Mixing Alfalfa Straw and Maize Straw to Enhance Nitrogen Mineralization, Microbial Biomass and Enzyme Activity: A Laboratory Study

The quality of straw affects N release after straw retention. As straw with high C： N ratio could result in N immobilization, additional N is needed to compensate N demand of crops. However, more and more N fertilizers have been applied to the soil to improve crop yields in China, which not only increases production cost but also reduces soil quality. Therefore, reasonable application of N fertilizer becomes a key problem after straw retention. This study aimed to assess the effects of applying maize straw with high quality alfalfa straw on mineral N content, microbial biomass and enzyme activity under controlled conditions. The effect of applying maize straw with alfalfa straw was compared with that of maize straw in combination with N fertilizer under the same C： N ratio （25：1）. The laboratory incubation experiment consisted of four treatments： （1） soil with no addition （CK）; （2） soil amended with maize straw （M）; （3） soil amended with alfalfa straw and maize straw with an adjusted C： N ratio of 25：1 （MM）; （4） soil amended with inorganic nitrogen fertilizer and maize straw with an adjusted C：N ratio of 25：1 （MF）. The results showed that application of maize straw leaded to an N immobilization during the 270 d of incubation. Combined application of alfalfa and maize straw and or mineral N fertilizer alleviates the N immobilization and increase soil mineral N content. Compared to MF treatment, MM treatment prolonged N availability during the incubation. MM and MF treatments increased the soil microbial biomass carbon and nitrogen contents, and soil invertase and β-glycosidase activities. There was no difference between MM and M treatment in soil urease activity. MF treatment had significantly negative influence on soil urease activity compared with M treatment. The amount of added N significantly affected mineral N content, soil microbial biomass and enzyme activity. The mixture of alfalfa straw and maize straw sustains higher level of mineral N content, microbial biomass and enzyme activity as it had high N input compared to maize straw in combination with N fertilizer. It is concluded that alfalfa straw may be a better N source than N fertilizer in alleviating N immobilization caused by maize straw retention.

Microbial Biomass Carbon and Total Organic Carbon of Soils as Affected by Rubber Cultivation

Soil samples were collected from different rubber fields in twenty-five plotsselected randomly in the Experimental Farm of the Chinese Academy of Tropical Agriculture Scienceslocated in Hainan, China, to analyse the ecological effect of rubber cultivation. The results showedthat in the tropical rubber farm, soil microbial biomass C (MBC) and total organic C (TOC) wererelatively low in the content but highly correlated with each other. After rubber tapping, soil MBCof mature rubber fields decreased significantly, by 55.5 percent. compared with immature rubberfields. Soil TOC also decreased but the difference was not significant. Ratios of MBC to TOCdecreased significantly. The decreasing trend of MBC stopped at about ten years of rubbercultivation. After this period, soil MBC increased relatively while soil TOC still kept indecreasing. Soil MBC changes could be measured to predict the tendency of soil organic matterchanges due to management practices in a tropical rubber farm several years before the changes insoil TOC become detectable.

Soil Microbial Biomass Nitrogen and Its Relationship to Uptake of Nitrogen by Plants

The contents of the soil microbial biomass nitrogen (SMBN) in the soils sampled from the Loess Plateau of China were determined using chloroform fumigation aerobic incubation method (CFAIM), chloroform fumigation anaerobic incubation method (CFANIM) and chloroform fumigation-extraction method (CFEM).The N taken up by ryegrass on the soils was determined after a glasshouse pot experiment. The flushes of nitrogen (FN) of the soils obtained by the CFAIM and CFANIM were higher than that by the CFEM, and there were significantly positive correlations between the FN obtained by the 3 methods. The N extracted from the fumigated soils by the CFAIM, CFANIM and CFEM were significantly positively correlated with the N uptake by ryegrass. The FN obtained by the 3 methods was also closely positively correlated with the plant N uptake. The contributions of the SMBN and mineral N and mineralized N during the incubation period to plant N uptake were evaluated with the multiple regression method. The results showed that the N contained in the soil microbial biomass might play a noticeable role in the N supply of the soils to the plant.

Relationship Between Soil Microbial Biomass C and N and Mineralizable Nitrogen in Some Arable Soils on Loess Plateau

The chloroform fumigation-incubation method was used to measure the soil microbial biomass C (SMBC)and N (SMBN) in 16 loessial soils sampled from Ausai, Yongshou and Yangling in Shaanxi Province. The SMBC contents in the soils ranged from 75.9 to 301.0 μg Cg-1 with an average of 206.1 μg C g-1, accounting for 1.36%～6.24% of the total soil organic C with an average of 3.07%, and the SMBN contents from 0.51 to 68.40 μg N g-1 with an average of 29.4 μg N g-1, accounting for 0.20%～5.65% of the total N in the soils with an average of 3.36%. A close relationship was found between SMBC and SMBN, and they both were positively correlated with total organic C, total N, NaOH hydrolizable N and mineralizable N. These results confirmed that soil microbial biomass had a comparative role in nutrient cycles of soils.

Effect of heat-disturbance on microbial biomass carbon and microbial respiration in Chinese fir(Cunninghamia lanceolata) forest soils

Prescribed fire has now become the usual management practice in the Chinese fir （Cunninghamia lanceolata （Lamb.） Hook.） plantation in southern China. Heat generated during fire may affect carbon （C） dynam- ics in soils. We investigated the microbial biomass C （MBC） and microbial respiration in two Chinese fir forest soils （one is not exposed to fire for the past 88 years, and the other is recently exposed to prescribed fire） after soil heating （100 and 200 ℃） under three moisture regimes [25, 50 and 75 % of water holding capacity （WHC）]. For both soils, significant reduction in MBC with increasing heating temperature was found. Soils without exposing to fire previously had significantly greater MBC concentra- tion than the fire-exposed soils when heated at 100 or 200 ℃. Lower soil water content resulted in higher MBC concentrations in both soils. In contrast, both soils had the highest soil microbial respiration rate at 50 % WHC. Soils without exposing to fire previously had the greatest microbial respiration rates at 200 ℃, while the fire-ex- posed soils when heated at 100 ℃ had greatest microbialrespiration rates. During 14-days post-heat incubation, soil MBC in both soils was greatest after heating at 200 ℃ and 25 % WHC. However, soil previously exposed to fire had the lowest CO2 evolution when incubated at 25 % WHC.

Effect of Long-Term Straw Incorporation on SoilMicrobial Biomass and C and N Dynamics

A study was performed on the long-term effect of straw incorporation on soil microbial biomass C contents, C and N dynamics in both Rothamsted and Woburn soils. The results showed that for both soils,the microbial biomass C contents were significantly different among all the treatments, and followed the sequence in treatments of straw chopped and incorporated into 10 cm (CI10) > straw burnt and incorporated into 10 cm (BI10) > straw chopped and incorporated into 20 cm (CI20) > straw burnt and incorporated into 20 cm (BI20). Laboratory incubation of soils showed that the cumulative CO2 evolution was closely related to the soil microbial biomass C content. Carbon dioxide evolution rates (CO2-C, μg (g d) -1 ) decreased rapidly in the first two weeks’ incubation, then decreased more slowly. The initial K2SO4-extractable NH4-N and NO3-N contents were low and similar in all the treatments, and all increased gradually with the incubation time. However, net N immobilization was observed in chopped treatments for Rothamsted soils during the first 4 weeks. Nevertheless, more N mineralization occurred in neatment CI10 than any other treatment at the end of incubation for both soils. The Woburn soils could more easily suffer from the leaching of nitrate because the soils were more permeable and more N was mineralized during the incubation compared to the Rothamsted soils.

Effect of land use on microbial biomass－C, －N and －P in red soils

Eleven red soils varying in land use and fertility status were used to examine the effect of land use on microbial biomass C, N and P. Microbial biomass C in the red soils ranged from about 68 mg C/kg to 225 mg C/kg, which is generally lower than that reported from other types of soil, probably because of low organic matter and high acidity in the red soils. Land use had considerable effects on the amounts of soil C mic . The C mic was the lowest in eroded fallow land, followed by woodland, tea garden, citrus grove and fallow grassland, and the highest in vegetable and paddy fields. There was significant correlation between C mic and organic matter content, suggesting that the influence of land use on C mic is mainly related to the input and accumulation of organic matter. Microbial biomass N in the soils ranged from 12.1 Nmg/kg to 31.7 Nmg/kg and was also affected by land use. The change of N mic with land use was similar to that of C mic . The microbial C/N ratio ranged from 5.2 to 9.9 and averaged 7.6. The N mic was significantly correlated with soil total N and available N. Microbial biomass P in the soils ranged from 4.5 mg P/kg to 52.3 mg P/kg. The microbial C/P ratio was in the range of 4-23. The P mic was relatively less affected by land use due to differences in fertilization practices for various land use systems.

Long-Term Impact of Soil Management on Microbial Biomass C, N and P in Rice-Based Cropping System

A 12-year field experiment was conducted to investigate the effect of different tillage methods and fertil- ization systems on microbial biomass C, N and P of a gray fluvo-aguic soil in rice-based cropping system. Five fertilization treatments were designed under conventional tillage (CT) or no tillage (NT) system: no fertilizer (CK); chemical fertilizer only (CF); combining chemical fertilizer with pig manure (PM); combining chemical fertilizer with crop straw (CS) and fallow (F). The results showed that biomass C, N and P were enriched in the surface layer of no-tilled soil, whereas they distributed relatively evenly in the tilled soil, which might result from enrichment of crop residue, organic manure and mineral fertilizer, and surficial development of root systems under NT. Under the cultivation system, NT had slightly greater biomass C, N and P at 0～5 cm depth, significantly less biomass C, N and P at 5～15 cm depth, less microbial biomass C, N and equivalent biomass P at 15 ～30 cm depth as compared to CT, indicating that tillage was beneficial for the multiplica tion of organisms in the plowed layer of soil. Under the fallow system, biomass C, N and P in the surface layer were significantly greater for NT than CT while their differences between the two tillage methods were negligible in the deeper layers. In the surface layer, biomass C, N and P in the soils amended with organic manure combined with mineral fertilizers were significantly greater than those of the treatments only with mineral fertilizers and the control. Soils without fertilizer had the least biomass nutrient contents among the five fertilization treatments. Obviously, the long-term application of organic manure could maintain the higher activity of microorganisms in soils. The amounts of biomass C, N and P in the fallowed soils varied with the tillage methods; they were much greater under NT than under CT, especially in the surface layer, suggesting that the frequent plowing could decrease the content of organic matter in the surface layer of the fallowed soil.

Effects of Lanthanum on Microbial Biomass Carbon and Nitrogen in Red Soil

The result of soil. culture experiment shows that lanthanum has inhibitory effect on the microbial biomass C and N in red soil, and the inhibition is strengthened with increasing concentration of La. The result of rice pot culture experiment shows that low concentration of La has slight stimulative effect on the microbial biomass C and N in red soil, but its high concentration has inhibitory effect and the inhibition is strengthened with increasing concentration of La. Soil microbial biomass is an important indicator for evaluating rare earths-polluted soil. It is assumed that the critical La concentration is 100 mg.kg(-1) at which red soil tends to be polluted.

Changes of Soil Microbial Biomass C and P During Wheat Growth After Application of Fertilizers

A pot experiment was carried out with a clay loam in a green house. The results showed that soil microbial biomass C increased with the application of organic manure at the beginning of the experiment and then gradually decreased with declining of the temperature. The soil biomass C increased at the tillering stage when the temperature gradually increased, and rose to the highest value at the anthesis stage, being about 554.9-794.4 mg C kg-1. The application of organic manure resulted in the highest increase in biomass C among the fertilization treatments while that of ammonium sulphate gave the lowest. At the harvest time the soil biomass C decreased to the presowing level. Like the soil biomass C the amount of biomass P was increased by the incorporation of organic manure and was the highest among the treatments, with the values of the check and ammonium sulphate treatments being the lowest. Meanwhile, the changing patterns of the C/P ratio of soil microbial biomass at stages of wheat growth are also described.

Contrasting resilience of soil microbial biomass,microbial diversity and ammonification enzymes under three applied soil fumigants

Fumigation is a widely applied approach to mitigate the soil-borne diseases.However,the potential effects of currently applied fumigants on ammonification remain unclear.An 84-day incubation experiment was conducted based on non-fumigated soil(CK)and fumigated soil using three common fumigants,i.e.,chloropicrin(CP),1,3-dichloropropene(1,3-D),and metam sodium(MS).The results showed that,the three fumigants all decreased the microbial C,and the largest reduction(84.7%)occurred with the application of CP.After fumigation,the microbial diversity in the CP treatment rapidly recovered,but that in the 1,3-D treatment decreased and did not recover by the end of the experiment.The application of MS showed no impact on the microbial diversity during the assay,indicating that significantly different microbial diversity can be achieved by choosing different fumigants.Futhermore,the three fumigants showed divergent effects on the enzymes involved in ammonification.The analysis showed that the enzyme variation with CP application was mainly associated with the changed microbial C and N(P<0.05),and not with the microbial community,which was different from the observed effects of 1,3-D or MS application.In addition,the soil quality index showed that CP was still significantly harmful at the end of incubation compared with the good resilience of MS,indicating that CP may not be a suitable fumigant.

The Fluxes of Organic C and N, and Microbial Biomass and Maize Yield in an Organically Manured Ultisol of the Guinea Savanna Agroecological Zone of Nigeria

Field experiments were conducted to evaluate the effects of integrated use of agricultural wastes and a compound mineral fertilizer on the fluxes of soil nutrients. Agricultural wastes applied were: livestock manure (cow dung and poultry litter), shoots of Chromolaena odorata and Parkia biglosa (locust bean), Neem (Azadiracta inidca) seed powder/cake and melon shell. These materials were applied at zero (control), 100% (i.e. organic wastes applied at the recommended rates of 10 t/ha) and 70% of their recommended rates plus 30% of the recommended rate of the mineral fertilizer (NPK: 400 Kg/ha). Average values of soil organic carbon (SOC) were 1.94, 1.68, 1.36 and 1.38 for organic wastes alone, organic waste plus mineral fertilizer (NPK) and unamended control. Mineral N ( N plus N) pools were relatively high at 30 and 60 days after planting, and were significantly higher for organically amended soils (550) and wastes applied at reduced rates combined with 120 kg/ha mineral NPK (470) than the unamended control (277). Across sampling dates, SOC values were the highest in poultry manure and neem seed cake. The values of N plus exchangeable N which constitutes plant available nitrogen (PAN) were significantly higher for organically amended soils and wastes applied at reduced rates combined with 120 kg/ha mineral NPK than the unamended control. The % C microbial to C organic ratio was higher in organically amended soils. The temporal profile of SOC, NH4-N and NO3-N showed declines with time, the relationship was linear for SOC (Y = 0.18x + 1.07;R2 = 0.34), by a power function for N (Y = 48.084x-1.79;R2 = 0.91) and a polynomial function for NH4-N (Y = -28.75x + 130.65x - 57.25;R2 = 0.61). The time dynamics of microbial population (cfu) followed trends obtained for SOC.

Responses of microbial activities and soil physical-chemical properties to the successional process of biological soil crusts in the Gurbantunggut Desert,Xinjiang

Biological soil crusts （BSCs） are capable of modifying nutrient availability to favor the establishment of biogeochemical cycles. Microbial activities serve as critical roles for both carbon and nutrient transformation in BSCs. However, little is known about microbial activities and physical-chemical properties of BSCs in the Gurbantunggut Desert, Xinjiang, China. In the present research, a sampling line with 1-m wide and 20-m long was set up in each of five typical interdune areas selected randomly in the Gurbantunggut Desert. Within each sampling line, samples of bare sand sheet, algal crusts, lichen crusts and moss crusts were randomly collected at the depth of 0-2 cm. Varia- tions of microalgal biomass, microbial biomass, enzyme activities and soil physical-chemical properties in different succession of BSCs were analyzed. The relationships between microalgal biomass, microbial biomass, enzymatic activities and soil physical-chemical properties were explored by stepwise regression. Our results indicate that micro- algal biomass, microbial biomass and most of enzyme activities increased as the BSCs developed and their highest values occurred in lichen or moss crusts. Except for total K, the contents of most soil nutrients （organic C, total N, total P, available N, available P and available K） were the lowest in the bare sand sheet and significantly increased with the BSCs development, reaching their highest values in moss crusts. However, pH values significantly decreased as the BSCs developed. Significant and positive correlations were observed between chlorophyll a and microbial biomass C. Total P and N were positively associated with chlorophyll a and microbial biomass C, whereas there was a significant and negative correlation between microbial biomass and available P. The growth of cyanobacteria and microorganism contributed C and N in the soil, which offered substrates for enzyme activities thus increasing enzyme activities. Probably, improvement in enzyme activities increased soil fertility and promoted the growth of cyanobacteria, eukary- otic algae and heterotrophic microorganism, with the accelerating succession of BSCs. The present research found that microalgal-microbial biomass and enzyme activities played important roles on the contents of nutrients in the successional stages of BSCs and helped us to understand developmental mechanism in the succession of BSCs.

Impacts of low-intensity prescribed fire on microbial and chemical soil properties in a Quercus frainetto forest

Prescribed fire is a common economical and effective forestry practice, and therefore it is important to understand the effects of fire on soil properties for better soil management. We investigated the impacts of low-intensity prescribed fire on the microbial and chemical properties of the top soil in a Hungarian oak(Quercus frainetto Ten.) forest. The research focused on microbial soil parameters(microbial soil respiration(RSM), soil microbial biomass carbon(Cmic) and metabolic quotient(qCO2) and chemical topsoil properties(soil acidity(pH),electrical conductivity(EC), carbon(C), nitrogen(N), C/N ratio and exchangeable cations). Mean annual comparisons show significant differences in four parameters(C/N ratio,soil pH, Cmic and qCO2) while monthly comparisons do not reveal any significant differences. Soil pH increased slightly in the burned plots and had a significantly positive correlation with exchangeable cations Mg, Ca, Mn and K.The mean annual C/N ratio was significantly higher in the burned plots(28.5:1) than in the control plots(27.0:1). The mean annual Cmic(0.6 mg g-1) was significantly lower although qCO2(2.5 lg CO2–C mg Cmic h-1) was significantly higher, likely resulting from the microbial response to fire-induced environmental stress. Low-intensity prescribed fire caused very short-lived changes. The annual mean values of C/N ratio, pH, Cmic and qCO2showed significant differences.

Soil microbial activity and nutrients of evergreen broad-leaf forests in mid-subtropical region of China

To better understand the effects of forest suc- cession on soil microbial activity, a comparison of soil microbial properties and nutrients was conducted between three forest types representing a natural forest succession chronosequence. The study compared a pine （Pinus mas- soniana） forest （PF）, a pine and broadleaf mixed forest （MF） and an evergreen broadleaf forest （BF）, in the Yingzuijie Biosphere Reserve, Hunan Province, China. Results showed that soil nutrients in the MF and BF plots were higher than in the PF plots. The range in microbial biomass carbon followed a similar pattem with Be havingthe greatest values, 522-1022 mg kg-1, followed by Mr 368-569 mg kg-1, and finally, PF 193--449 mg kg-1. Soil nutrients were more strongly correlated with microbial biomass carbon than basal respiration or metabolic quo- tient. Overall, forest succession in the study site improved soil microbial properties and soil fertility, which in turn can increase primary productivity and carbon sequestration.

Effect of carbon and nitrogen ratio control on Artemia growth, water quality, biofloc microbial diversity under high salinity and zero-water exchange culture condition

Biofloc technology has been applied successfully in the intensive aquaculture of several fish and shrimp species. The growth of heterotrophic microorganisms can be stimulated through adding extra carbon, which reduces the nitrogen level in the water and provides microbial protein to the animals. However, most of the studies and practical applications have been conducted in freshwater and marine environment. This paper focused on brine shrimp Artemia that lives in high salinity environment together with other halophilic or halotolerant microorganisms. The effect of carbon supplementation on Artemia growth, water quality, and microbial diversity of biofl ocs was studied in the closed culture condition without any water exchange. The salinity of the culture medium was 100. A 24-d culture trial was conducted through supplementing sucrose at carbon/nitrogen (C/N) ratio of 5, 15, and 30 (Su5, Su15, and Su30), respectively. The culture without adding sucrose was used as a control. Artemia was fed formulated feed at a feeding ration of 60% recommended feeding level. The results showed that sucrose supplementation at higher C/N ratio (15 and 30) signifi cantly improved the Artemia survival, growth and water quality ( P <0.05). Addition of sucrose at C/N ratio of 15 and 30 significantly increased biofloc volume (BFV)( P <0.05). The Illumina MiSeq sequencing analysis showed that supplementing carbon at C/N ratio of 15 had a better total bacterial diversity and richness, and shaped the microbial composition at genera level. This study should provide information for studying the mechanism of biofloc technology and its application in high salinity culture conditions.

Influence of chlorsulfuron herbicide on size of microbial biomass in the soil

A laboratory incubation experiment was conducted to study the effect of chlorsulfuron herbicide on the size of the microbial in loamy sand soil. The herbicide was applied, at four levels that were control, field rate 0\^01 (FR), 0\^1 (10FR) and 1 (100FR) μg/g. Determinations of microbial biomass C content and microbial biomass N content were carried out 1, 3, 5, 7, 10, 15, 25 and 45 days after herbicide application. In comparison to untreated soil, the microbial biomass carbon and biomass nitrogen decreased significantly in soil treated with herbicide in levels 10FR and 100FR within the first 10 days incubation. A more considerable increase in the microbial biomass C∶N ratio was observed in the herbicide treated soil than the non treated control. This effect was transitory and only at the higher rates of chlorsulfuron was significant.

Effect of Ammonium Fixation on Determination of N Mineralized from Soil Microbial Biomass

Two soils with relatively high (Soil 1) and low (Soil 2) ammonium fixation capacities were used in thisstudy to extalne the effect of ammonium fixation on the determination of N mineralised from soil ndcrobialbiomass. organism suspellsioll was quantitatively introduced to Soil 1 at various rates. Both fumigation-incubation (FI) and fumigation-ext raction (FE ) met hods were used to t reat t he soil. The amount of ffeedNH4+-N increased with increasing rate of organism-N addition. A close correlation was found between theamoun of fixed aznmonium and the rate of organism-N addition. The net increso of fixed NH4+-N wereequivalent to 38% and 12% of the added organism-N for FI and FE treatments, respectively in this specificsoil. To provide isotopic evidence, 15N-labelled organism-N was added to Soils 1 and 2 at 121.4 mg N kg-1.In FI treatment, 22 and 3 mg N kg-1 of labelled N were found in the fraction of fixed NH4+-N in Soils 1 and2 respectively; while in FE treatment, 9 mg N kg-1 of labelled N was found in the fraction of fixed NH4+-Nin Soil 1 only. There was no labelled N in the fraction of fixed NH4+-N in Soil 2. In all of the unfumigated(check) soils, there was little or no labelled N in the fixed fractions, probably because the organism-N addedwas easily mineralized and nitrified. A mean of 0.64 for KN value, the fraction of N ndneralized in the killedmicrobial biomass, was obtained with inclusion of the net increase of fixed NH4+-N. The corresponding valuecalculated with exclusion of the net increase of fixed NH4+-N was 0.46. It was concluded that ammniumfixation was a problem in determination of KN, particularly for soils with a high ammonium fixation capacity.Results also showed that microbial biomass N measurement by FE method was less affected by ammoniumprocess than that by FI method.

Ecological stoichiometry and biomass response of Agropyron michnoi Roshev.under simulated N deposition in a sandy grassland,China

Sandy grassland in northern China is a fragile ecosystem with poor soil fertility.Exploring how plant species regulate growth and nutrient absorption under the background of nitrogen(N)deposition is crucial for the management of the sandy grassland ecosystem.We carried out a field experiment with six N levels in the Hulunbuir Sandy Land of China from 2014 to 2016 and explored the Agropyron michnoi Roshev.responses of both aboveground and belowground biomasses and carbon(C),N and phosphorus(P)concentrations in the plant tissues and soil.With increasing N addition,both aboveground and belowground biomasses and C,N and P concentrations in the plant tissues increased and exhibited a single-peak curve.C:N and C:P ratios of the plant tissues first decreased but then increased,while the trend for N:P ratio was opposite.The peak values of aboveground biomass,belowground biomass and C concentration in the plant tissues occurred at the level of 20 g N/(m2•a),while those of N and P concentrations in the plant tissues occurred at the level of 15 g N/(m2•a).The maximum growth percentages of aboveground and belowground biomasses were 324.2%and 75.9%,respectively,and the root to shoot ratio(RSR)decreased with the addition of N.N and P concentrations in the plant tissues were ranked in the order of leaves>roots>stems,while C concentration was ranked as roots>leaves>stems.The increase in N concentration in the plant tissues was the largest(from 34%to 162%),followed by the increase in P(from 10%to 33%)and C(from 8%to 24%)concentrations.The aboveground biomass was positively and linearly correlated with leaf C,N and P,and soil C and N concentrations,while the belowground biomass was positively and linearly correlated with leaf N and soil C concentrations.These results showed that the accumulation of N and P in the leaves caused the increase in the aboveground biomass,while the accumulation of leaf N resulted in the increase in the belowground biomass.N deposition can alter the allocation of C,N and P stoichiometry in the plant tissues and has a high potential for increasing plant biomass,which is conducive to the restoration of sandy grassland.

Effects of Elevated CO_2 Concentration on the Biomasses and Nitrogen Concentrations in the Organs of Sainfoin(Onobrychis viciaefolia Scop.)

In forage grasses, the nitrogen concentration is directly related to the nutritional value. The studies examined the hypothesis that global elevation of CO2 concentration probably affects the biomass, nitrogen （N） concentration, and allocation and distribution patterns in the organs of forage grasses. While sainfoin （Onobrychis viciaefolia Scop.） seedlings grew on a low nutrient soil in closed chambers for 90 days, they were exposed to two CO2 concentrations （ambient or ambient＋350 μmol mol^-1 CO2） without adding nutrients to them. After 90 days exposure to CO2, the biomasses of leaves, stems, and roots, and N concentrations and contents of different parts were measured. Compared with the ambient CO2 concentration, the elevated CO2 concentration increased the total dry matter by 25.07%, mainly due to the root and leaf having positive response to the elevated CO2 concentration. However, the elevated CO2 concentration did not change the proportions of the dry matters in different parts and the total plants compared with the ambient CO2 concentration. The elevated CO2 concentration lowered the N concentrations of the plant parts. Because the dry matter was higher, the elevated CO2 concentration had no effect on the N content in the plants compared to the ambient CO2 concentration. The elevated CO2 concentration promoted N allocations of the different parts significantly and increased N allocation of the underground part. The results have confirmed the previous suggestions that the elevated CO2 concentration stimulates plant biomass production and decreases the N concentrations of the plant parts.