DOI: 10.5061/dryad.2rbnzs7r7
title: Leaf enzyme plays a more important role in leaf nitrogen resorption efficiency than soil properties along an elevation gradient

description: 1. Nitrogen (N) resorption is a strategy for plant N conservation through
 which plants withdraw N from senescing leaves prior to litterfall and its
 underlying mechanisms are important for better understanding of N cycling.
 However, most current studies focused on the impacts of soil and leaf
 nutrients on leaf N resorption efficiency (NRE), and plant physiological
 regulation that is species-dependent is still unclear. 2. Here, we
 conducted a field experiment to investigate the variations of leaf NRE
 along an altitudinal gradient in a temperate forest of Northeastern China.
 3. Results showed that leaf NRE of Q. mongolica and F. mandshurica
 increased with altitude, while leaf NRE of T. amurensis, A. mono and A.
 pseudosieboldianum exhibited an opposite trend, although the relationships
 were not significant for F. mandshurica and A. mono. The inconsistent
 responses of leaf NRE of different species to increasing altitude were
 primarily due to the effect of leaf Glutamate dehydrogenase (GDH), an
 enzyme responsible for N translocation. Leaf GDH activity in senescing
 leaves explained the variation of NRE more than soil and climate factors
 did, suggesting that different plant species had different physiological
 regulation strategies for their N conservation under similar environment.
 4. Synthesis. Our study highlights the role of leaf enzyme as a pivotal
 regulator of leaf NRE and helps us better understand and predict N cycling
 under climate change in forest ecosystems.
description: One
 tree per species per plot was selected, and the selected trees of the same
 species should have similar heights and diameter at breast height (DBH).
 In order to calculate leaf NRE, green mature leaves were sampled from the
 selected trees by climbing the trees during the peak growing season from
 July 20 to 23, 2019, while senesced leaves were collected from October 18
 to 21, 2019. Healthy green mature leaves were collected from lower and
 middle outer canopy. To minimize sample contamination, we collected green
 mature and senesced leaves with nitrile gloves. Each sampled tree was
 marked to facilitate collection of senesced leaves at the end of growing
 season. Senesced leaves were collected either directly from recently
 fallen leaves on the ground, or by gently shaking the branches and
 collecting fallen leaves. In all cases, non-degraded senesced leaves were
 acquired from the marked trees. The DBH of each sampled tree was measured
 to estimate leaf biomass. Another set
 of green mature and senescing leaves were collected separately for GDH
 measurement. Leaves were recognized as senescing leaves when they just
 started to turn yellow. Both green mature and senescing leaves were
 collected on the trees by climbing the trees. Green mature leaves were all
 sampled in July. Due to the different phenology of different species, it
 took two months from September to October to collect senescing leaves.
 All senescing leaves were
 collected at the same canopy position as that of green mature leaves from
 high to low altitude. Senescing leaves of
 <em>Q</em>.<em>
 mongolica</em>,
 <em>F</em>.<em>
 mandshurica</em>,
 <em>T</em>.<em>
 amurensis</em>,
 <em>A</em>.<em> mono</em> and
 <em>A</em>.<em>
 pseudosieboldianum</em>
 were obtained on October 21, September 27, October 5, and October
 10<sup> </sup>to 11, respectively, in 2019. The
 harvested green mature leaves were divided into two parts: one was used to
 measure leaf area immediately and the other was frozen in liquid nitrogen
 immediately and stored at -80 ℃ for GDH analysis.
 Because
 different soil horizons in temperate forests generally have distinct soil
 properties including soil nutrient, microbial community and soil
 texture, it is necessary
 to divide forest soil into organic (O) and mineral (A) layers. In each
 plot, after removing litter, five soil cores were acquired randomly using
 a soil auger (20 cm in depth). The O and A horizons were separated by
 visual observation of soil color. Generally, the depth of O horizon was
 approximately 5 cm, while that of A horizon was approximately 15 cm.
 The five soil cores were
 combined into one composite sample for each soil horizon. Additionally,
 two paired soil cores were obtained using stainless steel cores of 5 cm
 height and 100 cm3 volume in each plot from the O horizon and the A
 horizon, respectively. These soil cores were used for gross N
 ammonification (GA) and nitrification (GN) measurement using the 15N pool
 dilution mesocosm incubation method.