Vegetation is the main factor in nutrient retention in a constructed wetland buffer

Wetland buffers may play an important role in the retention of nitrogen (N) and phosphorus (P) that can be released in large quantities from forestry operations. In this study, we investigated the retention capacity of N and P of wetland vegetation comparing the control area with two experimental areas within one site before and after N and P pulse (45 kg N and 15 kg P) lasting one growing season (ca. 150 d). N and P pulse caused a significant increase in the plant biomass and N and P content in the upper experimental area, which received most of the added nutrients. Added N and P was mainly retained in the above and below ground parts of E. vaginatum, especially in storage organs and roots which form a long-term sink for nutrients. Total N retention in the plant biomass during the first year after N and P treatment ranged from 25.3 kg (equals to 126.7 kg N ha−1) in the upper experimental area to 6.1 kg (20.4 kg N ha−1) in the lower experimental area and 4.7 kg (15.7 kg N ha−1) in the control area. P retention ranged from 2.6 kg (13.1 kg P ha−1) in the upper experimental area to 1.0 kg (3.4 kg P ha−1) in the lower experimental area and 0.5 kg (1.8 kg P ha−1) in the control area. The retained proportions of N and P in the plant biomass in the two experimental areas were approximately 70% of the added N (45 kg N y−1) and approximately 25% of the added P (15 kg P y−1) during the first year after N and P addition in 1999. Our study shows that vigorously colonising and growing vegetation is the main factor in the retention of N, a significant factor in the retention of P in a constructed wetland buffer, and thus an important contributor to the prevention of detrimental effects of N and P leaching on watercourses.