Anthony's Science Blog

What forms of N are available?

October 5, 2011 – 7:06 pm

In the old days, it was assumed that the N uptake by plants and microbes was done via the labile currency of inorganic N ions: nitrate and ammonium. Later, it was discovered that plants and microbes could in fact take up amino acids as well, in a way “short-circuiting” the inorganic N part of the cycle. This leads to the question: what forms of N are actually taken up in the soil? A couple of interesting recent studies shed some light on this question and provide some good data suggesting that our understanding of labile N exchange is not complete.

Using pool dilution techniques, Wanek et al. show that:

…gross protein depolymerization exceeded gross N mineralization by >8 fold indicating that only a small fraction of amino acids released by extracellular enzymes was actually mineralized to ammonium.

Not only could litter microbes take up amino acids, but they seemed to snag them quickly enough that the inorganic N forms were never produced. These things are hard to measure so this is impressive.

Perhaps even more intriguing though, Farrell et al. conclude:

Our findings…point to a short-circuit whereby large peptides and proteins need only be extracellularly cleaved to short chain length peptides before direct assimilation by microbes.

This is a big deal because it means to understand labile N pools in soils, we may need to do a lot more than just measure inorganic N and amino acids; currently, most people don’t even measure amino acids.