Higher resistance of larch-broadleaf mixed forests than larch forests against soil acidification under experimental nitrogen addition

Growing evidence has shown that nitrogen (N) deposition can lead to soil acidification and tree nutrient imbalance. Tree species-specific differences in plant-soil interactions may render different forest types exhibiting contrasting responses to N deposition, yet this remains largely untested.

We conducted N addition experiments (0 and 50 kg N ha⁻¹ yr⁻¹) separately in a larch (Larix kaempferi) forest and an adjacent larch-broadleaf mixed forest, and examined whether soil N availability, soil acid–base chemistry, leaf nutrients and stoichiometry of these two forests responded differently to four-year N addition.

We found that N addition increased soil nitrate concentrations at four soil layers (i.e. Oa + e, 0–10, 10–20, and 20–40 cm), and resulted in soil acidification at Oa + e and 0–10 cm layers characterized by decreased pH and exchangeable base cations and increased hydrolyzing cations in the larch forest. In contrast to soil chemical properties, larch leaf nutrient stoichiometry except the C:N ratio showed no significant responses to N addition in the larch forest. Moreover, N addition did not significantly affect soil inorganic N concentration, soil acid–base chemistry, and tree leaf nutrients in the larch-broadleaf mixed forest.

Our results suggest the higher resistance of larch-broadleaf mixed forests than larch forests against soil acidification under N addition, and highlight the establishment of conifer-broadleaf mixed forests is an important silvicultural practice to alleviate soil acidification induced by N deposition.