Sodium shortage as a constraint on the carbon cycle in an inland tropical rainforest

Abstract

Sodium (Na) is uncommon in plants but essential to the metabolism of plant consumers, both decomposers and herbivores. One consequence, previously unexplored, is that as Na supplies decrease (e.g., from coastal to inland forests), ecosystem carbon should accumulate as detritus. Here, we show that adding NaCl solution to the leaf litter of an inland Amazon forest enhanced mass loss by 41%, decreased lignin concentrations by 7%, and enhanced decomposition of pure cellulose by up to 50%, compared with stream water alone. These effects emerged after 13-18 days. Termites, a common decomposer, increased 7-fold on ؉NaCl plots, suggesting an agent for the litter loss. Ants, a common predator, increased 2-fold, suggesting that NaCl effects cascade upward through the food web. Sodium, not chloride, was likely the driver of these patterns for two reasons: two compounds of Na (NaCl and NaPO 4) resulted in equivalent cellulose loss, and ants in choice experiments underused Cl (as KCl, MgCl 2, and CaCl2) relative to NaCl and three other Na compounds (NaNO 3, Na3PO4, and Na2SO4). We provide experimental evidence that Na shortage slows the carbon cycle. Because 80% of global landmass lies >100 km inland, carbon stocks and consumer activity may frequently be regulated via Na limitation.biogeochemistry ͉ biogeography ͉ decomposition ͉ fungi ͉ termites M odels of the carbon cycle start with the coavailability of water and solar energy as key constraints to photosynthesis and respiration (1-4). Lowland tropical forests, however, have ample sunshine, often ample moisture (5), and, frequently, weathered soils (6). In such forests, nutrient shortages, most notably the shortage of P, have been shown by experiment and comparative study to constrain both net primary productivity (NPP; g C/m 2 per y) and decomposition (7-13). Moreover, recent theory and experiment have suggested that the rate of decomposition, a collaborative process involving thousands of species, is unlikely to be constrained, Liebig style, by a single element (14,15). Here, we build on those studies and those of Chadwick et al. (16) to suggest that Na plays a key role in regulating decomposition in inland ecosystems.Of the 25 or so elements required for life (17), Na is unique. Most terrestrial plants have little need of Na (18). Herbivores and decomposers, in contrast, must amass Na in concentrations 100-to 1,000-fold over the plants they consume (19). In animals, costly sodium pumps maintain gradients of cell concentration and membrane voltage (Ͼ1% deviations of whole body sodium are signs of pathology; ref. 17). In plants, K, not Na, performs this function (20). This biogeochemical disconnect between plants and those that eat them suggests that consumers, but not plants, should suffer when Na inputs to ecosystems decline.Decomposers metabolize Ͼ90% of terrestrial plant biomass (21). Decomposition, the breakdown of necromass into CO 2 and inorganic compounds, is largely performed by microbes. Some are free living (e.g., basidiomycete and ascomycete fungi), and othe...

Resource discovery in ant communities: do food type and quantity matter?

“…Ant species can distinguish between different resources (Abrams, 1975;Savolainen, 1991;Sanders & Gordon, 2000Kay, 2002Kay, , 2004Bihn et al, 2008;Kaspari et al, 2009), although resource assessment was only known to take place after contact with the food item (de Biseau & Pasteels, 1994;Detrain & Deneubourg, 1997;Robson & Traniello, 1998;Mailleux et al, 2000;Lenoir, 2002;Cassill, 2003). This study is the first to address whether ants can detect food characteristics prior to contact and whether such food detection influences exploitative competition between species.…”

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

“…Several recent studies have shown that cations may be of considerable importance in constraining the soil food web in tropical forests (Kaspari et al 2008(Kaspari et al , 2009Waring 2012). Potassium constraints to decomposition may occur in lowland forests because K, like P, is primarily derived from bedrock and is lost during weathering (Kaspari et al 2008).…”

“…The scarcity of ocean-derived Na in the Western Amazon has led to the hypothesis of Na limitation to decomposers in these ecosystems, especially to ants, termites and fungi (Kaspari et al 2009). Consistent with this hypothesis, respiration positively responded to Na addition in our lowland site TAM-06 (Fig.…”

“…Fertilization and decomposition experiments have shown that phosphorus (P), nitrogen (N), sodium (Na) and potassium (K) all may limit soil microbial processes to some extent (Kaspari et al 2008(Kaspari et al , 2009Waring 2012). It remains unclear whether these variable responses to nutrient addition reflect different nutrient constraints to different groups of belowground organisms, different nutrient constraints to the decomposition of different organic matter substrates and/ or different environmental drivers of decomposition among sites.…”

“…However, in many of these tropical forest sites, strong colimitation of N and P together has been demonstrated Fanin et al 2012;Kaspari et al 2008) and additional influences of another rock-derived nutrient, K has been identified (Kaspari et al 2008). In the western Amazon, where oceanic Na deposition is low, there is also evidence that Na may limit heterotrophic metabolism, including fungi (Kaspari et al 2009). Therefore, while there is evidence to support P and N limitation of soil microbial processes in lowland and montane forests, respectively, the roles of other nutrients remain unclear, and it is uncertain whether the underlying paradigm applies to widely varying underlying soils (Quesada et al 2010).…”