Responses of CO2 emissions and soil microbial community structures to organic amendment in two contrasting soils in Zambia.

In sub-Saharan Africa, efforts have been made to increase soil carbon (C) content in agricultural ecosystems due to severe soil degradation. The use of organic materials is a feasible method for recovering soil organic C; however, the effects of organic amendments on soil microbial communities and C cycles under C-limited soil conditions are still unknown. In this study, we conducted field experiments in Zambia using organic amendments at two sites with contrasting C content. At both sites, temporal changes in soil carbon dioxide (CO2) emissions and prokaryotic community structures were monitored during the crop growing season (126 days). The organic amendments increased CO2 emissions and prokaryotic abundance at the Kabwe site, whereas no direct impacts were observed at the Lusaka site. We also observed a larger temporal variability in the soil microbial community structure at Kabwe than that at Lusaka. These contrasting results between the two soils may be due to the microbial community stability differences between each site. However, as organic amendments have considerable potential to enhance microbial abundance and consequently sequester C at the Kabwe site, site-specific strategies are required to address the issues of soil C depletion in drylands.

Abiotic and biotic factors controlling the dynamics of soil respiration in a coastal dune ecosystem in western Japan.

In this study, we examined the abiotic and biotic factors controlling the dynamics of soil respiration (Rs) while considering the zonal distribution of plant species in a coastal dune ecosystem in western Japan, based on periodic Rs data and continuous environmental data. We set four measurement plots with different vegetation compositions: plot 1 on bare sand; plot 2 on a cluster of young Vitex rotundifolia seedlings; plot 3 on a mixture of Artemisia capillaris and V. rotundifolia; and plot 4 on the inland boundary between the coastal vegetation zone and a Pinus thunbergii forest. Rs increased exponentially along with the seasonal rise in soil temperature, but summer drought stress markedly decreased Rs in plots 3 and 4. There was a significant positive correlation between the natural logarithm of belowground plant biomass and Rs in autumn. Our findings indicate that the seasonal dynamics of Rs in this coastal dune ecosystem are controlled by abiotic factors (soil temperature and soil moisture), but the response of Rs to drought stress in summer varied among plots that differed in dominant vegetation species. Our findings also indicated that the spatial dynamics of Rs are mainly controlled by the distribution of belowground plant biomass and autotrophic respiration.

Effects of short-term freezing on nitrous oxide emissions and enzyme activities in a grazed pasture soil after bovine-urine application.

Nitrous oxide (N2O) emissions from soils applied with livestock excreta have been widely reported previously. The highest N2O emissions from soils are also often reported during thawing periods in cold regions where soil freezing is common. However, the combined effects of cow urine application and freeze-thaw events on N2O emissions and the related enzyme activities are still not clear. Thus, we simulated a freeze-thaw event at -3 °C for 7 days, and then increased to 3 °C for 46 days using intact soil cores with cow urine (392 kg N ha-1). We compared the factors influencing the magnitudes of N2O emissions through soil microbial processes with and without the freeze-thaw event. Dicyandiamide (DCD), an inhibitor of nitrification, was added to investigate the significance of nitrification on N2O emissions. The N2O emission rates from the urine-applied soils peaked to approximately 1000 µg N2O-N m-2 h-1 immediately after the soils thawed. Soil freezing with urine application was significantly higher cumulative N2O emissions (537 mg N2O-N m-2), compared to non-frozen soils with urine (247 mg N2O-N m-2) during the incubation period (54 days). The effect of DCD application on N2O emissions was not clear during the freeze-thaw event, although nitrate production rates were reduced. After the freezing event, soil moisture (water-filled pore space) was significantly higher in the non-frozen soils compared to the frozen soils, due to a 9% decline in bulk density of frozen soils. Additionally, the impact of thawing on urease and denitrification enzyme activities was influenced by the urine application. Urine application increased the urease activity, while the freezing event decreased the magnitudes. The physical changes in the soils were also important controlling factors of the N2O emissions from cow urine-applied soils in cold regions.