The effects of global change factors on the stoichiometric composition of green and senesced plant tissues are critical determinants of ecosystem feedbacks to anthropogenic-driven global change. So far, little is known about species stoichiometric responses to these changes. We conducted a manipulative field experiment with nitrogen (N; 17.5 g m(-2) year(-1)) and water addition (180 mm per growing season) in a temperate steppe of northern China that is potentially highly vulnerable to global change. A unique and important outcome of our study is that water availability modulated plant nutritional and stoichiometric responses to increased N availability. N addition significantly reduced C:N ratios and increased N:P ratios but only under ambient water conditions. Under increased water supply, N addition had no effect on C:N ratios in green and senesced leaves and N:P ratios in senesced leaves, and significantly decreased C:P ratios in both green and senesced leaves and N:P ratios in green leaves. Stoichiometric ratios varied greatly among species. Our results suggest that N and water addition and species identity can affect stoichiometric ratios of both green and senesced tissues through direct and interactive means. Our findings highlight the importance of water availability in modulating stoichiometric responses of plants to potentially increased N availability in semi-arid grasslands.

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Although microtopographic heterogeneity is common in bottomland hardwood forests, it is rarely considered in bottomland restoration efforts. The objective of this study was to determine the responses of hydrologic condition, soil physiochemical properties, and introduced and colonizing vegetation to created microtopography and soil treatments at a landfill borrow pit in northern Texas. A series of mounds and pools were created and planted with fast-growing pioneer species as well as more desirable, later-successional species. Erosion control mats were installed on half the plots as a source of organic matter. Erosion control mats had little influence on introduced seedling survival or colonizing species abundance, but microtopography strongly influenced hydrologic condition, soil properties, seedling survival and growth, and colonizing species abundance and distribution. Pools were flooded during much of the summer months and had significantly higher nitrate and total nitrogen concentrations than mounds. Topographic position had little effect on survival of pioneer species, but mortality of most latersuccessional species was highest in pools. Colonizing species distribution and abundance were also strongly related to topographic position. Despite differences in soil nutrient concentration among topographic zones, hydrologic condition likely had the strongest influence on growth and survival of planted species and distribution of colonizing species. Creating microtopography resulted in a spatially heterogeneous system that reflected variations in natural bottomlands, and introducing a mix of species (pioneer and later-successional) across topographic and hydrologic gradients may improve the establishment and survival of a diverse community when hydrologic condition is highly variable or difficult to predict.

Determining the variation in roots traits within a grass root system is important for understanding the role of fine roots in carbon and nutrient cycling in grassland ecosystems, where the majority of biomass and litter accumulation occur belowground. However, few studies have been conducted in this regard. In this study, the structural and chemical traits of shoot-derived and root-derived roots were examined in three perennial grassesCleistogenes squarrosa, Achnatherum sibiricum and Stipa grandis-aiming to explore structural differences, responses to nitrogen and water addition in different types of roots and their correlations with aboveground plant nitrogen. Our results showed significant differences between these two root types, with root-derived roots having higher N concentration, tissue density, and specific root length, but lower C: N and diameter than shoot-derived roots. Trait relationships between root N concentration and tissue density for the two root types differed from that reported among species. These traits in different types of roots were insensitive to resource addition. Furthermore, N concentration in shoot-derived roots was more strongly linked to aboveground plant N concentration than rootderived roots. The results of this study demonstrate structural differences within the root system that may reflect functional heterogeneity in grass roots.