Long-term 15N balance after single-dose input of 15N-labeled NH4+ and NO3- in a subtropical forest under reducing N deposition

The N manipulation experiment was conducted in an N-saturated Masson pine (Pinus massoniana) forest in the TSP Forest Park, located approximately 25 km northeast of Chongqing city (106° 41.24’E, 29° 37.42’N). It has a typical subtropical monsoon climate, with an average annual precipitation and temperature of 1176 mm and 18.1 °C, respectively (average from 2001 to 2020, data from Chongqing Meteorological Service). The atmospheric N deposition and NH4+ / NO3- ratio in 2005 – 2014 were approximately 5.28 g N m-2 yr-1 and 1.23, respectively, and declined to 3.40 g N m-2 yr-1 and 0.91, respectively, in 2015 – 2018 (Huang et al., 2015; Liu et al., 2017; Xie et al., 2018a). The vegetation is a mixed coniferous-broadleaf forest dominated by Masson pine and includes some other trees, such as Cuninhama lanceolate, Sapium sebiferum, and Litsea mollis. The dominant species of ground vegetation are Woodwardia japonica, Dryopteris fuscipes, Dicranopteris dichotoma, and Loropetalum chinense. The Masson pine forest was naturally regenerated after the original forest was cut in the early 1960s. The soil is locally called yellow earth (corresponding to a Haplic Acrisol in FAO; IUSS Working Group, 2014) with sandstone as the parent material. The TSP soil has a dense argic B horizon at a soil depth of 20 – 30 cm and a strongly limited rooting depth, which reduces its hydraulic conductivity, creating interflow (Sørbotten, 2011).