Differential immediate and long-term effects of nitrogen input on denitrification N₂O/(N₂O +N₂) ratio along a 0–5.2 m soil profile

High nitrogen (N) input to soil can cause higher nitrous oxide (N2O) emissions, that is, a higher N2O/(N2O+N2) ratio, through an inhibition of N2O reductase activity and/or a decrease in soil pH. We assumed that there were two mechanisms for the effects of N input on N2O emissions, immediate and long-term effect. The immediate effect (field applied fertilizer N) can be eliminated by decreasing the N input, but not the long-term effect (soil accumulated N caused by long–term fertilization). Therefore, it is important to separate these effects to mitigate N2O emissions. To this end, soil samples along a 0–5.2 m profile were collected from a long-term N fertilization experiment field with two N application rates, that is, 600 kg N ha-1 year-1 (N600) and no fertilizer N input (N0). External N addition was conducted for each subsample in the laboratory incubation study to produce two additional treatments, which were denoted as N600+N and N0+N treatments. The results showed that the combined immediate and long-term effects led to an increase in the N2O/(N2O+N2) ratio by 6.8%. Approximately 32.6% and 67.4% of increase could be explained by the immediate and long-term effects of N input, respectively. Meanwhile, the long-term effects were significantly positively correlated to soil organic carbon (SOC). These results indicate that excessive N fertilizer input to the soil can lead to increased N2O emissions if the soil has a high SOC content. The long-term effect of N input on the N2O/(N2O+N2) ratio should be considered when predicting soil N2O emissions under global environmental change scenarios. </p

Anthropogenic N input increases global warming potential by awakening the “sleeping” ancient C in deep critical zones

Even a small net increase in soil organic carbon (SOC) mineralization will cause a substantial increase in the atmospheric CO2 concentration. It is widely recognized that the SOC mineralization within deep critical zones (2 to 12 m depth) is slower and much less influenced by anthropogenic disturbance when compared to that of surface soil. Here, we showed that 20 years of nitrogen (N) fertilization enriched a deep critical zone with nitrate, almost doubling the SOC mineralization rate. This result was supported by corresponding increases in the expressions of functional genes typical of recalcitrant SOC degradation and enzyme activities. The CO2 released and the SOC had a similar 14C age (6000 to 10,000 years before the present). Our results indicate that N fertilization of crops may enhance CO2 emissions from deep critical zones to the atmosphere through a previously disregarded mechanism. This provides another reason for markedly improving N management in fertilized agricultural soils. </p