*Review* **Nitrogen Gap Amelioration Is a Core for Sustainable Intensification of Agriculture—A Concept**

**Witold Grzebisz \* and Remigiusz Łukowiak**

> Department of Agricultural Chemistry and Environmental Biogeochemistry, Poznan University of Life Sciences, Wojska Polskiego 28, 60-637 Poznan, Poland; remigiusz.lukowiak@up.poznan.pl **\***

 Correspondence: witold.grzebisz@up.poznan.pl; Tel.: +48-618-487-788

**Abstract:** The main reason for the development of the yield gap in crop production is the inefficient managemen<sup>t</sup> of nitrogen (N). The nitrogen gap (NG) cannot be ameliorated without an indication and quantification of soil characteristics that limit N uptake by a crop plant. The insufficient supply of N to a plant during its cardinal stages of yield formation is a result of two major-variabilities. The first is spatial variability in the soil characteristics responsible for water supply to a plant, also serving as a nutrient carrier. The second is a vertical variability in soil factors, decisive for pools of available nutrients, and their in-season accessibility to the grown crop. The long-term strategy for NG cover should focus first on soil characteristics (humus stock, pH, nutrient content) responsible for water storage and its availability to the currently grown plant. Diagnostics of plant nutrient availability should deliver data on their contents both in the topsoil and subsoil. The combined use of both classical diagnostic tools and spectral imagery is a way to divide a single field into units, differing in productivity. Remote-sensing techniques offer a broad number of tools to define the in-season crop canopy requirement for fertilizer N in homogenous field units.

**Keywords:** climatic potential yield; yield gap; nitrogen use efficiency; soil constraints; subsoil; spatial variability; remote sensing-techniques; field

#### **1. Food Gap and Sustainable Intensification of Agriculture**

The global human population, depending on the scenario, will reach 9–10 billion in 2050. Some prognoses of food requirements in 2050, based on the level of food production in 2005, fluctuate from 50% to 110% [1,2]. The analysis made by Hunter et al. [3] showed a much lower level of food demand in 2050 compared to 2010, ranging from 25% to 70%. The effective managemen<sup>t</sup> of global food demand in 2050 should be a result of the simultaneous implementation of three complementary strategies, termed "mega-wedges" [4]. They are as follows: (i) filling the food production gap, (ii) decreasing food losses in the entire food chain, (iii) reduction in the worldwide food demand. The relative contribution of each particular mega-wedge in food security control has been assessed as 46%, 33.6%, and 20.4%, respectively. The second strategy does not include only direct losses of energy and proteins in the food chain, but also reduced productivity of soil and water (10.4%). This means that above 55% of the future food demand directly depends on efforts oriented on covering the food production gap. With respect to food losses, it is necessary to stress that the net excess of the global protein supply is almost equal to its intake by humans (36% vs. 44%) [5]. It can be, therefore, concluded that the managemen<sup>t</sup> of the food production chain requires significant changes in agriculture. The conceptual (diagnostic, management, techniques, technology) preparation of the agriculture sector for a considerable increase in foodproductioninthecoming30yearsisthekeychallengeforthepresentgeneration.

 The current level of food production is a function of two main factors, i.e., (i) actual crop yield, (ii) crop yield improvement. The actual yield of a particular crop in a strictly defined locality (field) is a result of the efficiency of production inputs under the course of

**Citation:** Grzebisz, W.; Łukowiak, R. Nitrogen Gap Amelioration Is a Core for Sustainable Intensification of Agriculture—A Concept. *Agronomy* **2021**, *11*, 419. https://doi.org/ 10.3390/agronomy11030419

Academic Editor: Claudio Ciavatta

Received: 21 January 2021 Accepted: 22 February 2021 Published: 25 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

meteorological conditions during the growing season [6,7]. The realization of the second target depends on progress in (i) breeding of new cultivars, (ii) improvement in water efficiency (WUE), (iii) improvement in nitrogen use efficiency (NUE). It is necessary to remember that the success of the Green Revolution, resulting in the significant increase of yields of major crops (cereals, rice, maize) was a result of the induced synergism between new, high-yielding cultivars, high rates of applied fertilizer nitrogen (Nf), and a high level of crop protection, based on agrochemicals [8]. An insufficient level of the required synergism between these three main factors has resulted in stagnation in the world average yields of main crops during the last two decades [9]. The necessary annual rate of yield increase of four global crops. i.e., maize, rice, wheat, and soybean to cover the food gap in 2050 would have to reach 2.4%. The current yearly rate of yield increase for these four crops is far below the assumed threshold, being at the level of 1.6%, 1.0%, 0.9%, and 1.3%, respectively [10]. For example, the genetic progress in nitrogen use efficiency for wheat in the years 1985–2010 was only in the range of 0.30–0.37% y<sup>−</sup>1. This was a result of the increased value of the Nitrogen Harvest Index (NHI), i.e., the relative amount of N accumulated in grain [11].

Future progress in food production, in fact, will depend on two key drivers. The first is high-tech intensification, based on highly productive cultivars of main crop plants, and the efficient use of mineral fertilizers and other agro-chemicals. This strategy has been responsible for about 3 4 of the global food production increase in the last 85 years. The remaining 1 4 was due to the increase in the area of arable land, mainly in less-developed countries. The productivity of the newly cultivated soils is to a grea<sup>t</sup> extent driven by their natural fertility. In the future, this option will be strongly limited due to the lack of high or even medium fertile soils. The primary resources of potential arable land are extensive pastures and tropical forests. The key disadvantage of the first is a shortage of water and of the second, low fertility, mainly due to high acidity and a shortage of essential nutrients. In addition, this option requires a considerable financial input to increase soil fertility, as a prerequisite of an economically and environmentally sound level of production [12,13].

The progressively increasing demand for food, resulting from the permanently growing human population, is in contradiction to the concept of sustainable development of agricultural production. The most advanced scenarios of a sustainable approach to food production assume a massive reduction or even the elimination of N fertilizers and pesticides. The main objective of this restrictive view of agriculture is to arrest the degradation of both local and global environments [14]. In the current and future reality, food demand is too high to completely abandon modern production measures [15,16]. A realistic view of sustainable agricultural development, termed as sustainable intensification of agriculture (SIA), defines this concept as a process or production system where yields are increased without an adverse impact on the environment and without the cultivation of low-quality land [17]. A less sophisticated, but at the same time a more practical definition of SIA, has been proposed by Smith [18]. The core of this definition is to produce more, and high-quality food per unit of used and applied production measures, taking into account both soil fertility and externally applied measures (fertilizers, pesticides, fuel) on the one hand, and protection of the existing ecosystems against damage on the other hand. These opinions are summarized in the concept of *twin objectives*, which relies on the assumption that agricultural development, including new tools, both implemented as new production technologies, production systems, and management, has to ensure the stability of the global ecosystem [19,20].

The area of arable land and the amount of available water during the growing season of a particular crop plant are key factors that limit food production for a country, region, or field. A rise in food production cannot be based solely on the increased efficiency of water usage. As reported recently by Grafton et al. [21], based on an analysis of numerous methods of crop plant irrigation, no direct substitution between water and nitrogen was observed. The key factor limiting yield, as results from this study, is nitrogen. The authors clearly stated that the greatest challenge to the progress of crop plant productivity depends

on the improvement in N management. The required rate of food production increase to cover the food gap by 2050 can be achieved by an adequate supply of fertilizers, both nitrogenous, and those balancing N, i.e., containing P, K, Mg, S, and micronutrients. Scenarios of nitrogen fertilizers (Nf) consumption in agriculture to cover the food production gap vary 3-fold, i.e., from 85 Mt N y<sup>−</sup><sup>1</sup> to 260 Mt N y<sup>−</sup><sup>1</sup> [22]. In the model of N flows by Conijn et al. [23], the total consumption of Nf will increase to 181 Mt y<sup>−</sup>1, i.e., by 76% in 2050 as compared to 2010 (103 Mt N y<sup>−</sup>1). The increased Nf consumption will lead, however, to a simultaneous increase in losses of its active compounds into the environment. The N loss in 2050 is projected to lie in the range of 102–156% with respect to 2010 [24].

Any increase in the production efficiency of both key agronomic factors, i.e., water and nitrogen, depends on the soil status of all the other production factors decisive for their efficiency. The production of phosphorus fertilizers to cover crop plant requirements for this nutrient is set to increase in the period 2010–2050 by 32% (from 17.9 to 23.7 Mt P y<sup>−</sup>1) [23]. An analysis made by Pradhan et al. [25] showed that to fulfill food production goals in 2050 the consumption of N, P, and K fertilizers will increase in the range of 45–73% for N, 22–46% for P2O5, and 2–3-fold for K2O, respectively, compared to 2010. The sustainable intensification of agriculture cannot be realized without taking into account other nutritional factors that limit the productivity of water and nitrogen, such as magnesium, sulfur, and micronutrients [26,27].

The principal challenge to the concept of sustainable intensification in agriculture is to develop effective diagnostic and managemen<sup>t</sup> tools oriented to the increased efficiency of the applied fertilizer nitrogen. Its realization is a prerequisite for decreasing the pressure of losses of its active forms to the local and, as a consequence to the global environment. These twin objectives can be successfully realized, provided there is a recognition and a simultaneous amelioration of factors constraining the productivity of nitrogen both in the critical stages of yield formation by the currently grown crop, and resulting from the spatial variability of its supply to plants.

#### **2. Yield Potential and Yield Gap**
