[Response of active nitrogen to salinity in a soil from the Yellow River Delta].

Soil salinity can inhibit the processes of nitrogen cycle, and the active nitrogen is the important indicator to reflect the turnover of nitrogen. A laboratory experiment was conducted to study the effect of soil salinity on the active nitrogen in a soil of the Yellow River Delta incubated aerobically under 25 degrees C for 45 days. Four levels of salinity (S1: 0.1%, S2: 0.5%, S3: 0.9%, S4: 1.3%) were imposed using NaCl (mass fraction), and glucose with or without NH4Cl were added to the soils. NO3(-) -N, NH4(+) -N, total soluble nitrogen (TSN) and microbial biomass nitrogen (MBN) were monitored. Results showed that NO3(-)-N was significantly higher in the low salinity soil (S1, S2) than in the high salinity soil (S3, S4) under the control and with NH4Cl addition, and especially the difference was larger with NH4Cl addition. Comparing with the control, NO3(-) -N was increased significantly in S1 and S2. NO3(-) -N was decreased significantly with glucose addition, and there was no difference among the four salinity soils during the whole incubation period. NH4(+) -N was significantly higher in the high salinity soil (S3, S4) than in the low salinity soil (S1, S2), and it was increased particularly in S4 after day 5. With the addition of NH4Cl, NH4(+) -N was increased in S3 and S4. MBN was higher in the low salinity soil than in the high salinity soil, and it was not increased with NH4Cl addition, though TSN was increased. With glucose addition, MBN was increased by 89.9% - 130.9% in the low salinity soil (S1, S2) and 36.9% - 79.5% in the high salinity soil (S3, S4). It was suggested that soil salinity had influence on N transformation, and high salinity inhibited the transformation and assimilation of N by microorganism. The addition of C depressed the effect of salinity, and improved the microbial activity. The application of organic matter is an effective measure to improve N transformation in saline soils.

[Roles of soil dissolved organic carbon in carbon cycling of terrestrial ecosystems: a review].

Soil dissolved organic carbon (DOC) is an active fraction of soil organic carbon pool, playing an important role in the carbon cycling of terrestrial ecosystems. In view of the importance of the carbon cycling, this paper summarized the roles of soil DOC in the soil carbon sequestration and greenhouse gases emission, and in considering of our present ecological and environmental problems such as soil acidification and climate warming, discussed the effects of soil properties, environmental factors, and human activities on the soil DOC as well as the response mechanisms of the DOC. This review could be helpful to the further understanding of the importance of soil DOC in the carbon cycling of terrestrial ecosystems and the reduction of greenhouse gases emission.

[Dynamics of labeled substrate N by microorganism and soil clay immobilized and its residue fractions in typical paddy soils in Dongting Lake floodplain].

Dynamics of immobilization of the labeled substrate N by microorganism and soil clay and its residue fractions in soils from the plough layers of two subtropical paddy soils (Reddish clayey soil, Purple alluvial soil) in the Dongting Lake floodplain were studied. A laboratory-flooded incubation experiment was composed of three treatments: control (CK), labeled ammonium sulphate group (15NA) and the combined of labeled ammonium sulphate and rice straw group (S + 15NA). During the incubation, microbial biomass N (BN) increased firstly and then those decreased and tended to stable subsequently, while the content of fixed ammonium changed little. Native BN acted as the major N pool because the percentage-of labeled substrate BN in reddish clayey soil and purple alluvial soil were 0.30%-6.67% and 1.00%-3.47%, respectively. The combined application of rice straw and chemical fertilizer (S + 15NA) improved the immobilization of inorganic N by microorganism, because the immobilization ratio of substrate N in reddish soil and purple alluvial soil were 6.78% and 10.78%, respectively, for S + 15NA treatment, higher than those for 15NA treatment. The immobilization ratio of substrate N by soil clay in reddish clayey soil and purple alluvial soil were 2.48%-10.57% and 12.55%-30.04%, respectively. However, the immobilization ratio of substrate N by soil clay in the two soils were 7.14% and 21.53%, respectively, for S + 15NA treatment, lower than those for NA treatment. The incorporation of chemical fertilizer and rice straw increased the N remain percent. The main residue formation of the labeled substrate N was total hydrolysable N (> 72%) in Reddish clayey soil, while it was total hydrolysable N (44.0%-53.2%) and fixed ammonium (35.2%-37.5%) in Purple alluvial soil. The residue of mineral nitrogen ranged 10%-20% in the two soils. In conclusion, fertilization method and the type of soil clay had important effects on the immobilization and mineralization of substrate N. The combined application of chemical fertilizer and straws increased the immobilization of inorganic nitrogen by microorganism and decreased immobilization of inorganic nitrogen by soil clay. The combined application of chemical fertilizer and straws decreased the loss of chemical fertilizer N, increased residue of nonhydrolysable N, and decreased residue of mineral nitrogen.

[Effects of long-term application of fertilizers on soil microbial biomass nitrogen and organic nitrogen components in subtropical paddy soils].

The effect of long-term fertilization on soil organic nitrogen components and microbial biomass nitrogen (B(N)) in paddy soils from two experiment sites in Hunan province were studied. Soil samples were collected from the plough layers of different fertilizer treatments. Soil B(N) was measured by the fumigation-extraction method, and soil organic N was fractionated by acid hydrolysis-distillation method according to the scheme of Bremner (1965). Results showed that the soil N increased 40 mg x kg(-1) every year at Ningxiang site (low N level) for 17 years under the application combined of fertilizers and manure, while that at Nanxian site (high N level) was 55 mg x kg(-1). Soil total nitrogen (T(N)), total hydrolysable nitrogen (THN) and microbial biomass nitrogen(B(N)) were increased by long-term combined application of chemical fertilizer and manure (NPKM). NPKM significantly increased the content of T(N), B(N), total hydrolysable nitrogen (THN), ammonia acid nitrogen (AAN), hydrolysable unidentified nitrogen (HUN) and the percentage of B(N) to T(N). Besides, NPKM increased the easily mineralizable B(N), AAN, and low decomposed HUN. There was positive correlated relationship between B(N) and THN and different THN components, and the effect of AAN and HUN on B(N) was biggest. It is obvious that NPKM increased soil fertility and enhanc the nitrogen-supplying capability of paddy soils. NPKM had the effect on increasing soil nitrogen capability of paddy soils, both easily decomposed fractions and difficultly decomposed ones.