Obviously, the most efficient approach to reducing P losses from agriculture employs a combination of (1) the best management practices to reduce P losses from the source soils, including management to improve water infiltration for diminishing surface runoff and (2) the establishment of buffer zones to further decrease the P load entering the waterways.
The effect of buffer strips to trap PP is mostly attributable to reduction of waterflow velocity. As buffer zones, the lower ends of concave slopes, where waterflow velocity is markedly reduced, could act as sedimentation areas for incoming PP. In contrast, buffer strips established at the lower ends of convex slopes would primarily reduce losses of PP only from that particular slope area. For maximum P retention efficiency, the buffer zone soil should have high porosity to enable infiltration of large amounts of water. Moreover, a dense, native vegetation with high species diversity and deep-rooted plants would best promote trapping of incoming P by plants. Buffer zones are able to decrease quite efficiently the load of particle-bound P, which is the main form of P in agricultural runoff, though not necessarily the main source of available P. Therefore, buffer strips and sedimentation ponds should be established at the lower ends of fields with high erosive cultivation producing relatively large losses of PP. In particular, depressions which channel water from large areas should be considered as potential sites for these zones.
Compared with PP, the retention of DP in buffer zones is much less efficient, and in some cases the losses of DP are even increased. Moreover, though the buffer zones are efficient initially in trapping P, this may decrease over time.
Drawing a conclusion from the processes of P retention in the buffer zones discussed above, one may state that an acidic soil rich in oxide material and clay and low in adsorbed P and organic matter would best serve as a chemical sink for P. These requirements are usually fulfilled by a fine-textured subsoil material, provided that the permeability is high enough to allow infiltration. It should be taken into account, however, that the accumulation of erosion material can change the mineralogical, chemical and physical properties of the buffer zone soil.
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