Monday, October 1, 2012

Global patterns of soil C concentration--an index for relative decomposition rates

Patterns of soil C concentrations as a function of mean annual temperature and precipitation in surface soils for 600+ soils from around the world.

One of the keys to understanding global N cycling patterns is understanding the global patterns of rates of decomposition. There are a few syntheses of litter bag studies, but the net result of inputs and outputs of C for soils are hard to interpret.

One of the best indices (I think) of the rate of decomposition relative to inputs is just soil concentration.

Most syntheses to date have examined soil C content, not concentration. Mostly because the goal is to determine soil C storage. Concentration only helps to determine content.

The major syntheses I can think of--Mac Posts 1982 and Batjes 1996--examine patterns for different soil orders. temperature or precipitation is never on the x-axis.

Bearing that, there should be an advance possible by putting climate on the x-axis and %C on the y-axis.

It seems like this would have been done, but I can't find a graph like it and people that are likely to know about it seems surprised when I show them.

For the 570 mineral soils I looked at, MAT and MAP explain about 60% of the variation in log-transformed soil C concentrations. That's a high r2 given all the variability out there in the world and other factors like how much clay is in soil, the quality of the plants,  and whether plants are eaten or not.

The interpretation of the patterns essentially is that if soil %C is an index of the amount of decomposition of plant biomass relative to primary productivity, then hot, dry places have soils with highly processed C.

The patterns of relative decomposition now seem pretty clear.

The question now becomes whether those hot, dry places consistently are elevated in 15N, such that soil 15N patterns are being caused by relative decomposition rates.

Two tests here.

1) Soil %C and soil 15N should scale across sites.
2) If soil 15N increases with increasing MAT and decreasing MAP, the relationship should disappear after accounting for variation in soil %C.

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