Twelve stands of snowbrush were selected for a biomass study to estimate nitrogen fixation under field conditions west of the summit of the Oregon Cascades. Total nitrogen in the upper 2 ft of soil was higher under snowbrush than in the open, but the difference may have been caused by loss of nitrogen from open areas rather than by nitrogen fixation. Total nitrogen in the upper 15 cm of soil under snowbrush did not differ from that found under nonfixing shrub species. Various shrub species may increase the total soil nitrogen under their canopies, however, if only by accumulation from sites that lack vegetation. More nitrogen may be tied up in the biomass of mature snowbrush stands than in stands of other shrubs. The difference could be explained by nitrogen fixation, which may range from zero to about 20 kg/ha per year under conditions of this study. Nodulated snowbrush seedlings produced 2.5 times the dry weight that non—nodulated seedlings produced in a nitrogen deficient soil. Sixty—one per cent of the nitrogen in nodulated greenhouse seedlings was fixed. Such fixation rates may be reached on infertile soils in the field, but they seem unlikely on soils of medium or better fertility. This point seems to be substantiated by delay in nodulation of snowbrush in soils with increased levels of organic matter. Bioassay tests using Douglas fir seeds and hemlock seedlings showed that snowbrush did not add to the soil any significant amount of nitrogen by nitrogen fixation. The species contributes to the formation of a new organic layer, however, through large amounts of nitrogen—rich litter.

Techniques using a plastic tent to contain C¹ ⁴O₂ gas over herbaceous vegetation were effective for applying a tracer to Missouri tallgrass prairie. The grasses incorporated 67 to 41 μCi of C¹ ⁴/m² from a single exposure to 151 μCi of C¹ ⁴O₂/m² for 6 hr on clipped and unclipped areas, respectively. Regenerated shoots, which had been clipped 6 weeks earlier, possessed 8 times higher specific activity than unclipped shoots (0.134 vs. 0.017 μCi/g of carbon in foliage biomass). The root systems of both areas possessed over 50% and the roots of mature vegetation as much as 85% of the assimilated C¹ ⁴ remaining in the plant biomass 8 weeks after tagging, thus illustrating the tendency for food reserves to accumulate in underground storage organs late in the growing season. Specific activities in the root systems were similar (0.02 vs. 0.016 μCi/b C) for both revegetated and mature areas and were adequate for study of later translocation, transfer to soil, and in situ decomposition of organic material in soil.

Net primary production of a 1—year—old field on the New Jersey Piedmont was 1.08 kcal/cm² or 10% of the radiant energy (0.4—0.7 @m) intercepted by the vegetation from the last spring frost to the latest date a dominant producer reached its peak standing crop biomass; 3.8% of the energy available above the vegetation during the same period; 7.5% of the energy intercepted from the last spring frost to the first fall frost; 3.1% of the energy available above the vegetation during this period; and 1.8% of the energy available yearly. These results are among the first direct determinations of efficiency of net primary production based on interception of radiant energy under field conditions. Interception, the difference between radiant energy available above and below the vegetation, was measured with the Yellott solarimeter, whose small size made possible below—vegetation measurements with minimum disturbance to the cover. Net primary production for shoots was determined on a species basis by the short—term harvest method. Root production was estimated on a community basis by extracting roots from soil samples by a soil—dispersion and chemical flotation technique.