Organic fertilizers derived from plant materials Part I: Turnover in soil at low and moderate temperatures

The aim was to investigate different organic fertilizers derived from plant materials with respect to their nitrogen and carbon turnover in soil in comparison with organic fertilizers derived from animal‐waste products. In a 64‐day incubation study at 5°C and 15°C, the following fertilizers were used: coarse faba bean–seed meal (Vicia faba L.), coarse meals of yellow and white lupin seeds (Lupinus albus L. and Lupinus luteus L.), Phytoperls^® (waste products of maize [Zea mays L.] processing), coarse meal of castor cake (Ricinus communis L.) as a widely used organic fertilizer, and horn meal as a reference fertilizer‐derived from animal waste products. At 15°C, horn meal showed the highest apparent net N mineralization of fertilizer‐derived N, followed by castor cake and the two lupin meals. At 5°C, apparent net N mineralization of fertilizer‐derived N from horn meal and coarse meal of yellow lupin seeds was nearly identical, followed by castor‐cake meal. Net N mineralization from legume‐seed meals showed no or even a negative temperature response, at least temporarily. In contrast, the other fertilizers showed a positive temperature response of net N mineralization. The content in recalcitrant structural components and the decoupling of decomposition of N‐rich and C‐rich tissue components in time are discussed as controlling factors of fertilizer‐N turnover at low temperature. Microbial residues seem to be an important temporary sink of fertilizer‐derived C and N. Legume‐seed meals induced considerable N‐priming effects. Temperature induced differences in the decomposition of total fertilizer C, indicated by changes in the sum of cumulative CO₂‐C evolution, total K₂SO₄‐soluble organic C and microbial‐biomass C were much smaller than indicated by cumulative CO₂‐C evolution alone. Our results indicate that legume‐seed meals have the potential to replace horn meal and castor‐cake meal in organic vegetable production, especially when soil temperatures in early spring are still low.     

Carbon mineralization rates at different soil depths across a network of European forest sites (FORCAST)

Most of the carbon (C) in terrestrial ecosystems is stored in the mineral soil layers. Thus, the response of the mineral soil to potential increases in temperature is crucial for the prediction of the impact of climate change on terrestrial ecosystems. Samples from three mineral soil layers were collected from eight mature forest sites in the European network CARBOEUROFLUX and were incubated at four temperatures (4, 10, 20 and 30°C) for c. 270 days. Carbon mineralization rates were related to soil and site characteristics. Soil water holding capacity, C content, nitrogen (N) content and organic matter all decreased with soil depth at all sites, with significantly larger amounts of organic matter, C and N in the top 0–5 cm of mineral soil than in the deeper layers. The conifer forest soils had significantly lower pH, higher C/N ratios and carbon contents in the top 5 cm than the broadleaf forest soils. Carbon mineralization rates decreased with soil depth and time at all sites but increased with temperature, with the highest rates measured at 30°C for all sites. Between 50 and 70% of the total C respired after 270 days of incubation came from the top 5 cm. The percentage C loss was small in all cases, ranging from 1 to 10%. A two‐compartment model was fitted to all data to derive the labile/active and slow/recalcitrant fractions, as well as their decomposition constants. Although the labile fraction was small in all cases, we found significantly larger amounts of labile C in the broadleaf forest soils than in the conifer forest soils. No statistically significant differences were found in the temperature sensitivity parameter Q₁₀ among sites, soil layers or between conifer and broadleaf soils. The average Q₁₀ for all soils was 2.98 (± 0.10). We found that despite large differences among sites, C mineralization can be successfully predicted as a combined function of site leaf area index, mean annual temperature and content of labile carbon in the soil (R² = 0.93).     

Nitrogen turnover of legume seed meals as affected by seed meal texture and quality at different temperatures

The aim was to investigate how legume seed meal texture and corresponding quality affects N turnover at different temperatures. Therefore, the effect of size fractionation ‘fine’ and ‘coarse’ of seed meals of yellow lupin (Lupinus luteus L.), blue lupin (Lupinus angustifolius L.) and faba bean (Vicia faba L.) on net N mineralization and turnover was investigated in an incubation experiment at 5°C, 12°C and 20°C. The differences in N release from the two particle size fractions could not be detected at 12 and 20°C incubation temperature. Moreover, net N mineralization at 5°C was higher during incubation of the coarse particle size fractions than during incubation of the fine fraction. In contrast to the common understanding of temperature dependence of microbial processes, the overall influence of incubation temperature on net N mineralization was less expressed. The formation of microbial biomass was highest at 5°C. The subsequent decrease of soil microbial biomass was only partly reflected by net N mineralization suggesting the formation of microbial residues as a preliminary N sink. The control of the N release from legume seed meals seems to be dominated by the N-immobilizing effects of polyphenols at lower temperatures and of C-rich polymers (hemicelluloses) at higher temperatures.     

A warmer climate reduces the bioreactivity of isolated boreal forest soil horizons without increasing the temperature sensitivity of respiratory CO2 loss

Changes in the carbon (C) balance of boreal forest ecosystems may impact the global C cycle and climate. The degree to which antecedent temperature regime and mineral protection of soil organic matter (OM) influence the temperature response of boreal soil C pools remains unknown, however. To investigate these phenomena on time scales relevant to anthropogenic climate change, we quantified the temperature response of four soil C pools (L, F and H organic horizons and B mineral horizon) within soil profiles collected from replicated sites representing two regions along a climate transect (“regional warming”) during a 480-day incubation at 5 and 15 °C (“experimental warming”). We hypothesized that 1) warmer region soils would exhibit reduced bioreactivity, a measure of C lability assessed via cumulative soil C mineralization, relative to colder region soils, paralleling a decrease in bioreactivity with depth in both regions, and 2) temperature sensitivity of C mineralization (denoted as Q₁₀) would increase with decreasing bioreactivity congruent with the “C quality-temperature” (CQT) hypothesis, with a smaller effect in mineral soil where physico-chemical protection likely occurs. Cumulative C mineralization decreased from surface L to deeper horizons and from the cold to warm region for organic F and H horizons only. This decrease in soil bioreactivity with depth was paralleled by an increase in Q₁₀ with depth as expected, except in mineral soil where Q₁₀ was similar to or lower relative to the overlying organic layer. The lower bioreactivity in F and H horizons of the warm relative to the cold region was not, however, associated with a greater Q_10. A warmer regional climate in these otherwise similar forests thus resulted in reduced bioreactivity of isolated soil C pools without increasing the temperature sensitivity of soil C mineralization. This suggests that assumptions about temperature sensitivity of C mineralization based on the propensity for isolated organic C pools to undergo mineralization may not be valid in some organic-rich, boreal forest soils.     

高寒草甸土CO2流通和氮矿化之间的关系以及对外加碳或氮的响应

In nutrient-limited alpine meadows,nitrogen(N) mineralization is prior to soil microbial immobilization;therefore,increased mineral N supply would be most likely immobilized by soil microbes due to nutrient shortage in alpine soils.In addition,low temperature in alpine meadows might be one of the primary factors limiting soil organic matter decomposition and thus N mineralization.A laboratory incubation experiment was performed using an alpine meadow soil from the Tibetan Plateau.Two levels of NH4NO3(N) or glucose(C) were added,with a blank without addition of C or N as the control,before incubation at 5,15,or 25 ℃ for 28 d.CO2 efflux was measured during the 28-d incubation,and the mineral N was measured at the beginning and end of the incubation,in order to test two hypotheses:1) net N mineralization is negatively correlated with CO2 efflux for the control and 2) the external labile N or C supply will shift the negative correlation to positive.The results showed a negative correlation between CO2 efflux and net N immobilization in the control.External inorganic N supply did not change the negative correlation.The external labile C supply shifted the linear correlation from negative to positive under the low C addition level.However,under the high C level,no correlation was found.These suggested that the correlation of CO2 efflux to net N mineralization strongly depend on soil labile C and C:N ratio regardless of temperatures.Further research should focus on the effects of the types and the amount of litter components on interactions of C and N during soil organic matter decomposition.     

Repeated freeze–thaw events affect leaching losses of nitrogen and dissolved organic matter in a forest soil

Freezing and thawing may substantially influence the rates of C and N cycling in soils, and soil frost was proposed to induce NO losses with seepage from forest ecosystems. Here, we test the hypothesis that freezing and thawing triggers N and dissolved organic matter (DOM) release from a forest soil after thawing and that low freezing temperatures enhance the effect. Undisturbed soil columns were taken from a soil at a Norway spruce site either comprising only O horizons or O horizons + mineral soil horizons. The columns were subjected to three cycles of freezing and thawing at temperatures of –3°C, –8°C, and –13°C. The control columns were kept at constant +5°C. Following the frost events, the columns were irrigated for 20 d at a rate of 4 mm d^–1. Percolates were analyzed for total N, mineral N, and dissolved organic carbon (DOC). The total amount of mineral N extracted from the O horizons in the control amounted to 8.6 g N m^–2 during the experimental period of 170 d. Frost reduced the amount of mineral N leached from the soil columns with –8°C and –13°C being most effective. In these treatments, only 3.1 and 4.0 g N m^–2 were extracted from the O horizons. Net nitrification was more negatively affected than net ammonification. Severe soil frost increased the release of DOC from the O horizons, but the effect was only observed in the first freeze–thaw cycle. We found no evidence for lysis of microorganisms after soil frost. Our experiment did not confirm the hypothesis that soil frost increases N mineralization after thawing. The total amount of additionally released DOC was rather low in relation to the expected annual fluxes.     

Litter decomposition and nitrogen mineralization in newly formed gaps in a Danish beech (Fagus sylvatica) forest

Gap formation is suggested as an alternative forest management approach to avoid extreme changes in the N cycle of forest ecosystems caused by traditional management practises. The present study aimed to investigate the effect of gap formation on N availability in beech litter and mineral soil on sites, which experienced only little soil disturbance during tree harvest. N pools, litter decomposition, and N mineralization rates in mineral soil were studied in two gaps (17 and 30 m in diameter) in a 75-year-old managed European beech (Fagus sylvatica L.) forest in Denmark and related to soil temperature (5 cm depth) and soil moisture (15 cm depth). Investigations were carried out during the first 2 years after gap formation in measurement plots located along the north-south transect running through the centre of each gap and into the surrounding forest.An effect of gap size was found only for soil temperatures and litter mass loss: soil temperatures were significantly increased in the northern part of the large gap during the first year after gap formation, and litter mass loss was significantly higher in the smaller gap. All other parameters investigated revealed no effect of gap size. Nitrification, net mineralization, and soil N concentrations tended to be increased in the gaps. Cumulative rates of net mineralization were two fold higher in the gaps during the growing season (June-October), but a statistically significant increase was found only for soil NH₄-N concentrations during this period. Forest floor parameters (C:N ratios, mass loss, N release) were not significantly modified during the first year after gap formation, neither were the total C content nor the C:N ratio in mineral soil at 0-10 cm depth.     

Can ectomycorrhizal fungi circumvent the nitrogen mineralization for plant nutrition in temperate forest ecosystems?

Nitrogen (N) limits plant growth in many forest ecosystems. The largest N pool in the plant-soil system is typically organic, contained primarily within the living plants and in the humus and litter layers of the soil. Understanding the pathways by which plants obtain N is a priority for clarifying N cycling processes in forest ecosystems. In this review, the interactions between saprotrophic microorganisms and ectomycorrhizal fungi in N nutrition with a focus on the ability of ectomycorrhizal fungi to circumvent N mineralization for the nutrition of plants in forest ecosystems will be discussed. Traditionally, it is believed that in order for plants to fulfill their N requirements, they primarily utilize ammonium (NH₄⁺) and nitrate (NO₃⁻). In temperate forest ecosystems, many woody plants form ectomycorrhizas which significantly improves phosphorus (P) and N acquisition by plants. Under laboratory conditions, ectomycorrhizal fungi have also been proven to be able to obtain N from organic sources such as protein. It was thus proposed that ectomycorrhizal fungi potentially circumvent the standard N cycle involving N mineralization by saprotrophic microorganisms. However, in many forest ecosystems the majority of the proteins in the forest floor form complexes with polyphenols. Direct access of N by ectomycorrhizal fungi from a polyphenol-protein complex may be limited. Ectomycorrhizal fungi may depend on saprotrophic microorganisms to liberate organic N sources from polyphenol complexes. Thus, interactions between saprotrophic microorganisms and ectomycorrhizal fungi are likely to be essential in the cycling of N within temperate forest ecosystems.     

Nitrogen transformations and pools in N-saturated mountain spruce forest soils

Nitrogen leaching persists in mountain forests of Europe even in the presence of decreasing N depositions. We have hypothesized that this leaching is linked to soil N transformations occurring over the whole year, even at 0°C temperatures. The aims were to estimate (1) the effect of temperature on N transformations and (2) N pools and fluxes. The study sites are situated in the Bohemian Forest (Czech Republic). Litter, humus, and 0–10-cm mineral layers were sampled in early spring, and the effect of temperature on net nitrification, net ammonification, and microbial N immobilization were measured in a short-term incubation experiment without substrate addition. Nitrogen pools were calculated from the concentrations of N forms in the soil and soil pool weights, while daily N fluxes were calculated from daily net rates of processes and soil pool weights. Relationships between temperature and net nitrification, net ammonification, and microbial N immobilization did not follow the Arrhenius type equation; all processes were active close to 0°C, indicating that microbial N transformations occur over the whole year. Microbial N immobilization rate was generally greater than N mineralization rate. The microbial N pool was significantly larger than mineral N pools. Organic layers containing tens of grams of available N per square meter contributed more than 70% to the available N in the soil profile. Daily N fluxes were related to N pools. On average, N fluxes represented daily mineral and microbial N pool changes of 1.14 and 1.95%, respectively. The effect of microbial composition on the C/N ratio of microbial biomass and respiration is discussed.     

Low levels of nitrogen addition stimulate decomposition by boreal forest fungi

Climate warming and associated increases in nutrient mineralization may increase the availability of soil nitrogen (N) in high latitude ecosystems, such as boreal forests. These changes in N availability could feed back to affect the decomposition of litter and organic matter by soil microbes. Since fungi are important decomposers in boreal forest ecosystems, we conducted a 69-day incubation study to examine N constraints on fungal decomposition of organic substrates common in boreal ecosystems, including cellulose, lignin, spruce wood, spruce needle litter, and moss litter. We added 0, 20, or 200 μg N to vials containing 200 mg substrate in factorial combination with five fungal species isolated from boreal soil, including an Ascomycete, a Zygomycete, and three Basidiomycetes. We hypothesized that N addition would increase CO₂ mineralization from the substrates, particularly those with low N concentrations. In addition we predicted that Basidiomycetes would be more effective decomposers than the other fungi, but would respond weakly or negatively to N additions. In support of the first hypothesis, cumulative CO₂ mineralization increased from 635 ± 117 to 806 + 108 μg C across all fungal species and substrates in response to 20 μg added N; however, there was no significant increase at the highest level of N addition. The positive effect of N addition was only significant on cellulose and wood substrates which contained very little N. We also observed clear differences in the substrate preferences of the fungal species. The Zygomycete mineralized little CO₂ from any of the substrates, while the Basidiomycetes mineralized all of the substrates except spruce needles. However, the Ascomycete (Penicillium) was surprisingly efficient at mineralizing spruce wood and was the only species that substantially mineralized spruce litter. The activities of β-glucosidase and N-acetyl-glucosaminidase were strongly correlated with cumulative respiration (r = 0.78 and 0.74, respectively), and Penicillium was particularly effective at producing these enzymes. On moss litter, the different fungal species produced enzymes that targeted different chemical components. Overall, our results suggest that fungal species specialize on different organic substrates, and only respond to N addition on low N substrates, such as wood. Furthermore, the response to N addition is non-linear, with the greatest substrate mineralization at intermediate N levels.     

A high soil potential net nitrification was observed at 4 m depth: What are the driving factors?

Studies about nitrogen (N) mineralization and nitrification in deep soil layers are rare because N processes are considered to occur mainly in topsoil that hosts active and diverse microbial communities. This study aimed to measure the soil potential net N mineralization (PNM) and nitrification (PNN) down to 4 m depth and to discuss factors controlling their variability. Twenty-one soil cores were collected at the Restinclières agroforestry experimental site, where 14-year-old hybrid walnut trees were intercropped with durum wheat. Soil cores were incubated in the dark in the laboratory at both 6 and 25°C. The soil was a deep calcic fluvisol with a fluctuating water table. It featured a black layer that was very rich in organic matter and permanently water saturated at depths between 3.0 and 4.0 m. The mean soil mineral N content was 3 mg N kg⁻¹ soil in the upper 0.0–0.2 m layer, decreasing until a depth of 2 m and increasing to the maximum value of 25.8 mg N kg⁻¹ soil in the black layer. While nitrate (NO₃⁻) was the dominant form of mineral N (89%) in the upper 0.0–0.2 m layer, its proportion progressively decreased with depth until ammonium (NH₄⁺) became almost the only form of mineral N (97%) in the saturated black layer. Laboratory soil incubation revealed that PNM and PNN occurred at all depths, although the latter remained low at 6°C. The soil nitrate content in the black layer was multiplied by 48 times after 51 days of incubation at 25°C, whereas it was almost inexistent at the sampling date. While the soil total N, the pH and the incubation temperature explained 84% of the variation in PNM, only 29% of the percent nitrification variance was explained by the incubation temperature (T_inc) and the soil C-to-N ratio. These results point out the necessity to consider soil potential net N mineralization and nitrification of deep soil layers to improve model predictions.     

Decomposition and nitrogen dynamics of litter in peat soils from two climatic regions under different temperature regimes

Soil temperature is a major factor affecting organic matter decomposition and thus, global warming may accelerate decomposition processes. However, it remains unclear whether the effects will be similar in climatically different regions. The effects of soil temperatures of 5, 10 and 15 °C on the decomposition of Scots pine (Pinus sylvestris L.) needles were assessed in a 1-year (360 days) growth chamber experiment. Intact peat cores from two climatically different peatland sites (southern and northern Finland) were used as the incubation environments. Needles were incubated in litter bags beneath the living moss layer, and mass loss and nitrogen (N) concentration were determined at 60-day intervals. The rate of mass loss from the needles over time was clearly lower in the 5 °C treatment than at the higher temperatures. Mass loss was strongly related to the accumulated soil temperature sum. In temperatures higher than 5 °C, mass losses were higher in the northern peat. Also, the limit value of decomposition (asymptotic maximum mass loss) was slightly higher in the northern peat (92%), than in the southern peat (87%). The N concentration increased up to a mass loss of 50–60%, whereupon it decreased, while the amount of N (as a percentage of the original amount) remained unchanged until a mass loss of 50–60%, whereupon it decreased linearly. It seems that increasing soil temperatures may result in slightly higher rates of needle litter mass loss and consequent N release in northern peat than in southern peat. The faster decomposition in higher temperatures in the northern peat, together with the slightly higher maximum mass loss value, imply that with climatic warming, susceptibility of boreal peatlands for becoming sources of carbon to the atmosphere may increase towards north.     

Temperature effects on N mineralization: changes in soil solution composition and determination of temperature coefficients by TDR

We studied the changes in composition of the soil solution following mineralization of N at different temperatures, with a view to using TDR to calculate temperature coefficients for the mineralization of N. Mineralization from soil organic nitrogen was measured during aerobic incubation under controlled conditions at six temperatures ranging from 5.5 to 30°C, and at constant water content in a loamy sand soil. We also monitored during the incubation the concentrations of SO₄^2–, Cl^–, HCO₃^–, Ca²⁺, K⁺, Mg²⁺ and Na⁺, and the pH and the electrical conductivity in 1:2 soil:water extracts. Zero‐order N mineralization rates ranged between 0.164 at 5.5°C and 0.865 mg N kg^?1 soil day^?1 at 30°C. There was a significant decrease in soil pH during incubation, of up to 0.6 pH units at the end of the incubation at 30°C. The electrical conductivity of the soil extracts increased significantly at all temperatures (the increase between the start and the end of the incubation was 4‐fold at 30°C) and was strongly correlated with N mineralization. The ratio of bivalent to monovalent cations increased markedly during mineralization (from 2.2 to 5.9 at 30°C), and this increase influenced the evolution of the electrical conductivity of the soil solution through the differences in molar‐limiting ion conductivity between mainly Ca²⁺ and K⁺. Zero‐order mineralization rate constants, k, for NO₃^– concentrations calculated from TDR varied between 0.070 (at 5.5°C) and 0.734 mg N kg^?1 soil day^?1 (at 30°C), which were slightly smaller, but in the same range, as the measured rates. Underestimation of the measured N mineralization rates was due, at least in part, to differences in cation composition of the soil solution between calibration and mineralization experiments. A temperature‐dependence model for N mineralization from soil organic matter was fitted to both the measured and the TDR‐calculated mineralization rates, k and k_TDR, respectively. There were no significant differences between the model parameters from the two. Our results are promising for further use of TDR to monitor soil organic N mineralization. However, the influence of changing cation ratios will also have to be taken into account when trying to predict N mineralization from measured electrical conductivities.     

Decomposition in forest and fallow subarctic soils

Summary Large-scale argicultural development in high latitude regions could lead to large losses of soil C due to accelerated decomposition. Changes in decomposition rates of forest floor material upon land clearing in interior Alaska were simulated by measuring, over a 2-year period, changes in mass, cellulose, lignin, and N of forest floor materials and in mass of filter papers and wood in a forest floor and a fallowed field. All materials decomposed slowly at the surface, with about 90% of the original weight remaining after 2 years. Decomposition rates were higher for materials buried in the field than the forest. Cellulose loss in forest floor materials closely followed mass loss, whereas lignin loss was not significant. However, weight loss of wood was rapid when buried in the field, with about 20% of the initial mass remaining after 2 years. Relationships between mass loss of buried forest floor materials and soil degree days were significant (r=70%–80%). Temperature was a major, but not the only factor, controlling decomposition rates. Forest floor materials showed significant N losses, indicating net N mineralization and that N deficiency was not a factor affecting decomposition. C loss to the atmosphere due to decomposition of forest floor materials after forest clearing will be minimal and similar to that in the undisturbed forest if left on the soil surface, but will be substantial if incorportated into the soil. Incorporation is necessary for cropping; thus some accelerated decomposition is unavoidable in clearing subarctic forests for cultivation.     

Potential for biodegradation of anthropogenic organic compounds at low temperature in boreal soils

The effect of temperatures of −2.5 to +20 °C on the biodegradation of concentrations 0.2-50 μg cm⁻³ of pentachlorophenol (PCP), phenanthrene, pyrene and 2,4,5-trichlorophenol (TCP) was studied in soils sampled from an agricultural field and a relatively pristine forest in Helsinki, Finland. At the temperatures simulating seasonal variation of boreal soil temperatures [Heikinheimo, M., Fougstedt, B., 1992. Statistic of Soil Temperature in Finland. Meteorological Publications 22. Finnish Meteorological Institute, Helsinki, Finland], the response of mineralization of PCP, phenanthrene and 2,4,5-TCP was the most effective in the rhizosphere fraction of the forest humus soil at the substrate concentrations of ?5 μg cm⁻³. In the control incubation, performed at constant temperature of +20 °C, the mineralization yields of the model pollutants were highest in the agricultural soil with the highest applied substrate concentration (50 μg cm⁻³). The results suggest that the high level of pollutant mineralization at +20 °C resulted from the apparent adaptation of the soil microbial community to the high substrate concentration. No such adaptation occurred when the soils were incubated at temperatures simulating the actual boreal soil temperatures. The present results stress the role of adjusting the incubation conditions to environmentally relevant values, when assessing biodegradation of anthropogenic organic compound in boreal soils.     

Significance of Enzyme Activities in Soil Nitrogen Mineralization

This study was undertaken to assess the relationship between nitrogen (N) mineralization in soils treated with eight lime application rates, with four field replications, and the activities of six amidohydrolases involved in N cycling and four glycosidases involved in carbon (C) cycling in soils. Nitrogen mineralization was studied at 20 or 30 °C for 20 weeks, and with the exception of N‐aceyl‐β‐D‐glucosaminidase (NAGase; EC 3.2.1.30) activity, which was assayed at both temperatures, the enzyme activities were assayed at 30 °C at their optimal pH values. Results showed that among the eight enzyme activities studied, NAGase activity was the most significantly correlated with the cumulative amounts of N mineralized in 32 soil samples at 20 °C (r = 0.87***) and at 30 °C (r = 0.95***). The cumulative amounts of N mineralized at 30 °C were also significantly correlated with arylamidase and L‐aspartase activities, with r values of 0.61*** and 0.52**, respectively. Because NAGase activity is involved in both N and C cycling, the cumulative amounts of N mineralized at 30 °C were also significantly correlated with the activities of β‐glucosidase (r = 0.80***) and β‐galactosidase (r = 0.58***). Activities of other N enzymes that were significantly correlated with the cumulative amounts of N mineralized at 30 °C in 20 weeks were those of L‐asparaginase (r = 0.61***), urease (r = 0.57***), amidase (r = 0.54**), and L‐glutaminase (r = 0.41*). It seems that the activity of NAGase can be used as an index of N mineralization in soils.     

Multiple effects of reindeer grazing on the soil processes in nutrient-poor northern boreal forests

Reindeer grazing has a great influence on the ground vegetation of nutrient-poor northern boreal forests dominated by Cladonia lichens in Fennoscandia. Grazing may influence the soil processes in these systems either by influencing the quality of plant litter, or by indirect effects through the soil microclimate. In order to investigate the mechanisms underlying the effects of reindeer on boreal forest soils, we analyzed litter decomposition, soil and microbial C and N, microbial community composition, and soil organic matter quality in three forest sites with old reindeer exclosures adjacent to grazed areas. There was no effect of grazing on soil C/N ratio, inorganic N concentrations, microbial biomass C, microbial community structure analyzed by phospholipid fatty acid (PLFA) analysis, and organic matter quality analyzed by sequential fractionation, in the soil organic layer. However, microbial N was enhanced by grazing at some of the sampling dates and was negatively correlated with soil moisture, which indicates that increased microbial N could be a stress response to drought. The effect of grazing on litter decomposition varied among the decomposition stages: during the first 1.5 months, the litter C loss was significantly higher in the grazed than the ungrazed areas, but the difference rapidly levelled out and, after one year, the accumulated litter C loss was higher in the ungrazed than the grazed areas. Litter N loss was, however, higher in the grazed areas. Our study demonstrates that herbivores may influence soil processes through several mechanisms at the same time, and to a varying extent in the different stages of decomposition.     

Mineralization of Nitrogen in Soils Amended with Dairy Manure as Affected by Wetting/Drying Cycles

Abstract

Interest in manure management and its effects on nitrogen (N) mineralization has increased in recent years. The focus of this research was to investigate the N‐mineralization rates of different soil types in Coastal Plain soils and compare them to a soil from Illinois. Soils with and without dairy composted manure addition were subjected to different wetting/drying cycles [constant moisture at 60% water‐filled pore space (WFPS) and cycling moisture from 60 to 30% WFPS] under laboratory conditions at three different temperatures (11°C, 18°C, and 25°C). Samples were collected from three different soil types: Catlin (Mollisols), Bama (Ultisols), and Goldsboro (Utilsols). Soil chemical and physical properties were determined to help assess variations in N-mineralization rates. Addition of composted manure greatly impacted the amount of N mineralized. The amount of manure‐derived organic N mineralized to inorganic forms was mainly attributed to the soil series, with the Catlin (silt loam) producing the most inorganic N followed by the Goldsboro (loam) and then Bama (sandy loam). This was probably due to soil texture and the native climatic conditions of the soil. No significant differences were observed between the constant and cycling moisture regimens, suggesting that the imposed drying cycle may not have been sufficient to desiccate microbial cells and cause a flush in N mineralization upon rewetting. Nitrogen mineralization responded greatly to the influence of temperature, with the greatest N mineralization occurring at 25°C. The information acquired from this study may aid in predicting the impact of manure application to help increase N‐use efficiency when applied under different conditions (e.g., climate season) and soil types.     

Nitrogen fertilization reduces diversity and alters community structure of active fungi in boreal ecosystems

Nitrogen (N) availability is increasing in many ecosystems due to anthropogenic disturbance. We used a nucleotide analog technique and sequencing of ribosomal RNA genes to test whether N fertilization altered active fungal communities in two boreal ecosystems. In decaying litter from a recently burned spruce forest, Shannon diversity decreased significantly with N fertilization, and taxonomic richness declined from 44 to 33 operational taxonomic units (OTUs). In soils from a mature spruce forest, richness also declined with N fertilization, from 67 to 52 OTUs. Fungal community structure in litter differed significantly with N fertilization, primarily because fungi of the order Ceratobasidiales increased in abundance. We observed similar changes in fungal diversity and community structure with starch addition to litter, suggesting that N fertilization may affect fungal communities by altering plant carbon inputs. These changes could have important consequences for ecosystem processes such as decomposition and nutrient mineralization.     

The CLIMEX soil-heating experiment: soil response after 2 years of treatment

 Most model predictions concerning the response of boreal forest ecosystems to climate change are inferred from small-scale experiments on artificial, simplified systems. Whole-ecosystem experiments designed to validate these models are scarce. We experimentally manipulated a small forested catchment in southern Norway by increasing soil temperature (+3  °C in summer to +5  °C in winter) using heating cables installed at 1 cm depth in the litter layer. Especially nitrification in the 0 to 10-cm soil layer increased as a result of the climate manipulation. Betula litter, produced after exposing trees for 2 years to ambient and elevated CO₂ in greenhouses, was incubated for 1 year in the manipulated catchment. Exposure to elevated CO₂ did not affect the C/N ratio or decomposition of the Betula litter, but lignin content decreased by 10%. We found no effect of elevated temperature on litter decomposition, probably due to desiccation of the litter. The heating cables caused a permanent increase in soil temperature in this soil layer, but when soils were dry, the temperature difference between control and heated plots decreased with increasing distance from the cables. When soils were wet, no gradients in temperature increase occurred. Received: 25 November 1997