**1. Introduction**

Greenhouse vegetable production has the advantages of less planting area requirement and relatively higher yield [1]. Increased vegetable consumption and farmers' increased per-capita earning expectations have promoted greenhouse vegetable production globally [2,3]. In 2009, the area under greenhouse vegetable cultivation in China was 3.35 million hectares, and the country had the highest yield of greenhouse vegetables globally [4]. By 2013, Chinese production accounted for 50% of world vegetable production, and the area

**Citation:** Geng, Y.; Bashir, M.A.; Zhao, Y.; Luo, J.; Liu, X.; Li, F.; Wang, H.; Raza, Q.-U.-A.; Rehim, A.; Zhang, X.; et al. Long-Term Fertilizer Reduction in Greenhouse Tomato-Cucumber Rotation System to Assess N Utilization, Leaching, and Cost Efficiency. *Sustainability* **2022**, *14*, 4647. https://doi.org/ 10.3390/su14084647

Academic Editor: Domenico Ronga

Received: 20 March 2022 Accepted: 8 April 2022 Published: 13 April 2022

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

**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/).

under cultivation in China for greenhouse vegetables had increased to more than 3.7 million hectares [5]. In 2016, the area under greenhouse vegetable cultivation in China was 3.91 million hectares, which accounted for 21.5% of the total planting area and produced 30.5% of the total yield in China [6,7].

To ensure maximum production from greenhouse vegetables, surplus fertilization is a common practice, which results in low fertilizer use efficiency [8,9]. Nitrogen (N) is the most important nutrient required by vegetables to maximize production; thus, it is essential to determine the appropriate fertilization rate. The rate of N uptake from different N sources by vegetables was observed to be less than 400 kg N ha−<sup>1</sup> [10]. The average soil N application rate for greenhouse vegetables was 1732 kg ha−<sup>1</sup> year−<sup>1</sup> in Beijing, which is nearly four times the rate at which vegetables can absorb N [7]. The average N fertilization rate for greenhouse vegetables reached 4088 kg ha−<sup>1</sup> year−<sup>1</sup> , with an NUE less than 10%, in a vegetable production area in Shouguang, Northern China [11,12]. For vegetable production in Northern China, Ju et al. [13] reported annual average inputs of N from chemical fertilizer, manure, and irrigation water, of 1358, 1881, and 402 kg ha−<sup>1</sup> , respectively, totaling 3641 kg ha−<sup>1</sup> ; this is nearly nine times more N than vegetables can absorb [13]. Excessive N input causes N to accumulate in the soil [14]. Whether from natural or anthropogenic, this leads to N leaching and volatilization losses that are the major sources of non-point source pollution [15,16]. Furthermore, it also is the major cause of severe nitrate leaching and increases the risk of groundwater pollution [17,18].

To address these problems, fertilizer reduction technology (FRT) has been investigated in the United States and other developed countries since the 1980s [19]. Reducing N fertilization to 200 kg ha−<sup>1</sup> resulted in satisfactory production and good vegetable quality [20]. A reduction of 40% N fertilization in greenhouse vegetable production reduces N leaching loss by 39.6% without affecting the yield [21]. Reducing N application from 360 kg ha−<sup>1</sup> to 240 kg ha−<sup>1</sup> can enhance yield (8.8%) and N agronomic efficiency (51.3%) in a greenhouse cucumber experiment [21,22]. Fertilizer N reductions of 20% and 50% can reduce total N (TN) leaching by 18.3% and 43.0%, respectively in the cucumber–cabbage season [22]. Therefore, FRT is an effective technique to not only improve economic benefit but also ensure sustainable vegetable development with improved N use efficiency (NUE) and less N leaching.

The Ningxia Plain is a vital vegetable production area located upstream of the Yellow River and is developing rapidly in recent years. Higher demands for vegetables have forced local farmers to apply a higher dose of chemical fertilizers in this area [23]. The adaptation of FRT is difficult in Ningxia Plain as well as other rural regions in China due to fear of less production, ineffective soil testing services, ignorance of environmental conditions, and lack of expertise [24]. Some recent cases have proved that training/seminars are effective tools in guiding farmers to reduce N fertilizer input for agricultural production [25–27]. Nevertheless, after training, they may restart the previous practice of excessive N fertilizer application, believing that the continuous reduction of N fertilizer application in the long term may reduce the TN supply in the soil and have a negative impact on crop yield [28]. This gives rise to the need for this study to identify the long-term influences of FRT vegetable yield, environmental pollution, and profitability of vegetable production systems.

Keeping an eye on the need of this study, we conducted a six-year in situ study aiming to measure the effects of conventional, reduced fertilization, and organic fertilization. The major objectives of this study are to (1) measure the influence of reduced fertilization on vegetable yield and NUE, (2) identify the key characteristics of reduced fertilization with respect to N leaching, and (3) estimate the N fertilizer economic benefit.

#### **2. Materials and Methods**

## *2.1. Site Description*

The greenhouse experiment was conducted at Yellow River Irrigation Region of Ningxia Plain at the NXL01 land block (38.4475 N, 106.3575 E), in Ningxia province, China, at an altitude of 1000 m. This experiment was launched over six years (December 2008

to November 2013). The mean annual rainfall and air temperatures were 233 mm and 9 ◦C, respectively. The tomato–cucumber rotation vegetable system was used: tomatoes were grown and harvested first, then fallow period, followed by cucumbers, with one rotation per year. The soil is classified as Sandy Loam Soil (USDA system) in the study area. The physicochemical properties of the soil were as: clay 14%, silt 30%, sand 56%, bulk density 1.37 g cm−<sup>3</sup> , moisture content 10.3%, pH 8.27, 30.10 g kg−<sup>1</sup> organic matter content, 2.42 g kg−<sup>1</sup> total N, 2.14 g kg−<sup>1</sup> total P, 302.40 mg kg−<sup>1</sup> Olsen-P, and 390.00 mg kg−<sup>1</sup> extractable K.
