大气CO_2浓度上升和氮添加对南亚热带模拟森林生态系统土壤碳稳定性的影响

Effects of elevated CO_2 concentration and nitrogen addition on soil carbon stability in southern subtropical experimental forest ecosystems

大气CO2浓度升高和氮(N)添加对土壤碳库的影响是当前国际生态学界关注的一个热点。为阐述土壤不同形态有机碳的抗干扰能力,运用大型开顶箱,研究了4种处理((1)高CO2浓度(700μmol·mol–1)和高氮添加(100 kg N·hm–2·a–1)(CN);(2)高CO2浓度和背景氮添加(CC);(3)高氮添加和背景CO2浓度(NN);(4)背景CO2和背景氮添加(CK))对南亚热带模拟森林生态系统土壤有机碳库稳定性的影响。近5年的试验研究表明:(1)CN处理能明显地促进各土层中土壤总有机碳含量的增加,其中,下层土壤(5–60 cm土层)中的响应达到统计学水平。(2)活性有机碳库各组分对处理的响应有所差异:不同土层中微生物生物量碳(MBC)的含量对各处理的响应趋势基本一致,各土层中的MBC含量均为CN>CC>NN>CK,其中0–5 cm、5–10 cm、10–20cm 3个土层的处理间差异都达到了显著水平;10–20 cm与20–40 cm两个土层中的易氧化有机碳处理间有显著差异;而对于各土层中水溶性有机碳,处理间差异均不明显。(3)各团聚体组分中的有机碳含量的响应也有所差异:20–40 cm与40–60 cm土层中250–2 000μm组分的有机碳含量存在处理间差异;40–60 cm土层中53–250μm组分的有机碳对各处理响应敏感,CC处理和NN处理都有利于该组分碳的深层积累,尤其CN处理下的效果最为明显;在各处理10–20 cm、20–40 cm及40–60 cm土壤中,<53μm组分中的碳含量间差异显著。大气CO2浓度上升和N添加促进了森林生态系统中土壤有机碳的增加,尤其有利于深层土壤中微团聚体与粉粒、黏粒团聚体等较稳定组分中有机碳的积累,增加了土壤有机碳库的稳定性。

Aims The influence of elevated atmospheric CO2 concentration and nitrogen（N） addition on soil carbon pool is one of the foci among international ecological research communities. The changes of soil carbon pool induced by atmospheric CO2 concentration and/or N deposition will lead to changes in atmospheric carbon pool and thus the global climate change. However, few studies have been carried out in the subtropical China. Our objective was to understand the effect of elevated CO2 concentration and N addition on soil carbon stability in south subtropical experimental forests. Methods Experimental forest ecosystems were constructed in open top chambers. Six native tree species in southern China were planted in these experimental forest ecosystems. The species were exposed to elevated CO2 and N addition in the open top chambers beginning in May 2005. The four treatments were： elevated CO2 and high N addition（CN）, elevated CO2 and ambient N deposition（CC）, high N addition and ambient CO2（NN）, and ambient CO2 and ambient N deposition（CK）. The elevated CO2 was（700 ± 20） μmol·mol–1. The total amount of added NH4NO3-N was 100 kg N·hm–2·a–1. In January 2010, soil samples were collected from the open top chambers and then relevant variables were measured. Important findings Elevated CO2 concentration and N addition（CN） effectively increased the soil total organic carbon in different soil layers, among which the increases in the lower soil layers（5–60 cm） were statistically significant. Different components of the active organic carbon pool differed in the responses to treatments. The differences in microbial biomass carbon were significant in the 0–5 cm, 5–10 cm and 10–20 cm soil layers amongthe treatments, and the readily oxidized carbon showed significant responses to the elevated CO2 concentration and N addition treatments in the 10–20 cm and 20–40 cm soil layers, while there was no significant difference in the dissolved organic carbon among different treatments in all the soil layers. The responses of carbon in different aggregate fractions differed among the treatments. The carbon in the 250–2 000 μm aggregates was significantly different among treatments in the 20–40 cm and 40–60 cm soil layers. The carbon in the 53–250 μm aggregates was susceptible to treatments in the 40–60 cm soil layer as both the CC and NN treatments facilitated the chronic carbon accumulation in deeper soil layers, especially under the CN treatment. Carbon in the 53 μm fraction showed significant differences among treatments in deeper soil layers（10–20 cm, 20–40 cm and 40–60 cm）. In conclusion, elevated CO2 concentration and N addition increased soil organic carbon in the experimental forest ecosystems, and facilitated the accumulation of carbon in micro-aggregates and silt-clay fraction in deep soil layers, thus strengthened the stability of soil organic carbon pool.