Component Characteristics Of Soil Labile And Recalcitrant Carbon&Nitrogen In Four Typical Soils Of China Under Different Long-term Fertilization Systems

To assess the component characteristics of soil labile and recalcitrant carbon&nitrogen under different long-term fertilization systems in China, black soil, fluvo-aquic soil, red soil and paddy soil was choosen from different regions. The fractions of labile and recalcitrant carbon&nitrogen were separated by particle-density method and the conversion rate between nitrogen pools were obtained by a 15 N tracing study in combination with a model. The characteristic of content, ratio and N mineralization in soil labile and recalcitrant carbon&nitrogen were analyzed, which will lay the foundation for study the knietics of nitrogen pools and offer theoretical reference for soil fertility improvement. The main results and conclusions are showed below:(1) In upland soils(balck soil, fluvo-aquic soil and red soil), labile carbon content under NPKM treatment was incresed significantly than other treatments, which increased by 98.4%, 43.7% and 71.2% compared to NPK treatments, respectively. There were no significant differences of labile carbon content among NPK, NPKS and CK treatments in black soil and fluvo-aquic soil, while labile carbon content in red soil under NPK and NPKS treatments were significantly lower than that under CK treatment(P<0.05) by 33.1% and 29.6%, respectively; There were no significant differences among four treatments of paddy soil in labile organic carbon content. Recalcitrant carbon content in four soils was consistent with their total organic carbon under four treatments as NPKM>NPKS>NPK>CK. Comepared with the NPK treatment, the recalcitrant carbon content in the four soils were greatly improved under NPKM treatment, incerased the proportion by 68.8%, 42.7%, 17.6% and 17.2%, respectively, in the meantime, the Rec-C/TOC was reduced. In conclusion, compared with paddy soil, the labile organic carbon content in upland soil was more susceptible to fertilization systerm. Long-term application of NPKS or NPKM could improve the quantity of soil labile/recalcitrant organic carbon contents; meanwhile it could increase the Lab-C/TOC ratio. The improvement effect was in order of NPKM, NPKS and NPK.(2) After 23 years, the TN of the upland soils under CK treatment decreased significantly(7.5%~9.7%) comparing with the beginning of the experiment, while in the paddy soil the TN increased 11.5%. The TN in all soils under NPK、NPKS and NPKM treatment were significantly higher than that of CK treatment; The Lab-N of red soil under NPK treatment was significantly lower than that of CK treatment, while for other three soils there were no significant differences under two treatments. The Lab-N of red soil under NPKS treatment decreased significantly comparing with CK treatment, while the Lab-N of fluvo-aquic soil increased significantly. The TN, Lab-N and the Lab-N/TN in the upland soils under NPKM treatment increased significantly. And the black soil increased the most by 85.5%, 106.5% and 4.2%; there was no significant difference of the Lab-N and Lab-N/TN in the paddy soil between NPKM and CK treatment. Both the Rec-N and the TN in four soils under four different treatments were in the order of NPKM>NPKS>NPK>CK; The Rce-N/TN under NPKM decreased comparing with CK treatment. In conclusion, compared with paddy soil, the labile organic nitrogen content in upland soil was more susceptible to fertilization systerm. Long-term application of NPKS or NPKM could improve the quantity of soil labile/recalcitrant organic nitrogen contents; meanwhile it could increase the Lab-N/TN ratio. The improvement effect was in order of NPKM, NPKS and NPK.(3)In the same soil, the C/N have no significantly difference under the long-term different fertilizations; while in the same treatment, the C/N of labile fraction was significantly higher than that of undisturbed soil and recalcitrant fraction.(4)The characteristics of the nitrogen conversion were different under long-term chemical fertilizer : the black soil showed a rate of middle mineralization, high nitrification and high net desorption characteristics; fluvo-aquic soil has low mineralization, middle nitrification, and low net immobilization rate; red soil showed high mineralization, low nitrification and high net immobilization rate. The paddy has the characteristics of high mineralization, middle nitrification and low net immobilization rate.