Inhibition of N2 Fixation by N Fertilization of Common Bean (Phaseolus vulgaris L.) Plants Grown on Fields of Farmers in the Eastern Cape of South Africa, Measured Using 15N Natural Abundance and Tissue Ureide Analysis

Abstract

Inhibition of N2 fixation in N-fertilized common bean (Phaseolus vulgaris L.) plants growing on the fields of farmers in the Eastern Cape of South Africa was measured using 15N natural abundance and tissue ureide analysis. The N-fertilized bean plants revealed greater soil N uptake, higher concentrations of nitrate in organs, low tissue ureide levels, and much lower percent relative ureide-N abundance when compared with unfertilized plants. In contrast, the unfertilized plants showed greater nodule fresh weight, higher N derived from fixation (e.g., 84.6, 90.4, and 97.1% at Lujecweni fields 2, 3, and 4, respectively), increased amount of N-fixed (e.g., 163.3, 161.3, and 140.3 kg ha−1 at Lujecweni fields 2, 3, and 4, respectively), greater ureide concentration in stems and petioles, higher % relative ureide-N abundance, and low soil N uptake. We also found that the percent N derived from fixation (%Ndfa) was very high for some bean plants receiving a double dose of N fertilizer [e.g., Lujecweni field 1 (51.8%) and Tikitiki field 1 (53.3%], and quite high for others receiving a single dose of N fertilizer [e.g., Tikitiki field 2 (50.1%), Mfabantu fields 1 and 2 (45.5 and 79.9%, respectively), and St. Luthberts field 1 (58.9%)]. Though not assessed in this study, it is likely that the rhizobia that effectively nodulated the N-fertilized bean plants and fixed considerable amounts of symbiotic N had constitutive and/or inducible nitrate reductase genes for reducing nitrate in nodules and bacteroids, hence their ability to form root nodules and derived high %Ndfa in bean with added N. While single- and double-dose N fertilizer applications increased plant growth and grain yield compared to unfertilized bean plants, the single-dose N fertilizer application produced much greater grain yield than the double dose. This indicates that farmers should stop using a double dose of N fertilizers on bean production, as it decreases yields and can potentially pollute the environment. This study has however shown that government supply of free N fertilizers to resource-poor farmers in South Africa increased bean yields for food/nutritional security.

Hydroponic Common-Bean Performance under Reduced N-Supply Level and Rhizobia Application

“…The inhibitory effects of standard levels of inorganic-N supply on BNF of the common bean found in this study are in accordance with many previous studies e.g., [ 19 , 20 , 31 ]. Moreover, the recorded %Ndfa values under either standard (100%) or deficit inorganic-N regimes here are in accordance with those recorded by Reinprecht et al [ 19 ], who cultivated different climbing-common-bean genotypes under similar inorganic-N regimes.…”

“…This is due to the poor BNF-capacity of the common bean, and optimal productivity relies on externally applied inorganic-N input [ 18 ]. However, a high inorganic-N application inhibits plant nodulation and BNF, further increasing the crop’s dependency on mineral-N addition, if yield is to be maintained [ 19 , 20 ]. Research has therefore been conducted to develop low inorganic-N-use management-strategies, based on optimising BNF without compromising the productivity of the common bean.…”

Lotus japonicus regulates root nodulation and nitrogen fixation dependent on the molecular form of nitrogen fertilizer

“…However, the relationship between fertilization and nodule count is complex, and exceptions can be instructive. In the present study, nodulation was never suppressed under the fertilization regimes tested, as has been shown for other legumes (Habinshuti et al 2021;Streeter 1988). Diverse empirical outcomes require cautious interpretation, as it can be difficult to compare among varied experimental conditions, nitrogen concentrations, forms of nitrogen, and delivery methods.…”