Friday, August 10, 2007 - 9:20 AM

COS 153-5: Hawaiian agriculture prior to Cook: Sources of nutrients to windward agricultural systems

Peter M. Vitousek1, Molly A. Palmer1, Oliver A. Chadwick2, Anthony S. Hartshorn2, Thegn N. Ladefoged3, Michael W. Graves4, and Ka`eo Duarte5. (1) Stanford University, (2) University of California, (3) Auckland University, (4) University of New Mexico, (5) Kamehameha Schools

We evaluated sources of the nutrients that sustained intensive agriculture prior to European contact in windward Kohala, Hawai`i, focusing on three potential sources – weathering of primary and secondary minerals in upland soils, enhancement of  weathering supply on eroded colluvial and alluvial landforms, and transport of rock-derived nutrients to crops in irrigation water. Extensive sampling across windward Kohala demonstrated that weathering was insufficient to sustain intensive agriculture in upland soils; all areas were more depleted in cations and phosphorus than were soils at a fertility threshold that bounds nearby rain-fed leeward agricultural systems. In contrast, we found enhanced supplies of rock-derived nutrients on the slopes and alluvial/colluvial areas of a large valley (>200 m deep); these areas retained 40-60% of Ca from parent material, versus ~ 3% for little-eroded upland surfaces, and they supported pre-contact agriculture. However, alluvial/colluvial soils of smaller valleys are less enriched (8-13% of parent material Ca retained), and many of these smaller valleys contained pre-contact irrigated agricultural systems. Analyses of streamwater in these valleys demonstrated that most dissolved cations derive from rock rather than atmospheric sources; for example, analyses of 87Sr/86Sr show that >90% of dissolved strontium comes from basalt sources at low and moderate streamflows.  We calculate that irrigation water could supply >50 times more Ca to taro crops than could in situ weathering on upland soils.