Influences of various forms of nitrogen additions on carbon mineralization in natural secondary forests and adjacent larch plantations in Northeast China

Soil organic matter decomposition, a major process that affects soil carbon (C) storage, is controlled by the available nitrogen (N) in soils. However, little is known about the effects of the various forms of N input on organic matter decomposition in typical temperate forest types such as secondary forests and larch plantations. A 56-day laboratory incubation experiment was performed to determine the effects of dominant N forms (ammonium dominant = NH 4 + ; ammonium nitrate dominant = NH 4 NO 3 ; and nitrate dominant = NO 3 − ) and four N levels (control = no N added; low N = 25 mg N·kg soil −1 ; medium N = 50 mg N·kg soil −1 ; and high N = 75 mg N·kg soil −1 ) on soil C mineralization in secondary forest and larch plantation soils. The results indicated that the addition of N inhibits C mineralization, regardless of the form of N applied in the secondary forest soil, whereas NH 4 + -dominant soil decreased C mineralization in the larch plantation soil. Furthermore, among the various forms of N, the addition of NH 4 + reduced C mineralization the most compared with NO 3 – and NH 4 NO 3 additions in the secondary forest soil. Additional N generally suppressed phenol oxidase activity but had no effects on activities of exoglucanase, β-glucosidase, and N-acetyl-β-glucosaminidase or soluble organic C in the secondary forest soil. The decrease in phenol oxidase activity that was associated with the addition of N is likely to have an effect on soil C mineralization. We also observed that soil pH decreased with the increasing rate of N input in the secondary forest soil, which indicates that soil C mineralization may be sensitive to the amount of N through changes in soil pH. Overall, the addition of N resulted in changes in soil C mineralization that depended on the form of the N input and the forest type. The application of NH 4 + -dominant N influenced soil C dynamics in the secondary forest and larch plantation soils in this short-term experiment.