Nitrogen Fixation Inputs in Pasture and Early Successional Forest in the Brazilian Amazon Region: Evidence From a Claybox Mesocosm Study

The role of biological nitrogen fixation (BNF) during secondary forest succession and in tropical pastures has been investigated and debated for several decades. Here we present results of a replicated experimental study in a degraded cattle pasture of eastern Amazonia using mass balance and a 15N tracer in lined soil pit mesocosms with three treatments: (1) plant‐free control plots, (2) pasture grass Brachiaria brizantha, and (3) regrowth of early successional secondary forest species. Accumulation of N in grass biomass slightly exceeded estimates of net N mineralization from the plant‐free control plots but was within the margin of error, so inputs of BNF may not have been needed. In contrast, the secondary forest vegetation accumulated about 3 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 stocks of soil N. Plain Language Summary Replacing tropical forests with cattle pastures can alter the amount of nitrogen available to support the growth of pasture grasses or regrowing trees. Pastures are often abandoned when they are no longer productive, allowing native forests to regrow. Here we present the results of an experiment located in a degraded cattle pasture in the Brazilian Amazon Basin, where we measured the amount of nitrogen flowing in and out of the soil. Three soil plots were kept free of plants as a control; three were planted in a common pasture grass; and three were planted with seedlings of native shrubs and trees. Some of the tree species can use nitrogen from the air in a process called biological nitrogen fixation. A special nitrogen label was added to the soil to distinguish between nitrogen from soil and air. Grasses did not take up nitrogen from the air, but nearly one third of the nitrogen in the forest regrowth plants came from the air. These results demonstrate that nitrogen fixation can be important for the recuperation of tropical forests following deforestation and pasture abandonment, although the majority of the nitrogen taken up by both grasses and forest plants arose