Nitrogen effects on seed germination and seedling growth

Recent evidence associates the persistence of invasive plant species with disturbance and fluctuations in distinct forms of mineral N in soils. We conducted soil and hydroponic experiments to investigate the influence of N form and availability on germination and seedling development of 2 invasive annual grasses, cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) and 6 perennial grasses, blue-bunch wheatgrass (Pseudoroegneria spicata), crested wheatgrass (Agropyron cristatum x A. desertorum), Sand Hollow and Seaman's Gulch big squirreltail (Elymus multisetus), and Little Camas and Little Wood bottlebrush squirreltail (E. elymoides ssp. brevifolius and E. elymoides ssp. elymoides, respectively). Seeds were sown in soils with no soil additions, barley straw (1 mg kg(-1)), NH4+ = 10 mg N kg(-1), NH4+ + I (nitrification inhibitor) = 10 mg N kg(-1) + 37 ml nitrapyrin, or NO3- = 10 mg N kg(-1) to evaluate cumulative germination percentage for 20 days in an incubator. For the hydroponic experiment, grass seedlings were exposed to distinct forms and uniform concentrations of mineral N to monitor root and shoot growth for 21 days. Treatments were no N added, NH4+ (1 mM), NO3- (1 mM), and NH4NO3 (0.5 mM). Treatments did not alter germination in the soil experiment. Lack of soil N effect on seed germination is attributed to the absence of seed dormancy in the populations of grasses we evaluated. Initial root length and overall shoot growth of grasses were greater in the NO3- than in the NH4+ treatment more frequently for perennial grasses. Root and shoot growth of medusahead and cheatgrass generally exceeded that of the other grasses except crested wheatgrass. However, relative decreases in root dry mass for the no N treatment were greater for the invasive annual grasses than the perennial grasses when compared to the N-addition treatments.