电超滤法（EUF）浸提测定^（15）N标记植物残体有机氮和无机氮的转化

EXTRACTABILITY OF￣(15)N LABELLED PLANT RESIDUES IN SOIL BYELECTRO-ULTRAFILTRATION(EUF)

采用电超滤法浸提土壤，分析浸提液中的Ｎ—ＮＨ＋４、Ｎ—ＮＯ－３和有机氮。测试土壤采自德国汉森的斑状砂土，用￣（１５）Ｎ标记的植物残体（油菜）施入土壤，２０℃恒温培养８０天，以不施（油菜）为对照，培养期间共取样１２次。结果表明，ＥＵＦ法对￣（１５）Ｎ标记的有机Ｎ的浸提能力比土壤全Ｎ中的有机Ｎ的浸提能力高出７．７倍。ＥＵＦ浸提的有机氮比率较常规浸提过程的更高。前４０天，培养试验中３８％的有机氮被矿比，其后没有发生大的矿化。标记的有机氮的矿化在第２天达到高峰之后一直减少，直到第４０天，随后处于稳定状态。标记的ＥＵＦ有机氮对标记的无机Ｎ只有很微小的积累。标记的有机氮主要在第２～９天减少，但大约８０％的标记无机氮在９天后明显增加。原土氮的矿化过程是明显的零级动力学，基础浓度任何时候不限制矿化。说明易矿化氮化合物总是不断得到土壤相对稳定的有机氮的补充。如果稳定状态在Ｎ矿化过程中起主要作用的话，就不可希望ＥＵＦ浸提的易矿化有机氮的减少。通过氮矿化库的变化量对预测氮矿化更重要，因此，寻找适当的矿化指标来制约土壤有机氮形式和变化量显得更为紧迫。

Aerob soil incubation carried out with￣(15)N labelled green manure was focused on three major aims:1) to determine the extractability of organic and inorganic N fractions from decomposing plant residues in the soil,(2 )to provide evidence that EUF extractable organic N was actually mineralized during the soil in-cubation, and(3)to examine why the course of extractable organic N did not always cortespond tO the accu-mulation of inorganic N. The extractability of￣(15)N tagged organic N by EUF was on average 7.7 times higher than the share of to-tal tagged organic N on the total soil orgunic N(tagged+untagged).This result indicates that the EUF tech-nique extracts with high selectivity a biological meaningful soil N fractionl The extractability ratio of tagged or-ganic N for the EUF method was subetantialy higher than ratios found in other studies for some conventional extraction procedures。During the first 40 days of incubation 38% of the applied organic N had been mineralized , but no major mineralization took place after this date. The course of tagged EUF一Norg pecked on day 2 of incubetion fol-lowed by a decrease until day 40. After day 40 tagged EUF一Norg remained stabl, This course clearly indi-cates that some of the EUF一Norg had been actually mineralized, Nevertheless , the dectease of tagged EUF一Norg cortesponded only weakly with the accumulation of tagsed inorganic N. The major decrease of tagged EUF一Norg took place from day 2 to day 9, but about 80%of the increase of tagged inorganic N was ob-served after day 9. The course of mincralization of indigenous soil N was sufficiently described by ziro order kinetics,which indicates that no decreasing concentration of substrate( e.g.easily mineralizable indigenous soil organic N)lim-ited mineralization at any tine.It is therefore supposed that steady state conditions could establish in the soil. This means that the Pool of easily mineralizable N compounds was al ways replenished by the decomposition preducts derived from the large pool of relatively stabel soil organic N,If this steady state conditions play a ma-jor rcle during the ptocess of N mineralization,a decrease of easily mineralizable organic N extracted by EUF could not be expected. But more importantely ,the flux through any pool rather than the size of a pool at any time would be of major importance for the prediction of N mineralization. Consequently ,instead of seatchingfor a more suitablc mineralization index a process orientied view is urgently needed describing the conditions which control the flux of N through possibly variously shaped pools of organic N compounds in soils.