Nitrogen Fixation Inputs in Pasture and Early Successional Forest in the Brazilian Amazon Region

I’ve always told students that data are not like wine – they don’t grow better with age. So it was with some embarrassment that I finally returned to a dataset that was more than a decade old to work on publishing it. The data hadn’t gotten better, but my coauthors and I were older and perhaps wiser, and we did have a better perspective on it.

The role of biological nitrogen fixation (BNF) during secondary forest succession and in tropical pastures has been investigated and debated for several decades. Back in 1999, Dan Markewitz, Ricardo Figueiredo, Plinio Camargo, and I tried to do something about that by designing a field experiment where we were working among cattle pastures and forest of the eastern Amazon region of Brazil. We lined soil pits with plastic in one of the degraded pastures, and we installed drains to measure rainwater leaching through them. We mixed the soil with a 15N tracer in each of three treatments: (1) plant-free control plots; (2) pasture grass Brachiaria brizantha, and (3) regrowth of early successional secondary forest species.

We had initially planned on analyzing the data as a pool dilution experiment, but for a variety of reasons that would take too long to explain here, that didn’t work out as we had expected. Although it had been a heck of a lot of work, we didn’t see a clear way forward for analyzing the data back in the early 2000s. With the best of intentions to get back to it “before too long” the data were put aside. Fast forward about 15 years and the perspective afforded by that time, along with a realization that the BNF quantification issue is just as important now as ever, and also the opportunity I was afforded through NSF’s Opportunities for Promoting Understanding through Synthesis (OPUS) program, I rang up my old colleagues about a year ago and we dove into the well-aged data again.

Re-analyzing those data, we found that accumulation of N in grass biomass during the two years after installing the experiment slightly exceeded estimates of net N mineralization from the plant-free control plots, but was within the margin of error. Therefore, inputs of BNF probably were not needed for the observed grass growth. In contrast, the secondary forest vegetation accumulated about three times as much biomass N annually as the net N mineralization estimate, suggesting at least some role for BNF. Based on isotopic and mass measurements of N-fixing species, BNF was estimated to contribute at least 27 ± 3 % of mean annual plant uptake in the secondary forest regrowth vegetation plots. Although BNF is probably important for recuperation of tropical secondary forests following land use change, the majority of the N taken up by both grasses and secondary forest regrowth arose from mineralization of the large stocks of soil N.

This work will also feed into another synthesis of that body of work in Brazil. My former post-doc, Rachel Nifong, and I are working on a stoichiometric model of changing C-N-P ratios in soils and vegetation with forest clearing, pasture management, and secondary forest regrowth. In that work, we are also finding that the secondary forest needs a bit more N that we think can come from the soil in order to support the observed rates of regrowth. We hope to publish that work in the coming year, which will provide another line of evidence of significant BNF in a tropical secondary forest.

Davidson, E.A., D. Markewitz, R d O. Figueiredo, and P.B. d Camargo. 2018 Nitrogen fixation inputs in pasture and early successional forest in the Brazilian Amazon region: evidence from a claybox mesocosm study. Journal of Geophysical Research: Biogeosciences, 123, 712-721.