Evaluation of yield and nitrogen utilisation with urease and nitrification inhibitors in sweet potato crop (Ipomoea batatas L.)

Sweet potato (Ipomoea batatas L.) is a crop that has been cultivated since ancient times. Nowadays, it is considered as an innovative crop mainly in the dietary area due to the new trends in human nutrition in developing countries. Furthermore, scientific concerns about environmental pollution and research on low environmental impact crops have led to an even greater positive evaluation of sweet potato crop. From a nutritional aspect, it contains many important nutrients for human health, as in all vegetables, as well as for animal feed. This is the main reason that sweet potato is a very significant crop in the food industry. Logestein (2009) reported that sweet potato is regarded as a safe food crop due to its ability to survive and grow under difficult tropical conditions and in infertile soils. The high rate of sweet potato productivity increases the importance of its economic cultivation and contribution in the food safety sector (Tumwegamire et al., 2004). According to FAOSTAT (2020), the worldwide sweet potato cultivation area in the year 2018 was 8,062,737 ha and the yield was 114,037 Kg · ha⁻¹.

Furthermore, the main profitable part of the sweet potato plant is the root tubers. Thus, fertilizer should be applied efficiently in order to boost both aboveground and underground growth. Nitrogen is an essential factor for plant growth. Bouwkamp (1985) reported that increased nitrogen uptake may also increase the growth of storage roots. In addition, other studies support that nitrogen increases root yield and nutritional value. Concerning the quality characteristics of sweet potato, nitrogen increases tuber dry matter (DM), carotenoid and protein content.

However, the use of larger nitrogen-based fertilizer quantities than needed by the plants results in incapability of nitrogen assimilation, thus causing environmental problems (Guarda et al., 2004; Zheng et al., 2007). For this particular reason, the need for nitrogen indicators, which will be used to determine the optimum amount of nitrogen that can positively affect plant growth without causing any adverse effects on the environment, is essential. In the international reports, many researchers emphasise using indicators to assess fertilizer performance (Kakabouki et al., 2018). In addition, it is noteworthy that the indicators express the yield of fertilizers and they are, therefore, useful in improving fertilizers. One of these indicators is nitrogen use efficiency (NUE), an indicator that expresses the ratio of yield to nitrogen content in the soil as well as nitrogen utilisation efficiency (Kakabouki et al., 2014). Other indicators are nitrogen harvest index (NHI), referring to the ratio of the nitrogen content of the tubers to the nitrogen content of the whole plant, and nitrogen agronomic efficiency (NAE), representing the increase in biomass in the applied nitrogen unit in the soil. Moreover, Arregui and Quemada (2008) reported that NAE is important because it promotes the use of nitrogen fertilizers in plant growth, without adverse environmental effects.

According to Mullen (2011), nitrogen fertilizer should be applied almost a month before planting sweet potatoes to assure the most efficient use of nitrogen by the crop and reduce the losses caused by volatilisation and denitrification procedures. Urease inhibitor (UI), N-(n-butyl)thiophosphoric triamide (NBPT), and the nitrification inhibitor (NI), dicyandiamide (DCD), are the most frequently used inhibitors. They are all used as additives in urea. NI is designed to inhibit the microbes that break down ammonium (NH₄⁺) to nitrate (NO₃⁻) ion, whereas UI relays urea (CO (NH₂)₂) conversion to NO₄⁺ ion. This way, the NH₃⁻ losses are better controlled.

While the use of new types of fertilizers is consolidated in the market and sweet potato cultivation gains space as a preferable new crop in Greece, the need for further exploring the way the fertilizers affect yield and nitrogen concentration in tubers and aboveground part has arose. Additionally, in order to evaluate the quantity of nitrogen that must be used, research must be conducted to examine the amount of nitrogen that is disposed of by the plant, both the underground and the aboveground part. For this purpose, NUE, NAE and NHI nitrogen indicators were calculated. As there is no experimental data of using fertilizers with inhibitors and no literature is available concerning their effect on sweet potato cultivation, the objective of this study is to determine the inhibitors’ influence on the sweet potato cultivation and the performance of nitrogen inhibitors.

Regarding LAI, values ranged from 5 to 5.14 during the first year of cultivation (2018) and from 4.9 to 5.35 during the second year (2019) (Table 1). The maximum LAI value was 5.35, when urea with double inhibitors treatment was applied, and the lowest value was 4.9 in control, during the second year (2019). In both years A and B, urea and urea with inhibitors urease and nitrification treatment had statistically significant difference with the urea + UI treatment. Regarding the number of tubers per plant, the highest value was 6.39 per plant in urea with double inhibitors treatment (year 2018) and the lowest 5.25 per plant in control (year 2019). In both years, urea + NI + UI and the urea with inhibitor urease treatment had statistically significant difference with all other treatments. Concerning the marketable yield values, a range from 18,412 to 23,230 Kg · ha⁻¹ during year A and a range from 18,180 to 21,273 Kg · ha⁻¹ during year B were marketed. The highest value estimated was 23,230 Kg · ha⁻¹ in year A and the lowest was 18,180 Kg · ha⁻¹ in year B. During the first year of experimenting, urea + UI and the urea + NI + UI treatments had statistically significant difference with all the other treatments but during the second year urea with double inhibitors treatment had statistically significant difference with the urea and control treatment (Table 1). It is worth noting that in both non-marketable yield and marketable percentage, none of the treatments had statistically significant difference during both years. Also, the values of tubers ranged from 175,304 to 204,522 ha⁻¹ in year A and from 168,240 to 194,800 ha⁻¹ in year B. Urea + NI + UI and the urea + UI treatment had statistically significant difference with all other treatments. The highest value was 204,522 ha⁻¹ in urea with double inhibitors treatment, year A, and the lowest was 168,240 ha⁻¹ in control, year B.

In Table 1, the treatments in mean weight fresh tuber did not mark statistically significant difference between them in both years. The values ranged from 0.144 to 0.151 Kg during year A and from 0.109 to 0.160 Kg during year B. Moreover, the yield's values ranged from 25,433 to 30,923 Kg · ha⁻¹ in year A and from 18,440 to 29,515 Kg · ha⁻¹ in year B. Urea with double inhibitors and urea + UI treatment had statistically significant difference with all other treatments in both years. The highest value was marked at 30,923 Kg · ha⁻¹ for urea + NI + UI treatment during year A. Also, the values for DM leaves and stems, ranged from 5,111 to 5,411 Kg · ha⁻¹ during the first year and from 5,017 to 5,488 Kg · ha⁻¹ during the second year. The highest value was 5,488 Kg · ha⁻¹ in urea with inhibitor urease treatment and the lowest was 5,017 Kg · ha⁻¹ in control, during year B (Table 1). In both years, urea did not mark statistically significant difference with control and urea + UI treatment. Urea with double inhibitors marked statistically significant difference with control and urea treatment (Table 1). Concerning the percentage of nitrogen in tuber, the highest value was 1.330% in urea + NI + UI treatment (year A) and the lowest was 0.825% in control (year B). During the first and second years, urea with double inhibitors treatment showed statistically significant difference with all other treatments. However, control showed no statistically significant difference with urea and with urea + UI treatment. The values of the nitrogen percentage in upper parts of sweet potato ranged from 0.436 to 0.490% in year A and from 0.325 to 0.493% in year B. The highest value was 0.493% in urea + NI + UI treatment (year A) and the lowest was 0.325% in control (year B). Urea with double inhibitors and urea with inhibitor urease showed statistically significant difference with all other treatments (Table 1).

Urea + NI + UI and urea + UI treatment concerning nitrogen uptake in upper parts showed statistically significant difference with urea and with control treatment in both years A and B. The values ranged from 22,494 to 26,540 Kg · ha⁻¹ in the first year and from 16,112 to 27,546 Kg · ha⁻¹ in the second year. The highest value was 27,546 Kg · ha⁻¹ and the lowest value was 16,112 Kg · ha⁻¹ in the second year (Table 2). As for the nitrogen uptake in tuber, in both experiments, urea showed no statistically significant difference with control. However, urea with double inhibitors showed statistically significant difference with urea + UI, and with all other treatments. The highest value was 169 Kg · ha⁻¹ in urea + NI + UI treatment (year A) and the lowest value was 81 Kg · ha⁻¹ (year B).

Regarding the total nitrogen uptake, the values ranged from 132 to 191 Kg · ha⁻¹ in the first year and from 97 to 180 Kg · ha⁻¹ in the second year. The highest value was 191 Kg · ha⁻¹ in year A and the lowest was 81 Kg · ha⁻¹ in year B. About NUE, urea had no statistically significant difference with control. Urea + NI + UI treatment had statistically significant difference with the urea + UI treatment in both years A and B. The highest value was 0.492 in urea with double inhibitors treatment and the lowest value was 0.016 in urea treatment, in the first year (Table 2). With regard to NHI, urea + NI + UI treatment had statistically significant difference with urea, whereas urea had no statistically significant difference with urea + UI in both years A and B. Also, urea with double inhibitors had no statistically significant difference with urea + UI treatment. The lowest value was 0.625 in control, in the second year, and the highest value was 0.861 in urea + NI + UI treatment, in the first year. With respect to NAE, the values ranged from 17.237 to 47.047 in year A and from 23,542 to 38,979 in year B. The lowest value was 17,237 in urea and the highest was 47,047 during year A. Urea + NI + UI treatment had no statistically significant difference with urea + UI treatment. Moreover, urea had no statistically significant difference with control, in both first and second years (Table 2).

LAI was significantly increased by all applied inhibitors through fertilizers and was affected by the years. For fertilization rate of 125 Kg · N · ha⁻¹, Pushpalatha et al. (2018) evaluated LAI at 2.14, 120 days after planting. In the results of the present study, LAI ranged from 5 to 5.14 for 120 Kg · N · ha⁻¹ application in the year 2018 and 5.11 to 5.35 in the year 2019. The marked increase in leaf area is also confirmed by Lebot's (2010) study. Still, other researchers suggest that the leaf surface index is affected by a variety of factors rather than only by fertilizer (Vosawai et al., 2015; Costantine, 2016).

As specified by our results, the marketable yield (Kg · ha⁻¹) was significantly affected by fertilizers and experimental year. These results also agree with the ones by Walker and Woodson (1987), Mortley and Hill (1990) and Fernandes et al. (2018). More specifically, Fernandes et al. (2018) and Mortley and Hill (1990) suggest that the nitrogen application greatly increases the marketable yield up to 13.5 t · ha⁻¹. However, these results disagree with Hartemink et al. (2000), who suggest that the marketable sweet potato yield was highest at 100 Kg · N · ha⁻¹ application and lowest at 400 Kg · N · ha⁻¹ application. There is a definite relationship among marketable yield, percentage of nitrogen in tuber and nitrogen uptake in tuber as shown in Table 3. Fernandes et al. (2018) mention that the relationship between marketable storage root and nitrogen uptake (Kg · ha⁻¹) was higher (R² = 0.63) than the one in storage root (R² = 0.6).

Table 1 presents that non-marketable yield (Kg · ha⁻¹) was significantly affected by the experimental year and not by the fertilizer. This is something that is not confirmed by Hartemink et al.'s (2000) study, where it is noticed that nitrogen application significantly decreases non-marketable yield during two experimental years. In general, the performance of the inhibitor that significantly affected the sweet potato yield presents a slight decrease in the second year.

The results of this study observed that DM of leaves and stems (Kg · ha⁻¹) was affected by the fertilizer and not by year. Vosawai et al. (2015) refer that the DM of upper part of the sweet potato is not affected by nitrogen fertilizer.

Nitrogen content was increased in upper parts of the plant compared with control in both years. Similar results were observed by Osaki et al. (1995). During 2018, we observed that the highest nitrogen content in upper parts was under urea plus two inhibitors (0.49%) treatment and the difference with control was 0.054. Nevertheless, in the year 2019, the highest nitrogen content in upper parts was noticed under urea fertilization treatment (0.493%) and the difference with control was 0.168%. These differences are lower than the ones in Pushpalatha et al.'s (2018) study, who observed that the difference of nitrogen content in the leaf is at the rate of 125 Kg · N · ha⁻¹ and control at 0.409%. However, in both cases, the nitrogen fertilizer application increases the nitrogen content in the above ground part of the sweet potato.

The percentage of nitrogen content in tuber was affected only by fertilizer. The significant influence of nitrogen fertilizers in tuber nitrogen content (%) was also observed by Pushpalatha et al.'s (2018) study. Specifically, the results of the current study for both years showed that higher nitrogen content in tuber was mentioned under urea + NI + UI treatment, 1.33% (2018) and 1.293% (2019). Similar results were observed by Pushpalatha et al. (2018), where the highest percentage of tuber nitrogen was noticed at 125 Kg · N · ha⁻¹ with 1.595% N.

In coherence with our results, Villagarcia et al. (1998) also mentioned that nitrogen uptake in tuber was significantly affected by fertilizers. However, Pushpalatha et al. (2018) noticed a 95.6 Kg · ha⁻¹ nitrogen uptake in tuber at the rate of 125 Kg · N · ha⁻¹ application, which is opposite to the results of the present study, where tuber nitrogen uptake values of 110 Kg · ha⁻¹ and 81 Kg · ha⁻¹ were noticed in control. Under the fertilization of 120 Kg · N · ha⁻¹, whether it was with inhibitors or not, nitrogen uptake in tuber ranged from 112 (urea treatment) to 165 (urea NI + UI treatment) in 2018 and 120 (urea treatment) to 153 (urea NI + UI treatment) in 2019. Table 3 presents an upward trend of nitrogen uptake in the tuber. The difference in the measurements between 2000 and 2001 might be related to differences in rainfall patterns. This increase is also referred in Phillips et al.'s (2005) study. Researches around reduced levels of nitrogen reveal the increase in sweet potato nitrogen uptake (Du et al., 2019). Contrariwise were the results for total uptake nitrogen marked by Pushpalatha et al. (2018). In our data, total nitrogen update was noticed at 191 Kg · N · ha⁻¹ application (under urea + NI + UI fertilizers in the year 2018) and 197.09 Kg · N · ha⁻¹ application at Pushpalatha et al.'s (2018) study.

According to our findings, the tuber yield of sweet potato was significantly affected only by fertilizers and ranged from 25,433 Kg · ha⁻¹ to 30,923 Kg · ha⁻¹ in the year 2018. Loebenstein et al. (2009) mention that for 37,000–52,500 Kg · ha⁻¹ the need is almost 200 Kg · N · ha⁻¹. The lowest tuber yield (control 25,433 Kg · ha⁻¹) is close to the findings by Pushpalatha et al. (2017), 22,398 Kg · ha⁻¹ tuber yield under 125 Kg · N · ha⁻¹. Hartemink et al. (2000) disconfirm the increase of yield due to nitrogen fertilizer whereas Abidin et al. (2017) and Bourke (1985) confirm it. Loebenstein et al. (2009) and Vosawai et al. (2015) state that fertilization rose tuber yield up to a limit. The fact that the sweet potato is not affected by nitrogen fertilization may be right as Stathers et al. (2013) and Dumbuya (2015) refer that potassium is more important. However, they state that the potassium nitrogen ratio should be 1:3.

Some researchers say that the availability of nitrogen helps the growth of shoots and leaves (Hartemink et al., 2000; Oliveira et al., 2010). The use of inhibitors in fertilizers promotes the growth of the tuber more than the growth of the aboveground part in sweet potato. Our results are in accordance with the studies of O'Sullivan et al. (1993) and Kays and Bouwkamp (1985), who noted that the reduction of nitrogen causes a reduction of the leaf surface and consequently a reduction of the production by 15–25%.

In this study, the number of tubers per plant that were recorded ranged from 5.5 to 6.39 in 2018 and from 5.25 to 6.08 in 2019. Pushpalatha et al. (2017) observed almost three tubers more per plant. Instead, Duan et al. (2019) observed more tubers per plant in control (4.23) and less under 150 Kg fertilizer ha⁻¹ (2.9 tubers/plant).

Table 2 reveals information about tuber fresh weight. The highest was noted under urea + NI + UI fertilizer (0.151 Kg). Pushpalatha et al. (2017) beheld quadruple tuber fresh weight under 125 Kg · N · ha⁻¹ (0.45 Kg) application. Additionally, Bourke (1985) mentioned that more the nitrogen is added to the fertilizer, the greater the fresh weight of the tuber produced. Duan et al. (2019) confirm the above statement and mentioned that fresh storage root yield (Kg · ha⁻¹) under 150 Kg · N · ha⁻¹ application was 36,265.72.

Many researchers have studied different factors (rates, dates of application) to increase nitrogen utilization in crops such as sweet potato. One of these research, Du et al. (2019), describes the application of nitrogen to various combinations. Our research mentions another important aspect that affects NUE, i.e., fertilizer inhibitors, such as NI and UI. Urea + NI + UI treatment enumerated the highest NUE value (0.492 and 0.425 in 2018 and 2019, respectively). In Phillips et al.'s (2005) study, the use of 84 nitrogen ha⁻¹ rate of NUE ranged from 28.4% to 23.6% in the first year. Other results by Phillips et al. (2005) and Fernandes et al. (2018) marked a decrease in NUE when increasing the rate during the two experimental years. On the other hand, Hill et al. (1990) present information on increasing NUE by raising the nitrogen rate. Specifically, NUE was noticed around 55% for cultivars with high nitrogen uptake levels (Hill et al., 1990). In relation to our results, NUE index for urea + NI + UI treatment is 49.2% in 2018 and 4.25% in 2019. Also, there are cultivars that do not significant affected by nitrogen rate so NUE (Martí and Mills, 2002).

Figure 2 depicts the correlation among fertilizers, marketable yield Kg · ha⁻¹ and yield Kg · ha⁻¹, where it was noticed that when yield (Kg · ha⁻¹) rose the marketable yield also increased. The marketable yield (Kg · ha⁻¹) and yield (Kg · ha⁻¹) are increased by adding inhibitors in fertilizers. Comparing these two yields we noted that the marketable yield was influenced more by inhibitors in fertilizer than yield (R² = 0.75 and R² = 0.54, respectively).

Figure 2

Figure 3 presents the linear relationship between NUE, NAE nitrogen indicators and fertilizers with inhibitors. It was noted that the difference between NUE and NAE under urea fertilization in sweet potato crop was high. However, using the inhibitors NI + UI this difference is reduced and eventually the two aforementioned markers are almost identical. In other words, we observed that in sweet potato cultivation, the use of inhibitors results in greater nitrogen utilisation, which is enough to affect the agricultural efficiency.

Figure 3

Figure 4 illustrates the upward trend of total nitrogen uptake (Kg · ha⁻¹) and nitrogen uptake in tuber (Kg · ha⁻¹) from control, which marked the lowest amount of nitrogen uptake under urea + NI + UI treatment, which had the highest amount. Concluding, inhibitors boost not only the total nitrogen uptake of the plant but also the tuber nitrogen uptake.

Figure 4

The significant increase observed in DM of the upper part of sweet potato increased the nitrogen uptake in tuber (165 Kg · ha⁻¹) in the year 2018 and 153 Kg · ha⁻¹ in the year 2019 when inhibitors NI and UI were added into nitrogen fertilizer (p = 0.11, Table 3).

Reviewing the data, yield (Kg · ha⁻¹) marked a significant correlation with NUE (p = 0.00), NHI (p = 0.014) and NAE (p = 0.00) inhibitors. We observed that total nitrogen uptake correlates responses to yield (p = 0.00, Table 3).

From the whole analysis, it is observed that only inhibitors mark a significant effect on yields of sweet potato and not the year.

This study has identified that using fertilizers with inhibitors will result in increasing the nitrogen exploitation for the growth and yield of sweet potato. The comparative data among several urea combinations enumerates urea + NI + UI treatment as the fertilizer causing higher yields. Urea with double inhibitors treatment resulted in better plant growth and nitrogen indices such as NUE, compared to urea treatment.

In particular, nitrogen indices were more efficient in urea + NI + UI treatment and more specifically NUE and NAE marked a better performance, thus resulting in higher yields. UI treatment showed the immediate best results in sweet potato cultivation. It is emphasised that all the above apply to the first and the second year when the experiments were conducted.