Water-nitrogen effects on below-ground primary production in a Chihuahuan Desert grassland

Background/Question/Methods

Arid lands comprise approximately 40% of the Earth's land surface, and contribute 30-35% of terrestrial net primary productivity. Recent studies indicate that fine roots (< 2 mm in diameter) account for the majority of plant carbon allocation in these regions, potentially playing a significant role in the global C cycle. However, due to high turnover of fine roots and difficulties excavating them, our understanding of the mechanisms governing belowground dynamics lags far behind that of aboveground processes. The objective of this study was to assess the effect of the independent and interactive effects long-term water and nitrogen availability on belowground net primary production in a Chihuahuan desert grassland in southwest New Mexico.  Experimental design was a full factorial design of water and N manipulation; treatments were  reduced and enhanced precipitation either -80%, +80%, or ambient rainfall and N fertilization ammonium nitrate 10 g m-2 yr-1. Plots were 2.5 X 2.5 m and N = 6. A Minirhizotron camera was used to non-destructively monitor root growth every 3 weeks through the growing season, July-September, in 2012 & 2013. Belowground net primary production was estimated from root growth using calculated turnover rates and allometric equations relating biomass to diameter classes of roots. 

Results/Conclusions

Belowground net primary production responded to the experimental changes in precipitation that occurred due to the experimental manipulations and natural differences between 2012 and 2013. On the contrary, nitrogen showed no effect on belowground net primary production under all precipitation treatments (β=6.17, t₍₂₀₆₎=0.34, p=0.73) and there were no interactions with precipitation (β=0.11, t₍₂₀₆₎=0.25, p=0.80). These results suggest that even in wet conditions N is not a limiting factor. There was a positive relationship between production and precipitation (β=0.16, t₍₂₀₆₎=1.99, p=0.047), suggesting that, as with above ground production, precipitation is a dominant driver of below ground production in this region.