Riparian buffer zones rescue water in agricultural landscapes

Agriculture is the dominant form of land use in Poland, currently occupying (according to current CSO data) almost 60% of the country’s land area. Despite a steady decline in the proportion of agricultural land, mainly in favor of transportation and residential development, mineral fertilizer use is increasing significantly. According to CSO data, 131 kg of mineral fertilizers were used per hectare of farmland in the 2019/2020 marketing year, while 20 years earlier the figure was 86 kg. The intensification of agricultural production, and the associated increase in the use of phosphate and nitrogen fertilizers, promotes the introduction of additional nutrients into the aquatic environment through their leaching from the soil with precipitation. This process contributes to increased eutrophication of waters.

This is particularly noticeable in the case of arable land directly adjacent to the banks of watercourses or standing water. The importance of the problem is indicated by numerous works by Polish limnologists and ecohydrologists, proving that the threat of surface runoff of nutrients and groundwater pollution posed by agriculture is one of the main pressures affecting inland waters in Poland, followed by coastal and marine waters of the Baltic [1-4].

An opportunity to protect aquatic ecosystems in the agricultural landscape is proper management of land directly adjacent to water. Areas straddling the boundary between the aquatic and terrestrial ecosystems are called the ecotone (ecotone), and if covered by shoreline vegetation – the buffer zone. It is the most effective “biological filter”, limiting the load of nutrients reaching water from the catchment area, through the incorporation of compounds into plant tissues, their sedimentation and sorption, and in the case of nitrogen, denitrification carried out by soil microorganisms [4-6]. The postulate of introducing buffer zones as an effective tool for protecting waters and improving their ecological status is one of the basic tenets of ecohydrology, the concept of reducing surface runoff of pollutants by shaping coastal biocoenoses [3].

In addition to reducing the immission of area pollutants, buffer zones perform a number of functions through which they ensure the protection of biodiversity and landscape, as well as promote agricultural cultivation. The expected effects of the presence of buffer zones along watercourses and reservoirs include:

1. Retention of fertilizer components and other pollutants (pesticides, fungicides, plant hormones) flowing from the fields;
2. purification of shallow groundwater from nutrients (nitrogen and phosphorus) as a result of uptake by plants and their incorporation into tissues;
3. Counteracting soil erosion by expanding the root system of plants;
4. Creation of ecological corridors and habitats for many species;
5. Beneficial effect on neighboring crops by increasing moisture (raising the groundwater level in the field), shielding from wind and protecting from erosion, counteracting frost, stabilizing snow cover;
6. Increase crop protection against pests, even narrow buffer zones provide refuge for predatory and parasitic insects, natural enemies of pests.

Buffer zone, or what kind of buffer zone?

The effectiveness of the ecotone in reducing nitrogen and phosphorus compounds, as well as other pollutants, depends on several factors, among which the width of the riparian vegetation belt is an important one. The width of the buffer is important for the level of nutrient reduction, while it varies depending on the conditions of the site. With a zone of mixed structure and species composition of vegetation with a width of 16 m, nitrogen reductions of up to 50% and phosphorus reductions of up to 30% were recorded. Literature on the design of buffer zones recommends a width of 7 to 100 m [7], and with a width of less than 3 – 4 m there is very little effectiveness [8].

Due to the faster development of soil microflora capable of carrying out the denitrification process, wetlands show particularly effective nitrogen reduction. A comparative study of 60- and 45-m-wide zones showed that due to greater hydration, it was the narrower zone that achieved higher (at 89%) nitrogen reduction [3].

The effectiveness of the buffer zone is affected by the species composition of the vegetation covering it. Many reports indicate that buffers overgrown with woody vegetation are more effective at removing pollutants than those built by grasses and dicotyledonous perennials [3,8]. However, the nitrogen retention capacity of grass-wood buffers increases to only 10 – 15% compared to purely grassy zones [9], and herbaceous vegetation, especially in buffers of considerable width and in wetlands, is a significant barrier to pollution.

Of great importance for the effectiveness of the zone is its integrity, especially if the plants occur as continuous, wide bands or a dense mosaic of patches in the immediate vicinity of the reservoir [10]. Even a relatively narrow but continuous zone of ecotone more effectively protects the aquatic ecosystem than a zone that is wide but with significant losses. Severe fragmentation significantly reduces the effectiveness of nutrient capture and water protection from surface runoff. Particular attention should also be paid to any kind of permanent or temporary breaks (e.g., furrows, lowering of the terrain), which are runoff paths for water and can nullify the beneficial effect of the buffer.

The width of buffer zones, their spatial structure and the species composition of the vegetation forming them are a separate, complex issue, and guidelines in this regard can be found in numerous publications and studies, both foreign [11-14], and domestic [3, 15-16].

Activities to promote buffer zones

The term “introduction of buffer zones” can be understood as a whole range of activities directed at preserving, restoring or creating ecotone zones, from passive (involving the abandonment of all agricultural activities and leaving a specific riparian area to spontaneous succession) through the restriction of agricultural activity in coastal zones to specific activities that favor the preservation of riparian vegetation and active restoration of wetlands and marshy areas through plantings.

• Exclude from use a strip of land along the water’s edge and leave it to natural succession. This is an example of a passive action; it does not require expenditures to restore the ecotone zone, but it involves a reduction in the possibility of benefiting from the use of part of the agricultural land, i.e. a loss of potential revenue.
• Restoration of the ecotone zone through controlled plantings. The measure involves planting in a strip of land of a certain width along the water’s edge; it involves incurring costs for the purchase of appropriate plant material. preparing the ground for planting and making the plantings themselves, sometimes tending them. In addition, the area excluded from use does not yield agricultural benefits.
• Restoration of marshlands through removal of hydraulic structures to prevent periodic flooding. The activity involves renaturalizing the hydrological regime of a reservoir or watercourse to restore the natural cycle of flooding and leave flooded areas to natural succession.

Formal and legal empowerment of buffer zones

The creation of buffer zones is a requirement resulting from, among other things. from Council Regulation (EC) 73/2009, establishing common rules for direct support schemes for farmers under the common agricultural policy, which in Annex III indicates “the establishment of buffer zones along watercourses” as one of the mandatory standards for the protection of water resources. The regulation’s buffer zone regulations indicate an obligation to meet, at the very least, the requirements for the conditions of agricultural use of nitrogen fertilizer near watercourses, but without defining any other guidelines for these areas and leaving it up to the discretion of the recommended solutions.

In national law, the regulations specifying the requirements for buffer zones, are contained in the Decree of the Council of Ministers of January 31, 2023. On the adoption of the “Program of measures to reduce water pollution by nitrates from agricultural sources and to prevent further pollution” (Journal of Laws 2023, item 244). The program introduces exclusions of coastal areas of a certain width (5, 10 or 20 meters depending on the type and size of the reservoir and the fertilizer used) from the use of nitrogen fertilizer, without indicating any other measures directed at creating or restoring these zones.

In addition, if there is crop cultivation on agricultural land and fertilizers are applied directly to the soil or full doses are divided into at least three doses applied at an interval of not less than 3 days, the widths of the zones may be halved with a minimum distance of 3 meters. This means the possibility of reducing an already relatively narrow strip of protection. A wider buffer zone is envisioned by the Waste Law, which in Art. 96 indicates a ban on the use of municipal sewage sludge in a strip of land 50 meters wide immediately adjacent to the shore of a lake or watercourse. However, the ban does not extend to any other activities in the immediate vicinity of the waters.

Good agricultural practices (GAEC), consistent with environmental protection in the establishment of buffer zones along watercourses (GAEC 4), indicate only a prohibition on the application of fertilizers (including nitrogen) and plant protection products on agricultural land near surface waters at distances of 5, 10 or 20 meters from the shoreline (according to the “Action Program”). This provision in no way exhausts the definition of a buffer zone.

Exclusion from fertilization is a basic requirement, but at the same time usually insufficient to ensure proper water quality. The range of activities recommended to be implemented when creating buffer zones should additionally include:

• Prohibition of use for raising/grazing livestock or as a parking and/or washing area for agricultural vehicles and equipment;
• Conducting (especially during the first years of operation) maintenance activities consisting of mowing in late summer and removing the mowed biomass to prevent it from entering waterways;
• Removal of alien species, especially invasive species, such as, for example, American uvula, Canadian goldenrod, and spiny thorn (species listed in the Decree of the Council of Ministers of December 9, 2022, on the list of invasive alien species posing a threat [Journal of Laws 2022, item 2649] and in scientific studies);
• Restoration of the buffer zone through controlled plantings in the case of a heavily degraded ecotone zone. An important element is the selection of vegetation so as to provide as much species diversity as possible. The most effective in removing nutrients is a zone made up of a belt of grasses or dicotyledonous perennials, but connected to deep-rooting shrubs and trees. It is assumed that the buffer zone should also protect the plant communities developed on them, species of natural value (even protected species) and a wealth of biodiversity.

Other mechanisms for introducing buffer zones

In the case of lakes, indications of the need to restore/maintain buffer zones can be introduced in the protective areas of inland water bodies established under Art. 141 of the Water Law. However, their implementation is possible only if such areas are established by a governor’s decree, and only if such practices are included in the catalog of activities for a specific area.

Buffer zone measures are sometimes indicated in the conservation task plans (PZOs) of Natura2000 areas aimed at protecting aquatic habitats. The records are very different in nature and very different in degree of categoricality. If an agricultural area is located in Natura2000 areas, activities in the zones could be identified as key within the Priority Action Framework (PAF).

The National Water Restoration Program [17] includes a whole series of measures referring to the restoration or preservation of buffer zones, but the document has no binding force so far.

It follows that in Poland, except for the established protection areas of inland water bodies and areas covered by the relevant provisions of the PZO, the action related to buffer zones in a broader sense than just exclusion from the use of fertilizers and plant protection products is optional and voluntary. This is a pity, because it would be worthwhile to more widely publicize and put into practice this relatively inexpensive and very effective measure from the point of view of water protection.

The author is a doctoral candidate in the discipline of biological sciences and a professor at the Institute of Environmental Protection – National Research Institute. Phycologist and ecologist of inland waters, with a particular fondness for still waters, from lakes to ponds. He has more than a dozen years of teaching work at the UMCS and JPII Catholic University of Lublin. For nearly 20 years he has worked at the Department of Water Protection of the IOŚ-PIB (formerly the Department of Water Assessment and Monitoring Methods) taking part in work related to the implementation of the Water Framework Directive, mainly in the field of biological methods of monitoring and assessment of the ecological status of waters. She is the author and co-author of dozens of scientific publications in the field of hydrobiological research and the implementation of scientific and research work commissioned by government bodies responsible for water management. A proponent of natural philosophy as a fundamental reflection on life.

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