Effects of repeated severe dry–wet cycles on nitrogen mineralization processes and associated microbial genes in humid temperate forest soils

Ongoing climate change may increase soil dry–wet cycles (DWCs) in humid temperate forest ecosystems that seldom experienced such stress before, yet their effects on nitrogen (N) dynamics remain unclear. Here, we investigated the effects of repeated DWCs on soil N mineralization and related microbial gene abundance (bacterial and archaeal amoA genes, as well as prokaryotic and fungal genes) in two temperate forests: one with volcanic ash (HSB) and the other with non-volcanic ash soil (AKZ). Soils were incubated for 75 d with (dry–wet) or without (control) five DWCs. Nitrogen mineralization rates were measured for 5 d following the first, third, and fifth rewetting periods. Microbial gene abundance was measured before and after rewetting during the first, third, and fifth DWCs. A significantly higher N mineralization rate in the dry–wet treatment than in the control was found in HSB after first rewetting. The inorganic N concentrations on day 75 were approximately 4.0 and 1.5 times higher in the dry–wet treatment than in the control for HSB and AKZ, respectively. Short-term nitrification rates did not differ among treatments. However, archaeal amoA gene abundance was lower in the dry–wet treatment than in the control during the third and fifth DWCs, and nitrate-N concentration at the end of incubation in HSB was significantly lower in the dry–wet treatment than in the control. Thus, repeated extremely severe soil DWCs may increase soil inorganic N while suppressing nitrification in currently humid forest ecosystems, although further in situ confirmations are needed.