**1. Introduction**

The efficient use of fertilisers relies on optimising the time of application to meet crop nutrient requirements, and minimising nutrient losses through processes such as leaching and surface water runoff. With the intensification of Australian agriculture in response to increased global food requirements, the use of nitrogen-based fertilisers has concomitantly increased dramatically [1–4]. However, <50% of applied nitrogen (N) is effectively utilised by the growing plant in many cropping situations, and this unutilised N represents a significant economic loss and can pose a high risk of environmental pollution [4–8].

At the same time, the need for developing more agricultural land to meet global food requirements has seen an increased research focus on the role of sandy soils in cropping systems, and on identifying management strategies to overcome cropping constraints [9]. Despite sands being perceived as highly leachable, this characteristic is of less importance in shallow sands underlain by low-permeability layers due to increasing clay content and/or mechanical impedance/compaction [10]. Therefore, any decline in infiltrability or deep drainage can encourage other water loss mechanisms, such as runoff [11].

There has been renewed interest in the use of organic fertilisers for supplying plant nutrients [12,13], and as a feedback mechanism for improving soil health [14]. The term

**Citation:** Phillips, I.;

Paungfoo-Lonhienne, C.; Tahmasbian, I.; Hunter, B.; Smith, B.; Mayer, D.; Redding, M. Combination of Inorganic Nitrogen and Organic Soil Amendment Improves Nitrogen Use Efficiency While Reducing Nitrogen Runoff. *Nitrogen* **2022**, *3*, 58–73. https://doi.org/ 10.3390/nitrogen3010004

Academic Editor: Jacynthe Dessureault-Rompré

Received: 19 November 2021 Accepted: 18 January 2022 Published: 20 January 2022

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**Copyright:** © 2022 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/).

"organic fertiliser" covers a wide range of substrates including manures, composts, and plant stubble and root residues [13,15,16], and these substrates vary considerably in nutrient content in both form and concentration [17]. However, there are issues relating to the slow rate of N release from organic substrates. The subsequent slow kinetics of mineralisation and nitrification rates have brought into question the sole reliance on organic fertilisers for satisfying the plant nutrient requirements [18,19]. Paungfoo-Lonhienne et al. (2019) [20] reported less biomass for kikuyu grass (*Pennisetum clandestinum*) grown in a poultry manure-based organic fertiliser (CropUpTM) compared with an inorganic fertiliser (urea). These researchers suggested that the rate of N supply (including resident NH4 and NO3, and mineralised N) from the organic substrate alone was insufficient to meet the N demand of the growing plant. To address this N limitation, plants were grown in soil receiving a combination of 50% urea and 50% CropUpTM. This combination significantly decreased mineral N in leachates compared to urea alone. However, in contrast to leachate N, the soil mineral N levels for the combined (organic + urea N sources) and urea-only treatments were similar, indicating that little N mineralisation from the organic substrate had occurred. Although organic fertilisers are capable of supplying N for plant use, factors such as the organic substrate and soil chemical and physical characteristics, as well as microbial diversity and functionality, play key roles in the mineralisation and nutrient release rates from these organic materials [13,15,16,19].

The retention of inorganic and organic forms of N closer to the soil surface due to reduced leaching and mineralisation rates can expose this nutrient to loss mechanisms such as wind and water erosion, depending on climatic and soil physical conditions. Soluble forms of both organic and inorganic N are the most prone to loss, particularly in surface runoff. As global demand for agricultural land increases, sandy soils with low nutrient and water retention properties are expected to play an increasing role in food production. The combined use of inorganic and organic sources of N may provide one strategy to minimise nutrient loss and maximise N use efficiency. Although the leaching of N supplied as combined inorganic/organic substrates has been investigated [20], losses from runoff, particularly for sandy soil, are scarce [21–23].

The aim of this study was to investigate the suitability of a combined inorganic and organic fertiliser to improve nitrogen use efficiency (NUE) in a sand. Specifically, this study targeted (1) N losses in surface runoff from well-controlled rainfall simulation experiments for a combined (organic + inorganic sources) and an inorganic N-based fertiliser, and (2) laboratory-scale N release from inorganic and organic N sources.
