Effects of snow absence on available N pools and enzyme activities within soil aggregates in a spruce forest on the eastern Tibetan Plateau

Winter climate change has great potential to affect the functioning of terrestrial ecosystems. In particular, increased soil frost associated with reduced insulating snow cover may impact the soil nitrogen (N) dynamics in cold ecosystems, but little is known about the variability of these effects among the soil aggregates. A snow manipulation experiment was conducted to investigate the effects of snow absence on N cycling within soil aggregates in a spruce forest on the eastern Tibetan Plateau of China. The extractable soil available N (ammonium and nitrate), net N mineralization rate, and N cycling-related enzyme activities (urease, nitrate reductase, and nitrite reductase) were measured in large macroaggregate ( > 2 mm), small macroaggregate (0.25–2 mm), and microaggregate ( < 0.25 mm) during the early thawing period in the years of 2016 and 2017. Snow absence increased soil N availabilities and nitrite reductase activity in microaggregate, but did not affect net N mineralization rate, urease or nitrate reductase activities in any of the aggregate fractions. Regardless of snow manipulations, both soil inorganic N and nitrate reductase were higher in small macroaggregate than in the other two fractions. The effect of aggregate size and sampling year was significant on soil mineral N, net N mineralization rate, and nitrite reductase activity. Our results indicated that snow cover change exerts the largest impact on soil N cycling within microaggregate, and its effect is dependent on winter conditions (e.g., snow cover and temperature). Such findings have important implications for soil N cycling in snow-covered subalpine forests experiencing pronounced winter climate change.