Rapidly growing tropical trees mobilize remarkable amounts of nitrogen, in ways that differ surprisingly among species

Abstract

Fast-growing forests such as tropical secondary forests can accumulate large amounts of carbon (C), and thereby play an important role in the atmospheric CO 2 balance. Because nitrogen (N) cycling is inextricably linked with C cycling, the question becomes: Where does the N come from to match high rates of C accumulation? In unique experimental 16-y-old plantations established in abandoned pasture in lowland Costa Rica, we used a mass-balance approach to quantify N accumulation in vegetation, identify sources of N, and evaluate differences among tree species in N cycling. The replicated design contained four broadleaved evergreen tree species growing under similar environmental conditions. Nitrogen uptake was rapid, reaching 409 (±30) kg·ha −1 ·y −1 , double the rate reported from a Puerto Rican forest and greater than four times that observed at Hubbard Brook Forest (New Hampshire, USA). Nitrogen amassed in vegetation was 874 (±176) kg·ha −1 , whereas net losses of soil N (0-100 cm) varied from 217 (±146) to 3,354 (±915) kg·ha −1 (P = 0.018) over 16 y. Soil C:N, δ 13 C values, and N budgets indicated that soil was the main source of biomass N. In Vochysia guatemalensis, however, N fixation contributed >60 kg·ha −1 ·y −1 . All species apparently promoted soil N turnover, such that the soil N mean residence time was 32-54 y, an order of magnitude lower than the global mean. High rates of N uptake were associated with substantial N losses in three of the species, in which an average of 1.6 g N was lost for every gram of N accumulated in biomass.carbon sequestration | forest regrowth | nitrogen cycle | soil organic nitrogen | species effects R ates of C accumulation can be rapid in secondary forests, to the extent that forest regrowth following harvest, abandonment of agricultural lands, and deforestation serves as an important sink for atmospheric CO 2 (1). The magnitude is on the order of 2.8 Pg C·y −1 (2), and thereby mitigates climate change. Moist tropical forests are especially productive: Tropical tree plantations capture 7-10 Mg C·ha −1 ·y −1 (3, 4). Abandoned agricultural and pasture land represents roughly half the landscape in the tropics (5); given its high C sequestration potential, if even half of these degraded ecosystems were reforested, it would substantially reduce rates at which CO 2 accumulates in the atmosphere. Tropical forests accounted for 55% of the 861 Pg C of the world's forest C stocks in 2007, a decrease from 58% in 1990 (6). From 1990 to 2007, deforestation of 284 Mha of intact tropical forests resulted in a loss of 58 Pg C while tropical forest regrowth on 110 Mha accrued 27 Pg C (6).Rapid C cycling is integrally linked to N cycling, because N-rich enzymes underlie photosynthesis and high N availability promotes plant productivity. The question becomes: Where does the N come from to match the observed high rates of C accumulation in biomass? Further, do tree species differ in this coupling of C and N cycling? From a conceptual framework, speciesspecific differences in the capture and ...

Native tree species regulate nitrous oxide fluxes in tropical plantations

“…Emissions from beneath Virola were on par with those from Penthaclethra, despite the fact that Virola did not have particularly N-rich tissues or elevated N flows within the plant-soil system (Russell and Raich 2012). However, Virola was among the least productive of the studied species and displayed notably low fine-root growth (Russell et al 2010;Fig.…”

“…And while natural forest regeneration following pasture and cropland abandonment is still common, an increasing fraction of the globe's tropical secondary forests are becoming actively managed, species-specific plantations (4% of global forest area; Lugo 1997, Mayaux et al 2005, Chazdon 2008). Tropical tree species have been shown to affect ecosystem-level C and N cycling due to differences in species traits and ecological strategies that control growth, N acquisition, and litter decomposability (e.g., Vitousek et al 1987, Binkley and Ryan 1998, Russell et al 2010, Russell and Raich 2012. Results to date also suggest that individual tree species may affect rates of N 2 O emission in both humid tropical monocultures (Erickson et al 2002) and mature mixed forests (van Haren et al 2010).…”

“…Pentaclethra, like many leguminous trees, has low litter and root C:N ratios ). This species, with its N-rich tissues, low N-use efficiency, and nodulated roots, sustains high fluxes of detrital N (Russell and Raich 2012). All of these traits point to high N fluxes through the plant-soil system, during the course of which more nitrogen can be lost.…”

“…4a and b, triangles) and high above-and belowground biomass Fisher 1994, Russell et al 2010). As such, it had high N uptake (Russell and Raich 2012), leaving soil N less vulnerable to losses from the system. Hieronyma, the other plantation species with low N 2 O fluxes, had the fastest average belowground growth rates (Russell et al 2010; Fig.…”

Improving Farming Practices for Sustainable Soil Use in the Humid Tropics and Rainforest Ecosystem Health

“…Species-specific differences in both the organic matter inputs and their chemical composition and decomposition dynamics can affect the SOM contents [42,58,59], though these effects may be limited to the labile fractions of SOM [60]. Elevated contents of polyphenols and other secondary components in leaf and root tissue retard litter decomposition and nutrient release and favor mycorrhizal fungi in oligotrophic humid tropical vegetation [61].…”