**It doesn't mean it is important on the absolute scale, just the relative scale...
I've said before that scientists have two jobs. The first is to create intellectual tension. The second is to resolve intellectual tension.
For global ecologists, some of the most important intellectual tension right now resides in two opposing ideas regarding nitrogen availability. Some theories suggest that global N availability is rising. Other theories suggest that it should be declining.
Given the central role of N availability in the functioning of the ecosphere, this is some of the most important intellectual tension we have to resolve. There is no doubt that excessive nutrient availability is damaging some aquatic ecosystems. Yet, in terrestrial ecosystems, whether N availability is increasing or decreasing is uncertain. The trajectory of terrestrial N availability determines how the ecosphere will respond to elevated CO2 and what types of policies we must impose. The difference couldn't be starker. If N availability is increasing, then we need to begin to limit anthropogenic N fixation. If the opposite is true, policies that limit N fixation might actually have deleterious effects.
The question about whether N availability to terrestrial plants is increasing or decreasing has suffered from a lack of data. Quite simply, there are no long-term measurements of terrestrial N availability across broad spatial scales that can be used to assess this question. We do not know the trajectory of N availability in grasslands. We do not know the trajectory of N availability in forests.
Paper #99 does not directly assess any of these, but it does one of the next best things for grasslands. It utilizes a unique long-term dataset on forage quality for cattle across the US.
I'll leave the details to the paper, but data suggest that N availability is declining in grasslands across the US. Not one or two experimental plots, but the whole of the Great Plains. Across this broad region, cattle are becoming more protein stressed as their forage is showing declines in protein. It appears that something is causing N availability to decline to plants and plants are responding by reducing their N (and protein) concentrations.
How much has protein declined over 20 years? The equivalent of a decline that causes plant N concentrations to decline about 10 mg protein g-1, or 0.3% N.
How much protein is that for US cattle? It would take about $2B in soybeans to replace all of that protein. Or about half of the soybeans produced in all of Iowa in a year.
By no means is paper #99 the final word on the topic. A fair amount of data support the thesis that N is declining in terrestrial ecosystems. Paper #100, which is still being reviewed, is going to be a major line of evidence in favor of declining terrestrial N availability.
As for this:
Let's just say that there are some intellectual tensions that aren't resolved.
I wouldn't believe it yet.
Post a Comment