土地利用方式对湿润亚热带土壤硝化作用的影响

在土壤最大持水量60%和30℃条件下对采自江西的自然土壤(森林和灌丛)和农业利用土壤(稻田、旱地和茶园)进行了实验室培养,研究土地利用对硝化作用的影响。结果表明,由于土壤呈酸性(pH4.2～6.3,平均为4.9),供试土壤的硝化作用很弱甚至缺失。当无外加铵态氮时,土壤的硝化速率与有机氮矿化速率呈显著的线性关系(p<0.01),而与土壤pH无关;当外加铵态氮使基质饱和时,硝化速率与土壤pH显著相关(p<0.01)。农业利用显著提高土壤的硝化作用能力,绝大部分自然土壤(78%)的净硝化速率小于净矿化速率,无机氮以铵态氮为主,而绝大部分农业利用土壤(74%)的净硝化速率大于净矿化速率。农业利用通过提高土壤pH、氮肥施用刺激硝化作用及改善土壤磷素供应状况等途径促进土壤的硝化作用。农业利用土壤硝化作用能力的提高增加了氮肥以硝态氮形态淋失的风险。

Effects of land use types on nitrification in humid subtropical soils of China

Soil samples were collected from non-agricultural (forest and shrub lands) and agricultural lands (tea, rice paddy and upland) in Jiangxi Province, China and incubated at 60% water-holding capacity and 30℃. The results showed that the nitrification activity was weak or absent in the studied soils due to be acid (pH 4.2～6.3 with a mean of pH 4.9). The nitrification rate was significantly correlated with mineralization rate of organic N, but not with soil pH if NH+ 4was not amended into soils before incubation. In contrast, the nitrification rate was significantly correlated with soil pH if NH+4 was amended into soils before incubation. For majority of the non-agricultural soils (14 of 18 samples), the nitrification rates were smaller than their net mineralization rate. Consequently inorganic N was dominated by ammonium rather than nitrate even after 56-days incubation. Nitrification activity was stimulated by agricultural use, dependent on land use types, with the greatest in upland soils followed by tea and paddy soils. Enhancement of soil pH by liming, application of N fertilizers, and improvement of available P status were the main factors stimulating nitrification activity in agricultural soils. The stimulation effect of agricultural use on nitrification weakens the capacity of humid subtropical soils to retain inorganic N. The NO-3-N produced through nitrification may threaten the environment by leaching and run-off, particularly when N fertilizers are applied into agricultural soils in seasons with higher temperatures and heavy rainfall.