Effects of nitrogen addition on soil organic carbon mineralization after maize stalk addition

Nitrogen (N) fertilization or increasing N deposition could significantly increase soil N availability, which could alter soil carbon cycling. Our understanding of the effects of N addition on the priming effect (PE) of carbon inputs on soil organic matter (SOM) mineralization (i.e. change of SOM mineralization after the addition of exogenous substrate) is still limited. Here we compared the effects of ammonium nitrate (NH4NO3) and urea (CO(NH2)2) on the PE of maize stalk on SOM mineralization. Results showed that during the 209-d incubation, maize stalk addition induced a SOM priming of 135 mg C kg−1 soil. Both NH4NO3 and urea addition significantly decreased SOM mineralization when there was no maize stalk addition. Compared to control, treatments of NH4NO3 + stalk and urea + stalk increased the 209-d cumulative SOM mineralization by 8.4% and 30.2%, respectively. The 55.0% lower positive PE under NH4NO3 + stalk treatment than stalk alone treatment hinted that N-mining mechanism prevailed under low N availability, while the 63.0% higher cumulative PE under urea + stalk treatment might be attributed to the function of co-metabolism mechanism. Nevertheless, net C sequestration (stalk-C incorporation into the soil minus primed soil C) among different stalk + N treatments was not statistically different because new C incorporation under urea + stalk treatment tended to be higher than that under NH4NO3 + stalk treatment. Our results suggested that NH4NO3 and urea had contrasting effects on SOM priming when maize stalk was added, which might complicate the selection of N fertilizers during straw returning practice in field conditions.