Discrepant responses of soil organic carbon dynamics to nitrogen addition in different layers: a case study in an agroecosystem

Empirical research indicates that heightened soil nitrogen availability can potentially diminish microbial decomposition of soil organic carbon(SOC).Nevertheless, the relationship between SOC turnover response to N addition and soil depth remains unclear. In this study, soils under varying N fertilizer application rates were sampled up to 100 cm deep to examine the contribution of both new and old carbon to SOC across different soil depths,using a coupled carbon and nitrogen isotopic approach. The SOC turnover time for the plot receiving low N addition(250 kg·ha^(-1)·yr^(-1) N) was about 20-40 years. Conversely, the plot receiving high N(450 kg·ha^(-1)·yr^(-1) N) had a longer SOC turnover time than the low N plot, reaching about 100 years in the upper 10-20 cm layer. The rise in SOC over the entire profile with low N addition primarily resulted from an increase in the upper soil(0-40 cm)whereas with high N addition, the increase was mainly from greater SOC in the deeper soil(40-100 cm). Throughout the entire soil layer, the proportion of new organic carbon derived from maize C_4 plant sources was higher in plots treated with a low N rate than those treated with a high N rate. This implies that, in contrast to low N addition agricultural practices, high N addition predominantly enhances the soil potential for fixing SOC by transporting organic matter from surface soils to deeper layers characterized by more stable properties. This research offers a unique insight into the dynamics of deep carbon under increased N deposition, thereby aiding in the formulation of policies for soil carbon management.

Lignin-phenol monomers govern the pyrolytic conversion of natural biomass from lignocellulose to products

The effect of the interaction between lignin-phenol monomers and holocellulose in natural biomass on the distribution of pyrolysis products remains unknown.The results of this study showed that the interaction between lignin and holocellulose during the pyrolysis of natural biomass became more pronounced as the pyrolysis temperature increased.The interaction between lignin and holocellulose in the natural crosslinked structure promoted the generation of CO and inhibited the generation of CO2 at 750
C.Lignin inhibited the decarboxylic reaction of hemicellulose during pyrolysis but was important for the generation of levoglucosan during cellulose pyrolysis.Holocellulose slowed the demethoxyreaction of lignin guaiacol but promoted the removal of aliphatic hydrocarbon substituents from the aromatic ring.The cinnamyl phenol monomers of lignin increased the rates of change of biomass pyrolysis products with the lignin mass fraction at 400
C.However,when the pyrolysis temperature increased to 750
C,all types of lignin phenol monomers increased the rates of change of the biomass pyrolysis products.Our results provide new insights that have implications for the development of pyrolysis techniques for the resource recycling of various types of biomass for the preparation of high-grade gaseous and liquid fuels.