The mineralization of nitrogen and phosphorus from plant residues provides an important input of inorganic nutrients to the soil, which can be taken up by plants. The dynamics of nutrient mineralization or immobilization during decomposition are controlled by different biological and physical factors. Decomposers sequester carbon and nutrients from organic substrates and exchange inorganic nutrients with the environment to maintain their stoichiometric balance. Additionally, physical losses of organic compounds from leaching and other processes may alter the nutrient content of litter. In this work, we extend a stoichiometric model of litter nitrogen mineralization to include (1) phosphorus mineralization, (2) physical losses of organic nutrients, and (3) chemical heterogeneity of litter substrates. The enhanced model provides analytical mineralization curves for nitrogen and phosphorus as well as critical litter carbon : nutrient ratios (the carbon : nutrient ratios below which net nutrient release occurs) as a function of the elemental composition of the decomposers, their carbon-use efficiency, and the rate of physical loss of organic compounds. The model is used to infer the critical litter carbon : nutrient ratios from observed nitrogen and phosphorus dynamics in about 2600 litterbag samplings from 21 decomposition data sets spanning artic to tropical ecosystems. At the beginning of decomposition, nitrogen and phosphorus tend to be immobilized in boreal and temperate climates (i.e., both C:N and C:P critical ratios are lower than the initial ratios), while in tropical areas nitrogen is generally released and phosphorus may be either immobilized or released, regardless of the typically low phosphorus concentrations. The critical carbon : nutrient ratios we observed were found to increase with initial litter carbon : nutrient ratios, indicating that decomposers adapt to low-nutrient conditions by reducing their carbon-use efficiency. This stoichiometric control on nutrient dynamics appears ubiquitous across climatic regions and ecosystems, although other biological and physical processes also play important roles in litter decomposition. In tropical humid conditions, we found high critical C:P ratios likely due to high leaching and low decomposer phosphorus concentrations. In general, the compound effects of stoichiometric constraints and physical losses explain most of the variability in critical carbon : nutrient ratios and dynamics of nutrient immobilization and release at the global scale.

Stoichiometric controls on carbon, nitrogen, and phosphorus dynamics in decomposing litter

Many geoscientists now recognize stemflow as an important phenomenon which can exert considerable effects on the hydrology, biogeochemistry, and ecology of wooded ecosystems and shrublands. Despite the explosive growth of stemflow research, until this review there has been no comprehensive attempt to summarize and synthesize this literature since 2003. Topical areas of substantive new knowledge in stemflow research include the following: (1) the interrelationships among stemflow and meteorological conditions, especially within individual rain events; (2) the dynamic interplay between stemflow and canopy structure; (3) stemflow and the cycling of solutes and transport of particulate matter; (4) stemflow and its interactions with canopy fungi and corticolous lichens; and (5) stemflow-soil interactions. Each of these five topical areas of substantive new stemflow research is summarized and synthesized, with areas of future research opportunities discussed. In addition, we have reviewed the parameters which can be used to describe stemflow and critically evaluate their utility for different purposes. This review makes a call for scientists studying stemflow to utilize common metrics in an effort to increase the cross-site comparability of stemflow studies. Capitalizing on the insights of prior research, exciting research opportunities await hydrologists, biogeoscientists, and forest ecologists who will conduct studies to deepen our knowledge of stemflow which will enable a better and more accurate framing of stemflow in the larger context of watershed hydrology and biogeochemistry.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2015RG000479

A review of stemflow generation dynamics and stemflow-environment interactions in forests and shrublands

Tropical forests play a substantial role in the global carbon (C) cycle and are projected to experience significant changes in climate, highlighting the importance of understanding the factors that control organic matter decomposition in this biome. In the tropics, high temperature and rainfall lead to some of the highest rates of litter decomposition on earth, and given the near-optimal abiotic conditions, litter quality likely exerts disproportionate control over litter decomposition. Yet interactions between litter quality and abiotic variables, most notably precipitation, remain poorly resolved, especially for the wetter end of the tropical forest biome. We assessed the importance of variation in litter chemistry and precipitation in a lowland tropical rain forest in southwest Costa Rica that receives >5000 mm of precipitation per year, using litter from 11 different canopy tree species in conjunction with a throughfall manipulation experiment. In general, despite the exceptionally high rainfall in this forest, simulated throughfall reductions consistently suppressed rates of litter decomposition. Overall, variation between species was greater than that induced by manipulating throughfall and was best explained by initial litter solubility and lignin:P ratios. Collectively, these results support a model of litter decomposition in which mass loss rates are positively correlated with rainfall up to very high rates of mean annual precipitation and highlight the importance of phosphorus availability in controlling microbial processes in many lowland tropical forests.

/pdf/controls-over-leaf-litter-decomposition-in-wet-tropical-485y2e48bl.pdf

Controls over leaf litter decomposition in wet tropical forests

Litter decomposition represents one of the largest annual fluxes of carbon (C) from terrestrial ecosystems, particularly for tropical forests, which are generally characterized by high net primary productivity and litter turnover. We used data from the Long-Term Intersite Decomposition Experiment (LIDET) to (1) determine the relative importance of climate and litter quality as predictors of decomposition rates, (2) compare patterns in root and leaf litter decomposition, (3) identify controls on net nitrogen (N) release during decay, and (4) compare LIDETrates with native species studies across five bioclimatically diverse neotropical forests. Leaf and root litter decomposed fastest in the lower montane rain and moist forests and slowest in the seasonally dry forest. The single best predictor of leaf litter decomposition was the climate decomposition index (CDI), explaining 51% of the variability across all sites. The strongest models for predicting leaf decomposition combined climate and litter chemistry, and included CDI and lignin (R 2 50.69), or CDI, N and nonpolar extractives (R 2 50.69). While we found no significant differences in decomposition rates between leaf and root litter, drivers of decomposition differed for the two tissue types. Initial stages of decomposition, determined as the time to 50% mass remaining, were driven primarily by precipitation for leaf litter (R 2 50.93) and by temperature for root litter (R 2 50.86). The rate of N release from leaf litter was positively correlated with initial N concentrations; net N immobilization increased with decreasing initial N concentrations. This study demonstrates that decomposition is sensitive to climate within and across tropical forests. Our results suggest that climate change and increasing N deposition in tropical forests are likely to result in significant changes to decomposition rates in this biome.

/pdf/controls-on-long-term-root-and-leaf-litter-decomposition-in-5giy06qz5z.pdf

Controls on long-term root and leaf litter decomposition in neotropical forests

Litter decomposition, governing nutrient and C cycling, is strongly influenced by the chemical litter quality. In order to determine the interspecific variation in leaf decomposition rates and to understand the chemical basis for such variation, decomposition dynamics of seven common canopy species was investigated over 2 year using the litterbag technique in a subtropical evergreen broad-leaved forest on Okinawa Island, Japan. The species studied are representatives of the vegetation in the study area and differed significantly in their chemical litter quality. Dry mass loss at the end of study varied in the order: Distylium racemosum< Quercus miyagii< Rapanea neriifolia< Symplocos confusa< Castanopsis sieboldii< Schima wallichii< Daphniphyllum glaucescens. All species showed a pattern characterized by a rapid initial decomposition followed by lower rates except for D. glaucescenswhich decomposition rate appeared to be rather constant. In the late phase, decomposition rates were correlated positively to initial N and ash contents and negatively to lignin content, lignin:N, C:N, and C:P ratios. The effects of N and lignin content or lignin:N ratio were stronger than other quality parameters. There was a wide range in patterns of N and P concentrations, from a net accumulation to a rapid loss in decomposition. The correlation between N and P release suggests that N and P dynamics may have influenced each other during litter decomposition. Analysis of initial quality for species showed that the C:P ratios were extremely high (range 1639–3811) but the N:P ratios were from 28 to 56, indicating a likely P-limitation for this forest. Our results suggest that P is an important control of litter decomposition and N and P dynamics.

Decomposition patterns of leaf litter of seven common canopy species in a subtropical forest: N and P dynamics

Partition des precipitations et flux de nutriments dans une foret subtropicale d'Okinawa au Japon. La partition des precipitations entre eau arrivant directement au sol et ecoulement le long des troncs et flux hydrochimiques lies a ete etudie pendant 3 ans de janvier 1998 a decembre 2000 dans une foret feuillue a feuilles persistantes de l'ile d'Okinawa au Japon. Mensuellement des echantillons d'eau etaient collectes pour determiner les concentrations et les flux des bioelements. Les precipitations annuelles moyennes pendant la periode etudiee ont atteint 3325 mm. Les typhons jouent un role central dans l'hydrologie de la foret etudiee avec 11 typhons contribuant a 29,1 % des precipitations totales pour les 3 ans de la periode d'etude. L'eau arrivant directement au sol et l'ecoulement le long des troncs representent respectivement 53,9 % et 30,9 % des precipitations incidentes annuelles, impliquant une interception des precipitations de 15,2 %. La part importante de l'ecoulement le long des troncs pourrait etre du a la morphologie des couronnes de l'espece dominante Castanopsis sieboldii qui a des branches inclinees et des feuilles de formes concaves. Le pH des precipitations incidentes etait de 6,22, il baissait significativement avec la traversee de l'eau a travers les couronnes. Les concentrations totales de N, DOC (carbone organique dissous), K, Na, Ca et Mg montrent un modele evident d'enrichissement au niveau de l'eau traversant directement les couronnes et de l'ecoulement le long des troncs comparativement aux precipitations incidentes. La proximite de l'Ocean Pacifique influence fortement lees flux de nutriments dans le site etudie comme le montre les flux eleves de Na apportes par les precipitations incidentes (213 kg Na ha -1 an -1 ) et l'eau arrivant directement au sol et l'ecoulement le long des troncs (291 kg Na ha -1 an -1 ). La moyenne annuelle des apports de nutriments etait de (en kg ha): N total 43, P 2,6, K 76, Ca 49, Mg 30, Na 291, Al 1,8, Fe 1,0 et Mn 1,1. L'apport de DOC a atteint 361 kg de carbone par hectare et par an. L'apport eleve de nutriments par l'ensemble des precipitations arrivant au sol (eau traversant directement plus ecoulement le long des troncs) particulierement en ce qui concerne N peut etre le reflet des stress induits par l'eau salee et les insectes herbivores.

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Precipitation partitioning and related nutrient fluxes in a subtropical forest in Okinawa, Japan

La decomposition des chutes d'aiguilles normales et occasionnees par un typhon et leur modele de liberation des nutriments ont ete etudies pour 10 elements (C, N, P, K, Ca, Mg, S, Fe, Al et Mn). L'etude a ete faite dans une plantation de pin (Pinus luchensis Mayr.) a Okinawa (SO du Japon). Elle a dure 3 ans et a utilise la technique des bacs a litiere. Apres 3 ans les taux de decomposition etaient de 0,361 et 0,323 an -1 respectivement pour les chutes d'aiguilles liees au typhon et les chutes normales d'aiguilles. Les chutes aiguilles liees au typhon se decomposent significativement plus rapidement que les chutes normales d'aiguilles, ce qui est du a une plus forte concentration en N et P. Les modeles de transfert des elements varient fortement. Toutefois, des differences significatives dans les modeles de dynamique de masse des elements entre les deux types de chute d'aiguille ont seulement ete mises en evidence pour N et P (P Ca≥Mg≥C>S≥N≥Mn>P» Fe ≥ Al. Une decomposition rapide avec une liberation rapide de P et N pour les aiguilles dont la chute a ete occasionnee par le typhon indique que la perturbation par le typhon peut conduire le cycle de P et N a un taux quelque peu plus rapide, ce qui est important du point de vue de la productivite de la foret puisque P et N sont des nutriments limitants dans les forets subtropicales d'Okinawa.

https://www.afs-journal.org/articles/forest/pdf/2006/07/f6072.pdf

Xiaoniu Xu

Papers

Decomposition patterns of leaf litter of seven common canopy species in a subtropical forest: N and P dynamics

Precipitation partitioning and related nutrient fluxes in a subtropical forest in Okinawa, Japan

Nutrient dynamics in decomposing needles of Pinus luchuensis after typhoon disturbance in a subtropical environment

Litterfall and the Nutrient Returns in Evergreen Broadleaved Forests in Northern Okinawa Island