ALBERT

Description: We investigated effects of tree species planted in replicated plots of a common garden on five indices of soil organic matter (SOM) stability (heterotrophic soil respiration, bulk soil δ15N and ∆14C, and C in particulate and mineral associated organic matter). Our results suggest tree species regulate SOM stability via the chemical composition of their tissues, especially roots. Some of our stability indices (C in mineral associated SOM and bulk soil ∆14C), though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change. Abstract Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40 year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15N and ∆14C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.

Description: Labile, ‘high quality’, plant litters are hypothesized to promote soil organic matter (SOM) stabilization in mineral soil fractions that are physico-chemically protected from rapid mineralization. However, the effect of litter quality on SOM stabilization is inconsistent. High quality litters, characterized by high N concentrations, low C/N ratios and low phenol/lignin concentrations, are not consistently stabilized in SOM with greater efficiency than ‘low quality’ litters characterized by low N concentrations, high C/N ratios and high phenol/lignin concentrations. Here, we attempt to resolve these inconsistent results by developing a new conceptual model that links litter quality to the soil C saturation concept. Our model builds on the Microbial Efficiency-Matrix Stabilization framework (Cotrufo et al ., 2013) by suggesting the effect of litter quality on SOM stabilization is modulated by the extent of soil C saturation such that high quality litters are not always stabilized in SOM with greater efficiency than low quality litters. This article is protected by copyright. All rights reserved.

Description: Grazing intensity elicits changes in the composition of plant functional groups in both shortgrass steppe (SGS) and northern mixed-grass prairie (NMP) in North America. How these grazing intensity-induced changes control aboveground net primary production (ANPP) responses to precipitation remains a central open question, especially in light of predicted climate changes. Here, we evaluated effects of four levels (none, light, moderate and heavy) of long-term (〉30 yrs) grazing intensity in SGS and NMP on 1) ANPP, 2) precipitation use efficiency (PUE, ANPP:precipitation) and 3) precipitation marginal response (PMR, slope of a linear regression model between ANPP and precipitation). We advance prior work by examining 1) the consequences of a range of grazing intensities (more than grazed vs. ungrazed), and 2) how grazing-induced changes in ANPP and PUE are related both to shifts in functional group composition and physiological responses within each functional group. Spring (April-June) precipitation, the primary determinant of ANPP, was only 12% higher in NMP than in SGS, yet both ANPP and PUE were 25% higher. Doubling grazing intensity in SGS and nearly doubling it in NMP reduced ANPP and PUE by only 24% and 33%, respectively. Increased grazing intensity reduced C 3 graminoid biomass and increased C 4 grass biomass in both grasslands. Functional group shifts affected PUE through biomass reductions, as PUE was positively associated with the relative abundance of C 3 species and negatively with C 4 species across both grasslands. At the community level, PMR was similar between grasslands and unaffected by grazing intensity. However, PMR of C 3 graminoids in SGS was eight-fold higher in the ungrazed treatment than under any grazed level. In NMP, PMR of C 3 graminoids was only reduced under heavy grazing intensity. Knowledge of the ecological consequences of grazing intensity provides valuable information for mitigation and adaptation strategies in response to predicted climate change. For example, moderate grazing (the recommended rate) in SGS would sequester the same amount of aboveground carbon as light grazing because ANPP was nearly the same. In contrast, reductions in grazing intensity in NMP from moderate to light intensity would increase the amount of aboveground carbon sequestrated by 25% because of increased ANPP. This article is protected by copyright. All rights reserved.

Description: Ecology, Volume 0, Issue 0, Ahead of Print.

Description: Ecology, Volume 0, Issue 0, Ahead of Print.

Description: Understanding why species respond to climate change is critical for forecasting invasions, diversity, and productivity of communities. Although researchers often predict species’ distributions and productivity based on direct physiological responses to environments, theory suggests that striking shifts in community composition could arise if global change alters indirect feedbacks mediated by resources, mutualists, or antagonists. To test whether global change influences plant communities via soil-mediated feedbacks, we grew model communities in soils collected from a seven-year field manipulation of CO 2 , warming, and invasion. We evaluated mechanisms underlying variation in the model communities by comparing species’ growth in equivalent soil histories with, and without, experimentally reduced soil biota (via sterilization) and nutrient limitation (via fertilization). We show that grasses performed consistently across all soil history scenarios and that soil biota limited grasses more than nutrients. In contrast, forbs were differentially sensitive to soil history scenarios, with the magnitude and direction of responses to soil biota and nutrients dependent upon plant species and global change scenario. The asymmetry in importance of soil history for grasses and forbs is likely explained by differences in life history strategy. We conclude that accounting for species’ growth strategies will improve predictions of species sensitivity to altered soil feedbacks in future climates.

Description: The effects of global environmental changes on soil nitrogen (N) pools and fluxes have consequences for ecosystem functions such as plant productivity and N retention. In a 13-year grassland experiment, we evaluated how elevated atmospheric carbon dioxide (CO 2 ), N fertilization, and plant species richness alter soil N cycling. We focused on soil inorganic N pools, including ammonium and nitrate, and two N fluxes, net N mineralization and net nitrification. In contrast with existing hypotheses, such as progressive N limitation, and with observations from other, often shorter, studies, elevated CO 2 had relatively static and small, or insignificant, effects on soil inorganic N pools and fluxes. Nitrogen fertilization had inconsistent effects on soil N transformations, but increased soil nitrate and ammonium concentrations. Plant species richness had increasingly positive effects on soil N transformations over time, likely because in diverse subplots the concentrations of N in roots increased over time. Species richness also had increasingly positive effects on concentrations of ammonium in soil, perhaps because more carbon accumulated in soils of diverse subplots, providing exchange sites for ammonium. By contrast, subplots planted with 16 species had lower soil nitrate concentrations than less diverse subplots, especially when fertilized, probably due to greater N uptake capacity of subplots with 16 species. Monocultures of different plant functional types had distinct effects on N transformations and nitrate concentrations, such that not all monocultures differed from diverse subplots in the same manner. The first few years of data would not have adequately forecast the effects of N fertilization and diversity on soil N cycling in later years; therefore, the dearth of long-term manipulations of plant species richness and N inputs is a hindrance to forecasting the state of the soil N cycle and ecosystem functions in extant plant communities. © 2012 Blackwell Publishing Ltd