Litter removal reduced soil nitrogen mineralization in repeated freeze-thaw cycles.

Repeated freeze-thaw cycles (FTCs) can alter the relationships between plant litter and soil nitrogen (N) mineralization in subalpine ecosystems, but little information is available about the underlying mechanisms. Therefore, a controlled soil incubation experiment was carried out to study the effects of litter removal on soil N mineralization during FTCs, and the results indicated that FTCs promoted soil N mineralization more than the continuously frozen or nonfrozen condition did. Litter removal promoted soil ammonium N (NH-N) and dissolved organic N (DON) as well as the cumulative N mineralization (CNM) and ammonification, but it reduced the soil microbial biomass N (MBN) in the early stage of FTCs. With an increasing number of FTCs, litter removal significantly reduced the CNM but increased the soil MBN. The modified first-order kinetics model was verified under incubation conditions and predicted a lower soil N mineralization rate in FTCs with litter removal. In addition, the dominant factor impacting soil N mineralization was soil NO-N, and soil MBN had a greater influence on soil N mineralization when litter remained than when it was removed. These results further clarify the mechanism driving the effect of plant residues on soil N cycling.