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To examine the mechanisms of earthworm effects on forest soil C and N, we double-labeled leaf litter with 13C and 15N, applied it to sugar maple forest plots with and without earthworms, and traced isotopes into soil pools. The experimental design included forest plots with different earthworm community composition (dominated by Lumbricus terrestris or L. rubellus). Soil carbon pools were 37% lower in earthworm-invaded plots largely because of the elimination of the forest floor horizons, and mineral soil C:N was lower in earthworm plots despite the mixing of high C:N organic matter into soil by earthworms. Litter disappearance over the first winter-spring was highest in the L. terrestris (T) plots, but during the warm season, rapid loss of litter was observed in both L. rubellus (R) and T plots. After two years, 22.0% +/- 5.4% of 13C released from litter was recovered in soil with no significant differences among plots. Total recovery of added 13C (decaying litter plus soil) was much higher in no-worm (NW) plots (61-68%) than in R and T plots (20-29%) as much of the litter remained in the former whereas it had disappeared in the latter. Much higher percentage recovery of 15N than 13C was observed, with significantly lower values for T than R and NW plots. Higher overwinter earthworm activity in T plots contributed to lower soil N recovery. In earthworm-invaded plots isotope enrichment was highest in macroaggregates and microaggregates whereas in NW plots silt plus clay fractions were most enriched. The net effect of litter mixing and priming of recalcitrant soil organic matter (SOM), stabilization of SOM in soil aggregates, and alteration of the soil microbial community by earthworm activity results in loss of SOM and lowering of the C:N ratio. We suggest that earthworm stoichiometry plays a fundamental role in regulating C and N dynamics of forest SOM.

We maintained a factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment for 11 years in a humid lowland forest growing on a relatively fertile soil in Panama to evaluate potential nutrient limitation of tree growth rates, fine-litter production, and fine-root biomass. We replicated the eight factorial treatments four times using 32 plots of 40 x 40 m each. The addition of K was associated with significant decreases in stand-level fine-root biomass and, in a companion study of seedlings, decreases in allocation to roots and increases in height growth rates. The addition of K and N together was associated with significant increases in growth rates of saplings and poles (1-10 cm in diameter at breast height) and a further marginally significant decrease in stand-level fine-root biomass. The addition of P was associated with a marginally significant (P = 0.058) increase in fine-litter production that was consistent across all litter fractions. Our experiment provides evidence that N, P, and K all limit forest plants growing on a relatively fertile soil in the lowland tropics, with the strongest evidence for limitation by K among seedlings, saplings, and poles.

Abstract: We present a meta‐analysis of plant responses to fertilization experiments conducted in lowland, species‐rich, tropical forests. We also update a key result and present the first species‐level analyses of tree growth rates for a 15‐yr factorial nitrogen (N), phosphorus (P), and potassium (K) experiment conducted in central Panama. The update concerns community‐level tree growth rates, which responded significantly to the addition of N and K together after 10 yr of fertilization but not after 15 yr. Our experimental soils are infertile for the region, and species whose regional distributions are strongly associated with low soil P availability dominate the local tree flora. Under these circumstances, we expect muted responses to fertilization, and we predicted species associated with low‐P soils would respond most slowly. The data did not support this prediction, species‐level tree growth responses to P addition were unrelated to species‐level soil P associations. The meta‐analysis demonstrated that nutrient limitation is widespread in lowland tropical forests and evaluated two directional hypotheses concerning plant responses to N addition and to P addition. The meta‐analysis supported the hypothesis that tree (or biomass) growth rate responses to fertilization are weaker in old growth forests and stronger in secondary forests, where rapid biomass accumulation provides a nutrient sink. The meta‐analysis found no support for the long‐standing hypothesis that plant responses are stronger for P addition and weaker for N addition. We do not advocate discarding the latter hypothesis. There are only 14 fertilization experiments from lowland, species‐rich, tropical forests, 13 of the 14 experiments added nutrients for five or fewer years, and responses vary widely among experiments. Potential fertilization responses should be muted when the species present are well adapted to nutrient‐poor soils, as is the case in our experiment, and when pest pressure increases with fertilization, as it does in our experiment. The statistical power and especially the duration of fertilization experiments conducted in old growth, tropical forests might be insufficient to detect the slow, modest growth responses that are to be expected. [ABSTRACT FROM AUTHOR]

To explore the importance of 12 elements in litter production and decomposition, we fertilized 36 1600 m2-plots with combinations of N, P, K, or micronutrients (i.e. B, Ca, Cu, Fe, Mg, Mn, Mo, S, Zn) for 6 years in a lowland Panamanian forest. The 90% of litter falling as leaves and twigs failed to increase with fertilization, but reproductive litter (fruits and flowers) increased by 43% with N. K enhanced cellulose decomposition; one or more micronutrients enhanced leaf-litter decomposition; P enhanced both. Our results suggest tropical forests are a non-Liebig world of multiple nutrient limitations, with at least four elements shaping rates of litterfall and decomposition. Multiple metallomic enzymes and cofactors likely create gradients in the break down of leaf litter. Selection favours individuals that make more propagules, and even in an N-rich forest, N is a non-substitutable resource for reproduction. [ABSTRACT FROM AUTHOR]

Lythrum salicaria (purple loosestrife) has invaded wetlands across North America and is known to have negative effects on native plant species and diversity. The effects of this invasion on ecosystem processes are less well known. We examined decomposition of L. salicaria stems versus that of native Typha latifolia (broad-leaved cattail) in a previously cultivated, freshwater marsh in central New York State. We expected different decomposition rates and differences in litter quality of L. salicaria stems and T. latifolia shoots. We also quantified ergosterol concentration, an index of fungal biomass. However, standing-dead T. latifolia shoots and L. salicaria stems had lost similar quantities of mass (20–25%) after 1 year. Subsequent heavy snowfall and winter winds caused tissue fragmentation and much greater mass loss, but still similar for both plant species. Ergosterol concentrations were greater in T. latifolia shoots than in L. salicaria stems; yet, changes in tissue concentrations of nitrogen (N) and Klason lignin were very similar for both plant species. Therefore, although native T. latifolia may be more favorable in wetlands, the results of this study suggest that L. salicaria stems and T. latifolia shoots decompose in a similar fashion and have similar influence on the ecosystem-level processes measured in this study. [Copyright &y& Elsevier]

Denitrification is the most poorly constrained term in the nitrogen (N) budgets of northern hardwood forests due to extraordinary complexity in soil, hydrology and ecosystem processes. This anaerobic microbial process is controlled by the availability of oxygen (O 2 ), nitrate (NO 3 − ) and available carbon (C) and produces three end products; nitric oxide (NO), nitrous oxide (N 2 O) and dinitrogen (N 2 ). Expression of these controlling factors is regulated by ecosystem, landscape and regional factors that influence soil moisture and N and C fluxes that interact to create complexity and extreme variability in flux rates. We investigated these controls and the capacity of topographic index (TI) and foliar N (FN), indicators of moisture and N richness, respectively, to function as integrated predictors of N 2 and N 2 O fluxes at ecosystem, landscape and regional scales at three sites in New Hampshire, USA, including the Hubbard Brook Experimental Forest (HBEF) long-term ecological research site. We found high N 2 fluxes, low N 2 O fluxes, and high N 2 :N 2 O ratios at all sites, with best estimates of ∼3–4 kg N ha −1 growing season −1 for N 2 and ∼0.005–0.05 kg N ha −1 growing season −1 , for N 2 O fluxes. N flux rates exhibited considerable spatial variability both within and among sites, as well as temporal variability within a season and among seasons. Indicators of N richness (e.g., net nitrification, C:N ratio, and soil ammonium (NH 4 + ) concentrations) were the strongest predictors of N gas fluxes at the ecosystem scale suggesting that these fluxes are controlled by N limitation in this region. Regressions between fluxes and TI and FN were used to create maps of seasonal N flux for the HBEF landscape for 2005 and 2006. At the regional scale, there were marked and coherent patterns between site history, N cycling variables and N gas fluxes. Variation in the relationship between FN and N gas fluxes between years suggests that while the proximal, organismal-scale controls of these fluxes do not vary, the distal factors that influence the proximal controls show marked temporal and spatial variation. Higher resolution and/or more dynamic data may be necessary to improve the accuracy of landscape and regional scale assessments of these fluxes. [ABSTRACT FROM AUTHOR]

Accurate measurement of decomposition of fine roots in forests is important for understanding nutrient cycling, carbon dynamics and biotic responses; however, various sources of error or bias interfere with commonly used approaches. Most studies of fine root decomposition have used the buried root litter bag method, but this method may underestimate decay rates based on comparisons with the less disruptive, intact soil core approach. We compared these two methods for one year for seven temperate forest trees growing in twelve monospecific plantations in central New York. The experimental design included four species that form arbuscular mycorrhizae (sugar maple, red maple, cherry, tulip poplar) and three with ectomycorrhizae (pine, spruce, oak). We also examined macronutrients (N, P) release and fungal community composition of decaying fine roots using Illumina sequencing. Across the seven species and 12 plots, fine root decay rate was significantly higher for intact cores than litter bags, with the difference being approximately 12 %, a smaller disparity than that observed in previous comparisons. However, one species (Norway spruce) showed the opposite trend. Similarly, N and P release from decaying roots was significantly greater for the intact core method, but again with an exception (red oak). No consistent effects of mycorrhizal type on the method effect or decay rate and nutrient release were detected. Fungal communities in decaying roots differed between the two methods; nevertheless, Mycena spp. dominated most plots for both methods. These results further emphasize the notion that decay rates and nutrient mobilization of fine roots in forests are usually underestimated by the litter bag approach. • Fine root decay rate was greater for roots in intact cores than in litter bags. • Nutrient release from decaying roots was greater for intact soil cores. • A fungal Mycena species dominated fine root decay. • Decay of fine roots in forests is underestimated by the litter bag approach. [ABSTRACT FROM AUTHOR]

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of 13C and 15N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more 13C and 15N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions. [ABSTRACT FROM AUTHOR]