北京东灵山地区常见灌丛生长及凋落物生产对氮添加的响应

Responses of growth and litterfall production to nitrogen addition treatments from common shrublands in Mt. Dongling,Beijing,China

我国北方灌丛土壤瘠薄,近几十年来的氮沉降显著提高了北方灌丛土壤的可利用氮水平。灌木生长是灌丛碳吸存的重要组成部分,凋落物在土壤和植物间充当着至关重要的纽带作用,是陆地生态系统养分与能量循环的关键,灌丛生长和凋落物生产受氮添加的影响很大。然而,大气氮沉降对灌丛碳吸存和凋落物生产的影响人们知之甚少。该研究以荆条(Vitex negundo var.heterophylla)和绣线菊(Spiraea salicifolia)灌丛为例,通过0(N0)、20(N1)、50(N2)、100(N3)kg N·hm^(–2)·a^(–1)施氮实验,研究了短期(2012^(–2)013年)氮添加对东灵山地区典型灌丛生长及凋落物生成的影响。研究结果显示:在4种氮添加处理中,荆条灌丛灌木基径年增长率分别为1.69%、2.78%、2.51%和1.80%,相应处理中,绣线菊灌丛灌木基径年增长率分别为1.38%、1.37%、1.59%和2.05%;与之对应的株高年增长率分别为8.36%、8.48%、9.49%和9.83%(荆条灌丛)和2.12%、2.86、2.36%、2.52%(绣线菊灌丛)。虽然处理之间的差异没有达到显著性水平,但N沉降在一定程度上促进了灌木的生长。不同处理间,荆条地上生物量增加了0.19、0.23、0.14、0.15 t C·hm^(–2)·a^(–1),绣线菊灌丛地上生物量增加了0.027、0.025、0.032、0.041 t C·hm^(–2)·a^(–1)。在自然条件下,荆条和绣线菊灌丛2013年凋落物的年产量分别为135.7和129.6 g·m^(–2)。短期氮沉降对凋落物及组分的年产量有一定的促进作用,但处理之间的差异没有达到显著性水平。研究结果表明施肥时间短、土壤含水量低等因素导致土壤可利用氮的利用效率很低,从而使灌丛对施肥的响应比较缓慢。

Aims The shrublands of northern China have poor soil and nitrogen （N） deposition has greatly increased the local soil available N for decades. Shrub growth is one of important components of C sequestration in shrublands and litterfall acts as a vital link between plants and soil. Both are key factors in nutrient and energy cycling of terrestrial ecosystems, which greatly affected by nitrogen （N） addition （adding N fertilizer to the surface soil directly）. However, the effects and significance of N addition on C sequestration and litterfall in shrublands remain unclear. Thus, a study was designed to investigate how N deposition and related treatments affected shrublands growth related to C sequestration and litterfall production of Vitex negundo var. heterophylla and Spiraea salicifolia in Mt. Dongling region of China. Methods A N enrichment experiment has been conducted for V. negundo var. heterophylla and S. salicifolia shrublands in Mt. Dongling, Beijing, including four N addition treatment levels （control （N0, 0 kg N·hm^–2·a^–1）, low N （N1, 20 kg N·hm^–2·a^–1）, medium N （N2, 50 kg N·hm^–2·a^–1） and high N （N3, 100 kg N·hm^–2·a^–1））. Basal diameter and plant height of shrub were measured from 2012–2013 within all treatments, and allometric models for different species of shrub’s live branch, leaf and root biomass were developed based on independent variables of basal diameter and plant height, which will be used to calculate biomass increment of shrub layer. Litterfall （litterfall sometimes is named litter, referring to the collective name for all organic matter produced by the aboveground part of plants and returned to the surface, and mainly includes leaves, bark, dead twigs, flowers and fruits.） also was investigated from 2012–2013 within all treatments. Important findings The results showed 1） mean basal diameter of shrubs in the V. negundo var. heterophylla and S. salicifolia shrublands were increased by 1.69%, 2.78%, 2.51%, 1.80% and 1.38%, 1.37%, 1.59%, 2.05% every year; 2） The height growth rate （the shrub height relative growth rate is defined with the percentage increase of plant height） of shrubs in the V. negundo var. heterophylla and S. salicifolia shrublands were 8.36%, 8.48%, 9.49%, 9.83% and 2.12%, 2.86%, 2.36%, 2.52% every year, respectively. Thee results indicated that N deposition stimulated growth of shrub layer both in V. negundo var. heterophylla and S. salicifolia shrublands, but did not reach statistical significance among all nitrogen treatments. The above-ground biomass increment of shrub layer in the V. negundo var. heterophylla and S. salicifolia shrublands were 0.19, 0.23, 0.14, 0.15 and 0.027, 0.025, 0.032, 0.041 t C·hm^–2·a^–1 respectively, which demonstrated that short-term N addition had no significant effects on the accumulation of C storage of the two shrublands. The litter production of the V. negundo var. heterophylla and S. salicifolia communities in 2013 were 135.7 and 129.6 g·m^–2 under natural conditions, respectively. Nitrogen addition promoted annual production of total litterfall and different components of litterfall to a certain extent, but did not reach statistical significance among all nitrogen treatments. Above results indicated that short-term fertilization, together with extremely low soil moisture content and other related factors, lead to inefficient use of soil available nitrogen and slow response of shrublands to N addition treatments.