DOI: 10.5061/dryad.z8w9ghxct
title: Effects of long-term nitrogen addition on water use by Cunninghamia lanceolate in a subtropical plantation

description: The deposition of reactive nitrogen (N) has substantially increased in
 subtropical regions due to human activities. However, the effects of
 long-term N addition on the water-use efficiency of subtropical forests
 are poorly understood. Here, we conducted an 11-year experiment in a
 subtropical Cunninghamia lanceolate plantation with four N-addition
 levels: N0, N1, N2, and N3 (equivalent to 0, 6, 12, and 24 g of N m-2
 yr-1, respectively). A thermal dissipation probe system was used to
 calculate sap flow and plant biomass carbon was assessed by field
 investigation. The whole-plant water use and water-use efficiency were
 estimated. In addition, the δ13C of tree rings was used to indicate the
 plant intrinsic water-use efficiency. The results showed that N3
 significantly increased the annual sap flow velocity, especially in summer
 and winter. Annual water use, plant growth, and water-use efficiency did
 not significantly differ among the N treatments, but water use tended to
 be higher in N3 than in N0. Furthermore, the reduction of δ13C between the
 pre-N treatment period and the post-N treatment period was 3.02%, 3.26%,
 3.58%, and 5.28% for N0, N1, N2 and N3, respectively, which supported the
 inference that N addition could enhance water use. We conclude that
 long-term addition of high levels (but not of low levels) of N increased
 whole-plant water use in C. lanceolate plantations. Our results indicate
 that N deposition accompanied by high temperature and drought events may
 negatively affect water balance in subtropical forests.
description: <b>Sap flow measurement and water use</b>
 In 2014, a thermal dissipation probe system (TDP) for sap flow
 measurement was installed in one sample tree per plot, so that there were
 three replicate sample trees for each treatment. The DBH of the sample
 trees were similar to the average (about 22.5 cm) in the plot. We assumed
 that that the sap flow of a sample tree with average DBH represented
 average sap flow for the plot. Two 2-cm-long probes separated by 10 cm in
 the vertical direction were inserted into the sapwood of the sample trees.
 The top probe was equipped with a heating component, which was heated
 continually at 0.2 W. The temperature difference between the two probes
 was influenced by the sap flow density near the heated probe. Therefore,
 the recorded difference in the temperature (<i>△</i>T) between
 the two probes was converted to sap flow velocity V (g
 cm<sup>-2</sup> s<sup>-1</sup>) according to
 equations 1 and 2 (see next paragraph) (Granier 1987; Zhu et al.
 2012). Probes
 were uniformly installed on the same side of sample trees at a height of
 1.3 m and were covered with aluminum foil to minimize the effects of solar
 heating on sap flow density. Hourly mean values were automatically
 recorded and stored on a data logger (CR10X, Campbell Scientific, Inc.,
 Logan, USA). Every 3 months, the probes were removed and reinserted on a
 previously unused side of each tree in order to ensure the well-condition
 of probes. In this study, data collected from 1 January 2015 to 31
 December 2015 were used. To better show the variation in sap flow velocity
 among N treatments, we multiplied each value for sap flow velocity by
 10<sup>6</sup>, but we used original values to estimate the
 water use by the sample trees. Sap flow through the trunk can be
 approximately equivalent to the transpiration from the canopy (Kozlowski
 and Pallardy 1997). We cut down 12 trees (without probes) in each plot to
 estimate sapwood area (A<sub>S</sub>; Table 1) as calculated
 with equation 1: A<sub>S</sub>
 =3.577DBH<sup>1.328                       
  </sup>(1) We then calculated the annual water
 use in each plot based on estimates of A<sub>S</sub> per plot,
 tree number per plot, and the following equations:
       K = (dT<sub>M</sub> – dT) / dT  
              (2)        V = 0.0119 ×
 K<sup>1.231</sup>                 (3)    W
 = As × V×3600                  (4) where K is a
 dimensionless value, dT<sub>M</sub> is the maximum temperature
 difference between two probes, dT is the instantaneous temperature
 difference, V is sap flow velocity (g cm<sup>-2</sup>
 s<sup>-1</sup>), and W is water use (g
 h<sup>-1</sup>). The whole-tree water use per year (kg
 year<sup>-1</sup> tree<sup>-1</sup>) was
 calculated by summing water use in the year. We assumed that no sap flow
 occurred at night when the temperature difference reached its
 minimum.