Effects of nitrogen additions on above- and belowground carbon dynamics in two tropical forests

Abstract

Anthropogenic nitrogen (N) deposition is increasing rapidly in tropical regions, adding N to ecosystems that often have high background N availability. Tropical forests play an important role in the global carbon (C) cycle, yet the effects of N deposition on C cycling in these ecosystems are poorly understood. We used a field N-fertilization experiment in lower and upper elevation tropical rain forests in Puerto Rico to explore the responses of above-and belowground C pools to N addition. As expected, tree stem growth and litterfall productivity did not respond to N fertilization in either of these Nrich forests, indicating a lack of N limitation to net primary productivity (NPP). In contrast, soil C concentrations increased significantly with N fertilization in both forests, leading to larger C stocks in fertilized plots. However, different soil C pools responded to N fertilization differently. Labile (low density) soil C fractions and live fine roots declined with fertilization, while mineral-associated soil C increased in both forests. Decreased soil CO 2 fluxes in fertilized plots were correlated with smaller labile soil C pools in the lower elevation forest (R 2 = 0.65, p \ 0.05), and with lower live fine root biomass in the upper elevation forest (R 2 = 0.90, p \ 0.05). Our results indicate that soil C storage is sensitive to N deposition in tropical forests, even where plant productivity is not N-limited. The mineral-associated soil C pool has the potential to respond relatively quickly to N additions, and can drive increases in bulk soil C stocks in tropical forests.

Seasonal changes in soil organic matter after a decade of nutrient addition in a lowland tropical forest

“…Nutrient addition to tropical forest soils can influence soil organic matter concentrations, CO 2 fluxes, and priming effects Cusack et al 2011;Nottingham et al 2012), yet we found no significant effects of a decade of N, P, K, or micronutrient additions on C and N concentrations in soil organic matter. This is despite significant effects of P addition on litter fall and microbial biomass, of K addition on root growth, of N plus K addition on trunk growth, and of K, P, and dolomite/micronutrient addition on litter decomposition (Kaspari et al 2008;Turner and Wright 2014;Wright et al 2011;Yavitt et al 2011).…”

“…On the other hand, N addition is predicted to increase soil organic C by increasing forest productivity (Sayer et al 2012) and reducing priming effects through a decline in the microbial demand for N from soil organic matter (Nottingham et al 2012). Indeed, the addition of N for several years to tropical forests in Puerto Rico increased soil organic C stocks, primarily in the mineral-associated pool (Cusack et al 2011). Such changes are of potential significance given the magnitude of the soil C stock in tropical forests, because even small changes in soil organic C could exert a considerable influence on atmospheric CO 2 concentrations and therefore future climate.…”

Nutrient limitations to bacterial and fungal growth during cellulose decomposition in tropical forest soils

“…For example, in different lowland tropical forests, P addition accelerated rates of decomposition (Kaspari et al 2008) and microbial growth (Fanin et al 2015;Turner and Wright 2014). In contrast, N limitation in tropical montane forest soils has been indicated by low concentrations of exchangeable N (Teh et al 2013;Wolf et al 2011) and increased soil CO 2 efflux or microbial biomass following N fertilization (Cusack et al 2011b;Fisher et al 2013). In a study that included the same tropical forest sites as in this paper, a shift from P to N constraints to microbial growth with increased elevation was inferred by differences in the stoichiometry of N/P-degrading enzymes (Nottingham et al 2015b).…”

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research

“…N-C-P interactions in soil vary among biomes. Where P limits primary production, such as in some tropical ecosystems or acid alpine grasslands, increases in N deposition may have little impact on productivity (Matson et al 1999), a finding that has recently been documented by field experiments at ILTER sites (Bowman et al 2008;Cusack et al 2011). In both N-and P-limited tundra ecosystems, C fluxes were found to respond positively to additions of both elements, although responses to P tended to be stronger than to N (Shaver et al 1998).…”