Wetlands in agricultural landscapes for nitrogen attenuation and biodiversity enhancement: Opportunities and limitations

Riparian zones along streams and other small wetlands perform important ecosystem services in agricultural landscapes. They improve water quality by retaining nutrients and sediment from through-flowing water, sequester carbon and attenuate flood peaks at high discharge. At the same time, they have been shown to enhance biodiversity at the landscape as well as regional scale. However, there is concern about loss of biodiversity and enhanced greenhouse gas emissions where wetlands have become overloaded with nutrient-rich surface water or groundwater. This paper discusses conditions for the best performance of riparian zones and other small wetlands in agricultural landscapes in terms of nutrient and greenhouse gas retention and biodiversity.High nitrous oxide emissions have been identified in riparian zones used for N removal and could be a concern which would diminish their overall environmental benefit. However, it has been shown in riparian zones in the Rhine and Scheldt catchment that nitrous oxide emissions mainly occur when soil pH values are lower than 5. Such conditions occur mostly in stream headwater areas only where sandy sediments have a low buffering capacity, whereas higher-order streams and their riparian zones often have fine silty clay or loam sediments with higher pH values, supporting effective denitrification with low nitrous oxide emission.The question whether the nutrient attenuation service comes at the expense of biodiversity losses should be answered differently for existing, (semi-)natural wetlands and for new wetlands created in the agricultural landscape. Existing wetlands are quite vulnerable for nutrient loading, depending on their natural nutrient richness. Oligotrophic and mesotrophic wetlands such as bogs and fens are negatively affected by nutrient loadings > 4 g N m−2 y−1 and 0.5 g P m−2 y−1, which lead to loss of diversity and characteristic species. Wetlands in riparian zones are naturally more nutrient-rich and can tolerate much higher nutrient loadings without consequences for ecosystem health and diversity, their loading limits are 50 g N m−2 y−1 and 4 g P m−2 y−1. For nitrogen the relatively high limit is explained by significant denitrification activities in wetlands, transforming nitrate to mostly inert elemental nitrogen gas. There are also indications that only one of the major nutrients needs to be in short supply (i.e. below the limits indicated) in order to avoid major eutrophication problems. In agricultural areas where large numbers of wetlands have been created for N removal and biodiversity enhancement, the experience of this ‘dual purpose’ are generally positive, particularly in landscapes where not much of the original wetland resource remains.