The Characteristic Of Microbial Biomass N And Its Relationship With The Turnover Of Added N In Wetland And Paddy Soil Of Dongting Lake Region

The wetland and paddy soil have been recognized for importance in the retention ofactive nutrition and the environment protection nowadays. Studies on the forms and theturnover of N and the loss of N added in soil have been permanent research topics by soil,plant nutrition, and environmental scientists since nitrogen is distinguished to be one ofthe essential macronutrients for plant growth. The Dongting Lake plain is one of thelargest areas of wetland in lake regions in China and it is the important output area offarm production. Meanwhile, it is one of the districts where the input of chemicalfertilizer N is more excessive and the environmental pollution is more serious. Therefore,it is significant to evaluate the capacity of soil N-supplying, increase the recovery of Nand reduce the environmental pollution through revealing the situation and distribution ofsoil microbial biomass N (B_N) in natural and the artificial wetland, reveal the relationshipbetween B_N transformation and soil organic nitrogen components as well as the effect oforganic and inorganic fertilizers on soil N transformation and so on.Based on field investigation, long-term field experiments, laboratory-floodedincubation and pot experiment, the content of soil organic carbon(T_C), total nitrogen (T_N),microbial biomass carbon (B_C), B_N, mineral N, mineralizable N, amino acid N (AAN),ammonium N (AN), amino sugar N (ASN), hydrolysable unidentified N (HUN), totalhydrolysable N (THN), nonhydrolysable N(NHN) and fixed ammonium were measured.The main results were as following:(1) There are eight profiles of three types of wetland, including Carexspp-dominated,Phragmites-dominated and paddy soil, for field investigation in Dongting floodplainwetland. Results showed that content of B_N in the top layer (0-10 cm) is 57.90-259.47mg/kg, the proportion of microbial biomass nitrogen in soil total nitrogen in DongtingLake is 3.1-6.4%. Vertical distribution of soil C, N, B_C and B_N were very similar in soil profiles (0～100 cm) of the three types, and decreasing gradually with the increase indepth. The soil organic C, N and B_N in the Carexspp-dominated floodplain were higherthan those of paddy soil and the Phragmites-dominated floodplain at 0～10 cm; The B_C inCarexspp-dominated floodplain and paddy soil were higher than that ofPhragmites-dominated floodplain at 0～10 cm. There were significant logarithmcorrelations between organic carbon, soil N, B_C, B_N, and soil clay granules(＜0.001mm) inour studied three type of soil (p＜0.01). There was also significant power correlationbetween organic carbon, soil N, B_C, B_N, and bulk density in our studied soil (p＜0.01).(2) Field investigation on the farming soil derived from wetlands (purple alluvial soil)were conducted in a selected landscape unit (112°16′- 112°56′E, 28°42′- 29°11′N) in theDongting lake region of Hunan province. The results showed that the content of B_Nranges from 70 to 160 mg/kg (average of 115.47 mg/kg) in different land-use types andcropping systems, and approximately, the percent of B_N in soil total nitrogen in DongtingLake was 3.1-6.4%. The amount of soil C, N were significant different in variousland-use types and cropping systems, such as paddy soil (double rice, single rice), upland(rape, ramie). The T_C and T_N in the double rice paddy soil were higher than those ofsingle rice, rape, and ramie, the same as the content of B_C and B_N in double rice paddysoil. The percentages of soil B_C, B_N in organic C, total nitrogen in farmland of DongtingLake region were 0.6～7.2% (average of 3.00%), 1.0～7.4% (3.8%), respectively. Theratios of T_C to T_N and B_C to B_N were 3.87-17.31 (9.15), 4.06～9.29 (7.26), respectively.There were significant correlations between T_C, T_N and B_C, B_N(p＜0.01). And B_C, B_Ncould responses to the changes of T_C, T_N in farmland of Dongting Lake region.(3) Soil samples were collected from two long-term field fertilizer experiments sites,which had been for 18 years. One soil derived from wetlands lacustrine sediment and theother derived from alluvial sediment in the Dongting lake region. Data suggested that T_C,T_N, organic N components and B_N has changed significantly after the long-term combinedapplication of organic and chemical fertilizers (NPKM). NPKM increased the content ofT_N, B_N, total hydrolysable N (THN) and the ratio of B_N to T_N. The majority of organic Ncomponents in soil were total hydrolysable N (THN). In THN components, thepercentage of ammonia acid N (AAN) was the highest, followed by ammonium N (AN),then hydrolysable undefined N (HUN), and ammonia sugar N (ASN) was the lowest. B_Nhad significant positive correlation with THN and all THN components (p＜0.01). Theavailability of AAN and HUN for B_N were the highest in THN components.(4)The laboratory-flooded incubation experiment, using the paddy soil developed from Quaternary red soil (reddish clayey soil) and derived from wetlands (purple alluvialsoil), showed that the majority of B_N was native, and the percentage of labelled substrateB_N in reddish clayey soil and purple alluvial soil were 0.30～6.7% and 1.0～3.5%,respectively. The combined application of rice straw and chemical fertilizer improved theimmobilization of inorganic N by microbes. In addition, the maximum of labeledsubstrate N assimilated by microbe in reddish clayey soil and purple alluvial soil were20.6% and 15.9%, respectively. The assimilation of labeled substrate N in NPKMtreatments was higher than those in chemical fertilizer treatment. The maximum oflabeled substrate N immobilized by clay in reddish clayey soil and purple alluvial soilwere 10.6% and 30.0%, respectively. B_N and fixed ammonium decreased with theincubation time. NPKM increased the remained percentage of organic N and decreasedthe remained percentage of mineral N and fixed ammonium.(5) For pot experiments, there was dynamic change for B_N during the period of ricegrowth. B_N was the lowest at booting stage, and then re-increased at matured stage. Thenative B_N was the majority part, and the percentage of labeled substrate B_N in reddishclayey soil and purple alluvial soil were 1.6～24.3% and 1.15～17.6%. The capability ofmicrobe assimilating inorganic nitrogen increased in the combined application of urea andrice straw treatments. The maximum of immobilization of labeled substrate N by microbein reddish clayey soil and purple alluvial soil were 8.3% and 19.2%, respectively. Theassimilation of labeled substrate N by microbe in NPKM treatments was higher thanthose in chemical fertilizer treatment. The contents of fixed ammonium in fertilizerstreatments in the two soils were lowest at tillering stage. The-maximum of immobilizationof labeled substrate N in clay in the studied soils were 2.03% and 18.69%, respectively.The immobilization of labeled substrate N by clay in NPKM treatments was lower thanthose in chemical fertilizer treatment. The remained majority status of labeled substrate N(urea and straw) were organic N, and the percentage accounted for 80% in reddish clayeysoil and 72% in purple alluvial soil, respectively. The status of labeled substrate N inremained inorganic N (urea) was mineral N (12%) in reddish clayey soil and fixed-N(20%) in purple alluvial soil.