Advances in NANI and NAPI accounting for the Baltic drainage basin: spatial and temporal trends and relationships to watershed TN and TP fluxes

What is it about?

We constructed nitrogen and phosphorus budgets for countries around the Baltic Sea. These budgets include fertilizer imports and the trade of food and livestock feed. Overall nitrogen and phosphorus budgets decreases were small (6% for nitrogen and 4% for phosphorus), but there was much regional variation. For example, there were 25% and 40% decreases for nitrogen and phosphorus, respectively, in areas draining to the Danish Straights. We also found a strong, positive relationship between nitrogen and phosphorus budgets and inputs of these nutrients by rivers to the sea. In other words, leakage increases with the size of the budget. Lastly, we found that the nitrogen and phosphorus in livestock manure could provide most nutrients that crops need, suggesting the potential to reduce imported fertilizers. Nitrogen and phosphorus are critical for life; they are also important fertilizers used in agriculture. Nitrogen and phosphorus from farm land and sewage treatment can leak to the environment and have a fertilizing effect (called eutrophication) on water bodies. In the Baltic Sea, eutrophication has resulted in harmful algal blooms, reduced water clarity, and large "dead zones" (bottom areas where there is not enough oxygen to support life).

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Why is it important?

In the Baltic Sea region, a number of policies have been implemented to reduce nitrogen and phosphorus leakage from land to the sea. One way to track progress to the eutrophication management goals is to examine nitrogen and phosphorus flows on land, because this is the ultimate source of nutrients to the sea. We found that decreased leakage to the sea between 2000 and 2010 was directly proportional to the decrease in the nitrogen and phosphorus budgets for the same years. This finding was unexpected and interesting, because the response was relatively fast and occurred in a number of regions around the sea. Many scientific studies have found that decreases in nutrient budgets don't result into reduced leakage because of the large accumulation of nutrients from earlier years that continues to leak.

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