Tuesday, January 20, 2009

Understanding limitation by N and P

One of the most fundamental questions to understanding the adaptations of plants to low resource availability has been to understand the covariation in resource availability across sites. Do environments that have relatively low P availability have an excess of N relative to plant demand? Are plants that are limited by water also limited by N, or is N in excess for those plants and available for secondary purposes like defense?

I’m in the middle of an experiment to begin to understand the covariation in N and P limitation across grasslands in the US. Generally, researchers have attacked this question by doing field factorial fertilizations and comparing results across sites. Instead of this approach, I asked people to send me soils from 100 grasslands across the US. I grew the same species of grass (Schizachyrium scoparium) in all 100 soils with a factorial fertilization: control, +N, +P, +NP. After 10 weeks, the plants were harvested.

The most notable result at this point as the patterns of the response of biomass to N addition relative to soil P availability. As seen below, soils with low P availability had the least response to N addition, while soils with high P availability had the greatest. Yet, there was no relationship between P availability and the response to P addition. One interpretation of the data is that 1) P availability limits N response, and 2) P availability by itself does not limit P response. My guess is that plants in low P soils are not just limited by N, but by N and P. Any N supplied in excess in plant demands is lost from the ecosystem, whereas P isn’t lost when supplied in excess. Hence, I would guess that over evolutionary time scales, grassland plants would either be limited by N, or N and P, but not by P alone. Field fertilizations seem to bear this out as no grassland has ever been found to be limited by P alone.

We’re still collecting data on the plants and soils, but likely will be working the data up soon.

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