PARENT SESSION
Oral Session #35: Nitrogen cycling: Response to inputs, variation in time and space. Presiding: M. Fenn.
Tuesday, August 7, 2001. 1:00 PM to 5:00 PM. Hall of Ideas G.
Fate of 15N nitrate additions in Hawaiian wet tropical forest soils.
Lohse, Kathleen 1, Matson, Pamela2, 1 2
ABSTRACT- We investigated the relative importance of soil biotic and abiotic factors in retaining 15N nitrate additions in Hawaiian wet tropical forests that differed in nutrient status and soil age. While biological demand for N is often considered the primary factor retaining chronic N inputs in many temperate forest ecosystems, anion exchange capacity may be an important mechanism retaining nitrate in highly weathered tropical soils. Following 15N nitrate additions in a simulated rainfall event, we determined and compared the fate of 15N label into different soil N pools in a N-limited forest growing on 300 year old soil and a P-limited forest growing on 4.1 million year old soil. While 84% of the applied nitrate was lost as soil solution losses in the young site, only 38% was lost in the old site. A majority of the remaining nitrate label was recovered in the surface soils in the young site (14% of total added), while 15N recovery occurred in the surface and at depth in the old site. Relatively small proportions of N were recovered in microbial biomass in both sites, although microbial uptake was greater in the young N-limited site. In the old site, a range of evidence suggests that anion exchange may be reducing nitrate loss. While stabilization of nitrogen into soil organic matter appears to be the primary mechanism retaining nitrate additions in the young N-limited site, anion exchange appears to regulate nitrate losses from the highly weathered tropical soil.
KEY WORDS: nitrogen, tropical, AEC