Soil chemical and microbiological properties in hay production systems: residual effects of contrasting N fertilization of swine lagoon effluent versus ammonium nitrate

This study characterized soil chemical and microbiological properties in hay production systems that received from 0 to 600 kg plant-available N (PAN) ha⁻¹ year⁻¹ from either swine lagoon effluent (SLE) or ammonium nitrate (AN) from 1999 to 2001. The forage systems contained plots planted with bermudagrass (Cynodon dactylon L.) or endophyte-free tall fescue (Festuca arundinaceae Schreb.). In March 2004, the plots were sampled for measurements of a suite of soil chemical and microbiological properties. Nitrogen fertilization rates were significantly correlated with soil pH and K₂SO₄-extractable soil C but not with total soil C, soil C/N ratio, electrical conductivity, or Mehlich-3-extractable nutrients. Soil supplied with SLE had significantly lower Mehlich-3-extractable nutrients than the soil supplied with AN. Two indicators of soil N-supplying capacity (potentially mineralizable N and amino sugar N) varied with plant species and the type of N fertilizer. However, they generally peaked at an application rate of 200 or 400 kg PAN ha⁻¹ year⁻¹. Soil microbial biomass C also peaked at an application rate of 200 or 400 kg PAN ha⁻¹ year⁻¹. Nitrification potential was significantly higher in soil supplied with AN than in the unfertilized control but was similar between SLE-fertilized and unfertilized soils. Our results indicated that an application rate as high as 600 kg PAN ha⁻¹ year⁻¹ did not benefit soil microbial biomass, microbial activity, and N transformation processes in these forage systems.     

Evaluation of press mud cake as a source of nitrogen and phosphorus for rice–wheat cropping system in the Indo-Gangetic plains of India

Press mud cake (PMC) is an important organic source available for land application in India. Adequate information regarding availability of nitrogen and phosphorous contained in PMC to rice–wheat (RW) cropping system is lacking. In field experiments conducted for 4 years to study the effect of PMC application to rice as N and P source in RW system, application of 60 kg N ha⁻¹ along with PMC (5 t ha⁻¹) produced grain yield of rice similar to that obtained with the 120 kg N ha⁻¹ in unamended plots. In the following wheat, the residual effects of PMC applied to preceding rice were equal to 40 kg N and 13 kg P ha⁻¹. Immobilization of soil and fertilizer N immediately after the application of PMC was observed in laboratory incubation. The net amount of N mineralized from the PMC ranged from 16% at 30 days to 43% at 60 days after incubation. Available P content in the soil amended with PMC increased by about 60% over the unamended control within 10 days of its application. The P balance for the no-PMC treatment receiving recommended dose of 26 kg P ha⁻¹ year⁻¹ was −13.5 kg P ha⁻¹ year⁻¹. The P balance was positive (+42.3 to 53.5 kg P ha⁻¹ year⁻¹) when PMC was applied to rice. Application of PMC increased total N, organic carbon, and available P contents in the soil.     

N<Subscript>2</Subscript> fixation by faba bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.) in a gypsum-amended sodic soil

The response of faba bean to the application of four rates of gypsum (0, 2.5, 5.0, 10.0 t ha⁻¹) to a non-saline, alkaline sodic soil was measured in terms of grain yield, dry matter (DM) production, N accumulation and the proportional dependence of the legume on symbiotic N₂ fixation (P _atm). A yield-independent, time-integrated ¹⁵N-dilution model was used to estimate symbiotic dependence. A significant decrease in the exchangeable sodium percentage and significant increases in exchangeable Ca⁺⁺ and the Ca⁺⁺:Mg⁺⁺ ratio in the 0–10-cm soil layer were measured 30 months after application of 10 t ha⁻¹ gypsum. Despite low and erratic rainfall during crop growth, faba bean DM and N uptake responded positively to gypsum application. The symbiotic dependence of the legume at physiological maturity was little affected by sodicity (P _atm = 0.74 at zero gypsum and 0.81–0.82 at 2.5–10 t ha⁻¹ gypsum). The increase in fixed N due to gypsum application was mainly due to increases in legume DM and total N uptake. At 10 t ha⁻¹ of gypsum, faba bean fixed more than 200 kg N ha⁻¹ in above-ground biomass.     

Leaching of soil organic carbon and nitrogen in sandy soils after cultivating grass-clover swards

Several studies have focused on the formation and losses of dissolved organic matter in forest systems, whereas a limited number have dealt with this aspect in agricultural soils. The purpose of this study was to estimate the leaching of dissolved organic carbon (DOC) and nitrogen (DON), with focus on the period after cultivating grass-clover swards. Grass-clovers were ploughed in the spring prior to sowing cereals followed by either catch crops or bare soil. The concentrations of DOC and DON decreased with soil depth and ranged at 90-cm soil depth between 7 and 21 mg C L⁻¹ and between 1 and 3 mg N L⁻¹, respectively, in a sandy loam soil, and between 16 and 63 mg C L⁻¹ and between 1 and 10 mg N L⁻¹, respectively, in a coarse sandy soil. The resulting DOC/DON ratios were in the range between 2 and 42, with higher values in the coarse sandy soil than in the sandy loam soil. The total percolation was 218 mm in the sandy loam soil and 596–645 mm in the coarse sandy soil, which resulted in an annual leaching of 22–40 kg DOC ha⁻¹ year⁻¹ and 3–4 kg DON ha⁻¹ year⁻¹ in the sandy loam soil, and 174–310 kg DOC ha⁻¹ year⁻¹ and 10–31 kg DON ha⁻¹ year⁻¹ in the coarse sandy soil. It was shown that higher amounts of DOC were lost by leaching under the catch crops than from bare soil, that losses of DON were higher from bare soil than from soils with catch crops and that DON contributed significantly to the total N loss. Thus, DON needs to be taken into account in N-balance calculations.     

Irrigation practices may affect denitrification more than nitrogen mineralization in warm climatic conditions

Predicting the impact of irrigation practices on soil N mineralization and N balance is an important issue to optimize N fertilization and reduce the N losses towards the environment. The effect of summer irrigation on N dynamics was investigated in two arable fields in Southern France. Net N mineralization was assessed by combining frequent measurements of water and mineral N contents in soil and the use of a calculation model (LIXIM). It was first calculated assuming that denitrification was negligible. This hypothesis led to inconsistent results, apparent net N mineralized being smaller under irrigated than non-irrigated conditions and net mineralization kinetics being erratic. The occurrence of denitrification was confirmed by the use of ¹⁵NO₃ tracing in an experiment carried out in summer, including three irrigated treatments. The average ¹⁵N recovery varied from 45% to 85% and was smallest in the most frequently irrigated treatment. Over the 8-week experiment, the N losses varied from 30 to 38 kg ha⁻¹ in the irrigated treatments. They were satisfactorily simulated by a simple denitrification model (NEMIS). Combining the LIXIM model and the simulated or calculated denitrification allowed to predict satisfactorily the evolution of soil mineral N accounting for the effects of temperature and moisture. The net N mineralized for 8 weeks varied from 34 kg N ha⁻¹ in the un-irrigated to 46 kg N ha⁻¹ in the irrigated treatments. The drying–rewetting cycles did not induce a flush of N mineralization. Our results suggest that denitrification has to be accounted for in irrigated systems, particularly in warm conditions and when the topsoil contains high nitrate contents.     

Long-term effects of fertilizer managements on crop yields and organic carbon storage of a typical rice–wheat agroecosystem of China

The present paper summarizes the results from a long-term experiment setup in 1980 in the Taihu Lake region, China, to address the yield sustainability, the dynamic changes of soil organic carbon (SOC) storage, and soil fertility in the rice–wheat ecosystem. Treatments in three replicates comprising manure-treated and chemical fertilizer-treated groups (two factors), each having seven sub-treatments of different combinations of inorganic nitrogen (N), phosphorus (P), potassium (K), and rice straw, were randomly distributed. Results showed that the treatments of manure (pig manure from 1980 to 1996 and oil rape cake thereafter) + N + P + K (MNPK) and chemical fertilizer + N + P + K (CNPK) produced the highest and the most stable yields for both rice and wheat within the respective fertilizer treatment group. Potassium fertilization was necessary for yield sustainability in the ecosystem. Treatments of straw (as rice straw) + N (CRN) and manure + straw + N (MRN) produced more stable yield of rice but less stable of wheat. It was therefore recommended that straw should be only incorporated during the rice season. SOC contents in all treatments showed increasing trends over the period, even in the control treatment. Predicted SOC in chemical fertilizer-treated plots (mostly yet attainable) ranged from 16 to 18 g C kg⁻¹, indicating the high carbon (C) sequestration potential of the soil as compared to the initial SOC. SOC in manure- or straw-treated plots ranged from 17 to 19 g C kg⁻¹, which had been attained roughly 10 years after the experiment was initiated. Nutrient balance sheet showed that there was P surplus in all P-treated plots and a steady increase in Olsen-P over a 24-year period in 0–15 cm soil, which contributed little to crop yield increases. It was therefore suggested that P fertilization rate should be decreased to 30–40 kg P ha⁻¹ year⁻¹. Comparison of yields among the treatments showed that wheat was more responsive to P fertilizer than rice. Thus P fertilizer should be preferably applied to wheat. Soil pH decrease was significant over the 24-year period and was not correlated with fertilizer treatments. The overall recommendation is to incorporate straw at 4,500 kg ha⁻¹ year⁻¹ during the rice season only, with additional 190 kg N ha⁻¹ year⁻¹, 30–40 kg P ha⁻¹ year⁻¹ mainly during the rice season, and 150–160 kg K ha⁻¹ year⁻¹. Further research on the unusual P supply capacity of the soil is needed.     

Nitrogen Balance and Fertigation Use Efficiency in a Field Coffee Crop

Brazil is a major world coffee producer, using increasing quantities of nitrogen (N) fertilizer as the monoculture expands across the savannas. The fate and efficiency of this fertilizer N were evaluated for one complete cropping cycle using ¹⁵N tracer, permitting an N balance at harvest. Annual rates of 200, 400, 600, and 800 kg N ha^?1 year^?1 of ¹⁵N-labeled urea and an unfertilized control were applied every 14 days via fertigation. The N concentration, percentage of N derived from fertilizer, quantity of N derived from fertilizer, and percentage of nitrogen derived from fertilizer per N rate was assessed for 8-year-old coffee trees. The most efficient N use was with 200 kg ha^?1 year^?1 because it presented the lowest losses and highest N recoveries in the crop. Conversely, the least sustainable rate was 800 kg ha^?1 year^?1, which presented the greatest losses and the lowest N recovery in the whole plant.     

Effects of long-term mineral fertilization on microbial biomass,microbial activity,and the presence of <Emphasis Type="Italic">r</Emphasis>- and <Emphasis Type="Italic">K</Emphasis>-strategists in soil

Long-term effects of mineral fertilization on microbial biomass C (MBC), basal respiration (R _B), substrate-induced respiration (R _S), β-glucosidase activity, and the r–K-growth strategy of soil microflora were investigated using a field trial on grassland established in 1969. The experimental plots were fertilized at three rates of mineral N (0, 80, and 160 kg ha⁻¹ year⁻¹) with 32 kg P ha⁻¹ year⁻¹ and 100 kg K ha⁻¹ year⁻¹. No fertilizer was applied on the control plots (C). The application of a mineral fertilizer led to lower values of the MBC and R _B, probably as a result of fast mineralization of available substrate after an input of the mineral fertilizer. The application of mineral N decreased the content of C extracted by 0.5 M K₂SO₄ (C _ex). A positive correlation was found between pH and the proportion of active microflora (R _S/MBC). The specific growth rate (μ) of soil heterotrophs was higher in the fertilized than in unfertilized soils, suggesting the stimulation of r-strategists, probably as the result of the presence of available P and rhizodepositions. The cessation of fertilization with 320 kg N ha⁻¹ year⁻¹ (NF) in 1989 also stimulated r-strategists compared to C soil, probably as the result of the higher content of available P in the NF soil than in the C soil.     

Nitrogen conservation in soil and crop residues as affected by crop rotation and soil disturbance under Mediterranean conditions

We investigated conservation and cycling of N under oat–oat and lupine–oat rotations in disturbed and undisturbed soil, when roots or roots plus aboveground residues were retained. Crop residues were labelled with ¹⁵N in Year 1, and differential soil disturbance was imposed after harvest. In Year 2, plant growth, N transfer from residue into the various sinks of the second crop (plant, soil, and residual residues), and changes in microbial activity and numbers were determined. Oat biomass was greater after lupine than after oat due to differences in supply of N from these residues. Buried residues of both crops appeared to decompose faster than when left on the soil surface. Lupine residues decomposed faster than oat residues. Oat biomass was not affected by soil disturbance if grown after lupine but decreased when oat straw was buried in the soil. More residue N was recovered from soil than from the crop. Most ¹⁵N was recovered from disturbed soil, which also had greater dehydrogenase activity and more culturable fungi. At the end of the oat–oat rotation, 20 and 5 kg N ha⁻¹ were derived from the roots of the first crop in undisturbed or disturbed soil, respectively. Equivalent values for the lupine–oat rotation were 18 and 44 kg N ha⁻¹. Returning aboveground residues provided an extra 52–80 kg N ha⁻¹ for oat and 61–63 kg N ha⁻¹ for lupine relative to treatments where they were removed. Over a year, lupine contributed 9 to 20 kg N ha⁻¹ more to the agroecosystem than did oat.     

Fate of Fertilizer-15N to a Maize Crop Grown in Northern Zambia

Abstract

A field study with maize (Zea mays L.) was conducted in the 1988/89 cropping season to investigate the fate of ¹⁵NO₃-N-labelled NH₄ ¹⁵NO₃ applied at 40, 80 and 120 kg N ha^?1 (unlabelled N applied at 0, 80, 160 and 240 N ha^?1) with and without lime. The investigations were conducted in northern Zambia at Misamfu Regional Research Centre, Kasama on a Misamfu red sandy loam soil. The experimental design was a split plot arrangement with four replications with main plots receiving 0 and 2 Mg ha^?1 dolomitic limestone, while subplots received fertilizer N at various rates. Significant (p < 0.001) grain and DM yield responses to applied N up to 160 kg ha^?1 were observed. At higher rates little or no crop responses were observed and fertilizer use efficiency declined. Partitioning of amounts of total N and ¹⁵N in plants was in the order of seed = tassel > leaf> cob = earleaf> stem. Fertilizer N rates showed a highly significant (p < 0.001) effect on plant uptake of labelled N. Lime and its interaction with N rates had no effect on all measured parameters. Leaching of NO₃-N fertilizer to lower soil depths was in proportion to the rate of N applied, with highly significant (p < 0.001) differences among soil depths. Although higher concentrations of fertilizer-¹⁵N were recovered in the 0–20 cm depth the recovered portion at lower soil depths was still significant. Total recovery of labelled N by plant and by soil after crop harvest averaged 75, 55 and 54% of originally applied fertilizer-¹⁵N at 40, 80 and 120 kg N ha^?1, respectively. Corresponding unaccounted for ¹⁵N was 25, 45 and 46%. The most probable loss mechanism could have been by leaching to depths greater than 60 cm, gaseous losses to the atmosphere and root assimilation.     

Effects of NPK fertilizer combinations on yield and nitrogen balance in sorghum or pigeonpea on a vertisol in the semi-arid tropics

A long-term experiment was carried out on a Vertisol from 1986 to 1992 to examine the combined effects of NPK fertilizers on yield using sorghum (Sorghum bicolor L. Moench cv. CSH 5) and short-duration pigeonpea (Cajanus cajan L. Millsp. cv. ICPL 87). The fertilizer treatments were as follows: 0 (no fertilization), N (150 kg N ha^-1 ), P (65.5 kg P₂O₅ ha^-1), K (124.5 kg K₂O ha^-1), and all possible combinations (NP, NK, PK, and NPK). In this study we continued this experiment during the period 1993 to 1994 and analyzed the crop yield response to fertilizers and the N balance. The amount of N derived from the atmosphere and fertilizer was estimated by the ¹⁵N natural abundance method and ^l5N isotope dilution method, respectively. A combined application of Nand P fertilizers gave the highest grain yield for the two crops under the 8th and 9th continuous croppings, unlike the application of K fertilizer. The values of total N for the two crops were significantly higher in the NP and NPK plots. These crops took up N mainly from soil. There was a significant positive relationship between the uptake of N_dff and N_dfs by each crop. Pigeonpea or sorghum took up more N from the soil in the N fertilizer plots than in the plots without N, suggesting that soil N fertility was enhanced and the amount of N supplied from soil increased in the plots with consecutive application of N fertilizer for 7 y. Even pigeonpea, which fixes atmospheric N inherently, needed N fertilizer to achieve high grain yield, suggesting that N fixation by the nodules was not always sufficient to meet the N requirements of the crop under these conditions. Although fertilizer N exerted a beneficial effect on plant growth and yield in the two crops, the values of fertilizer N recovery (FNR) by the two crops were considerably low. Therefore, it is suggested that the development of N fertilizer management which could maximize FNR of each crop should be promoted.     

Poultry litter as a nitrogen and phosphorous source for the rice–wheat cropping system

Poultry litter (PL) is an important nutrient source; however, no information is available regarding its value in supplying N and P in rice–wheat (RW) production. A three-year field study was conducted at Ludhiana, Punjab, India on a loamy sand soil to identify optimum combination of PL and N and P fertilizers for a sustainable RW production. The litter was applied to rice at 5 Mg ha⁻¹ as a single application and supplemented with different rates of N. The residual effect of PL and the direct effects of the different combinations of N and P were studied in the following wheat. Nitrogen and P mineralization from PL was studied under controlled conditions in the laboratory, and macronutrient input–output balances were estimated from field results. About 46% of the N from PL was released after 60 days of incubation. The release of P from the PL occurred mainly during the initial 20 days after incubation, accounting for 15–17% of the total P. Combining PL with fertilizer N (40 kg ha⁻¹) increased rice yield and nutrient uptake similar to what was obtained with the application of recommended fertilizer N (120 kg ha⁻¹). In the following wheat, the residual effect of PL was equal to 30 kg N ha⁻¹ and 13 kg P ha⁻¹. After three annual cropping cycles and PL application, mean soil organic C increased by 17%, Olsen-P by 73%, and NH₄OAc-extractable-K by 24%. Most treatments had positive P but negative K balances. About 11% of the net P balance was recovered from the soil as Olsen-P. The study showed that optimum N and P fertilizer doses for an RW system receiving 5 Mg ha⁻¹ of PL are 40 kg N ha⁻¹ for rice and 90 kg N + 13 kg P ha⁻¹ for the following wheat. Safe and effective management of PL should be based on P balance, particularly when regular applications of PL are to be made in the RW system.     

Biological nitrogen fixation by common beans (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) increases with bio-char additions

This study examines the potential, magnitude, and causes of enhanced biological N₂ fixation (BNF) by common beans (Phaseolus vulgaris L.) through bio-char additions (charcoal, biomass-derived black carbon). Bio-char was added at 0, 30, 60, and 90 g kg⁻¹ soil, and BNF was determined using the isotope dilution method after adding ¹⁵N-enriched ammonium sulfate to a Typic Haplustox cropped to a potentially nodulating bean variety (CIAT BAT 477) in comparison to its non-nodulating isoline (BAT 477NN), both inoculated with effective Rhizobium strains. The proportion of fixed N increased from 50% without bio-char additions to 72% with 90 g kg⁻¹ bio-char added. While total N derived from the atmosphere (NdfA) significantly increased by 49 and 78% with 30 and 60 g kg⁻¹ bio-char added to soil, respectively, NdfA decreased to 30% above the control with 90 g kg⁻¹ due to low total biomass production and N uptake. The primary reason for the higher BNF with bio-char additions was the greater B and Mo availability, whereas greater K, Ca, and P availability, as well as higher pH and lower N availability and Al saturation, may have contributed to a lesser extent. Enhanced mycorrhizal infections of roots were not found to contribute to better nutrient uptake and BNF. Bean yield increased by 46% and biomass production by 39% over the control at 90 and 60 g kg⁻¹ bio-char, respectively. However, biomass production and total N uptake decreased when bio-char applications were increased to 90 g kg⁻¹. Soil N uptake by N-fixing beans decreased by 14, 17, and 50% when 30, 60, and 90 g kg⁻¹ bio-char were added to soil, whereas the C/N ratios increased from 16 to 23.7, 28, and 35, respectively. Results demonstrate the potential of bio-char applications to improve N input into agroecosystems while pointing out the needs for long-term field studies to better understand the effects of bio-char on BNF.     

Impact of grazed grassland management on total N accumulation in soil receiving different levels of N inputs

Nitrogen balances and total N and C accumulation in soil were studied in reseeded grazed grassland swards receiving different fertilizer N inputs (100–500 kg N ha^?1 year^?1) from March 1989 to February 1999, at an experimental site in Northern Ireland. Soil N and C accumulated linearly at rates of 102–152 kg N ha^?1 year^?1 and 1125–1454 kg C ha^?1 year^?1, respectively, in the top 15 cm soil during the 10 year period. Fertilizer N had a highly significant effect on the rate of N and C accumulation. In the sward receiving 500 kg fertilizer N ha^?1 year^?1 the input (wet deposition + fertilizer N applied) minus output (drainflow + animal product) averaged 417 kg N ha^?1 year^?1. Total N accumulation in the top 15 cm of soil was 152 kg N ha^?1 year^?1. The predicted range in NH₃ emission from this sward was 36–95 kg N ha^?1 year^?1. Evidence suggested that the remaining large imbalance was either caused by denitrification and/or other unknown loss processes. In the sward receiving 100 kg fertilizer N ha^?1 year^?1, it was apparent that N accumulation in the top 15 cm soil was greater than the input minus output balance, even before allowing for gaseous emissions. This suggested that there was an additional input source, possibly resulting from a redistribution of N from lower down the soil profile. This is an important factor to take into account in constructing N balances, as not all the N accumulating in the top 15 cm soil may be directly caused by N input. N redistribution within the soil profile would exacerbate the N deficit in budget studies.     

Quantification of N2 fixation and annual N benefit from N2 fixation in soybean accrued to the soil under soybean‐wheat continuous rotation

A computational exercise was undertaken to quantify the percent N derived from atmosphere %Ndfa) in soybean and consequent N benefit from biological N₂‐fixation process annually accrued to the soil by the soybean crop using average annual N‐input/‐output balance sheet from a 7 yr old soybean‐wheat continuous rotational experiment on a Typic Haplustert. The experiment was conducted with 16 treatments comprised of combinations of four annual rates of farmyard manure (FYM ? 0, 4, 8, and 16 t ha^–1) and four annual rates of fertilizer N (? 0, 72.5, 145, and 230 kg N ha^–1) applications. The estimated N contributed through residual biomass of soybean (RBN_S) consisting of leaf fall, root, nodules, and rhizodeposition varied in the ranges of 7.02–16.94, 11.65–28.83, 3.31–8.91, and 11.3–23.8 kg N ha^–1 yr^–1, respectively. A linear relationship was observed between RBN_S and harvested biomass N (HBN_S) of soybean in the form of RBN_S = 0.461 × HBN_S – 20.67 (r = 0.989, P < 0.01), indicating that for each 100 kg N assimilated by the harvested biomass of soybean, 25.4 kg N was added to the soil through residual biomass. The Ndfa values ranged between 13% and 81% depending upon the annual rates of application of fertilizer N and FYM. As per the main effects, the %Ndfa declined from 76.4 to 26.0 with the increase in annual fertilizer‐N application from 0 to 230 kg N ha^–1, whereas %Ndfa increased from 40.8 to 65.8 with the increase in FYM rates from 0 to 16 t ha^–1, respectively. The N benefit from biological N₂ fixation accrued to the soil through residual biomass of soybean ranged from 7.6 to 53.7 kg N ha^–1 yr^–1. The treatments having %Ndfa values higher than 78 showed considerable annual contribution of N from N₂ fixation to the soil which were sufficient enough to offset the quantity of N removed from the soil (i.e., native soil N / FYM‐N / fertilizer‐N) with harvested biomass of soybean.     

Predicting plant sulfur deficiency in soils: results from Ohio

We developed a model for plant available sulfur (S) in Ohio soils to predict potential crop plant S deficiency. The model includes inputs of plant available S due to atmospheric deposition and mineralization of soil organic S and output due to leaching. A leaching index was computed using data on annual precipitation; soil pH and clay content that influence sulfate adsorption; and pore water velocity based upon percent sand, silt, and clay. There are five categories of S status ranging from highly deficient to highly sufficient, and the categories are defined based on whether the crop S requirement was 15 or 30kg S ha⁻¹ year⁻¹. The final database derived from the model includes 1,473 soil samples representing 443 of the 475 soil series in Ohio. For a crop requiring 15kg S ha⁻¹ year⁻¹, most soils (68.6%) were classified as variably deficient, which implies that the response to S fertilization will be variable but often positive depending on specific crop conditions. For a crop requiring 30kg S ha⁻¹ year⁻¹, 43.2% of soils were classified as variably deficient, but 49.7% were classified as moderately or highly deficient, implying that a response to S fertilization will usually or always occur. The model predicts crop S status for a single state in the USA, but with proper inputs, it should be applicable to other areas.     

A comparison of nitrogen fixation in genotypes of groundnut (Arachis hypogaea L.) using 15N-isotope dilution

Summary Nitrogen fixation in seven groundnut genotypes was measured by ¹⁵N-isotope dilution using a non-nodulating cultivar of groundnut as the nonfixing reference plant. Nitrogen fixation varied between 100 kg N ha^–1 in genotype J-11 and 153 kg N ha^–1 in Robut 33-1. The amount of plant-available soil N was small, so that 86%–92% of plant nitrogen was derived from N₂-fixation. Thus differences in N₂-fixation between genotypes closely reflected differences in their total N accumulation.ICRISAT Journal Article no. 600     

Dinitrogen (C2H2) fixation in relation to nitrogen fertilization of grey alder [Alnus incana (L.) Moench.] plantations in a peat bog

Summary Nitrogenase activity was measured in young grey alder plantations in a peat bog in central Sweden. The stands were treated in three ways: (1) daily irrigation during the growing season with a complete nutrient solution, including N; (2) application of bark ash or wood ash before planting; and (3) fertilization every second year with solid PK fertilizers. Acetylene reduction assays were performed on (1) detached nodules and attached nodules, either on (2) whole enclosed plants or (3) enclosed nodules. The acetylene reduction rate for the enclosed plants showed a maximum in July when mean values of nearly 80 mol C₂H₄ (g nodule dry matter)^-1 h^-1 were reached. No diurnal patterns were observed. The irrigated stands, with an N supply, showed overall nitrogenase activities that corresponded well with those of the other treatments. Only in the case of temporarily increased soil nutrient concentrations in the irrigated stands did the nitrogenase activity fall considerably. In 6-to 7-year-old intensively managed irrigated stands N₂-fixation was estimated as 85–115 kg N ha^-1 year^-1 which was about 55% of the total N uptake of the trees.     

Assessment of cowpea and groundnut contributions to soil fertility and succeeding sorghum yields in the Guinean savannah zone of Burkina Faso (West Africa)

Atmospheric biological nitrogen fixation (BNF) by cowpea (Vigna unguiculata) and groundnut (Arachis hypogea) was evaluated using a 2-year (2000–2001) experiment with different fertilizer treatments. The ¹⁵N isotopic dilution method with a nonfixing cowpea as test reference crop was used. The effects of the two legumes on soil N availability and succeeding sorghum (Sorghum bicolor) yields were measured. Groundnut was found to fix 8 to 23 kg N ha-1 and the percentage of N derived from the atmosphere varied from 27 to 34%. Cowpea fixed 50 to 115 kg N ha⁻¹ and the percentage of N derived from the atmosphere varied from 52 to 56%. Compared to mineral NPK fertilizer alone, legumes fixed more N from the atmosphere when dolomite or manure was associated with mineral fertilizers. Compared to soluble phosphate, phosphate rock increased BNF by cowpea. Significant correlation (p<0.05, R ²=0.94) was observed between total N yields of legumes and total N derived from the atmosphere. Compared to monocropping of sorghum, the soils of cowpea–sorghum and groundnut–sorghum rotations increased soil mineral N from 15 and 22 kg N ha⁻¹, respectively. Cowpea–sorghum and groundnut–sorghum rotations doubled N uptake and increased succeeding sorghum yields by 290 and 310%, respectively. Results suggested that, despite their ability to fix atmospheric nitrogen, N containing fertilizers (NPK) are recommended for the two legumes. The applications of NPK associated with dolomite or cattle manure or NK fertilizer associated with phosphate rock were the better recommendations that improved BNF, legumes, and succeeding sorghum yields.     

Effect of soil treatment with cereal straw and method of crop establishment on field pea (Pisum sativum L.) N2 fixation

The effects of soil incorporation with cereal straw (nil, 2.5, 5 and 10 t straw ha^–1) and direct drilling on the proportion and amount of pea N derived from biological N fixation were investigated in three field experiments. Fixed N was determined by¹⁵N dilution using barley as a reference plant. The three sites were on acidic, red clay-loams in the cropping zone of southeastern Australia. Seasonal plant available soil N, as determined by the N accumulated in barley, was 31, 56 and 158 kg N ha^–1, for the three sites. Incorporated straw reduced soil nitrate at sowing by 10–50 kg N ha^–1 (0–30 cm), and 5 or 10 t straw ha^–1 reduced barley uptake of N by 10–38 kg N ha^–1. However, reducing plant available soil N was generally ineffective for increasing the N fixed by pea. Fixed N increased only at the site with the least plant-available N, and only one-third of the increase could be attributed to lower soil N uptake by pea. There was no evidence that direct drilling pea increased fixed N by decreasing crop uptake of soil N. It is proposed that a lower requirement for soil N by pea as compared to barley, and availability of mineral N beneath the soil layer treated with straw, minimise the effectiveness of straw incorporation for increasing the N fixed by pea.