Higher relative performance at low soil nitrogen and moisture predicts field distribution of nitrogen-fixing plants
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Background and Aims
Symbiotic associations between vascular plants and nitrogen-fixing bacteria are expected to be costly except when N availability is low. We tested the prediction that in low-N soils in dry climates, plants with nitrogen-fixing symbioses (N-fixing species) have higher growth rates, and occur relatively more frequently, than non-fixing species,
In a pot experiment, we measured the growth and survival of 6 N-fixing and 8 non-fixing species across nitrogen and moisture gradients. Using plot survey data from the South Island, New Zealand, we then modelled the relative occurrence of N-fixing species using derived measures of temperature, soil N and moisture.
Non-fixing species had higher relative growth rates than N-fixing species except when both total N and soil moisture were low. Low soil moisture increased the root:shoot ratio in N-fixing species more than twice that observed in non-fixing species. Soil moisture had a strong effect on mortality, which was slightly lower for N-fixing species. Survey data showed that a higher proportion of N-fixing species were present at cool, dry sites with low levels of soil N.
In temperate climates, with geologically young landscapes, the influences of soil N and water on N availability are key factors determining the relative success of N-fixing and non-fixing species.
KeywordsFabaceae Nitrogen fixation Nutrient availability Phosphorus Relative growth rate Spatial modelling
This paper was funded by the New Zealand Foundation for Research, Science and Technology. We thank Chris Berg for helping with the maintenance of the experiment; Peter Keller and Lester Davey for provision of seeds and seedlings; Guy Forrester for statistical advice; David Purcell and Stuart Oliver for nursery advice; Jagath Ekanayake for help with soil physical measurements; Ian Lynn, Allan Hewitt and Ian Dickie for discussions and advice on soil and plant–soil micro-organism interactions; and Peter Bellingham, Joe Craine, Bill Lee, Duane Peltzer and anonymous reviewers for helpful discussions and comments that greatly improved the manuscript. We thank the many people who provided data, and also acknowledge the use of data drawn from the National Vegetation Survey Database (NVS).
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