Balancing potato yield, soil nutrient supply, and nitrous oxide emissions: An analysis of nitrogen application trade-offs.

Potato has been promoted as a national key staple food to alleviate pressure on food security in China. Appropriate nitrogen (N) application rate is prerequisite and is crucial for increasing yield, improving fertilizer efficiency, and reducing N losses. In the present study, we determined the optimum N application rates by analyzing field trial data from the main potato producing areas of China between 2004 and 2020. We considered the equilibrium relationships between potato yield, N uptake, partial N balance (PNB), and N2O emission under different soil indigenous N supply (INS) scenarios. The results showed that N rate, INS, and their interactions all significantly affect potato yield and nutrient uptake increment. On average, N application increased potato yield and N uptake by 29.5 % and 56.7 %, respectively. The relationship between N rate and yield increment was linear-plateau, while the relationship between N rate and N uptake increment was linear-linear. Soil INS accounted for 63.5 % of total potato N requirement. Potato yield increment and nutrient uptake increment were exponentially negatively correlated with INS and had a significant parabolic-nonlinear relationship with the interaction of N fertilizer application rate and INS. PNB was negatively correlated with fertilizer N supply intensity as a power function. Based on our analysis, a N application rate of 166 kg N ha-1 was found to be sufficient when the target yield was <34 t ha-1. However, when the target yield reached 40, 50 and 60 t ha-1, the recommended N application rate increased to 182, 211, and 254 kg N ha-1, respectively, while ensuring N2O emissions low with an emission factor of 0.2 %. Our findings will help guide potato farming toward cleaner production without compromising environmental benefit.

Long-term combined organic manure and chemical fertilizer application enhances aggregate-associated C and N storage in an agricultural Udalfs soil.

Little information is known on whether carbon (C) and nitrogen (N) immobilization is synchronized in different sizes of aggregates under different agricultural management practices. Carbon and N concentrations and the C/N ratios in different sizes of aggregates down to 40 cm depth were determined after long-term application of chemical fertilizers combined with manure or without manure in a wheat-rice cropping system. Manure application usually produced significantly (P < 0.05) higher C and N concentrations and lower C/N ratios in bulk soil and in different sizes of aggregates down to 20 cm depth than the other treatments, and the 1.5 MNPK treatment at 0-10 cm depth had the highest SOC concentration of 26.3 g/kg. The C and N concentrations in bulk soil and all aggregate fractions decreased markedly with increasing soil depth. Among water stable aggregates in all soil depths, the highest C (48.2-66.4%) and N (47.8-68.3%) concentrations as a percentage of SOC were found in the small macroaggregates (2000-250 µm, SM). Manure application significantly (P < 0.05) increased the mass and C and N concentrations of SM and SM fractions down to 20 depth. The mean C/N ratios of silt-clay within large and small macroaggregates (inter-SC) were 1.57 and 1.46 units lower than those of silt-clay particles, respectively, indicating that inter-SC had relative high N availability. Moreover, the C and N content of SM down to 40 cm depth tended to saturation with increasing C input rate. Overall, manure application effectively improved soil structure, SM were the dominant particles involved in soil C and N storage, and inter-SC were the main particles responsible for N availability.

Characteristics of rice straw decomposition and bacterial community succession for 2 consecutive years in a paddy field in southeastern China.

The characteristics of long-term rice straw decomposition and succession in the bacterial community in the double-rice system are still unclear. Here a 2-year continuous straw bag decomposition experiment was conducted to explore changes in nutrient release, enzyme activity, and bacterial community composition during rice straw decomposition in the double-rice system in Southeast China. After burial in soil, the cumulative dry matter loss rates of rice straw were 38.9%, 72.6%, and 82.7% after 2, 12, and 24 months, respectively. The change in the release rate of straw nitrogen and phosphorus was similar to the dry matter loss, but 93.5% of straw potassium was released after 1st month. Bacterial abundance and community diversity in straw increased rapidly, reaching peaks after 7 and 12 months, respectively. Straw extracellular enzyme activities were the highest in the first 2 months and then gradually decreased over time, and they significantly and positively correlated with straw decomposition rate. Straw decomposition was dominated by copiotrophic Bacilli and Flavobacteriia in the early stages and evolved to be dominated by oligotrophic Acidobacteria, Anaerolineae, Deltaproteobacteria, Saccharibacteria, and Sphingobacteriia in the later stages. Changes in the C/N and K content of straw are the main reasons for bacterial community succession during rice straw decomposition. This study can provide a scientific basis for developing efficient decomposing bacteria agents for rice straw.

Novel ecological ditch system for nutrient removal from farmland drainage in plain area: Performance and mechanism.

The loading of nitrogen (N) and phosphorus (P) from agricultural drainage as the non-point sources is a worldwide environmental issue for aquatic ecosystem. However, how to remove these nutrients effectively from agricultural drainage remains a big challenge with increasing cemented ditches for better management. Here, we designed a novel ecological ditch system which integrated an earth ditch and a cemented ditch with iron-loaded biochar in the Chengdu Plain to reduce the loss of N and P from farmland. After a two-year monitoring, the removal efficiency of total N and total P reached 24.9% and 36.1% by the earth ditch and 30.7% and 57.8% by the integrated ditch system, respectively. The water quality was evidently improved after passing through the ditch system with the marked decrease in the concentrations of N and P. Dissolved organic N, nitrate, and particulate P became the dominant fractions of N and P loss. Rainfall soon after fertilization increased the concentrations of N and P in the ditch system and markedly affected their removal efficiency. The iron-loaded biochar effectively removed N and P from the drainage, especially at the high concentrations, which was mainly attributed to its high adsorption of the dissolved N and P fractions and the interception of the particulate nutrients. Our results indicate that the designed ecological ditch system has a high potential for alleviating agricultural non-point source pollution in the plain area and can be extended to other lowland agricultural ecosystems.

Differences in total stored C and N in dryland red soil caused by different long-term fertilization practices.

Fertilizer application is important to achieve sustainable agriculture. However, it remains unclear about the effects of long term fertilization on C and N immobilization as well as C/N ratios in soil aggregates at different depths. Samples taken at depths of 0 to 40 cm from dryland red soil subjected to long-term fertilization were analyzed. Four treatments were involved in the long term fertilization including no fertilizer (control), chemical fertilizer applied at two different rates, and manure combined with chemical fertilizers (MNPK). The C and N concentrations in the soil aggregates of different sizes were significantly higher (P < 0.05) and the C/N ratios in the particulate organic matter were significantly lower (P < 0.05) for soil to 20 cm deep for the MNPK treatment than for the other treatments. ANOVA indicated that the C and N concentration and C/N ratios in different sizes of aggregates significantly varied with soil depth (P < 0.05). Microaggregates contained most of the C and N, and the C/N ratios for silt-clay particles in macroaggregates were 1.37 unit (ranging - 0.25 to 2.44) lower than for other soil particles with diameters < 53 µm. The C and N contents in aggregates of different sizes increased as the C input rate increased to a depth of 40 cm because of the fertilization practices. Overall, both increased C input and deep application of C sources promoted the storage of C and N in microaggregates, which in turn increased C and N sequestration in dryland red soils.

Effects of chicken manure substitution for mineral nitrogen fertilizer on crop yield and soil fertility in a reduced nitrogen input regime of North-Central China.

Organic manure has been proposed to substitute part of the chemical fertilizers. However, past research was usually conducted in regimes with excessive nitrogen (N) fertilization, which was not conducive to the current national goal of green and sustainable development. Therefore, exploring the potential of organic fertilizer substitution for mineral N fertilizer under regimes with reduced N inputs is important to further utilize organic fertilizer resources and establish sustainable nutrient management recommendations in the winter wheat (Triticum aestivum L.) - summer maize (Zea mays L.) rotation system in North-central China. In this study, a 4-year field experiment was conducted to investigate the effects of different chicken manure substitution ratios on crop yield, N recovery efficiency (REN), soil N and soil organic matter contents, to clarify the optimal organic substitution ratio of N fertilizer under reduced N application (from 540 kg N ha-1 year-1 to 400 kg N ha-1 year-1). Six substitution ratios were assessed: 0%, 20%, 40%, 60%, 80% and 100% under 200 kg N ha-1 per crop season, respectively, plus a control with no N application from chemical fertilizer or chicken manure. Results showed that the highest yield was achieved under the 20% substitution ratio treatment, with 1.1% and 2.3% higher yield than chemical N alone in wheat season and maize seasons, respectively. At the chicken manure substitution ratios of 20% in wheat season and 20%-40% in maize season, the highest REN reached to 31.2% and 26.1%, respectively. Chicken manure application reduced soil residual inorganic N with increasing substitution ratio. All organic substitution treatments increased soil organic matter and total N content. Implementing 20% organic substitution in wheat season and 20%-40% in maize season under the reduced N application regime in the North-central China is therefore recommended in order to achieve high crop yields and REN, improve soil fertility and enhance livestock manure resource utilization.

Altitudinal-modulated sediment inputs rather than the land-uses determine the distribution of lead in the riparian soils of the Three Gorges Reservoir.

Lead (Pb) as a toxic metal has potential ecological hazards for aquatic quality. However, the variation in the distribution patterns of Pb and its fractions in flooding soils with frequent and anti-seasonal water-level fluctuation and various human disturbances remains unclear. In this study, the distribution of Pb and its fractions in the riparian soils of the Three Gorges Reservoir (TGR) were delineated based on the differences in altitude and land-uses including farmland, orchard, forest and residential area. Then, we assessed the contamination and eco-risk of Pb in the soils and deciphered the key factors determining the distribution of Pb and its fractions. The results showed that the concentrations of Pb and its fractions in the soils decreased significantly with altitude, while the significant difference was not observed among the land-uses. The contamination of Pb in the soils reached a moderate level, and its eco-risk was very low by the potential eco-risk index and mobile Pb fraction. The source of soil Pb at the upper zone (> 160 m) was mainly from natural inputs, while the source at the lower zone (≤ 160 m) was attributed to anthropogenic contributions including ores mining, fossil fuel combustion, vehicle emissions and atmospheric deposition indicated by Pb isotopic ratios. With the limited effect of land-uses, the sediment inputs regulated by frequent water-level fluctuation determined the altitudinal distribution of Pb and its fractions in the flooding soils. The soil particle size dominated the migration and transformation of Pb over other soil properties such as pH and organic matters. The results of this study indicate that the anthropogenic Pb mainly exists in the soils of lower riparian zone in the TGR, and the frequent and anti-seasonal dry and rewetting alternation aggravates the potential for the Pb migration downstream due to the determinant of soil particles.

Investigations of the effect of the amount of biochar on soil porosity and aggregation and crop yields on fertilized black soil in northern China.

The combination of chemical fertilizer and biochar is regarded as a useful soil supplement for improving the properties of soil and crop yields, and this study describes how the biochar of maize straw can be used to improve the quality of the degraded black soil. This has been achieved by examining the effects of combining different amounts of biochar with chemical fertilizer on the porosities and aggregate formation of soil and exploring how these changes positively impact on crop yields. A field trial design combining different amounts of maize straw biochar [0 (NPK), 15.75 (BC1), 31.5 (BC2), and 47.25 t ha-1 (BC3)] with a chemical fertilizer (NPK) has been used to investigate changes in the formation of soil aggregate, clay content, soil organic carbon (SOC), and crop yields in Chinese black soil over a three year period from 2013 to 2015. The results of this study show that the addition of fertilizer and biochar in 2013 to black soil results in an increased soybean and maize yields from 2013 to 2015 for all the treatments, with BC1/BC2 affording improved crop yields in 2015, while BC3 gave a lower soybean yield in 2015. Total porosities and pore volumes were increased for BC1 and BC2 treatments but relatively decreased for BC3, which could be attributed to increased soil capillary caused by the presence of higher numbers of fine soil particles. The addition of biochar had a positive influence on the numbers and mean weight diameters (MWD) of soil macroaggregates (>0.25 mm) that were present, with the ratio of SOC to TN in soil macroaggregates found to be greater than in the microaggregates. The most significant amount of carbon present in macroaggregates (>2 mm and 0.25-2 mm) was observed when BC2 was applied as a soil additive. Increasing the levels of maze straw biochar to 47.25 t ha-1 led to an increase in the total organic carbon of soil, however, the overall amount of macroaggregates and MWD were decreased, which is possibly due to localized changes in microbial habitat. The supplementation of biochar increased in the amount of aromatic C present (most significant effect observed for BC2), with the ratio of aliphatic C to aromatic C found to be enhanced due to a relative reduction in the aliphatic C content with >2 mm particle fraction. These changes in organic carbon content and soil stability were analyzed using univariate quadratic equations to explain the relationship between the type of functional groups (polysaccharide C, aliphatic C, aromatic C, aliphatic C/aromatic C) present in the soil aggregates and their MWDs, which were found to vary significantly. Overall, the results of this study indicate that the use of controlled amounts of maize-straw biochar in black soil is beneficial for improving crop yields and levels of soil aggregation, however, the use of excessive amounts of biochar results in unfavorable aggregate formation which negatively impacts the yields of crop growth. The data produced suggest that aromatic C content can be used as a single independent variable to characterize the stability of soil aggregate when biochar/fertilizer mixtures are used as soil additives to boost growth yields. Analysis of soil and crop performance in black soil revealed that the application of maize-straw biochar at a rate of 15.75 and 31.5 t ha-1 had positive effects on crop yields, soil aggregation and accumulation of aromatic C in the aggregate fractions when a soybean-maize rotation system was followed over three years.

Influences of Multilayer Graphene and Boron Decoration on the Structure and Combustion Heat of Al3Mg2 Alloy.

To improve the engine-driven performance of propellants, high-energy alloys such as Al and Mg are usually adopted as annexing agents. However, there is still room for improvement in the potential full utilization of alloy energy. In this study, we investigated how to improve combustion efficiency by decorating Al3Mg2 alloy with multilayer graphene and amorphous boron. Scanning electron microscopy and Raman tests showed that decorating with multilayer graphene and amorphous boron promoted the dispersion of Al3Mg2 alloy. The results showed that decorating with 1% boron and 2% multilayer graphene improved the combustion heat of Al3Mg2 alloy to 32.8 and 30.5 MJ/kg, respectively. The coexistence of two phases improved the combustion efficiency of the matrix Al3Mg2 alloy.

South India projected to be susceptible to high future groundnut failure rates for future climate change and geo-engineered scenarios.

With an increase in global mean temperature predicted for this century accompanied by more frequent extremes, will farming communities need to brace for increased crop failures and hardship? Solar dimming climate geoengineering has been proposed as a possible solution to combat rising global temperature but what effect will it or other climate related adaptation have on crop failures? We performed a crop modelling study using future climate and geoengineering projections to investigate these questions. Our results indicate that groundnut crop failure rates in Southern India are very sensitive to climate change, and project an increase of approximately a factor of two on average over this century, affecting one out of every two to three years instead of one in every five years. We also project that solar dimming geoengineering will have little impact on reducing these failure rates. In contrast, the projections for the rest of Indian regions show decreasing failure rates of 20-30%. In this research, we indicate why south India is more susceptible than the rest of the country and show that neither Solar dimming geoengineering nor reducing heat or water stress are able to fully counteract the increase in failure rates for this region. Thus our modelling projections indicate the potential for a grountnut crop failure crisis for the South India.

Soybean yield, nutrient uptake and stoichiometry under different climate regions of northeast China.

Climate and soil fertility influence seed yield, nutrient uptake, and nutrient stoichiometry in the plant. We collected soybean [Glycine max (L.) Merr.] data were collected from field experiments in northeast China (warm and cold regions) to study the effect of temperature variations during the crop growing season on seed yield, nutrient uptake and stoichiometry from 2001 to 2017. Soybean seed yield has been increased in the cold region but not in the warm region, where average seed yield was higher. The indigenous nitrogen (N) supply followed the same trend as yield, greater in warmer environments but also increasing over time. The internal efficiency (IE) of N and potassium (K) performed similarly in both climate regions, but phosphorus (P) IE was 30% greater in the warm region than the cold region. For soybean nutrient uptake ratio, the N/K ratio was similar between both regions; however, the N/P ratio was greater in the warmer region relative to the colder region. Overall, the higher temperature experienced in the warm region increased soybean seed yield relative to the cold region, and high soil P accumulation caused soybean P luxury uptake in the cold region of northeast China.

Spatial variation of yield response and fertilizer requirements on regional scale for irrigated rice in China.

A large number of on-farm experiments (n = 5556) were collected for the period 2000-2015 from the major rice (Oryza sativa L.) producing regions in China, to study the spatial variability of attainable yield, yield response, relative yield and fertilizer requirements at regional scale, by coupling geographical information system with the Nutrient Expert for Rice decision support system. Results indicated that average attainable yield was 8.8 t ha-1 across all sites, with 18.3% variation. There were large variations in yield response to nitrogen (N), phosphorus (P), and potassium (K) fertilizer application with coefficients of variation of 39.2%, 57.0%, and 53.4%, and the sites of 73.4%, 85.8%, and 87.6% in the study area ranged from 2.0 to 3.0, from 0.7 to 1.3, and from 0.7 to 1.3 t ha-1, respectively. Mapping the spatial variability of relative yield to N, P, and K indicated that the sites of 78.6%, 92.4%, and 88.7% in the study area ranged from 0.65 to 0.75, from 0.80 to 0.92, and from 0.84 to 0.92, respectively. The high yield response and low relative yield to N and P were mainly located in the Northeast (NE), Northwest (NW), and north of the Middle and Lower Reaches of Yangtze River (MLYR) regions. The spatial distribution of N, P, and K fertilizer requirements ranged 140-160 kg N ha-1, 50-70 kg P2O5 ha-1 and 35-65 kg K2O ha-1 which accounted for 66.4%, 85.5% and 73.0% of sites in the study area, respectively. This study analyzed the spatial heterogeneity of attainable yield, soil nutrient supply capacity and nutrient requirements based on a large database at regional or national scale by means of geographical information systems and fertilizer recommendation systems, which provided a useful tool to manage natural resources, increase efficiency and productivity, and minimize environmental risk.

Yield Gap, Indigenous Nutrient Supply and Nutrient Use Efficiency for Maize in China.

Great achievements have been attained in agricultural production of China, while there are still many difficulties and challenges ahead that call for put more efforts to overcome to guarantee food security and protect environment simultaneously. Analyzing yield gap and nutrient use efficiency will help develop and inform agricultural policies and strategies to increase grain yield. On-farm datasets from 2001 to 2012 with 1,971 field experiments for maize (Zea mays L.) were collected in four maize agro-ecological regions of China, and the optimal management (OPT), farmers' practice (FP), a series of nutrient omission treatments were used to analyze yield gap, nutrient use efficiency and indigenous nutrient supply by adopting meta-analysis and ANOVA analysis. Across all sites, the average yield gap between OPT and FP was 0.7 t ha-1, the yield response to nitrogen (N), phosphorus (P), and potassium (K) were 1.8, 1.0, and 1.2 t ha-1, respectively. The soil indigenous nutrient supply of N, P, and K averaged 139.9, 33.7, and 127.5 kg ha-1, respectively. As compared to FP, the average recovery efficiency (RE) of N, P, and K with OPT increased by percentage point of 12.2, 5.5, and 6.5, respectively. This study indicated that there would be considerable potential to further improve yield and nutrient use efficiency in China, and will help develop and inform agricultural policies and strategies, while some management measures such as soil, plant and nutrient are necessary and integrate with advanced knowledge and technologies.

[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.

[Dynamics of soil microbial biomass and dissolved organic carbon and nitrogen under flooded condition].

With reddish yellow soil (RYS) and alluvial purple soil (APS), the two typical paddy soils in the Dongting Lake floodplain of China as test soils, an incubation test was conducted at 25 degrees C to study the dynamic changes of soil microbial biomass and dissolved organic carbon and nitrogen under flooded condition. Three treatments were installed, i.e., control (CK), ammonium sulfate (N), and rice straw powder plus ammonium sulfate (S-N). The results showed that during incubation, soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), soil dissolved organic carbon (SDOC), and soil dissolved organic nitrogen (SDON) reached their maximum initially, decreased thereafter, and tended to be stable. After amending the substrates to the two soils, the averages of SMBC to soil total carbon, SMBN to soil total nitrogen, SDOC to soil total carbon, and SDON to soil total nitrogen were 2% - 3%, 2% - 3%, 1% or so, and 5% - 6%, respectively. In the two soils, the peak values of SMBC in treatment N and those of SMBN, SDOC and SDON in treatment S-N were the highest, while those of SMBC in treatments N and S-N had no significant difference. The peak values of SMBN, SDOC and SDON in RYS were significantly different between treatments N and S-N, while no significant difference was observed between the peak values of SMBN and SDOC in APS, because the fertility of RYS was lower than that of APS. In the first 7 days of incubation, SMBC/SMBN ratio was < 10, while after 14 days of incubation, this ratio was higher in treatment N than in treatment S-N at the same time in the same soil. The SDOC/SDON ratio in all treatments was the highest at the 3rd d, and the lowest at the 28th d of incubation.

[Soil C, N and P contents and their relationships with soil physical properties in wetlands of Dongting Lake floodplain].

Eight representative soil profiles were installed on three types of wetland (two profiles on Carex spp. -dominated floodplain, four on Phragmites-dominated floodplain, and two on paddy soil) in Dongting Lake floodplain of China in 2004, and their C, N and P contents, microbial biomass C, N and P, <0.001 mm clay particles, and bulk density were measured. The results indicated the spatial distribution of soil C and N and soil microbial biomass C, N, and P were very similar in the profiles (0-100 cm) of three types of wetland, being decreased gradually with depth, except for soil TP which was constant in the profiles. The percentages of soil microbial biomass C, N and P to soil organic C, total nitrogen and total phosphorus decreased gradually with depth. In top layer (0-10 cm), the contents of soil organic C and microbial biomass C and the percentage of soil microbial biomass C to organic C were 19.63-50.20 g x kg(-1), 424.63 - 1 597.36 mg kg(-1), and 3.17% - 4.82%, respectively, the contents of soil total N and microbial biomass N and the percentage of soil microbial biomass N to total N were 1.85-4.45 g x kg(-1), 57.90 - 259.47 mg x kg(-1), and 3.13% - 6.42%, respectively, and the content of soil microbial biomass P and the percentage of soil microbial biomass P to soil total P was 24.16 - 200.99 mg x kg(-1) and 1.09% - 11.20%, respectively. The bulk density of soil top layer (0 -10 cm) was 0.65 - 1.04 g x cm(-3), and the content of < 0.001 mm clay particles was 26.24% - 39.48%. The contents of soil organic C and N and microbial biomass N and P in 0 - 10 cm layer were the highest in Carex spp.-dominated floodplain, followed by paddy soil, and Phragmites-dominated floodplain. Also in 0 - 10 cm layer, the soil microbial biomass C in Carex spp. dominated floodplain and paddy soil was higher than that in Phragmites-dominated floodplain, while the soil bulk density in Phragmites-dominated floodplain was higher than that in paddy soil, and much higher than that in Carex spp. -dominated floodplain. The amount of soil < 0.001 mm clay particles in Carex spp. -dominated floodplain and Phragmites-dominated floodplain was higher than that in paddy soil. In these three types of wetland, soil organic C and N and microbial biomass C, N and P had a significant logarithm correlation (P < 0.01) with <0.001 mm clay particles, and a significant index correlation (P <0.01) with bulk density.