We assembled a globally-derived data set for site-averaged foliar
δ15N, the δ15N of
whole surface mineral soil and corresponding site factors (mean annual
rainfall and temperature, latitude, altitude and soil pH). The
δ15N of whole soil was related to all of the site
variables (including foliar δ15 N) except altitude
and, when regressed on latitude and rainfall, provided the best model of these
data, accounting for 49% of the variation in whole soil
δ15N. As single linear regressions, site-averaged
foliar δ15N was more strongly related to rainfall
than was whole soil δ15N. A smaller data set
showed similar, negative correlations between whole soil
δ15N, site-averaged foliar
δ15N and soil moisture variations during a single
growing season. The negative correlation between water availability (measured
here by rainfall and temperature) and soil or plant
δ15N fails at the landscape scale, where wet spots
are δ15N-enriched relative to their drier
surroundings. Here we present global and seasonal data, postulate a proximate
mechanism for the overall relationship between water availability and
ecosystem δ15N and, newly, a mechanism accounting
for the highly δ15N-depleted values found in the
foliage and soils of many wet/cold ecosystems. These hypotheses are
complemented by documentation of the present gaps in knowledge, suggesting
lines of research which will provide new insights into terrestrial N-cycling.
Our conclusions are consistent with those of Austin and Vitousek (1998) that
foliar (and soil) δ15N appear to be related to the
residence time of whole ecosystem N.
The spread of farming from western Asia to Europe had profound long-term social and ecological impacts, but identification of the specific nature of Neolithic land management practices and the dietary contribution of early crops has been problematic. Here, we present previously undescribed stable isotope determinations of charred cereals and pulses from 13 Neolithic sites across Europe (dating ca. 5900-2400 cal B.C.), which show that early farmers used livestock manure and water management to enhance crop yields. Intensive manuring inextricably linked plant cultivation and animal herding and contributed to the remarkable resilience of these combined practices across diverse climatic zones. Critically, our findings suggest that commonly applied paleodietary interpretations of human and herbivore δ 15 N values have systematically underestimated the contribution of crop-derived protein to early farmer diets.agriculture | prehistoric | husbandry | paleodiet
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