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Publication costs assisted by the U. S. Energy Research and Development AdminlstrationRate constants for the reactions of H atoms, formed by mercury photosensitization of H2-substrate mixtures at 32 O C , with (CH3),SiH4-, and (C2HS),SiH4-, for n = 0, 1,2,3; with (CH3),SiD4-, for n = 0, 1; and with Si2H6, Si2D6, GeH4, and Ge2H6 have been determined in experiments involving the competitive reaction of H atoms with C2H4 and i-C4HB. Activation energies, estimated from these rate constants, have been combined with thermochemical data by means of a bond-energy-bond-order (BEBO) method of calculation to yield a consistent set of dissociation energies for Si-H bonds in the R,SiH4-, series of silanes for n = 0, 1, 2, 3.

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A direct 15N method was used to measure N2 and N2O flux in flooded rice (Oryza sativa L.) fields. The method involved application of a highly 15N‐enriched source to a 0.4‐m by 0.4‐m microplot, collection of evolved N2 and N2O in a chamber placed over the flood‐water, removal of NH3 from the air sample, conversion of some (N2 + N2O)‐N in the air sample to NH+4‐N, and determination of 15N content with a mass spectrometer. The flux of (N2 + N2O)‐15N was measured for 17 d following urea application by three methods, each replicated four times, on a Vertic Tropaquept. In all cases, the (N2 + N2O)‐15N flux was much smaller than total gaseous 15N loss as estimated from unaccounted for 15N in the 15N balance after 18 d. The (N2 + N2O)‐15N flux and total 15N loss were 1.1 and 40%, respectively, following basal broadcast and incorporation of 58 kg urea‐N ha−1 with 2‐cm standing water; they decreased to 0.2 and 26%, respectively, when the urea basal incorporation was without standing water. The (N2 + N2O)‐15N flux and total 15N loss were 0.5 and 46%, respectively, following application of 44 kg urea‐N ha−1 to 50‐mm floodwater at 10 d after transplanting. The methodology could detect a flux of 10 g N ha−1 d−1 from the added 59 to 64 atom % 15N‐labeled urea. Additional studies with 15N‐labeled NO‐3 evaluated the possibility that the low (N2 + N2O)‐15N flux was due to incomplete recovery of N2 and N2O formed from added fertilizer. Direct recovery of (N2 + N2O)‐15N following addition of labeled NO‐3 ranged from 41 to 73% of the total 15N loss. No 15N was lost by leaching or runoff, and dissolved N2O in the floodwater calculated from published solubility data was less than 1% of the added 15N.