A N tracing model to analyse N transformations in old grassland soil

A new ¹⁵N tracing model was developed to analyse nitrogen (N) transformations in old grassland soil. There was a need to develop a new model because existing models such as FLUAZ were not able to simulate the observed N dynamics. The new features of the model are: (a) simulation of heterotrophic nitrification, (b) simulation of dissimilatory nitrate (NO₃⁻) reduction to ammonium (NH₄⁺) (DNRA), (c) release of adsorbed or stored fertiliser N into the available mineral N pools and (d) immobilisation of NH₄⁺ and NO₃⁻ into two separate organic N pools with different re-mineralisation characteristics. The tracing model contains six N pools and nine simultaneous N transformations either at zero- or first-order kinetics. The model is set up in the modelling software ModelMaker which contains non-linear optimisation routines based on the Marquardt-Levenberg algorithm. The model is able to simulate data obtained from triple labelling studies where either the NH₄⁺, the NO₃⁻ or both pools were labelled with ¹⁵N. The flexible modelling environment allows the user to develop the model further.     

Comment on the comment by Amthor et al. on “Appropriate experimental ecosystem warming methods” by Aronson and McNulty

In a recent comment on the paper by Aronson and McNulty (2009) about “Appropriate experimental ecosystem warming methods by ecosystem, objective, and practicality”, Amthor et al. (2010) state that infrared lamps do not warm open-field plots by the mechanism expected with global warming. While this statement is correct, in the aftermath of their comment, confusion exists about how warming with infrared heaters can be related to global warming. This comment illustrates how infrared heating at “constant temperature rise” relates in a quantitative way to anticipated global warming. Amthor et al. (2010) also state that changes in vapor pressure gradient from leaf to atmosphere are an issue with infrared heating, but this problem can be minimized with supplemental irrigations in controlled amounts. Therefore, “constant temperature rise” infrared warming experiments with supplemental irrigations are a viable T-FACE (temperature free-air controlled enhancement) that can be used in combination with CO₂-FACE to produce conditions more representative of future open-fields than can be done with chambers with their many known artifacts.     

Aluminium and uptake of base cations by tree roots: A critique of the model proposed by Sverdrup et al.

Sverdrupet al. (1992) proposed a model intended to evaluate effects of soil solution (Ca²⁺+Mg²⁺/Al³⁺ ratios on the uptake of Ca²⁺ and Mg²⁺ (here called base cations=BC) by tree roots. They considered the adsorption of BC on exchange sites on root surfaces to be a major step, and the competition between Al³⁺ and BC for exchange sites on roots to be crucial. Although this may certainly affect the rates of uptake, it will not exclude base cations from entering the root cells through ion channels or by carriers. Above all, the model overlooks important gradients in the rhizosphere, e.g. pH-gradients and root exudates, which have profound influences on the speciation of Al. One should instead consider modifying existing models accounting for processes in a buffered soil medium and handling gradients in the rhizosphere (e.g. Nye and Tinker, 1977). This would also require considerations on mycorrhizas and possible changes in the capacity for uptake of certain ions, which could be induced by deficiencies.     

Does specialization explain rarity and decline among British bumblebees? A response to Goulson et al.

Recent studies of British bumblebees have proposed a seemingly simple explanation for the decline in some species: that greater dietary specialization among the rarer species has put them at greater risk. However, comparisons of dietary specialization require: (1) that bees have access to the same dietary options among which to make their choices; (2) that the differing relative breadths of dietary choices made are not obscured by the differing sample sizes among bee species. Using one of the few suitable data sets, I find no evidence for a relationship between, on the one hand, rarity or declines in British bumblebees and, on the other, their dietary breadths, the strengths of their dietary preferences, or their proboscis lengths (which influence dietary choices). In contrast, there is support for a relationship between rarity or declines within Britain and the sizes of species’ European ranges, particularly when these measures are adjusted to represent their ranges near sea level. Adjusted range sizes may reflect overall niche breadth and perhaps climatic and habitat specialization. This is not to say that climate change is the driving factor for declines or that changing food-plant availability is unimportant, but that climatic and habitat specialization may be a better indicator of risk of decline, which deserves further study.     

Response to the Views and Opinions of Maningat,Seib, and Bassi Regarding McCleary et al (2013)

Phosphate cross‐linked starch, referred to as resistant starch 4 (RS₄) is hydrolyzed much more rapidly under physiological conditions (i.e., pH 6, 37°C, pancreatic α‐amylase) than under the conditions used in AOAC dietary fiber method 985.29/AACC International Approved Method 32‐45.01 (the Prosky method), in which samples are incubated with thermostable bacterial α‐amylase at 95–100°C. For this reason, the author concludes that the Prosky method overestimates the dietary fiber content of these materials. A more accurate estimate of dietary fiber content is obtained with AOAC Method 2009.01/AACCI Approved Method 32‐45.01.