The relationships between metal uptake and antioxidant enzyme activities or a response to membrane lipid peroxidation (i.e., malondialdehyde production) in Chlorella vulgaris exposed to Cu and Cd compounds singly and in combination were investigated. The results showed that bioaccumulation of a single metal was influenced by the presence of the other metal. The activities of superoxide dismutase and peroxidase increased to more than fivefold of the control after exposure to Cu(1.5 μM) alone or to Cu(1.5 μM) with Cd mixtures. Malondialdehyde levels in C. vulgaris also increased to approximately twofold of the control after exposure to high concentration of Cu(1.5 μM) alone or to Cu and Cd mixtures. However, Cd alone did not significantly increase the levels of antioxidant enzymes or malondialdehyde.

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The combination of nitrogen (N) fertilization and straw incorporation has complex influences on soil N transformations and derived nitrous oxide (N2O) emissions. This study aimed to reveal the coupled effects of straw returning forms and N fertilizer management on soil N2O emissions. Here, carbon (C) sources with varied availabilities were supplied by different straw returning forms, including straw, charred straw, and combined application of straw and charred straw. The time gaps between additions of exogenous N and C sources were carried out by managing the timing of N fertilization, including N application simultaneous with or delayed after straw return. Soil N2O emissions under different straw returning forms and their response relationships to soil C, N, microbial biomass, as well as soil pH and oxygen (O2) were explored. Co‐application of straw and N fertilizer provided sufficient C and N sources, enhanced the microbial biomass, and consequently increased N2O emissions. Delayed N fertilization could decouple the response of N2O emissions to straw addition, because of constrained N availability and limited pH decline, which decreased the cumulative N2O emissions significantly. Possibly due to straw induced N immobilization coupled with charred straw induced pH elevation, the combined application of straw and charred straw constrained soil N2O emissions compared with straw only application, regardless of N fertilizer management. By shaping the microbial biomass and soil C, N, and O2 dynamics, delayed application of N fertilizer could further enhance the inhibition effect of the charred straw and straw combination on soil N2O emissions. Therefore, the varied straw returning forms and the timing of N fertilization could affect the supply of available C and N, influencing the hot moments of N2O emissions, and the joint addition of straw and charred straw with a few weeks delayed N application could possibly reduce the risk of soil N2O emissions from straw return systems. Such mitigation potential should be evaluated further under field conditions. Highlights Delayed N fertilization attenuated the magnitude of N2O emissions after straw additions. Combination of charred straw with straw mitigated N2O emissions, further enhanced by delayed N fertilization. Soil pH was the chief regulator for N2O emissions under different straw returning forms and N application.