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Kraft N.J.B.,University of Maryland University College | Godoy O.,University of California at Santa Barbara | Godoy O.,CSIC - Institute of Natural Resources and Agriculture Biology of Seville | Levine J.M.,ETH Zurich
Proceedings of the National Academy of Sciences of the United States of America | Year: 2015

Understanding the processes maintaining species diversity is a central problem in ecology, with implications for the conservation and management of ecosystems. Although biologists often assume that trait differences between competitors promote diversity, empirical evidence connecting functional traits to the niche differences that stabilize species coexistence is rare. Obtaining such evidence is critical because traits also underlie the average fitness differences driving competitive exclusion, and this complicates efforts to infer community dynamics from phenotypic patterns. We coupled fieldparameterized mathematical models of competition between 102 pairs of annual plants with detailed sampling of leaf, seed, root, and whole-plant functional traits to relate phenotypic differences to stabilizing niche and average fitness differences. Single functional traits were often well correlated with average fitness differences between species, indicating that competitive dominance was associated with late phenology, deep rooting, and several other traits. In contrast, single functional traits were poorly correlated with the stabilizing niche differences that promote coexistence. Niche differences could only be described by combinations of traits, corresponding to differentiation between species in multiple ecological dimensions. In addition, several traits were associated with both fitness differences and stabilizing niche differences. These complex relationships between phenotypic differences and the dynamics of competing species argue against the simple use of single functional traits to infer community assembly processes but lay the groundwork for a theoretically justified trait-based community ecology.

Knicker H.,CSIC - Institute of Natural Resources and Agriculture Biology of Seville
Soil Biology and Biochemistry | Year: 2011

The availability of Soil Organic Nitrogen (SON) determines soil fertility and biomass production to a great extent. SON also affects the amounts and turnover rates of the soil organic carbon (SOC) pools. Although there is increasing awareness of the impact of the nitrogen (N) cycle on the carbon (C) cycle, the extent of this interaction and the implications for soil organic matter (SOM) dynamics are still under debate. Therefore, present knowledge about the inter-relationships of the soil cycles of C and N as well as current ideas about SON stabilization are summarized in this paper in order to develop an advanced concept of the role of N on C sequestration. Modeling global C-cycling, it was already recognized that SON and SOC are closely coupled via biomass production and degradation. However, the narrow C/N ratio of mature soil organic matter (SOM) shows further that the impact of SON on the refractory SOM is beyond that of determining the size of the active cycling entities. It affects the quantity of the slow cycling pool and as a major contributor it also determines its chemical composition. Although the chemical nature of SON is still not very well understood, both improved classical wet chemical analyses and modern spectroscopic techniques provide increasing evidence that almost the entire organic N in fire-unaffected soils is bound in peptide-like compounds and to a lesser extent in amino sugars. This clearly points to the conclusion, that such compounds have greater importance for SOM formation than previously assumed. Based on published papers, I suggest that peptides even have a key function in the C-sequestration process. Although the mechanisms involved in their medium and long-term stabilization are far from understood, the immobilization of these biomolecules seems to determine the chemistry and functionality of the slow cycling SOM fraction and even the potential of a soil to act as a C sink. Pyrogenic organic N, which derives mostly from incomplete combustion of plant and litter peptides is another under-rated player in soil organic matter preservation. In fire-prone regions, its formation represents a major N stabilization mechanism, leading to the accumulation of heterocyclic aromatic N, the stability of which is still not elaborated. The concept of peptide-like compounds as a key in SOM-sequestration implies that for an improved evaluation of the potential of soils as C-sinks our research focus as to be directed to a better understanding of their chemistry and of the mechanisms which are responsible for their resistance against biochemical degradation in soils. © 2011 Elsevier Ltd.

Gomez-Aparicio L.,CSIC - Institute of Natural Resources and Agriculture Biology of Seville | Valiente-Banuet A.,National Autonomous University of Mexico
Proceedings of the Royal Society B: Biological Sciences | Year: 2012

Biotic interactions assembling plant communities can be positive (facilitation) or negative (competition) and operate simultaneously. Facilitative interactions and posterior competition are among the mechanisms triggering succession, thus representing a good scenario for ecological restoration. As distantly related species tend to have different phenotypes, and therefore different ecological requirements, they can coexist, maximizing facilitation and minimizing competition. We suggest including phylogenetic relatedness together with phenotypic information as a predictor for the net effects of the balance between facilitation and competition in nurse-based restoration experiments. We quantify, by means of a Bayesian meta-analysis of nurse-based restoration experiments performed worldwide, the importance of phylogenetic relatedness and life-form disparity in the survival, growth and density of facilitated plants. We find that the more similar the life forms of neighbouring plants are the greater the positive effect of phylogenetic distance is on survival and density. This result suggests that other characteristics beyond life form are also contained in the phylogeny, and the larger the phylogenetic distance, the less is the niche overlap, and therefore the less is the competition. As a general rule, we can maximize the success of the nurse-based practices by increasing life-form disparity and phylogenetic distances between the neighbour and the facilitated plant. © 2012 The Royal Society.

Knicker H.,CSIC - Institute of Natural Resources and Agriculture Biology of Seville
Organic Geochemistry | Year: 2010

The chemical and thermal stability of organic nitrogen (N) in pyrogenic organic matter (PyOM) was examined by charring casein at 350 °C and 450 °C. The alteration was compared with that observed for char derived from lignin, cellulose, grass and wood. With respect to heating, casein showed a considerably higher stability than cellulose. Comparable proportions of carbon (C) and N were recovered, supporting the idea that black nitrogen (BN) represents an integral part of the char structure. Although some amides were still present, they lost importance with increasing temperature. Charring of grass revealed an enrichment in N compounds because of the low thermal stability of cellulose. The similarity in the resulting nuclear magnetic resonance (NMR) spectra to those of casein char confirmed that BN can play a major role in the chemical composition of plant char. Subjecting the chars to oxidation with acidic dichromate demonstrated that, in spite of their relatively high resistance to heat, the N-containing compounds of the chars were less recalcitrant than the components of the cellulose char. Thus, in soil, N-rich chars are likely to be underestimated on the basis of this method. On the other hand, for an ancient paddy soil whose N-containing char compounds were calculated to account for ca. 25% of the total organic C in the soil. This clearly underlines the pedogenic stability of BN and confirms that it has the potential to contribute significantly to the refractory soil organic matter pool. © 2010 Elsevier Ltd.

Hilscher A.,TU Munich | Knicker H.,CSIC - Institute of Natural Resources and Agriculture Biology of Seville
Soil Biology and Biochemistry | Year: 2011

The present study focuses on the microbial recalcitrance of pyrogenic organic material (PyOM) on a molecular scale. We performed microcosm incubation experiments using 13C- and 15N-enriched grass-derived PyOM mixed with a sub soil material taken from a Haplic Cambisol. Solid-state 13C and 15N NMR studies were conducted to elucidate the humification processes at different stages of PyOM degradation. The chemical structure of the remaining PyOM after incubation was clearly different from the initial pyrogenic material. The proportion of O-containing functional groups was increased, whereas that of aryl C and of N-containing heterocyclic structures had decreased, probably due to mineralisation and conversion to other C and N groups. After 20 months of incubation the aryl C loss reached up to 40% of the initial amount and up to 29% of the remaining PyOM C was assigned to carboxyl/carbonyl C and O-aryl C. These reactions alter the chemical and physical properties of the char residue and make it more available for further microbial attack but also for adsorption processes. Our study presents direct evidence for the degradation of N-heterocyclic domains in charred plant remains adding new aspects to the understanding of the N cycling in fire-affected ecosystems. © 2010 Elsevier Ltd.

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