Comparing root and shoot responses to nutrient additions and mowing in a restored semi-arid grassland

Abstract

Although the use of minirhizotrons is increasing, many gaps in our scientific understanding of root systems remain. For example, relatively few studies have examined root responses to nutrient additions and mowing treatments in restored North American grasslands. In this project, we employed a combination of methods to evaluate the above- and belowground portions of restored grassland plots in western North Dakota (mixed grass prairie; 405 mm average annual precipitation). The plots were planted with various species combinations (1–20) and subjected to 5 years of nutrient additions (N, P, or control) and 3 years of mowing treatments (yearly fall mowing or no mowing). In general, we found the belowground variables (root length, surface area, number of tips, and number of branches) to be relatively unresponsive to the nutrient and mowing treatments. For instance, average root length was 309, 315, and 300 cm in the nitrogen, phosphorus, and control plots, respectively. In contrast, the aboveground variables exhibited pronounced responses to nitrogen applications (200 kg/ha year). Nitrogen applications increased aboveground biomass (660 vs. 265 g/m2, respectively) and reduced average species richness compared to the control (3.25 species vs. 7.29 species, respectively). Our results indicated that the root systems in the plots that received nitrogen were able to support more aboveground biomass than were similarly sized root systems in plots that did not receive nitrogen. However, the observed increase in aboveground biomass came at a cost, as species richness was lower in the plots that received nitrogen.