凯氏全氮与固定态铵之间的关系

Relationships Between Total Nitrogen Determined by Kjeldahl Method and Nitrogen Fixed by Soil Minerals

从中国各地采集了40种不同类型的土壤,以凯氏法、包括粘土矿物固定态铵的HF-凯氏法和凯氏法测定全氮后再用HF处理测定固定态铵的两步法分别测定了这些土壤全氮含量,并用Silva-Bremner法测定了其粘土矿物固定态铵含量,确定凯氏法测定的全氮是否包括固定态铵在内。结果表明,土壤全氮平均含量凯氏法测定的为1.622 gkg-1,HF-凯氏法为1.633 gkg-1,而两步法为1.656 gkg-1;两步法测定的全氮量明显高于凯氏法和HF-凯氏法。由此可见,凯氏法无法释放2∶1粘土矿物中的固定态铵,在2∶1型粘土矿物含量高的土壤上测定值也不是真正的全氮。供试土壤固定态铵含量平均为166 mgkg-1,占凯氏全氮10.1%;而用两步法测定的残余固定态铵含量平均为30.4 mgkg-1,占凯氏全氮的1.9%,固定态铵的18.3%。相关分析表明,固定态铵与残余固定态铵之间显著相关(r =0.598)。残余固定态铵高的土壤正是固定态铵含量高的土壤,而固定态铵含量高的土壤正是2∶1型粘土矿物含量高的土壤,因而凯氏法测定的全氮不能反映这些土壤全氮的真实情况。但对于2∶1型粘土矿物含量低的或无此类粘土矿物的土壤,凯氏法与其它方法的测定值无显著差异。

Forty soils from top layer (0-20 cm) were sampled in different regions of China and Kieldahl, HF-Kieldahl and double treatment methods were used to determine total N, total N plus fixed ammonium, and total N and the residual fixed ammonium left in soil after determination of total N, respectively, to evaluate if Kieldahl抯 method could include the fixed N by soil minerals. The fixed N by soil minerals was measured by Silva-Bremner procedure to make comparison. Results showed that total N determined by Kieldahl抯 method averaged 1.622 g穔g-1, while that by HF- Kieldahl抯 method 1.633 g穔g-1, and that of double procedure 1.666 g穔g 1. Obviously, the results obtained by the last two methods, particularly the double treatment method, were higher than Kieldahl抯, showing that Kieldahl抯 method could not or not fully release N fixed by 2∶1 minerals in soil, and therefore the determined results would not be the true 搕otal N?for soils that contained large amount of the fixed N. The mineral fixed N averaged 166 mg穔g-1, accounting for 10.1% of the total N while the residual fixed N amounted to 30.4 mg穔g-1, equivalent to 1.9% of the total N or 18.3% of the total fixed N. The residual fixed N was correlated neither to organic matter nor to total N, but closely related to the total fixed N with a correlation coefficient of 0.598 (n = 40), showing that the fixed N was the sole source of the residues. Soils having high residues of the fixed N were just those containing high fixed N, and soils containing high fixed N were just those containing high amount of 2∶1 minerals. As a result, Kieldahl抯 method could not give a true value of the total N for such soils. However, for those containing small or little amount of 2∶1 minerals, there was no significant difference in results measured by these methods.