Tom Addiscott

When Does Nitrate Become a Risk for Humans

Is nitrate harmful to humans? Are the current limits for nitrate concentration in drinking water justified by science? There is substantial disagreement among scientists over the interpretation of evidence on the issue. There are two main health issues: the linkage between nitrate and (i) infant methaemoglobinaemia, also known as blue baby syndrome, and (ii) cancers of the digestive tract. The evidence for nitrate as a cause of these serious diseases remains controversial. On one hand there is evidence that shows there is no clear association between nitrate in drinking water and the two main health issues with which it has been linked, and there is even evidence emerging of a possible benefit of nitrate in cardiovascular health. There is also evidence of nitrate intake giving protection against infections such as gastroenteritis. Some scientists suggest that there is sufficient evidence for increasing the permitted concentration of nitrate in drinking water without increasing risks to human health. However, subgroups within a population may be more susceptible than others to the adverse health effects of nitrate. Moreover, individuals with increased rates of endogenous formation of carcinogenic N-nitroso compounds are likely to be susceptible to the development of cancers in the digestive system. Given the lack of consensus, there is an urgent need for a comprehensive, independent study to determine whether the current nitrate limit for drinking water is scientifically justified or whether it could safely be raised.

Quantitative methods to evaluate and compare Soil Organic Matter (SOM) Models

Evaluation and comparison of the performance of soil organic matter models is often based upon visual/graphical comparison of the simulated values produced by the model with actual values from field experiments. Such methods provide an immediate qualitative description of the differences, highlighting trends, different types of errors and distribution patterns of simulated and measured values. However, model evaluations or comparisons should ideally incorporate both a qualitative visual/graphical assessment and a quantitative statistical appraisal. Statistical methods have been selected that are suitable for quantitative evaluation and comparison of soil organic matter models. The methods included each provide information on some distinct aspect of the accuracy of the simulation.

Emergence or self-organization? Look to the soil population

Emergence is not well defined, but all emergent systems have the following characteristics. The whole is more than the sum of the parts, they show bottom-up rather top-down organization and, if biological, they involve chemical signalling. Self-organization can be understood in terms of the second and third stages of thermodynamics enabling these stages used as analogues of ecosystem functioning. The second stage system was suggested earlier to provide a useful analogue of the behaviour of natural and agricultural ecosystems subjected to perturbations, but for this it needs the capacity for self-organization. Considering the hierarchy of the ecosystem suggests that this self-organization is provided by the third stage, whose entropy maximization acts as an analogue of that of the soil population when it releases small molecules from much larger molecules in dead plant matter. This it does as vigorously as conditions allow. Through this activity, the soil population confers self-organization at both the ecosystem and the global level. The soil population has been seen as both emergent and self-organizing, supporting the suggestion that the two concepts are are so closely linked as to be virtually interchangeable. If this idea is correct one of the characteristics of a biological emergent system seems to be the ability to confer self-organization on an ecosystem or other entity which may be larger than itself. The beehive and the termite colony are emergent systems which share this ability.