Effects of co-addition of ammonium, nitrite, and glucose with methionine on soil nitrogen.

Methionine is one of the many amino acids in the soil. In order to study the role of methionine in acidic forest soil, the effect of methionine (Met) was compared with control together with addition of ammonium (Met + A), nitrite (Met + N), and glucose (Met + C) under 60% or 90% water holding capacity (WHC), because ammonium and nitrite are important factors in nitrification, and glucose affect the heterotrophic nitrification and nitrogen immobilization. We found that methionine addition significantly reduced NO3- concentration in acidic forest soil. Compared to Met, Met + A and Met + N treatments non-significantly enhanced nitrification; however, Met + C treatment decreased NH4+ concentration which suggested that soil autotrophic and heterotrophic nitrification were limited in the presence of methionine at 60% WHC. Further, our findings of 15N-labeled treatment showed the impact and priming effect of methionine was negative for NO3- concentration and positive for N2O emission, which were observed mainly from the soil N source rather than methionine. At 90% WHC, Met + C treatment significantly lessened concentrations of NH4+ and NO3-, nonetheless improved N2O compared to Met treatment. Therefore, besides the denitrification and dissimilatory NO3- reduction to ammonia, the immobilization might be the key factor to explain this decrease in NO3- concentration at 90% WHC, while these processes were induced with the C addition. This study indicated that the positive role of amino acids in soil N cycling might be overrated.

N2O emission from a temperate forest soil during the freeze-thaw period: A mesocosm study.

Nitrous oxide (N2O) is an important greenhouse gas and is involved in the destruction of ozone layer. However, the underlying mechanisms of the high soil N2O emission during the freeze-thaw (FT) period are still unclear. Here, we conducted a mesocosm study with high frequency in situ measurements to explore the responses of soil microbes to the FT cycles and their influences on soil N2O emission. We found the high N2O emission rate during the FT period was mainly due to the release of substrates, the maintenance of high enzyme activities at the freezing stage, and the fast recovery of microbial biomass nitrogen (MBN) and high microbial activities at the thawing stage. Physical isolation of previously produced N2O was an important mechanism for the higher N2O flux at the thawing stage. With increasing numbers of the FT cycles, MBN at the thawing stage remained stable and potential dehydrogenase activities at the thawing stage also remained stable after the first eight FT cycles and only declined during the last two cycles, suggesting the sustainability of the biological mechanisms. Our study suggests that although MBN declined, microbial enzymes could maintain high activities at a few degrees Celsius below zero in this temperate forest soil and produce high N2O fluxes even at the freezing stage, which were trapped under the ice layer and released at the thawing stage, resulting in high soil N2O emission during the FT period.

[Effects of L-methionine on nitrification and N2O emission in subtropical forest soil].

The objective of this study was to investigate the influence of L-methionine on nitrification and nitrous oxide emission in a red soil under laboratory incubation experiments. A subtropical broad-leaved forest soil sample was collected from Wanmulin natural reserve in Fujian Province, Southeast China. Five treatments were carried out with three replications, i. e., control (CK), L- methionine addition (M), L-methionine and NH(4+)-N addition (MA), L-methionine and NO(2-)-N addition (MN), L-methionine and glucose addition (MC). The soil moisture was maintained at 60% WHC or 90% WHC. The results indicated that the soil NH(4+)-N content in the M treatment significantly increased by 0.8%-61.3%, while the soil NO(3-)-N content reduced by 13.2%-40.7% compared with CK. Under 60% WHC condition, soil NO(2-)-N content in the MC treatment was higher than in the M treatment, soil NO(3-)-N content in the MA and MN treatments were greater than that in the M treatment, and greater in the MN treatment than in the MA treatment. The soil NO(3-)-N content was lowest in the M treatment after incubation. These results suggested that L-methionine could inhibit nitrosation process of autotrophic nitrification. To some extent, carbon addition as glucose with L-methionine decreased the NH(4+)-N content, inhibited the autotrophic nitrification and their effects were dependent on water level. Under 90% WHC condition, carbon addition improved denitrification more obviously, but the decrease of NO(3-)-N content was not sufficient to prove the inhibition of hetero-nitrification due to carbon addition in the presence of L-methionine. The nitrous oxide emission from soil was increased by L-methionine addition. Compared with 60% WHC condition, the nitrous oxide emission was higher under 90% WHC condition, and the promotion of L-methionine addition on N2O was greater when glucose added.