Are low-productive plant communities responsive to nutrient addition? Evidence from dry pioneer grassland

Question: What is the effect of nutrient addition on primary production, phytodiversity and succession of a very unproductive plant community (ca. 100 g. m(-2). a (-1))? Location: A nutrient- poor, calcareous inland sand ecosystem in the northern upper Rhine valley, Germany. Methods: Within a five-fold replicated randomized block design, 10- m(2) plots were given six single or combined applications of nitrogen, phosphorus, potassium and other essential nutrients for four years. An organic carbon treatment was included as a measure to immobilize soil nitrogen. Data were analysed by mixed linear models. Results: Productivity of above-ground vascular plant species doubled after nitrogen addition. Additional nutrient elements did not increase productivity further. The cover of ten species and the height of 15 ( out of 19 examined) species were significantly enhanced by nutrient addition. Centaurea stoebe is nitrogen- limited and exhibits a decisive impact on total above-ground vascular species productivity. Nutrients did not affect phytodiversity. Low-dosage nitrogen addition ( 25 kg. ha(-1) a(-1) ) and organic carbon treatment had no significant impact. However, long-term effects cannot be excluded. Conclusions: The productivity of the examined plant community is responsive to nutrient addition. Although the response is actually more pronounced than in plant communities with higher initial productivity, productivity remained at a low level. Nutrient limitation ( mostly phosphorus + nitrogen co-limitation) of many individual species across all life forms is shown. Total above-ground vascular plant productivity is nitrogen- limited. No species were suppressed completely, nor has there yet been an encroachment of new species. However, high-dosage nitrogen addition resulted in accelerated succession