Fire Affects Asymbiotic Nitrogen Fixation in Southern Amazon Forests

Abstract

In this study, we investigate the biogeochemical consequences of fire in seasonally flooded Amazon forests, where recent declines in forest cover have been linked to increases in fire frequency and severity. Previous studies have hypothesized that a quasi‐permanent state‐shift transition from typical Amazon forests to open savannas can occur when fire results in further depletion of already impoverished soil nutrient pools. Asymbiotic N2 fixation (ANF) is an essential pathway for fire‐affected forests to acquire nitrogen (N) after disturbance, but ANF response to fire has yet to be quantified in Amazonia. Here, we quantify ANF through field sampling and laboratory incubations using 15N‐labeled dinitrogen (15N2) and measurement of 14 biogeochemical parameters in surface (0–10 cm) and subsurface (10–30 cm) soils. Our data represent burned and unburned replicated sampling sites, across five stands, spanning a gradient from infrequent (once in 13 years) to frequent (five times in 13 years) fire occurrences. ANF did not vary with fire frequency but was, on average, 24% lower in burned than in unburned surface soils across all stands. Burned and unburned subsurface soils had similar ANF rates. About 58% of ANF variance was explained by the joint effect of carbon (C):N ratio and available phosphorus (P) in burned and unburned soils. ANF increased linearly with C:N and P availability in unburned soils, but a highly non‐linear relationship was observed in burned soils. Our findings show that fire alters soil C‐to‐nutrient stoichiometry, which resulted in lower N inputs via ANF into burned relative to unburned tropical forest soils.

Sixteen hundred years of increasing tree cover prior to modern deforestation in Southern Amazon and Central Brazilian savannas

“…We found evidence of increasing tree cover near forest–savanna ecotones as well as a significant densification of tree cover within most sampled forests and savannas with grass cover expanding only in a few locations characterized by dry climates and poorly developed soils. Several biophysical and biogeochemical mechanisms may have contributed to forest expansion and densification of woody vegetation in savannas, including positive feedbacks between tree cover and precipitation which can dampen fire disturbance and promote nutrient accumulation, which favor the persistence of closed canopies (Bomfim et al., 2019, 2020; Silva & Lambers, 2020). The observed expansion of forests into savannas could have significantly impacted carbon–water relations throughout the region, potentially affecting the balance between carbon sequestration, evapotranspiration, and precipitation (Garcia et al., 2016).…”

Feedback in tropical forests of the Anthropocene

“…Usually, in undisturbed forests, nitrogen is abundant in the organic topsoil, but after disturbances that remove or burn topsoils, nitrogen can become limiting, until its concentration recovers along with secondary succession (Davidson et al, 2007; Pellegrini et al, 2018). Although symbiotic N 2 fixation may contribute to increase carbon accumulation in secondary forests (Levy‐Varon et al, 2019), after fire, N 2 fixation can be reduced (Bomfim et al, 2020). Soil phosphorus and soil moisture can be strong limiting factors of N 2 fixation rates in tropical forests (Van Langenhove et al, 2020), implying that if extreme drought events and disturbances by fire and deforestation happen more often, intensifying soil erosion (Borrelli et al, 2017; Flores et al, 2020), N availability may severely decrease.…”

Impacts of land-use change on soil microbial communities and their function in the Amazon Rainforest

“…An initial flush of surface layer (0-10 cm) extractable C and N compounds immediately following forest burn commonly coincides with microbial biomass losses through heat sterilization; in the medium-term (2 months to several years following burn), surface soil concentrations of organic matter are typically similar to, or lower than, unburned forest stands (Navarrete et al, 2015a;Neill et al, 1995;Prieto-Ferna ´ndez et al, 1998). Additionally, fire is likely to lower the ratio of available C:N (Bomfim et al, 2020;Prieto-Ferna ´ndez et al, 1998) given the volatility of C compounds. This shift in C and N availability may impact the composition and diversity of recovering microbial communities, particularly if burns are repeated (Zarin et al, 2005).…”

“…The latter finding may serve as an indication of survival and maintenance of genetic material during periods of unfavorable conditions. In the Amazonia-Cerrado transition zone, a study on the effects of fire on seasonally-flooded forest soils indicated that rates of a key soil microbial process, ANF, were on average 24% lower in burned compared to unburned surface soils (0-10 cm; Bomfim et al, 2020). However, variable frequency of burn does not appear to add significant effect to this difference, and rates below 10 cm are unaffected by burning of any frequency.…”