Non-additive effects of leaf litter and insect frass mixture on decomposition processes

Although there is a growing body of evidence that herbivorous insects have a significant impact on decomposition and soil nutrient dynamics through frass excretion, how mixtures of leaf litter and insect frass influence such ecosystem processes remains poorly understood. We examined the effects of mixing of leaf litter and insect frass on decomposition and soil nutrient availability, using a study system consisting of a willow, Salix gilgiana Seemen, and a herbivorous insect, Parasa consocia Walker. The chemical characteristics of insect frass differed from those of leaf litter. In particular, frass had a 42-fold higher level of ammonium–nitrogen (NH₄ ⁺–N) than litter. Incubation experiments showed that the frass was decomposed and immobilized with respect to N more rapidly than the litter. Furthermore, litter and frass mixtures showed non-additive enhancement of decomposition and reduction of NH₄ ⁺–N, depending on the litter–frass mixing ratio. These indicate that, while insect frass generally accelerated decomposition, the effect of frass on soil nutrient availability was dependent largely on the relative amounts of litter and frass.     

Positive and negative impacts of insect frass quality on soil nitrogen availability and plant growth

Frass deposition to soil is an important pathway by which herbivorous insects impact decomposition and soil nutrient availability. However, little is known about how frass quality influences ecosystem properties. Here, we examined the effects of frass quality on the decomposition process, soil nitrogen (N) availability, and plant growth, using frass of Mamestra brassicae (L.) that fed on fertilized or unfertilized Brassica rapa L. var. perviridis Bailey. The frass quality was largely dependent on the host plant quality. Frass excreted by larvae that fed on the fertilized plants had higher N than that of larvae that fed on the unfertilized plants. The decomposition rate of the frass did not differ between N-rich and N-poor frass, except during the early decomposition period. The inorganic N concentration decreased during decomposition in both frass types. However, difference in the initial inorganic N concentration led to different consequences regarding soil N availability. Furthermore, addition of frass to the soil differently influenced the growth of B. rapa plants depending on the frass quality: plant biomass was increased by N-rich frass addition but decreased by N-poor frass addition, compared to the biomass without frass addition. These results indicate that frass quality is an important factor in determining the impact of herbivorous insects on nutrient dynamics, and that frass positively or negatively influences soil N availability and plant growth, depending on its quality.     

Insect herbivores and their frass affect Quercus rubra leaf quality and initial stages of subsequent litter decomposition

Defoliation‐induced changes in plant foliage are ubiquitous, though factors mediating induction and the extent of their influence on ecosystem processes such as leaf litter decomposition are poorly understood. Soil nitrogen (N) availability, which can be affected by insect herbivore frass (feces), influences phytochemical induction. We conducted experiments to test the hypotheses that insect frass deposition would (1) reduce phytochemical induction following herbivory and (2) increase the decomposition and nutrient release of the subsequent leaf litter. During the 2002 growing season, 80 Quercus rubra saplings were subjected to a factorial experiment with herbivore and frass manipulations. Leaf samples were collected throughout the growing season to measure the effects of frass deposition on phytochemical induction. In live foliage, herbivore damage increased tannin concentrations early, reduced foliar N concentrations throughout the growing season, and lowered lignin concentrations in the late season. Frass deposition apparently reduced leaf lignin concentrations, but otherwise did not influence leaf chemistry. Following natural senescence, litter samples from the treatment groups were decomposed in replicated litterbags for 18 months at the Coweeta Hydrologic Laboratory, NC. In the dead litter samples, initial tannin concentrations were lower in the herbivore damage group and higher in the frass addition group relative to their respective controls. Tannin and N release rates in the first nine months of decomposition were also affected by both damage and frass. However, decomposition rates did not differ among treatment groups. Thus, nutrient dynamics important for some ecosystem processes may be independent from the physical loss of litter mass. Overall, while lingering effects of damage and even frass deposition can therefore carry over and affect ecosystem processes during decomposition, their effects appear short lived relative to abiotic forces that tend to homogenize the decomposition process.     

Bemisia tabaci (Gennadius) population density and pupal size are dependent on rootstock and nitrogen in hydroponic tomato crop

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Differential effects of mineral and organic N sources,and of ectomycorrhizal infection by Hebeloma cylindrosporum,on growth and N utilization in Pinus pinaster

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH₄⁺ or NO₃^?) or organic N (L ‐glutamate) and at 5 mol m^?3. Plants were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non‐mycorrhizal plants grew better with NH₄⁺ than with NO₃^? and grew very slowly when supplied with L ‐glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L ‐glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change ¹⁵NH₄⁺ uptake rate by roots, suggesting that the growth decrease of the host‐plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH₄⁺ acquisition. In contrast, in NO₃^?‐grown plants, in addition to draining carbon for NO₃^? reduction the fungus competed with the root for NO₃^? uptake. With NH₄⁺ or NO₃^? feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L ‐glutamate‐grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.     

Nitrogen source and water regime effects on durum wheat photosynthesis and stable carbon and nitrogen isotope composition

Water stress and nitrogen (N) availability are the main constraints limiting yield in durum wheat (Triticum turgidum L. var. durum). This work investigates the combined effects of N source (ammonium–NH₄⁺, nitrate–NO₃^– or a mixture of both–NH₄⁺:NO₃^–) and water availability (well‐watered vs. moderate water stress) on photosynthesis and water‐use efficiency in durum wheat (cv. Korifla) flag leaves grown under controlled conditions, using gas exchange, chlorophyll fluorescence and stable carbon isotope composition (δ¹³C). Under well‐watered conditions, NH₄⁺‐grown plants had lower net assimilation rates (A) than those grown with the other two N forms. This effect was mainly due to lower stomatal conductance (g_s). Under moderate water stress, differences among N forms were not significant, because water regime (WR) had a stronger effect on g_s and A than did N source. Consistent with lower g_s, δ¹³C and transpiration efficiency (TE) were the highest in NH₄⁺ leaves in both water treatments. These results indicate higher water‐use efficiency in plants fertilized with NH₄⁺ due to stomatal limitation on photosynthesis. Moreover, leaf δ¹³C is an adequate trait to assess differences in photosynthetic activity and water‐use efficiency caused by different N sources. Further, the effect of these growing conditions on the nitrogen isotope composition (δ¹⁵N) of flag leaves and roots was examined. Water stress increased leaf δ¹⁵N in all N forms. In addition, leaf δ¹⁵N increased as root N decreased and as leaf δ¹³C became less negative. Regardless of WR, the leaf δ¹⁵N of NO₃^–‐grown plants was lowest. Based on stepwise and canonical discriminant analyses, we conclude that plant δ¹⁵N together with δ¹³C and other variables may reflect the conditions of N nutrition and water availability where the plants were grown. Thus well‐watered plants grown with NH₄⁺:NO₃^– resembled those grown with NO3^–, whereas under water stress they were closer to plants grown with NH₄⁺.     

Segregation of nitrogen use between ammonium and nitrate of ectomycorrhizas and beech trees

Here, we characterized nitrogen (N) uptake of beech (Fagus sylvatica) and their associated ectomycorrhizal (EM) communities from NH₄⁺ and NO₃^?. We hypothesized that a proportional fraction of ectomycorrhizal N uptake is transferred to the host, thereby resulting in the same uptake patterns of plants and their associated mycorrhizal communities. ¹⁵N uptake was studied under various field conditions after short‐term and long‐term exposure to a pulse of equimolar NH₄⁺ and NO₃^? concentrations, where one compound was replaced by ¹⁵N. In native EM assemblages, long‐term and short‐term ¹⁵N uptake from NH₄⁺ was higher than that from NO₃^?, regardless of season, water availability and site exposure, whereas in beech long‐term ¹⁵N uptake from NO₃^? was higher than that from NH₄⁺. The transfer rates from the EM to beech were lower for ¹⁵N from NH₄⁺ than from NO₃^?. ¹⁵N content in EM was correlated with ¹⁵N uptake of the host for ¹⁵NH₄⁺, but not for ¹⁵NO₃^?‐derived N. These findings suggest stronger control of the EM assemblage on N provision to the host from NH₄⁺ than from NO₃^?. Different host and EM accumulation patterns for inorganic N will result in complementary resource use, which might be advantageous in forest ecosystems with limited N availability.     

Do native and invasive herbivores have an effect on Brassica rapa pollination?

• Mutualistic (e.g. pollination) and antagonistic (e.g. herbivory) plant–insect interactions shape levels of plant fitness and can have interactive effects.
• By using experimental plots of Brassica rapa plants infested with generalist (Mamestra brassicae) and specialised (Pieris brassicae) native herbivores and with a generalist invasive (Spodoptera littoralis) herbivore, we estimated both pollen movement among treatments and the visiting behaviour of honeybees versus other wild pollinators.
• Overall, we found that herbivory has weak effects on plant pollen export, either in terms of inter‐treatment movements or of dispersion distance. Plants infested with the native specialised herbivore tend to export less pollen to other plants with the same treatment. Other wild pollinators preferentially visit non‐infested plants that differ from those of honeybees, which showed no preferences. Honeybees and other wild pollinators also showed different behaviours on plants infested with different herbivores, with the former tending to avoid revisiting the same treatment and the latter showing no avoidance behaviour. When taking into account the whole pollinator community, i.e. the interactive effects of honeybees and other wild pollinators, we found an increased avoidance of plants infested by the native specialised herbivore and a decreased avoidance of plants infested by the invasive herbivore.
• Taken together, our results suggest that herbivory may have an effect on B. rapa pollination, but this effect depends on the relative abundance of honeybees and other wild pollinators.

     

A critical experimental evaluation of methods for determination of NH4+ in plant tissue, xylem sap and apoplastic fluid

Ammonium (NH₄⁺) is a central intermediate in the N metabolism of plants, but the quantitative importance of NH₄⁺ in transporting N from root to shoot and the capability of plants to store NH₄⁺ in leaves are still matters of substantial controversy. This paper shows that some of these controversies have to be related to the use of inadequate analytical procedures used for extraction and quantification of NH₄⁺ in plants. The most frequently used methods for determination of NH₄⁺, viz. colorimetric methods based on the classical Berthelot reaction, suffered severely from interference caused by amino acids, amines, amides and proteins. For some of these metabolites the interference was positive, while for others it was negative, making correction impossible. Consequently, colorimetric analysis is inapplicable for determination of NH₄⁺ in plants. Results obtained by ion chromatography may overestimate the NH₄⁺ concentration due to co‐elution of NH₄⁺ with amines like methylamine, ethylamine, ethanolamine and the non‐protein amino acid Γ‐aminobutyric acid. Derivatization of NH₄⁺ with o‐phthalaldehyde at alkaline pH and subsequent quantification of NH₄⁺ by fluorescence spectroscopy was also associated with interference. However, when pH was lowered to 6.8 during derivatization and 2‐mercaptoethanol was used as reductant, NH₄⁺ could be determined with a high selectivity and sensitivity down to a detection limit of 3.3 μM in a 10‐μl sample volume. Derivatization was performed on‐line using a column‐less HPLC system, enabling rapid quantification of NH₄⁺ in a few minutes. Flow injection analysis with on‐line gas dialysis was, likewise, free from interference, except when applied on highly senescent plant material containing volatile amines. Labile N metabolites in leaf tissue extract, xylem sap and apoplastic fluid were degraded to NH₄⁺ during extraction and subsequent instrumental analysis if the samples were not stabilised. A simple and efficient stabilisation could be obtained by addition of 10 mM ice‐cold HCOOH to the plant extraction medium or to the samples of apoplastic fluid or xylem sap. We conclude that significant concentrations of NH₄⁺, exceeding 1 mM, may occur in xylem sap, leaf apoplastic fluid and leaf tissue water of nitrate‐grown tomato and oilseed rape plants. The measured NH₄⁺ concentrations were not a result of excessive N supplies, as even plants grown under mildly N‐deficient conditions contained NH₄⁺.     

Elevated temperature may intensify the positive effects of nutrients on microbial decomposition in streams

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Molecular identification and functional characterization of GhAMT1.3 in ammonium transport with a high affinity from cotton (Gossypium hirsutum L.)

Ammonium (NH₄⁺) represents a primary nitrogen source for many plants, its effective transport into and between tissues and further assimilation in cells determine greatly plant nitrogen use efficiency. However, biological components involved in NH₄⁺ movement in woody plants are unclear. Here, we report kinetic evidence for cotton NH₄⁺ uptake and molecular identification of certain NH₄⁺ transporters (AMTs) from cotton (Gossypium hirustum). A substrate‐influx assay using ¹⁵N‐isotope revealed that cotton possessed a high‐affinity transport system with a K_m of 58 μM for NH₄⁺. Sequence analysis showed that GhAMT1.1–1.3 encoded respectively a membrane protein containing 485, 509 or 499 amino acids. Heterologous functionality test demonstrated that GhAMT1.1–1.3 expression mediated NH₄⁺ permeation across the plasma membrane (PM) of yeast and/or Arabidopsis qko‐mutant cells, allowing a growth restoration of both mutants on NH₄⁺. Quantitative PCR measurement showed that GhAMT1.3 was expressed in roots and leaves and markedly up‐regulated by N‐starvation, repressed by NH₄⁺ resupply and regulated diurnally and age‐dependently, suggesting that GhAMT1.3 should be a N‐responsive gene. Importantly, GhAMT1.3 expression in Arabidopsis improved plant growth on NH₄⁺ and enhanced total nitrogen accumulation (～50% more), conforming with the observation of 2‐fold more NH₄⁺ absorption by GhAMT1.3‐transformed qko plant roots during a 1‐h root influx period. Together with its targeting to the PM and saturated transport kinetics with a K_m of 72 μM for NH₄⁺, GhAMT1.3 is suggested to be a high‐affinity NH₄⁺ permease that may play a significant role in cotton NH₄⁺ acquisition and utilization, adding a new member in the plant AMT family.     

Fungal contributions to carbon flow and nutrient cycling during decomposition of standing Typha domingensis leaves in a subtropical freshwater marsh

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Positive effect of shredders on microbial biomass and decomposition in stream microcosms

1. Animals play a major role in nutrient cycling via excretory processes. Although the positive indirect effects of grazers on periphytic algae are well understood, little is known about top‐down effects on decomposers of shredders living on leaf litter. 2. Nutrient cycling by shredders in oligotrophic forest streams may be important for the microbial‐detritus compartment at very small spatial scales (i.e. within the leaf packs in which shredders feed). We hypothesised that insect excretion may cause local nutrient enrichment, so that microorganism growth on leaves is stimulated. 3. We first tested the effect of increasing concentration of ammonium (+10, +20 and +40 μg NH₄⁺ L^?1) on fungal and bacterial biomass on leaf litter in a laboratory experiment. Then we performed two experiments to test the effect of the presence and feeding activity of shredder larvae. We used two species belonging to the trichopteran family Sericostomatidae: the Palaearctic Sericostoma vittatum and the Neotropical Myothrichia murina, to test the effect of these shredders on fungal and bacterial biomass and decomposition on leaves of Quercus robur and Nothofagus pumilio, respectively. All experiments were run in water with low ammonium concentrations (2.4 ± 0.34 to 14.47 ± 0.95 μg NH₄⁺ L^?1). 4. After 5 days of incubation, NH₄ concentrations were reduced to near‐ambient streamwater concentrations in all treatments with leaves. Fungal biomass was positively affected by increased ammonium concentration. On the other hand, bacteria abundance was similar in all treatments, both in terms of abundance (bacteria cells mg^?1 leaf DW) and biomass. However, there was a tendency towards larger mean cell size in treatments with 20 μg NH₄ L^?1. 5. In the experiment with S. vittatum, fungal biomass in the treatment with insects was more than twice that in the control after 15 days. Bacteria were not detected in treatments with insects, where hyphae were abundant, but they were abundant in treatments without larvae. In the decomposition experiment run with M. murina, leaf‐mass loss was significantly higher in treatments with larvae than in controls. 6. Our hypothesis of a positive effect of shredders on fungal biomass and decomposition was demonstrated. Insect excretion caused ammonium concentration to increase in the microcosms, contributing to microbial N uptake in leaf substrata, which resulted in structural and functional changes in community attributes. The positive effect of detritivores on microbes has been mostly neglected in stream nutrient‐cycling models; our findings suggest that this phenomenon may be of greater importance than expected in stream nutrient budgets.     

Variability of leaf photosynthetic characteristics in rice and its relationship with resistance to water stress under different nitrogen nutrition regimes

The negative effects of water stress on rice can be alleviated by NH₄⁺ nutrition. However, the effects of mixed nitrogen (N) nutrition (NO₃^? + NH₄⁺) on resistance to water stress are still not well known. To investigate the response of rice growth to water stress and its relationship with photosynthetic characteristics, a hydroponic experiment supplying different N forms was conducted. Compared with NO₃^? nutrition, mixed‐N and NH₄⁺ nutrition greatly alleviated the reduction of leaf area, chlorophyll content, and photosynthesis under water stress, whilst subsequently maintaining higher biomass. In contrast, water stress inhibited the root‐shoot ratios in NH₄⁺‐ and mixed‐N‐supplied plants, indicating reduced root growth and higher photosynthate availability to shoots. The following key observations were made: (1) a similar stomatal limitation and low proportion of activated Rubisco were observed among the three different N nutrition regimes; (2) increased mesophyll conductance in NH₄⁺‐ and mixed‐N‐supplied plants simultaneously stimulated leaf photosynthesis and improved the water use efficiency and (3), the maximum carboxylation rate and actual photochemical efficiency of photosystem II in NH₄⁺‐ and mixed‐N‐supplied plants were significantly higher than that in NO₃^?‐supplied plants, thus resulting in higher photochemical efficiency under water stress. In conclusion, mixed‐N and NH₄⁺ nutrition may be used to develop strategies for improved water stress resistance and stimulated biomass production under conditions of osmotic stress and possibly drought.     

Asymmetric plant-mediated cross-effects between a herbivorous insect and a phytopathogenic fungus

• 1 Cross‐effects between a herbivorous insect and a phytopathogenic fungus on their common host plant were examined. Specifically, we addressed the questions whether (i) infection of Chinese cabbage leaves by the fungus Alternaria brassicae affects the development and host selection behaviour of the leaf beetle Phaedon cochleariae and (ii) whether herbivory influences host suitability of Chinese cabbage for A. brassicae.
• 2 Feeding on fungus‐infected leaves prolonged larval development and reduced pupal weight of P. cochleariae. Adult beetles avoided feeding and egg deposition on fungus‐infected leaves. In contrast to these local effects, no systemic effect of phytopathogenic infection on the herbivore was detected.
• 3 Herbivory did not influence fungal growth neither locally nor systemically.
• 4 Thus, our results demonstrate an asymmetric relationship between herbivore and fungus. Whereas herbivory had no visible impact on fungal growth, fungal infection of the plant induced local resistance against P. cochleariae.

     

Fertilization rates and clay fixed ammonium in two Quebec soils

Clay fixed NH₄ ⁺ can provide a significant sink for fertilizer N, as well as a source of N for plant uptake. Knowledge or soil NH₄ ⁺ fixing capacity and release for crops is necessary to develop long-term fertilizer programs. Field experiments with corn (Zea mays L.) were carried out to investigate soil NH₄ ⁺ fixing capacity and subsequent release as influenced by fertilizer rates using ¹⁵N in a Ste. Rosalie clay (fine, mixed, frigid, Typic Humaquept) and a Chicot sandy clay loam (fine-loamy, mixed, frigid, Typic Hapludalf). With high N rates increased NH₄ ⁺ fixation occurred only in the Ste. Rosalie soil. At the end of the first growing season, fertilizer N recovery as clay fixed NH₄ ⁺ for high and normal rates of fertilizer in the Ste. Rosalie soil was 17.8% and 28.7%, respectively and the recovery for the high and normal rates in the Chicot soil was 4.6 and 10.5%, respectively. Significant amounts of clay fixed NH₄ ⁺-N were released in the soil profile in the second year after ¹⁵N application on the Chicot soil. Recently clay fixed fertilizer NH₄ ⁺N was released more rapidly than that of the native fixed NH₄ ⁺, from the surface layer of the Ste. Rosalie soil. The fertilizer fixed NH₄ ⁺ seems to be in a more labile N pool than the native fixed NH₄ ⁺-N in the Chicot soil.     

氮素形态及配比对5个种源菘蓝的生物碱类成分和多糖的影响

以5份不同种源的菘蓝为材料,采用田间小区试验,设置不施氮(CK)、硝态氮(NO3--N)、铵态氮(NH4+-N)、NH4+-N/NO3--N=75/25、NH4+-N/NO3--N=50/50、NH4+-N/NO3--N=25/75和酰胺态氮等7个处理,分析比较了不同种源植株的靛蓝、靛玉红和总生物碱含量、(R,S)-告依春及多糖含量等指标的差异,为菘蓝栽培生产中氮素的高效利用提供理论参考。结果表明:氮素处理有利于提高山西运城菘蓝和陕西商洛菘蓝叶内靛蓝含量,以及安徽亳州菘蓝和陕西商洛菘蓝叶内的总生物碱含量;NH4+-N/NO3--N=50/50处理对山西运城菘蓝,以及酰胺态氮处理对山西运城菘蓝和陕西商洛菘蓝叶内生物碱类成分的积累均有促进作用;与对照相比,氮素处理亦能有效地提高甘肃张掖菘蓝和陕西商洛菘蓝根内的(R,S)-告依春及安徽亳州菘蓝根内的多糖含量;安徽阜阳菘蓝(R,S)-告依春含量在任一氮处理下均远远高于其他种质菘蓝。研究表明,不同种源菘蓝对氮素处理的响应存在较大的差异,建议生产中综合考虑菘蓝的来源和需肥规律,采用经济有效的施氮组合,以提高其活性成分含量。     

Opposing effects of organic and conventional fertilizers on the performance of a generalist and a specialist aphid species

• 1 Sustainable and conventional farming systems use fertilizers that differ in the availability of nitrogen, which may affect plant quality to alter the abundance and performance of potential pest species.
• 2 We grew brassica plants in several types of fertilizer, including those commonly used in conventional and sustainable farming systems, and an unfertilized control. The effects of fertilizer type on the performance of two aphid species and foliar glucosinolate content were investigated.
• 3 Both aphid species performed poorly (with reduced fecundity) on the unfertilized treatment compared with those feeding on fertilized host plants.
• 4 Brevicoryne brassicae, the brassica specialist, performed best on Brassica oleracea plants fertilized with an organic animal manure, with a 72% increase in fecundity and an 18% increase in intrinsic rate of increase compared with plants fertilized with ammonium nitrate.
• 5 By contrast, the generalist Myzus persicae had an intrinsic rate of increase that was reduced by 15% on plants growing in the animal manure compared with those growing in ammonium nitrate.
• 6 These results may explain earlier findings on the effects of fertilizer type on aphid populations in the field, and are discussed in the context of pest species' responses to sustainable and conventional agricultural systems.

     

γ‐Aminobutyric acid addition alleviates ammonium toxicity by limiting ammonium accumulation in rice (Oryza sativa) seedlings

Excessive use of nitrogen (N) fertilizer has increased ammonium (NH₄⁺) accumulation in many paddy soils to levels that reduce rice vegetative biomass and yield. Based on studies of NH₄⁺ toxicity in rice (Oryza sativa, Nanjing 44) seedlings cultured in agar medium, we found that NH₄⁺ concentrations above 0.75 mM inhibited the growth of rice and caused NH₄⁺ accumulation in both shoots and roots. Use of excessive NH₄⁺ also induced rhizosphere acidification and inhibited the absorption of K, Ca, Mg, Fe and Zn in rice seedlings. Under excessive NH₄⁺ conditions, exogenous γ‐aminobutyric acid (GABA) treatment limited NH₄⁺ accumulation in rice seedlings, reduced NH₄⁺ toxicity symptoms and promoted plant growth. GABA addition also reduced rhizosphere acidification and alleviated the inhibition of Ca, Mg, Fe and Zn absorption caused by excessive NH₄⁺. Furthermore, we found that the activity of glutamine synthetase/NADH‐glutamate synthase (GS; EC 6.3.1.2/NADH‐GOGAT; EC1.4.1.14) in root increased gradually as the NH₄⁺ concentration increased. However, when the concentration of NH₄⁺ is more than 3 mM, GABA treatment inhibited NH₄⁺‐induced increases in GS/NADH‐GOGAT activity. The inhibition of ammonium assimilation may restore the elongation of seminal rice roots repressed by high NH₄⁺. These results suggest that mitigation of ammonium accumulation and assimilation is essential for GABA‐dependent alleviation of ammonium toxicity in rice seedlings.