土壤及凋落物源氮对中亚热带森林土壤SON的影响

Effects of soil and litter source nitrogen on SON in mid-subtropical forest soil

土壤可溶性有机氮(SON)含量虽低,却是土壤氮库中最活跃的组分之一;主要来源于凋落物分解和土壤氮素转化。但是它们各自对土壤的影响还不清楚。通过添加杉木和^(15)N标记的阔叶凋落物于土壤表面,研究针阔叶凋落物分解对土壤SON的影响,及与土壤氮的关系。结果表明:由于没有降水的淋溶影响,培养期间,凋落物SON的显著降低,并没有直接增加土壤SON。与对照比较,杉木凋落物添加显著增加了土壤无机氮的含量,而较高C/N比的阔叶凋落物在其分解初期首先需要吸收更多的土壤氨态氮。添加^(15)N标记的阔叶凋落物提高了土壤SON在培养90—210天来自凋落物的比例,在第210天高达74.8%;来自凋落物的氨态氮比例在实验30天开始增加,到第210天高达39.8%;但是对硝态氮的影响不大。结果表明,土壤SON在培养初期因受凋落物的影响,主要来自土壤有机质的分解,而来自凋落物的SON更容易矿化;且土壤源的氮更容易发生硝化作用。可见,土壤中的SON是与凋落物分解动态、以及对土壤的影响有关。

Soil soluble organic nitrogen( SON) content is low,but is one of the most active components in the soil nitrogen pool. Litter decomposition and soil nitrogen transformation contribute considerably to SON; however,their effects on soil SON are unclear. To investigate the role of litter in soil nitrogen transformation,after seven days of pre-incubation,an experiment was carried out to study the effects of litter decomposition on soil SON and its relationship with soil nitrogen by adding contrasting fresh litter,Chinese fir,and broad-leaved 15N-labeled( atom 6.1%) litter to the soil surface at 25℃ for 210 days. Soil moisture was kept at 60% water-holding capacity by adding deionized water every two days to the surface. The concentration of soluble inorganic and organic nitrogen in 0.5 mol/L K2SO4 solution extracts of soil and litter were analyzed at each sampling time. In addition,for the treatment with addition of broad-leaved 15N-labeled litter,the 15N abundance in ammonium and nitrate nitrogen was analyzed and the SON was calculated,and their proportion in soil or litter was also calculated. The results showed that soil SON did not increase when it decreased significantly in litter under litter addition,independent of litter type,which may be owing to no leaching. Compared to the control,Chinese fir litter addition increased soil inorganic nitrogen significantly,whereas for higher C/N ratios,broad-leaved litter needed more soil ammonium nitrogen from soil at the early stage of its decomposition. Therefore,ammonium nitrogen in soil under broad-leaved litter addition didnot increase. However,soil nitrate nitrogen increased with broad-leaved litter addition,similarly to that of Chinese fir litter addition. Results showed that a considerably higher proportion of soil SON was from 15N labeled broad-leaved litter in 90—210 days of incubation,and was 74. 8% on day 210. In addition,the proportion of soil ammonium nitrogen from litter increased in the first 30 days of the experiment,and then increased to 39.8% by day 210. However,soil nitrate nitrogen was high and increased with incubation time,and its proportion from litter was low,evidenced by the calculation of 15N isotope data,which indicated that soil nitrate nitrogen was mainly from soil nitrogen nitrification rather than litter source.These results suggested that in the early stages of incubation,soil SON mainly comes from the decomposition of soil organic matter from broad-leaved litter,and the SON from litter possibly mineralized to become ammonium nitrogen. However,the nitrogen of the soil source is more prone to nitrification to form nitrate. Although 15N-labeled Chinese fir litter was not considered in this experiment,compared to the effect of broad-leaved litter on soil,the biggest proportion of soil SON from Chinese fir litter may occur earlier and the proportion of soil ammonium nitrogen may be higher. These results suggested that soil SON is related to the litter decomposition dynamics and the effect of litter on soil.