Iron and zinc levels in Vigna unguiculata (L.) Walp under varying phosphorus and fixed nitrogen treatment conditions

Abstract

Using phosphate fertilizer (particularly at high levels) to improve the phosphorus (P) level of soils during the cultivation of legumes and other cereals unfortunately has negative effect on the levels of micronutrients like zinc (Zn) in grains. This study and cowpea is also at the heart of the fight against micronutrient deficiency due to its ability to accumulate Fe and Zn in both grains and leaves (Mamiro, Mbwaga, Mamiro, Mwanri, & Kinabo, 2011).However, micronutrient-deficient soils are widespread; many millions of hectares of arable land worldwide are deficient in one or more micronutrient elements (Rengel, 2015). Further compounding this is the low phosphorus (P) utilization efficiency of the cowpea (Emechebe, Singh, Leleji, Atokple, & Adu, 1991). In addition, legumes store P as phytate (PA) which, together with inositol penta phosphate, is one of the main inhibitors of Zn absorption (Kaya, Küçükyumuk, & Erdal, 2009). For diffusion-supplied micronutrients, the uptake rate is governed by the soil nutrient supply, and fertilization with micronutrients (more so in case of Zn than Fe) can be effective in increasing the concentration of micronutrients at the soil-root interface (Rengel, 2015). There are growing concerns, however, that the emphasis on increasing cereal yields by the application of macronutrients alone may lead to deficiencies of micronutrients, mainly Fe and Zn, and also vitamins, in diets of the poor who rely on a cereal-based food supply (Graham & Welch, 1996). It is common practice to use P fertilizer to improve the P level of the soil during the cultivation of legumes. Unfortunately, high level of phosphate fertilization of soil has been reported to have negative effect on the levels of Zn in sorghum (Buerkert, Haake, & Ruckwied, 1998) and other cereals. Earlier reports (Zhu, Smith, & Smith, 2001;Gianquinto, Abu-Rayyan, Tola, Piccotino, & Pezzarossa, 2000;Mandal & Mandal, 1990;Haldar & Mandal, 1981;Orabi, Mashadi, Abdallah, & Morsy, 1981;Takkar, Mann, Bansal, Randhawa, & Singh, 1975), and more recently Ova, Kutman, Ozturk, and Cakmak (2015) and Drissi, AïtHoussa, Bamouh, Coquant, and Benbella (2015) have all drawn negative correlations between P fertilization and Zn levels in various cereal (wheat and maize) and leguminous plants. Benvindo, Prado, Nóbrega, and Flores (2014) even reported a corresponding decrease in foliar Zn levels with increase in P levels.There is also the well-documented issue of arbuscular mycorrhizal fungi (AMF) that greatly enhance P uptake in low P soil. Although, P application reportedly and significantly increases grain and fodder yields, it decreases AMF colonization of cowpea roots (Saidou, Singh, Abaidoo, Iwuafor, & Sanginga, 2012). This inverse relationship between P fertilization and levels of micronutrients in crops is a source of concern, and as noted by Graham and Welch (1996), may lead to deficiencies of micronutrients, mainly Fe and Zn, and also vitamins, in diets of the poor who rely on a cereal-based food supply of these micronutrients. Also, as ...

High soil phosphorus application significantly increased grain yield, phosphorus content but not zinc content of cowpea grains

“…Application of Zn containing fertilizers has been observed elsewhere to increased cowpea's grain Zn content and yield under integrated soil fertility management (Manzeke et al, 2017). Ayeni et al (2018) observed that some cowpea genotypes had a high content of Zn at all levels of P, a nding similar to our observation in this study where certain genotypes such as IT82E-18, UCR779, CB46, and IT97K-556-6 had increased grain Zn content with increasing supply of P in the growth environment. This was con rmed by the GGE biplots that displayed genotypes like 16 (IT89KD-288), 8 (Danila), and 22 (SAMPEA-9) with high grain Zn content under high soil P fertilization while genotypes like 3 (58-77), 21(SAMPEA-17), 22 (SAMPEA-9) and 23 (Sanzi) had stable grain Zn across all P levels.…”

“…While a more recent study on the impact of contrasting P rates and constant level of N fertilizers on iron and Zn contents of cowpea grains made a contrary observation. Ayeni et al (2018) observed that application of P alone at high rates do not signi cantly decrease the content of Zn and Fe on cowpea grains, but a combination of high P (60 kg/ha) and N was associated with reduced Zn content of the grains. The authors provided some explanation for this observation suggesting that the lack of decrease in Zn content when only high P rate was applied may be due to applied P being xed in certain soil compounds.…”

“…In contrast to these results, we observed that a high rate of P applications had no signi cant effect on the Zn content of cowpea genotypes investigated or and did not caused appearance of Zn de ciency symptoms in cowpea. There are few reports on cowpea addressing the relationship between high P application and Zn content of grains or other edible parts (Ayeni et al, 2018). The few reports of P-Zn relationship on the crop are sometimes confusing.…”

“…These include the application of Zn fertilizers either as a foliar spray (Zhang et al, 2012) or applied directly to the soil (Imran et al, 2016;Liu et al, 2020;Manzeke et al, 2017), improved agronomic practices such as integrated soil fertility management and intercropping in which cereals can increase Zn bioavailability of the soil and bene t the companion legumes (Manzeke et al, 2017;Xue et al, 2016), adopting multiple strategies such as dietary diversi cation (Manzeke et al, 2017) which could mean supplementing cowpea meal with high micronutrient-rich crops like common beans, a legume also rich in Zn content (Blair et al, 2010;Cichy et al, 2009) and cultivation of varieties with inherently high Zn concentration at any soil P rates. Other workers (Ayeni et al, 2018) have recommended application of medium P rates such as 40 kg P 2 O 5 /ha even without application of Zn fertilizer to obtain high yield and Zn content in the grains (Benvindo et al, 2014;Oseni, 2009;Saidou et al, 2012). Application of Zn containing fertilizers has been observed elsewhere to increased cowpea's grain Zn content and yield under integrated soil fertility management (Manzeke et al, 2017).…”

“…Loneragan & Webb, (1993) reported that there is confusing literature on P-Zn interactions for crop plants and noted that the subject of P-Zn interactions remains complex, involving many phenomena in both soils and plants, such as observations that P applications can decrease, or not decrease, Zn concentrations in plant shoots. For cowpea, the application of phosphate fertilizer is required for increased productivity in areas with low soil available P. However, its in uences on micronutrient availability on the grains and leaves have not adequately been investigated (Ayeni et al, 2018;Gonçalves et al, 2016;Santos & Boiteux, 2013). The application of P, especially at a high rate, may be counterproductive, if P application negatively affects Zn availability, as has been found in cereals (Ashok Kumar et al, 2013;Gaddameedi et al, 2018;Genc et al, 2005;Zhang et al, 2012).…”