Effects of white clover (Trifolium repens L.) on plant and soil nitrogen and soil organic matter in mixtures with perennial ryegrass (Lolium perenne L.)

To increase our insight into the above- and belowground N flows in grass and grass-clover swards relations between crop and soil parameters were studied in a cutting trial with perennial ryegrass (Lolium perenne) monocultures and ryegrass–white clover (Trifolium repens) mixtures. The effects of clover cultivar on herbage yield, the amount of clover-derived nitrogen, apparent N transfer to companion grass, dynamics of N and organic matter in the soil were estimated.The grass monocultures had very low DM yields (<2.1 t ha^-1) and a low N concentration in the harvested herbage. During 1992–1995 the annual herbage DM yield in the mixtures ranged from 7.0 to 14.3 t ha^-1, the white clover DM yield from 2.4 to 11.2 t ha^-1 and the mean annual clover content in the herbage DM harvested from 34 to 78%. Mixtures with the large-leaved clover cv. Alice yielded significantly more herbage and clover DM and had a higher clover content than mixtures with small/medium-leaved cvs. Gwenda and Retor. Grass cultivar did not consistently affect yield, botanical composition or soil characteristics.The apparent N₂ fixation was very high, ranging from 150 to 545 kg N ha^-1 in the different mixtures. For each tonne of clover DM in the harvested herbage 49 to 63 kg N was harvested, while the apparent N transfer from clover to grass varied between 55 and 113 kg N ha^-1 year^-1.The net N mineralization rate was lower under monocultures than under mixtures. The C mineralization and the amounts of C and N in active soil organic matter fractions were similar for monocultures and mixtures, but the C:N ratio of the active soil organic matter fractions were higher under grass than under mixtures. This explains the lower N mineralization under grass.     

An assessment of the contribution of net mineralization to N cycling in grass swards using a field incubation method

Measurements of net mineralization using a field incubation method were made over a full growing season (180 d). Soil cores, taken from cut swards which for many years had been previously grazed by cattle, were placed in jars in the field for successive incubation periods of 14 d. Acetylene was added to the incubation jars to inhibit nitrification in the soil cores and thereby prevent losses of N through denitrification. Net mineralization over 180 d amounted to 415, 321 and 310 kg N ha^–1 under grass/clover, unfertilized grass and grass receiving 420 kg N ha^–1 y^–1, respectively. At the start of the growing season, an index of potentially mineralizable N in the soil was estimated by a chemical extraction method, but this index was <50% of the estimates obtained by field incubation. The amount of N in herbage harvested regularly from the swards also under-estimated the supply of N from the soil, with apparent recoveries of 53, 82 and 74% and total yields of N of 240, 263 and 538 (kg N ha^–1) from grass/clover, unfertilized grass and fertilized grass, respectively. Mineralization rates varied significantly with seasonal soil temperature fluctuations, but the incubation method was apparently less sensitive in relation to changes in soil water content. Rates of N-turnover (as % of total soil N) were highest under grass/clover (9%), but similar under fertilized and unfertilized grass swards (approximately 5%).     

The response of denitrifiers in a sandy loam soil affected by a long-term fertilization to organic carbon and nitrate

The response of denitrifiers to carbon in the form of glucose (Glc-C) and nitrate (NO ₃ ⁻ -N) amendments was studied in four differently fertilized plots of sandy-loam soil. Two basically different characteristics of denitrification activity were determined: (1) potential denitrification measured as nitrous oxide production during 1-d incubation in the presence of acetylene, and (2) denitrifying enzyme activity determined in soil slurries as a N₂O production in the presence of acetylene and chloramphenicol during 1 h of incubation. Potential denitrification was strongly influenced by both Glc-C and NO ₃ ⁻ -N amendments in their various combinations, but was also affected by the fertilization practice. The response of denitrifiers to Glc-C and NO ₃ ⁻ -N was generally lower in unfertilized and surprisingly also in highly fertilized soils than in organically and moderately fertilized soils. Denitrifying enzyme activity was stimulated by the fertilization and was, in contrast to potential denitrification, the highest in highly fertilized soil. The results indicate that although active denitrifiers were present in a highly fertilized soil, their ability to develop under optimal conditions was decreased (being similar to that of denitrifiers in unfertilized soil). This suggests long-term changes in soil microbial community in a highly fertilized soil, presumably connected to changes in soil chemistry caused by fertilization.     

Reduction of nitrate in clayey subsoils controlled by geochemical and microbial barriers

Vertical distribution of redox zones, concentrations of redox‐sensitive constituents, numbers of aerobic heterotrophic bacteria, and potential denitrification activity were studied in 1‐m cores taken at the transition between the oxidized and reduced layers in two Danish clayey subsoils. Based on the matrix soil colors, a redox sequence of oxidized, suboxic, and reduced zones was identified at both sites. The geochemical composition of the oxidized brown colored zone (to depths of 2.6 and 3.2 m) was characterized by high concentrations of NO₃ ^? and low amounts of total organic carbon, exchangeable forms of NH₄ ⁺, Fe²⁺, and Mn²⁺, and structural Fe(II) in the clay minerals. In the underlying 20‐ to 30‐cm‐deep suboxic zone, decreasing NO₃ ^? concentrations were observed together with increasing amount of exchangeable forms of Fe²⁺ and Mn²⁺, and structural Fe(II). Finally, in the reduced grey zone, NO₃ ^? was no longer present and maximum concentrations of other redox sensitive constituents occurred. Throughout the subsoils, the distribution of exchangeable Fe²⁺ corresponded most closely to changes in the colors of redox zones. The low‐organic Havrebjerg site displayed geochemical profiles indicating that NO₃ ^? was chemically reduced by structural Fe(II) in the clay minerals of the suboxic zone, and that the Fe(II) formed a geochemical barrier for the downward progression of NO₃ ^?. Aerobic heterotrophic bacteria occurred only in low numbers at this site and potential denitrification activity was very low. In contrast, the Sparresholm site had a significant population of bacteria in the suboxic zone, which also contained a heterogeneous distribution of potential denitrification activity. Specific microsites with facilitated transport of soluble organic substrates are proposed to support the denitrification activity in a heterogeneous distribution, constituting a microbial barrier for downward progression of NO₃ ^? in this subsoil.     

Effect of rice plants on nitrification-denitrification loss of nitrogen under greenhouse conditions

Summary Rice is unique among cereal crops in its ability to tolerate the anaerobic environment of waterlogged soils, but little is known about the influence of these plants on nitrogen loss by nitrification-denitrification. This problem was approached by loss of urea-N in cores with and without rice plants, using the acetylene inhibition method. Considerably greater denitrification was observed for surface-applied urea as compared to subsurface application in all cases. Regardless of the application point, however, the planted system yielded greater N₂O+N₂ accumulation in the first two days than from the nonplanted soil. After 4–6 days from fertilization no difference was observed in denitrification loss between planted and nonplanted systems. Inorganic NH ₄ ⁺ levels were observed to decrease rapidly in planted soils. Initial enhancement of gaseous N accumulation may occur because of the oxidized rice-root rhizosphere, however the appreciable denitrification in non-planted soil suggests that other N loss mechanisms are more important than the losses occurring in the root rhizosphere.     

Factors affecting nitrogenase activity associated with marsh grasses and their soils from eutrophic lakes

The acetylene reduction activity (ARA) of soil-plant cores and intact, soil-free plant-root systems was used to study the influence of soil moisture content, diurnal cycles of temperature and light, and inorganic N and P on the nitrogenase activity (AR) associated with Phalaris arundinacea L. and Phragmites australis (Cav.) Trin. from two eutrophic lakes (lochs), Balgavies and Forfar.A positive correlation (r = 0.81, n = 26) was established between AR and soil moisture content in individual soil-plant cores of Phalaris from Forfar Loch. Nitrogenase activity, soil moisture and NO₂⁻-N increased with decreasing distance from lake water in Balgavies Loch.Diurnal fluctuations in AR, probably attributable to a combined effect of soil temperature and illumination changes, were observed under field conditions for Phalaris and Phragmites. Under laboratory conditions, the shading and cutting of Phalaris shoots did not inhibit ARA, which suggested that new photosynthates did not necessarily supply substrate for activity in the short term.Partial and temporary inhibition of ARA was obtained in dissected soil-plant cores after a single application of NH₄⁺-N and NO₃⁻-N (350 μg N g⁻¹ fresh weight). At concentrations equivalent to 300 μg P g⁻¹ fresh weight of Phalaris cores, PO₄³⁻-P also caused partial and temporary inhibition of ARA of soil-free plant-root systems, but stimulated activity in intact simulated in situ systems.     

Denitrification and N mineralization from hairy vetch (Vicia villosa Roth) and rye (Secale cereale L.) cover crop monocultures and bicultures

N mineralization, N immobilization and denitrification were determined for vetch, rye and rye-vetch cover crops using large packed soil cores. Plants were grown to maturity from seed in cores. Cores were periodically leached, allowing for quantification of NO₃ ⁻ and NH₄ ⁺ production, and denitrification incubations were conducted before and after cover crop kill. Gas permeable tubing was buried at two depths in cores allowing for quantification of N₂O in the soil profile. Cover crops assimilated most soil N prior to kill. After kill, relative rates of N mineralization were vetch > rye-vetch mixture > fallow > rye. After correcting for N mineralization from fallow cores, net N mineralization was observed in vetch and rye-vetch cores, while net N immobilization was observed in rye cores. Denitrification incubations were conducted 5, 15 and 55 days after kill, with adjustment of cores to 75% water filled pore space (WFPS). The highest denitrification was observed in vetch cores 5 days after kill, when soil NO₃ ⁻ and respiration rates were high. Substantially lower denitrification was observed on subsequent measurement dates and in other treatments probably due to either limited NO₃ ⁻ or organic carbon in the soil. On day 5, 3%, 23%, 31% and 31% of the N₂O was recovered in the headspace of fallow, vetch, rye and rye-vetch cores, respectively. The rest was stored in the soil profile. In a field study using intact soil cores, denitrification rates also peaked 1 week after cover crop kill and decreased significantly thereafter. Results suggest greater potential N losses from vetch than rye or rye-vetch cover crops due to rapid N-mineralization in conjunction with denitrification and potential leaching, prior to significant crop N-assimilation. This revised version was published online in June 2006 with corrections to the Cover Date.     

N2 fixation and nitrogen allocation to above and below ground plant parts in red clover-grasslands

Leys, used for grazing or production of forage to be conserved as silage or hay, are very important crops in northern areas. In order to measure the N₂ fixation in leys of varying ages and during different parts of the season, detailed measurements were taken of yield, N₂ fixation and the amounts of N remaining in the field after harvesting red clover (Trifolium pratense L.)-grass leys at a site in northern Sweden, where they are generally harvested twice per growing season. Entire plants, including stubble and roots, were sampled at the time of first and second harvest and, in addition, at the end of the growing season in three neighbouring fields, carrying a first, a second and a third year ley, respectively. N₂ fixation was measured by both ¹⁵N isotope dilution (ID) and ¹⁵N natural abundance (NA) methods. The proportion of clover dry matter (DM) in the stands increased from the first to the second harvest, but the grasses dominated throughout the entire season, especially below ground. The N concentrations, in both herbage and whole plants, were about twice as high in the clover as in the grasses. Seasonal variations in N concentrations were minor, and total N contents followed the same trends as DM. The clover acquired nearly all of its N from N₂ fixation: the proportion of N in clover herbage derived from N₂ fixation was often >0.8 throughout the season. The variations in the amounts of N₂ fixed during the course of the season corresponded well to the seasonal changes in clover biomass. Amounts of fixed N₂ allocated to clover herbage during the whole season were in the range 4 to 6 g N m⁻² in this unusually rainy year. Calculations of daily N allocation rates to herbage showed that N uptake rates were similar, and high, in grasses during May–June and July–August, while N₂ fixation rates in clover were about 10-fold as high in July–August as in May–June, reflecting the need for N in clover growth. The proportion of N remaining in clover stubble and roots after the first and second harvests was about 60 and 25%, respectively, while about 60% of the N in grasses remained in stubble and roots after both harvests. The considerable amounts of biomass and N that were left in field after harvesting red clover-grass leys are important for re-growth of the plants and provide substantial N fertilization for the next crop in the crop rotation.     

Denitrification and nitrous oxide emissions from riparian forests soils exposed to prolonged nitrogen runoff

Compared to upland forests, riparian forest soils have greater potential to remove nitrate (NO₃) from agricultural runoff through denitrification. It is unclear, however, whether prolonged exposure of riparian soils to nitrogen (N) loading will affect the rate of denitrification and its end products. This research assesses the rate of denitrification and nitrous oxide (N₂O) emissions from riparian forest soils exposed to prolonged nutrient runoff from plant nurseries and compares these to similar forest soils not exposed to nutrient runoff. Nursery runoff also contains high levels of phosphate (PO₄). Since there are conflicting reports on the impact of PO₄ on the activity of denitrifying microbes, the impact of PO₄ on such activity was also investigated. Bulk and intact soil cores were collected from N-exposed and non-exposed forests to determine denitrification and N₂O emission rates, whereas denitrification potential was determined using soil slurries. Compared to the non-amended treatment, denitrification rate increased 2.7- and 3.4-fold when soil cores collected from both N-exposed and non-exposed sites were amended with 30 and 60 μg NO₃-N g⁻¹ soil, respectively. Net N₂O emissions were 1.5 and 1.7 times higher from the N-exposed sites compared to the non-exposed sites at 30 and 60 μg NO₃-N g⁻¹ soil amendment rates, respectively. Similarly, denitrification potential increased 17 times in response to addition of 15 μg NO₃-N g⁻¹ in soil slurries. The addition of PO₄ (5 μg PO₄-P g⁻¹) to soil slurries and intact cores did not affect denitrification rates. These observations suggest that prolonged N loading did not affect the denitrification potential of the riparian forest soils; however, it did result in higher N₂O emissions compared to emission rates from non-exposed forest soils.     

Allantoin-induced changes of microbial diversity and community in rice soil

The effects of slurry application method and weather conditions after application on ammonia volatilisation are well documented, however, the effect on slurry N recovery in herbage is less evident due to large variability of results. The objective of this field experiment was to determine the recovery of cattle slurry NH₄-N in herbage and soil in the year of application as affected by application method (trailing shoe versus broadcast) and season of application (spring versus summer), using ¹⁵N as a tracer. In 2007 and 2008, ¹⁵N enriched slurry was applied on grassland plots. N recovery in herbage and soil during the year of application was determined. Both spring and trailing shoe application resulted in significantly higher herbage DM yields, N uptake and an increased recovery of ¹⁵NH₄-N in herbage. Additionally, the recovery of slurry ¹⁵NH₄-N in the soil at the end of the growing season was increased. Spring and trailing shoe application reduced the losses of slurry ¹⁵NH₄-N by on average 14 and 18 percentage points, respectively, which corresponded closely to ammonia volatilisation as predicted by the ALFAM model. It was concluded that slurry N recovery in temperate pasture systems can be increased by adjusting the slurry application method or timing.     

Effects of forest fertilization on nitrogen leaching and soil microbial properties in the Northern Calcareous Alps of Austria

Several boreal and alpine forests are depleted in nutrients due to acidification. Fertilization may be a remedy, but rapidly-soluble salts (N, P, K, Mg) may pose nitrate problems for the groundwater or decrease microbial activity.With the aim to investigate potential nitrogen leaching after fertilization we set up an experiment employing intact soil cores (11 cm diameter, 20–40 cm long) from a mixed forest and a Picea abies stand (soil type Rendsina) in the Northern Calcareous Alps of Austria. The cores were fertilized with a commercial NPK fertilizer or a methylene-urea-apatite-biotite (MuAB) fertilizer at a rate corresponding to 300 kg N ha^-1 and incubated for 28 weeks together with unfertilized controls. Both soil water (retrieved 5 cm below the soil surface) and leachate were analyzed for nitrate and ammonium in regular intervals. After the incubation, soil microbial biomass and basal repiration were determined and a nitrogen mineralization assay was performed.For the control, in the soil water and leachate maximum NH₄ ⁺ and NO₃ ^- concentrations of 5 and 11 mg N L^-1, respectively, were found. Compared to that, MuAB fertilizer resulted in a slow increase of NH₄ ⁺ and NO₃ ^- in the soil water (up to 11 and 35 mg N L^-1 respectively) and in the leachate (4 mg NH₄ ⁺-N L^-1 and 44 mg NO₃ ^--N L^-1). Highest nitrogen loads were found for the fast release NPK fertilizer, with NH₄ ⁺ and NO₃ ^- concentrations up to 170 and 270 mg N L^-1, respectively, in the soil water. NH₄ ⁺-N levels in the leachate remained below 5, while NO₃-N levels were up to 190 mg L^-1. Fast- release NPK caused a significant decrease of microbial biomass and basal respiration. These parameters were not affected by MuAB fertilizer.The results suggest that the MuAB fertilizer may be an ecologically appropriate alternative to fast-release mineral fertilizers for improving forest soils.     

滇南喀斯特地区灌木群落和人工林土壤元素化学计量特征

在云南喀斯特地区,为提升退化灌木群落的生态系统服务功能,营造了不同树种的人工林分。这些人工林分如何影响土壤化学性质还未得到充分认识。以云南泸西县灌木群落及三种常见人工林(云南松(Pinus yunnanensis)、赤杨(Alnus japonica)和侧柏(Platycladus orientalis))土壤为研究对象探讨喀斯特地区在人工林建造后土壤的13种元素全量、可利用性含量和化学计量学特征变异格局,为喀斯特石漠化治理提供理论依据。结果表明,1)基于判别分析,四种群落土壤化学计量特征可以显著区分。土壤Fe、P、K、Mn全量及交换性Ca、交换性Mg和NH_4~+-N对区分四种群落土壤贡献最大。2)四种群落之间相比,侧柏林土壤C、N、S、Na全量和NO_3~--N含量均低于其他三种群落,土壤肥力较低;赤杨林铵态氮含量最高;云南松林有效Fe、有效Cu含量/N、C素具有显著相关性,占所有元素对数的38.5%,说明该地土壤元素积累的相互依赖性。与灌木群落相比,人工林土壤元素全量和可利用性含量相关性比例均更高。这些研究结果对今后基于适地适树人工林营造、生态系统服务功能提升和经营利用,均具有重要指导意义。     

Interactions between perennial ryegrass ( Lolium perenne L.) and white clover (Trifolium repens L.) under contrasting nitrogen availability: productivity,seasonal patterns of species composition,N2 fixation,N transfer and N recovery

Nitrogen (N) fertiliser and clover cultivar choice affect competition and productivity in grass-clover mixtures. Pure stands and mixtures of perennial ryegrass and white clover cultivars with contrasting growth habits were examined. The aim of this work was to study the effect of repetitive nitrogen (N) application and cultivar combination on competition and productivity, N yield in the harvested herbage, N₂ fixation in mixtures and pure stands, and transfer of N from clover to the companion grass. Large-leaved white clover cultivar Alice and small-leaved cv. Gwenda and perennial ryegrass cvs. Barlet (erect) and Heraut (prostrate) were sown in pure stands and as four binary grass-clover mixtures on a sandy soil in 1995. In the mixtures, two levels of N fertiliser were applied: 0 (-N) and 150 and 180 kg ha^-1 y^-1 N (+N) in 1996 and 1997, respectively, while the grass monocultures received three N levels (0, 140/180 and 280/360 kg ha^-1) in 1996 and 1997, respectively. No N was applied to pure clover. The plots were cut five times during 1996 and six times during 1997. Fertiliser N was applied in early spring and after every harvest. The treatments were continued until the summer of 1999. In pure grass, the applied N was effectively recovered. In mixtures, N application affected competition by enhancing grass growth and the overall effect of N application was 17 kg DM per kg N applied in 1996. However, there was no yield response to N fertilizer in 1997, because this was compensated for by a higher clover production in unfertilised mixtures. In 1997, -N mixtures yielded more N than +N mixtures, owing to the higher clover content and N₂ fixation. Large-leaved clover cv. Alice was better able to withstand the negative effect of repetitive N application on clover production in mixtures and increased its proportion during the growing season of the second harvest year. In 1997, mixtures with Alice yielded more N than mixtures with Gwenda, but in pure clover swards, there was no cultivar effect on N yield. Also, during the autumn of 1998 and the spring of 1999, the clover content was highest in mixtures with Alice. Harvested N and apparent N₂ fixation were almost twice as high in 1997 as in 1996. N yield and apparent N₂ fixation were higher in pure clover than in mixtures. In mixtures, the apparent N₂ fixation in 1996 was 142 kg N ha^-1, irrespective of cultivar or N treatment. In 1997, it was on average 337 kg N ha^-1, and higher in -N mixtures and in mixtures with Alice. For each tonne of clover DM in the harvested herbage, 65 and 57 kg N was harvested in 1996 and 1997 in -N mixtures, respectively. The apparent transfer of clover-derived N to grass was on average 29 and 70 kg N ha^-1 yr^-1 in 1996 and 1997, respectively. It was highest in +N mixtures and highest in mixtures with Gwenda in 1997. In contrast to clover, the grass cultivars were very similar in their productivity and seasonal patterns, despite their contrasting growth habits. Seasonal trends in N yield, N transfer and N recovery are discussed in relation to fertilizer application regimes and variation in production patterns in mixtures and pure stands. This revised version was published online in June 2006 with corrections to the Cover Date.     

Short-term effects of a dung pat on N2 fixation and total N uptake in a perennial ryegrass/white clover mixture

The short-term effects of a simulated cattle dung pat on N₂ fixation and total uptake of N in a perennial ryegrass/white clover mixture was studied in a container experiment using sheep faeces mixed with water to a DM content of 13%. We used a new ¹⁵N cross-labelling technique to determine the influence of dung-pat N on N₂ fixation in a grass/clover mixture and the uptake of dung N in grass and clover. The proportion of N in clover derived from N₂ fixation (%Ndfa) varied between 88–99% during the 16 weeks following application of the dung. There was no effect of dung on the %Ndfa in clover grown in mixture, whereas the %Ndfa in clover grown in pure stand decreased (nominal 2–3%) after dung application. Dung did not influence the amount of N₂ fixed, and the uptake of dung N in grass and clover proceeded at an almost constant rate. After 16 weeks, 10% of the applied dung N was taken up by grass and clover, 57% had been incorporated in the soil by faunal activity and 27% remained in residual dung on the soil surface. The dung N unaccounted for (7%) was probably lost by ammonia volatilisation and denitrification. The uptake of dung N in grass/clover mixtures in the field was similarly followed by using simulated ¹⁵N-labelled dung pats. The total dry matter production and N yields increased in the 0–30 cm distance from the edge of the dung patch, but the proportion of clover decreased. Thirteen months after application of the dung 4% of the applied dung N was recovered in the harvested herbage, 78% was recovered from the soil and the residual dung, and 18% was not accounted for. It is concluded that N₂ fixation in the dung patch border area in grass/clover mixtures is not influenced directly by the release of N from dung pats in the short term. However the amount of N₂ fixed may be reduced, if the growth of clover is reduced in the patch border area.     

Field calibration of liming responses of four crops using soil pH,Al and Mn

Liming trials were conducted at 28 sites in the western Great Plains of Canada for barley, rape, red clover and alfalfa. Yield increases from liming correlated with soil pH and Al but not with Mn. When all sites were included, yield increases from liming correlated closely (r=0.86 to 0.94) with exchangeable Al, percent Al saturation and 0.02M CaCl₂-Al for barley, rape and red clover, these responses having correlated less well (R=0.56 to 0.72) with soil pH. Alfalfa yield responses gave low correlations with both pH and the Al measurements. When only the sites with soil pH≥5 were used, the yield responses to lime of barley and rape still correlated better with the Al measurements than with pH even though the correlations, in general, were much lower than when all sites were included. For the sites with soil pH>-5, the correlations were highest for yield responses of barley and rape with 0.02M CaCl₂-Al. It is suggested that the use of toxic Al and Mn for routinely diagnosing the limiting factor by soil acidity could improve on the economy of liming. Contribution Number 653.     

Influence of pH,exchangeable aluminium and 0.02M CaCl2-extractable aluminium on the growth and nitrogen-fixing activity of white clover (Trifolium repens) in some New Zealand soils

The effects of soil acidity on the growth and N₂-fixing activity of white clover in seven acid topsoils and subsoils of New Zealand were investigated using a glasshouse experiment.The application of phosphate (Ca(H₂PO₄)₂) to the soils resulted in very large increases in white clover growth on all soils. The application of phosphate, as well as increasing P supply, also decreased 0.02M CaCl₂-extractable Al levels, but had little effect on exchangeable Al levels.Where adequate phosphate was applied, increasing rates of lime (CaCO₃) resulted in increased plant growth on most soils. N₂[C₂H₂]-fixing activity was increased by the first level of lime for one soil, but generally remained approximately constant or declined slightly at higher rates of lime. Up to the point of maximum yield, white clover top weight was more highly correlated with 0.02M CaCl₂-extractable soil Al than with exchangeable Al or pH. At pH values greater than 5.5, plant yield declined on some soils, apparently because of Zn deficiency. The data suggest that white clover is unlikely to be affected by Al toxicity at 0.02M CaCl₂-extractable Al levels of less than about 3.3 g g^–1. However, there were differences between soils in apparent plant tolerance to 0.02M CaCl₂-extractable Al, which appeared to be caused by differing C levels in the 0.02M CaCl₂ extracts.     

Influence of dissimilarities in temporal and spatial N-uptake patterns on15N-based estimates of fixation and transfer of N in ryegrass-clover mixtures

The temporal N-uptake patterns of white clover (Trifolium repens L.) mixed with perennial ryegrass (Lolium perenne L.) and of red clover (Trifolium pratense L.) mixed with Italian ryegrass (Lolium multiflorum Lam.) were determined in successive harvests of herbage within the growth cycles of a ley established near Zürich (Switzerland). Rooting patterns were examined by injecting¹⁵N-fertilizer at soil depths ranging from 10 to 40 cm. The results were analyzed to determine the effect of variations in time and depth of N-uptake on the¹⁵N-based measurement of N from symbiosis (N_sym) and N from transfer (N_trans).Grasses in mixture appeared to have deeper rooting systems than grass monocultures, which led to an overestimation of N transfer from white clover to perennial ryegrass if¹⁵N was spread on the soil surface.White clover generally lagged behind grass in soil N- uptake. Soil N-uptake of red clover slowed down before that of the grass because % N_sym almost reached 100% during the second half of each growth cycle. However, the effect of these dissimilarities on the seasonal average of %N_sym did not exceed 2%.It is concluded that at the observed high levels of N₂ fixation, failure to account for the N-uptake patterns of the test and reference crops only slightly affected the estimates of % N_sym and % N_trans, and did not invalidate the observed differences between species.     

Persistence of Denitrifying Enzyme Activity in Dried Soils

The effects of air drying soil on denitrifying enzyme activity, denitrifier numbers, and rates of N gas loss from soil cores were measured. Only 29 and 16% of the initial denitrifying enzyme activity in fresh, near field capacity samples of Maury and Donerail soils, respectively, were lost after 7 days of air drying. The denitrifying activity of bacteria added to soil and activity recently formed in situ were not stable during drying. When dried and moist soil cores were irrigated, evolution of N gas began, and it maximized sooner in the dried cores. This suggests that the persistence of denitrifying enzymes permits accelerated denitrification when dried soils are remoistened. Enzyme activity increased significantly in these waterlogged cores, but fluctuations in enzyme activity were small compared with fluctuations in actual denitrification rate, and enzyme activities were always greater than denitrification rates. Apparent numbers of isolatable denitrifiers (most-probable-number counts) decreased more than enzyme activity as the soils were dried, but after the soils were rewetted, the extent of apparent growth was not consistently related to the magnitude of N loss. We hypothesize that activation-inactivation of existing enzymes by soil O₂ is of greater significance in transient denitrification events than is growth of denitrifiers or synthesis of new enzymes.     

温度和水分变化对冻土区泥炭地土壤氮循环功能基因丰度的影响

以大兴安岭多年冻土区泥炭地为研究对象,通过室内模拟增温实验,研究温度升高对不同深度（0-150 cm）土壤氮循环功能基因丰度的影响。同时针对0-20 cm和20-40 cm土壤设置两个水分处理,分别为土壤原始含水量和淹水状态,研究水分变化对表层土壤氮循环功能基因丰度的影响。结果表明温度升高显著提高了活动层（0-60 cm）、过渡层（60-80 cm）、永冻层（80-100 cm）中nifH、nirK基因丰度,温度升高显著提高了活动层（0-40 cm）和过渡层（60-80 cm）中nirS基因丰度。温度升高显著提高了过渡层（60-80 cm）NH₄⁺-N和较深永冻层（140-150 cm）NO₃^--N的含量,但降低了过渡层（60-80 cm）NO₃^--N和较深永冻层（120-150 cm）NH₄⁺-N的含量,相关性分析表明,NH₄⁺-N含量与nifH和nirS基因丰度呈显著正相关,NO₃^--N含量与nirK基因丰度呈显著正相关,说明温度升高能够通过改变微生物丰度促进过渡层固氮作用和反硝化作用。在增温条件下,淹水处理使表层土壤nirS和nirK基因丰度及NH₄⁺-N含量降低,但提高了NO₃^--N含量,说明淹水造成了过度还原的条件使反硝化底物浓度降低,降低反硝化微生物活性进而抑制了土壤反硝化作用。该结果对于明确未来气候变化影响下冻土区泥炭地土壤氮循环过程具有重要意义。     

Transformation of iron and manganese in rice soils under different moisture regimes and organic matter applications

Summary Transformation of iron and manganese under three different moisture regimes,viz continuous waterlogged (W₁), continuous saturated (W₂) and alternate waterlogged and saturated (W₃) and three levels of organic matterviz 0, 0.5 and 1.0% in all possible combinations was studied in four soils. The results showed that under waterlogged moisture regime there was a sharp increase in the content of water soluble plus exchangeable manganese accompanied by significant decrease in the content of reducible manganese in all the soils excepting the acidic soil which was very poor in active manganese content. The increase in respect of iron in similar form was, however, very small. The increase in the content of water soluble plus exchangeable manganese as well as iron under the continuous saturated and alternate waterlogged and saturated moisture regimes was always much lower as compared to that under the continuous waterlogged condition. Application of organic matter brought about an increase in the content of water soluble plus exchangeable manganese in all the soils excepting the lateritic one irrespective of moisture regimes but did not cause any change in the content of iron and manganese in insoluble complex. The content of water soluble plus exchangeable iron and of insoluble ferrous iron although recorded some increase due to organic matter application, the increase was not so marked in any of the soils.