Land cover change effects on soil chemical and biological properties after planting Mongolian pine (Pinus sylvestris var. mongolica) in sandy lands in Keerqin, northeastern China

We compared soil moisture content, pH, total organic carbon (C _org), total nitrogen (TN), total phosphorus (TP) and inorganic N (NH₄ ⁺–N, NO₃ ^?–N) concentrations, soil potential C and N mineralization rates, soil microbial biomass C (C _mic), soil metabolic quotient (qCO₂), soil microbial quotient (C _mic/C _org) and soil enzyme (urease and invertase) activities in semiarid sandy soils under three types of land cover: grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, and elm (Ulmus punila)–grass savanna in southeastern Keerqin, in northeast China. Soil C _org, TN and TP concentrations (0–10, 10–20, 20–40 and 40–60 cm) were lower while soil C/N and C/P ratios were higher in the plantation than in grassland and savanna. The effects of land cover change on NH₄ ⁺–N and NO₃ ^?–N concentrations, soil potential nitrification and C mineralization rates in the surface soil (0–10 cm) were dependent on sampling season; but soil potential N mineralization rates were not affected by land cover type and sampling season. The effects of land cover change on C _mic and qCO₂ of surface soil were not significant; but C _mic/C _org were significantly affected by land cover change and sampling season. We also found that land cover change, sampling season and land cover type?×?sampling season interaction significantly influenced soil enzyme (urease and invertase) activities. Usually soil enzyme activities were lower in the pine plantations than in grassland and savanna. Our results suggest that land cover change markedly influenced soil chemical and biological properties in sandy soils in the semiarid region, and these effects vary with sampling season.     

Stimulation of microbial extracellular enzyme activities by elevated CO2 depends on soil aggregate size

Increased belowground carbon (C) transfer by plant roots at elevated CO₂ may change properties of the microbial community in the rhizosphere. Previous investigations that focused on total soil organic C or total microbial C showed contrasting results: small increase, small decrease or no changes. We evaluated the effect of 5 years of elevated CO₂ (550 ppm) on four extracellular enzymes: β‐glucosidase, chitinase, phosphatase, and sulfatase. We expected microorganisms to be differently localized in aggregates of various sizes and, therefore analyzed microbial biomass (C_mic by SIR) and enzyme activities in three aggregate‐size classes: large macro‐ (> 2 mm), small macro‐ (0.25–2 mm), and microaggregates (< 0.25 mm). To estimate the potential enzyme production, we activated microorganisms by substrate (glucose and nutrients) amendment. Although C_total and C_mic as well as the activities of β‐glucosidase, phosphatase, and sulfatase were unaffected in bulk soil and in aggregate‐size classes by elevated CO₂, significant changes were observed in potential enzyme production after substrate amendment. After adding glucose, enzyme activities under elevated CO₂ were 1.2–1.9‐fold higher than under ambient CO₂. This indicates the increased activity of microorganisms, which leads to accelerated C turnover in soil under elevated CO₂. Significantly higher chitinase activity in bulk soil and in large macroaggregates under elevated CO₂ revealed an increased contribution of fungi to turnover processes. At the same time, less chitinase activity in microaggregates underlined microaggregate stability and the difficulties for fungal hyphae penetrating them. We conclude that quantitative and qualitative changes of C input by plants into the soil at elevated CO₂ affect microbial community functioning, but not its total content. Future studies should therefore focus more on the changes of functions and activities, but less on the pools.     

Phytoremediation affects microbial development on a limestone quarry

Phytoremediation was used to regenerate a limestone quarry area. Plant growth mixed medium added over the quarry surface, consisting of a mixture of pyrolusite byproducts, natural soil, sand, and rice husk. Three different plant species: pine, cypress, and broom were planted at 9 randomized plots in order to assess the effects of vegetation on the microbial development, which was measured for the following 3 years. Substrate samples were analyzed for organic carbon content (C_org), microbial biomass (C_mic), basal CO₂ respiration activity (BR), alkaline phosphatase (ALP), and acid phosphatase activities at each plant specie and year. Furthermore, the ratio C_mic/C_org, the metabolic quotient (qCO₂), and the C mineralization quotient (qM) were determined. The highest survival rates occurred for broom (93.52%), followed by cypress and pine (82.41%) at the final year, while the content of C_mic, BR, and ALP was increased significantly under plants (pine, cypress, and broom) compared with control. C_mic content and BR was plant dependent. Cypress sites had the highest values of C_mic (214.9 μgCg^?1) and BR (112.8 μgCO₂-Cg^?1d^?1) at the 3^rd year. The plant root environment clearly enhances and regulates the microbial community, in correspondence to the species used. Below ground enhanced activity could fulfill the scope of phytoremediation strategies.     

Soil quality indicator response to land‐use change from annual to perennial bioenergy cropping systems in Germany

Production of biomass feedstock for methanation in Europe has focused on silages of maize and cereals. As ecological awareness has increased in the last several years, more attention is being focused on perennial energy crops (PECs). Studies of specific PECs have shown that their cultivation may enhance agrobiodiversity and increase soil organic carbon stocks while simultaneously providing valuable feedstock for methanation. This study was designed to compare soil quality indicators under annual energy crops (AECs), PECs and permanent grassland (PGL) on the landscape level in south‐western Germany. At a total 25 study sites, covering a wide range of parent materials, the cropping systems were found adjacent to each other. Stands were commercially managed, and PECs included different species such as the Cup Plant, Tall Wheatgrass, Giant Knotweed, Miscanthus, Virginia Mallow and Reed Canary Grass. Soil sampling was carried out for the upper 20 cm of soil. Several soil quality indicators, including soil organic carbon (C_org), soil microbial biomass (C_mic), and aggregate stability, showed that PECs were intermediate between AEC and PGL systems. At landscape level, mean C_org content for (on average) 6.1‐year‐old stands of PEC was 22.37 (±7.53) g kg^?1, compared to 19.23 (±8.08) and 32.08 (±10.11) for AEC and PGL. C_mic contents were higher in PECs (356 ± 241 μg C g^?1) compared to AECs (291 ± 145) but significantly lower than under PGL (753 ± 417). The aggregate stability increased by almost 65% in PECs compared to AEC but was still 57% lower than in PGL. Indicator differences among cropping systems were more pronounced when inherent differences in the parent material were accounted for in the comparisons. Overall, these results suggest that the cultivation of PECs has positive effects on soil quality indicators. Thus, PECs may offer potential to make the production of biomass feedstock more sustainable.     

Influence of climatypes of Scots pine on certain chemical and microbiological characteristics of soils

By using multivariate statistical analysis, the influence of Scots pine climatypes on a set of chemical and microbiological properties of soil, i.e., soil C/N, C_mic/C_org, and C_mic/N_mic, was revealed in a series of long-term (ca. 30 years) field experiments, which were carried out according to the same scheme under contrasting environmental and soil conditions of Siberian forestries.     

Mechanisms of Carbon Sequestration in Soil Aggregates

Soil and crop management practices have a profound impact on carbon (C) sequestration, but the mechanisms of interaction between soil structure and soil organic C (SOC) dynamics are not well understood. Understanding how an aggregate stores and protects SOC is essential to developing proper management practices to enhance SOC sequestration. The objectives of this article are to: (1) describe the importance of plants and soil functions on SOC sequestration, (2) review the mechanisms of SOC sequestration within aggregates under different vegetation and soil management practices, (3) explain methods of assessing distribution of SOC within aggregates, and (4) identify knowledge gaps with regards to SOC and soil structural dynamics. The quality and quantity of plant residues define the amount of organic matter and thus the SOC pool in aggregates. The nature of plant debris (C:N ratio, lignin content, and phenolic compound content) affects the rate of SOC sequestration. Mechanisms of interaction of aggregate dynamics with SOC are complex and embrace a range of spatial and temporal processes within macro- ( > 250 μ m e.c.d.) and microaggregates ( < 250 μ m e.c.d.). A relevant mechanism for SOC sequestration within aggregates is the confinement of plant debris in the core of the microaggregates. The C-rich young plant residues form and stabilize macroaggregates, whereas the old organic C is occluded in the microaggregates. Interactions of clay minerals with C rich humic compounds in correlation with clay mineralogy determine the protection and storage of SOC. Principal techniques used to assess the C distribution in aggregates include the determination of total organic C in different aggregate size fractions, isotopic methods to assess the turnover and storage of organic C in aggregates, and computed tomography and X-ray scattering to determine the internal porosity and inter-aggregate attributes. The literature is replete with studies on soil and crop management influences on total organic C and soil aggregation. However, research reports on the interactions of SOC within aggregates for C sequestration are scanty. Questions still remain on how SOC interacts physically and chemically with aggregates, and research is needed to understand the mechanisms responsible for the dynamics of aggregate formation and stability in relation to C sequestration.     

The influence of acid irrigation and liming on the soil microbial biomass in a Norway spruce (Picea abies [L.] K.) stand

Over a period of three years (1990–1992) microbial biomass-C (C_mic), CO₂ evolution, the C_mic:C_org ratio and the metabolic quotient for CO₂ (qCO₂) were determined in a Norway spruce stand (Höglwald) with experimentally acid-irrigated and limed plots since 1984. A clear relationship between soil pH and the level of microbial biomass-(C_mic) was noted, C_mic increasing with increasing soil pH in O_h or A_h horizons. More microbial biomass-C per unit C_{org} (C_mic:C_org ratio) was detected in limed plots with elevated pH of O_h or A_h horizons as compared to unlimed plots with almost 3 times more C_mic per unit C_org in the limed O_h horizon. Differences here are significant at least at the p=0.05 level. The positive effects of liming (higher pH) on the C_mic:C_org ratio was more pronounced in the upper horizon (O_h)). The total CO₂ evolution rate of unlimed plots was only half of that noted for limed plots which corresponded to the low microbial biomass levels of unlimed plots. The specific respiratory activity, qCO₂, was similar and not significantly different between the unlimed control plot and the limed plot.Acid irrigation of plots with already low pH did not significantly affect the level of microbial biomass, the C_mic:C_org ratio or qCO₂. An elevated qCO₂ could be seen, however, for the limed + acid irrigated plot. The biomass seemed extremely stressed, showing with 3.8 g CO₂-C mg_-1 C_mic h_-1 (O_h) the highest qCO₂ value of all treatments. This was interpreted as a reflection of the continuous adaptation processes to the H⁺ ions by the microflora. The negative effect of acid irrigation of limed plots was also manifested in a decreased C_mic:C_org ratio.     

Association of Soil Aggregation with the Distribution and Quality of Organic Carbon in Soil along an Elevation Gradient on Wuyi Mountain in China

Forest soils play a critical role in the sequestration of atmospheric CO₂ and subsequent attenuation of global warming. The nature and properties of organic matter in soils have an influence on the sequestration of carbon. In this study, soils were collected from representative forestlands, including a subtropical evergreen broad-leaved forest (EBF), a coniferous forest (CF), a subalpine dwarf forest (DF), and alpine meadow (AM) along an elevation gradient on Wuyi Mountain, which is located in a subtropical area of southeastern China. These soil samples were analyzed in the laboratory to examine the distribution and speciation of organic carbon (OC) within different size fractions of water-stable soil aggregates, and subsequently to determine effects on carbon sequestration. Soil aggregation rate increased with increasing elevation. Soil aggregation rate, rather than soil temperature, moisture or clay content, showed the strongest correlation with OC in bulk soil, indicating soil structure was the critical factor in carbon sequestration of Wuyi Mountain. The content of coarse particulate organic matter fraction, rather than the silt and clay particles, represented OC stock in bulk soil and different soil aggregate fractions. With increasing soil aggregation rate, more carbon was accumulated within the macroaggregates, particularly within the coarse particulate organic matter fraction (250–2000 μm), rather than within the microaggregates (53–250μm) or silt and clay particles (< 53μm). In consideration of the high instability of macroaggregates and the liability of SOC within them, further research is needed to verify whether highly-aggregated soils at higher altitudes are more likely to lose SOC under warmer conditions.     

Kinetics of net nitrification associated with soil aggregates under conventional and no-tillage in a subtropical rice soil

Tillage effects on soil nitrification kinetics at the aggregate scale were studied for a subtropical rice soil. Soil samples were separated into large aggregates (>2.0 mm), macro-aggregates (2.0–0.25 mm), micro-aggregates (0.25–0.053 mm) and silt + clay fractions (<0.053 mm) by wet-sieving. The net nitrification process was simulated by a zero- and first kinetics model. Conventional tillage (CT) increased the proportion of the silt + clay fraction by 60% and decreased large-aggregates by 35% compared to ridge with no-till (RNT). Regression analysis showed that the time-dependent kinetics of net nitrification were best fitted by a zero-order model for the large-aggregates and silt + clay fraction but a first-order kinetic model for macro- and microaggregates and whole soil, regardless of tillage regime. Both potential nitrification rates (V _p ) and net nitrification rates (V _a ) were higher for macroaggregates than microaggregates. The potential nitrification (N _p ) for whole soil under RNT was 38.7% higher than CT. The V _p and V _a for whole soil was 88.5% and 64.7% higher under RNT than CT, respectively. Although nitrification was stimulated under RNT, the kinetics model of nitrification was not affected by tillage. This inferred that the interaction between substrates and enzymes involved in nitrification associated with aggregates was not altered by tillage. For this soil, nitrifying microorganisms were mainly associated with macro- and microaggregates rather than large-aggregates and silt + clay fractions.     

Differences of Soil Microbial Biomass and Nitrogen Transformation under Two Forest Types in Central Germany

In order to observe the tree species effect on soil N status, soil microbial biomass C and N (C_mic, N_mic), potential N mineralization and potential nitrification (under laboratory incubation conditions, 22 °C) in different subhorizons (LOf₁, Of₂, Oh and mineral soil at 0–10 cm depth) were determined at three forest sites in central Germany. At each site, two contrasting stands (Beech, Norway spruce or Scots pine) were selected, where the initial soil conditions were similar. Three sampling dates that represented different stages of tree growth were selected: growing season - August, dormant season - November, after budbreak – April. In organic layers, C_mic-to-total C (C_t) ratios under beech and under conifer were 0.72–4.74% and 0.34–2.11%, respectively. N_mic-to-total N (N_t) ratios were 2.47–11.61% and 0.71–5.77%, respectively. Both concentrations of C_mic and N_mic were significantly affected by the stand type and sampling time. Potential N mineralization rates, ranging from 3.7 to 19.7 mg N kg⁻¹ d⁻¹, showed no clear pattern in relation to stand type. However, potential nitrification rates were mostly significantly higher under beech than under contrasting conifer. In mineral soils, concentrations of C_mic and N_mic showed a clear temporal pattern in the order: August>November>April. The average N_mic and N_mic-to-N_t were higher in soils from beech than conifer, while C_mic and C_mic-to-C_t ratios were similar between the two forest types. In organic layers, the highest values of C_mic-to-N_mic ratio and C_mic were found in November samples, especially under beech. By contrast, in mineral soils the highest value of C_mic-to-N_mic ratios were found in April samples, and at that time the C_mic concentrations were the lowest, especially under conifer. These results revealed the differences in microbial growth form and survival strategy associated with different tree species and soil layers.     

The role of soil cover in maintaining the structural and functional integrity of northern taiga ecosystems in Western Siberia

The soils of the studied area function in different temperature regimes: under the influence of permafrost (Turbic Histic Cryosol and Cryic Eutric Histosol) and long-term seasonal freezing (Albic Podsol). Sporadic permafrost defines the differences in temperature regimes. All soils are characterized by the low production of carbon dioxide (an average of 160 mg CO₂/(m² h)), indicating their low biological activity. The lack of easily available carbon for microorganisms is detected in all investigated soils by the C_mic: C_org ratio despite the high reserves of organic matter. The value of CO₂ emission and the C_mic: C_org ratio are “site-specific” for the region of investigation and may be used as indicators of environmental changes. Cryogenic peat soils represent a unique natural object and ensure the functional diversity and integrity of northern taiga ecosystems in Western Siberia.     

木论喀斯特自然保护区土壤微生物生物量的空间格局

土壤微生物是森林生态系统中的重要分解者,在森林生态系统物质循环和能量转换中占有特别重要的地位。以典型喀斯特峰丛洼地为试验对象,利用经典统计学和地统计方法分析了土壤微生物量的空间变异特征。结果表明:土壤微生物量的变异程度均很大,土壤微生物量碳(Cmic)、土壤微生物量氮(Nmic)、土壤微生物量磷(Pmic)的变化范围依次为:44.29—5209.63,20.91—1894.37,0.34—77.06 mg/kg。Cmic、Nmic呈极显著的相关关系,Cmic/Nmic为4.78,明显低于其它生态系统。半变异函数分析表明,Cmic和Nmic的最佳拟合模型为高斯模型,Pmic的最佳拟合模型为球状模型,Cmic/Nmic的最佳拟合模型为指数模型。土壤微生物量的块金值/基台值均介于25%—75%之间,表现为中等空间相关性,说明其受随机因素和结构因素的综合影响。Cmic、Nmic的自相关距离约为50 m,随着滞后距离的增大,自相关函数逐渐向负方向增长,达到显著的负相关。Pmic的Moran’s I在滞后距大于70 m后反而增大,表现为正相关。Cmic/Nmic的Moran’s I较小,在-0.2—0.2之间波动。Cmic、Nmic的空间分布具有很高的相似性,呈凸型片状分布,坡中含量高且向两边递减。Pmic表现为明显不同的分布格局,其在坡中上位和洼地含量较高。Cmic/Nmic呈相反的凹形零星斑块状分布。土壤微生物存在着一定的空间格局,受干扰后其含量急剧降低,因此应加强喀斯特原生生态系统的保护。     

云贵高原喀斯特坡耕地土壤微生物量 C、N、P空间分布

土壤微生物是地球生物演化进程中的先锋种类,具有重要的生态修复功能,但空间分布格局是否存在的争议很大。以云贵高原典型喀斯特坡耕地为对象,基于网格法取样,用经典统计学和地统计学综合分析了土壤微生物生物量的空间变异特征。结果表明,云贵高原喀斯特坡耕地土壤微生物生物量碳(C_mic)、磷(P_mic)以及碳氮比(C_mic/N_mic)适宜,氮(N_mic)的含量较低,变异均很大,空间自相关性明显,最佳拟合模型均为指数模型。块金值C₀较小(0.0016-0.0087),C₀/(C₀+C)均<25%(2.6%-10.2%),变程a较短(22.2-51.0 m),其强烈的空间变异主要由结构性变异引起。Kriging等值线图表明,C_mic、N_mic和C_mic/N_mic的高值区分布在坡的中上部,P_mic的高值区则在坡的中下部和坡脚。云贵高原喀斯特坡耕地土壤微生物不仅存在着小尺度的空间分布格局,而且不同土壤微生物属性的空间分布不同。     

Increases in soil organic carbon sequestration can reduce the global warming potential of long‐term liming to permanent grassland

The application of calcium‐ and magnesium‐rich materials to soil, known as liming, has long been a foundation of many agro‐ecosystems worldwide because of its role in counteracting soil acidity. Although liming contributes to increased rates of respiration from soil thereby potentially reducing soils ability to act as a CO₂ sink, the long‐term effects of liming on soil organic carbon (C_org) sequestration are largely unknown. Here, using data spanning 129 years of the Park Grass Experiment at Rothamsted (UK), we show net C_org sequestration measured in the 0–23 cm layer at different time intervals since 1876 was 2–20 times greater in limed than in unlimed soils. The main cause of this large C_org accrual was greater biological activity in limed soils, which despite increasing soil respiration rates, led to plant C inputs being processed and incorporated into resistant soil organo‐mineral pools. Limed organo‐mineral soils showed: (1) greater C_org content for similar plant productivity levels (i.e. hay yields); (2) higher ¹⁴C incorporation after 1950s atomic bomb testing and (3) lower C : N ratios than unlimed organo‐mineral soils, which also indicate higher microbial processing of plant C. Our results show that greater C_org sequestration in limed soils strongly reduced the global warming potential of long‐term liming to permanent grassland suggesting the net contribution of agricultural liming to global warming could be lower than previously estimated. Our study demonstrates that liming might prove to be an effective mitigation strategy, especially because liming applications can be associated with a reduced use of nitrogen fertilizer which is a key cause for increased greenhouse gas emissions from agro‐ecosystems.     

Interactions of tropospheric CO2 and O3 enrichments and moisture variations on microbial biomass and respiration in soil

Soil microbial biomass C (C_mic) is a sensitive indicator of trends in organic matter dynamics in terrestrial ecosystems. This study was conducted to determine the effects of tropospheric CO₂ or O₃ enrichments and moisture variations on total soil organic C (C_org), mineralizable C fraction (C_Min), C_mic, maintenance respiratory (qCO₂) or C_mic death (qD) quotients, and their relationship with basal respiration (BR) rates and field respiration (FR) fluxes in wheat‐soybean agroecosystems. Wheat (Triticum aestivum L.) and soybean (Glycine max. L. Merr) plants were grown to maturity in 3‐m dia open‐top field chambers and exposed to charcoal‐filtered (CF) air at 350 μL CO₂ L^?1; CF air + 150 μL CO₂ L^?1; nonfiltered (NF) air + 35 nL O₃ L^?1; and NF air + 35 nL O₃ L^?1 + 150 μL CO₂ L^?1 at optimum (? 0.05 MPa) and restricted soil moisture (? 1.0 ± 0.05 MPa) regimes. The + 150 μL CO₂ L^?1 additions were 18 h d^?1 and the + 35 nL O₃ L^?1 treatments were 7 h d^?1 from April until late October. While C_org did not vary consistently, C_Min, C_mic and C_mic fractions increased in soils under tropospheric CO₂ enrichment (500 μL CO₂ L^?1) and decreased under high O₃ exposures (55 ± 6 nL O₃ L^?1 for wheat; 60 ± 5 nL O₃ L^?1 for soybean) compared to the CF treatments (25 ± 5 nL O₃ L^?1). The qCO₂ or qD quotients of C_mic were also significantly decreased in soils under high CO₂ but increased under high O₃ exposures compared to the CF control. The BR rates did not vary consistently but they were higher in well‐watered soils. The FR fluxes were lower under high O₃ exposures compared to soils under the CF control. An increase in C_mic or C_mic fractions and decrease in qCO₂ or qD observed under high CO₂ treatment suggest that these soils were acting as C sinks whereas, reductions in C_mic or C_mic fractions and increase in qCO₂ or qD in soils under elevated tropospheric O₃ exposures suggest the soils were serving as a source of CO₂.     

Amendment Placement Directs Soil Carbon and Nitrogen Cycling in Severely Disturbed Soils

The recovery of ecosystem processes in severely disturbed systems is often limited by biological resources in the soil. The objective of this study was to direct soil microbial biomass (SMB) size and activity with organic amendments. These amendments were applied to the soil at different amendment locations (incorporated versus surface‐applied) and amounts (none, light, and heavy) in a 2 × 3 factorial design. The size and activity of SMB, soil nutrients, and aboveground biomass were monitored over 3 years to determine the rate and direction of change. Contrary to expectations that SMB and carbon mineralization potential (C‐MIN) would be larger with amendment incorporation, SMB‐carbon was greatest in the surface‐heavy treatment and lowest in the incorporated‐control treatment. SMB‐nitrogen, C‐MIN, and organic carbon were greater in the surface than in the incorporated treatments and in amended plots compared to controls. This departure from expectations suggests that other factors, such as microclimate or vegetation, are interacting with the amendment to affect SMB. The degree of contribution, however, is unclear. The treatments only affected planted aboveground biomass early in the experiment, with greater total biomass in the surface‐light treatment in fall 2003. There was also a significant positive relationship between aboveground biomass and SMB in fall 2004. Inorganic nitrogen, total nitrogen, and the soil quality indicators qCO₂ and C_mic/C_org did not vary systematically with amendment treatment. In general, amendment addition did enhance soil biotic properties and supported increased vegetation, but the complication of incorporating the amendment was not necessary for promoting biological development in disturbed soils.     

Plant diversity shapes microbe‐rhizosphere effects on P mobilisation from organic matter in soil

Plant species richness (PSR) increases nutrient uptake which depletes bioavailable nutrient pools in soil. No such relationship between plant uptake and availability in soil was found for phosphorus (P). We explored PSR effects on P mobilisation [phosphatase activity (PA)] in soil. PA increased with PSR. The positive PSR effect was not solely due to an increase in C_org concentrations because PSR remained significant if related to PA:C_org. An increase in PA per unit C_org increases the probability of the temporal and spatial match between substrate, enzyme and microorganism potentially serving as an adaption to competition. Carbon use efficiency of microorganisms (C_mic:C_org) increased with increasing PSR while enzyme exudation efficiency (PA:C_mic) remained constant. These findings suggest the need for efficient C rather than P cycling underlying the relationship between PSR and PA. Our results indicate that the coupling between C and P cycling in soil becomes tighter with increasing PSR.     

Soil nutrients and microbial biomass in three contrasting Mediterranean forests

Aims

The extent to which the spatial and temporal patterns of soil microbial and available nutrient pools hold across different Mediterranean forest types is unclear impeding the generalization needed to consolidate our understanding on Mediterranean ecosystems functioning.

Methods

We explored the response of soil microbial, total, organic and inorganic extractable nutrient pools (C, N and P) to common sources of variability, namely habitat (tree cover), soil depth and season (summer drought), in three contrasting Mediterranean forest types: a Quercus ilex open woodland, a mixed Q. suber and Q. canariensis woodland and a Pinus sylvestris forest.

Results

Soil microbial and available nutrient pools were larger beneath tree cover than in open areas in both oak woodlands whereas the opposite trend was found in the pine forest. The greatest differences in soil properties between habitat types were found in the open woodland. Season (drought effect) was the main driver of variability in the pine forest and was related to a loss of microbial nutrients (up to 75 % loss of N_mic and P_mic) and an increase in microbial ratios (C_mic/N_mic, C_mic/P_mic) from Spring to Summer in all sites. Nutrient pools consistently decreased with soil depth, with microbial C, N and P in the top soil being up to 208 %, 215 % and 274 % larger than in the deeper soil respectively.

Conclusions

Similar patterns of variation emerged in relation to season and soil depth across the three forest types whereas the direction and magnitude of the habitat (tree cover) effect was site-dependent, possibly related to the differences in tree species composition and forest structure, and thus in the quality and distribution of the litter input.     

Microbial diversity of topographical gradient profiles in Fushan forest soils of Taiwan

To evaluate the microbial diversity of Fushan forest soils, the variation of soil properties, microbial populations, and soil DNA with soil depth in three sites of different altitude were analyzed. Microbial population, moisture content, total organic carbon (C_org), and total nitrogen (N_tot) decreased with increasing soil depth. The valley site had the lowest microbial populations among the three tested sites due to the low organic matter content. Bacterial population was the highest among the microbial populations. The ratios of cellulolytic microbes to the total bacteria in organic layers were high, implying their roles in the carbon cycle. The microbial biomass carbon (C_mic) and nitrogen (N_mic) contents ranged from 130.5 to 564.1 μg g⁻¹ and from 16.7 to 95.4 μg g⁻¹, respectively. The valley had the lowest C_mic and N_mic. The organic layer had the highest C_mic and N_mic and decreased with soil depth. Analysis using denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplicons of 16S rDNA showed that the bacterial diversity of the three sites were very similar to each other in the major bands, and the variation was in the minor bands. However, the patterns in PCR-DGGE profile through gradient horizons were different, indicating the prevalence of specific microbes at different horizons. These results suggest that the microbial diversity in the deeper horizons is not simply the diluted analogs of the surface soils and that some microbes dominate only in the deeper horizons. Topography influenced the quantity and diversity of microbial populations.     

Soil organic carbon dynamics 75 years after land-use change in perennial grassland and annual wheat agricultural systems

The dynamics of roots and soil organic carbon (SOC) in deeper soil layers are amongst the least well understood components of the global C cycle, but essential if soil C is to be managed effectively. This study utilized a unique set of land-use pairings of harvested tallgrass prairie grasslands (C₄) and annual wheat croplands (C₃) that were under continuous management for 75 years to investigate and compare the storage, turnover and allocation of SOC in the two systems to 1 m depth. Cropland soils contained 25 % less SOC than grassland soils (115  and 153 Mg C ha^?1, respectively) to 1 m depth, and had lower SOC contents in all particle size fractions (2000–250, 250–53, 53–2 and <2 μm), which nominally correspond to SOC pools with different stability. Soil bulk δ¹³C values also indicated the significant turnover of grassland-derived SOC up to 80 cm depth in cropland soils in all fractions, including deeper (>40 cm) layers and mineral-associated (<53 μm) SOC. Grassland soils had significantly more visible root biomass C than cropland soils (3.2 and 0.6 Mg ha^?1, respectively) and microbial biomass C (3.7 and 1.3 Mg ha^?1, respectively) up to 1 m depth. The outcomes of this study demonstrated that: (i) SOC pools that are perceived to be stable, i.e. subsoil and mineral-associated SOC, are affected by land-use change; and, (ii) managed perennial grasslands contained larger SOC stocks and exhibited much larger C allocations to root and microbial pools than annual croplands throughout the soil profile.