SUMMARY Celery (Apium graveolens L.) plants were grown in pots in which the root system was separated from the soil in a side chamber by a fine mesh screen. The side chamber was treated with either an organic (ground plant tissue) or inorganic [(NH4)2SO4] source of 15N. Mycorrhizal (Glomus mosseae) and control (non‐mycorrhizal) plants were exposed to 15N over a period of 30 days (inorganic‐15N) or 88 days (organic‐16N). Mycorrhizal and control plants did not differ in shoot dry weight or shoot P content. Dry weight of root was reduced in the mycorrhiza treatments. Mycorrhizal plants derived significantly (P= 0.01) more 16N, from both N sources, than did control plants. In the inorganic‐N treatment, 15N in mycorrhizal plants was significantly (P= 0.001) and positively correlated with percent mycorrhizal fungal colonization (r= 0.58), number of hyphal crossings (±10 μ diameter) through the mesh into the area of 15N placement (r= 0.76), total length of hyphae per gram of soil (r= 0.74), and length of hyphae of 5 μ diameter in the soil (r= 0.77). No correlations were found between the 16N content of mycorrhizal plants and any parameter in the organic‐N treatment. The 16N content of control plants was not correlated with hyphal length in the outer chamber and there were no hyphal crossings of the size ( 10 μ diameter) which was counted for the mycorrhiza treatments. The presence of the organic matter (ground plant tissue) increased the total length of saprophytic hyphae per gram of soil but decreased the number of vesicular‐arbuscular mycorrhizal fungal hyphae crossing into the area of 16N placement. The mean flux of N through the hyphae of G. mosseae was 7.42 × 10−8 mol N cm−2 s−1 for the inorganic‐N treatment over a 30‐day period, and 1.74 × 10−8 mol N cm−2 s−1 for the organic‐N treatment over an 88‐day period.

The nitrogen‐sulfur ratios in the tops and roots of wheat (Triticum aestivum), corn (Zea mays), and beans (Phaseolus vulgaris) were studied in relation to N and S supplies in soil. Greenhouse studies showed that when S became limiting, additional N did not affect either the yield or protein level of the plants, but the nonprotein N (nitrates, amides, and amino acids) increased. There was a close relationship between the amounts of N and S metabolized in the plants. The data indicated that one part sulfur was required for every 12 to 15 parts nitrogen to insure maximum production of both dry matter and protein. As the use of nitrogen fertilizers increases, the possibility that S may become limiting also increases, and adequate consideration of S becomes necessary. This is especially true in greenhouse studies, where S may become limiting very quickly.

Chloride diffusivities were measured in medium‐ and fine‐textured soils at moisture tensions from 1/3 to 15 atm. Chloride diffusivities obtained from the soil systems were divided by the diffusivity of chloride in water to obtain transmission factors. These factors varied from 0.310 to 0.027 depending on the soil moisture content and soil texture. The results were interpreted in terms of volumetric moisture content, effective path length factors and clay‐water interactions.

The isotopic signal provided by differential discrimination against atmospheric carbon dioxide ( 13 CO 2 ) by C 3 and C 4 plant photosynthetic pathways is being widely used to study the processes of carbon (C) fixation, soil organic matter formation, and mineralization in nature. These studies have been facilitated by the availability of automated C and nitrogen (N) combustion analyzers (ANCA) combined with continuous flow isotope ratio mass spectrometers (CFIRMS). Analysis of 13 CO 2 in these instruments requires consistent sample mass for best precision, a requirement that is easily satisfied for soil and tissue samples by adjusting sample weight. Consistent CO 2 sample size is much more difficult to achieve using gas handling systems for samples of headspace gases when CO 2 concentrations vary widely. Long storage of gaseous samples also is difficult. Extended respiration studies are most easily conducted by trapping CO 2 in alkali and conversion to an insoluble carbonate. Thermal decomposition of the carbonate in an on-line ANCA allows consistent and optimal CO 2 sample mass to be obtained. The use of precipitated 1 Corresponding author (e-mail address: 22833mgr@msu.edu). carbonates also facilitates storage of samples and enables full automation of sample analysis using an ANCA interfaced to a CFIRMS. Calcium (Ca), strontium (Sr), and barium (Ba) carbonates were tested. Strontium carbonate (SrCO 3 ) with the addition of vanadium pentoxide (V 2 O 5 ) as a combustion catalyst was found most suitable.

Beans (Phase()las valgtais L.) often respond to N fertilization; however, N fertilization is not practiced for maximum seed production in southern Idaho. This suggests that the symbiotic relationship and/or soil N sources can provide most of the N needed by this legume. Our objective was to evaluate the relative contribution of the symbiotic-nonsymbiotic N sources by studying the effects of N fertilization on the symbiotic N. fixation and seed yields under field conditions. Experiments were conducted on silt loam soils belonging to the Portneuf series (Xerollic Calciortnids). An acetylene reduction (AR) method was used to determine the effect of N fertilization treatments on the relative seasonal N, (AR) fixation. The symbiotic N, fixation was also estimated by the equation, N. = N"" -(N, + N", N.) -aN., where N", is the accumulated N uptake measured near physiological maturity, N, and N,, are the amounts of soil NO,-N in the root zone before planting and near physiological maturity, N" is the N mineralized from soil organic N sources, and a is the recovery of the N fertilizer (N,) applied. Estimates of the N fertilizer recoveries were obtained from two experiments using 15N-depleted (NH4), SO,.The symbiotic N, relationship contributed up to 90 kg N/ha, which was 40 to 50% of the total N found in bean plants near physiological maturity. The amount of symbiotic N. fixed decreased as the available soil N or fertilizer N increased, and increased as the N required by the individual cultivars increased. The response to N fertilization depended upon the cultivar, as well as on the N_available from soil sources. Measured fertilizer N recoveries ranged from 7 to 33%. An average of 52% of the total N uptake near physiological maturity was taken up after the maximum symbiotic NAAR) rate occurred; while the seed contained an average of 60% of the total N uptake. A low N fertilization rate (< 50 kg Nilia) when the soil N, was low (<50 kg N/ha) ensured an early vi us plant growth but did not always increase seed yields. Higher N fertilization rates may be required on soils with lower amounts of mineralizable N.