground growth at more advanced growth stages (Zerihun et al. 1998; Cambui et al. 2011; Franklin et al. 2017). However, in the field, such effects can be negated by the transient availability of urea. On this basis, our objective is to demonstrate that supplying stabilised ureic amine to potato will eventually lead to increases in tuber yield in glasshouse experiments and in the field, by producing phenotypes which permit this: more root and tuber mass per unit of shoot mass. We have shown that potato yields are lifted by such technology in UK field trials (Marks et al. 2018).
The amount of resource consumed during assimilation also varies with nitrogen form. Assimilation of inorganic nitrate in leaves requires the operation of the nitrate reductase pathway, which consumes more of the plant’s energy and carbon (C) from photosynthesis, than any other N assimilation mechanism (Sunil et al. 2013; Franklin et al. 2017). More protein, energy and C will thus be available for photosynthesis and biomass growth under urea or ammonium nutrition. However, ammonium can be toxic to plants as it affects the pH balance. Resource-consuming processes are used by plants to mitigate this effect, such that ureic amine assimilation may be the most resource efficient, and provide the most protein.
Thus, stabilising urea amine has the potential to promote NUE, internal nitrogen utilisation efficiency, crop productivity per unit of applied nitrogen (partial factor productivity) and yielding per se, in multiple ways: (a) it can increase nitrogen longevity in the environment whilst reducing pollution; (b) cytosolic ureic nitrogen and its hydrolytic product, ammonium, induce the development of phenotypes which initially favour root growth over vegetative growth; and (c) less energy and carbon are used to assimilate urea amine nitrogen than nitrate and ammonium nitrogen, thus more is available for photosynthesis and biomass growth. We have shown that ureic amine stabilisation increases shoot biomass, leaf relative chlorophyll content and flower numbers in a range of horticultural species (Wilkinson et al. 2019).
That urea is preferentially assimilated in roots when applied to soil has been shown to be linked to increases in root to shoot weight ratio; however, foliarly applied urea is also at least partially assimilated in leaves (Witte et al. 2002). It is not known whether ureic amine will have the same desired effect on root to shoot (or tuber to shoot) ratio when applied foliarly, and here we investigate this possibility. The leaf cuticle is up to 10 times more permeable to urea than to inorganic nitrogen ions, and this may be an adaptation to the transient nature of urine urea availability in natural systems (Wojcik 2004). Urea entering plants from foliar applications is cycled through a range of tissues, and is allocated within 48 h to the strongest sinks (Klein and Weinbaum (1985), olive; Witte et al. (2002), potato).
Additionally, effects that ureic amine stabilisation may have on root to shoot ratio could be less easy to define in a tuber crop. Many of these, including potato, have relatively small, weak basal root systems. However, urea stabilisation should also benefit other developmental processes such as tuberisation and bulking more directly. Potato tubers are not formed from a root (Hannapel et al. 2017); they arise from specialised underground stems called stolons, the tips of which swell to form the tuber. The developing tubers are important sinks for nitrogen, and we propose that foliar ureic amine-sourced N can have positive effects at this stage by increasing shoot biomass and photosynthesis. When cells begin to divide and expand, tubers start to enlarge and form starch, or ‘bulk’. Bulking tubers are sinks that import phloem-mobile substances from leaves, such as sucrose for starch formation (Hannapel et al. 2017). We propose that bulking-stage applications of foliar urea N could increase biomass allocation to developing tubers via increased sucrose production and transport.
We describe here effects of a chemical method of stabilising urea-sourced amine N in foliar applications, developed by Levity Crop Science Ltd. (Preston, UK), on changes in physiology, form and yield of potato. We compare effects of this stabilised amine nitrogen (SAN) with those of non-stabilised urea and a commercially available industry-standard N–P–K, where all formulations contain identical amounts of N by weight. The variety Casablanca is used in greenhouse experiments. SAN is also tested on the physiology and tuber yields of Rooster and Shelford in the field in Ireland and England. Casablanca is generally a first early crop, and main crop Rooster tubers are large and are used for chipping, mashing and roasting. Shelford is an early main crop, and tubers are often used for crisping. We investigate whether the proposed yield increase induced by stabilisation is merely a result of reducing degradation and prolonging nitrogen availability, or whether alterations in biomass partitioning and chlorophyll concentration specifically induced by ureic amine N also make a contribution.