Sites sampled for grass nitrogen isotopes.
I'll admit I have a soft spot in my heart for expedition science. You start out with no real hypotheses. Instead you have a plan to measure something interesting along an interesting gradient not knowing what the ultimate patterns might be. You get a mess of data and then start to try and tell a story. It's an adventure to collect and an adventure to write.
In one recent study, over 400 grass samples were taken from what amounts to the entire continent of Australia. It's not a perfect study--the analyses could have been more complete and they could have spelled Ben Houlton's name right. That said, there is an interesting story that comes out. As water availability increased, del15N decreased, as we've seen before.
Yet, comparing C4 types, PCK C4's were enriched in 15N relative to other types. PCK's have always been a mystery ecologically. NADP's are the tallgrass C4's, NAD's are the shortgrass C4's and PCK's are the tropical C4's. So why the 15N enrichment? Are they less reliant on mycorrhizal fungi? Do they occupy higher N availability sites than the other types?
Hard to know, but with some broad surveys done, at least we know the patterns. Which is an important first step to understanding why the patterns are there.
Murphy, B. P. and D. Bowman. 2009. The carbon and nitrogen isotope composition of Australian grasses in relation to climate. Functional Ecology 23:1040-1049.
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