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The carbon isotope composition (delta C-13) 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 C-13 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 C-13 signatures were derived from the delta C-13 of vegetation and segments of tail switch hair of cattle grown while grazing pastures. Hair C-13 signatures provided an assimilation-weighted C-13 signal that integrated both spatial (paddock-scale) and temporal (grazing season) variation of C-13 signatures on a pasture. The delta C-13 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 C-13 of hair was 2.6 parts per thousand less negative than that of vegetation, reflecting the diet-hair isotopic shift. Furthermore, the delta C-13 of hair and vegetation on peat soil pastures was 0.5 parts per thousand more negative than on pastures situated on mineral soil. This may have resulted from a similar to 10 ppm CO2 enrichment of canopy air derived from ongoing peat mineralization. Community-scale season-mean C-13 discrimination (Delta) exhibited a saturation-type response towards season-mean modeled PAW (r(2)=0.78), and ranged between 19.8 parts per thousand on soils with low PAW capacity during the drought year of 2003, and 21.4 parts per thousand on soils with high PAW capacity in a wet year. This indicated relatively small variation in season-mean assimilation-weighted p(i)/p(a) (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.