Influence of Tree Spacing on Soil Nitrogen Mineralization and Availability in Hybrid Poplar Plantations

Abstract

Nitrogen (N) availability and mineralization are key parameters and transformation processes that impact plant growth and forest productivity. We hypothesized that suitable plantation spacing can lead to enhanced soil N mineralization and nitrification, which in turn promote tree growth. Studies were conducted to evaluate seasonal patterns of soil inorganic N pools as well as rates of nitrification and N mineralization of three soil layers under four tree spacing treatments. Results showed tree spacing significantly affected annual net N mineralization, whereas inorganic N content in surface soils was significantly affected by tree spacing only during the growing season. The total annual cumulative net N mineralization ranged from 80.3-136.0 mg·kg −1 in the surface soils (0-20 cm), whereas the cumulative net N mineralization of 6 × 6 m and 4.5 × 8 m spacings was 65% and 24% higher than that of the 5 × 5 m, respectively. In general, tree spacing would affect N availability in soil by altering N mineralization rates, while high annual N mineralization was found in soils of low density plantations, with higher rates in square spacing than rectangular spacing. The obtained results suggest that suitable spacing could lead to enhanced N mineralization, but seasonal variation of soil N mineralization may not only be directly related to plantation productivity but also to understory vegetation productivity. OPEN ACCESSForests 2015, 6 637

Thinning Intensity Affects Soil-Atmosphere Fluxes of Greenhouse Gases and Soil Nitrogen Mineralization in a Lowland Poplar Plantation

“…The detailed resin core method described by Yan et al [31] was exactly followed in this study during spring, autumn, and winter times, owing to the similar site conditions and both for the poplar plantations. To absorb N input from rainfall (about 10 kg¨ha´1¨year´1 at the study area) and litter decomposition, the upper resin layer covered with a 0.13 mm nylon mesh on the top was installed.…”

“…The procedure was repeated every three months over the course of a year. Additional soil cores were sampled in adjacent locations to quantify soil initial ammonium and nitrate concentrations [31]. The collected soil and resin-soil core samples were transported to the laboratory and stored at 4˝C within a month until analysis.…”

“…On 15 January 2014, after clipping and removal of plants and litter, sharp-edged PVC tubes were driven into two soil layers (0-10 and 10-20 cm) to sample soil cores and make resin-soil core combinations within each sub-plot. Based on the results from a similar site [31], the resin-soil core combinations were retrieved for chemical analysis, and new resin-soil core combinations were incubated at threemonth intervals. The procedure was repeated every three months over the course of a year.…”

“…The NH4 + -N and NO3 − -N concentrations were analyzed using a Flow-Injection Autoanalyzer (Bran+Luebbe, Hamburg, Germany). The resin bags were washed with distilled water and dried at room temperature (28 °C-32 °C), while the resin NH4 + -N and NO3 − -N were extracted by shaking the resin bags with 2 M KCl for 12 h [31].…”

“…Poplar plantations have markedly increased in China since 2004 (with only 3.9 million ha of poplar plantations in 2004), and poplar shave been incorporated into many managed systems for timber and fiber production as well as for environmental application throughout the south temperate zone in central China. Over the most recent ten years, most studies on poplar plantations have been focused on renewable bioenergy with short-rotation coppice [21,22], potential utilization of phytochemicals [23], the relationship between genetic diversity and productivity [24][25][26], and site productivity maintenance [27][28][29][30][31]. Although more attention has been paid to the role of poplar plantations in CO 2 sequestration in China [15,16,32], there has been no comprehensive study of how management practices affect the soil-atmosphere fluxes of GHGs in poplar plantations and how these GHG emissions impact the total GHG balance.…”