Volatile monoterpenes in soil atmosphere under birch and conifers: Effects on soil N transformations

The aim of this study was to examine the occurrence and concentrations of volatile organic compounds (VOCs), in particular, volatile monoterpenes, in soil atmosphere under silver birch (Betula pendula L.) and two conifers, Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.), and to determine the effects of the most relevant monoterpenes on transformations of soil N. The study site was a 70-year-old tree species experiment in Kivalo, northern Finland. VOCs were collected using two methods, passive air samplers and a chamber method. In soil atmosphere under spruce and especially under pine, the concentrations of monoterpenes were high, α- and β-pinene, Δ-3-carene and myrcene being the most abundant compounds, whereas concentrations of monoterpenes in soil atmosphere under birch were negligible. Samples of humus layer from the birch stand incubated in vitro and exposed to vapors from monoterpenes typical of coniferous forest soil showed decreased rates of net N mineralization but simultaneously increased rates of C mineralization. The response of soil microbial biomass C and N to different monoterpenes varied, but some monoterpenes considerably decreased soil microbial biomass. Altogether these results suggest that these compounds have negative effects on soil N transformations, but may serve as carbon and energy source for part of soil microbes.     

Effect of Baythroid on nitrogen transformations in soil

Laboratory incubation experiments were conducted in soil to study the influence of the insecticide Baythroid on immobilization-remineralization of added inorganic N, mineralization of organic N, and nitrification of added NH _inf4 ^su+ -N. Baythroid was applied at 0, 0.4, 0.8, 1.6, 3.2, and 6.4 g g^-1 soil (active ingredient basis). The treated soils were incubated at 30°C for different time intervals depending upon the experiment. The immobilization and mineralization of N were significantly increased in the presence of Baythroid, the effect being greater with higher doses of the insecticide. Conversely, nitrification was retarded at lower doses of Baythroid and significantly inhibited at higher doses. The results of these studies suggest that excessive amonts of insecticide residues affect different microbial populations differently, leading to changes in nutrient cycling.     

Soil amino acid composition across a boreal forest successional sequence

Soil amino acids are important sources of organic nitrogen for plant nutrition, yet few studies have examined which amino acids are most prevalent in the soil. In this study, we examined the composition, concentration, and seasonal patterns of soil amino acids across a primary successional sequence encompassing a natural gradient of plant productivity and soil physicochemical characteristics. Soil was collected from five stages (willow, alder, balsam poplar, white spruce, and black spruce) of the floodplain successional sequence on the Tanana River in interior Alaska. Water-extractable amino acid composition and concentration were determined by HPLC. Irrespective of successional stage, the amino acid pool was dominated by glutamic acid, glutamine, aspartic acid, asparagine, alanine, and histidine. These six amino acids accounted for approximately 80% of the total amino acid pool. Amino acid concentrations were an order of magnitude higher in coniferous-dominated late successional stages than in early deciduous-dominated stages. The composition and concentration of amino acids were generally constant throughout the growing season. The similar amino acid composition across the successional sequence suggests that amino acids originate from a common source or through similar biochemical processes. These results demonstrate that amino acids are important components of the biogeochemical diversity of nitrogen forms in boreal forests.     

Seasonal nitrification measurements with different species of forest litter applied to granite-sand-filled lysimeters in the field

Summary The biodegradation of litter from Festuca silvatica, Abies pectinata, Fagus silvatica, Calluna vulgaris, Picea abies associated with forest brown acid soils or with podzolic soils was studied in field lysimeters filled with granite sand. Analysis of the leachates collected during 2 years made it possible to determine NO _inf3 ^sup- , NH _inf4 ^sup+ , and soluble organic N production in order to investigate the specific influence of the different species of litter on the mineralization of organic N and the variations in nitrification. With Festuca silvatica (grass), active nitrification was observed after the addition of fresh litter in autumn (fall of leaves). Nitrification remained significant in winter, reached a maximum in spring until early summer, and then decreased after mineralization of the easily mineralizable organic N. Nitrification was the major N transformation process in this litter. The addition of fresh litter of Abies pectinata (fir), Fagus silvatica (beech), Calluna vulgaris (heather), and Picea abies (spruce) in autumn induced an inhibition of nitrification during winter and spring. With these litter species, nitrification started again by the end of spring and was at a maximum in summer and autumn until leaf fall. By comparison with Festuca, inhibition observed in winter and spring with the other litter species was definitely due to the chemical composition of the leaves. Simultaneously, a lower C mineralization of these plant material occured. These litter species, in particular Calluna and Picea released leachates containing significant amounts of soluble organic N that were only slightly decomposed. We conclude that NO _inf3 ^sup- production outside of the plant growth period can definitely be involved in soil acidification and weathering processes.     

Resistance of microbial and soil properties to warming treatment seven years after boreal fire

Boreal forests store a large fraction of global terrestrial carbon and are susceptible to environmental change, particularly rising temperatures and increased fire frequency. These changes have the potential to drive positive feedbacks between climate warming and the boreal carbon cycle. Because few studies have examined the warming response of boreal ecosystems recovering from fire, we established a greenhouse warming experiment near Delta Junction, Alaska, seven years after a 1999 wildfire. We hypothesized that experimental warming would increase soil CO₂ efflux, stimulate nutrient mineralization, and alter the composition and function of soil fungal communities. Although our treatment resulted in 1.20 °C soil warming, we found little support for our hypothesis. Only the activities of cellulose- and chitin-degrading enzymes increased significantly by 15% and 35%, respectively, and there were no changes in soil fungal communities. Warming resulted in drier soils, but the corresponding change in soil water potential was probably not sufficient to limit microbial activity. Rather, the warming response of this soil may be constrained by depletion of labile carbon substrates resulting from combustion and elevated soil temperatures in the years after the 1999 fire. We conclude that positive feedbacks between warming and the microbial release of soil carbon are weak in boreal ecosystems lacking permafrost. Since permafrost-free soils underlie 45-60% of the boreal zone, our results should be useful for modeling the warming response during recovery from fire in a large fraction of the boreal forest.     

Short-term biochar-induced increase in soil CO2 release is both biotically and abiotically mediated

The application of biochar to soil has been shown to cause an apparent increase in soil respiration. In this study we investigated the mechanistic basis of this response. We hypothesized that increased CO₂ efflux could occur by: (1) Biochar-induced changes in soil physical properties (bulk density, porosity, moisture content); (2) The biological breakdown of organic carbon (C) released from the biochar; (3) The abiotic release of inorganic C contained in the biochar; (4) A biochar-induced stimulation of decomposition of native soil organic matter (SOM) which could occur both biotically or abiotically; (5) The intrinsic biological activity of the biochar results in the liberation of CO₂. Our results show that most of the extra CO₂ produced after biochar addition to soil came from the equal breakdown of organic C and the release of inorganic C contained in the biochar. Using long-term ¹⁴C-labelled SOM, we show that biochar repressed native SOM breakdown, counteracting the release of CO₂ from the biochar. A range of mechanisms to describe this negative priming response is presented. Although biochar-induced significant changes in the physical characteristics of the soil, overall this made no contribution to changes in soil respiration. Similarly, the evidence from our study suggests that changes in soluble polyphenols do not help explain the respiration response. In summary, biochar induced a net release of CO₂ from the soil; however, this C loss was very small relative to the amount of C stored within the biochar itself (ca. 0.1%). This short-term C release should therefore not compromise its ability to contribute to long-term C sequestration in soil environments.     

Urea losses in flooded soils with established oxidized and reduced soil layers

Summary Laboratory batch incubation experiments were conducted to determine in fate of urea-¹⁵N applied to floodwater of four rice soils with established oxidized and reduced soil layers. Diffusion-dependent urea hydrolysis was rapid in all soils, with rates ranging from 0.0107 to 0.0159 h^-1 and a mean rate of 0.0131 h^-1. Rapid loss of 53%–65% applied urea-¹⁵N occurred during the first 8 days after application, primarily by NH₃ volatilization. At the end of 70 days, an additional 20%–30% of applied urea-¹⁵N was lost, primarily through nitrification-denitrification processes. The soil types showed significant differences in total applied urea-¹⁵ recovery. Conversion of urea-¹⁵N to N₂-¹⁵N provided direct evidence of urea hydrolysis followed by nitrification-denitrification in flooded soils.Joint contribution from the University of Florida and Louisiana State University. Florida Experimental Stations Journal Series No R-00501     

Gross nitrogen mineralization rates still high 14 years after suspension of N input to a N-saturated forest

Significant areas of temperate forests in Central Europe, NE America and E Asia receive high amounts of N deposition. According to the few studies available, suspension of the N load leads to reductions in both inorganic soil N and leaching of N within a few years. We report that, surprisingly, N is still mineralized at high rates 14 yr after suspension of a previous N-load of >100 kg N/ha yr for 20 yr. In this treatment, gross N mineralization rates exceeded those in control plots by a factor 3, but equaled those in still on-going (34 yr of) treatments with 30 and 60 kg N/ha yr, in which levels of extractable NH₄⁺ were up to 10 times higher.     

Organic matter characteristics and C and N transformations in the humus layer under two tree species, Betula pendula and Picea abies

The aim of this study was to compare the effects of silver birch (Betula pendula Roth) and Norway spruce (Picea abies (L.) Karst.) on soil C and N transformations and on the characteristics of organic matter. Soil samples were taken from the humus layer of a replicated 35-year-old birch-spruce field experiment growing on Vaccinium myrtillus site type in middle-eastern Finland. The soil was a podzol and humus type mor. Soil pH was higher under birch (4.7) than under spruce (4.1). The C-to-N ratio was lower under birch (17) than under spruce (23). Per unit organic matter, microbial biomass C and N, net N mineralization and net nitrification were all higher in birch soil than in spruce soil. The rate of C mineralization (CO₂ production) was, however, the same regardless of tree species. Water-extracts were analyzed for the concentrations of dissolved organic C (DOC) and N (DON) and characterized according to molecular size distribution by ultrafiltration and according to chemical composition using a resin fractionation technique. The concentration of DON, in particular, was higher in birch soil than in spruce soil. The distribution of DOC and DON into different fractions based on molecular size or chemical composition was rather similar in both soils. The concentration of total phenolics, expressed as tannic acid equivalents, was higher in the humus layer under birch than in the humus layer under spruce, because the birch humus layer contained significantly more low-molecular weight (about <0.5 kD) phenolics than the spruce humus layer did. The concentration of proanthocyanidins (condensed tannins) was higher in spruce soil than in birch soil. The concentrations of the five most abundant phenolic acids showed that ferulic and p-coumaric acids were more abundant in spruce soil. Birch soil tended to contain slightly more nonvolatile sesquiterpenes than the spruce soil. The concentration of diterpenes was similar in both soils; but birch soil contained significantly more triterpenes, mainly sterols, than spruce soil did.     

Microbial biomass and C and N transformations in forest floors under European beech, sessile oak, Norway spruce and Douglas-fir at four temperate forest sites

The purpose of this research was to compare soil chemistry, microbially mediated carbon (C) and nitrogen (N) transformations and microbial biomass in forest floors under European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Lieblein), Norway spruce (Picea abies (L.) Karst) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) at four study sites. We measured soil chemical characteristics, net N mineralization, potential and relative nitrification, basal respiration, microbial and metabolic quotient and microbial biomass C and N under monoculture stands at all sites (one mixed stand). Tree species affected soil chemistry, microbial activities and biomass, but these effects varied between sites. Our results indicated that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Differences in potential nitrification and relative nitrification might be related to the presence of ground vegetation through its influence on soil NH₄ and labile C availability. Our findings highlight the need to study the effects of tree species on microbial activities at several sites to elucidate complex N cycle interactions between tree species, ground vegetation, soil characteristics and microbial processes.     

添加硝化抑制剂DMPP对红壤水稻土硝化作用及微生物群落功能多样性的影响

通过室内培育试验,研究了不同施氮水平下添加硝化抑制剂(DMPP)处理对红壤水稻土NH4+-N、NO3–-N含量、微生物生物量碳及微生物群落功能多样性的影响。结果表明:56天培养期内,不同处理的NH4+-N含量总体呈下降趋势,而NO3–-N含量呈上升趋势。随施氮水平提高,培养期内NH4+-N平均含量从0 mg/kg处理的24.10 mg/kg增加到400 mg/kg处理的412.10 mg/kg,NO3–-N平均含量从0 mg/kg处理的41.88 mg/kg增加到400 mg/kg处理的99.83 mg/kg。添加DMPP显著抑制硝化作用进行,抑制效果随施氮量增加而提高,400 mg/kg施氮水平下,添加DMPP硝化率和硝化速率比不添加DMPP处理分别下降了29.0%和44.3%,下降幅度远大于其他施氮水平处理。施氮水平也影响土壤微生物生物量碳和微生物群落功能多样性。施氮量从0 mg/kg增加到400 mg/kg,土壤微生物生物量碳下降了12.5%,AWCD值下降了78.4%,Shannon指数下降了22.3%;与不添加DMPP处理相比,添加DMPP处理的土壤微生物生物量、AWCD值、Shannon指数分别提高了2.1%、23.9%、7.8%,尤其在400 mg/kg施氮水平下,提高的幅度更加明显。