Elsevier

Scientia Horticulturae

Volume 278, 27 February 2021, 109861
Scientia Horticulturae

Recovery efficiency of applied and residual nitrogen fertilizer in tomatoes grown on sandy soils using the 15N technique

https://doi.org/10.1016/j.scienta.2020.109861Get rights and content

Highlights

  • Higher rates on applied N above 168 kg N ha−1 did not show any positive responses with respect to plant and soil N fertilizer recoveries, fruit and whole plant dry matter yields, indicating that the recommended N rate of 224 kg N ha−1 is more than adequate.

  • Fertilizer N contributions to the tomato crop were approximately 62 %, making soil N contributions 38 %.

  • A 38 % reduction in recommended N application rate may help enhance N fertilizer recovery efficiency in fertigated tomatoes grown on sandy soils.

Abstract

Plant use efficiency of applied N is estimated to be around 50 % for most crops. In coastal plain sandy soils particularly, leaching along with volatilization in warmer climates may be a predominant pathway for N loss to the environment. A replicated field study to determine both crop N requirement and recovery efficiency of N (REN) in tomatoes (var. BHN 602) grown on sandy soils under a plastic-mulched bed system was conducted in north Florida. Isotope labeled-ammonium nitrate (15NH415NO3) was applied in spring at four different N rates (0, 168, 224, 280 kg N ha−1). A subsequent study in the fall was conducted in order to determine the recovery of residual N fertilizer in tomatoes. In spring, no appreciable response to applied fertilizer N rate above 168 kg N ha−1 (lowest rate) were observed, indicating recommended N rates may be more than what is required. On average, approximately, 62 % of N accumulation in the plant came from fertilizer, whereas 38 % came from soil N. In the fall, recovery of residual 15N fertilizer at harvest ranged from 1.9 to 5.4 kg N ha−1. At the end of both studies, a 15N balance was calculated to estimate total crop and soil recovery of N fertilizer, where approximately 15.4 % of applied 15N fertilizer was recovered. Unrecovered N in this study shows that optimizing N fertilizer management in warmer climates is critical in order to avoid unnecessary excess application of fertilizer and minimize loss of N to the environment.

Introduction

Florida is the second largest producer of vegetables in the USA (USDA-NASS, 2019), which are grown under intensive management of nutrients and water on typically sandy soils. Optimization of nutrient and water application and their use efficiencies in these sandy soils, therefore, is important for minimizing any potential losses of nutrients to the environment, particularly nitrogen (N). Use of predictive and diagnostic scientific tools such as soil and plant tissue testing are critical for determining crop nutrient requirements and supplemental nutrient application recommendations for sustainable crop production. Several agricultural best management practices (BMPs) have been identified, developed and implemented for enhancing plant use efficiency of nutrients such as N, phosphorus (P), and potassium (K) in conjunction with the 4R (right source, right rate, right time, and right place) nutrient stewardship principles; soil testing is inherently the first step (Hochmuth et al., 2014).

Nitrogen is the most limiting factor for plant growth and an essential nutrient with respect to increasing yields (in most agricultural settings), making the use of N fertilizer extensive worldwide. Nitrogen use efficiency (NUE) of agricultural systems is estimated at 50 % or less (Smil, 1999), where unused N fertilizer may remain in the soil or be subject to volatilization, denitrification, and leaching to groundwater (Carpenter et al., 1998). Excessive leaching of N into groundwater causes nitrate (NO3) pollution, which is hazardous to human health (Paltineanu et al., 1980) and degrades surface water bodies through eutrophication (Carpenter et al., 1998). In most agricultural soils, nitrate (NO3) is the dominant form available for crop uptake (Mengel and Kirkby, 2001), where NO3 has the potential to leach, especially on sandy soils that are managed for agricultural production. Loss of N necessitates the use of N fertilizer application to meet crop nutrient requirements (CNR). In 2019, out of the 199 million tonnes of fertilizer (N, P2O5, and K2O) utilized, approximately 118 million tonnes came from applications of N alone (FAO, 2016). Consumption and demand for N fertilizer will continue to grow as the human population increases (Xu et al., 2012), where the world demand for N fertilizer is rising at a 1.2 % annual growth rate (FAO, 2016). Thus, it is critical to produce information on fertilizer uptake by the crop in order to improve production practices that can increase NUE and decrease N fertilizer leaching.

Efforts from researchers and government agencies are attempting to look at optimizing NUE in agricultural production while minimizing negative impacts to the environment. For example, the Florida Department of Agriculture and Consumer Services (FDACS) implemented BMPs in order to reduce nutrient loading from agricultural production into bodies of water (FDACS, 2015). Efforts have included the improvement and implementation of soil test recommendations and other cultural practices to improve NUE of agricultural systems and reduce leaching (Way, 2007). For tomato production on sandy soils in Florida, BMPs include drip irrigation, splitting of N fertilizer applications into 13 weekly doses (Liu et al., 2018), and plastic mulched raised beds, all of which help improve the NUE.

Nitrogen undergoes rapid transformations in the soil (e.g. denitrification, volatilization, mineralization, and immobilization) making it challenging to determine the amount of N needed to meet CNR based on a soil test, particularly in subtropical regions such as Florida and other southeastern United States. Therefore, N recommendations are based on research data from CNR studies that assess crop response to various rates of applied N fertilizer (Meisinger et al., 2015). Since a soil test for N is not reliable for guiding N fertilizer recommendations, comprehensive data are needed to estimate fertilizer and native soil N contributions to the crop N requirement.

The 15N isotope, as a tracer, can be used to directly study plant fertilizer N uptake, residual fertilizer N in the soil, and to evaluate native soil N contributions (IAEA, 1983, 2001; Halitligil, 2004). The use of enriched 15N fertilizer to evaluate fertilizer recovery efficiency in tomatoes have been conducted in previous studies (Halitligil et al., 2002; Zuraiqi et al., 2002), but no recovery efficiency data are available for current standard tomato production systems on sandy soils. Also, data on the recovery of residual N fertilizer in the second crop using the 15N technique is not available in the literature for vegetable production systems and requires investigation. Therefore, a study where application of labeled N fertilizer on spring season tomatoes was conducted and followed by a fall season tomato crop to estimate residual recoveries of N fertilizer not used by spring tomato. The objectives of this study were 1) to determine the crop N requirement and N uptake in tomato, 2) to determine the recovery efficiency by tomato at four N rates, 3) to determine the uptake of residual 15N by the subsequent fall crop, and 4) to study the N balance in the plant and soil through the recovery of 15N fertilizer in the two seasons.

Section snippets

Field site description and experiment design

The 15N recovery of labelled ammonium nitrate fertilizer by tomatoes was evaluated through a field experiment conducted during the spring and fall growing seasons in 2017. The field site was located at the University of Florida (UF), Plant Science Research and Education Unit in Citra, Florida (29.402443 N, -82.175082 W), where the area is characterized as having a udic moisture regime, with high precipitation and humidity occurring most of the year. Mean annual precipitation is approximately

Fruit and dry matter yield

No significant difference was found for dry matter yield (1.5-2.5 Mg ha−1) between the control and three fertilizer N rates in the spring (data not shown). The lowest yield was found in the control (3.2 Mg ha−1), whereas higher though similar yields (24.3-30.5 Mg ha−1) were found among the three fertilizer N rates (Fig. 2). In the fall, yields were higher though similar (36.6-44.0 Mg ha−1) among the three N rates, when compared to control yield (5.8 Mg ha−1) (Fig. 2). No treatment effect was

Yield

Typical yields in Florida averages around 33.6 Mg ha−1 (USDA-NASS, 2019). The spring fruit yields were slightly lower than typical yields due to heat stress, whereas the fall yields improved due to cooler temperatures, which prolonged the harvest season. Therefore, higher fruit N with lower fruit yield in the spring and lower fruit N with higher fruit yield in the fall were recorded. Hence, the response to heat stress was enhanced in the treatments that received N fertilizer. Dry matter yields

Conclusions

Soil N fertilizer recoveries, N uptake in the whole plant, REN, APR, fruit and dry matter yields were not beneficially affected by higher applications of N fertilizer that were above 168 kg N ha−1. Therefore, recommended fertilizer N rates for tomato production are too high and need to be lowered by at least 38 % to take into account soil N contributions and see a fertilizer benefit, especially in fertilizer recovery efficiency by the crop.

With limited information available on 15N studies for

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Credit author statement

Rao Mylavarapu: Supervision, Conceptualization, Methodology, Laura Jalpa: Field work, Data Analysis and Writing, George Hochmuth: Advising treatments and methodology. Alan Wright: Quality Control and reviewing, Edzard van Santen: Statistical Analyses

Declaration of Competing Interest

The authors report no declarations of interest.

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