Effects of Different Land Cover Types on Soil Microbial Biomass Carbon and Nitrogen in the Lower Reaches of Niyang River

[Objectives]To comprehensively and deeply explore the effects of different land cover types in the lower reaches of Niyang River on soil microbial biomass carbon and nitrogen,and to provide a scientific basis for the rational use and sustainable management of land resources in this area.[Methods]Taking the 3 types of land cover(cultivated land,grass land and forest land)in the lower reaches of Niyang River in Tibet as the research object,the contents,distribution characteristics and relationships of soil organic carbon,organic nitrogen,microbial biomass carbon,microbial biomass nitrogen and readily oxidizable organic carbon,and their relationships were studied in 0-10,10-20,20-40,40-60,and 60-100 cm soil depth.[Results]The soil organic carbon content of forest land was higher than that of grass land and cultivated land;the vertical change trend of soil organic carbon content decreased with the increase of depth(P<0.05),and it was mainly concentrated in the soil with a depth of 0-20 cm.The soil organic carbon content was significantly different among forest land,grass land and cultivated land(P<0.05),but there was no significant difference between cultivated land and grass land(P>0.05).The soil organic nitrogen content was significantly different among cultivated land,grass land,and forest land(P<0.05),but there was no significant difference between grass land and forest land(P>0.05).The readily oxidizable organic carbon,microbial biomass carbon and nitrogen in forest land were higher than that in cultivated land and grass land.The change trend of soil readily oxidizable organic carbon,microbial biomass carbon and microbial biomass nitrogen was similar to the change of soil organic carbon content,showing a significant positive correlation.In addition to being subject to land cover,soil microbial biomass carbon and nitrogen content were also subject to the interaction of factors such as soil temperature,humidity,pH and vegetation types.[Conclusions]Changes in land cover significantly affect soil organic carbon and nitrogen,readily oxidizable organic carbon,microbial biomass carbon and nitrogen content.

Seasonal variation in soil microbial biomass carbon and nitrogen in an artificial sand-binding vegetation area in Shapotou, northern China

In this study, seasonal variation characteristics of surface soil microbial biomass carbon （MBC） and soil microbial biomass nitrogen （MBN） of an artificial vegetation area located in Shapotou for different time periods were studied using the chloroform fumigation method, and the results were compared with those of near-natural vegetation areas and mobile dunes. Results showed that the MBC and MBN levels in the 0-5 cm soil layer were higher in autumn than in summer and spring. As the prolongation of vegetation restoration raised the MBC and MBN levels in summer and autumn, no clear variation was found in spring. However, the MBC and MBN in 5-20 cm had no obvious seasonal variation. During summer and autumn, the variation trend of MBC and MBN in the vertical direction was shown to be 0-5 〉 5-10 〉 10-20 cm in the vegetation area, while for mobile dunes, the MBC and MBN levels increased as the depth increased. The natural vegetation area was shown to possess the highest MBC and MBN levels, and yet mobile dunes have the lowest MBC and MBN levels. MBC and MBN levels in artificial sand-binding vegetation increased with the prolongation of vegetation restoration, indicating that the succession of sand-binding vegetation will result in the ac- cumulation of soil carbon and nitrogen, as well as the restoration of soil fertility.

Soil microbial biomass carbon and nitrogen in forest ecosystems of Northeast China:a comparison between natural secondary forest and larch plantation

Aims Natural secondary forest(NSF)and larch plantation are two of the predominant forest types in Northeast China.However,how the two types of forests compare in sustaining soil quality is not well understood.This study was conducted to determine how natural secondary forest and larch plantation would differ in soil microbial biomass and soil organic matter quality.Methods Microbial biomass carbon(MBC),microbial biomass nitrogen(MBN),soil organic carbon(SOC)and total nitrogen(TN)in the 0-to 15-cm and 15-to 30-cm soil layers were investigated by making chemical and biological measurements in the montane region of eastern Liaoning Province,Northeast China,during the growing season of 2008 in stands of NSF and Larix olgensis plantation(LOP).Important Findings We found that soil MBC and MBN were significantly lower in the LOP than in the NSF.Both MBC and MBN declined significantly with increasing soil depth in the two types of stands.The ratios of MBC to SOC(MBC/SOC)and MBN to TN(MBN/TN)were also significantly lower in the LOP than in the NSF.Moreover,the values of MBC,MBC/SOC,and MBN/TN significantly varied with time and followed a similar pattern during the growing season,all with an apparent peak in summer.Our results indicate that NSF is better in sustaining soil microbial biomass and nutrients than larch plantation in the temperate Northeast China.This calls for cautions in large-scale conversions of the native forests to coniferous plantations as a forest management practice on concerns of sustaining soil productivity.

Effects of Carbon and Nitrogen Additions on Soil Microbial Biomass Carbon and Enzyme Activities Under Rice Straw Returning

The effects of different amounts of carbon and nitrogen sources on the soil microbial biomass carbon,dissolved organic carbon and related enzyme activities were studied by the simulation experiment of rice straw returning to the field,and the mechanism of the decomposition of rice straw returning to the field was discussed.Completely randomized experiment of the two factors of the three levels was designed,and a total of nine treatments of indoor soil incubation tests were conducted.Full amount of rice straw was applied to the soil in this simulation experiment and different amounts of brown sugar and urea were added in the three levels of 0(no carbon source and nitrogen source),1(low levels of carbon and nitrogen sources)and 2(high levels of carbon and nitrogen sources),respectively.The results showed that the addition of different amounts of carbon and nitrogen sources to the rice straw could increase the soil carbon content.Compared with T0N0,the microbial biomass carbon of T2N2 was increased significantly by 170.48%;the dissolved organic carbon content of T1N2 was significantly increased by 58.14%and the free humic acid carbon contents of T0N2,T1N1 and T2N0 were significantly increased by 56.16%and 45.55%and 47.80%,respectively;however,there were no significant differences among those of treatments at later incubation periods.The addition of different carbon and nitrogen sources could promote the soil enzyme activities.During the incubation period,all of the soil enzyme activities of adding sugar and urea were higher than those of T0N0 treatment.Therefore,the addition of different amounts of carbon and nitrogen sources to rice straw returning could improve soil microbial biomass carbon content,dissolved organic carbon and soil enzyme activities.

Impact of pH on Microbial Biomass Carbon and Microbial Biomass Phosphorus in Red Soils

The impact of pH changes on microbial biomass carbon (Cmic) and microbial biomass phosphorus (Pmic)were examined for 3 red soils under citrus production with different lengths of cultivation. Soil pH significantly affected Cmic and Pmic. The Cmic and Pmic changes, as a function of soil pH, appeared to follow a normal distribution with the original soil pH value at the apex and as pH increased or decreased compared to the original soil pH, Cmic and Pmic declined. Moreover, there were critical pH values at both extremes (3.0 on the acidic side and 8.0 to 8.5 on the alkaline side), beyond which most of microorganisms could never survive.The effect of pH on Cmic and Pmic was also related to the original soil pH. The higher the original soil pH was, the less Cmic or Pmic were affected by pH change. It is suggested that soil microorganisms that grow in a soil environment with a more neutral soil pH range (I.e. pH 5.5-7.5) may have a greater tolerance to pH changes than those growing in more acidic or more alkaline soil pH conditions.

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.

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 plots selected randomly in the Experimental Farm of the Chinese Academy of Tropical Agriculture Sciences located in Hainan, China, to analyse the ecological effect of rubber cultivation. The results showed that in the tropical rubber farm,soil microbial biomass C (MBC) and total organic C (TOC) were relatively low in the content but highly correlated with each other. After rubber tapping, soil MBC of mature rubber fields decreased significantly,by 55.5%, compared with immature rubber fields. Soil TOC also decreased but the difference was not significant. Ratios of MBC to TOC decreased significantly. The decreasing trend of MBC stopped at about ten years of rubber cultivation. After this period, soil MBC increased relatively while soil TOC still kept in decreasing. Soil MBC changes could be measured to predict the tendency of soil organic matter changes due to management practices in a tropical rubber farm several years before the changes in soil TOC become detectable.

Soil resource availability impacts microbial response to organic carbon and inorganic nitrogen inputs

Impacts of newly added organic carbon （C） and inorganic nitrogen （N） on the microbial utilization of soil organic matter are important in determining the future C balance of terrestrial ecosystems. We examined microbial responses to cellulose and ammonium nitrate additions in three soils with very different C and N availability. These soils included an organic soil（ 14.2% total organic C, with extremely high extractable N and low labile C）, a forest soi1（4.7% total organic C, with high labile C and extremely low extractable N）, and a grassland soil（1.6% total organic C, with low extractable N and labile C）. While cellulose addition alone significantly enhanced microbial respiration and biomass C and N in the organic and grassland soils, it accelerated only the microbial respiration in the highly-N limited forest soil. These results indicated that when N was not limited, C addition enhanced soil respiration by stimulating both microbial growth and their metabolic activity, New C inputs lead to elevated C release in all three soils, and the magnitude of the enhancement was higher in the organic and grassland soils than the forest soil. The addition of cellulose plus N to the forest and grassland soils initially increased the microbial biomass and respiration rates, but decreased the rates as time progressed. Compared to cellulose addition alone, cellulose plus N additions increased the total C-released in the grassland soil, but not in the forest soil. The enhancement of total C- released induced by C and N addition was less than 50% of the added-C in the forest soil after 96 d of incubation, in contrast to 87.5% and 89.0% in the organic and grassland soils. These results indicate that indigenous soil C and N availability substantially impacts the allocation of organic C for microbial biomass growth and/or respiration, potentially regulating the turnover rates of the new organic C inputs.

Bentonite-humic acid improves soil organic carbon,microbial biomass,enzyme activities and grain quality in a sandy soil cropped to maize(Zea mays L.) in a semi-arid region

A bentonite-humic acid(B-HA) mixture added to degraded soils may improve soil physical and hydraulic properties, due to effects such as improved soil structure and increased water and nutrient retention, but its effect on soil physicochemical and biological properties, and grain quality is largely unknown. The effect of B-HA, added at 30 Mg ha^(-1), was studied at 1,3, 5 and 7 years after its addition to a degraded sandy soil in a semi-arid region of China. The addition of B-HA significantly increased water-filled pore space and soil organic carbon, especially at 3 to 5 years after its soil addition to the soil. Amending the sandy soil with B-HA also increased the content of microbial biomass(MB)-carbon,-nitrogen and-phosphorus, and the activities of urease, invertase, catalase and alkaline phosphatase. The significant effect of maize(Zea mays L.) growth stage on soil MB and enzyme activities accounted for 58 and 84% of their total variation, respectively. In comparison, B-HA accounted for 8% of the total variability for each of the same two variables. B-HA significantly enhanced soil properties and the uptake of N and P by maize in semi-arid areas. The use of B-HA product would be an effective management strategy to reclaim degraded sandy soils and foster sustainable agriculture production in northeast China and regions of the world with similar soils and climate.

Land Use Effects on Soil Organic Carbon, Microbial Biomass and Microbial Activity in Changbai Mountains of Northeast China

Land use changes are known to alter soil organic carbon(SOC) and microbial properties, however, information about how conversion of natural forest to agricultural land use as well as plantations affects SOC and microbial properties in the Changbai Mountains of Northeast China is meager. Soil carbon content, microbial biomass carbon(MBC), basal respiration and soil carbon mineralization were studied in five selected types of land use: natural old-growth broad-leaved Korean pine mixed forest(NF); spruce plantation(SP) established following clear-cutting of NF; cropland(CL); ginseng farmland(GF) previously under NF; and a five-year Mongolian oak young forest(YF) reforested on an abandoned GF, in the Changbai Mountains of Northeast China in 2011. Results showed that SOC content was significantly lower in SP, CL, GF, and YF than in NF. MBC ranged from 304.4 mg/kg in CL to 1350.3 mg/kg in NF, which was significantly higher in the soil of NF than any soil of the other four land use types. The SOC and MBC contents were higher in SP soil than in CL, GF, and YF soils, yielding a significant difference between SP and CL. The value of basal respiration was also higher in NF than in SP, CL, GF, and YF. Simultaneously, higher values of the metabolic quotient were detected in CL, GF, and YF soils, indicating low substrate utilization of the soil microbial community compared with that in NF and SP soil. The values of cumulative mineralized carbon and potentially mineralized carbon(C0) in NF were significantly higher than those in CL and GF, while no significant difference was observed between NF and SP. In addition, YF had higher values of C0 and C mineralization rate compared with GF. The results indicate that conversion from NF into agricultural land(CL and GF) uses and plantation may lead to a reduction in soil nutrients(SOC and MBC) and substrate utilization efficiency of the microbial community. By contrast, soils below SP were more conducive to the preservation of soil organic matter, which was reflected in the comparison of microbial indicators among CL, GF, and YF land uses. This study can provide data for evaluating soils nutrients under different land use types, and serve as references for the rational land use of natural forest in the study area.

Assessment of soil quality using soil organic carbon and total nitrogen and microbial properties in tropical agroecosystems

Assessment of soil quality is an invaluable tool in determining the sustainability and environmental impact of agricultural ecosystems. The study was conducted to assess the quality of the soils under arable cultivation, locally irri-gated and non-irrigated, forestry plantations of teak (Tectona grandis Lin.) and gmelina (Gme- lina arborea Roxb.), and cashew (Anacardium occidentale Lin.) plantation agro ecosystems using soil organic carbon (SOC), soil total ni-trogen (STN) and soil microbial biomass C (SMBC) and N (SMBN) at Minna in the southern Guinea savanna of Nigeria. Soil samples were collected from soil depths of 0-5 cm and 5-10 cm in all the agro ecosystems and analyzed for physical, chemical and biological properties. All the agro ecosystems had similar loamy soil texture at both depths. The soils have high fer-tility status in terms of available phosphorus and exchangeable calcium, magnesium and po- tassium. The irrigated arable land had significantly (P 6.6 suggesting fungal domination in all the agroecosystems. The forestry plantation soils had higher SMBC and SMBN as a per-centage of SOC and STN respectively than the cultivated arable land soils. Burning for clearing vegetation and poor stocking of forestry planta-tions may impair the quality of the soil. The study suggests that the locally irrigated agro- ecosystem soil seems to be of better quality than the other agroecosystem soils.

Relations between the Underground Biomass and Soil Organic Carbon and Nitrogen of the Alpine Meadow at the Eastern Margin of the Qinghai-Tibet Plateau

This article, by combining field investigation with laboratorial analysis, studies diverse alpine meadow at the Eastern Margin of the Qinghai-Tibet Plateau for the underground biomass dynamics, vertical distribution of the content of soil carbon and nitrogen, the connection between the biomass and the content of carbon and nitrogen. The studies show that underground biomass in the herb layer of upland meadow is more than that in the terrace meadow, while underground biomass in the upland shrubland is the most. The vertical distribution of underground biomass of each type is obvious as in shape of＂T＂. As to the distribution of the content of soil organic carbon in the three sample grounds, it showed that the deeper the soil the less the content of soil organic carbon. In May, unlike at terrace meadow, the underground biomass and the content of soil organic carbon in positive proportion, such revelation at upland meadow and upland shrubland is not apparent. In July, at upland meadow and terrace meadow the underground biomass and the content of soil total nitrogen in positive proportion, such revelation at upland shrubland is not apparent either.

Effects of Soil Salinity on Microbial Biomass Nitrogen of Landscape Soil

Soil salinization can limit the development of agriculture in the Yellow River Delta.In this paper,saline and alkaline farmland in the Yellow River Delta was chosen as the research object,and effects of soil salinity on soil microbial biomass nitrogen(SMBN) under different conditions were investigated to study the response of soil nitrogen turnover to salt stress.There were four salinity gradients(S1:0.1%;S2:0.5%;S3:0.9%;S4:1.3%),and four substrates were added to the soil.The results showed that after the addition of various substrates,SMBN in treatments with high soil salinity(S3 and S4) was obviously lower than that in treatments with low soil salinity(S1 and S2).In comparison with treatment S1,the average of SMBN in treatments S3 and S4 decreased by 35.8% and 46.7% respectively when there was no substrate added to them;it declined by 55.6% and 56.1% respectively as the carbon source was added to them;it reduced by 24.6% and 28.3% when the nitrogen source was added to them;it dropped by 43.8% and 57.0% respectively as the carbon and nitrogen source were added to them.Compared with treatments without substrates,the addition of the nitrogen source could not improve SMBN;the addition of the carbon source or carbon and nitrogen source could enhance SMBN obviously,and it increased by 60.9% and 66.1%(or 110.8% and 140.2%) in treatments with low soil salinity(S1 and S2),while it changed slightly in treatments with high soil salinity(S3 and S4).In order to increase SMBN,it is needed to apply organic fertilizer or chemical fertilizer and organic fertilizer to maintain or improve soil fertility.

Role of Biochar Amendment on Soil Carbon Mineralization and Microbial Biomass

To understand the influence of biochar properties (pyrolysis temperature and types) on soil physicochemical properties, we investigated the changes of soil organic carbon mineralization, nutrient contents and microbial biomass after 135 d incubation. Results showed that both corn straw (CB) and rice straw (RB) derived biochars increase the mineralization of organic carbon and nitrogen in the soil, and these biochars pyrolysised at 500?C (CB500, RB500) significantly enhanced the mineralization of soil organic nitrogen. In comparison with control treatment, the application of biochar significantly increased the contents of soil organic carbon, available P and K in soil. Moreover, the activity of soil microbe was enhanced with biochar amendment. Among all treatments, RB500 significantly increased the content of soil microbial biomass carbon (379 ± 9 mg?kg?1) in soil. Our results suggested that the application of biochars to soil improve soil quality, while the biochar type and pyrolysis temperature should be taken into consideration before its application in agro-ecosystem.

Changes in the activities of key enzymes and the abundance of functional genes involved in nitrogen transformation in rice rhizosphere soil under different aerated conditions

Soil microorganisms play important roles in nitrogen transformation. The aim of this study was to characterize changes in the activity of nitrogen transformation enzymes and the abundance of nitrogen function genes in rhizosphere soil aerated using three different methods(continuous flooding(CF), continuous flooding and aeration(CFA), and alternate wetting and drying(AWD)). The abundances of amoA ammonia-oxidizing archaea(AOA) and ammonia-oxidizing bacteria(AOB), nirS, nirK, and nifH genes, and the activities of urease, protease, ammonia oxidase, nitrate reductase, and nitrite reductase were measured at the tillering(S1), heading(S2), and ripening(S3) stages. We analyzed the relationships of the aforementioned microbial activity indices, in addition to soil microbial biomass carbon(MBC) and soil microbial biomass nitrogen(MBN), with the concentration of soil nitrate and ammonium nitrogen. The abundance of nitrogen function genes and the activities of nitrogen invertase in rice rhizosphere soil were higher at S2 compared with S1 and S3 in all treatments. AWD and CFA increased the abundance of amoA and nifH genes, and the activities of urease, protease, and ammonia oxidase, and decreased the abundance of nirS and nirK genes and the activities of nitrate reductase and nitrite reductase, with the effect of AWD being particularly strong. During the entire growth period, the mean abundances of the AOA amoA, AOB amoA, and nifH genes were 2.9, 5.8, and 3.0 higher in the AWD treatment than in the CF treatment, respectively, and the activities of urease, protease, and ammonia oxidase were 1.1, 0.5, and 0.7 higher in the AWD treatment than in the CF treatment, respectively. The abundances of the nirS and nirK genes, and the activities of nitrate reductase and nitrite reductase were 73.6, 84.8, 10.3 and 36.5% lower in the AWD treatment than in the CF treatment, respectively. The abundances of the AOA amoA, AOB amoA, and nifH genes were significantly and positively correlated with the activities of urease, protease, and ammonia oxidase, and the abundances of the nirS and nirK genes were significantly positively correlated with the activities of nitrate reductase. All the above indicators were positively correlated with soil MBC and MBN. In sum, microbial activity related to nitrogen transformation in rice rhizosphere soil was highest at S2. Aeration can effectively increase the activity of most nitrogen-converting microorganisms and MBN, and thus promote soil nitrogen transformation.

Legume Green Manure and Intercropping for High Biomass Sorghum Production

Before the advent of cheap, synthetic fertilizers, legumes were commonly used as green manure crops for their ability to fix atmospheric nitrogen (N). A three-year study at Overton, TX examined legume integration into high-biomass sorghum (Sorghum bicolor L.) production systems on a Lilbert loamy fine sand recently cultivated after a fertilized bermudagrass [Cynodon dactylon (L.) Pers.] pasture. In this split-split plot design, ‘Dixie’ crimson clover (Trifolium incarnatum L.) and ‘Iron and Clay’ cowpea (Vigna unguiculata L.) were integrated into a high-biomass sorghum production system to evaluate impacts on N concentration, C concentration, and yield of high-biomass sorghum and their impacts on soil total N and soil organic carbon (SOC). Main plots were split into crimson clover green manure (CLGM) and winter fallow (FALL) followed by three sub-plots split into warm-season crop rotations: cowpea green manure (CPGM), cowpea-sorghum intercrop (CPSR), and sorghum monocrop (SORG). Three N fertilizer treatments (0, 45, 90 kg N∙ha−1) were randomized and applied as sub-sub plots. The CLGM increased (P sorghum biomass yield (16.5 t DM∙ha−1) 28% in year three but had no effect in the first two years. The CPSR treatment reduced sorghum yield up to 62% compared to SORG;whereas CPGM increased sorghum yield 56% and 18% the two years following cowpea incorporation. Rate of N fertilizer had no effect on sorghum biomass yield. Decrease in SOC and soil N over time indicated mineralization of organic N and may explain why no N fertilizer response was observed in sorghum biomass yield. Cowpea showed strong potential as a green manure crop but proved to be too competitive for successful intercropping in high-biomass sorghum production systems.

Soil microbial biomass and its controls

Microbial biomass represents a relatively small standing stock of nutrients, compared to soil organic matter, but it can act as a labile source of nutrients for plants, a pathway for incorporation of organic matter into the soil, and a temporary sink for nutrients. This review describes several factors controlling the dynamics of soil microbial biomass. These factors mainly include organic carbon and nitrogen limitation, residue and nutrient management, differences in plant species, soil texture, soil moisture and temperature. On the basis of detailed analysis, it is reasonable that future research would be focused on the impact of land use change on soil MB in tropical and subtropical ecosystems.

Seasonal dynamics of soil microbial biomass C and N of Keteleeria fortunei var. cyclolepis forests with different ages

Soil microbial biomass is an important indicator to measure the dynamic changes of soil carbon pool.It is of great signifi cance to understand the dynamics of soil microbial biomass in plantation for rational management and cultivation of plantation.In order to explore the temporal dynamics and infl uencing factors of soil microbial biomass of Keteleeria fortunei var.cyclolepis at diff erent stand ages,the plantation of diff erent ages(young forest,5 years;middle-aged forest,22 years;mature forest,40 years)at the Guangxi Daguishan forest station of China were studied to examine the seasonal variation of their microbial biomass carbon(MBC)and microbial biomass nitrogen(MBN)by chloroform fumigation extraction method.It was found that among the forests of diff erent age,MBC and MBN diff ered signifi cantly in the 0–10 cm soil layer,and MBN diff ered signifi cantly in the 10–20 cm soil layer,but there was no signifi cant diff erence in MBC for the 10–20 cm soil layer or in either MBC or MBN for the 20–40 cm soil layer.With increasing maturity of the forest,MBC gradually decreased in the 0–10 cm soil layer and increased fi rstly and then decreased in the 10–20 cm and 20–40 cm soil layers,and MBN increased fi rstly and then decreased in all three soil layers.As the soil depth increased,both MBC and MBN gradually decreased for all three forests.The MBC and MBN basically had the same seasonal variation in all three soil layers of all three forests,i.e.,high in the summer and low in the winter.Correlation analysis showed that MBC was signifi cantly positively correlated with soil organic matter,total nitrogen,and soil moisture,whereas MBN was signifi cantly positively correlated with soil total nitrogen.It showed that soil moisture content was the main factor determining the variation of soil microbial biomass by Redundancy analysis.The results showed that the soil properties changed continuously as the young forest grew into the middle-aged forest,which increased soil microbial biomass and enriched the soil nutrients.However,the soil microbial biomass declined as the middle-age forest continued to grow,and the soil nutrients were reduced in the mature forest.

Effects of continuous nitrogen addition on microbial properties and soil organic matter in a Larix gmelinii plantation in China

Continuous increases in anthropogenic nitrogen（N） deposition are likely to change soil microbial properties, and ultimately to affect soil carbon（C） storage.Temperate plantation forests play key roles in C sequestration, yet mechanisms underlying the influences of N deposition on soil organic matter accumulation are poorly understood. This study assessed the effect of N addition on soil microbial properties and soil organic matter distribution in a larch（Larix gmelinii） plantation. In a 9-year experiment in the plantation, N was applied at100 kg N ha-1 a-1 to study the effects on soil C and N mineralization, microbial biomass, enzyme activity, and C and N in soil organic matter density fractions, and organic matter chemistry. The results showed that N addition had no influence on C and N contents in whole soil. However,soil C in different fractions responded to N addition differently. Soil C in light fractions did not change with N addition, while soil C in heavy fractions increased significantly. These results suggested that more soil C in heavy fractions was stabilized in the N-treated soils. However,microbial biomass C and N and phenol oxidase activity decreased in the N-treated soils and thus soil C increased in heavy fractions. Although N addition reduced microbial biomass and phenol oxidase activity, it had little effect on soil C mineralization, hydrolytic enzyme activities, d13 C value in soil and C–H stretch, carboxylates and amides, and C–O stretch in soil organic matter chemistry measured by Fourier transform infrared spectra. We conclude that N addition（1） altered microbial biomass and activity without affecting soil C in light fractions and（2） resulted in an increase in soil C in heavy fractions and that this increase was controlled by phenol oxidase activity and soil N availability.

Effects of Grazing Exclusion on Plant Productivity and Soil Carbon, Nitrogen Storage in Alpine Meadows in Northern Tibet, China

Grazing exclusion is widely adopted in restoring degraded alpine grasslands on the Qinghai-Tibetan Plateau. However, its effectiveness remains poorly understood. In this study, we investigated the effects of grazing exclusion on plant productivity, species diversity and soil organic carbon(SOC) and soil total nitrogen(STN) storage along a transect spanning from east to west of alpine meadows in northern Tibet, China. After six years of grazing exclusion, plant cover, aboveground biomass(AGB), belowground biomass(BGB), SOC and STN were increased, but species diversity indices declined. The enhancement of AGB and SOC caused by grazing exclusion was correlated positively with mean annual precipitation(MAP). Grazing exclusion led to remarkable biomass increase of sedge species, especially Kobresia pygmaea, whereas decrease of biomass in forbs and no obvious change in grass, leguminous and noxious species. Root biomass was concentrated in the near surface layer(10 cm) after grazing exclusion. The effects of grazing exclusion on SOC storage were confined to shallow soil layer in sites with lower MAP. It is indicated that grazing exclusion is an effective measure to increase forage production and enhance soil carbon sequestration in the studied region. The effect is more efficient in sites with higher precipitation. However, the results revealed a tradeoff between vegetation restoration and ecological biodiversity. Therefore, carbon pools recover more quickly than plant biodiversity in the alpine meadows. We suggest that grazing exclusion should be combined with other measures to reconcile grassland restoration and biodiversity conservation.