Net Mineralization Rates Research Articles

Selective lignin decomposition and nitrogen mineralization in forest litter colonized by Clitocybe sp.

Fruiting bodies of Clitocybe sp. were encountered in association with partly decomposed litter materials that were bleached due to colonization by mycelia of this fungus. We clarified the impact of this fungus on the chemical and fungal properties of litter materials and quantitatively evaluated the potential ability of this fungus to cause selective decomposition of recalcitrant compounds such as lignin. The content of acid-unhydrolyzable residues (AUR) was lower, and the contents of soluble carbohydrates and nutrients (N, P, K, Ca, Mg) and net mineralization rate of N were higher in bleached litter materials (BLM) than in adjacent nonbleached litter materials (NBL). Total hyphal length was 5.4 times greater in BLM than in NBL. A total of 49 fungal taxa were isolated, 30 from BLM and 42 from NBL. In pure culture decomposition tests, Clitocybe sp. caused greater mass loss in partly decomposed leaves than in freshly fallen leaves, which was attributed to the greater mass loss of AUR and more selective decomposition of AUR in partly decomposed leaves. Fourier transform infrared (FT-IR) spectroscopy showed that Clitocybe sp. was responsible for the selective transformation of the chemical structure of lignin. These results showed that Clitocybe sp. had a marked ability to remove AUR and lignin selectively from partly decomposed leaves and to enhance N mineralization, contributing to small-scale heterogeneity of the decomposition within the forest floor.

Carbon dynamics of terrestrial ecosystems on the Tibetan Plateau during the 20th century: an analysis with a process‐based biogeochemical model

ABSTRACTAim The Tibetan Plateau accounts for about a quarter of the total land area of China and has a variety of ecosystems ranging from alpine tundra to evergreen tropics. Its soils are dominated by permafrost and are rich in organic carbon. Its climate is unique due to the influence of the Asian monsoon and its complex topography. To date, the carbon dynamics of the Tibetan Plateau have not been well quantified under changes of climate and permafrost conditions. Here we use a process‐based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), which was incorporated with a soil thermal model, to examine the permafrost dynamics and their effects on carbon dynamics on the plateau during the past century.Location The Tibetan Plateau.Methods We parameterize and verify the TEM using the existing data for soil temperature, permafrost distribution and carbon and nitrogen from the region. We then extrapolate the model and parameters to the whole plateau.Results During the 20th century, the Tibetan Plateau changed from a small carbon source or neutral in the early part of the century to a sink later, with a large inter‐annual and spatial variability due to changes of climate and permafrost conditions. Net primary production and soil respiration increased by 0.52 and 0.22 Tg C year−1, respectively, resulting in a regional carbon sink increase of 0.3 Tg C year−1. By the end of the century, the regional carbon sink reached 36 Tg C year−1 and carbon storage in vegetation and soils is 32 and 16 Pg C, respectively. On the plateau, from west to east, the net primary production, soil respiration and net ecosystem production increased, due primarily to the increase of air temperature and precipitation and lowering elevation. In contrast, the decrease of carbon fluxes from south to north was primarily controlled by precipitation gradient. Dynamics of air temperature and associated soil temperature and active layer depth resulted in a higher plant carbon uptake than soil carbon release, strengthening the regional carbon sink during the century.Main conclusions We found that increasing soil temperature and deepening active layer depth enhanced soil respiration, increasing the net nitrogen mineralization rate. Together with the effects of warming air temperature and rising CO2 concentrations on photosynthesis, the stronger plant nitrogen uptake due to the enhanced available nitrogen stimulates plant carbon uptake, thereby strengthening the regional carbon sink as the rate of increase net primary production was faster than that of soil respiration. Further, the warming and associated soil thermal dynamics shifted the regional carbon sink from the middle of July in the early 20th century to early July by the end of the century. Our study suggests that soil thermal dynamics should be considered for future quantification of carbon dynamics in this climate‐sensitive region.

Variability of indices of macronutrient availability in soils at different spatial scales along an elevation transect in tropical moist forests (NE Ecuador)

The availability of key plant nutrients may change with elevation in tropical mountains due to altitudinal gradients in temperature and moisture which affect pedogenesis and nutrient cycling. In a transect from upper lowland to montane forests in NE Ecuador, we tested the hypotheses that (1) the availability of P is low in low-elevation forests but increases upslope, while the availability of N is relatively high at low elevations but decreases with elevation, and (2) increasing amounts of calcium, magnesium and potassium are stored on top of the soil with progressive humus accumulation toward higher elevations, likely to improve nutrient availability. In each 20 plots in undisturbed natural forest at 500, 1000, 1500 and 2000 m a.s.l., we measured in situ N net mineralization and nitrification rate (NNM and NNI, buried bag method), plant-available phosphorus (Pa, resin-bag method), and salt-exchangeable calcium, potassium and magnesium concentrations (Caex, Kex, Mgex) in the organic and mineral topsoils. NNM and NNI, and the Caex, Kex and Mgex concentrations were much more variable at the plot level than across the four elevations, while Pa varied equally at small and large spatial scales. Pa increased more than 10fold from 500 to 2000 m. The net release of nitrate dominated over ammonium at all elevations. While mass-related NNM and NNI rates and also organic matter C/N ratio in the topsoil remained invariant along the slope, NNM and NNI rates per ground area decreased by about 40% from 500 to 2000 m. Thus, the NNM/Pa ratio decreased markedly with elevation proving our first hypothesis. In support of the second hypothesis, the pools of Caex, Mgex and Kex in the organic layers increased with elevation, demonstrating the key role that organic topsoil horizons are playing for forest nutrition at high elevations. We suggest that the large difference in N versus P availability of tropical (upper) lowland and montane forests is likely to be a key factor influencing the species composition and productivity along tropical mountain slopes.

Invasive Grasses Increase Nitrogen Availability in California Grassland Soils

AbstractAs Europeans colonized California, they introduced annual grasses from the Mediterranean Basin. These exotic annual grasses eventually invaded grasslands throughout the state, some of which were once dominated by native perennial grass species. Annual grasses differ from perennials in their phenology, longevity, rooting depth, litter chemistry, and interaction with the microbial community. As these traits may influence plant nitrogen (N) use, it is likely that the invasion by annual species resulted in changes in the availability and cycling of N in California grassland systems. We addressed the question of how invasive annual grasses influence rates of N cycling by measuring N pool sizes and rates of net and gross mineralization and nitrification, gross immobilization, and the denitrification potential of soils from experimentally planted annual and perennial-dominated grasslands. With an increase in annual grass cover, we saw increases in ammonium (NH4+) pool sizes and rates of N mineralization, nitrification, and denitrification in soils. These differences in N status suggest that N cycling in California grasslands was altered at sites where native perennial bunchgrasses were invaded by nonnative annual grasses. One consequence of annual grass invasion may be a legacy of NH4+-enriched soils that hinder the reestablishment of native perennial grass species.

Effects of summer catch crop, residue management, soil temperature and water on the succeeding cucumber rhizosphere nitrogen mineralization in intensive production systems

Nitrogen nutrient management is crucially important in shallow-rooted vegetable production systems characterized by high input and high environmental risk. To investigate the effects of summer catch crop (sweet corn, common bean, garland chrysanthemum and edible amaranth), residue management, and soil temperature and water on the succeeding cucumber rhizosphere nitrogen mineralization in intensive production systems, we determined the rates of net nitrogen mineralization and nitrification in a 4-year field experiment on greenhouse cucumber double-cropping systems. Summer catch crop and its residue significantly increased the succeeding cucumber rhizosphere mineral nitrogen contents, when compared to conventional practices. In general, summer catch crop and its residue significantly increased the rates of both net nitrogen mineralization and net nitrogen nitrification at 4 or 40°C, and increased the rates of net nitrogen immobilization (negative mineralization) and net nitrogen nitrification at 15 or 28°C, in succeeding cucumber rhizosphere after four-year treatment. Soil temperature and water had more influence than catch crops and residue management on N mineralization. The effect of carbon on nitrogen mineralization was more pronounced than that of nitrogen, and the effect of microbial carbon on the different forms of inorganic N was more pronounced than that of organic carbon. When the effects of soil temperature and water content were eliminated, cumulative net nitrogen mineralization and nitrification in catch crop and residue management plots were 296–784 and 57–84% higher, respectively, than conventional practices plots. Catch crops and residue management influenced change of ammonium-N more significantly than that of nitrate-N. Additionally, there were complex relationships between fruit yield and soil N mineralization in catch crop- and residue management-induced systems.

The interaction between decomposition, net N and P mineralization and their mobilization to the surface water in fens

Worldwide, fens and peat lakes that used to be peat-forming systems have become a significant source of C, N and P due to increased peat decomposition. To test the hypothesis that net nutrient mineralization rates may be uncoupled from decomposition rates, we investigated decomposition and net mineralization rates of nutrients in relation to sediment and pore water characteristics. We incubated 28 non-calcareous peat sediments and floating fen soils under aerobic and anaerobic conditions. We also tried to find a simple indicator to estimate the potential nutrient mobilization rates from peat sediments to the water layer by studying their relation with sediment and pore water characteristics in 44 Dutch non-calcareous peat lakes and ditches. Decomposition rates were primarily determined by the organic matter content, and were higher under aerobic conditions. However, highly decomposed peat sediments with low C:P and C:N ratios still showed high net nutrient mineralization rates. At Fe:PO 4 ratios below 1 mol mol −1, PO 4 mobilization from the sediment to the water layer was considerable and linearly related to the pore water PO 4 concentration. At higher ratios, there was a strong linear correlation between the Fe:PO 4 ratio and PO 4 mobilization. Hence, measuring Fe and PO 4 in anaerobic sediment pore water provides a powerful tool for a quick assessment of internal PO 4 fluxes. Mobilization of mineral N was largely determined by diffusion. Total sediment Fe:S ratios gave an important indication of the amount of Fe that is available to immobilize PO 4. Pore water Fe concentrations decreased at ratios <1 mol mol −1, whereas pore water PO 4 concentrations and PO 4 mobilization to the water layer increased. As PO 4 mobilization rates from the sediment to the water layer contribute to almost half of the total P load in Dutch peat lakes and fens, it is of pivotal importance to examine the magnitude of internal fluxes. Dredging of the nutrient-rich upper sediment layer will only be a useful restoration measure if both the influx of P-rich water and its internal mobilization from the newly exposed, potentially more reactive peat layer are sufficiently low.

Characteristics of N Mineralization in Urban Soils of Hefei, East China

Nitrogen cycling has been poorly characterized in urban ecosystems. An in-situ buried bag incubation technique was used to quantify net rates of N mineralization and nitrification in soils of two urban sites, a street greening belt and a university campus, and a suburban site, a forest park, in Hefei, East China. The average concentration of extractable NO − 3 in the surface soil (0–10 cm) was significantly higher at the urban sites than the suburban park site, whereas extractable NH + 4 concentration was significantly higher at the suburban park site than the urban sites. The forest park soil had greater potential N mineralization (148.1 μg N cm −3) than the soils from the campus (138.3 μg N cm −3)and street (99.8 μg N cm −3). The net mineralization rates varied between 1.63 and 2.69 μg N cm −3 d −1 and net nitrification rates between 0.82 and 1.02 μg N cm −3 d −1 at the suburban forest park site, but the rates varied from 1.27 to 2.41 μg N cm −3 d −1 and from 1.07 to 1.49 μg N cm −3 d −1, respectively, at the urban campus site. Both net mineralization and nitrification rates were lower during dry seasons. Results from regression analysis indicated that net N mineralization was significantly and positively correlated with soil moisture and soil C/N ratio, and was negatively correlated with soil pH. Relative nitrification was, however, significantly and negatively correlated with soil moisture and soil C/N ratio, and was positively correlated with soil pH. Mean relative nitrification was 0.763, indicating the dominance of nitrate cycling relative to ammonium cycling at the urban sites. The urban soils had the great potential for N losses compared to the suburban soils.

Soils as agents of selection: feedbacks between plants and soils alter seedling survival and performance

Soils are one of the first selective environments a seed experiences and yet little is known about the evolutionary consequences of plant-soil feedbacks. We have previously found that plant phytochemical traits in a model system, Populus spp., influence rates of leaf litter decay, soil microbial communities and rates of soil net nitrogen mineralization. Utilizing this natural variation in plant-soil linkages we examined two related hypotheses: (1) Populus angustifolia seedlings are locally adapted to their native soils; and (2) Soils act as agents of selection, differentially affecting seedling survival and the heritability of plant traits. We conducted a greenhouse experiment by planting seedlings from 20 randomly collected P. angustifolia genetic families in soils conditioned by various Populus species and measured subsequent survival and performance. Even though P. angustifolia soils are less fertile overall, P. angustifolia seedlings grown in these soils were twice as likely to survive, grew 24% taller, had 27% more leaves, and 29% greater above-ground biomass than P. angustifolia seedlings grown in non-native P. fremontii or hybrid soils. Increased survival resulted in higher trait variation among seedlings in native soils compared to seedlings grown in non-native soils. Soil microbial biomass varied significantly across soil environments which could explain more of the variation in seedling performance than soil texture, pH, or nutrient availability, suggesting strong microbial interactions and feedbacks between plants, soils, and associated microorganisms. Overall, these data suggest that a “home-field advantage” or a positive plant soil feedback helps maintain genetic variance in P. angustifolia seedlings.

Nitrogen availability from peat amendments used in boreal oil sands reclamation

Following surface mining, peat is typically used as an organic amendment to cap reconstructed soils in the Athabasca oil sands region of Alberta. Yet, very little is known about its ability to provide available nitrogen (N) in these soils. Hence, the overall objective of this study was to measure soil nitrogen (N) availability throughout the year in five peat amendments. Specific objectives were: (1) to examine seasonal variability in soil labile N pool sizes (nitrate, ammonium, dissolved organic N, and microbial biomass N), and (2) to determine in situ net nitrification, ammonification, and mineralization rates using the resin-core technique. Results from this field incubation method indicated a strong seasonal variability in net mineralization rates, with maximum positive values in the fall, and low or negative rates in winter. Net ammonification rates, which were significantly correlated to soil moisture content, were significantly smaller and showed smaller seasonal fluctuations and fewer differences among peat materials than net nitrification rates. Furthermore, the contribution of net nitrification to total net mineralization rates was characteristically higher than what is typically observed in undisturbed boreal forest soils. Taken together, results indicate that net nitrification processes may control nitrogen availability in these reclaimed soils.Key words: Soil nitrogen, soil reclamation, nitrification, mineralization, boreal soils

Net nitrogen mineralization and nitrification rates in forest soil in northeastern Taiwan

We used a laboratory incubation approach to measure rates of net N mineralization and nitrification in forest soils from Fu-shan Experimental Forest WS1 in northern Taiwan. Net mineralization rates in the O horizon ranged from 4.0 to 13.8 mg N kg−1 day−1, and net nitrification rates ranged from 2.2 to 11.6 mg N kg−1 day−1. For mineral (10–20 cm depth) soil, net mineralization ranged from 0.06 to 2.8 mg N kg−1 day−1 and net nitrification rates ranged from 0.02 to 2.8 mg N kg−1 day−1. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. We compared the rates of these processes in three soil horizons (to a soil depth of 30 cm) on a single sampling date and found a large decrease in both net N mineralization and nitrification with depth. We estimated that the soil total N pool was 6,909 kg N ha−1. The present study demonstrates the importance of the stock of mineral soil N in WS1, mostly organic N, which can be transformed to inorganic N and potentially exported to surface and ground water from this watershed. Additional studies quantifying the rates of soil N cycling, particularly multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in nitrogen cycling.

Organic materials: sources of nitrogen in the organic production of lettuce

This paper presents the results of 2 experiments: an incubation experiment and a subsequent field experiment. An incubation experiment was set up in order to determine the mineralization potential of different organic materials (OMs) (well-rotted farmyard manure [FTM], guano [G], soybean seed [S], and forage pea seed [P]), the kinetics of mineral nitrogen (N) release, and the correlation between OM content and the quantity of mineralized N. The results of the incubation experiment were checked under field conditions in which different OMs were used as N sources for lettuce (Lactuca sativa L.). The highest mineralization rate constant k (0.127 day^{-1}) and net mineralization rate (NM) (76.37% of total applied N) were obtained with G, and the lowest k (0.098 day^{-1}) and NM (30.12% of total applied N) were obtained with FYM. In the field experiment in 2007 and 2008 we applied different OM sources that contained the same amount of potentially mineralizable N (40 and 35 kg N ha^{-1}, respectively), calculated on the basis of k and NM. The use of OMs increased the concentration of mineral N in the soil and increased the yield of fresh lettuce. Nitrate content in the fresh lettuce was within acceptable limits both years (

10.1007/s11184-008-1004-x

The effect of afforestation on the activity of microbiological mineralization of soil organic matter has been studied in Siberia. The results show that this effect concerns mainly net nitrogen mineralization and net nitrification, while carbon mineralization (CO2 formation) does not depend on the type of ecosystem. It is proposed to use the rates of net nitrogen mineralization and nitrification as the most sensitive indicators of changes in an ecosystem.

Snow-induced changes in dwarf birch chemistry increase moth larval growth rate and level of herbivory

Changes in snow cover might influence arctic ecosystems to the same extent as increased temperatures. Although the duration of snow cover is generally expected to decrease in the future as a result of global warming, the amounts of snow might increase in arctic areas where much of the elevated precipitation will fall as snow. We examined the effects of an increased snow cover, as a result of a snow fence treatment, on soil nitrogen mineralization, plant phenology, plant chemistry (nitrogen and potential defense compounds), the level of invertebrate herbivory, and performance of invertebrate herbivores in an arctic ecosystem, using dwarf birch (Betula nana) and the autumnal moth (Epirrita autumnata) as study organisms. An enhanced and prolonged snow cover increased the level of herbivory on dwarf birch leaves. Larvae feeding on plants that had experienced enhanced snow cover grew faster and pupated earlier than larvae fed with plant material from control plots, indicating that plants from enhanced snow-lie plots produce higher-quality food to herbivores. The increased larval growth rate was strongly correlated with higher leaf nitrogen concentration in plants subjected to snow manipulation, and also to certain phenolic acids. Snow manipulation did not change net nitrogen mineralization rates in the soil or total carbon concentration in leaves, but it altered the within-season fluctuating pattern of leaf phenolic compounds. This study demonstrates a positive relationship between increased snow cover and level of herbivory on deciduous shrubs, thus proposing a negative feedback on the climate-induced dwarf shrub expansion in arctic areas.

Biomass allocation and nitrogen limitation in a Cryptomeria japonica plantation chronosequence

We investigated soil net nitrogen mineralization rate, above- and belowground biomass allocation, and nitrogen use in a Cryptomeria japonica plantation chronosequence. Total biomass accumulation showed an asymptotic accretion pattern, and the peak total biomass accumulation rate occurred approximately 30 years after afforestation. Soil net nitrogen mineralization rate was lowest 30 years after afforestation. Between years 30 and 88, net nitrogen mineralization increased again. These results indicate that an imbalance in soil nitrogen supply and plant nitrogen demand occurred approximately 30 years after afforestation. Furthermore, leaf nitrogen concentration, which was used as an index of plant nitrogen status, was lower in mature forest than in young forest, suggesting that mature stands did not take up nitrogen as successfully. If soil resources such as nitrogen limit plant growth, plants may increase biomass allocation to fine root structure; however, fine root biomass was not higher in 30- and 88-year-old stands than in younger stands, suggesting that changes in biomass allocation may not be effective against nitrogen deficiency in a C. japonica plantation chronosequence.

Nitrogen mineralisation in the soil of indigenous oak and pine plantation forests in a Mediterranean environment

Nitrogen mineralisation in soils of various forest sites (pine plantation, natural and thinned oak) at Uludağ University campus in Bursa, Turkey was investigated continuously over a year by the field incubation method. Net nitrogen mineralisation and nitrification rates varied depending on sampling dates. Although nitrogen mineralisation and nitrification rates increased in the spring and summer months, there was no seasonal variation in the soils of the examined forests. Annual net nitrate (NO 3 −–N) accumulation in the upper soil layer (0–5 cm) was higher in Oak I and Oak II (14 kg ha y −1 and 12 kg ha y −1) than in the pine plantation (8 kg ha y −1). While annual net NO 3 −–N accumulation (0–5 cm) varied between the oak forests (possibly due to forest management practices), annual net N min values were similar in these forests. No significant correlation was found between the examined soil parameters and net nitrification and mineralisation rates in the soils ( P > 0.05). These results indicate that tree species and forest management practices play important roles in N cycling in forest ecosystems.

The effects of changes in soil moisture on nitrogen cycling in acid wetland types of the New Jersey Pinelands (USA)

Wetlands are subject to changes in soil moisture as a result of both short-term seasonal climate variations and long-term changes in regional water resource management, both of which can modify the dynamics of ground and surface water inputs. In the New Jersey Pinelands, forested wetlands that differ in both plant communities and soil structure occur along a topographic and hydrological gradient associated with an unconfined aquifer. Proposed groundwater withdrawals may affect water content of soils along this gradient. We hypothesized that prolonged changes in soil moisture would alter net nitrogen mineralization and nitrification rates in proportion to the amount of moisture change, and that these changes would be similar for the different soils along the drainage catena. Soils from two catenary sequences of wetlands, including pine-dominated (driest landscape position), hardwood-dominated, and Atlantic white-cedar-dominated (wettest landscape position) communities were used in long-term laboratory incubations (36 weeks). Production of NH 4 +–N, NO 3 −–N, and dissolved organic N were measured under two sets of conditions: constant moisture levels of 100%, 60% and 30% water-holding capacity (WHC), and fluctuating moisture levels (alternating 2 week periods at 100% and 30% WHC). In soils from most of the wetlands, we observed increases in net mineralization and nitrification when constant low-moisture conditions were established, but not under fluctuating conditions. Contrary to expectations, responses to the drying treatments varied between wetland types and between replicate wetlands of each type. Under constant-moisture conditions, nitrification increased more in cedar swamps than in either type of pine wetland. Under all conditions, soils from all the wetlands within one of the catenas produced more inorganic and organic soluble N than did the wetlands from the other catena, suggesting that area-wide effects are as important as wetland type in regulating production of soluble N. Within both catenas, pine-hardwood wetlands generated more soluble N under all moisture conditions than did either pine-dominated or cedar wetlands. Our results suggest that changes in soil moisture due to management of water resources will affect N cycling in wetland soils, but that the magnitude of the effects, and the potential for large releases of nitrate, will depend on the specific soil properties of affected wetlands.

Changes in net N mineralization rates and soil N and P pools in a pine forest wildfire chronosequence

The concern that climate change may increase fire frequency and intensity has recently heightened the interest in the effects of wildfires on ecosystem functioning. Although short-term fire effects on forest soils are well known, less information can be found on the long-term effects of wildfires on soil fertility. Our objective was to study the 17-year effect of wildfires on forest net mineralization rates and extractable inorganic nitrogen (N) and phosphorus (P) concentrations. We hypothesize that (1) burned forest stands should exhibit lower net mineralization rates than unburned ones; (2) these differences would be greatest during the growing season; (3) differences between soil variables might also be observed among plots from different years since the last fire; and (4) due to fire-resistant geochemical processes controlling P availability, this nutrient should recover faster than N. We used a wildfire chronosequence of natural and unmanaged Pinus canariensis forests in La Palma Island (Canary Islands). Soil samples were collected during winter and spring at 22 burned and unburned plots. We found significantly higher values for net N mineralization and extractable N pools in unburned plots. These differences were higher for the winter sampling date than for the spring sampling date. Unlike extractable N and N mineralization rates, extractable P levels of burned plots exhibited a gradual recovery over time after an initial decrease. These results demonstrate that P. canariensis forest soils showed low resilience after wildfires, especially for N, and that this disturbance might induce long-term changes in ecosystem functioning.

Effects of prescribed burning and seasonal and interannual climate variation on nitrogen mineralization in a typical steppe in Inner Mongolia

Fires in grasslands significantly alter nutrient cycling processes. Seasonal climatic changes can interact with fire to further modify nutrient cycling processes. To investigate the effects of fire on soil nitrogen transformation processes and their seasonal change and interannual variability in a typical steppe in Inner Mongolia, we determined the rates of net nitrogen mineralization and nitrification over two growing seasons and a winter following a prescribed spring fire in May 2006. Fire significantly decreased rates of both net nitrogen mineralization and net nitrification during the first growing season and winter following burning. Cumulative net nitrogen mineralization in unburned and burned plots in the 2006 growing season was 133% and 183% higher, respectively, than in the drier 2007 growing season. Nitrogen mineralization apparently occurred in winter and the cumulative net nitrogen mineralization from October 2, 2006, to April 27, 2007 in unburned and burned plots amounted to 1.18 ± 0.25 g N m −2 and 0.51 ± 0.08 g N m −2, respectively. Cumulative net nitrogen mineralization was higher in a wet 2006 than in a dry 2007 growing season, indicating that the net N mineralization rate was sensitive to soil moisture in a dry season. Our study demonstrated that a one-time prescribed fire decreased net N mineralization rates only for a short period of time after burning while interannual variation in climate had more significant effects on the process of nitrogen mineralization.

Rapid Assessment of Urban Wetlands: Do Hydrogeomorphic Classification and Reference Criteria Work?

The Hydrogeomorphic (HGM) functional assessment method is predicated on the ability of hydrogeomorphic wetland classification and visual assessment of alteration to provide reference standards against which functions in individual wetlands can be evaluated. The effectiveness of this approach was tested by measuring nitrogen cycling functions in forested wetlands in an urbanized region in New Jersey, USA. Fourteen sites represented three HGM classes and were characterized as "least disturbed reference" or "non-reference" based on initial visual assessment. Water table levels and in situ rates of net nitrogen mineralization, net nitrification, and denitrification were measured over one year in each site. Hydrological alterations, resulting in consistently low or flashy water table levels, were not correlated with a priori designations as reference and non-reference. Although the flat-riverine wetland class had lower net nitrification and higher denitrification rates than riverine or mineral flat wetland classes, this difference was attributable to the lack of hydrologically-altered wetlands in the flat-riverine class, and thus more consistently wet conditions. Within all HGM classes, a classification based on the long-term hydrological record that separated sites with "normal," saturated hydrology from those with "altered," drier hydrology, clearly distinguished sites with different nitrogen cycling function. Based on these findings, current practices for designating reference standard sites to judge wetland functions, at least in urbanized regions, are ineffective and potentially misleading. At least one year of hydrological monitoring data is suggested to classify wetlands into groups that have different nutrient cycling functions, particularly in urban landscapes.

Rhizosphere soil microbial community structure and microbial activity in set-aside and intensively managed arable land

The effects of plant species which frequently occur in set-aside arable land on rhizosphere soil properties were assessed and compared to rhizosphere soil of Secale cereale (Rye) grown on an intensively managed arable soil (sandy Cambisol, Saxony, NE- Germany). On a 6 year old set-aside arable land rhizosphere soil samples were taken under Agropyron repens, Cirsium arvense and Rumex acetosa, the most frequent plant species, and under the leguminous plant species Vicia villosa. Phospholipid fatty acid analysis (PLFA) has been used to characterise the structure of the soil microbial community. Carbon mineralisation rates as well as gross ( 15 N isotope pool dilution method) and net nitrogen mineralisation rates were determined as indicator of microbial activity. In intensive managed plots a rhizosphere effect was obvious in higher nutrient contents, gross N mineral- isation rates and higher relative abundances of fungi and protozoa in Rye rhizosphere compared to bulk soil. Plant species altered rhizosphere microbial activity. Lowest gross N mineralisation and gross NH4 con- sumption rates were detected in Rye rhizosphere soil. Both rates revealed high positive correlations with dissolved organic carbon (extracted with KCl) and soil pH. The rhizosphere soil microbial communities of the three dominant plant species of the set-aside arable land (Agropyron, Cirsium, Rumex) were more similar to each other than to Vicia grown on the same set-aside plots and Rye grown on intensive managed plots. The highest number of non-identified PLFAs detected in Vicia rhizosphere soil suggests that microbial diversity was highest. Differences in quantity and quality of Vicia rhizodeposition, especially higher N contents, seem to induce the higher microbial activity and different microbial community structure. The rhizo- sphere soil of the dominant plant species on the set- aside and intensively managed arable land reflected the differences in bulk soil properties (obtained in a previous study) between the two management systems (e.g. pH, gross N mineralisation, metabolic quotient, PLFA marker characteristic of Gbacteria and fungi).