Study reveals delays in symbiotic nitrogen fixation in trees

Symbiotic nitrogen fixation (SNF), the process by which plants capture atmospheric nitrogen with the help of bacteria, occurs more slowly than previously thought. The revelation comes from a study conducted by researchers at Columbia University and the University of Texas, who quantified for the first time the delays in SNF adjustment in young trees. The results indicate that these delays compromise the performance of fixing species in environments where the supply of nitrogen in the soil varies rapidly.

The research involved four tree species with different climatic origins and types of symbiosis: two tropical (Gliricidia sepium e Morella cerifera) and two seasoned (Robinia pseudoacacia e Red Alnus).

Each belongs to one of two known symbiotic groups: rhizobial and actinorhizal. The plants were grown under controlled temperature conditions, in both cold and hot environments, and underwent abrupt changes in nitrogen supply to simulate natural challenges.

The results... The reduction in SNF after nitrogen addition took between 31 and 51 days to complete. The increase in fixation after nutrient removal took between 108 and 138 days, considering the entire cycle. After the first detection of activity, the system still took between 21 and 57 days to reach the maximum rate. These numbers contradict previous assumptions, which estimated adaptations in a few days.

The experiment revealed that plants from temperate climates and those grown in hot environments reduce SNF more quickly. Tropical species and those exposed to cold conditions showed a slower rate of reduction. This difference reinforces the hypothesis that climatic and evolutionary factors affect the physiological response capacity.

Another highlight is the differences between the symbioses. Plants with rhizobia initiate SNF earlier, but the growth rate of fixation is greater in actinorhizal plants, once the process begins. The structure of the root nodules helps to explain this behavior. Actinorhizal nodules are larger and more woody, which requires more time to build, but allows faster growth after activation.

According to the scientists, the methodology used in the study represents a breakthrough. They employed a continuous, non-destructive analysis system known as ARACAS, which measures the activity of nitrogenase, the enzyme responsible for fixation. The system provides real-time data on ethylene production and CO2 exchange, allowing SNF to be monitored over months with unprecedented precision.

From an ecological perspective, the data reinforce a well-known paradox: tropical forests, despite containing trees that regulate SNF in response to excess nitrogen, continue to lose large volumes of the nutrient.

The explanation may lie in the delays in regulation itself. When the SNF takes weeks to adjust to the new scenario, the plant continues to fix nitrogen even when it is not needed, generating surpluses that escape the ecosystem through leaching or gas emissions.

The theoretical model that underpinned the study predicted that delays of more than two days would compromise the competitiveness of nitrogen-fixing species in relation to non-fixing species. The empirical results, with lags of more than one month, confirm that nitrogen-fixing species may lose ground in ecosystems where nitrogen supply fluctuates rapidly.

This phenomenon may explain, at least in part, the lower presence of fixing trees in high latitudes, where cold weather prolongs the delays. Interestingly, even under high temperatures, the geographic origin of the species maintained an influence on the pace of adjustment. This suggests that evolutionary adaptation to the original environment persists as a determining factor in the physiological response.

The study also highlights important limits to phenotypic plasticity. Although SNF is theoretically adjustable to environmental conditions, the time required for this adjustment limits its effectiveness. Plants do not respond instantly, and this slowness can mean the difference between ecological success and failure.

More information at doi.org/10.1111/nph.70295