Abstract
Abstract In this article, by using carbon stable isotopes, we assessed the past and present land use influences that riparian areas are subject within agricultural landscapes. Emphasis is given to the understanding of the effects of the 2012 Brazilian Forest Act on such areas. We selected five riparian areas within a highly C4 dominated agricultural landscape. Three of them had 30 meters native riparian forest buffer (NRFB) and two of them had 8 meter and no NRFB. We used three 100 meter-transects located 5, 15 and 30 meters relative to stream channel to obtain soil samples (0 - 10 cm). All riparian areas presented soil carbon isotopic signatures that are not C3 (native forests) irrespective of having or not 30 meters NRFB. Two cases presenting less than 30 meters NRFB had higher C4 derived carbon contribution. All of the other three areas that followed the 30 meters NRFB presented, to some degree, C4 derived carbon, which was attributed to C4 organic matter deposition originated from cultivated areas and, in one case, to the persistence of former exotic grasses. With the 2012 Forest Act allowing narrower buffers (< 30 meters), we expect C4 contributions to soil organic matter to remain high in riparian areas and streams within agricultural landscapes dominated by C4 plants where 30 meter NRFB is no longer required. Such contributions will likely continue to have detrimental effects on stream water quality and biota.
Highlights
Riparian ecosystems generally perform many important ecological processes
In the watershed 1, even the 5-m sampling points that are located within the riparian forest remnant had approximately 75% of organic carbon derived from C4 plants, suggesting that the entire riparian zone had influence of C4 plants (Table 2)
In the riparian area of the watershed 2, the d13C of the soil near the spring resembled those values of C3 vegetation, being lower than À 24%
Summary
Riparian ecosystems generally perform many important ecological processes When these streamside ecosystems are under forest cover these areas are considered to be important due to a series of reasons: (i) protection of stream banks against erosion and bank sliding (Abernethy & Rutherford 2000); (ii) reduction of erosion and input of soil particles into the stream (Lowrance et al 1986, Verstraeten et al 2006, Pires et al 2009); (iii) shading and reduction of the water temperature (important for fish reproduction) (Imholt et al 2013); (iv) riparian forests provide nutrients and carbon to aquatic communities (Lowrance et al 1985); and they might increase the input of coarse woody debris to the stream channel, which is important in creating habitat diversity within the stream environment (De Paula et al 2011). If all these important processes are to be achieved in a single place at the same time, the process that requires a wider buffer might be the one that should be adopted
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