Relationships between soil microbial properties and aboveground stand characteristics of conifer forests in Oregon

Eight forest sites representing a large range of climate, vegetation, and productivity were sampled in a transect across Oregon to study the relationships between aboveground stand characteristics and soil microbial properties. These sites had a range in leaf area index of 0.6 to 16 m² m^–2 and net primary productivity of 0.3 to 14 Mg ha^–1 yr^–1.Measurements of soil and forest floor inorganic N concentrations and in situ net N mineralization, nitrification, denitrification, and soil respiration were made monthly for one year. Microbial biomass C and anaerobic N mineralization, an index of N availability, were also measured. Annual mean concentrations of NH ₄ ⁺ ranged from 37 to 96 mg N kg^–1 in the forest floor and from 1.7 to 10.7 mg N kg^–1 in the mineral soil. Concentrations of NO ₃ ^– were low ( < 1 mg N kg^–1) at all sites. Net N mineralization and nitrification, as measured by the buried bag technique, were low on most sites and denitrification was not detected at any site. Available N varied from 17 to 101 mg N kg^–1, microbial biomass C ranged from 190 to 1230 mg Ckg^–1, and soil respiration rates varied from 1.3 to 49 mg C kg^–1 day^–1 across these sites. Seasonal peaks in NH ₄ ⁺ concentrations and soil respiration rates were usually observed in the spring and fall.The soils data were positively correlated with several aboveground variables, including leaf area index and net primary productivity, and the near infrared-to-red reflectance ratio obtained from the airborne simulator of the Thematic Mapper satellite. The data suggest that close relationships between aboveground productivity and soil microbial processes exist in forests approaching semi-equilibrium conditions.Abbreviations IR infrared - LAI leaf area index - k _c proportion of microbial biomass C mineralized to CO₂ - NPP net primary productivity - TM Thematic Mapper     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

Effects of forest clear-cutting on the carbon and nitrogen fluxes through podzolic soil horizons

Effects of clear-cutting on the dissolved fluxes of organic C (DOC), organic N (DON), NO₃ ^– and NH₄ ⁺ through surface soil horizons were studied in a Norway spruce dominated mixed boreal forest in eastern Finland. Bulk deposition, total throughfall and soil water from below the organic (including understorey vegetation and, after clear-cutting, also logging residues), eluvial and illuvial horizons were sampled weekly from 1993 to 1999. Clear-cutting was carried out in September 1996. The removal of the tree canopy decreased the deposition of DOC and DON to the forest floor and increased that of NH₄ ⁺ and NO₃ ^– but did not affect the deposition of total N (DTN, <3 kg ha^–1 a^–1). The leaching of DOC and DON from the organic horizon increased over twofold after clear-cutting (fluxes were on an average 168 kg C and 3.3 kg N ha^–1 a^–1), but the increased outputs were effectively retained in the surface mineral soil horizons. Inorganic N deposition was mainly retained by the logging residues and organic horizon indicating microbial immobilization. Increased NO₃ ^– formation reflected as elevated concentrations in the percolate from below the mineral soil horizons were observed especially in the third year after clear-cutting. However, the changes were small and the increased leaching of DTN from below the illuvial horizon remained small (<0.4 kg ha^–1 a^–1) and mainly DON. Effects of clear-cutting on the transport of C and N to surface waters will probably be negligible.     

Land-use change effects on soil C and N transformations in soils of high N status: comparisons under indigenous forest, pasture and pine plantation

Globally, land-use change is occurring rapidly, and impacts on biogeochemical cycling may be influenced by previous land uses. We examined differences in soil C and N cycling during long-term laboratory incubations for the following land-use sequence: indigenous forest (soil age = 1800 yr); 70-year-old pasture planted after forest clearance; 22-year-old pine (Pinus radiata) planted into pasture. No N fertilizer had been applied but the pasture contained N-fixing legumes. The sites were adjacent and received 3–6 kg ha^–1 yr^–1volcanic N in rain; NO₃ ^–-N leaching losses to streamwater were 5–21 kg ha^–1 yr^–1, and followed the order forest < pasture = pine. Soil C concentration in 0–10 cm mineral soil followed the order: pasture > pine = forest, and total N: pasture > pine > forest. Nitrogen mineralization followed the order: pasture > pine > forest for mineral soil, and was weakly related to C mineralization. Based on radiocarbon data, the indigenous forest 0–10 cm soil contained more pre-bomb C than the other soils, partly as a result of microbial processing of recent C in the surface litter layer. Heterotrophic activity appeared to be somewhat N limited in the indigenous forest soil, and gross nitrification was delayed. In contrast, the pasture soil was rich in labile N arising from N fixation by clover, and net nitrification occurred readily. Gross N cycling rates in the pine mineral soil (per unit N) were similar to those under pasture, reflecting the legacy of N inputs by the previous pasture. Change in land use from indigenous forest to pasture and pine resulted in increased gross nitrification, net nitrification and thence leaching of NO₃ ^–-N.     

Factors affecting nitrogen dynamics in a semiarid woodland (Dry Chaco,Argentina)

In an effort to elucidate the factors affecting soil N dynamics in the Dry Chaco ecosystem, soil respiration and microbial biomass N were measured for one year underneath 5 vegetation types: a leguminous tree (Prosopis flexuosa DC), a non-leguminous tree (Aspidosperma quebracho-blanco Schlecht.), a non leguminous shrub (Larrea spp.), the open interspaces, and a pure grassland. Ammonifier and nitrifier densities and N content in litter were also measured in some cases. Results were compared with previously reported N mineralization rates and soil fertility.During the dry season microbial biomass N and net N mineralization were low, while accretion of easily mineralizable C occurred (estimated through soil respiration rates in lab under controlled temperature and moisture). With the onset of rain, microbial biomass N and N mineralization increased markedly, resulting in a decrease in easily mineralizable C. Throughout the wet season N mineralization varied with soil moisture while microbial biomass N remained consistently high. Mean values of immobilized N in this ecosystem were high (20–140 mg kg^–1), of about the same order of magnitude as accumulated net N mineralization (50–150 mg kg^–1 yr^–1). Microbial decay in the dry season, considered as a source of easily mineralizable N, accounted for only 40% of gross N mineralization increase at the beginning of the wet season. Ammonifier densities correlated significantly with soil moisture and N mineralization, but nitrifiers did not.The highest values of total N, N mineralization, inorganic N, microbial biomass N, nitrifier densities, N content in litter, total organic C and easily mineralizable C were found under Prosopis and the lowest values under shrubs and the interspaces. The main differences between tree species were in N mineralization at the beginning of the wet season, in total and inorganic N pools, and in nitrifier densities; all of which were significantly lower under Aspidosperma than under Prosopis.N mineralization in the pure grassland was very low despite high values of total N and C sources. Although N immobilized in microbial biomass was similarly high under Aspidosperma, Prosopis and the pure grassland, net N mineralization rates were quite different.     

Biomass,Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes

Tropical dry forest is the most widely distributed land-cover type in the tropics. As the rate of land-use/land-cover change from forest to pasture or agriculture accelerates worldwide, it is becoming increasingly important to quantify the ecosystem biomass and carbon (C) and nitrogen (N) pools of both intact forests and converted sites. In the central coastal region of México, we sampled total aboveground biomass (TAGB), and the N and C pools of two floodplain forests, three upland dry forests, and four pastures converted from dry forest. We also sampled belowground biomass and soil C and N pools in two sites of each land-cover type. The TAGB of floodplain forests was as high as 416 Mg ha^–1, whereas the TAGB of the dry forest ranged from 94 to 126 Mg ha^–1. The TAGB of pastures derived from dry forest ranged from 20 to 34 Mg ha^–1. Dead wood (standing and downed combined) comprised 27%–29% of the TABG of dry forest but only about 10% in floodplain forest. Root biomass averaged 32.0 Mg ha^–1 in floodplain forest, 17.1 Mg ha^–1 in dry forest, and 5.8 Mg ha^–1 in pasture. Although total root biomass was similar between sites within land-cover types, root distribution varied by depth and by size class. The highest proportion of root biomass occurred in the top 20 cm of soil in all sites. Total aboveground and root C pools, respectively, were 12 and 2.2 Mg ha^–1 in pasture and reached 180 and 12.9 Mg ha^–1 in floodplain forest. Total aboveground and root pools, respectively, were 149 and 47 kg ha^–1 in pasture and reached 2623 and 264 kg ha^–1 in floodplain forest. Soil organic C pools were greater in pastures than in dry forest, but soil N pools were similar when calculated for the same soil depths. Total ecosystem C pools were 306. The Mg ha^–1 in floodplain forest, 141 Mg ha^–1 in dry forest, and 124 Mg ha^–1 in pasture. Soil C comprised 37%–90% of the total ecosystem C, whereas soil N comprised 85%–98% of the total. The N pools lack of a consistent decrease in soil pools caused by land-use change suggests that C and N losses result from the burning of aboveground biomass. We estimate that in México, dry forest landscapes store approximately 2.3 Pg C, which is about equal to the C stored by the evergreen forests of that country (approximately 2.4 Pg C). Potential C emissions to the atmosphere from the burning of biomass in the dry tropical landscapes of México may amount to 708 Tg C, as compared with 569 Tg C from evergreen forests.     

Influence of Earthworm Invasion on Redistribution and Retention of Soil Carbon and Nitrogen in Northern Temperate Forests

We analyzed soil organic matter distribution and soil solution chemistry in plots with and without earthworms at two sugar maple (Acer saccharum)–dominated forests in New York State, USA, with differing land-use histories to assess the influence of earthworm invasion on the retention or loss of soil carbon (C) and nitrogen (N) in northern temperate forests. Our objectives were to assess the influence of exotic earthworm invasion on (a) the amount and depth distribution of soil C and N, (b) soil ¹³C and ¹⁵N, and (c) soil solution chemistry and leaching of C and N in forests with different land-use histories. At a relatively undisturbed forest site (Arnot Forest), earthworms eliminated the thick forest floor, decreased soil C storage in the upper 12 cm by 28%, and reduced soil C:N ratios from 19.2 to 15.3. At a previously cultivated forest site with little forest floor (Tompkins Farm), earthworms did not influence the storage of soil C or N or soil C:N ratios. Earthworms altered the stable isotopic signature of soil at Arnot Forest but not at Tompkins Farm; the alteration of stable isotopes indicated that earthworms significantly increased the loss of forest floor C but not N from the soil profile at Arnot Forest. Nitrate (NO₃^–) concentrations in tension and zero-tension lysimeters were much greater at Tompkins Farm than Arnot Forest, and earthworms increased NO₃^– leaching at Tompkins Farm. The results suggest that the effect of earthworm invasion on the distribution, retention, and solution chemistry of soil C and N in northern temperate forests may depend on the initial quantity and quality of soil organic matter at invaded sites.     

Relationship between soil organic carbon and microbial biomass on chronosequences of reclamation sites

The interrelationship between soil microorganisms and soil organic carbon was studied on an agricultural and on a forest chronosequence of open-pit mine reclamation soils. Thirty years after reclamation, soil carbon levels of 0.8% on the agricultural sites and 1.7% on the forest sites (A-horizon) were reached. Microbial biomass rose very fast to levels characteristic of undisturbed soils. Microbial carbon (C_mier) was 57 mg·100 g^–1 soil after 15 years on the agricultural sites and 43 mg·100 g^–1 on the forest sites. The contribution of C_mier to the total organic carbon (C_org) decreased with time, more rapidly on the forest sites than on the agricultural ones. From the C_mierr/C_org ratio it became evident that both chronosequences had not yet reached a steady state within the 50 years of reclamation. A significant decrease of the metabolic quotient qCO₂ (microbial respiration per unit biomass) with time was observed on the agricultural sites but not on the forest sites. The C_mier/C_org ratio proved to be a reliable soil microbial parameter for describing changes in man-made ecosystems. For evaluating reclamation efforts, the C_mier/C_org ratio can be considered superior to its single components (C_mier or C_org) and to other parameters.     

Carbon and nitrogen cycling during old-field succession: Constraints on plant and microbial biomass

Soil C and N dynamics were studied in a sequence of old fields of increasing age to determine how these biogeochemical cycles change during secondary succession. In addition, three different late-successional forests were studied to represent possible "steady state" conditions. Surface soil samples collected from the fields and forests were analyzed for total C, H₂O-soluble C, total N, potential net N mineralization, potential net nitrification, and microbial biomass. Above-and belowground plant biomass was estimated within each of the old field sites.Temporal changes in soil organic C, total N and total plant biomass were best described by a gamma function [y =at ^b e ^ctd +f] whereas a simple exponential model [y =a(l – e^–bt ) + c] provided the best fit to changes in H₂O-soluble C, C:N ratio, microbial C, and microbial N. Potential N mineralization and nitrification linearly increased with field age; however, rates were variable among the fields. Microbial biomass was highly correlated to soil C and N pools and well correlated to the standing crop of plant biomass. In turn, plant biomass was highly correlated to pools and rates of N cycling.Patterns of C and N cycling within the old field sites were different from those in a northern hardwood forest and a xeric oak forest; however, nutrient dynamics within an oak savanna were similar to those found in a 60-yr old field. Results suggest that patterns in C and N cycling within the old-field chronosequence were predictable and highly correlated to the accrual of plant and microbial biomass.     

Exotic Earthworm Invasion and Microbial Biomass in Temperate Forest Soils

Invasion of north temperate forest soils by exotic earthworms has the potential to alter microbial biomass and activity over large areas of North America. We measured the distribution and activity of microbial biomass in forest stands invaded by earthworms and in adjacent stands lacking earthworms in sugar maple-dominated forests in two locations in New York State, USA: one with a history of cultivation and thin organic surface soil horizons (forest floors) and the other with no history of cultivation and a thick (3–5 cm) forest floor. Earthworm invasion greatly reduced pools of microbial biomass in the forest floor and increased pools in the mineral soil. Enrichment of the mineral soil was much more marked at the site with thick forest floors. The increase in microbial biomass carbon (C) and nitrogen (N) in the mineral soil at this site was larger than the decrease in the forest floor, resulting in a net increase in total soil profile microbial biomass in the invaded plots. There was an increase in respiration in the mineral soil at both sites, which is consistent with a movement of organic matter and microbial biomass into the mineral soil. However, N-cycle processes (mineralization and nitrification) did not increase along with respiration. It is likely that the earthworm-induced input of C into the mineral soil created a microbial sink for N, preventing an increase in net mineralization and nitrification and conserving N in the soil profile.     

Accumulation of organic matter along a pollution gradient: Application of odum's theory of ecosystem energetics

Forest soil biology in Scots pine forests of the Empetrum-Vaccinium type was studied around the industrialized city of Oulu, northern Finland since 1987. The forest sites lie along a sulphur and nitrogen concentration gradient in the mor humus ranging from 1.6 to 3.9 mg S g^–1 organic matter (OM) and from 14 to 23 mg N g^–1 OM. A number of biological parameters have earlier been found to vary along this gradient, thus indicating that the ecosystems are subjected to a pollution stress. Total microbial biomass and various activity parameters were studied in 1991. The different methods are discussed and the results interpreted within the light of Odum's theory of the energetic stabilization of ecosystems. Microbial biomass C determined by the fumigation-extraction (FE) technique varied from 5 to 10 mg g^–OM, N from 0.5 to 1.0 mg g^–1OM, and basal respiration rate from 0.040 to 0.097 mg CO₂ h^–1 g^–1OM. All decreased along the pollution gradient. Substrate induced respiration values (SIR) varied from 0.025 to 0.085 mg CO₂-C h^–1 g^–1dw. SIR correlated well with the biomass values determined by the FE technique. The lag time of the microbial community after glucose addition (varying from 13 to 22 h) was shortened and the specific respiration increment of the microbial community in response to glucose addition increased along the gradient. The metabolic quotient (respiration/biomass) of the microflora strongly depended on the technique and equation used in calculating the microbial biomass. The results show a reduced biomass, but a more intensive regeneration and intensified activity per biomass unit of microorganisms in polluted forest soil. This in turn denotes an alteration in the microbial community in favor of a higher proportion of r-strategists under the disturbed conditions. In contrast, K-strategists may be more dominant under less polluted conditions. This interpretation is presented with some reservations concerning methodology. There is a need for the calibration of each method for determining microbial biomass in different types of soil.     

Biochemical properties of soils of undisturbed and disturbed mangrove forests of South Andaman (India)

Studies on soil quality of mangrove forests would be of immense use in minimizing soil degradation and in adopting strategies for soil management at degraded sites. Among the various parameters of soil quality, biological and biochemical soil properties are very sensitive to environmental stress and provide rapid and accurate estimates on changes in quality of soils subjected to degradation. In this study, we determined the general and specific biochemical characteristics of soils (0-30 cm) of inter-tidal areas of 10 undisturbed mangrove forest sites of S. Andaman, India. In order to determine the effects of disturbance, soils from the inter-tidal areas of 10 disturbed mangrove forest sites were also included in the study. The general biochemical properties included all the variables directly related to microbial activity and the specific biochemical parameters included the activities of extracellular hydrolytic enzymes that are involved in the carbon, nitrogen, sulfur and phosphorus cycles in soil. The pH, clay, cation exchange capacity, Al₂O₃ and Fe₂O₃ levels exhibited minimum variation between the disturbed and undisturbed sites. In contrast, organic C, total N, Bray P and K levels exhibited marked variation between the sites and were considerably lower at the disturbed sites. The study also revealed marked reductions in microbial biomass and activity at the disturbed sites. In comparison to the undisturbed sites, the levels of all the general biochemical parameters viz., microbial biomass C, microbial biomass N, N flush, basal respiration, metabolic quotient (qCO₂), ATP, N mineralization rates and the activities of dehydrogenase and catalase were considerably lower at the disturbed sites. Similarly, drastic reductions in the activities of phosphomonoesterase, phosphodiesterase, ß-g1ucosidase, urease, BAA-protease, casein-protease, arylsulfatase, invertase and carboxymethylcellulase occurred at the disturbed sites due mainly to significant reductions in organic matter/substrate levels. The data on CO₂ evolution, qCO₂ and ATP indicated the dominance of active individuals in the microbial communities of undisturbed soils and the ratios of biomass C:N, ATP:biomass C and ergosterol:biomass C ratios indicated low N availability and the possibility of fungi dominating over bacteria at both the mangrove sites. Significant and positive correlations between soil variables and biochemical properties suggested that the number and activity of soil microorganisms depend mainly on the quantity of mineralizable substrate and the availability of nutrients in these mangrove soils.     

A comparative account of the microbiological characteristics of soils under natural forest,grassland and cropfield from Eastern India

Microbiological and physico-chemical characteristics of tropical forest, grassland and cropfield soils from India were investigated. The study revealed that the conversion of natural forest led to a reduction of soil organic C (26–36%), total N (26–35%), total P (33–44%), microfungal biomass (44–66%) and total microbial biomass C, N and P (25–60%) over a period of 30–50 years. Comparative analysis of microbial activity in terms of basal soil respiration revealed maximum activity in the forest and minimum in the cropfield soil. Analysis of microbial metabolic respiratory activity (qCO₂) indicated relatively greater respiratory loss of CO₂-C per unit microbial biomass in cropfield and grassland than in forest soil. Considering the importance of the microbial component in soil, we conclude that the conversion of the tropical forest to different land uses leads to the loss of biological stability of the soil.     

Impact of Ecosystem Management on Microbial Community Level Physiological Profiles of Postmining Forest Rehabilitation

We investigated the impacts of forest thinning, prescribed fire, and contour ripping on community level physiological profiles (CLPP) of the soil microbial population in postmining forest rehabilitation. We hypothesized that these management practices would affect CLPP via an influence on the quality and quantity of soil organic matter. The study site was an area of Jarrah (Eucalyptus marginata Donn ex Sm.) forest rehabilitation that had been mined for bauxite 12 years previously. Three replicate plots (20 × 20 m) were established in nontreated forest and in forest thinned from 3,000–8,000 stems ha⁻¹ to 600–800 stems ha⁻¹ in April (autumn) of 2003, followed either by a prescribed fire in September (spring) of 2003 or left nonburned. Soil samples were collected in August 2004 from two soil depths (0–5 cm and 5–10 cm) and from within mounds and furrows caused by postmining contour ripping. CLPP were not affected by prescribed fire, although the soil pH and organic carbon (C), total C and total nitrogen (N) contents were greater in burned compared with nonburned plots, and the coarse and fine litter mass lower. However, CLPP were affected by forest thinning, as were fine litter mass, soil C/N ratio, and soil pH, which were all higher in thinned than nonthinned plots. Furrow soil had greater coarse and fine litter mass, and inorganic phosphorous (P), organic P, organic C, total C, total N, ammonium, microbial biomass C contents, but lower soil pH and soil C/N ratio than mound soil. Soil pH, inorganic P, organic P, organic C, total C and N, ammonium, and microbial biomass C contents also decreased with depth, whereas soil C/N ratio increased. Differences in CLPP were largely (94%) associated with the relative utilization of gluconic, malic (greater in nonthinned than thinned soil and mound than furrow soil), l-tartaric, succinic, and uric acids (greater in thinned than nonthinned, mound than furrow, and 5–10 cm than 0–5 cm soil). The relative utilization of amino acids also tended to increase with increasing soil total C and organic C contents but decreased with increasing nitrate content, whereas the opposite was true for carboxylic acids. Only 45% of the variance in CLPP was explained using a multivariate multiple regression model, but soil C and N pools and litter mass were significant predictors of CLPP. Differences in soil textural components between treatments were also correlated with CLPP; likely causes of these differences are discussed. Our results suggest that 1 year after treatment, CLPP from this mined forest ecosystem are resilient to a spring prescribed fire but not forest thinning. We conclude that differences in CLPP are likely to result from complex interactions among soil properties that mediate substrate availability, microbial nutrient demand, and microbial community composition.     

桂西北喀斯特土壤对生态系统退化的响应

在桂西北喀斯特地区选取玉米 红薯轮作地（KMS）、放牧+冬季火烧草地（KGB）、自然恢复地（KNR）和原生林地（KPF）4种典型生态系统,研究了土壤有机碳、全氮、全磷,微生物生物量碳、氮、磷和土壤结构对生态系统退化的响应.结果表明：KPF土壤有机碳、全氮、全磷和微生物生物量碳、氮、磷均极显著高于其他3种土壤;其他3种土壤中,有机碳和全氮为：KNR > KGB > KMS,但差异不显著;KMS土壤全磷含量（0.87g·kg^-1）分别是KNR和KGB的2.07和9.67倍 (P<0.01);KGB和KNR土壤微生物生物量碳、氮、磷含量均显著大于KMS;KGB中微生物生物量碳显著大于KNR,但二者间微生物生物量氮和磷含量差异不显著.说明减少人为干扰后喀斯特退化生态系统可以缓慢增加土壤有机碳含量,适当放牧和自然恢复都可以作为退化生态系统恢复的方式;土壤微生物生物量对生态系统的变化响应较灵敏,可以作为喀斯特地区土壤养分变化或生态系统退化的一个敏感指标.土壤结构以>0.25 mm水稳性大团聚体为主（>70%）(KMS除外,以2～0.25 mm团聚体为主),并以>2 mm团聚体为主;土壤结构破坏率KMS（51.62%）大于KGB（23.48%）,KNR和KPF较小（分别为9.09%和9.46%）.说明人为干扰或农业耕作破坏了土壤水稳性大团聚体,使其向小粒级转变,土壤结构破坏率增大.对喀斯特地区退化严重的生态系统应减少人为干扰,以自然恢复等保护性措施为主.     

The Role of Dissolved Organic Carbon, Dissolved Organic Nitrogen, and Dissolved Inorganic Nitrogen in a Tropical Wet Forest Ecosystem

Although tropical wet forests play an important role in the global carbon (C) and nitrogen (N) cycles, little is known about the origin, composition, and fate of dissolved organic C (DOC) and N (DON) in these ecosystems. We quantified and characterized fluxes of DOC, DON, and dissolved inorganic N (DIN) in throughfall, litter leachate, and soil solution of an old-growth tropical wet forest to assess their contribution to C stabilization (DOC) and to N export (DON and DIN) from this ecosystem. We found that the forest canopy was a major source of DOC (232 kg C ha^–1 y^–1). Dissolved organic C fluxes decreased with soil depth from 277 kg C ha^–1 y^–1 below the litter layer to around 50 kg C kg C ha^–1 y^–1 between 0.75 and 3.5m depth. Laboratory experiments to quantify biodegradable DOC and DON and to estimate the DOC sorption capacity of the soil, combined with chemical analyses of DOC, revealed that sorption was the dominant process controlling the observed DOC profiles in the soil. This sorption of DOC by the soil matrix has probably led to large soil organic C stores, especially below the rooting zone. Dissolved N fluxes in all strata were dominated by mineral N (mainly NO₃⁻). The dominance of NO₃^– relative to the total amount nitrate of N leaching from the soil shows that NO₃^– is dominant not only in forest ecosystems receiving large anthropogenic nitrogen inputs but also in this old-growth forest ecosystem, which is not N-limited.     

Microbial biomass C and N,and mineralizable-N,in litter and mineral soil under Pinus radiata on a coastal sand: Influence of stand age and harvest management

Microbial biomass C and N, and anaerobically mineralizable-N, were measured in the litter and mineral soil (0–10 cm and 10–20 cm depth) of Pinus radiata plantations in two trials on a nitrogen-deficient coastal sand. The trials comprised (a) stands of different age (1 to 33 years), with five of the seven stands studied being second rotation, and (b) a harvest-management trial, with stands established after different harvesting treatments of the first-rotation trees and understorey development controlled by manual weeding and chemical sprays. The harvest-management stands were sampled in the fifth year after the second-rotation establishment.In the stands of different age, the levels of microbial biomass C and N, and also mineralizable-N, in the litter and mineral soil showed no relationship with tree age and were similar to those in the oldest (33 years) stands of P. radiata. In the harvesting trial, five years after establishment of the second rotation, levels of microbial N and mineralizable-N in the litter and mineral soil were generally lowest where whole trees and the original forest floor had been removed; they were higher in associated plots in which the original forest floor had been removed but fertilizer N was regularly applied. No marked differences were then found between the other harvest treatments, viz. whole-tree harvest, stem-only harvest with slash remaining on site, and stem-only harvest plus extra added slash materials. In each trial, levels of microbial C and N and mineralizable-N were closely related to total C, and especially total N, in 0–10 cm depth mineral soil, but not generally in litter. Respiratory measurements strongly suggest that the microbial populations in mineral soil had a high metabolic activity.On an area basis in the harvest-management trial, total tree N and microbial N in the litter and mineral soil were lowest in stands where the original forest floor had been removed. In this particular treatment, microbial N in the litter plus mineral soil (0–20 cm depth) after five years of second-rotation growth comprised 7.3% of the total ecosystem N; values in the other treatments ranged between 5.6 and 6.0%.Our results emphasise the importance of slash and litter, and probably volunteer shrubs and herbaceous under-storey species, in conserving pools of potentially available N during the early stages of tree development.     

Impact of NH4NO3 on microbial biomass C and N and extractable DOM in raised bog peat beneath Sphagnum capillifolium and S. recurvum

Regular bi-weekly additions of NH₄NO₃, equivalent to a rate of 3 g N m^–2 yr^–1, were applied to cores of Sphagnum capillifolium, inhabiting hummocks and S. recurvum a pool and hollow colonizer, in a raisedbog in north east Scotland. Microbial biomass C and N,both measured by chloroform extraction, showed similarseasonal patterns and, for most depths, the effects ofadded N on microbial biomass C and N changed withtime. The addition of inorganic N had greatest effectduring October when the water table had risen to thesurface and microbial C and N in the untreated coreshad decreased. Microbial C and N were maintained at75 g C m^–2 and 8.3 g N m^–2 above the values in the untreated cores and far exceeded the amounts of N that had been added up to that date (1 g N m^–2) as NH₄NO₃. This increased microbial biomass was interpreted as leaching of carbonaceous material from the NH₄NO₃ treated moss resulting in greater resistance of the microbialbiomass to changes induced by the rising water table.Treatment with N also caused significant reductions inextractable dissolved organic N (DON) at 10–15 cmdepth, beneath the surface of the moss, but at lowerdepths to 25 cm no changes were observed. Extracteddissolved organic carbon (DOC) was not affected by Ntreatment and showed less seasonal variation than DON,such that the C:N ratio of dissolved organic matter(DOM) in all depths increased from approximately 4 inJuly to around 30 in December.     

Interactive Effects of Nitrogen and Phosphorus on Soil Microbial Communities in a Tropical Forest

Elevated nitrogen (N) deposition in humid tropical regions may exacerbate phosphorus (P) deficiency in forests on highly weathered soils. However, it is not clear how P availability affects soil microbes and soil carbon (C), or how P processes interact with N deposition in tropical forests. We examined the effects of N and P additions on soil microbes and soil C pools in a N-saturated old-growth tropical forest in southern China to test the hypotheses that (1) N and P addition will have opposing effects on soil microbial biomass and activity, (2) N and P addition will alter the composition of the microbial community, (3) the addition of N and P will have interactive effects on soil microbes and (4) addition-mediated changes in microbial communities would feed back on soil C pools. Phospholipid fatty acid (PLFA) analysis was used to quantify the soil microbial community following four treatments: Control, N addition (15 g N m⁻² yr⁻¹), P addition (15 g P m⁻² yr⁻¹), and N&P addition (15 g N m⁻² yr⁻¹ plus 15 g P m⁻² yr⁻¹). These were applied from 2007 to 2011. Whereas additions of P increased soil microbial biomass, additions of N reduced soil microbial biomass. These effects, however, were transient, disappearing over longer periods. Moreover, N additions significantly increased relative abundance of fungal PLFAs and P additions significantly increased relative abundance of arbuscular mycorrhizal (AM) fungi PLFAs. Nitrogen addition had a negative effect on light fraction C, but no effect on heavy fraction C and total soil C. In contrast, P addition significantly decreased both light fraction C and total soil C. However, there were no interactions between N addition and P addition on soil microbes. Our results suggest that these nutrients are not co-limiting, and that P rather than N is limiting in this tropical forest.     

Soil Biochemical Responses to Nitrogen Addition in a Bamboo Forest

Many vital ecosystem processes take place in the soils and are greatly affected by the increasing active nitrogen (N) deposition observed globally. Nitrogen deposition generally affects ecosystem processes through the changes in soil biochemical properties such as soil nutrient availability, microbial properties and enzyme activities. In order to evaluate the soil biochemical responses to elevated atmospheric N deposition in bamboo forest ecosystems, a two-year field N addition experiment in a hybrid bamboo (Bambusa pervariabilis × Dendrocalamopsis daii) plantation was conducted. Four levels of N treatment were applied: (1) control (CK, without N added), (2) low-nitrogen (LN, 50 kg N ha⁻¹ year⁻¹), (3) medium-nitrogen (MN, 150 kg N ha⁻¹ year⁻¹), and (4) high-nitrogen (HN, 300 kg N ha⁻¹ year⁻¹). Results indicated that N addition significantly increased the concentrations of NH₄ ⁺, NO₃ ⁻, microbial biomass carbon, microbial biomass N, the rates of nitrification and denitrification; significantly decreased soil pH and the concentration of available phosphorus, and had no effect on the total organic carbon and total N concentration in the 0–20 cm soil depth. Nitrogen addition significantly stimulated activities of hydrolytic enzyme that acquiring N (urease) and phosphorus (acid phosphatase) and depressed the oxidative enzymes (phenol oxidase, peroxidase and catalase) activities. Results suggest that (1) this bamboo forest ecosystem is moving towards being limited by P or co-limited by P under elevated N deposition, (2) the expected progressive increases in N deposition may have a potential important effect on forest litter decomposition due to the interaction of inorganic N and oxidative enzyme activities, in such bamboo forests under high levels of ambient N deposition.