Intercropping of Wheat and Pea as Influenced by Nitrogen Fertilization

The effect of sole and intercropping of field pea (Pisum sativum L.) and spring wheat (Triticum aestivum L.) on crop yield, fertilizer and soil nitrogen (N) use was tested on a sandy loam soil at three levels of urea fertilizer N (0, 4 and 8 g N m⁻²) applied at sowing. The ¹⁵ N enrichment and natural abundance techniques were used to determine N accumulation in the crops from the soil, fertilizer and symbiotic N₂ fixation. Intercrops of pea and wheat showed maximum productivity without the supply of N fertilizer. Intercropping increased total dry matter (DM) and N yield, grain DM and N yield, grain N concentration, the proportion of N derived from symbiotic N₂ fixation, and soil N accumulation. With increasing fertilizer N supply, intercropped and sole cropped wheat responded with increased yield, grain N yield and soil N accumulation, whereas the opposite was the case for pea. Fertilizer N enhanced the competitive ability of intercropped wheat recovering up to 90% of the total intercrop fertilizer N acquisition and decreased the proportion of pea in the intercrop, but without influencing the total intercrop grain yield. As a consequence, Land Equivalent Ratios calculated on basis of total DM production decreased from a maximum of 1.34 to as low as 0.85 with increased fertilizer N supply. The study suggests that pea–wheat intercropping is a cropping strategy that use N sources efficiently due to its spatial self-regulating dynamics where pea improve its interspecific competitive ability in areas with lower soil N levels, and vice versa for wheat, paving way for future option to reduce N inputs and negative environmental impacts of agricultural crop production.     

Detectability of15N-depleted fertilizer N in soil and plant tissue during a corn-rye crop rotation

Use of¹⁵N-depleted fertilizer materials have been primarily limited to fertilizer recovery studies of short duration. The objective of this study was to determine if¹⁵N-depleted fertilizer N could be satisfactorily used as a tracer of residual fertilizer N in plant tissue and various soil N fractions through a corn (Zea mays L.) -winter rye (Secale cereale L.) crop rotation. Nitrogen as¹⁵N-depleted (NH₄)₂SO₄ was applied at five rates (0, 84, 168, 252, and 336 kg N ha^–1) to corn. Immediately following corn harvest a winter rye cover crop treatment was initiated. Residual fertilizer N was easily detected in the soil NO ₃ ^- -N fraction following corn harvest (140-d after application). Low levels of exchangeable NH ₄ ⁺ -N (<2.5 mg kg^–1) did not permit accurate isotope-ratio analysis. Fertilizer-derived N recovered in the soil total N fraction following corn harvest was detectable in the 0 to 30-cm depth at each N rate and in the 30 to 60 and 60 to 90-cm depths at the 336 kg ha^–1 N rate. Atom %¹⁵N concentrations in the nonexchangeable NH ₄ ⁺ -N fraction did not differ from the control at each N rate. Nitrogen recovery by the winter rye cover crop reduced residual soil NO ₃ ^- -N levels below the 10 kg ha^–1 level needed for accurate isotope-ratio analysis. Atom %¹⁵N concentrations in the soil total N fraction (approximately one yr after application) were indistinguishable from the control plots below the 168, 252, and 336 kg ha^–1 N rate at the 0 to 30, 30 to 60, and 60 to 90-cm depths, respectively. Recovery of residual fertilizer N by the winter rye cover crop was verified by measuring significant decreases in atom %¹⁵N concentrations in rye tissue with increasing N rates. The greatest limitation to the use of¹⁵N-depleted fertilizer N as a tracer of residual fertilizer N in a corn-rye crop rotation appears to be its detectibility from native soil N in the total N pool.Research partially supported by grants from the National Fertilizer and Environmental Research Center/TVA and the Virginia Division of Soil and Water Conservation.     

The effect of plant residues on plant uptake and leaching of soil and fertilizer nitrogen in a tropical red earth soil

Two field experiments, in which differing amounts and types of plant residues were incorporated into a red earth soil, were conducted at Katherine, N.T., Australia. The aim of the work was to evaluate the effect of the residues on uptake of soil and fertilizer N by a subsequent sorghum crop, on the accumulation and leaching of nitrate, and on losses of N.Stubble of grain sorghum applied at an exceptionally high rate (~ 18 000 kg ha^–1) reduced uptake of N by sorghum by 13% and depressed the accumulation of nitrate under a crop and particularly under a fallow.Loss of fertilizer N, movement of nitrate down the profile, and uptake by the crop was studied in another experiment after application of N as¹⁵NH₄ ¹⁵NO₃ to field microplots. By four weeks after fertilizer application 14% had been lost from the soil-plant system and by crop maturity 36 per cent had been lost. The pattern of¹⁵N distribution in the profile suggested that losses below 150 cm had occurred during crop growth. The recovery of¹⁵N by the crop alone ranged from 16 to 32 per cent. There was an apparent loss of N from the crop between anthesis and maturity. Residue levels common to sorghum crops in the region (~ 2000 kg ha^–1) did not significantly affect uptake by a subsequent sorghum crop, N losses, or distribution of nitrate in the profile.     

N relationship in a legume nonlegume association grown in an intercropping system

Sorghum grown in a mixture with legumes viz. groundnut, mungbean and cowpeas took up more N than sorghum grown as sole crop. In a mixture with mungbean the total N uptake by sorghum was 8.65 g m^–2, while with sole sorghum it was 6.79 g m^–2. The per cent N derived from fertilizer (% Ndff) was highest with sole sorghum and the lowest when grown in mixture with legumes. It is possible that sorghum derived part of the N from the soil pool enriched by concurrently grown legumes in the mixture.     

15N-labelled urea recovery by different crops in the Sudan Gezira soil

The aim of this work was to provide practical information on the efficient utilization of urea-nitrogen by wheat, cotton and sorghum using¹⁵N isotopic tracer method. The utilization efficiency of applied nitrogen was found to be dependent on the crop ability to remove nitrogen from the soil solution over a relatively short period. Mixing 42 kg P per ha with 84 kg N increased wheat grain yield by about 30%, and N recovery by about 50% over the same N treatment without P. Placement of urea 10–20 cm deep increased %Ndff by cotton over surface-application by about 45% and the % N recovery by about 55%. N recovery by sorghum was low, and was improved slightly by application of urea four weeks after sowing compared to application at sowing. Loss of N as NH₃ from surface applied urea was presumably the major avenue of loss which reduced N recovery by the different crops, especially sorghum and cotton. Urea-N recoveries calculated by the difference method and the¹⁵N isotopic method show the more reliability of the latter method.     

Effects of improved drainage and nitrogen source on yields, nutrient uptake and utilization efficiencies by maize (Zea mays L.) on Vertisols in sub-humid environments

Nitrogen is the most limiting plant nutrient in Vertisols in Kenya. Soil properties, climatic conditions and management factors as well as fertilizer characteristics can influence fertilizer nitrogen (N) use efficiency by crops. Vertisols, characterized by low-basic water infiltration rate, are prone to waterlogging under sub-humid and humid conditions. We determined effects of drainage, N source and time of application on yields, nutrient uptake and utilization efficiencies by maize grown on Vertisols in sub-humid environments. Treatments comprised two furrows (40 cm and 60 cm deep) and a check (i.e., no furrow), calcium nitrate to furnish NO₃-N, ammonium sulphate to supply NH₄-N at 100 kg N ha⁻¹, a control (i.e., no fertilizer N), and fertilizer N application at sowing, 40 days after sowing, and split (i.e., half the rate at sowing and half 40 days after sowing). A split-plot design was used in which drainage formed the main plots and N source × time of N application formed the sub-plots. Higher grain and total dry matter yields, harvest index, leaf N content, uptake of N, P and K, as well as N agronomic (NAE) and recovery (NRE) efficiencies were obtained from drained compared to undrained plots. The increase ingrain yields as a result of drainage varied from 31 to 45% for control, 35 to 43% for NO₃-N, and 16 to 21% for NH₄-N treatments. Drainage resulted in total N uptake increases from 50 to 80 kg N ha⁻¹ in control plots, 80 to 130 kg N ha⁻¹ in NO₃-N treated plots, and 90 to 130kg N ha⁻¹ in NH₄-N treated plots. Ammonium-N source was superior to NO₃-N source in terms of higher yields, NAE, and NRE in undrained plots, but the two N sources behaved similarly in drained plots. Delayed or split NO₃-N application gave higher yields, NAE and NRE than when all N was applied at sowing in undrained plots. There was no difference between 40 cm and 60 cm deep furrows in terms of crop yields and nutrient use efficiencies. Thus, draining excess water with furrows at least 40 cm deep is essential for successful crop production in these Vertisols under sub-humid conditions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Nitrogen cycling assessment in a hedgerow intercropping system using 15N enrichment

Nitrogen (N) cycling was determined in monocultures of Sorghumbicolor (L.) Moench and alley cropped sorghum with Acaciasaligna (Labill.) H. Wendl. in semiarid Northern Kenya. N inputthrough biological N₂ fixation of the acacia, N transfer from thelegume to the intercrop and losses of applied N through harvest and leachingwere estimated using ¹⁵N enrichment. The biological N₂fixation and N transfer estimates clearly demonstrated the limitations of¹⁵N enrichment techniques in field experiments showing even highertransfer than actually fixed N. Therefore, N transfer in the hedgerowintercropping system could not be determined by the ¹⁵N dilutionmethodology. The ¹⁵N balance approach, however, yielded reliableresults even 1.5 years after ¹⁵N application. 74 to 88% of theapplied ¹⁵N was recovered after three cropping cycles, most of it inthe soil (0–1.2 m). Only about 10% of the¹⁵N was taken up by the above-ground vegetation of both monocultureand agroforestry. The trees took up more of the applied ¹⁵N(8.4%) than the sorghum (1.3%) in the agroforestry system,indicating nutrient competition between tree and crop. Leaching losses below 1.2m depth were low in this semi-arid environment with 3 and 6%of the applied ¹⁵N in the monoculture and agroforestry system,respectively. ¹⁵N losses from leaching were 2.5 times higher in thealley than under the tree row. Incorporating the leguminous tree into thesorghum cropping system had no effect on total leaching and total uptake ofapplied ¹⁵N in above-ground biomass.     

Effect of green manure on the yield,N uptake and floodwater properties of a flooded rice,wheat rotation receiving15N urea on a highly permeable soil

Field studies were conducted for two years on a rapidly percolating loamy sand (Typic Ustochrept) to evaluate the effect of green manure (GM) on the yield,¹⁵N recovery from urea applied to flooded rice, the potential for ammonia loss and uptake of residual fertilizer N by succeeding crops. The GM crop ofSesbania aculeata was grownin situ and incorporated one day before transplanting rice. Urea was broadcast in 0.05 m deep floodwater, and incorporated with a harrow. Green manure significantly increased the yield and N uptake by rice and substituted for a minimum of 60 kg fertilizer N ha^–1. The recovery of fertilizer N as indicated by¹⁵N recovery was higher in the GM + urea treatments. The grain yield and N uptake by succeeding wheat in the rotation was slightly higher with GM. The recovery of residual fertilizer N as indicated by the¹⁵N recovery in the second, third and fourth crops of wheat, rice and wheat was only 3, 1 and 1 per cent of the urea fertilizer applied to the preceding rice crop. Floodwater chemistry parameters showed that the combined use of the GM and 40 kg N ha^–1 as urea applied at transplanting resulted in a comparatively higher potential for NH₃ loss immediately after fertilizer application. The actual ammonia loss as suggested by the¹⁵N recoveries in the rice crop, however, did not appear to be appreciably larger in the GM treatment. It appeared the ammonia loss was restricted by low ammoniacal-N concentration maintained in the floodwater after 2 to 3 days of fertilizer application.     

Residual N and P fertilizer effect and fertilizer recovery on intercropped and sole-cropped corn and bean in semiarid northeast Brazil

The effects of a single ¹⁵N and P fertilizer application (16 and 12 kg ha^–1) on intercropped and sole-cropped corn and beans was followed over three consecutive years. Grain (0.1–0.9 ton ha^–1 yr^–1) and straw productions (0.2–2.5 ton ha^–1 yr^–1) were limited by rainfall and showed small responses to fertilizer. In the first year, plant N uptake was more than twice the fertilizer amounts, while P uptake was less than half the fertilizer amounts. Plant N derived from fertilizer was low (9–19%). Sole corn took up more (34%) than beans (16%) and the combined intercrop (26%) and also had higher recovery of fertilizer in the soil than single beans (50% against 28%). The distribution of fertilizer N and P in the soil showed a similar pattern in all treatments, with a high concentration around the application spot and decreasing concentrations at greater distances and above and below this point. Total P increases in a soil volume 10 cm around the application spot corresponded to 60% of the amount applied. Fertilizer contributions to the second crop were < 3% of total plant N and represented <6% of the applied amount. Therefore, the residual fertilizer effect on production was attributable to P. The patterns of fertilizer N and P distribution in the soil remained similar but N recoveries decreased 14–18%. Despite low rainfall, low productivities and reasonable proportions of fertilizer N remaining in the soil, the residual effects of the applied fertilizer N were too low to justify a fertilizer recommendation based on economic returns on the investment.     

The comparison of nitrogen use and leaching in sole cropped versus intercropped pea and barley

The effect of sole and intercropping of field pea (Pisumsativum L.) and spring barley (Hordeum vulgareL.) and of crop residue management on crop yield,NO₃ ^– leaching and N balance in the cropping systemwas tested in a 2-year lysimeter experiment on a temperate sandy loam soil. Thecrop rotation was pea and barley sole and intercrops followed by winter-rye anda fallow period. The Land Equivalent Ratio (LER), which is defined as therelative land area under sole crops that is required to produce the yieldsachieved in intercropping, was used to compare intercropping performancerelative to sole cropping. Crops received no fertilizer in the experimentalperiod. Natural ¹⁵N abundance techniques were used to determine peaN₂ fixation. The pea–barley intercrop yielded 4.0 Mg grainha^–1, which was about 0.5 Mg lowerthan theyields of sole cropped pea but about 1.5 Mg greater than harvestedin sole cropped barley. Calculation of the LER showed thatplant growth resources were used from 17 to 31% more efficiently by theintercrop than by the sole crops. Pea increased the N derived fromN₂fixation from 70% when sole cropped to 99% of the total aboveground Naccumulation when intercropped. However, based upon aboveground N accumulationthe pea–barley intercrop yielded about 85 kg Nha^–1, which was about 65 kg lower thansolecropped pea but about three times greater than harvested in sole croppedbarley.Despite different preceding crops and removal or incorporation of straw, therewas no significant difference between the subsequent non-fertilized winter-ryegrain yields averaging 2.8 Mg ha^–1, indicating anequalization of the quality of incorporated residue by theNO₃ ^– leaching pattern.NO₃ ^– leaching throughout the experimental periodwas61 to 76 kg N ha^–1. Leaching dynamics indicateddifferences in the temporal N mineralization comparing lysimeters previouslycropped with pea or with barley. The major part of this N was leached duringautumn and winter. Leaching tended to be smaller in the lysimeters originallycropped with the pea–barley intercrops, although not significantly differentfromthe sole cropped pea and barley lysimeters. Soil N balances indicated depletionof N in the soil–plant system during the experimental period, independent ofcropping system and residue management. N complementarity in the croppingsystemand the synchrony between residual N availability and crop N uptake isdiscussed.     

Recovery of 15N-labelled fertilizer applied to winter wheat and perennial ryegrass crops and residual 15N recovery by succeeding wheat crops under different crop residue management practices

The recovery of ¹⁵N-labelled fertilizer applied to a winter wheat (120 kg N ha^–1) and also a perennial ryegrass (60 kg N ha^–1) crop grown for seed for 1 year in the Canterbury region of New Zealand in the 1993/94 season was studied in the field. After harvests, ryegrass and wheat residues were subjected to four different residue management practices (i.e. ploughed, rotary hoed, mulched and burned) and three subsequent wheat crops were grown, the first succeeding wheat crop sown in 1994/95 to examine the effects of different crop residue management practices on the residual ¹⁵N recovery by succeeding wheat crops. Total ¹⁵N recoveries by the winter wheat and ryegrass (seed, roots and tops) were 52% and 41%, respectively. Corresponding losses of ¹⁵N from the crop-soil systems represented by un-recovered ¹⁵N in crop and soil were 12% and 35%, respectively. These losses were attributed to leaching and denitrification. The proportions of ¹⁵N retained in the soil (0-400 mm depth) at the time of harvest of winter wheat and ryegrass were 36% and 24%, respectively. Although the soil functioned as a substantial sink for fertilizer N, the recovery of this residual fertilizer by subsequent three winter wheat crops was low (1-5%) and this was not affected by different crop residue management practices.     

Response of dryland wheat to fertilizer nitrogen in relation to stored water,rainfall and residual farm yard manure

Yield response of dryland wheat to fertilizer N application in relation to components of seasonal water (available soil moisture and rainfall) and residual farm yard manure (FYM) was studied for five years (1983–84 to 1987–88) on a maize-wheat sequence on sandy loam soils in Hoshiarpur district of Punjab, India. Four rates of N viz. 0, 40, 60 and 80 kg ha^–1 in wheat were superimposed on two residual FYM treatments viz. no FYM (F₀) and 15 t ha^–1 (F₁₅) to preceding maize. FYM application to maize increased the residual NO₃-N content by 19–30 kg ha^–1 in the 180 cm soil profile. For a given moisture distribution, F₁₅ increased attainable yields. Over the years, F₁₅ increased wheat yield by 230 to 520 kg ha^–1. Response to fertilizer N was lower in FYM amended plots than in unamended plots. Available soil moisture at wheat seeding and amount and distribution of rainfall during the vegetative and the reproductive phases of crop development affected N use efficiency by wheat. Available soil moisture at seeding alone accounted for 50% variation in yield. The residual effect of FYM on wheat yield could be accounted for by considering NO₃-N in 180 cm soil profile at seeding. The NO₃-N and available soil moisture at wheat seeding along with split rainfall for two main phases of crop development and fertilizer N accounted for 96% variation in wheat yield across years and FYM treatments.     

Light fraction organic N, ammonium, nitrate and total N in a thin Black Chernozemic soil under bromegrass after 27 annual applications of different N rates

Anadequate supply of N for a crop depends among others on the amounts of N thataremineralized from the soil organic matter plus the supply of ammonium andnitrateN already present in the soil. The objective of this study was to determine thebehaviour of light fraction organic N (LFN), NH₄-N, NO₃-Nand total N (TN) in soil in response to different rates of fertilizer Napplication. The 0–5, 5–10, 10–15 and 15–30cm layers of a thin Black Chernozemic soil under bromegrass(Bromus inermis Leyss) at Crossfield, Alberta, Canada,weresampled after 27 annual applications of ammonium nitrate at rates of 0, 56,112,168, 224 and 336 kg N ha^–1. The concentration andmass of TN and LFN in the soil, and the proportion of LFN mass within the TNmass usually increased with N rates up to 224 kg Nha^–1. The increase in TN mass and LFN mass per unit ofNadded was generally maximum at 56 kg N ha^–1 anddeclined with further increases in the rate of N application. The percentchangein response to N application was much greater for the LFN mass than for the TNmass for all the N rates and all soil depths that were sampled. Mineral N intheform of NH₄-N and NO₃-N did not accumulate in the soil at 112 kg N ha^–1 rates, whereas theiraccumulation increased markedly with rates of 168 kg Nha^–1. In conclusion, long-term annual fertilization at 112 kg N ha^–1 to bromegrass resulted insubstantial increase in the TN and LFN in soil, with no accumulation ofNH₄-N and NO₃-N down the depth. The implication of thesefindings is that grasslands for hay can be managed by appropriate Nfertilization rates to increase the level of organic N in soil.     

Effect of poultry litter and composts on soil nitrogen and phosphorus availability and corn production

Environmental problems associated with raw manure application might bemitigated by chemically or biologically immobilizing and stabilizing solublephosphorus (P) forms. Composting poultry litter has been suggested as a means tostabilize soluble P biologically. The objectives of this study were to assessthe nutrient (N, P) value of different-age poultry litter (PL) compostsrelativeto raw poultry litter and commercial fertilizer and determine effects ofpoultrylitter and composts on corn (Zea mays) grain yield andnutrient uptake. The research was conducted for two years on Maryland'sEastern Shore. Six soil fertility treatments were applied annually to aMatapeake silt loam soil (Typic Hapludult): (1) a check without fertilizer, (2)NH₄NO₃ fertilizer control (168 kg Nha^–1), (3) raw poultry litter (8.9 Mgha^–1), (4) 15-month old poultry litter compost (68.7Mg ha^–1), (5) 4-month old poultry litter compost(59 Mg ha^–1) and (6) 1-month old poultry littercompost (64 Mg ha^–1). We monitored changes inavailable soil NO₃-N and P over the growing season and post harvest.We measured total aboveground biomass at tasseling and harvest and corn yield.We determined corn N and P uptake at tasseling.Patterns of available soil NO₃-N were similar between raw PL-and NH₄NO₃ fertilizer-amended soils. LittleNO₃-N was released from any of the PL composts in the first year ofstudy. The mature 15-month old compost mineralized significant NO₃-Nonly after the second year of application. In contrast, available soil P washighest in plots amended with 15-month old compost, followed by raw PL-amendedplots. Immature composts immobilized soil P in the first year of study. Cornbiomass and yields were 30% higher in fertilizer and raw PL amendedplotscompared to yields in compost-amended treatments. Yields in compost-amendedplots were greater than those in the no-amendment control plots. Corn N and Puptake mirrored patterns of available soil NO₃-N and P. Corn Puptakewas highest in plots amended with 15-month old compost and raw PL, even thoughother composts contained 1.5–2 times more total P than raw PL. There wasalinear relationship between amount of P added and available soil P, regardlessof source. The similar P availabilities from either raw or composted PL,coupledwith limited crop P uptake at high soil P concentrations, suggest that raw andcomposted PL should be applied to soils based on crop P requirements to avoidbuild-up of available soil P.     

Effect of climate on the recovery in crop and soil of15N-labelled fertilizer applied to wheat

Data was assembled from experiments on the fate of¹⁵N-labelled fertilizer applied to wheat (Triticum spp.) grown in different parts of the world. These data were then ranked according to the annual precipitation-evaporation quotient for each experimental location calculated from the average long-term values of precipitation and potential evaporation. Percentage recovery of¹⁵N fertilizer in crop and soil varied with location in accordance with the precipitation-evaporation quotient. In humid environments more¹⁵N fertilizer was recovered in the crop than in the soil, while in dry environments more¹⁵N fertilizer was recovered in the soil than in the crop. Irrespective of climatic differences between locations 20% (on average) of the¹⁵N fertilizer applied to wheat crops was unaccounted for at harvest. Most of the¹⁵N fertilizer remaining in the soil was found in the 0–30 cm layer. The most likely explanation of these differences is that wheat grown in dry environments has a greater root:shoot ratio than wheat grown in humid environments and, further, that the residue of dryland crops have higher C/N ratios. Both factors could contribute to the greater recovery of¹⁵N fertilizer in the soil in dry environments than in humid ones.     

Sugarcane residue management and grain legume crop effects on N dynamics,N losses and growth of sugarcane

To reduce greenhouse gas emissions farmers are being encouraged not to burn sugarcane residues. An experiment was set up in NE Thailand, where sugarcane residues of the last ratoon crop were either burned, surface mulched or incorporated and subsequently the field left fallow or planted to groundnut or soybean. The objectives of the current experiment were to evaluate the residual effects of these treatments during the following new sugarcane crop on (i) microbial and mineral N dynamics, (ii) performance of sugarcane and (iii) effectiveness of recycled legume residues compared to mineral N fertilizer on N use efficiencies, ¹⁵N recovery in the system and in soil particle size and density fractions (using ¹⁵N labelled legume residues and fertilizer). The millable cane and sugar yield were positively affected by sugarcane residue mulching and incorporation compared to burning suggesting microbial remobilization of previously immobilized N. Residual effects of legumes increased sugarcane tillering and yield (127 and 116 Mg ha⁻¹ for groundnut and soybean, respectively) compared to the fallow treatment without N fertilizer (112 Mg ha⁻¹). Soybean residues of higher C:N ratio (33:1) and lignin content (13%) compared to groundnut residues (21:1 C:N, 5% lignin) decomposed slower and improved N synchrony with cane N demand. This led to a better conservation of residue N in the system with proportionally less ¹⁵N losses (15–17%) compared to the large losses from groundnut residues (50–57%) or from mineral N fertilizer (50–63%). ¹⁵N recoveries in soil were larger from residues (41–80%) than from fertilizer (30%) at final harvest. Recycled legume residues were able to substitute basal fertilizer N application but not topdressing after 6 months.     

Use of15N for fertilizer N recovery and N mineralization studies in semi-arid Kenya

Maize and beans were grown on a ferralsol at Kiboko, Kenya, with up to 120 kg N ha^–1. Within the 10 kg N ha^–1 plots,¹⁵N labelled fertilizer was applied in microplots. There was no significant response in yield to fertilizer N and labelled N recovery was low, being 7.5% or less in one season and 17.7% or less in the second season. Samples of Kiboko soil at four different water contents were incubated and the rate of gross N mineralization over 7 days was calculated, utilizing¹⁵N labelling of the mineral N. Gross N mineralization increased greatly with soil moisture and a fitted relationship between gross N mineralization rate and soil water content was obtained. Using measurements of soil water content at the field site, daily values of the soil N supply by gross mineralization were calculated. On average, modelled gross soil N mineralized could supply much (> 69%) of the N removed from the plots. It is suggested that the lack of response to fertilizer N may be explained by the coincidence of a high rate of N mineralization, and increased crop demand, caused by the onset of rain.     

Management of nitrogen by fertigation of potato in Lebanon

Reports on soil and groundwater contamination with nitrates in Lebanon and other developing countries could be related to the mismanagement of water and fertilizer inputs. The objective of this work was to determine the N requirements and N-use efficiency of a main-crop potato in Lebanon, irrigated by a drip system, compared to the farmer's practice of macro-sprinkler. In the drip irrigation, fertilizers input was as soil application at the time of sowing or added continuously with the irrigation water (fertigation). Nitrogen-fertilizer recovery was determined using ¹⁵N-labeled ammonium sulfate. Fertigation with continuous N feeding based on actual N demands and available sources allowed for 55% N recovery. For a total N uptake of 197 kg ha^–1 per season in the lower N rate, the crop removed 66 kg N ha^–1 from fertilizers. The spring potato crop in this treatment covered 44.8% of its N need from the soil and 21.8% from irrigation water. Higher N input increased not only N derived from fertilizers, but also residual soil N. Buildup of N in the soil with the traditional potato fertilization practice reached 200 kg N ha^–1 per season. With increasing indications of deteriorating groundwater quality, we monitored the nitrate leaching in these two watering regimes using soil solution extractors (tensionics). Nitrate leaching increased significantly with the macro-sprinkler technique. But N remained within the root zone with the drip irrigation. The crop response to applied N requires a revision of the current fertilizer recommendation in semi-arid regions, with an improved management of fertilizer and water inputs using fertigation to enhance N recovery.     

Long-term tillage,straw and N rate effects on some chemical properties in two contrasting soil types in Western Canada

Long-term use of soil, crop residue and fertilizer management practices may affect some soil properties, but the magnitude of change depends on soil type and climatic conditions. Two field experiments with barley, wheat, or canola in a rotation on Gray Luvisol (Typic Cryoboralf) loam at Breton and Black Chernozem (Albic Argicryoll) loam at Ellerslie, Alberta, Canada, were conducted to determine the effects of 19 or 27 years (from 1980 to 1998 or 2006 growing seasons) of tillage (zero tillage [ZT] and conventional tillage [CT]), straw management (straw removed [S_Rem] and straw retained [S_Ret]) and N fertilizer rate (0, 50 and 100 kg N ha^?1 in S_Ret, and 0 kg N ha^?1 in S_Rem plots) on pH, extractable P, ammonium-N and nitrate–N in the 0–7.5, 7.5–15, 15–30 and 30–40 cm or 0–15, 15–30, 30–60, 60–90 and 90–120 cm soil layers. The effects of tillage, crop residue management and N fertilization on these chemical properties were usually similar for both contrasting soil types. There was no effect of tillage and residue management on soil pH, but application of N fertilizer reduced pH significantly (by up to 0.5 units) in the top 15 cm soil layers. Extractable P in the 0–15 cm soil layer was higher or tended to be higher under ZT than CT, or with S_Ret than S_Rem in many cases, but it decreased significantly with N application (by 18.5 kg P ha^?1 in Gray Luvisol soil and 20.5 kg P ha^?1 in Black Chernozem soil in 2007). Residual nitrate–N (though quite low in the Gray Luvisol soil in 1998) increased with application of N (by 17.8 kg N ha^?1 in the 0–120 cm layer in Gray Luvisol soil and 23.8 kg N ha^?1 in 0–90 cm layer in Black Chernozem soil in 2007) and also indicated some downward movement in the soil profile up to 90 cm depth. There was generally no effect of any treatment on ammonium-N in soil. In conclusion, elimination of tillage and retention of straw increased but N fertilization decreased extractable P in the surface soil. Application of N fertilizer reduced pH in the surface soil, and showed accumulation and downward leaching of nitrate–N in the soil profile.