Needle chemistry in young Norway spruce stands after application of crushed wood ash

Nutrient concentrations in current and 1-year-old needles were analyzed annually for 5 years after application of hardened wood ash in 1–4-year-old Norway spruce (Picea abies (L.) Karst.) stands within a range of climate and fertility gradients. At each site, 3000 kg ha^–1 hardened wood ash of two types, Nymölla and Perstorp, was applied in a randomized block design. Wood ash Nymölla contained 12 kg ha^–1 P, 30 kg ha^–1 K, 891 kg ha^–1 Ca, 72 kg ha^–1 Mg and wood ash Perstorp contained 12 kg ha^–1 P, 60 kg ha^–1 K, 486 kg ha^–1 Ca, and 60 kg ha^–1 Mg. The ash was intended to compensate for nutrients removed at the preceding harvest when logging residues were collected and removed from the site (whole-tree harvesting). The climate gradient included four climate zones throughout Sweden and each of these included a fertility gradient of three sites classified according to their ground vegetation type. There were no effects on nutrient concentrations in the needles 1 year after the application of wood ash. Five years after ash application, the concentrations of P, K and Ca in current and 1-year-old needles were higher than in the control plots. The results were consistent over all stands, irrespective of climate zone and fertility status. P and K concentrations were higher in spruce needles from plots treated with Perstorp wood ash, whereas Ca concentrations were higher in those of Nymölla treated plots. Analyses across all study sites revealed a treatment effect in terms of increased ratios of P:N, K:N and Ca:N in 1-year-old needles. The ratio P:N tended to increase with time in the Perstorp wood ash treatment compared with the control. The needle concentrations of Mg and S were not affected by the ash applications. The increase in needle nutrient concentrations after application of hardened wood ash suggests that wood ash recycling could be used in order to replace nutrients removed at whole-tree harvesting.     

Effect of soil-applied NPK fertilizers on severity of black spot disease (Alternaria brassicae) and yield of oilseed rape

Effect of soil application of eight combinations of NPK fertilizers on the severity of black spot disease (BSD), caused by Alternaria brassicae (Sacc.) Berk., and yield of short duration oilseed rape (Brassica campestris L) were investigated under both pot and field conditions in 1987–88, 1988–89 and 1990–91. The severity of BSD was significantly greater (36–48%) on plants grown in ground treated with NP (N 90 kg ha^–1+P 40 kg ha^–1) applied as urea and single superphosphate respectively than on plants from the unfertilized control (NoPoKo) (o). However, the severity of BSD was significantly smaller (25–33%) when K (40 kg ha^–1) was applied as muriate of potash than in plants from control and NP treatments. The effect of NK (N 90 kg ha^–1+K 40 kg ha^–1) in decreasing the severity of BSD was increasingly more pronounced than the effects of PK (P 40 kg ha^–1+K 40 kg ha^–1), NP and K (40 kg ha^–1) applications. The decrease in the severity of BSD due to K was due to increased production in plants of phenolics which inhibited conidial germination and decreased sporulation of A. brassicae.The decrease in the severity of BSD due to NK application gave consistently increased seed yield 68% more than those of control and other treatments. The K-fertilized plants also showed increased resistance to lodging, increased 1000-seed weight and decreased seed infection. Seeds obtained from K-fertilized plants showed good seed germinability and vigorous seeding growth.     

Mineral cycling in a tropical palm forest

Nutrient cycling and biomass characteristics of a tropical palm forest dominated byOrbignya cohune were found to be different from thsoe of hardwood dominated forests. The cohune palm forest had a high proportion of biomass in leaves (5%), a reduced sapling layer, a large amount of standing forest litter and an exceptionally low decomposition rate factor (0.1 year^–1). Mineral concentrations in palm leaves were generally lower than in hardwood species with the exception of Na, which was exceptionally high inOrbignya cohune. Biomass was estimated at 226 tons ha^–1 containing 1173 kg ha^–1 N; 126 kg ha^–1 P; 437 kg ha^–1 K; 1869 kg ha^–1 Mg; 125 kg ha^–1 Ca, and 2177 kg ha^–1 Na. Soils of cohune association did not differ significantly from those of neighbouring hardwood dominated associations with the exception of Na which occurred in higher concentration because of bioaccumulation in the dominant. The results suggest that the growth habits and physiology of a dominant can strongly influence some of the ecological parameters used to describe aforest association.     

Exchange of N-gases at the Höglwald Forest – A summary

During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Höglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N₂-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO₂-deposition was in a range of –1.6 –2.9 kg N ha^–1 yr^–1 and no huge differences between the different sites could be demonstrated. In contrast to NO₂-deposition, NO- and N₂O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4–9.1 kg N ha^–1 yr^–1), lowest at the beech site (2.3–3.5 kg N ha^–1 yr^–1) and intermediate at the limed spruce site (3.4–5.4 kg N ha^–1 yr^–1). With regard to N₂O-emissions, the following ranking between the sites was found: beech (1.6–6.6 kg N ha^–1 yr^–1) >> spruce limed (0.7–4.0 kg N ha^–1 yr^–1) > spruce control (0.4–3.1 kg N ha^–1 yr^–1). Average N-trace gas emissions (NO, NO₂, N₂O) for the years 1994–1997 were 6.8 kg N ha^–1 yr^–1 at the spruce control site, 3.6 kg N ha^–1 yr^–1 at the limed spruce site and 4.5 kg N ha^–1 yr^–1 at the beech site. Considering N₂-losses, which were significantly higher at the beech (12.4 kg N ha^–1 yr^–1) than at the spruce control site (7.2 kg N ha^–1 yr^–1), the magnitude of total gaseous N losses, i.e. N₂-N + NO-N + NO₂-N + N₂O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha^–1 yr^–1 at the spruce control site and 15.5 kg N ha^–1 yr^–1 at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.     

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha^–1 and received urea at 0, 200 or 400 kg N ha^–1 yr^–1 in 8–10 split applications. A fourth farmlet received 400 kg N ha^–1 yr^–1 and had 4.4 cows ha^–1.There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha^–1 yr^–1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (P_N; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha^–1 yr^–1 to 39–53 kg N ha^–1 yr^–1. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease P_N from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.     

The role of <Emphasis Type="Italic">Calamagrostis</Emphasis> communities in preventing soil acidification and base cation losses in a deforested mountain area affected by acid deposition

The effects of grass growth and N deposition on the leaching of nutrients from forest soil were studied in a lysimeter experiment performed in the Moravian-Silesian Beskydy Mts. (the Czech Republic). It was assumed that the grass sward formed on sites deforested due to forest decline would improve the soil environment. Lysimeters with growing acidophilous grasses (Calamagrostis arundinacea and C. villosa), common on clear-cut areas, and with unplanted bare forest soil were installed in the deforested area affected by air pollution. Wet bulk deposition of sulphur in SO₄^2– corresponded to 21.6–40.1 kg ha^–1 and nitrogen in NH₄⁺ and NO₃^– to 8.9–17.4 kg N ha^–1, with a rain water pH of 4.39–4.59 and conductivity of 18.6–36.4 S cm^–1 during the growing seasons 1997–1999. In addition, the lysimeters were treated with 50 kg N ha^–1 yr^–1 as ammonium nitrate during the 3 years of the experiment. Rapid growth of planted grasses resulted in a very fast formation of both above- and below-ground biomass and a large accumulation of nitrogen in the tissue of growing grasses. The greatest differences in N accumulation in aboveground biomass were observed at the end of the third growing season; in C. villosa and C. arundinacea, respectively, 2.66 and 3.44 g N m^–2 after addition of nitrogen and 1.34 and 2.39 g N m^–2 in control. Greater amounts of nitrogen were assessed in below-ground plant parts (9.93–12.97 g N m^–2 in C. villosa and 4.29–4.39 g N m^–2 in C. arundinacea). During the second and third year of experiment, the following effects were the most pronounced: the presence of growing grasses resulted in a decrease of both the acidity and conductivity of lysimetric water and in a lower amount of leached nitrogen, especially of nitrates. Leaching of base cations (Ca²⁺ and Mg²⁺) was two to three times lower than from bare soil without grasses. An excess of labile Al³⁺ was substantially eliminated in treatments with grasses. Enhanced N input increased significantly the acidity and losses of nutrients only in unplanted lysimeters. The leaching of N from treatments with grasses (3.9–5.6 kg N ha^–1) was 31–46% of the amount of N in wet deposition. However, the amount of leached N (4.2–6.0 kg N ha^–1) after N application was only 7.1–8.9% of total N input. After a short three year period, the features of soil with planted grasses indicated a slight improvement: higher pH values and Ca²⁺ and Mg²⁺ contents. The ability of these grass stands to reduce the excess nitrogen in soil is the principal mechanism modifying the negative impact on sites deforested by acid depositions. Thus it is suggested that grass sward formation partly eliminates negative processes associated with soil acidification and has a positive effect on the reduction of nutrient losses from the soil.     

Response of corn (Zea mays L.) to manganese application on Atlantic Coastal Plain soils

Field research was conducted on four Atlantic Coastal Plain soils in the United States to evaluate response of corn (Zea mays L.) plants to Mn application. The soils under study were classified as either Aeric or Typic Ochraquults. Manganese application increased corn grain yields by an average of 1195 kg ha^–1 on the four soils. The average grain yields on the soils were 7955 kg ha^–1 for the control and 9150 kg ha^–1 for the +Mn treatment. A Mitscherlich plant growth model was used to establish relationships between percent maximum grain yield and Mn concentration in the ear leaf at early silk (r=0.87, =0.01) and in the mature grain (r=0.58, =0.01). Based on 90% of maximum yield as the definition of the critical deficiency level, the critical Mn deficiency levels calculated with parameters from the Mitscherlich model were 10.6 mg kg^–1 in the ear leaf and 4.9 mg kg^–1 in the grain.     

Destructive and non-destructive measurements of residual crop residue and phosphorus effects on growth and composition of herbaceous fallow species in the Sahel

Little is known about the residual effects of crop residue (CR) and phosphorus (P) application on the fallow vegetation following repeated cultivation of pearl millet [Pennisetum glaucum (L.) R. Br.] in the Sahel. The objective of this study, therefore, was (i) to measure residual effects of CR, mulched at annual rates of 0, 500, 1000 and 2000 kg CR ha^–1, broadcast P at 0 and 13 kg P ha^–1 and P placement at 0, 1, 3, 5 and 7 kg P ha^–1 on the herbaceous dry matter (HDM) 2 years after the end of the experiment and (ii) to test a remote sensing method for the quantitative estimation of HDM. Compared with unmulched plots, a doubling of HDM was measured in plots that had received at least 500 kg CR ha^–1. Previous broadcast P application led to HDM increases of 14% compared with unfertilised control plots, whereas no residual effects of P placement were detected. Crop residue and P treatments caused significant shifts in flora composition. Digital analysis of colour photographs taken of the fallow vegetation and the bare soil revealed that the number of normalised green band pixels averaged per plot was highly correlated with HDM (r = 0.86) and that red band pixels were related to differences in soil surface crusting. Given the traditional use of fallow vegetation as fodder, the results strongly suggest that for the integrated farming systems of the West African Sahel, residual effects of soil amendments on the fallow vegetation should be included in any comprehensive analysis of treatment effects on the agro-pastoral system.     

Rice yields as influenced by Azolla N2 fixation and urea N-fertilization

Application of 0, 30, 60, 90 and 120 kg N ha^–1 of urea (U) in split doses with (and without)Azolla pinnata, R. Brown was studied for three consecutive seasons under planted field condition. Fresh weight (FW), acetylene reduction activity (ARA) and N yield of Azolla were found to be maximum 14 days after inoculation (DAI). Among the different treatments, maximum Azolla growth was recorded in no N control. The FW, ARA and N yield of Azolla were inhibited increasingly with the increase in N levels. Irrespective of season, FW and N yield of Azolla were inhibited only a small extent with 90 kg N ha^–1 U, beyond which the inhibition was pronounced. ARA was inhibited only slightly up to 60 kg N ha^–1 of U. Grain yield and crop N uptake of rice increased significantly up to 90 kg N ha^–1 of U (alone or in combination with Azolla) in the dry seasons (variety IR 36) and up to 60 kg N ha^–1 U in the wet season (variety CR 1018).     

Effects of planted tree fallows on soil nitrogen dynamics,above-ground and root biomass,N2-fixation and subsequent maize crop productivity in Kenya

The effects of planted fallows of Sesbania sesban (L.) Merr. and Calliandra calothyrsus (Meissner) on soil inorganic nitrogen dynamics and two subsequent maize crops were evaluated under field conditions in the highlands of eastern Kenya. Continuous unfertilised maize, maize/bean rotation and natural regrowth of vegetation (weed fallow) were used as control treatments. The proportion of symbiotic N₂-fixation was estimated by measuring both leaf ¹⁵N enrichment and whole-plant ¹⁵N enrichment by the ¹⁵N dilution technique for Sesbania and Calliandra, using Eucalyptus saligna (Sm.) and Grevillea robusta (A. Cunn) as reference species. Above- and below-ground biomass and N contents were examined in Sesbania, Calliandra, Eucalyptus and Grevillea 22 months after planting. Both the content of inorganic N in the topsoil and the quantity of N mineralised during rainy seasons were higher after the Sesbania fallows than after the other treatments. Compared to the continuous unfertilised maize treatment, both residual crop yields were significantly higher when mineral N (one application of 60 kg N ha^–1) was added. Furthermore, the second crop following the Sesbania fallow was significantly higher than the continuous maize crop. The above-ground biomass of the trees at final harvest were 31.5, 24.5, 32.5 and 43.5 Mg ha^–1 for the Sesbania, Calliandra, Grevillea and Eucalyptus, respectively. For the total below-ground biomass the values for these same tree species were 11.1, 15.5, 17.7, and 19.1 Mg ha^–1, respectively, of which coarse roots (>2 mm), including tap roots, amounted to 70–90%. About 70–90% of the N in Sesbania, and 50–70% in Calliandra, was derived from N₂-fixation. Estimates based on leaf ¹⁵N enrichment and whole-plant ¹⁵N enrichment were strongly correlated. The N added by N₂-fixation amounted to 280–360 kg N ha^–1 for Sesbania and 120–170 kg N ha^–1 for Calliandra, resulting in a positive N balance after two maize cropping seasons of 170–250 kg N ha^–1 and 90–140 kg N ha^–1, for Sesbania and Calliandra, respectively. All the other treatments gave negative N balances after two cropping seasons. We conclude that Sesbania sesban is a tree species well suited for short duration fallows due to its fast growth, high nutrient content, high litter quality and its ability to fix large amounts of N₂ from the atmosphere.     

Efficiency of soil and fertilizer nitrogen of a sod–potato system in the humid,acid and cool environment

It was hypothesized that soil N variability, and fertilization and cropping management affect potato (Solanum tuberosum L.) growth and fertilizer N efficiency. Following a 20-year sod breakup on a loamy soil in eastern Quebec, Canada (46°37 N, 71°47 W), we conducted a 3-year (1993–1995) study to investigate the effects of soil pool N and fertilizer N management on non-irrigated potato (cv. Superior) tuber yield, fertilizer N recovery (NRE), and residual N distribution in soils under humid, cool and acid pedoclimatic conditions. The fertilizer N treatments consisted of a control, side-dress at rates of 70, 105 and 140 kg ha^–1, and split applications (at seeding and bloom) at rates of 70+70, 105+70 and 140+70 kg ha^–1, respectively. Soil acidity was corrected with limestone following the plow down of the sod. Years of cropping, main effect of N treatment, and year and fertilizer N interaction were significant on total and marketable tuber yields and N uptake, which were significantly related to soil N, and root growth. Apparent NRE ranged between 29 and 70%, depending on years and N rates. Total tuber yield, N uptake, soil N use and NRE were significantly higher in the first (sod–potato) year, but decreased by 41.8, 22.7, 21.4 and 14.7%, respectively, in the third (sod–potato–potato–potato) year. Initial soil N pool was declined by 75% following the 3-year cropping. In 2–3 years, the side-dress N (140 kg ha^–1) increased significantly tuber yields (11.4–19.8%) compared to the split N (70+70 kg ha^–1). Higher split N had no effect on tuber yield and N uptake but increased residual N at harvest. Unused fertilizer N was strongly linked (R ²=0.98) to fertilizer N rates. Time factor and N treatment had significant effects (P<0.0001) on loss of N to below the root zone. Smaller scale rate and timing of split N need to be further determined. Increasing fertilizer N use efficiency could be expected with sod breakup and 75% of regional recommendation rate under humid, cool and acid pedoclimatic conditions.     

Nitrogen cycling in an acid forest ecosystem in the Netherlands under increased atmospheric nitrogen input

Within a long-term research project studying the biogeochemical budget of an oak-beech forest ecosystem in the eastern part of the Netherlands, the nitrogen transformations and solute fluxes were determined in order to trace the fate of atmospherically deposited NH₄ ⁺ and to determine the contribution of nitrogen transformations to soil acidification.The oak-beech forest studied received an annual input of nitrogen via throughfall and stemflow of 45 kg N ha^–1 yr^–1, mainly as NH₄ ⁺, whereas 8 kg N ha^–1 yr^–1 was taken up by the canopy. Due to the specific hydrological regime resulting in periodically occurring high groundwater levels, denitrification was found to be the dominant output flux (35 kg N ha^–1 yr^–1). N₂0 emmission rate measurements indicated that 57% of this gaseous nitrogen loss (20 kg N ha^–1 yr^–1) was as N₂O. The forest lost an annual amount of 11 kg N ha^–1 yr^–1 via streamwater output, mainly as N0₃ ^–.Despite the acid conditions, high nitrification rates were measured. Nitrification occurred mainly in the litter layer and in the organic rich part of the mineral soil and was found to be closely correlated with soil temperature. The large amount of NH₄ ⁺ deposited on the forest floor via atmospheric deposition and produced by mineralization was to a large extent nitrified in the litter layer. Almost no NH₄ ⁺ reached the subsurface soil horizons. The N0₃ ^– was retained, taken up or transformed mainly in the mineral soil. A small amount of N0₃ ^– (9 kg N ha^–1 yr^–1) was removed from the system in streamwater output. A relatively small amount of nitrogen was measured in the soil water as Dissolved Organic Nitrogen.On the basis of these data the proton budget of the system was calculated using two different approaches. In both cases net proton production rates were high in the vegetation and in the litter layer of the forest ecosystem. Nitrogen transformations induced a net proton production rate of 2.4 kmol ha^–1 yr^–1 in the soil compartment.     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.     

Transformation of15N-labelled urea in rice-wheat cropping system

Summary In a udic chromusterts the transformation of an initial application of¹⁵N-urea @ 80 kg N ha^–1 to rice (Oryza sativa L.) in rice-wheat (R-W) and to wheat (Triticum aestivum L.) in wheat-rice (W-R) rotations was followed in 6 successive crops in each rotation. All rice crops were grown in irrigated wetland and wheat in irrigated upland conditions.The first wheat crop in W-R rotation utilized 22 kg fertilizer N ha^–1 as compared to 19 kg by the corresponding rice crop in R-W rotation. But the latter absorbed more soil N than the former. About 69% of the total N uptake in rice was derived from mineralization of soil organic N as compared to 61% in wheat.The succeeding wheat crop in R-W rotation utilized 6.7% of the residual fertilizer N in the soil but the corresponding rice crop in W-R rotation only 2.2%. The higher utilization appeared to be related to a greater incorporation of labelled fertilizer N in mineral and hexosamine fractions of the soil N. After the second crop in each rotation, the average residual fertilizer N utilization in the next 4 crops ranged between 3 and 4%.The total recovery of¹⁵N-urea in all crops amounted to 21.7 and 24.3 kg N ha^–1 in R-W and W-R rotation, respectively. At the end of the experiment, about 9 to 10 kg ha^–1 of the applied labelled N was found in soil upto 60 cm depth. Most of the labelled soil N (69–76%) was located in the upper 0–20 cm soil layer indicating little movement to lower depths despite intensive cropping for 4 years.     

Evaluation of the availability ofAzolla-N and urea-N to rice using15N

Summary The symbiotic association of the water fernAzolla with the blue-green algaAnabaena azollae can fix 30–60 kg N ha^–1 per rice cropping season. The value of this fixed N for rice production, however, is only realized once the N is released from theAzolla biomass and taken up by the rice plants. The availability of N applied asAzolla or as urea was measured in field experiments by two¹⁵N methods. In the first,Azolla caroliniana (Willd.) was labelled with¹⁵N in nutrient solution and incorporated into the soil at a rate of 144 kg N ha^–1. The recovery ofAzolla-N in the above ground parts of rice [Oryza sativa (L) cv. Nucleoryza] was found to be 32% vs. 26% for urea applied at a rate of 100 kg N/ha; there was no significant difference in recovery. In the second, 100 kg N/ha of¹⁵N-urea was applied separately or in combination with either 250 or 330 kg N ha^–1 of unlabelledAzolla. At the higher rate, the recovery ofAzolla-N was significantly greater than that of urea. There was a significant interaction when both N sources were applied together, which resulted in a greater recovery of N from each source in comparison to that source applied separately. Increasing the combined urea andAzolla application rate from 350 kg N ha^–1 to 430 kg N ha^–1 increased the N yield but had no effect on the dry matter yield of rice plants. The additional N taken up at the higher level of N application accumulated to a greater extent in the straw compared to the panicles. Since no assumptions need to be made about the contribution of soil N in the method using¹⁵N-labelledAzolla, this method is preferable to the¹⁵N dilution technique for assessing the availability ofAzolla-N to rice. Pot trials usingAzolla stored at –20°C or following oven-drying showed that both treatments decreased the recovery of N by one third in comparison to freshAzolla.     

Growing vetches (Vicia villosa Roth) in irrigated cotton systems: inputs of fixed N, N fertiliser savings and cotton productivity

We compared symbiotic N₂ fixation by winter forage legumes (clovers, medics and vetches) using the ¹⁵N natural abundance technique in three experiments. Vetches (Vicia spp.) were the most productive legumes, and woollypod vetch fixed (shoot+root) up to 265 kg N ha^–1 (mean 227 kg N ha^–1) during a 4–5 months period over winter and early spring. Balansa and Berseem clovers, and Gama medic were highly productive in the first experiment, but fixed significantly less N than woollypod vetch in the second experiment. A 6-year study (1997–2003) compared cotton (Gossypium hirsutum L.) systems with and without vetch, or with faba beans (Vicia faba L.) to assess the effects of these crops on cotton production. Woollypod vetch was grown either between annual cotton crops, or between wheat (Triticum aestivumL.) and cotton crops. Vetch added 230 kg N ha^–1 (174 kg fixed N ha^–1) to the soil when incorporated as a green manure. Faba bean shoot residues and nodulated roots contributed 108 kg fixed N ha^–1 to the soil, following the removal of 80 kg N ha^–1 in the harvested seed (meaned over three crops). Lablab (Lablab purpureus L. – summer-growing and irrigated) added 277 kg N ha^–1 (244 kg fixed N ha^–1) before incorporation as a green manure in the first year of the experiment. The economic optimum N fertiliser rate for each cropping system was determined every second year when all systems were sown to cotton. Cotton following cotton required 105 kg fertiliser N ha^–1, but only 40 kg N ha^–1 when vetch was grown between each cotton crop. Cotton following wheat required 83 kg fertiliser N ha^–1 but no N fertiliser was needed when vetch was grown after wheat (the highest yielding system). Cotton following faba beans also required no N fertiliser. The vetch-based systems became more N fertile over the course of the experiment and produced greater lint yields than the comparative non-legume systems, and required less N fertiliser. While no cash flow was derived from growing vetch, economic benefits accrued from enhanced cotton yields, reduced N fertiliser requirements and improved soil fertility. These findings help explain the rotational benefits of vetches observed in other regions of the world.     

Effects of agricultural production on phosphorus losses from paddy soils: a case study in the Taihu Lake Region of China

A field plot experiment was conducted on two types of paddy soils in the Taihu Lake Region of China from June 2000 through 2002 to assess phosphorus (P) losses by runoff and drainage flow and the effectiveness of rice–wheat double cropping on reducing P losses from paddy soils. Commercial NPK compound fertilizer and single superphosphate fertilizer were applied to furnish 0, 30, 150, and 300 kg P ha^–1 for rice season trials, and 0, 20, 80, and 160 kg P ha^–1 for wheat season trials. The experiments consisted of four replicates (plots of 5 × 6 m in a randomized block design) of each treatment in Argic stagnic anthrosols (Anzhen site) and six replicates in Cumulic stagnic anthrosols (Changshu site). P30 and P20 treatments (30 and 20 kg P ha^–1 in rice and wheat seasons, respectively) were considered as conventional P application rates in this area. Higher P treatments, such as P150 and P300 for rice and P80 and P160 for wheat, were intended to simulate the status of soil P in ~10–20 years with an application of P30 or P20 kg P ha^–1 each season. Results revealed that the average concentration of total P (TP) in runoff samples was 0.870 mg P l^–1 from P30 plots during the rice season, and 0.763 mg P l^–1 from P20 plots during the wheat season in both years at the Anzhen site, while it was 0.703 and 1.292 mg P l^–1, respectively, at the Changshu site. Average TP load (mass loss) at the Anzhen site with conventional P application rates was 220.9 and 439.5 g P ha^–1 during rice season in 2000/2001 and 2001/2002, respectively, but was 382.3 and 709.4g P ha^–1 during wheat season, respectively. Mass loss at the Changshu site was 140.4 and 165.7 g P ha^–1 during the rice season and 539.1 and 1184.6 g P ha^–1 during the wheat season, respectively. P losses from paddy soils were significantly greater during the wheat season, especially at the Changshu site, indicating that planting rice reduced P. Phosphate fertilizer levels significantly affected P concentrations and P loads in runoff both seasons. Both mean concentrations and average seasonal P loads from the P150/P80 plots were lower than that from the P300/P160 plots, but significantly higher than that from the P30/P20 and P0 plots. This implied that runoff P loads would be greatly increased in 10–20 years as a result of the accumulation of soil P if 50 kg P ha^–1 (rice season plus wheat season) is applied each year.     

Populations and production of fish in two small tributaries of the Paraná River,Paraná, Brazil

Mean biomass (153-1) and production (P) of fish in two small tributaries of the Paraná River (Paraná, Brazil) were 61 kg ha^–1 and 48 kg ha^–1 yr^–1 in the Caracu River and 29 kg ha^–1 and 26 kg ha^–1 yr^–1 in the Agua do Rancho River, respectively. Matrix correlation analysis revealed high positive correlations of both 153-2 and P to maximum depth and hiding places and, at a lower level of significance, to mean depth, pH and oxygen level. Lower 153-3 and P values were found in the Agua do Rancho River, whose valley has retained a more natural character, rich canopy and scarcity of macrophytes, but also lower conductivity and nitrogen and phosphate levels than those in the Caracu River.Address for correspondence     

Production,standing biomass and natural abundance of <Superscript>15</Superscript>N and <Superscript>13</Superscript>C in ectomycorrhizal mycelia collected at different soil depths in two forest types

Nutrient uptake by forest trees is dependent on ectomycorrhizal (EM) mycelia that grow out into the soil from the mycorrhizal root tips. We estimated the production of EM mycelia in root free samples of pure spruce and mixed spruce-oak stands in southern Sweden as mycelia grown into sand-filled mesh bags placed at three different soil depths (0–10, 10–20 and 20–30 cm). The mesh bags were collected after 12 months and we found that 590±70 kg ha^–1 year^–1 of pure mycelia was produced in spruce stands and 420±160 kg ha^–1 year^–1 in mixed stands. The production of EM mycelia in the mesh bags decreased with soil depth in both stand types but tended to be more concentrated in the top soil in the mixed stands compared to the spruce stands. The fungal biomass was also determined in soil samples taken from different depths by using phospholipid fatty acids as markers for fungal biomass. Subsamples were incubated at 20°C for 5 months and the amount of fungal biomass that degraded during the incubation period was used as an estimate of EM fungal biomass. The EM biomass in the soil profile decreased with soil depth and did not differ significantly between the two stand types. The total EM biomass in the pure spruce stands was estimated to be 4.8±0.9×10³ kg ha^–1 and in the mixed stands 5.8±1.1×10³ kg ha^–1 down to 70 cm depth. The biomass and production estimates of EM mycelia suggest a very long turnover time or that necromass has been included in the biomass estimates. The amount of N present in EM mycelia was estimated to be 121 kg N ha^–1 in spruce stands and 187 kg N ha^–1 in mixed stands. The ¹³C value for mycelia in mesh bags was not influenced by soil depth, indicating that the fungi obtained all their carbon from the tree roots. The ¹³C values in mycelia collected from mixed stands were intermediate to values from pure spruce and pure oak stands suggesting that the EM mycelia received carbon from both spruce and oak trees in the mixed stands. The ¹⁵N value for the EM mycelia and the surrounding soil increased with soil depth suggesting that they obtained their entire N from the surrounding soil.     

N deposition, N transformation and N leaching in acid forest soils

Nitrogen deposition, mineralisation, uptake and leaching were measured on a monthly basis in the field during 2 years in six forested stands on acidic soils under mountainous climate. Studies were conducted in three Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantations (D20: 20 year; D40: 40 yr; D60: 60 yr) on abandoned croplands in the Beaujolais Mounts; and two spruce (Picea abies Karst.) plantations (S45: 45 yr; S90: 90 yr) and an old beech (Fagus sylvatica L.) stand (B150: 150 yr) on ancient forest soils in a small catchment in the Vosges Mountains. N deposition in throughfall varied between 7–8 kg ha^–1 year^–1 (D20, B150, S45) and 15–21 kg ha^–1 yr^–1 (S90, D40, D60). N in annual litterfall varied between 20–29 kg ha^–1 (D40, D60, S90), and 36–43 kg ha^–1 (D20, S45, B150). N leaching below root depth varied among stands within a much larger range, between 1–9 kg ha^–1 yr^–1 (B150, S45, D60) and 28–66 kg ha^–1 yr^–1 (D40, S90, D20), with no simple relationship with N deposition, or N deposition minus N storage in stand biomass. N mineralisation was between 57–121 kg ha^–1 yr^–1 (S45, D40, S90) and between 176–209 kg ha^–1 yr^–1 in (B150, D60 and D20). The amounts of nitrogen annually mineralised and nitrified were positively related. Neither general soil parameters, such as pH, soil type, base saturation and C:N ratio, nor deposition in throughfall or litterfall were simply related to the intensity of mineralisation and/or nitrification. When root uptake was not allowed, nitrate leaching increased by 11 kg ha^–1 yr^–1 at S45, 36 kg ha^–1 yr^–1 at S90 and between 69 and 91 kg ha^–1 yr^–1 at D20, D40, B150 and D60, in relation to the nitrification rates of each plot. From this data set and recent data from the literature, we suggest that: high nitrification and nitrate leaching in Douglas-fir soils was likely related to the former agricultural land use. High nitrification rate but very low nitrate leaching in the old beech soil was related to intense recycling of mineralised N by beech roots. Medium nitrification and nitrate leaching in the old spruce stand was related to the average level of N deposition and to the deposition and declining health of the stand. Very low nitrification and N leaching in the young spruce stand were considered representative of fast growing spruce plantations receiving low N deposition on acidic soils of ancient coniferous forests. Consequently, we suggest that past land use and fine root cycling (which is dependent on to tree species and health) should be taken into account to explain the variability in the relation between N deposition and leaching in forests.