Water-nitrogen effects on fine-root production in a Chihuahuan Desert grassland

Arid lands comprise approximately 40% of the Earth's land surface, and contribute 30-35% of the terrestrial net primary productivity. Recent studies indicate that fine roots (< 2 mm in diameter) can account for a major portion of plant C allocation. Since the majority of net primary production in grasslands occurs below ground, fine roots have the potential to play a significant role in the global C cycle. However, due to the high turnover of fine roots and difficulties excavating and isolating them, our understanding of belowground dynamics lags far behind that of aboveground processes. We hypothesize that fine root production increases (1) as water availability increases, and (2) as soil N increases; and that (3) N and water availability interact synergistically such that their combined effects promote fine root production to a greater extent than their additive effects.

We utilized non-destructive minirhizotron cameras in a long-term water manipulation experiment to quantify the effects of water and N availability on fine root production in a semi-arid Chihuahuan desert grassland. Rainout shelters and sprinkler systems were installed in 2.5 x 2.5m plots in 2006 to decrease (n=12 plots) or increase (n=12) ambient precipitation by 80%. An additional 12 plots receiving ambient precipitation served as controls. Half of the plots were fertilized with 10 g m^-2 of N. Fine root production was quantified in all plots using a minirhizotron (BTC-2 camera system) consisting of  observation tubes (90 cm length) installed in 2007 and 2009 at a 45^o angle. This allowed root observations to a depth of 32 cm. During the 2012 growing season, images were captured at three-week intervals and analyzed (RooTracker ver 2.0.3, Duke University) to quantify the root biomass using allometric relationships between root diameter and area.