Ammonia/Ammonium Toxicity Root Symptoms Induced by Inorganic and Organic Fertilizers and Placement

Abstract

Ammoniacal fertilizers can cause seedling damage. The present aims were to characterize spatial and temporal, root morphological NH3/NH4+ toxicity symptoms, assess the extent of the toxicity zone, and relate species‐specific responses to their root architecture. Wheat (Triticum aestivum L.) and canola (Brassica napus L.) were exposed to seed and deep placed urea. Faba (Vicia faba L) seedlings were grown above organic amendments. Time‐sequential images of canola root apex and root hair die‐back, discoloration, and accelerated lateral rooting were captured with soil‐buried, high resolution digital scanners. Seed‐placed urea stunted wheat shoot and root radicles, while slow‐release urea reduced these symptoms. Primary axes of all three species were damaged by encountering the deep fertilizer zones. The multiple seminal axes and lateral root growth away from the fertilizer allowed greater wheat seedling survival, while toxicity‐damage to a single tap root of the germinating canola and faba often resulted in seedling mortality. Urea and chicken manure developed expanded NH3/NH4+ toxicity zones 1.5 to 5 cm, eliciting similar toxicity symptoms initiated at the root apex. Within 3 d after planting, canola tap root elongation stopped, followed by progressive basal directed necrosis and shrinking of the root axis, root hairs. These characteristic symptoms may be used for future toxicity diagnostics of soil‐grown plants. Elevated pH in the soil zone above the chicken manure suggested NH3 gas transported through soil pores followed by H+ consumption and elevated NH4+. Ammonia gas toxicity and species‐specific root system architecture should be considered in N placement and source selection.Core Ideas NH3/NH4+ toxicity initiates at the root apex and moves basipetally. Symptoms include tissue discoloration, axis shrinkage, root hair disfigurement, and seedling death. Toxicity zones ranged from 1 to 5 cm from the ammonia sources. Putative upward movement of ammonia raised soil pH and NH4+ above chicken manure. Most wheat axes avoided NH3/NH4+ toxicity zones, improving survival over tap‐rooted species.

Assessment of Relative Potencies of Nitrogen Sources on Seedling Root Systems

“…The non‐proliferation zone as defined by the distance of root halt above the fertilizer band indicates the ability of the fertilizer to be transported up through the soil profile and be present in toxic concentrations at a given distance above the fertilizer band. Previous research demonstrates that the zone of non‐proliferation has been found to vary from 35 mm in maize with a below and to the side placed urea at 75 mg N cm –1 to 12 mm with urea banded directly below the seed at 19 mg N cm –1 (Pan et al, 2016; Creamer and Fox, 1980). The urea zone of non‐proliferation modeled in this study aligns well with previously reported results for canola (Fig.…”

“…Toxicity symptoms have been detected both on the roots and shoots of many crop species (Dowling, 1998). The root apex and its axis has been shown to dieback on direct encounter with the fertilizer reaction zone (FRZ) (Pan et al, 2016). As a result, placement of ammoniacal N fertilizer below the seed increases the likelihood for root dieback to occur.…”

“…Controlling the rate and the source of the fertilizer being applied has been shown to mitigate the effects of the toxicity (Creamer and Fox, 1980; Passioura and Wetselaar 1972; Pan et al, 2016). A number of different N sources have been used in toxicity experiments, including anhydrous ammonia, urea ammonium nitrate (UAN), urea, di‐ammonium phosphate, mono‐ammonium phosphate, and ammonium sulfate (AS) (Passioura and Wetselaar, 1972; Creamer and Fox, 1980; Abbès et al, 1995; Kosegarten et al, 1997; Dowling, 1998; Coskun et al, 2013, Angus et al, 2014).…”

“…In addition to modifications of RSA, signs of physiological stress have also been detected in some studies. The signs of physiological damage which have been detected are a discoloration or browning of the root and shrinkage of the root width (Passioura and Wetselaar, 1972; Su et al, 2015; Pan et al, 2016).…”

“…The utility of document scanners for imaging extracted and in situ roots to track the growth and development of roots and their interactions with the soil is well documented (Pan and Bolton, 1991; Pan et al, 1998, Hammac et al, 2012; Nahar and Pan, 2018). Recently, document scanners were used to quantify ammoniacal‐N toxicity symptoms in terms of canola RSA and root morphological responses to banded urea rates of 34 kg N ha –1 and 67 kg N ha –1 (Pan et al, 2016). While rate affected the distance of apical halt, no effects on lateral emergence and root shrinkage due to increasing fertilizer rate were reported.…”