Labile,recalcitrant, microbial carbon and nitrogen and the microbial community composition at two Abies faxoniana forest elevations under elevated temperatures

We investigated the interactions of altitude and artificial warming on the soil microbial community structure in a subalpine Abies faxoniana forest in southwestern China after four years of warming. Open top chambers (OTCs) at two elevations (3000 m and 3500 m) were established, and their soil microbial characteristics, organic carbon (C) and nitrogen (N) were measured. The microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis. A two-step sulfuric acid hydrolysis was used to quantify the labile and recalcitrant C fractions in the soil organic matter. The results showed that bacterial PLFAs and gram-negative bacterial PLFAs increased and the fungal PLFAs and the fungi/bacteria ratio decreased with warming at the high altitude. By contrast, the warming effects on those parameters at low altitude were small. The higher proportion of labile easily decomposable soil C may explain the different responses of the microbial community composition at the two altitudes. An RDA analysis confirmed that the variations in the soil community structure were significantly associated with soil organic matter properties such as the sizes of the soil labile N pool (LP-N), the recalcitrant N pool (RP-N), and the labile C pool as well as dissolved organic C (DOC) and dissolved organic N concentrations (DON). Our results also showed that labile C and N pools increased with the altitude, but the microbial biomass C as measured with chloroform fumigation techniques decreased. Warming increased only the recalcitrant C pools at the high altitude. Given the longer mean residence time for recalcitrant C and the much greater size of this soil organic carbon pool, the results indicated that a rise in temperature in our case increased soil C pools at higher altitudes, at least during the early stages of experimental soil warming. Warming could also cause changes in the composition of the microbial community and enzyme activities, consequently leading to functional changes in soil ecosystem processes at the high altitude.     

不同培肥措施下种植制度及撂荒对土壤微生物量碳氮的影响

以黄土高原南部半湿润易旱区已进行17年的田间定位试验为研究对象,研究了不同培肥措施(不施肥、施用氮磷钾及氮磷钾与有机肥配合施用)下两种种植制度(一年1熟及一年两熟)和撂荒对土壤微生物量碳、氮(SMBC、SMBN)及可溶性有机碳、氮(SOC、SON)等含量的影响.结果表明,与一年1熟的小麦一休闲种植制度相比,一年两熟小麦一玉米轮作提高了0～10 cm土层SMBC、SMBN、有机碳(TOC)、全氮(TN)和土壤SOC、SON的含量,而对10～20 cm土层上述测定指标影响不大.与不施肥(CK)或单施化肥处理(NPK)下小麦-休闲和小麦-玉米轮作方式相比,撂荒处理显著提高了0～10 cm土层各测定指标的含量.不同培肥措施相比,氮磷钾配施有机肥显著提高了0～10 cm、10～20 cm土层SMBC、SMBN含量;NPK处理0～10 cm土层SMBN含量显著增加,10～20 cm土层SMBN和0～10 cm、10～20 cm土层SMBC含量增加但未达显著水平.不同培肥措施和种植制度对SMBC/TOC和SMBN/TN的比例无明显影响.     

Effects of litter addition and warming on soil carbon, nutrient pools and microbial communities in a subarctic heath ecosystem

Climatic warming leads to the expansion of deciduous shrubs and trees in the Arctic. This leads to higher leaf litter inputs, which together with warming may alter the rate of carbon and nutrient cycling in the arctic ecosystems. We assessed effects of factorial warming and additional litter on the soil ecosystem of a subarctic heath in a 7-year-long field experiment. Fine root biomass, dissolved organic carbon (DOC) and total C concentration increased in response to warming, which probably was a result of the increased vegetation cover. Litter addition increased the concentration of inorganic P in the uppermost 5 cm soil, while decreasing the pool of total P per unit area of the organic profile and having no significant effects on N concentrations or pools. Microbial biomass C and N were unaffected by the treatments, while the microbial biomass P increased significantly with litter addition. Soil ergosterol concentration was also slightly increased by the added litter in the uppermost soil, although not statistically significantly. According to a principal component analysis of the phospholipid fatty acid profiles, litter addition differed from the other treatments by increasing the relative proportion of biomarkers for Gram-positive bacteria. The combined warming plus litter addition treatment decreased the soil water content in the uppermost 5 cm soil, which was a likely reason for many interactions between the effects of warming and litter addition. The soil organic matter quality of the combined treatment was also clearly different from the control based on a near-infrared reflectance (NIR) spectroscopic analysis, implying that the treatment altered the composition of soil organic matter. However, it appears that the biological processes and the microbial community composition responded more to the soil and litter moisture conditions than to the change in the quality of the organic matter.     

Soil microbial biomass and nitrogen dynamics in a turfgrass chronosequence: A short-term response to turfgrass clipping addition

A mechanistic understanding of soil microbial biomass and N dynamics following turfgrass clipping addition is central to understanding turfgrass ecology. New leaves represent a strong sink for soil and fertilizer N, and when mowed, a significant addition to soil organic N. Understanding the mineralization dynamics of clipping N should help in developing strategies to minimize N losses via leaching and denitrification. We characterized soil microbial biomass and N mineralization and immobilization turnover in response to clipping addition in a turfgrass chronosequence (i.e. 3, 8, 25, and 97 yr old) and the adjacent native pines. Our objectives were (1) to evaluate the impacts of indigenous soil and microbial attributes associated with turf age and land use on the early phase decomposition of turfgrass clippings and (2) to estimate mineralization dynamics of turfgrass clippings and subsequent effects on N mineralization of indigenous soils. We conducted a 28-d laboratory incubation to determine short-term dynamics of soil microbial biomass, C decomposition, N mineralization and nitrification after soil incorporation of turfgrass clippings. Gross rates of N mineralization and immobilization were estimated with ¹⁵N using a numerical model, FLAUZ. Turfgrass clippings decomposed rapidly; decomposition and mineralization equivalent to 20-30% of clipping C and N, respectively, occurred during the incubation. Turfgrass age had little effect on decomposition and net N mineralization. However, the response of potential nitrification to clipping addition was age dependent. In young turfgrass systems having low rates, potential nitrification increased significantly with clipping addition. In contrast, old turfgrass systems having high initial rates of potential nitrification were unaffected by clipping addition. Isotope ¹⁵N modeling showed that gross N mineralization following clipping addition was not affected by turf age but differed between turfgrass and the adjacent native pines. The flush of mineralized N following clipping addition was derived predominantly from the clippings rather than soil organic N. Our data indicate that the response of soil microbial biomass and N mineralization and immobilization to clipping addition was essentially independent of indigenous soil and microbial attributes. Further, increases in microbial biomass and activity following clipping addition did not stimulate the mineralization of indigenous soil organic N.     

长期施肥对土壤微生物生物量碳、氮及矿质态氮含量动态变化的影响

利用位于陕西杨凌的17年长期定位试验研究了长期不施肥(CK)、单施化肥(F)、化肥配施有机肥(F+M)和化肥加秸秆还田(F+S)处理对小麦-玉米轮作体系中作物不同生长时期土壤微生物生物量碳、氮(SMBC、SMBN)和矿质态氮含量的影响。结果表明,0—10 cm土层土壤SMBC、SMBN和矿质态氮含量的变化范围分别为264.8~752.2、37.51~14.8和3.83~8.5 mg/kg。不同处理相比,F+M处理中各采样时期(小麦苗期、拔节期、灌浆期及玉米播种期、大喇叭口期、灌浆期和收获后)土壤SMBC和SMBN含量均为最高,分别为不施肥对照的1.382~.65和1.892~.50倍;F+S处理矿质态氮含量最高,SMBC和SMBN也高于F和CK处理,大部分采样时期的差异达显著水平(P0.05);与CK相比,长期单施化肥也使各时期SMBC和SMBN含量提高。在小麦拔节期到灌浆期的旺盛生长阶段各施肥处理土壤SMBN含量均下降,而矿质态氮含量变化不大,处于较低水平;在玉米大喇叭口期到灌浆期的旺盛生长阶段,F+M、F+S和F处理土壤矿质态氮含量显著下降,而SMBN含量均有所升高。表明在土壤矿质态氮含量较高时,作物首先利用矿质态氮,而在土壤矿质态氮含量处于较低水平时,微生物固持的氮素可能会释放出来供作物吸收利用。     

Immobilization and mineralization of nitrogen in a saline and alkaline soil during microbial use of sugarcane filter cake amended with glucose

A 42-day incubation was conducted to study the effect of glucose and ammonium addition adjusted to a C/N ratio of 12.5 on sugarcane filter cake decomposition and on the release of inorganic N from microbial residues formed initially. The CO₂ evolved increased in comparison with the non-amended control from 35% of the added C with pure +5 mg g⁻¹ soil filter cake amendment to 41% with +5 mg g⁻¹ soil filter cake +2.5 mg g⁻¹ soil glucose amendment to 48% with 5 mg g⁻¹ soil filter cake +5 mg g⁻¹ soil glucose amendment. The different amendments increased microbial biomass C and microbial biomass N within 6 h and such an increase persisted. The fungal cell-membrane component ergosterol initially showed a disproportionate increase in relation to microbial biomass C, which completely disappeared by the end of the incubation. The cellulase activity showed a 5-fold increase after filter cake addition, which was not further increased by the additional glucose amendment. The cellulase activity showed an exponential decline to values around 4% of the initial value in all treatments. The amount of inorganic N immobilized from day 0 to day 14 increased with increasing amount of C added, in contrast to the control treatment. After day 14, the immobilized N was re-mineralized at rates between 1.3 and 1.5 μg N g⁻¹ soil d⁻¹ in the treatments being more than twice as high as in the control treatment. This means that the re-mineralization rate is independent of the actual size of the microbial residues pool and also independent of the size of the soil microbial biomass.     

A meta-analysis of the effects of experimental warming on soil carbon and nitrogen dynamics on the Tibetan Plateau

Alpine ecosystems at high altitudes and latitudes are notably sensitive to climatic warming and the Tibetan Plateau is a widely distributed alpine ecosystem. The magnitude of climatic warming on the Tibetan Plateau is expected to be considerably greater than the global average. However, a synthesis of the experimental warming soil carbon and nitrogen data is still lacking and whether forest soils are more sensitive to warming than grassland soils remains unclear. In this study, we used a meta-analysis approach to synthesise 196 observations from 25 published studies on the Tibetan Plateau. Warming significantly increased microbial biomass carbon (MBC) by 14.3% (95% CI: 2.9–24.6%), microbial biomass nitrogen (MBN) by 20.1% (95% CI: 2.0–45.1%), net nitrogen mineralization by 49.2% (95% CI: 38.1–62.3%) and net nitrification by 56.0% (95% CI: 51.4–66.1%), but did not significantly affect soil carbon (95% CI: −13.9 to 2.7%) or nitrogen (95% CI: −12.4 to 2.6%). The mean annual air temperature was negatively correlated with the warming effects on MBC and MBN. Grasslands exhibited significant MBC and MBN responses to warming. Specifically, soil microbial biomass was more responsive to warming in colder environments. Moreover, forest soils are not always more sensitive to warming than grassland soils as previous studies have suggested. These findings indicate that clarifying the effect of warming on alpine soils need consider ecosystem types and their local climate.     

Responses of microbial biomass and respiration of soil to topography,burning, and nitrogen fertilization in a temperate steppe

Temporal dynamics of microbial biomass and respiration of soil and their responses to topography, burning, N fertilization, and their interactions were determined in a temperate steppe in northern China. Soil microbial indices showed strong temporal variability over the growing season. Soil microbial biomass C (MBC) and N (MBN) were 14.8 and 11.5% greater in the lower than upper slope, respectively. However, the percentage of organic C present as MBC and the percentage of total N present as MBN were 16.9 and 26.2% higher in the upper than lower slope, respectively. Neither microbial respiration (MR) nor metabolic quotient (qCO₂) was affected by topography. Both MBC and MBN were increased by burning, on average, by 29.8 and 14.2% over the growing season, and MR and qCO₂ tended to reduce depending on the sampling date, especially in August. Burning stimulated the percentage of organic C present as MBC and the percentage of total N present as MBN in the upper slope, but did not change these two parameters in the lower slope. No effects of N fertilization on soil microbial indices were observed in the first growing season after the treatment. Further research is needed to study the long-term relationships between changes in soil microbial diversity and activity and plant community in response to burning and N fertilization.     

Soil microbial biomass, activity and nitrogen transformations in a turfgrass chronosequence

Understanding the chronological changes in soil microbial properties of turfgrass ecosystems is important from both the ecological and management perspectives. We examined soil microbial biomass, activity and N transformations in a chronosequence of turfgrass systems (i.e. 1, 6, 23 and 95 yr golf courses) and assessed soil microbial properties in turfgrass systems against those in adjacent native pines. We observed age-associated changes in soil microbial biomass, CO₂ respiration, net and gross N mineralization, and nitrification potential. Changes were more evident in soil samples collected from 0 to 5 cm than the 5 to 15 cm soil depth. While microbial biomass, activity and N transformations per unit soil weight were similar between the youngest turfgrass system and the adjacent native pines, microbial biomass C and N were approximately six times greater in the oldest turfgrass system compared to the adjacent native pines. Potential C and N mineralization also increased with turfgrass age and were three to four times greater in the oldest vs. the youngest turfgrass system. However, microbial biomass and potential mineralization per unit soil C or N decreased with turfgrass age. These reductions were accompanied by increases in microbial C and N use efficiency, as indicated by the significant reduction in microbial C quotient (qCO₂) and N quotient (qN) in older turfgrass systems. Independent of turfgrass age, microbial biomass N turnover was rapid, averaging approximately 3 weeks. Similarly, net N mineralization was ∼12% of gross mineralization regardless of turfgrass age. Our results indicate that soil microbial properties are not negatively affected by long-term management practices in turfgrass systems. A tight coupling between N mineralization and immobilization could be sustained in mature turfgrass systems due to its increased microbial C and N use efficiency.     

Effect of bamboo harvest on dynamics of nutrient pools,N mineralization,and microbial biomass in soil

The effect of harvesting bamboo savanna on the dynamics of soil nutrient pools, N mineralization, and microbial biomass was examined. In the unharvested bamboo site NO _inf3 ^sup- -N in soil ranged from 0.37 to 3.11 mg kg^-1 soil and in the harvested site from 0.43 to 3.67 mg kg^-1. NaHCO₃-extractable inorganic P ranged from 0.55 to 3.58 mg kg^-1 in the unharvested site and from 1.01 to 4.22 mg kg^-1 in the harvested site. Over two annual cycles, the N mineralization range in the unharvested and harvested sites was 0–19.28 and 0–24.0 mg kg^-1 soil month^-1, respectively. The microbial C, N, and P ranges were 278–587, 28–64, and 12–26 mg kg^-1 soil, respectively, with the harvested site exhibiting higher values. Bamboo harvesting depleted soil organic C by 13% and total N by 20%. Harvesting increased N mineralization, resulting in 10 kg ha^-1 additional mineral N in the first 1st year and 5 kg ha^-1 in the 2nd year following the harvest. Microbial biomass C, N and P increased respectively by 10, 18, and 5% as a result of bamboo harvesting.     

Effect of cultivation on microbial carbon and nitrogen in dry tropical forest soil

Summary Fifteen- and forty-year-old cropfields developed from a dry tropical forest were examined for soil organic C and total N and soil microbial C and N. The 15-year-old field had never been manured while the 40-year-old field had been fertilized with farmyard manure every year. The native forest soil was also examined. The results indicated that the native forest soil lost about 57% and 62% organic C and total N, respectively, in the 0–10 cm layer after 15 years of cultivation. The microbial C and N contents of the forest soil were greater than those of the cultivated soils. Application of farmyard manure increased the biomass-C and -N levels in the cultivated soil but the values were still markedly lower than in the forest soil. There was an appreciable seasonal variation in biomass C and N, the values being highest in summer and lowest in the rainy season. During an annual cycle, biomass-C contents varied from 180 to 727 g g^–1 and N from 20 to 80 g g^–1 dry soil, and both were linearly related. Microbial biomass C represented 1.6%–3.6% of total soil organic C and microbial biomass N represented 1.7% 1–4.4% of soil organic N.     

A comparison of soil and microbial carbon,nitrogen, and phosphorus contents,and macro-aggregate stability of a soil under native forest and after clearance for pastures and plantation forest

Total, extractable, and microbial C, N, and P, soil respiration, and the water stability of soil aggregates in the F-H layer and top 20 cm of soil of a New Zealand yellow-brown earth (Typic Dystrochrept) were compared under long-term indigenous native forest (Nothofagus truncata), exotic forest (Pinus radiata), unfertilized and fertilized grass/clover pastures, and gorse scrub (Ulex europaeus). Microbial biomass C ranged from 1100 kg ha^-1 (exotic forest) to 1310kg ha^-1 (gorse scrub), and comprised 1–2% of the organic C. Microbial N and P comprised 138–282 and 69–119 kg ha^-1 respectively, with the highest values found under pasture. Microbial N and P comprised 1.8–7.0 and 4.9–18% of total N and P in the topsoils, and 1.8–4.4 and 23–32%, respectively, in the F-H material. Organic C and N were higher under gorse scrub than other vegetation. Total and extractable P were highest under fertilized pasture. Annual fluxes through the soil microbial biomass were estimated to be 36–85 kg N ha^-1 and 18–36 kg P ha^-1, sufficiently large to make a substantial contribution to plant requirements. Differences in macro-aggregate stability were generally small. The current status of this soil several years after the establishment of exotic forestry, pastoral farming, or subsequent reversion to scrubland is that, compared to levels under native forest, there has been no decline in soil and microbial C, N, and P contents or macro-aggregate stability.     

Remnant tree effects on soil microbial carbon and nitrogen in tropical seasonal pasture in western Mexico

Selection of plant species for agro-silvo-pastoral or ecological reclamation programs must be based on a deeper knowledge of the existing relationships between plant species and soil nutrient dynamics in each ecosystem. We evaluated the seasonal pattern of soil microbial carbon (C) and nitrogen (N) under two remnant tree species (Caesalpinia eriostachys and Cordia elaeagnoides) in a tropical seasonal pasture dominated by Panicum maximum in western Mexico. Soil samples were taken from under two arboreal species and P. maximum in rainy and dry seasons. The soil C:N ratio was higher under P. maximum [17] than under both tree species [15]. The soil microbial C (Cm) was higher under C. elaeagnoides than under C. eriostachys and P. maximum. Magnitude and direction of effect of the two remnant tree species on soil biogeochemistry changed with seasonal rainfall. The interaction of plant species and seasonal rainfall did have an effect on soil microbial N (Nm). Soil samples from April and July had the lowest microbial N concentrations under the three plant species, increasing four fold in September under C. elaeagnoides and P. maximum. At the end of the wet season, C. elaeagnoides clearly had the highest Nm values (130 μg N g⁻¹), suggesting that this tree species has a higher capacity to protect soil N within microbial biomass than C. eriostachys, because under C. elaeagnoides the soil had more organic matter due a higher input of litter and root chemical quality. Therefore, C. elaeagnoides would be the best plant species to implement in agro-silvo-pastoral programs or ecological reclamation of TDF pastures.     

Spatial patterns of soil biological and physical properties in a ridge tilled and a ploughed Luvisol

The present study was conducted to determine the spatial heterogeneity of bulk density, soil moisture, inorganic N, microbial biomass C, and microbial biomass N in the ridge tillage system of Turiel compared to conventional mouldboard ploughing on three sampling dates in May, July, and August. The soil sampling was carried out under vegetation representing the ridge in a high spatial resolution down the soil profile. Bulk density increased with depth and ranged from 1.3 g cm⁻³ at 10 cm depth to 1.6 g cm⁻³ at 35 cm in ploughed plots and from 1.0 g m⁻³ at 5 cm to 1.4 g m⁻³ at 35 cm in the ridges. In the ploughed plots, the contents of microbial biomass C and microbial biomass N remained roughly constant at 215 and 33 μg g⁻¹ soil, respectively, throughout the experimental period. The microbial biomass C/N ratio varied in a small range around 6.4. In the ridged plots, the contents of microbial biomass C and microbial biomass N were 5% and 6% higher compared to the ploughed plots. Highest microbial biomass C contents of roughly 300 μg g⁻¹ soil were always measured in the crowns in July. The lowest contents of microbial biomass C of 85–137 μg g⁻¹ soil were measured in the furrows. The ridges showed strong spatial heterogeneity in bulk density, soil water content, inorganic nitrogen and microbial biomass.     

Effects of long-term litter manipulation on soil carbon,nitrogen, and phosphorus in a temperate deciduous forest

Changes in above-ground litterfall can influence below-ground biogeochemical processes in forests. In order to examine how above-ground litter inputs affect soil carbon (C), nitrogen (N) and phosphorus (P) in a temperate deciduous forest, we studied a 14-year-old small-scale litter manipulation experiment that included control, litter exclusion, and doubled litter addition at a mature Fagus sylvatica L. site. Total organic C (TOC), total N (TN) and total P (TP), total organic P (TOP), bioavailable inorganic P (Pi), microbial C, N and P, soil respiration and fine root biomass were analyzed in the A and in two B horizons. Our results showed that litter manipulation had no significant effect on TOC in the mineral soil. Litter addition increased the bioavailable Pi in the A horizon but had no significant effect on N in the mineral soil. Litter exclusion decreased TN and TP in the B horizon to a depth of 10 cm. In the A horizon of the litter exclusion treatment, TP, TOP and bioavailable Pi were increased, which is most likely due to the higher root biomass in this treatment. The high fine root biomass seems to have counteracted the effects of the excluded aboveground litter. In conclusion, our study indicates that aboveground litter is not an important source for C in the mineral soil and that P recycling from root litter might be more important than from above-ground litter.     

夜间增温对冬小麦土壤微生物量碳氮及其活性的影响

全球气候变暖存在明显的昼夜不对称性,夜间气温升高幅度显著高于白天,但目前关于夜间增温对土壤微生物影响的田间研究尚较少。为此,本研究采用夜间被动式增温系统(passive nighttime warming,PNW),在我国冬小麦主产区(石家庄、徐州、许昌和镇江)进行全生育期田间增温试验,于2008—2010年监测了土壤微生物对夜间增温的响应。结果显示,与不增温对照相比,夜间增温可显著降低土壤微生物量碳、氮含量和微生物活性。冬小麦整个生育期中,夜间增温分别使石家庄、徐州、许昌和镇江试验点土壤微生物量碳平均降低11.4%、7.8%、10.9%和8.5%,微生物量氮平均降低15.2%、16.7%、13.8%和8.4%,微生物呼吸速率平均下降6.6%、9.6%、7.0%和11.1%。在整个增温过程中,石家庄、徐州、许昌和镇江试验点土壤水分含量分别下降8.8%、3.7%、3.8%和2.9%,与对照相比差异不显著。同时,该夜间增温系统使相应试验点0~5 cm土层的温度分别提高1.2℃、0.7℃、0.7℃和0.7℃。本试验表明,夜间增温将可能通过改变土壤微生物特性而影响土壤碳/氮循环,从而影响到土壤养分供应和冬小麦生长;且表现出了一定的纬度差异性。     

Long-term exclusion of plant-inputs to soil reduces the functional capacity of microbial communities to mineralise recalcitrant root-derived carbon sources

Microbial communities in soil are highly species-rich, recognition of which has led to the view that functional redundancy within communities may buffer many impacts of altered community structure on soil functions. In this study we investigated the impact of long-term (>50 years) exclusion of plant-inputs (bare-fallow treatment) on soil microbial community structure and on the ability of the microbial biomass to mineralise tracer additions of ¹³C-labelled plant-derived C-substrates. Exclusion of plant-inputs resulted in depletion of soil organic matter (SOM) and a reduction in microbial biomass size. The microbial community structure was also strongly affected, as indicated by the distinct phospholipid fatty acid (PLFA) profiles in bare-fallow and grassland soils. Mineralisation of labile plant-derived substrates was not perturbed by the bare-fallow treatment. The incorporation of labile plant-derived C into PLFA biomarkers was found to differ between soils, reflecting the distinct community structures of the soils and indicating that these substrates were utilised by a broad range of microbial groups. In contrast, the mineralisation of recalcitrant plant-derived substrates was reduced in bare-fallow soil and the fate of substrate-derived C within PLFA biomarkers was, initially, similar between the soils. These results indicate that utilisation of these recalcitrant substrates was a function restricted to specific groups, and that exclusion of plant-derived inputs to soil had reduced the capacity of bare-fallow microbial communities to utilise this substrate type. Therefore, the study suggests that long-term selective pressure on microbial communities, resulting in altered community structure, may also result in altered functional attributes. This structure-function relationship was apparent for utilisation of recalcitrant plant-derived substrates, but not for the more widely distributed attribute of labile C-substrate utilisation.     

Microbial reaction in activity, biomass, and community structure after long-term continuous mixing of a grassland soil

Long-term continuous mixing at 40% water holding capacity (WHC) or as slurry at 400% WHC should result in increased soil organic matter decomposition rates in comparison to a control treatment at 40% WHC, but may have strong impacts on soil microbial indices for activity, biomass, and community structure. The amount of extractable inorganic N (NO₃-N+NH₄-N) accumulated in the soil solution after 40 weeks of incubation at 25 °C was 3% of total N in the control treatment and 4% in the two continuous mixing treatments. However, in the treatment mixing at 40% WHC, this 33% increase compared to the control treatment might be explained solely by the decrease in microbial biomass N. In the control treatment, microbial indices decreased in the order microbial biomass C (−10%), microbial biomass N (−40%), ergosterol (−45%) and ATP (−60%). In the treatment mixing at 40% WHC, all four microbial biomass indices were significantly lower than the respective index in the control treatment. This was especially true for microbial biomass N. In the treatment mixing as slurry, only the contents of microbial biomass C and ATP were significantly lower in comparison to the control treatment. The correspondence analysis ordination biplot of the phospholipid fatty acid (PLFA) profiles showed distinct clusters for the three treatments at the end of the incubation. The strongest relative decline of 64% was observed for the fungi-specific PLFA 18:3ω6 in the treatment mixing as slurry in comparison to the control treatment. The content of total bacterial PLFA decreased only by 23%. The differences between the control treatment and the treatment mixing at 40% WHC were less apparent. Fungi represent on average 21% of total microbial biomass C at the end of the incubation if the ergosterol content is recalculated into fungal biomass C. In accordance with this percentage, 22% of the group-specific PLFA could be attributed to fungi.     

Effects of fertilizer application and fire regime on soil microbial biomass carbon and nitrogen,and nitrogen mineralization in an Australian subalpine eucalypt forest

The effects of a range of fertilizer applications and of repeated low-intensity prescribed fires on microbial biomass C and N, and in situ N mineralization were studied in an acid soil under subalpine Eucalyptus pauciflora forest near Canberra, Australia. Fertilizer treatments (N, P, N+P, line + P, sucrose + P), and P in particular, tended to lower biomass N. The fertilizer effects were greatest in spring and smaller in summer and late actumn. Low-intensity prescribed fire lowered biomass N at a soil depth of 0–5 cm with the effect being greater in the most frequently burnt soils. No interactions between fire treatments, season, and depth were significant. Only the lime + P and N+P treatments significantly affected soil microbial biomass C contents. The N+P treatment increased biomass C only at 0–2.5 cm in depth, but the soil depth of entire 0–10 cm had much higher (>doubled) biomass C values in the line + P treatment. Frequent (two or three times a year) burning reduced microbial boomass C, but the reverse was true in soils under forest burn at intervals of 7 years. Soil N mineralization was increased by the addition of N and P (alone or in combination), line + P, and sucrose + P to the soil. The same was true for the ratio of N mineralization to biomass N. Soil N mineralization was retarded by repeated fire treatments, especially the more frequent fire treatment where rates were only about half those measured in unburnt soils. There was no relationship between microbial biomass N (kg N ha^-1) and the field rates of soil N mineralization (kg N ha^-1 month^-1). The results suggest that although soil microbial biomass N represents a distinct pool of N, it is not a useful measure of N turnover.