Variable effects of biochar application to soils on nitrification-mediated N2O emissions.

Although a meta-analysis on biochar's effects on N2O emission reported an overall reduction in N2O emission by adding biochar to the soils, there are still variations in the changes in N2O emission, especially from field results. The objectives of this study are 1) to compare the effects of biochar addition on N2O emission between three agricultural upland field experiments, where soil water status was dry favoring nitrification and 2) to identify main factors explaining biochar's variable effects on N2O emission. Three field experiments were conducted: Exp A in the cultivated grassland treated with rice husk biochar at 2 ton ha-1 + urea (CHAR) and with urea only (CON); Exp B in the cabbage field with CHAR and CON treatments; and Exp C in the pepper field with CHAR, CON, and CHAR + DCD (dicyandiamide, nitrification inhibitor) treatments. In Exp A and C, cumulative N2O emissions significantly increased by 82.5% and 55.8% in the CHAR than CON treatments, respectively, while in Exp B, there was no difference in cumulative N2O emission between the CHAR and CON. Based on results from using nitrification inhibitor and soil % water filled pore space (WFPS), we assumed that the main N2O production mechanism was nitrification. Our results suggest that soil water status right after urea application is the primary determinant of different effects of biochar on N2O emission in addition to soil C status and biochar's adsorption. Principal component analysis using the 25 compiled data also supported our results. This study identified the specific field conditions under which biochar could have stimulating effects on N2O emission. Mitigation potential of biochar application should be reconsidered if biochar and urea were amended to dry soils with low C contents.

Novel Pathways in the Reactions of Superoxometal Complexes.

The reaction of a 1:1 mixture of (H(2)O)(5)Cr((16)O(2))(2+) and (H(2)O)(5)Cr((18)O(2))(2+) at pH 1 did not yield measurable amounts of (16)O(18)O. This result rules out a Russell-type mechanism (2(H(2)O)(5)CrO(2)(2+) --> 2(H(2)O)(5)CrO(2+) + O(2)) for the bimolecular decomposition reaction. Evidence is presented in support of unimolecular (S(H)1) and bimolecular (S(H)2) homolyses as initial steps in the decomposition of (H(2)O)(5)CrO(2)(2+) in strongly acidic solutions (pH

Hydrocarbon oxidation by Bis-mu-oxo manganese dimers: electron transfer, hydride transfer, and hydrogen atom transfer mechanisms.

Described here are oxidations of alkylaromatic compounds by dimanganese mu-oxo and mu-hydroxo dimers [(phen)(2)Mn(IV)(mu-O)(2)Mn(IV)(phen)(2)](4+) ([Mn(2)(O)(2)](4+)), [(phen)(2)Mn(IV)(mu-O)(2)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(2)](3+)), and [(phen)(2)Mn(III)(mu-O)(mu-OH)Mn(III)(phen)(2)](3+) ([Mn(2)(O)(OH)](3+)). Dihydroanthracene, xanthene, and fluorene are oxidized by [Mn(2)(O)(2)](3+) to give anthracene, bixanthenyl, and bifluorenyl, respectively. The manganese product is the bis(hydroxide) dimer, [(phen)(2)Mn(III)(mu-OH)(2)Mn(II)(phen)(2)](3+) ([Mn(2)(OH)(2)](3+)). Global analysis of the UV/vis spectral kinetic data shows a consecutive reaction with buildup and decay of [Mn(2)(O)(OH)](3+) as an intermediate. The kinetics and products indicate a mechanism of hydrogen atom transfers from the substrates to oxo groups of [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+). [Mn(2)(O)(2)](4+) is a much stronger oxidant, converting toluene to tolyl-phenylmethanes and naphthalene to binaphthyl. Kinetic and mechanistic data indicate a mechanism of initial preequilibrium electron transfer for p-methoxytoluene and naphthalenes because, for instance, the reactions are inhibited by addition of [Mn(2)(O)(2)](3+). The oxidation of toluene by [Mn(2)(O)(2)](4+), however, is not inhibited by [Mn(2)(O)(2)](3+). Oxidation of a mixture of C(6)H(5)CH(3) and C(6)H(5)CD(3) shows a kinetic isotope effect of 4.3 +/- 0.8, consistent with C-H bond cleavage in the rate-determining step. The data indicate a mechanism of initial hydride transfer from toluene to [Mn(2)(O)(2)](4+). Thus, oxidations by manganese oxo dimers occur by three different mechanisms: hydrogen atom transfer, electron transfer, and hydride transfer. The thermodynamics of e(-), H(*), and H(-) transfers have been determined from redox potential and pK(a) measurements. For a particular oxidant and a particular substrate, the choice of mechanism is influenced both by the thermochemistry and by the intrinsic barriers. Rate constants for hydrogen atom abstraction by [Mn(2)(O)(2)](3+) and [Mn(2)(O)(OH)](3+) are consistent with their 79 and 75 kcal mol(-)(1) affinities for H(*). In the oxidation of p-methoxytoluene by [Mn(2)(O)(2)](4+), hydride transfer is thermochemically 24 kcal mol(-)(1) more facile than electron transfer; yet the latter mechanism is preferred. Thus, electron transfer has a substantially smaller intrinsic barrier than does hydride transfer in this system.