Invasive perennial forb effects on gross soil nitrogen cycling and nitrous oxide fluxes depend on phenology.

Invasive plants can increase soil nitrogen (N) pools and accelerate soil N cycling rates, but their effect on gross N cycling and nitrous oxide (N2 O) emissions has rarely been studied. We hypothesized that perennial pepperweed (Lepidium latifolium) invasion would increase rates of N cycling and gaseous N loss, thereby depleting ecosystem N and causing a negative feedback on invasion. We measured a suite of gross N cycling rates and net N2 O fluxes in invaded and uninvaded areas of an annual grassland in the Sacramento-San Joaquin River Delta region of northern California. During the growing season, pepperweed-invaded soils had lower microbial biomass N, gross N mineralization, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification-derived net N2 O fluxes (P < 0.02 for all). During pepperweed dormancy, gross N mineralization, DNRA, and denitrification-derived net N2 O fluxes were stimulated in pepperweed-invaded plots, presumably by N-rich litter inputs and decreased competition between microbes and plants for N (P < 0.04 for all). Soil organic carbon and total N concentrations, which reflect pepperweed effects integrated over longer time scales, were lower in pepperweed-invaded soils (P < 0.001 and P = 0.04, respectively). Overall, pepperweed invasion had a net negative effect on ecosystem N status, depleting soil total N to potentially cause a negative feedback to invasion in the long term

The role of soils in delivering Nature's Contributions to People

Data accessibility. This article does not contain any additional data. Funding Information:The input of P.S. contributes to Soils-R-GRREAT (NE/ P019455/1) and the input of P.S. and S.D.K. contributes to the European Union’s Horizon 2020 Research and Innovation Programme through project CIRCASA (grant agreement no. 774378). Acknowledgements. T.K.A. acknowledges the support of ‘Towards Integrated Nitrogen Management System (INMS)’ funded by the Global Environment Facility (GEF), executed through the UK’s Natural Environment Research Council (NERC).Peer reviewedPostprin

On the shoulders of giants: Continuing the legacy of large-scale ecosystem manipulation experiments in Puerto Rico

There is a long history of experimental research in the Luquillo Experimental Forest in Puerto Rico. These experiments have addressed questions about biotic thresholds, assessed why communities vary along natural gradients, and have explored forest responses to a range of both anthropogenic and non-anthropogenic disturbances. Combined, these studies cover many of the major disturbances that affect tropical forests around the world and span a wide range of topics, including the effects of forest thinning, ionizing radiation, hurricane disturbance, nitrogen deposition, drought, and global warming. These invaluable studies have greatly enhanced our understanding of tropical forest function under different disturbance regimes and informed the development of management strategies. Here we summarize the major field experiments that have occurred within the Luquillo Experimental Forest. Taken together, results from the major experiments conducted in the Luquillo Experimental Forest demonstrate a high resilience of Puerto Rico’s tropical forests to a variety of stressors

Experimentally induced root mortality increased nitrous oxide emission from tropical forest soils

We conducted an experiment on sand and clay tropical forest soils to test the short‐term effect of root mortality on the soil‐atmosphere flux of nitrous oxide, nitric oxide, methane, and carbon dioxide. We induced root mortality by isolating blocks of land to 1 m using trenching and root exclusion screening. Gas fluxes were measured weekly for ten weeks following the trenching treatment. For nitrous oxide there was a highly significant increase in soil‐atmosphere flux over the ten weeks following treatment for trenched plots compared to control plots. N2O flux averaged 37.5 and 18.5 ng N cm−2 h−1 from clay trenched and control plots and 4.7 and 1.5 ng N cm−2 h−1 from sand trenched and control plots. In contrast, there was no effect for soil‐atmosphere flux of nitric oxide, carbon dioxide, or methane

Fine root dynamics and trace gas fluxes in two lowland tropical forest soils

Fine root dynamics have the potential to contribute significantly to ecosystem-scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 ( 35) and 153 ( 27) gm 2 yr 1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k5 0.96 year 1) than in the sandy loam soil (k5 0.61 year 1),leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N2O) emissions were significantly greater in the clay soil (13 1ngNcm 2 h 1) than in the sandy loam (1.4 0.2 ngNcm 2 h 1). Root mortality and decay following trenching doubled rates of N2O emissions in the clay and tripled them in sandy loam over a 1-year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 gCm 2 yr 1, accounting for 24% to 35% of the total soil CO2 efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521 206 gCm2 yr 1) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land-use change can contribute significantly to increased rates of nitrogen trace gas emissions

Cross-biome transplants of plant litter show decomposition models extend to a broader climatic range but lose predictability at the decadal time scale

We analyzed results from 10-year long field incubations of foliar and fine root litter from the Long-term Intersite Decomposition Experiment Team (LIDET) study. We tested whether a variety of climate and litter quality variables could be used to develop regression models of decomposition parameters across wide ranges in litter quality and climate and whether these models changed over short to long time periods. Six genera of foliar and three genera of root litters were studied with a 10-fold range in the ratio of acid unhydrolyzable fraction (AUF, or ‘lignin’) to N. Litter was incubated at 27 field sites across numerous terrestrial biomes including arctic and alpine tundra, temperate and tropical forests, grasslands and warm deserts. We used three separate mathematical models of first-order (exponential) decomposition, emphasizing either the first year or the entire decade. One model included the proportion of relatively stable material as an asymptote. For short-term (first-year) decomposition, nonlinear regressions of exponential or power function form were obtained with r 2 values of 0.82 and 0.64 for foliar and fine-root litter, respectively, across all biomes included. AUF and AUF : N ratio were the most explanative litter quality variables, while the combined temperature-moisture terms AET (actual evapotranspiration) and CDI (climatic decomposition index) were best for climatic effects. Regressions contained some systematic bias for grasslands and arctic and boreal sites, but not for humid tropical forests or temperate deciduous and coniferous forests. The ability of the regression approach to fit climate-driven decomposition models of the 10-year field results was dramatically reduced from the ability to capture drivers of short-term decomposition. Future work will require conceptual and methodological improvements to investigate processes controlling decadal-scale litter decomposition, including the formation of a relatively stable fraction and its subsequent decomposition.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78615/1/j.1365-2486.2009.02086.x.pd

Redox Fluctuations Control the Coupled Cycling of Iron and Carbon in Tropical Forest Soils.

Oscillating redox conditions are a common feature of humid tropical forest soils, driven by an ample supply and dynamics of reductants, high moisture, microbial oxygen consumption, and finely textured clays that limit diffusion. However, the net result of variable soil redox regimes on iron (Fe) mineral dynamics and associated carbon (C) forms and fluxes is poorly understood in tropical soils. Using a 44-day redox incubation experiment with humid tropical forest soils from Puerto Rico, we examined patterns in Fe and C transformations under four redox regimes: static anoxic, "flux 4-day" (4d oxic, 4d anoxic), "flux 8-day" (8d oxic, 4d anoxic) and static oxic. Prolonged anoxia promoted reductive dissolution of Fe-oxides, and led to an increase in soluble Fe(II) and amorphous Fe oxide pools. Preferential dissolution of the less-crystalline Fe pool was evident immediately following a shift in bulk redox status (oxic to anoxic), and coincided with increased dissolved organic C, presumably due to acidification or direct release of organic matter (OM) from dissolving Fe(III) mineral phases. The average nominal oxidation state of water-soluble C was lowest under persistent anoxic conditions, suggesting that more reduced organic compounds were metabolically unavailable for microbial consumption under reducing conditions. Anoxic soil compounds had high H/C values (and were similar to lignin-like compounds) whereas oxic soil compounds had higher O/C values, akin to tannin- and cellulose-like components. Cumulative respiration derived from native soil organic C was highest in static oxic soils. These results show how Fe minerals and Fe-OM interactions in tropical soils are highly sensitive to variable redox effects. Shifting soil oxygen availability rapidly impacted exchanges between mineral-sorbed and aqueous C pools, increased the dissolved organic C pool under anoxic conditions implying that the periodicity of low-redox events may control the fate of C in wet tropical soils

Evaluating the Classical Versus an Emerging Conceptual Model of Peatland Methane Dynamics

We appreciate discussions with M. Firestone and S. Blazewicz. We received assistance in the field and lab from K. Smetak, H. Dang, and A. McDowell. This research was funded by grants to W.L.S. from the U.S. National Science Foundation (ATM-0842385 and DEB-0543558) and the California Department of Fish and Wildlife (CDFW) and California Department of Water Resources (DWR) contract 4600011240. The data used are listed in the references, tables, supporting information, and the Illinois Digital Environment for Access to Learning and Scholarship (IDEALS) repository at https://www.ideals.illinois.edu/.Peer reviewedPublisher PD