Molybdenum, phosphorus, and pH do not constrain free-living nitrogen fixation in a tropical forest in the southeastern Amazon
Abstract
Biological nitrogen (N) fixation allows ecosystems to obtain new N, and some of the highest documented N-fixation rates are found in tropical forests. However, the low availability of molybdenum (Mo) and phosphorus (P) can constrain free-living N-fixation. The southeastern Amazon is a region where N-fixation has not yet been quantified and where Mo and P concentrations are likely low in the dominant, highly-weathered oxisols. We hypothesized that (1) Mo and P limitation of free-living N-fixation would be more pronounced in this region, compared with locations like Costa Rica and Panama where Mo and P inputs are higher from atmospheric sources (dust and sea-salt spray) and the soils are less weathered, and that (2) the low soil pH reduces Mo and P availability, further constraining N-fixation. We conducted a series of block-design field experiments at Tanguro Ranch in Mato Grosso, Brazil by adding Mo and P alone and in combination with lime, and measured N-fixation in the soil and litter immediately, and then weeks and months after the applications. To verify that our experiment altered soil conditions, we measured total and resin-extractable Mo and P along with soil pH one day and one month after the applications. We also conducted a short-term laboratory experiment by adding the same treatments to soil and litter and measured N-fixation after one day.
Despite the higher concentrations of resin-extractable Mo and P and higher pH in the Mo, P, and lime addition plots in the field experiment, none of the treatments had any significant effect on free-living N-fixation either initially or over time. We found an almost identical response in the laboratory experiment. While we did find low Mo and P concentrations in the soil, we found high concentrations of soil extractable ammonium and nitrate in surface soils and abundant nitrate in deep soils (1 to 4 m). We postulate that the high availability of ammonium and nitrate, rather than Mo or P soil concentrations, controls rates of N-fixation in this forest. The low rates of free-living N-fixation have important implications for understanding the role of N-fixation in tropical forests more generally, because large areas of tropical forest occur in lowland, interior basins on highly-weathered oxisols.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B53K2211W
- Keywords:
-
- 0444 Evolutionary geobiology;
- BIOGEOSCIENCESDE: 0465 Microbiology: ecology;
- physiology and genomics;
- BIOGEOSCIENCESDE: 0469 Nitrogen cycling;
- BIOGEOSCIENCESDE: 4870 Stable isotopes;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL