Patterns of soil moisture in the lowlands of an annually burned watershed at Konza Prairie. Soil moisture is expressed on a relative basis at 6 depths for 1993 (wet year) and 1994 (dry year).
Earlier, I had discussed a potentially interesting case of transitive limitation, i.e. when the low availability of one resource reduces the availability of another. In the case of water and nitrogen, it is unclear in grasslands whether the limitation ascribed to water could actually be due to low N availability. N mineralization is known to decrease with decreasing soil moisture. As such, as soil moisture declines, so should N mineralization.
The correlation between soil moisture and N mineralization does not necessarily mean that the two should co-limit across a range of soil moistures. In a given soil profile, soil organic N is generally concentrated in shallow depths, while soil moisture is more evenly distributed throughout the soil profile, if not greater at depth. As such, plants can have access to plenty of water at depth even if shallow soils have dried out. Soil N mineralization and moisture might be correlated for a given volume of soil, but not over the whole soil profile.
Konza is an interesting example. At different times, productivity is said to be limited by water and nitrogen, but the two have never been rectified. Do they simultaneously limit production? Does limitation vary over the course of a season, or across years? Or is it transitive?
If it is transitive, disentangling the two is not easy. Standard factorial resource addition experiments do not work since adding water would also increase N availability. Is there a way to add water without increasing N? Not easily from above. But you could add it from below.
Inferentially, if you look at Konza soil moisture patterns, there is always plenty of water at depth, even in dry times. In the above example, in 1994, soil moistures are depleted in shallow soils, but there is very little draw down of deep soils. Proximally, this could be due to the lack of roots at depth, but we are only talking 1 m. The dominant species could easily produce roots at 1 m--if there was a benefit to doing so. If productivity was water limited, there would be a benefit. Yet, if productivity was actually N limited, accessing deep water provides little benefit when N is not being mineralized.
There are other lines of evidence that support the dominant role of transitive limitation at Konza. For example, regardless of whether you add N or water, the same species--Panicum virgatum--comes to dominate. If N was limiting, wouldn't adding N dry out the soils more and favor a low-water, high-N species?
One of the tough things to demonstrate is the roll that soil water potential plays in productivity. I'll likely expand on this later, but there are no relationships between water potential and productivity, only conductance. If we could show that productivity should not be diminished by lowering soil water potential to say -2 MPa, we might be able to demonstrate that it is not water that is limiting directly, but transitively by reducing N supplies.
There are still multiple pieces to assemble before the story is complete, but transitive limitation is likely a linchpin in understanding grasslands.