Recovery of soil organic matter,organic matter turnover and nitrogen cycling in a post-mining forest rehabilitation chronosequence

Recovery of soil organic matter, organic matter turnover and mineral nutrient cycling is critical to the success of rehabilitation schemes following major ecosystem disturbance. We investigated successional changes in soil nutrient contents, microbial biomass and activity, C utilisation efficiency and N cycling dynamics in a chronosequence of seven ages (between 0 and 26 years old) of jarrah (Eucalyptus marginata) forest rehabilitation that had been previously mined for bauxite. Recovery was assessed by comparison of rehabilitation soils to non-mined jarrah forest references sites. Mining operations resulted in significant losses of soil total C and N, microbial biomass C and microbial quotients. Organic matter quantity recovered within the rehabilitation chronosequence soils to a level comparable to that of non-mined forest soil. Recovery of soil N was faster than soil C and recovery of microbial and soluble organic C and N fractions was faster than total soil C and N. The recovery of soil organic matter and changes to soil pH displayed distinct spatial heterogeneity due to the surface micro-topography (mounds and furrows) created by contour ripping of rehabilitation sites. Decreases in the metabolic quotient with rehabilitation age conformed to conceptual models of ecosystem energetics during succession but may have been more indicative of decreasing C availability than increased metabolic efficiency. Net ammonification and nitrification rates suggested that the low organic C environment in mound soils may favour autotrophic nitrifier populations, but the production of nitrate (NO₃^?) was limited by the low gross N ammonification rates (≤1 μg N g^?1 d^?1). Gross N transformation rates in furrow soils suggested that the capacity to immobilise N was closely coupled to the capacity to mineralise N, suggesting NO₃^? accumulation in situ is unlikely. The C:N ratio of the older rehabilitation soils was significantly lower than that of the non-mined forest soils. However, variation in ammonification rates was best explained by C and N quantity rather than C:N ratios of whole soil or soluble organic matter fractions. We conclude that the rehabilitated ecosystems are developing a conservative N cycle as displayed by non-mined jarrah forests. However, further investigation into the control of nitrification dynamics, particularly in the event of further ecosystem disturbance, is warranted.     

Impact of land-use types on soil nitrogen net mineralization in the sandstorm and water source area of Beijing,China

Changes of land-use type (LUT) can affect soil nutrient pools and cycling processes that relate long-term sustainability of ecosystem, and can also affect atmospheric CO₂ concentrations and global warming through soil respiration. We conducted a comparative study to determine NH₄⁺ and NO₃⁻ concentrations in soil profiles (0–200 cm) and examined the net nitrogen (N) mineralization and net nitrification in soil surface (0–20 cm) of adjacent naturally regenerated secondary forests (NSF), man-made forests (MMF), grasslands and cropland soils from the windy arid and semi-arid Hebei plateau, the sandstorm and water source area of Beijing, China. Cropland and grassland soils showed significantly higher inorganic N concentrations than forest soils. NO₃⁻-N accounted for 50–90% of inorganic N in cropland and grassland soils, while NH₄⁺-N was the main form of inorganic N in NSF and MMF soils. Average net N-mineralization rates (mg kg⁻¹ d⁻¹) were much higher in native ecosystems (1.51 for NSF soils and 1.24 for grassland soils) than in human disturbed LUT (0.15 for cropland soils and 0.85 for MMF soils). Net ammonification was low in all the LUT while net nitrification was the major process of net N mineralization. For more insight in urea transformation, the increase in NH₄⁺ and, NO₃⁻ concentrations as well as C mineralization after urea addition was analyzed on whole soils. Urea application stimulated the net soil C mineralization and urea transformation pattern was consistent with net soil N mineralization, except that the rate was slightly slower. Land-use conversion from NSF to MMF, or from grassland to cropland decreased soil net N mineralization, but increased net nitrification after 40 years or 70 years, respectively. The observed higher rates of net nitrification suggested that land-use conversions in the Hebei plateau might lead to N losses in the form of nitrate.     

Effect of freezing on soil nitrogen mineralization under different plant communities in a semi-arid area during a non-growing season

The influences of winter climate on terrestrial ecosystem processes have been the subject of growing attention, which is necessary to make the predictions about ecological responses to global warming in the future. However, little information can be found about the impacts of a large range of soil temperature fluctuation (e.g. −10 to 5 °C) over winter on the soil nitrogen (N) dynamics in the field. In the present study, we employed an intact soil core in situ incubation technique, and measured soil N mineralization and nitrification rates under three plant communities, i.e. a grassland, a shrub and a plantation, during the non-growing season (October 2004–April 2005) in Inner Mongolia, China. Our results demonstrate the significant effects of different plant communities on soil net N mineralization and the great temporal variations of soil N dynamics during the incubation period. The mean soil net N mineralization rates were 0.93, 0.77 and −1.28 mg N m⁻² d⁻¹, respectively, in the grassland, shrub and plantation. The mean soil NH₄⁺-N in the three plant communities declined by 40%, but the mean soil NO₃⁻-N increased by 190% by the end of the incubation compared with their initial concentrations at the beginning of incubation. The differences in plant communities significantly affected their soil N mineralization rates, accumulations and turnover rates, which followed the order: grassland > shrub > plantation. During the winter time, the studied soils experienced the three phases consisting of mild freezing (−7 to −2 °C soil), deep freezing (approximately −10 °C soil) and freeze–thaw (−2 to 5 °C soil). The results suggest that temporal variations of soil N mineralization are positively affected by the soil temperature and the soil nitrification is dominant in the N transformation process during the non-growing season. Our study indicates that the soil N mineralization over winter can make a substantial contribution to the mineral N pool that plants are able to utilize in the upcoming spring, but may also pose a great risk of mineral N leaching loss if great rainfalls occur during spring and early summer.     

Moisture pulses,trace gas emissions and soil C and N in cheatgrass and native grass-dominated sagebrush-steppe in Wyoming,USA

Effects of large-scale weed invasion on the nature and magnitude of moisture-pulse-driven soil processes in semiarid ecosystems are not clearly understood. The objective of this study was to monitor carbon dioxide (CO₂) and nitrous oxide (N₂O) emissions and changes in soil carbon (C) and nitrogen (N) following the application of a water pulse in Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) communities dominated by the exotic annual grass cheatgrass (Bromus tectorum) and by the native perennial grass western wheatgrass (Pascopyrum smithii). Sampling locations were established in shrub interspaces dominated by B. tectorum and P. smithi and beneath shrub canopies adjacent to interspaces dominated by B. tectorum and P. smithi, where no grass was present. Soils were classified as fine-loamy, mixed, Borollic Haplargids. Soil samples (0–10 cm) and air samples were collected at 0, 4, 8, 24, 49, 72, and 216 h following additions of 25.4 mm of water. Soil samples were analyzed for dissolved organic carbon (DOC), microbial biomass carbon (MBC), extractable ammonia (NH₄⁺), extractable nitrate (NO₃^?), and dissolved organic nitrogen (DON). Grass species induced differences in soil nitrification, N₂O and CO₂ emissions, and the quantity and timing of labile C available to microbial populations responding to increased moisture availability. In the first 8-h phase after wetting P. smithii soils had the greatest CO₂ emissions compared to other soils but B. tectorum soils had the greatest N₂O emissions and the greatest increases in CO₂ emissions relative to before wetting. Microbial biomass C in B. tectorum interspace soils increased rapidly but the response was short-lived despite sufficient water availability. After the first 8 h of soil response to wetting, the observed MBC declines in B. tectorum interspace coincided with disproportional DOC and DON concentration increases. Similar DOC and DON increases were also observed in B. tectorum soils beneath shrub canopy. In contrast, DOC and DON concentrations in P. smithii soils remained unaffected by soil wetting and small MBC increases observed during the first 8-h phase did not decline as rapidly as in B. tectorum interspace soils. In conclusion, summer drying-wetting cycles that occur frequently in areas invaded by B. tectorum can accelerate rates of nitrification and C mineralization, and contribute significantly to trace gas emissions from sagebrush-steppe grasslands. With frequent summer rainfall events, the negative consequences B. tectorum presence in the ecosystem can be significant.     

Effects of soil moisture on gross N transformations and N2O emission in acid subtropical forest soils

Soil moisture changes, arising from seasonal variation or from global climate changes, could influence soil nitrogen (N) transformation rates and N availability in unfertilized subtropical forests. A ¹⁵?N dilution study was carried out to investigate the effects of soil moisture change (30–90 % water-holding capacity (WHC)) on potential gross N transformation rates and N₂O and NO emissions in two contrasting (broad-leaved vs. coniferous) subtropical forest soils. Gross N mineralization rates were more sensitive to soil moisture change than gross NH₄ ⁺ immobilization rates for both forest soils. Gross nitrification rates gradually increased with increasing soil moisture in both forest soils. Thus, enhanced N availability at higher soil moisture values was attributed to increasing gross N mineralization and nitrification rates over the immobilization rate. The natural N enrichment in humid subtropical forest soils may partially be due to fast N mineralization and nitrification under relatively higher soil moisture. In broad-leaved forest soil, the high N₂O and NO emissions occurred at 30 % WHC, while the reverse was true in coniferous forest soil. Therefore, we propose that there are different mechanisms regulating N₂O and NO emissions between broad-leaved and coniferous forest soils. In coniferous forest soil, nitrification may be the primary process responsible for N₂O and NO emissions, while in broad-leaved forest soil, N₂O and NO emissions may originate from the denitrification process.     

The influence of soluble carbon and fertilizer nitrogen on nitric oxide and nitrous oxide emissions from two contrasting agricultural soils

Contradictory effects of simultaneous available organic C and N sources on nitrous oxide (N₂O), carbon dioxide (CO₂) and nitric oxide (NO) fluxes are reported in the literature. In order to clarify this controversy, laboratory experiments were conduced on two different soils, a semiarid arable soil from Spain (soil I, pH=7.5, 0.8%C) and a grassland soil from Scotland (soil II, pH=5.5, 4.1%C). Soils were incubated at two different moisture contents, at a water filled pore space (WFPS) of 90% and 40%. Ammonium sulphate, added at rates equivalent to 200 and 50 kg N ha^?1, stimulated N₂O and NO emissions in both soils. Under wet conditions (90% WFPS), at high and low rates of N additions, cumulative N₂O emissions increased by 250.7 and 8.1 ng N₂O–N g^?1 in comparison to the control, respectively, in soil I and by 472.2 and 2.1 ng N₂O–N g^?1, respectively, in soil II. NO emissions only significantly increased in soil I at the high N application rate with and without glucose addition and at both 40% and 90% WFPS. In both soils additions of glucose together with the high N application rate (200 kg N ha^?1) reduced cumulative N₂O and NO emissions by 94% and 55% in soil I, and by 46% and 66% in soil II, respectively. These differences can be explained by differences in soil properties, including pH, soil mineral N and total and dissolved organic carbon content. It is speculated that nitrifier denitrification was the main source of NO and N₂O in the C-poor Spanish soil, and coupled nitrification–denitrification in the C-rich Scottish soil.     

Differing N status and N retention processes of soils under old-growth lowland forest in Eastern Amazonia,Caxiuanã, Brazil

Indirect evidence of the nitrogen (N) status of tropical forests strongly suggests that in heavily weathered soils under old-growth lowland tropical forests nitrogen is in relative excess. However, within the lowland forests of the Amazon basin, there is substantial evidence that soil texture influences soil NH₄⁺ and NO₃^? concentrations and hence possibly N availability and retention in the soil. Here, we evaluate the soil N status of two heavily weathered soils which contrast in texture (sandy versus clay Oxisol). Using ¹⁵N pool dilution, we quantified gross rates of soil N cycling and retention. We also measured the δ¹⁵N signatures from the litter layer down to 50-cm depth mineral soil and calculated the overall ¹⁵N enrichment factor (ε) for each soil type. The clay soil showed high gross N mineralization and nitrification rates and a high overall ¹⁵N enrichment factor, signifying high N losses. The sandy soil had low gross rates of N cycling and ¹⁵N enrichment factor, manifesting a conservative soil N cycling. Faster turnover rates of NH₄⁺ compared to NO₃^? indicated that NH₄⁺ cycles faster through microorganisms than NO₃^?, possibly contributing to better retention of NH₄⁺ than NO₃^?. However this was opposite to abiotic retention processes, which showed higher conversion of NO₃^? to the organic N pool than NH₄⁺. Our combined results suggest that clay Oxisol in Amazonian forest have higher N availability than sandy Oxisol, which will have important consequences for changes in soil N cycling and losses when projected increase in anthropogenic N deposition will occur.     

好氧条件下两种水稻土的氮素总转化速率比较

Although to date individual gross N transformations could be quantified by ~(15)N tracing method and models,studies are still limited in paddy soil.An incubation experiment was conducted using topsoil(0-20 cm) and subsoil(20-60 cm) of two paddy soils,alkaline and clay(AC) soil and neutral and silt loam(NSL) soil,to investigate gross N transformation rates.Soil samples were labeled with either ~(15)NH4_NO_3 or NH_4~(15)NO_3,and then incubated at 25 °C for 168 h at 60%water-holding capacity.The gross N mineralization(recalcitrant and labile organic N mineralization) rates in AC soil were 1.6 to 3.3 times higher than that in NSL soil,and the gross N nitrification(autotrophic and heterotrophic nitrification) rates in AC soil were 2.4 to 4.4 times higher than those in NSL soil.Although gross NO_3~- consumption(i.e.,NO_3~- immobilization and dissimilatory NO_3~- reduction to NH_4~+ rates increased with increasing gross nitrification rates,the measured net nitrification rate in AC soil was approximately 2.0 to 5.1 times higher than that in NSL soil.These showed that high NO_3~- production capacity of alkaline paddy soil should be a cause for concern because an accumulation of NO_3~- can increase the risk of NO_3~- loss through leaching and denitrification.     

Bacterial and fungal contributions to soil nitrogen cycling under Douglas fir and red alder at two sites in Oregon

A study was conducted at two experimental tree plantations in the Pacific Northwest to assess the roles of bacteria and fungi in nitrogen (N) cycling. Soils from red alder (Alnus rubra) and Douglas-fir (Pseudotsuga menziesii) plots in low- (H.J. Andrews) and high- (Cascade Head) productivity stands were sampled in 2005 and 2006. Fungal:bacterial ratios were determined using phospholipid fatty acid (PLFA) profiles and quantitative (Q)-PCR. Ratios from these two molecular methods were highly correlated and showed that microbial biomass varied significantly between the two experimental sites and to a lesser extent between tree types with fungal:bacterial biomass ratios lower in more N-rich plots. ¹⁵N isotope dilution experiments, with ammonium (NH₄⁺) and nitrate (NO₃^?), were paired with antibiotics that blocked bacterial (bronopol) and fungal (cycloheximide) protein synthesis. This modified isotope dilution technique was used to determine the relative contribution of bacteria and fungi to net N mineralization and gross rates of ammonification and nitrification. When bacterial protein synthesis was blocked NH₄⁺ consumption and nitrification rates decreased in all treatments except for NH₄⁺ consumption in the Douglas-fir plots at H.J. Andrews, suggesting that prokaryotic nitrifiers are a major sink for mineral NH₄⁺ in forest soils with higher N availability. Cycloheximide consistently increased NH₄⁺ consumption, however the trend was not statistically significant. Both antibiotics additions also significantly increased gross ammonification, which may have been due to continued activity of extra- and intracellular enzymes involved in producing NH₄⁺ combined with the inhibition of NH₄⁺ assimilation into proteins. The implication of this result is that microorganisms are likely a major sink for soil dissolved organic N (DON) in soils.     

Nitrogen Saturation and Soil N Availability in a High-Elevation Spruce and Fir Forest

A field study was conducted during the summer of 1995 to gain abetter understanding of the causes of nitrate (NO₃-N)leaching and ongoing changes in soil nitrogen (N) availabilityin high-elevation (1524–2000 m) spruce (Picea rubens) andfir (Abies fraseri) forests of the Great Smoky MountainsNational Park, Tennessee and North Carolina, U.S.A. Indicatorsof soil N availability (total soil N concentrations,extractable NH₄-N, extractable NO₃-N, and C/N ratios)were measured in Oa and A horizons at 33 study plots. Dynamicmeasures included potential net soil N mineralization determinedin 12-week aerobic laboratory incubations at 22 °C.Potential net nitrification in the A horizon was correlated (r =+0.83, P < 0.001) with total soil N concentrations. Mostmeasures of soil N availability did not exhibit significanttrends with elevation, but there were topographic differences.Potential net soil N mineralization and net nitrification in theA horizon were higher in coves than on ridges. Relative amountsof particulate and organomineral soil organic matter influencedpotential net N mineralization and nitrification in the Ahorizon. Calculations indicate that soil N availability andNO₃-N leaching in high-elevation spruce and fir forests ofthe Great Smoky Mountains National Park will increase inresponse to regional warming.     

Effects of tillage and nitrogen management on soil chemical and physical properties after 23 years of continuous sorghum

Long-term tillage and nitrogen (N) management practices can have a profound impact on soil properties and nutrient availability. A great deal of research evaluating tillage and N applications on soil chemical properties has been conducted with continuous corn (Zea Mays L.) throughout the Midwest, but not on continuous grain sorghum (Sorghum bicolor (L.) Moench). The objective of this experiment was to examine the long-term effects of tillage and nitrogen applications on soil physical and chemical properties at different depths after 23 years of continuous sorghum under no-till (NT) and conventional till (CT) (fall chisel-field cultivation prior to planting) systems. Ammonium nitrate (AN), urea, and a slow release form of urea were surface broadcast at rates of 34, 67, and 135 kg N ha⁻¹. Soil samples were taken to a depth of 15 cm and separated into 2.5 cm increments. As a result of lime applied to the soil surface, soil pH in the NT and CT plots decreased with depth, ranging from 6.9 to 5.7 in the NT plots and from 6.5 to 5.9 in the CT plots. Bray-1 extractable P and NH₄OAc extractable K was 20 and 49 mg kg⁻¹ higher, respectively, in the surface 2.5 cm of NT compared to CT. Extractable Ca was not greatly influenced by tillage but extractable Mg was higher for CT compared to NT below 2.5 cm. Organic carbon (OC) under NT was significantly higher in the surface 7.5 cm of soil compared to CT. Averaged across N rates, NT had 2.7 Mg ha⁻¹ more C than CT in the surface 7.5 cm of soil. Bulk density (Δ_b) of the CT was lower at 1.07 g cm⁻³ while Δ_b of NT plots was 1.13 g cm⁻³. This study demonstrated the effect tillage has on the distribution and concentration of certain chemical soil properties.     

Nitrogen mineralization in the ruderal sub-alpine communities in Mount Uludağ, Turkey

Nitrogen mineralization and nitrification in the soil of sub-alpine ruderal community of Mount Uludağ, Bursa, Turkey was measured for 1 year, under field conditions with Verbascum olympicum and Rumex olympicus being the dominant pioneer species under dry and wet sites, respectively. Seasonal fluctuations were observed in N mineralization and nitrification. The net N mineralization and nitrification were high in early summer and winter, due to high moisture. The annual net N mineralization rate (for the 0–15 cm soil layer) was higher under R. olympicus (188 kg N ha⁻¹ yr⁻¹) than under V. olympicum (96 kg N ha⁻¹ yr⁻¹). A significant positive correlation between net N mineralization and soil organic C (r² = 0.166), total N (r² = 0.141) and water content (r² = 0.211) was found. Our results indicate that N mineralization rate is high in soils of ruderal communities on disturbed sites and varies with dominant species and, a difference in net N mineralization rate can be attributed to organic C, total N and moisture content of soils.     

Gross nitrogen mineralization-, immobilization-, and nitrification rates as a function of soil C/N ratio and microbial activity

A laboratory experiment was designed to challenge the idea that the C/N ratio of forest soils may control gross N immobilization, mineralization, and nitrification rates. Soils were collected from three deciduous forests sites varying in C/N ratio between 15 and 27. They were air-dried and rewetted to induce a burst of microbial activity. The N transformation rates were calculated from an isotope dilution and enrichment procedure, in which ¹⁵NH₄Cl or Na¹⁵NO₃ was repeatedly added to the soils during 7 days of incubation. The experiments suggested that differences in gross nitrogen immobilization and mineralization rates between the soils were more related to the respiration rate and ATP content than to the C/N ratio. Peaks of respiration and ATP content were followed by high rates of mineralization and immobilization, with 1-2 days of delay. The gross immobilization of NH₄⁺ was dependent on the gross mineralization and one to two orders of magnitude larger than the gross NO₃⁻ immobilization. The gross nitrification rates were negatively related to the ATP content and the C/N ratio and greatly exceeding the net nitrification rates. Taken together, the observations suggest that leaching of nitrate from forest soils may be largely dependent on the density and activity of the microbial community.     

Partitioning soil surface CO2 efflux into autotrophic and heterotrophic components,using natural gradients in soil δ13C in an undisturbed savannah soil

We used natural gradients in soil and vegetation δ¹³C signatures in a savannah ecosystem in Texas to partition soil respiration into the autotrophic (Ra) and heterotrophic (Rh) components. We measured soil respiration along short transects from under clusters of C₃ trees into the C₄ dominated grassland. The site chosen for the study was experiencing a prolonged drought, so an irrigation treatment was applied at two positions of each transect. Soil surface CO₂ efflux was measured along transects and CO₂ collected for analysis of the δ¹³C signature in order to: (i) determine how soil respiration rates varied along transects and were affected by localised change in soil moisture and (ii) partition the soil surface CO₂ efflux into Ra and Rh, which required measurement of the δ¹³C signature of root- and soil-derived CO₂ for use in a mass balance model.The soil at the site was unusually dry, with mean volumetric soil water content of 8.2%. Soil respiration rates were fastest in the centre of the tree cluster (1.5 ± 0.18 μmol m^?2 s^?1; mean ± SE) and slowest at the cluster–grassland transition (0.6 ± 0.12 μmol m^?2 s^?1). Irrigation produced a 7–11 fold increase in the soil respiration rate. There were no significant differences (p > 0.5) between the δ¹³C signature of root biomass and respired CO₂, but differences (p < 0.01) were observed between the respired CO₂ and soil when sampled at the edge of the clusters and in the grassland. Therefore, end member values were measured by root and soil incubations, with times kept constant at 30 min for roots and 2 h for soils. The δ¹³C signature of the soil surface CO₂ efflux and the two end member values were used to calculate that, in the irrigated soils, Rh comprised 51 ± 13.5% of the soil surface CO₂ efflux at the mid canopy position and 57 ± 7.4% at the drip line. In non-irrigated soil it was not possible to partition soil respiration, because the δ¹³C signature of the soil surface CO₂ efflux was enriched compared to both the end member values. This was probably due to a combination of the very dry porous soils at our study site (which may have been particularly susceptible to ingress of atmospheric CO₂) and the very slow respiration rates of the non-irrigated soils.     

Microbial immobilization of ammonium and nitrate in relation to ammonification and nitrification rates in organic and conventional cropping systems

Agricultural systems that receive high or low organic matter (OM) inputs would be expected to differ in soil nitrogen (N) transformation rates and fates of ammonium (NH₄⁺) and nitrate (NO₃⁻). To compare NH₄⁺ availability, competition between nitrifiers and heterotrophic microorganisms for NH₄⁺, and microbial NO₃⁻ assimilation in an organic vs. a conventional irrigated cropping system in the California Central Valley, chemical and biological soil assays, ¹⁵N isotope pool dilution and ¹⁵N tracer techniques were used. Potentially mineralizable N (PMN) and hot minus cold KCl-extracted NH₄⁺ as indicators of soil N supplying capacity were measured five times during the tomato growing season. At mid-season, rates of gross ammonification and gross nitrification after rewetting dry soil were measured in microcosms. Microbial immobilization of NO₃⁻ and NH₄⁺ was estimated based on the uptake of ¹⁵N and gross consumption rates. Gross ammonification, PMN, and hot minus cold KCl-extracted NH₄⁺ were approximately twice as high in the organically than the conventionally managed soil. Net estimated microbial NO₃⁻ assimilation rates were between 32 and 35% of gross nitrification rates in the conventional and between 37 and 46% in the organic system. In both soils, microbes assimilated more NO₃⁻ than NH₄⁺. Heterotrophic microbes assimilated less NH₄⁺ than NO₃⁻ probably because NH₄⁺ concentrations were low and competition by nitrifiers was apparently strong. The high OM input organic system released NH₄⁺ in a gradual manner and, compared to the low OM input conventional system, supported a more active microbial biomass with greater N demand that was met mainly by NO₃⁻ immobilization.     

Relationships of soil properties with Mn and Zn distribution in acidic soils and their uptake by a barley crop

The extractability and solid-phase fractionation of manganese (Mn) and zinc (Zn) in acid-to-neutral agricultural soils from Central Spain was evaluated by sampling and analysing twenty-nine representative soils and by greenhouse cropping eleven of them with spring barley (Hordeum vulgare, L.). All soil samples were extracted with three chemical extractants commonly used for soil fertility evaluation (0.43 M HOAc, DTPA and Mehlich-3). The soil samples were also operationally determined in six steps with the following extractants: 1 M Mg(NO₃)₂ extractable (WSEX, water soluble plus exchangeable), 0.7 M NaOCl extractable (OC, organically complexed), 0.1 M NH₂OH·HCl extractable (MnOX, Mn-oxide), 0.2 M (NH₄)₂C₂O₄ + 0.2 M H₂C₂O₄ extractable (AFeOX, amorphous Fe-oxide), 0.2 M (NH₄)₂C₂O₄ + 0.2 M H₂C₂O₄ + 0.1 M ascorbic acid extractable (CFeOX, crystalline Fe-oxide), and HCl, HNO₃, and HF in mixture (RES, residual). Soil-extractable amounts for the three single extractants were highly correlated with each other for both metals. Distributions among metal fractions showed that Mn was mainly found in the MnOX fraction (30.9%, ranging from 13.0 to 51.2%), whereas Zn was predominantly found in the RES fraction (44.3%, ranging from 26.4 to 56.8%). The proportion of Mn fractions extracted from the soils was in the order as follows: CFeOX ≤ WSEX = OC ≤ AFeOX = RES < MnOX, whereas Zn was in the order: WSEX ≤ OC ≤ AFeOX < MnOX < CFeOX < RES. The soil properties that correlated best with the distribution of Mn and Zn forms in these soils were soil organic matter and pH. The “availability factor” values [AF = (WSEX + OC) 100 / total metal], were higher for Mn than for Zn in these soils. Plant metal concentrations (Y) and soil-extractable and sequential extracted fractions showed few significant correlations. However, it was possible to significantly predict the phytoavailability of Mn and Zn for barley using a series of empirical equations involving extractable metals, solid-phase fractions and soil properties as components. The R² values of the best-fit regression models ranging from 0.50 [Y-Zn = 19.3 + 6.32 (WSEX + OC)-Zn] to 0.92 [Y-Zn = 57.3 + 0.23 P − 8.56 pH + 20.6 DTPA-Zn].     

Microbial biomass and activity in salt affected soils under arid conditions

The effects of salinity on the size, activity and community structure of soil microorganisms in salt affected arid soils were investigated in Shuangta region of west central Anxi County, Gansu Province, China. Eleven soils were selected which had an electrical conductivity (EC) gradient of 0.32–23.05 mS cm⁻¹. There was a significant negative exponential relationship between EC and microbial biomass C, the percentage of soil organic C present as microbial biomass C, microbial biomass N, microbial biomass N to total N ratio, basal soil respiration, fluorescein diacetate (FDA) hydrolysis rate, arginine ammonification rate and potentially mineralizable N. The exponential relationships with EC demonstrate the highly detrimental effect that soil salinity had on the microbial community. In contrast, the metabolic quotient (qCO₂) was positively correlated with EC, and a quadratic relationship between qCO₂ and EC was observed. There was an inverse relationship between qCO₂ and microbial biomass C. These results indicate that higher salinity resulted in a smaller, more stressed microbial community which was less metabolically efficient. The biomass C to biomass N ratio tended to be lower in soils with higher salinity, reflecting the bacterial dominance in microbial biomass in saline soils. Consequently, our data suggest that salinity is a stressful environment for soil microorganisms.     

Interactions of nitrogen fertilizers and forest humus—II. Beech and oak

Laboratory incubation experiments with and without added urea or NH₄NO₃ were performed on humus from stands of beech (Fagus silvatica) grown on soils from limestone, schists, flysch and peridotites and on humus from oak (Quercus conferta) stands on soils from limestone and schists.Beech and oak humus from stands grown on soils from limestone and flysch showed considerable nitrification with a concurrent high mobilization rate of the nutrient elements Ca, Mg and K, especially in the presence of increasing urea concentrations, but no net humus N mineralization was observed. Beech humus from stands grown on soils from schists and peridotites showed no nitrification and increasing concentrations of added urea did not modify their inability to nitrify. Non-nitrifying types of humus showed considerable ammonification but their Ca, Mg and K mobilization rates were about one-tenth those observed in nitrifying humus and were inversely correlated with urea concentrations.Exchangeable Al³⁺ and extractable Mn were present in high concentrations in the underlying inorganic soils in all cases where nitrification was absent from the overlying humus but addition of 500 parts Al³⁺ and 1000 parts Mn/10⁶ separately or in combination to a nitrifying humus failed to inhibit nitrification.An interpretation of these findings is attempted with reference to the possibility of absence of nitrification in climax vegetations and the preference of certain forest species for NH⁺₄ or NO^?₃.     

Soil application of meat and bone meal. Short-term effects on mineralization dynamics and soil biochemical and microbiological properties

Meat and bone meal (MBM) utilization for animal production was banned in the European Union since 2000 as a consequence of the appearance of transmissive spongiform encephalopathies. Soil application could represent a lawful and effective strategy for the sustainable recycling of MBM due to its relevant content of nutritive elements and organic matter. The effectiveness of MBM as organic fertilizer needs to be thoroughly investigated since there is a lack of knowledge about the mineralization dynamics of MBM in soil and the impact of such residues, in particular the high content of lipids, on soil biochemical and microbiological properties. For this aim, a defatted (D) and the correspondent non-defatted (ND) MBM were added at two rates (200 and 400 kg N ha^?1) to two different moist soils and incubated at 15 and 20 °C for 14 d. MBM mineralization dynamics was studied by measuring CO₂ evolution. Water extractable organic C, K₂SO₄-extractable NO₃^? and NH₄⁺, microbial biomass ninhydrin-reactive N, enzymatic activities (FDA, urease, protease, alkaline phosphatase) and microbial composition (aerobic and anaerobic bacteria, fungi) were measured 2 and 14 d after MBM addition to the soil. The rate of CO₂ evolution showed a maximum 2–3 d after the addition of MBM, followed by a decrease approaching the control. MBM mineralization was fast with, on average, 54% of total CO₂ evolved in the first 4 d of incubation at 20 °C. The percentage of added C which was evolved as CO₂ at the end of the incubation period ranged between 8% and 16% and was affected by temperature, soil type and MBM treatment (ND > D). Soil amendment with MBM caused a noteworthy increase in both extractable NH₄⁺ and NO₃^? (about 50% of added N) which was higher for ND. The addition of MBM also enhanced microbial content and activity. Microbial biomass increased as a function of the rate of application and was higher for ND with respect to D. The increase in numbers of aerobic and anaerobic bacteria and fungi caused by MBM addition was, in general, more pronounced with ND. Enzymatic activity in amended soils showed an enhancement in nutrient availability and element cycling. At the rate of application of present work, lipids did not cause adverse effects on soil microorganisms.The potential of MBM as effective organic fertilizer was supported by the large increase in available N and the enhancement of the size and activity of soil microorganisms.