Abstract
Introduction Accumulating evidence indicates that fertilizing soybean with nickel (Ni) can enhance biological nitrogen fixation (BNF) and plant productivity. Seed application is ideal for promoting nodule formation and early plant development, but this practice raises the risk of toxicity by reducing the difference between beneficial and harmful doses. Unfortunately, studies of the effects of Ni on the survival of Bradyrhizobium spp. applied as inoculants or on BNF have not yielded a consensus on an optimal dose. Methods The objective of this study was to establish Ni thresholds that maximize physiological and productivity benefits for soybean, balancing Ni’s positive effects on BNF and plant growth against risks of phytotoxicity and bacterial inhibition. Soybean seeds were treated with nickel sulfate (NiSO 4 ·6H 2 O) at six doses: 0, 60, 120, 180, 240, and 300 mg Ni kg −1 . We then assessed the effects of seed treatment with Ni on the recovery of Bradyrhizobium cells from treated seeds, BNF as assessed by continuous-flow analysis of acetylene reduction activity (ARA), and soil CO 2 evolution in greenhouse experiments. In addition, we evaluated the impact of Ni dose on the physiological, nutritional, agronomic traits, and grain yield of soybean in multi-site field trials over two cropping seasons. Results and Discussion Ni doses of up to 60 mg kg −1 enhanced nitrogenase activity, nodulation, nodule biomass, and grain yield without compromising Bradyrhizobium viability. Doses exceeding this threshold reduced bacterial survival, nodulation, and yield, indicating Ni toxicity. The field trials exhibited a natural gradient in soil Ni levels and texture (0.4–0.6 mg dm −3 sandy to clayey), which helps explain differences in response magnitude and reinforces the need for contextualized recommendations. Applying a second-order polynomial regression to mean standardized Z-scores of integrated agronomic traits revealed a significant quadratic response ( p < 0.05). Consequently, an agronomically optimal range of 50–100 mg Ni kg −1 is recommended to sustainably optimize soybean growth and N fixation by balancing the benefits and risks of Ni application.
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Claims 7
Claim · model-written summary
Ni seed treatment up to 60 mg kg-1 enhanced nitrogenase activity, nodulation and nodule biomass in soybean.
- Taxon
- soybean
Glycine maxt0 - Treatment
- Ni doses of up to 60 mg kg −1e1
- Outcome
- nitrogenase activity, nodulation, nodule biomasse1
- Direction
- enhancede1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Nickel treatment of soybean seeds”
e1“Ni doses of up to 60 mg kg −1 enhanced nitrogenase activity, nodulation, nodule biomass, and grain yield without compromising Bradyrhizobium viability”
Claim · model-written summary
Ni seed treatment up to 60 mg kg-1 enhanced soybean grain yield.
- Taxon
- soybeanGlycine maxt0
- Treatment
- Ni doses of up to 60 mg kg −1e1
- Outcome
- grain yielde1
- Direction
- enhancede1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Nickel treatment of soybean seeds”
e1“Ni doses of up to 60 mg kg −1 enhanced nitrogenase activity, nodulation, nodule biomass, and grain yield without compromising Bradyrhizobium viability”
Claim · model-written summary
Ni seed treatment up to 60 mg kg-1 did not compromise Bradyrhizobium viability.
- Taxon
- soybeanGlycine maxt0
- Treatment
- Ni doses of up to 60 mg kg −1e1
- Interactor
- Bradyrhizobium
Bradyrhizobiume1symbiont - Outcome
- Bradyrhizobium viabilitye1
- Direction
- no_changeinferrede1
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Nickel treatment of soybean seeds”
e1“Ni doses of up to 60 mg kg −1 enhanced nitrogenase activity, nodulation, nodule biomass, and grain yield without compromising Bradyrhizobium viability”
Claim · model-written summary
Ni seed doses above the threshold reduced Bradyrhizobium survival.
- Taxon
- soybeanGlycine maxt0
- Treatment
- Doses exceeding this thresholde2
- Interactor
- Bradyrhizobium spp.Bradyrhizobiume5symbiont
- Outcome
- bacterial survivale2
- Direction
- reducede2
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Nickel treatment of soybean seeds”
e2“Doses exceeding this threshold reduced bacterial survival, nodulation, and yield, indicating Ni toxicity”
e5“survival of Bradyrhizobium spp. applied as inoculants”
Claim · model-written summary
Ni seed doses above the threshold reduced soybean nodulation and yield.
- Taxon
- soybeanGlycine maxt0
- Treatment
- Doses exceeding this thresholde2
- Outcome
- nodulation, and yielde2
- Direction
- reducede2
Not reportedComparator
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Nickel treatment of soybean seeds”
e2“Doses exceeding this threshold reduced bacterial survival, nodulation, and yield, indicating Ni toxicity”
Claim · model-written summary
A second-order polynomial regression of standardized agronomic traits showed a significant quadratic response to Ni dose in soybean.
- Taxon
- soybeanGlycine maxt0
- Outcome
- quadratic responsee3
- Value
- p < 0.05e3
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Nickel treatment of soybean seeds”
e3“revealed a significant quadratic response ( p < 0.05 )”
Claim · model-written summary
An agronomically optimal Ni range of 50–100 mg kg-1 is recommended for soybean.
- Taxon
- soybeanGlycine maxt0
- Outcome
- agronomically optimal rangee4
- Value
- 50–100 mg Ni kg −1e4
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Nickel treatment of soybean seeds”
e4“an agronomically optimal range of 50–100 mg Ni kg −1 is recommended to sustainably optimize soybean growth and N fixation”
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Claims are grouped by organism, treatment and what was measured or targeted. A paper counts as positive when any of its claims in the group is positive.