Nutrient‐specific solubility patterns of leaf litter across 41 lowland tropical woody species

Abstract

Leaching is a mechanism for the release of nutrients from litter or senesced leaves that can drive interactions among plants, microbes, and soil. Although leaching is well established in conceptual models of litter decomposition, potential nutrient solubility of mineral elements from recently senesced litter has seldom been quantified. Using a standardized extraction (1:50 litter-to-water ratio and four-hour extraction) and recently senesced leaf litter of 41 tropical tree and liana species, we investigated how solubility varies among elements, and whether the solubility of elements could be predicted by litter traits (e.g., lignin, total element concentrations). In addition, we investigated nutrient forms (i.e., inorganic and organic) and ratios in leachate. Water-soluble elements per unit litter mass were strongly predicted by total initial litter element concentrations for potassium (K; r2 = 0.79), sodium (Na; r2 = 0.51) and phosphorus (P; r2 = 0.66), while a significant but weaker positive relationship was found for nitrogen (N; r2 = 0.36). There was no significant relationship for carbon (C) or calcium (Ca). Element-specific solubility varied markedly. On average 100% of total K, 35% of total P, 28% of total Na, 5% of total N, 4% of total Ca, and 3% of total C were soluble. For soluble P, 90% was inorganic orthophosphate. The high solubility of K, Na, and P as inorganic orthophosphate suggests that these nutrients can become rapidly available to litter microbes with no metabolic cost. Few common predictors of decomposition rates were correlated with element solubility, although soluble C (milligrams per gram of litter) was negatively related to lignin content (r2 = 0.19; P < 0.004). Solubility of elements was linked within a species: when a species ranked high in the soluble fraction of one element, it also ranked high in the solubility of other elements. Overall nutrient-specific patterns of solubility from recently senesced litter emphasize that litter elements cannot be treated equally in our conceptual and empirical models of decomposition. The relatively high potential solubility of P as orthophosphate from fresh litter advances our understanding of ecological stoichiometric ratios and nutrient bioavailability in tropical forests.

Controls on fallen leaf chemistry and forest floor element masses in native and novel forests across a tropical island

“…Our findings are consistent with the predictions, except for: (1) rainfall effects on P (but see below), (2) fallen leaf Na concentrations were opposite what was predicted (as above), and (3) fallen leaf K concentrations did not show any patterns. Perhaps the exceptionally high mobility of K in leaf litter (e.g., Schreeg et al 2013) contributed to high variation in our data and an inability to detect differences.…”

“…A potential caveat is that uneven leaching of nutrient elements (cf. Schreeg et al 2013) and inconsistent decomposition across environmental gradients may contribute to unaccounted-for variation in fallen leaf chemistry; we address this in the discussion. We focus on the chemistry (concentrations) of fallen leaves in our analyses because (1) litterfall is usually dominated by leaves (except during storms, Silver et al 2004), (2) fallen leaf chemistry likely reflects nutrient availability better than total forest floor chemistry or element mass, and (3) fallen leaf chemistry is less variable than forest floor element mass (see Results).…”

Seasonal Changes and Treatment Effects on Soil Inorganic Nutrients Following a Decade of Fertilizer Addition in a Lowland Tropical Forest

“…Although we found significant temporal variation in a number of soil properties, only extractable K followed the expected pattern of higher concentrations in the early wet season. This is presumably due to rapid leaching of K from litter (Attiwill, 1968;Schreeg et al, 2013b) compared to the slower release of less soluble nutrients like Ca that occur in structural parts of the leaves and are released more slowly throughout the wet season (Cornejo et al, 1994;Yavitt and Wright, 1996). It is likely that our monthly sampling regime missed short-term nutrient pulses, although the absence of a broad increase in nutrient availability at the onset of the wet season provides some support for the suggestion that nutrient pulses are more common in dry tropical forests with a 6-mo dry season (Singh et al, 1989;Campo et al, 1998) than in semi-deciduous forests with a 4-mo dry season (Yavitt et al, 1993).…”

“…For example, N is relatively transient in tropical forest soils, with large inputs through biological fixation and atmospheric deposition (Hietz et al, 2011;Reed et al, 2011) and large losses through leaching and denitrification (Houlton et al, 2006;Koehler et al, 2009;Corre et al, 2010). Similarly, K is mobile in the plant-soil system, being rapidly leached from leaves and retained only weakly in the soil (Schreeg et al, 2013b). We therefore expected N and K concentrations to increase moderately but rapidly to a new steady state under chronic fertilizer addition, and to show pronounced seasonality.…”

Potassium: a neglected nutrient in global change

“…As noted below, this is due to large losses of K by leaching that causes nutrient imbalances. Moreover, other studies have observed that K concentrations in the soil solution and in runoff are more sensitive to changes in land use and fertilization than N or P concentrations (Watmough et al, 2005;Anguelov et al, 2011), and also that K is more quickly leached from litter than N or P and has a much shorter residence time in soil organic matter (Florez-Florez et al, 2013;Schreeg et al, 2013).…”