Effects of long term precipitation and nitrogen manipulation on root growth in a semi-arid grassland

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Arid lands comprise approximately 40% of the Earth's land surface, and contribute 30-35% of terrestrial net primary productivity. 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 carbon cycle. However, due to high turnover of fine roots and difficulties excavating them, our understanding of the mechanisms governing belowground dynamics lags behind that of aboveground processes. The objective of this study was to assess independent and interactive effects of long-term water and nitrogen availability on fine root growth in a Chihuahuan desert grassland in southwest New Mexico.  The experimental design was a full factorial design of water and N manipulation. 2.5 x 2.5 m plots (N=6) have received reduced or enhanced precipitation, either -80%, +80%, or ambient rainfall and no fertilization or nitrogen fertilization in the form of ammonium nitrate at 10 g m-2 yr-1. Treatments have been maintained for 9 years. A minirhizotron camera was used to non-destructively monitor root growth every 3 weeks through the growing season, July-September, in 2012 - 2014. Precipitation showed a positive effect on root growth. Root growth showed no response to nitrogen fertilization. However, nitrogen and precipitation showed a synergistic interaction, suggesting that nitrogen plays a roll only under wetter conditions.