Hair of grazing cattle provides an integrated measure of the effects of site conditions and interannual weather variability on C of temperate humid grassland
Schnyder, H.; Schwertl, M.; Auerswald, K.; Scheaufele, R.
Global change biology 12(7): 1315-1329
ISSN/ISBN: 1354-1013 DOI: 10.1111/j.1365-2486.2006.01169.x
The carbon isotope composition ([delta][superscript 13]C) of C3 ecosystems is sensitive to water availability, and provides important information for the assessment of terrestrial carbon (C) sink/source activity. Here, we report the effects of plant available soil water (PAW) on community [superscript 13]C signatures of temperate humid grassland. The 5-year study was conducted on pastures exhibiting a large range of PAW capacity that were located on two site types: peat and mineral soils. The data set included the centennial drought year 2003, and data from wet years (2000 and 2002). Seasonal variation of PAW was modeled using PAW capacity of each pasture, precipitation inputs and evapotranspiration estimates. Community [superscript 13]C signatures were derived from the [delta][superscript 13]C of vegetation and segments of tail switch hair of cattle grown while grazing pastures. Hair [superscript 13]C signatures provided an assimilation-weighted [superscript 13]C signal that integrated both spatial (paddock-scale) and temporal (grazing season) variation of [superscript 13]C signatures on a pasture. The [delta][superscript 13]C of hair and vegetation increased with decreasing modeled PAW in the same way on mineral and peat soils. But, at a given PAW, the [delta][superscript 13]C of hair was 2.6[per thousand] less negative than that of vegetation, reflecting the diet-hair isotopic shift. Furthermore, the [delta][superscript 13]C of hair and vegetation on peat soil pastures was 0.5[per thousand] more negative than on pastures situated on mineral soil. This may have resulted from a ~10 ppm CO[subscript 2] enrichment of canopy air derived from ongoing peat mineralization. Community-scale season-mean [superscript 13]C discrimination ([Delta]) exhibited a saturation-type response towards season-mean modeled PAW (r[superscript 2]=0.78), and ranged between 19.8[per thousand] on soils with low PAW capacity during the drought year of 2003, and 21.4[per thousand] on soils with high PAW capacity in a wet year. This indicated relatively small variation in season-mean assimilation-weighted pi/pa (0.68-0.75) between contrasting sites and years. However, this range is similar to that reported in other studies, which encompass the range from subtropical arid to humid temperate grassland. Furthermore, the tight relationship between season-mean [Delta] and modeled mean PAW suggests that PAW may be used as proxy for [Delta].