Juice,sugar, and bagasse response of sweet sorghum (Sorghum bicolor (L.) Moench cv. M81E) to N fertilization and soil type

The objective of this research was to determine the optimum nitrogen fertilizer rate for producing sweet sorghum (a promising biofuel crop) juice, sugar, and bagasse on silt loam, sandy loam, and clay soils in Missouri. Seven nitrogen fertilization rates were applied, ranging from 0 to 134 kg N ha^?1. Regardless of the soil and year, the juice content of sweet sorghum stalk averaged 68.8% by weight. The juice yield ranged from 15.2 to 71.1 m³ ha^?1. Soil and N rate significantly impacted the juice yield (P < 0.0001). The pH and the density of the juice were not affected by the soil or N. The sugar content (Brix) of the juice varied between 10.7% and 18.9%. N fertilization improved the sugar content of the juice. A negative correlation existed between the sugar concentration and the juice yield. In general, the lowest sugar content was found in the clay soil and the impact of the N fertilization on juice sugar content was most pronounced in that soil. The juice sugar yield ranged between 2 and 9.9 Mg ha^?1, with significant differences found between years, N rates, and soils. N fertilization always increased the sugar yield in the clay soil, whereas in loam soil, a significant sugar response was recorded when the sweet sorghum was planted after corn. The average juice water content was 84% by weight. The dry bagasse yield fluctuated between 3.2 and 13.8 Mg ha^?1 with significant difference found with N rate, soil, and year. When sweet sorghum was grown after soybean or cotton, its N requirement was less than after a corn crop was grown the previous year. In general, a minimum of 67 kg N ha^?1 was required to optimize juice, sugar, and bagasse yield in sweet sorghum.     

The partitioning of fertilizer-N between soil and crop: Comparison of ammonium and nitrate applications

Field experiments were carried out in 1987 on winter wheat crops grown on three types of soil. ¹⁵N-labelled urea, ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ (80 kg N ha^-1) was applied at tillering. The soils (chalky soil, hydromorphic loamy soil, sandy clay soil) were chosen to obtain a range of nitrogen dynamics, particularly nitrification. Soil microbial N immobilization and crop N uptake were measured at five dates. Shortly after fertilizer application (0–26 days), the amount of N immobilized in soil were markedly higher with labelled urea or ammonium than that with nitrate in all soils. During the same period, crop ¹⁵N uptake occurred preferentially at the expense of nitrate. Nitrification differed little between soils, the rates were 2.0 to 4.7 kg N ha^-1 day^-1 at 9°C daily mean temperature. The differences in immobilization and uptake had almost disappeared at flowering and harvest. ¹⁵N recovery in soil and crop varied between 50 and 100%. Gaseous losses probably occurred by volatilization in the chalky soil and denitrification in the hydromorphic loamy soil. These losses affected the NH₄ ⁺ and NO₃ ^- pools differently and determined the partitioning of fertilizer-N between immobilization and absorption.     

Fate of nitrogen fertilizer applied on two main arables crops,winter wheat (Triticum aestivum) and sugar beet (Beta vulgaris) in the loam region of Belgium

Since 1986, the fate of fertilizer N (NH₄NO₃ or NaNO₃) applied in field conditions on two main arable crops, winter wheat (Triticum aestivum) and sugar beet (Beta vulgaris), has been studied using ¹⁵N. Up to a rate of 200 kg ha^-1 of N, mean recovery of fertilizer by winter wheat was 70%, provided it had been split applied. Single application (with or without dicyandiamid) was less effective. For sugar beet, in 1990, 1991 and 1992, 40% of fertilizer N was found in the crop at harvest when NH₄NO₃ had been broadcast at 100 to 160 kg N ha^-1 at sowing time. For the same N rate, recovery was 50% when row applied near the seeds and 60% for 80 kg N ha^-1. For the two experimental crops, residual fertilizer N in soil was exclusively organic. It ranged from 15 to 30% of applied N and was located in the 30 cm upper layer. Losses were generally lower with winter wheat (12%) than with sugar beet (20–40%) and could be ascribed to volatilization and denitrification. Soil derived N taken up by the plant was site and year dependent.     

Prediction of long-term fertilizer nitrogen requirements of maize in the tropics using a nitrogen balance model

A simple N balance model was used to calculate fertilizer requirement for a target N uptake by maize. Nitrogen uptake from soil sources and target uptake of N with fertilizer N additions were obtained from fertilizer trials in Africa and Latin America. Most experiments had data for only one cropping period, although some from Latin America had data for four to six crops. The transfer coefficient of fertilizer N to the crop was adjusted to realize maximum recovery of fertilizer N under best methods of fertilizer application. The time constants of transfer of soil N to the crop were allowed to vary and were affected mainly by soil texture. Where 4 to 6 cropping periods were available good agreement between actual and predicted fertilizer N requirements was obtained. With this approach long-term fertilizer N requirements for 14 sites were predicted using first cropping period N uptake. This study showed that pools of organic N in more coarse-textured soils were usually smaller and declined more rapidly than in fine-textured soils. Labile organic N pools declined with time under all simulations, but approached equilibrium within 10 croppings seasons. Equilibrium N uptake from the soil organic N pool was predicted to be 31 kg ha^–1 for the more coarse-textured soils and 36 kg ha^–1 for the fine-textured soils. Long-term projections of fertilizer requirements using input data of the field experiments were reasonable, and effects of legume green manures and other amendments could be clearly evaluated.     

Soil nitrous oxide and methane fluxes are low from a bioenergy crop (canola) grown in a semi-arid climate

Understanding nitrous oxide (N₂O) and methane (CH₄) fluxes from agricultural soils in semi‐arid climates is necessary to fully assess greenhouse gas emissions from bioenergy cropping systems, and to improve our knowledge of global terrestrial gaseous exchange. Canola is grown globally as a feedstock for biodiesel production, however, resulting soil greenhouse gas fluxes are rarely reported for semi‐arid climates. We measured soil N₂O and CH₄ fluxes from a rain‐fed canola crop in a semi‐arid region of south‐western Australia for 1 year on a subdaily basis. The site included N fertilized (75 kg N ha^?1 yr^?1) and nonfertilized plots. Daily N₂O fluxes were low (?1.5 to 4.7 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 128 g N₂O‐N ha^?1 (standard error, 12 g N₂O‐N ha^?1) from N fertilized soil and 80 g N₂O‐N ha^?1 (standard error, 11 g N₂O‐N ha^?1) from nonfertilized soil. Daily CH₄ fluxes were also low (?10.3 to 11.9 g CH₄‐C ha^?1 day^?1), and did not differ with treatments, with an average annual net emission of 6.7 g CH₄–C ha^?1 (standard error, 20 g CH₄–C ha^?1). Greatest daily N₂O fluxes occurred when the soil was fallow, and following a series of summer rainfall events. Summer rainfall increased soil water contents and available N, and occurred when soil temperatures were >25 °C, and when there was no active plant growth to compete with soil microorganisms for mineralized N; conditions known to promote N₂O production. The proportion of N fertilizer emitted as N₂O, after correction for emissions from the no N fertilizer treatment, was 0.06%; 17 times lower than IPCC default value for the application of synthetic N fertilizers to land (1.0%). Soil greenhouse gas fluxes from bioenergy crop production in semi‐arid regions are likely to have less influence on the net global warming potential of biofuel production than in temperate climates.     

Fate of 15N-labelled fertilizer applied to spring barley grown on soils of contrasting nutrient status

An experiment with ¹⁵N-labelled fertilizer was superimposed on the Rothamsted Hoosfield Spring Barley Experiment, started in 1852. Labelled ¹⁵NH₄ ¹⁵NO₃ was applied in spring at (nominal) rates of 0, 48, 96 and 144 kg N ha^-1. The labelled fertilizer was applied to microplots located within four treatments of the original experiment: that receiving farmyard manure (FYM) annually, that receiving inorganic nutrients (PK) annually and to two that were deficient in nutrients: applications were made in two successive years, but to different areas within these original treatments. Maximum yields in 1986 (7.1 t grain ha^-1) were a little greater than in 1987. In 1987, microplots on the FYM and PK treatments gave similar yields, provided enough fertilizer N was applied, but in 1986 yields on the PK treatment were always less than those on the FYM treatment, no matter how much fertilizer N was applied. In plots with adequate crop nutrients, about 51% of the labelled N was present in above-ground crop and weed at harvest, about 30% remained in the top 70 cm of soil (mostly in the 0–23 cm layer) and about 19% was unaccounted for, all irrespective of the rate of N application and of the quantity of inorganic N in the soil at the time of application. Less than 4% of the added fertilizer N was present in inorganic form in the soil at harvest, confirming results from comparable experiments with autumn-sown cereals in south-east England. Thus, in this experiment there is no evidence that a spring-sown cereal is more likely to leave unused fertilizer in the soil than an autumn-sown one. With trace applications (ca. 2 kg N ha^-1) more labelled N was retained in the soil and less was in the above-ground crop. Where P and K were deficient, yields were depressed, a smaller proportion of the labelled fertilizer N was present in the above-ground crop at harvest and more remained in the soil.Although the percentage uptake of labelled N was similar across the range of fertilizer N applications, the uptake of total N fell off at the higher N rates, particularly on the FYM treatment. This was reflected in the appearance of a negative Added Nitrogen Interaction (ANI) at the highest rate of application. Fertilizer N blocked the uptake of soil N, particularly from below 23 cm, once the capacity of the crop to take up N was exceeded. Denitrification and leaching were almost certainly insufficient to account for the 19% loss of spring-added N across the whole range of N applications and other loss processes must also have contributed.     

A meta‐analysis of fertilizer‐induced soil NO and combined NO+N2O emissions

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N₂O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N₂O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha^?1 yr^?1, with the greatest (9.75 kg ha^?1 yr^?1) in vegetable croplands and the lowest (0.11 kg ha^?1 yr^?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF_NO) and combined NO+N₂O (EF_c) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EF_NO (2.39%). Within the croplands, the EF_NO (1.71%) and EF_c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF_NO (2.93%) and EF_c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.     

Can intensification reduce emission intensity of biofuel through optimized fertilizer use? Theory and the case of oil palm in Indonesia

Closing yield gaps through higher fertilizer use increases direct greenhouse gas emissions but shares the burden over a larger production volume. Net greenhouse gas (GHG) footprints per unit product under agricultural intensification vary depending on the context, scale and accounting method. Life cycle analysis of footprints includes attributable emissions due to (i) land conversion (‘fixed cost’); (ii) external inputs used (‘variable cost’); (iii) crop production (‘agronomic efficiency’); and (iv) postharvest transport and processing (‘proportional’ cost). The interplay between fixed and variable costs results in a nuanced opportunity for intermediate levels of intensification to minimize footprints. The fertilizer level that minimizes the footprint may differ from the economic optimum. The optimization problem can be solved algebraically for quadratic crop fertilizer response equations. We applied this theory to data of palm oil production and fertilizer use from 23 plantations across the Indonesian production range. The current EU threshold requiring at least 35% emission saving for biofuel use can never be achieved by palm oil if produced: (i) on peat soils, or (ii) on mineral soils where the C debt due to conversion is larger than 20 Mg C ha^?1, if the footprint is calculated using an emission ratio of N₂O–N/N fertilizer of 4%. At current fertilizer price levels in Indonesia, the economically optimized N fertilizer rate is 344–394 kg N ha^?1, while the reported mean N fertilizer rate is 141 kg N ha^?1 yr^?1 and rates of 74–277 kg N ha^?1 would minimize footprints, for a N₂O–N/N fertilizer ratio of 4–1%, respectively. At a C debt of 30 Mg C ha^?1, these values are 200–310 kg N ha^?1. Sustainable weighting of ecology and economics would require a higher fertilizer/yield price ratio, depending on C debt. Increasing production by higher fertilizer use from current 67% to 80% of attainable yields would not decrease footprints in current production conditions.     

Fertilizer vs. organic matter contributions to nitrogen leaching in cropping systems of the Pampas: 15N application in field lysimeters

Nitrogen (N) export from soils to streams and groundwater under the intensifying cropping schemes of the Pampas is modest compared to intensively cultivated basins of Europe and North America; however, a slow N enrichment of water resources has been suggested. We (1) analyzed the fate of fertilizer N and (2) evaluated the contribution of fertilizer and soil organic matter (SOM) to N leaching under the typical cropping conditions of the Pampas. Fertilizer N was applied as ¹⁵N-labeled ammonium sulfate to corn (in a corn/soybean rotation) sown under zero tillage in filled-in lysimeters containing two soils of different texture representative of the Pampean region (52 and 78 kg N ha^-1, added to the silt loam and sandy loam soil, respectively). Total fertilizer recovery at corn harvest averaged 84 and 64% for the silt loam and sandy loam lysimeters, respectively. Most fertilizer N was removed with plant biomass (39%) or remained immobilized in the soil (29 and 15%, for the silt loam and sandy loam soil, respectively) whereas its loss through drainage was negligible (<0.01%). We presume that the unaccounted fertilizer N losses were related to volatilization and denitrification. Throughout the corn growing season, subsequent fallow and soybean crop, which took place during an exceptionally dry period, the fertilizer N immobilized in the organic pool remained stable, and N leaching was scarce (7.5 kg N ha^-1), similar at both soils, and had a low contribution of fertilizer N (0–3.5%), implying that >96% of the leached N was derived from SOM mineralization. The inherent high SOM of Pampean soils and the favorable climatic conditions are likely to propitiate year-round production of nitrate, favoring its participation in crop nutrition and leaching. The presence of ¹⁵N in drainage water, however, suggests that fertilizer N leaching could become significant in situations with higher fertilization rates or more rainy seasons.     

Biomass Yield and Nutrient Responses of Switchgrass to Phosphorus Application

Increasing desire for renewable energy sources has increased research on biomass energy crops in marginal areas with low potential for food and fiber crop production. In this study, experiments were established on low phosphorus (P) soils in southern Oklahoma, USA to determine switchgrass biomass yield, nutrient concentrations, and nutrient removal responses to P and nitrogen (N) fertilizer application. Four P rates (0, 15, 30, and 45?kg?P?ha^?1) and two N fertilizer rates (0 and 135?kg?N?ha^?1) were evaluated at two locations (Ardmore and Waurika) for 3?years. While P fertilization had no effect on yield at Ardmore, application of 45?kg?P?ha^?1 increased yield at Waurika by 17% from 10.5 to 12.3?Mg?ha^?1. Across P fertilizer rates, N fertilizer application increased yields every year at both locations. In Ardmore, non-N-fertilized switchgrass produced 3.9, 6.7, and 8.8?Mg?ha^?1, and N-fertilized produced 6.6, 15.7, and 16.6?Mg?ha^?1 in 2008, 2009, and 2010, respectively. At Waurika, corresponding yields were 7.9, 8.4, and 12.2?Mg?ha^?1 and 10.0, 12.1, and 15.9?Mg?ha^?1. Applying 45?kg?P?ha^?1 increased biomass N, and P concentration and N, P, potassium, and magnesium removal at both locations. Increased removal of nutrients with N fertilization was due to both increased biomass and biomass nutrient concentrations. In soils of generally low fertility and low plant available P, application of P fertilizer at 45?kg?P?ha^?1 was beneficial for increasing biomass yields. Addition of N fertilizer improves stand establishment and biomass production on low P sites.     

Relationship between rate of crop growth at date of fertiliser N application and fate of fertiliser N applied to winter wheat

To investigate the relationship between the timing of fertiliser N applications and the N use efficiency of wheat, three field experiments with ¹⁵N were set up on winter wheat, on three different soils in France. Different crop N demands on the day of fertiliser application were obtained by varying either crop densities or date of fertiliser application. Labelled ¹⁵NH₄ ¹⁵NO₃ was applied at tillering and during stem elongation. The ¹⁵N recovered from plant and soil at different dates after ¹⁵N addition and at maturity of wheat was measured. The fate of fertiliser N was rapidly determined, most of the fertiliser N accumulated in the wheat at maturity having been taken up within a few days of application. ¹⁵N recovery by the crop at final harvest (%) varied greatly (19–55% N applied) according to crop density, soil type and date of application. It was linearly related to the instantaneous crop growth rate calculated at the day of ¹⁵N application. The amount of fertiliser N immobilised in the soil was constant at 20 kg N ha⁻¹, for all soil types and crop densities. Because residual mineral ¹⁵N in the soil at harvest was negligible and immobilisation was constant, the level of total ¹⁵N measured in the different N pools (soil+plant) reflected the% ¹⁵N uptake by the plant. There was consequently a negative linear relationship between the percentage of ¹⁵N not recovered for measurement, and crop growth rate (i.e. crop N demand) at date of fertiliser application. These results suggest that crop N demand at the time of N application determines the ability of the crop to compete for N with other processes, and may be a major factor determining the division of N between soil and crop. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in on‐farm corn crops of the US Midwest

Row‐crop agriculture is a major source of nitrous oxide (N₂O) globally, and results from recent field experiments suggest that significant decreases in N₂O emissions may be possible by decreasing nitrogen (N) fertilizer inputs without affecting economic return from grain yield. We tested this hypothesis on five commercially farmed fields in Michigan, USA planted with corn in 2007 and 2008. Six rates of N fertilizer (0–225 kg N ha^?1) were broadcast and incorporated before planting, as per local practice. Across all sites and years, increases in N₂O flux were best described by a nonlinear, exponentially increasing response to increasing N rate. N₂O emission factors per unit of N applied ranged from 0.6% to 1.5% and increased with increasing N application across all sites and years, especially at N rates above those required for maximum crop yield. At the two N fertilizer rates above those recommended for maximum economic return (135 kg N ha^?1), average N₂O fluxes were 43% (18 g N₂O–N ha^?1 day^?1) and 115% (26 g N₂O–N ha^?1 day^?1) higher than were fluxes at the recommended rate, respectively. The maximum return to nitrogen rate of 154 kg N ha^?1 yielded an average 8.3 Mg grain ha^?1. Our study shows the potential to lower agricultural N₂O fluxes within a range of N fertilization that does not affect economic return from grain yield.     

Integrated Nutrient Management Improves Productivity and Quality of Sugarcane (<i>Saccharum Officinarum</i> L.)

Sugarcane is one of the major important sugar yielding crops in Bangladesh. As an exhaustive crop, sugarcane removes a huge amount of plant nutrients from the soil. However, the combined use of organic and inorganic fertilizers can be a good approach to deal with nutrient depletion and promote sustainable crop production as well as improve soil health. Therefore, an attempt was made to identify the most fruitful and profitable integrated nutrient management on the aspects of growth, yield and quality of sugarcane in two consecutive growing seasons. Seven treatments: T₁=Control, T₂=165:55:120:30:10:2.5:4 kg N:P:K:S:Mg:Zn:B ha⁻¹, T₃=Poultry Litter (PL) at 5 t ha⁻¹+95:51:87:9:10:2.5:4 kg N:P:K:S:Mg:Zn:B ha⁻¹, T₄=Cow Dung (CD) at 15 t ha⁻¹+ 36:52:60:17:10:2.5:4 kg N:P:K:S:Mg:Zn:B ha⁻¹, T₅=Press Mud (PM) at 15 t ha⁻¹+10:50:43:0:10:2.5:4 kg N:P:K:S:Mg:Zn:B ha⁻¹, T₆=Mustard Oil Cake (MOC) at 0.5 t ha⁻¹+140:54:115:25:10:2.5:4 kg N:P:K:S:Mg:Zn:B ha⁻¹ and T₇=GM (Green Manure) at 5 t ha⁻¹+140:53:100:28:10:2.5:4 kg N:P:K:S:Mg:Zn:B ha⁻¹ were used in this experiment. Two years data showed that treatment T₃ produced the maximum amount of tillers, total dry matter yield, millable sugarcane, cane yield and sugar yield, followed by the T₄ treatment. The highest stalk heights were recorded in the T₃ treatment, which was statistically similar to all other treatments except T₁ and T₂. The juice quality parameters viz., brix and pol in cane were found significant in treatment T₃ while the highest purity was obtained in the T₇ treatment. All the data of Jaggery (goor) quality parameters, the highest sucrose content, color transmittance, Jaggery (goor) recovery and the lowest ash content of Jaggery (goor) were observed in the T₃ treatment, which was statistically similar to the T₄ treatment in both seasons. The highest cost of production was obtained from the T₆ treatment while the highest gross return, net return and BCR were recorded in the T₃ treatment. No significant changes were found in one cycle of sugarcane in initial and post-harvest soil characteristics viz., pH, organic carbon, total N, and available P, K and S contents due to integrated use of different fertilizer packages. From the experimental findings, it was concluded that treatment T₃ followed by T₄ treatment would be the better productive and profitable integrated nutrient management technology for ensuring higher yields and quality of sugarcane without soil fertility degradation in the High Ganges River Floodplain soils.     

Nitrous oxide emissions from a cropped soil in a semi-arid climate

Understanding nitrous oxide (N₂O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N₂O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha^?1 yr^?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N₂O using gas chromatography. Daily N₂O emissions were low (?1.8 to 7.3 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 0.11 kg N₂O‐N ha^?1 from N‐fertilized soil and 0.09 kg N₂O‐N ha^?1 from nonfertilized soil. Over half (55%) the annual N₂O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N₂O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N₂O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N₂O emission, demonstrating that a proportion of N₂O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N₂O emissions.     

Transformation of15N-labelled urea and its modified forms in tropical wetland rice culture

Summary The fate of 100 kg N ha^–1 applied as¹⁵N-urea and its modified forms was followed in 4 successive field-grown wetland rice crops in a vertisol. The first wet season crop recovered about 27 to 36.6% of the applied N depending upon the N source. In subsequent seasons the average uptake was very small and it gradually decreased from 1.4 to 0.5 kg N ha^–1 although about 18 to 20, 12 to 17 and 14 to 18 kg ha^–1 residual fertilizer N was available in the root zone after harvest of first, second and third crops, respectively. The average uptake of the residual fertilizer N was only 7.6% in the second crop and it decreased to 4.5% in the third and to 3.2% in the fourth crop although all these crops were adequately fertilized with unlabelled urea. The basal application of neem coated urea was more effective in controlling the leaching loss of labelled NH₄+NO₃–N than split application of uncoated urea. In the first 3 seasons in which¹⁵N was detectable, the loss of fertilizer N through leaching as NH₄+NO₃–N amounted to 0.5 kg ha^–1 from neem-coated urea, 1.5 kg from split urea and 4.1 kg from coal tar-coated urea. At the end of 4 crops, most of the labelled fertilizer N (about 69% on average) was located in the upper 0–20 cm soil layer showing very little movement beyond this depth. In the profile sampled upto 60 cm depth, totally about 13.8 kg labelled fertilizer N ha^–1 from neem-coated urea, 12.7 kg from coal-tar coated urea, and 11.8 kg from split urea were recovered. The average recovery of labelled urea-N in crops and soil during the entire experimental period ranged between 42 and 51%. After correcting for leaching losses, the remaining 47 to 56% appeared to have been lost through ammonia volatilization and denitrification.     

The importance of initial exchangeable ammonium in the nitrogen nutrition of lowland rice soils

Summary The importance of initial exchangeable soil NH ₄ ⁺ in nitrogen nutrition and grain yield of rice was studied in a number of representative lowland rice soils in the Philippines. The initial exchangeable soil NH ₄ ⁺ +fertilizer N plotted against nitrogen uptake by the crop resulted in a highly significant linear relationship (R²=0.91), suggesting that the presence of exchangeable NH ₄ ⁺ in the soil at transplanting behaved like fertilizer nitrogen. The correlation between N fertilizer rate and N uptake by the rice crop was relatively poor (R²=0.73). On the other hand, relative grain yield was more closely correlated with the initial exchangeable soil NH ₄ ⁺ +fertilizer N than with fertilizer nitrogen applied alone. These results indicate that the initial exchangeable NH ₄ ⁺ in the soil contributed substantially to the nitrogen uptake of the crop.Critical nitrogen levels in the soil defined as the initial exchangeable soil NH ₄ ⁺ +fertilizer N at which the optimum grain yield (95% of the maximum yield) is obtained, varied from 60 to 100 kg N/ha in the wet season and from 100 to 120 kg N/ha in the dry season for the different fertilizer treatments. The results further suggest that the initial exchangeable soil NH ₄ ⁺ should serve as a guide in selecting an optimum nitrogen fertilizer rate for high grain yields.     

Do nitrogen fertilizers stimulate or inhibit methane emissions from rice fields?

In rice cultivation, there are controversial reports on net impacts of nitrogen (N) fertilizers on methane (CH ₄) emissions. Nitrogen fertilizers increase crop growth as well as alter CH ₄ producing (Methanogens) and consuming (Methanotrophs) microbes, and thereby produce complex effects on CH ₄ emissions. Objectives of this study were to determine net impact of N fertilizers on CH ₄ emissions and to identify their underlying mechanisms in the rice soils. Database was obtained from 33 published papers that contained CH ₄ emissions observations from N fertilizer (28–406 kg N ha^?1) treatment and its control. Results have indicated that N fertilizers increased CH ₄ emissions in 98 of 155 data pairs in rice soils. Response of CH ₄ emissions per kg N fertilizer was significantly (P < 0.05) greater at < 140 kg N ha^?1 than > 140 kg N ha^?1 indicating that substrate switch from CH ₄ to ammonia by Methanotrophs may not be a dominant mechanism for increased CH ₄ emissions. On the contrary, decreased CH ₄ emission in intermittent drainage by N fertilizers has suggested the stimulation of Methanotrophs in rice soils. Effects of N fertilizer stimulated Methanotrophs in reducing CH ₄ emissions were modified by the continuous flood irrigation due to limitation of oxygen to Methanotrophs. Greater response of CH ₄ emissions per kg N fertilizer in urea than ammonia sulfate probably indicated the interference of sulfate in the CH ₄ production process. Overall, response of CH ₄ emissions to N fertilizers was correlated with N‐induced crop yield (r = +0.39; P < 0.01), probably due to increased carbon substrates for Methanogens. Using CH ₄ emission observations, this meta‐analysis has identified dominant microbial processes that control net effects of N fertilizers on CH ₄ emissions in rice soils. Finally, we have provided a conceptual model that included microbial processes and controlling factors to predict effects of N fertilizers on CH ₄ emissions in rice soils.     

Nitrous oxide emissions from silage maize fields under different mineral nitrogen fertilizer and slurry applications

Intensive dairy farming systems are a large source of emission of the greenhouse gas nitrous oxide (N₂O), because of high nitrogen (N) application rates to grasslands and silage maize fields. The objective of this study was to compare measured N₂O emissions from two different soils to default N₂O emission factors, and to look at alternative emission factors based on (i) the N uptake in the crop and (ii) the N surplus of the system, i.e., N applied minus N uptake by the crop. Twelve N fertilization regimes were implemented on a sandy soil (typic endoaquoll) and a clay soil (typic endoaquept) in the Netherlands, and N₂O emissions were measured throughout the growing season. Highest cumulative fluxes of 1.92 and 6.81 kg N₂O-N ha^–1 for the sandy soil and clay soil were measured at the highest slurry application rate of 250 kg N ha^–1. Background emissions from unfertilized soils were 0.14 and 1.52 kg N₂O-N ha^–1 for the sandy soil and the clay soil, respectively. Emission factors for the sandy soil averaged 0.08, 0.51 and 0.26% of the N applied via fertilizer, slurry, and combinations of both. For the clay soil, these numbers were 1.18, 1.21 and 1.69%, respectively. Surplus N was linearly related to N₂O emission for both the sandy soil (R²=0.60) and the clay soil (R²=0.40), indicating a possible alternative emission factor. We concluded that, in our study, N₂O emission was not linearly related to N application rates, and varied with type and application rate of fertilizer. Finally, the relatively high emission from the clay soil indicates that background emissions might have to be taken into account in N₂O budgets.     

Conventionally Tilled and Permanent Raised Beds with Different Crop Residue Management: Effects on Soil C and N Dynamics

Conservation tillage in its version of permanent bed planting under zero-tillage with crop residue retention has been proposed as an alternative wheat production system for northwest Mexico. However, little is known about the dynamics of C and N in soils under wheat/maize on permanent beds (PB) where straw was burned, removed, partly removed or retained, as opposed to conventionally tilled beds (CTB) where straw was incorporated. We investigated the dynamics of soil C and N and normalized difference vegetative index (NDVI) crop values in zero-tilled PB and CTB after 26 successive maize and wheat crops. Organic C and total N were respectively, 1.15 and 1.17 times greater in PB with straw partly removed and with straw retained on the surface, than in CTB with straw incorporated. Organic C and total N were 1.10 times greater in soils with 300 kg N ha⁻¹ added than in unfertilized soil. Cumulative production of CO₂ was lower under CTB with straw incorporated than under PB treatments, and CO₂ production increased with increments in inorganic fertilizer. The N-mineralization rate was 1.18 times greater than in unamended soils when 150 kg inorganic N ha⁻¹ was applied, and 1.48 times greater when 300 kg inorganic N ha⁻¹ was added. The N-mineralization rate was significantly (1.66 times) greater in PB where the straw was burned or retained on the surface than in CTB where the straw was incorporated, but significantly (1.25 times) lower than in PB with straw partly removed. The NDVI values reached a maximum 56 days after planting and decreased thereafter. The NDVI for unfertilized soil were similar for CTB with straw incorporated, PB with straw partly removed, and PB with straw retained on the surface, but significantly lower for PB with straw burned and PB with straw removed. In soils to which 150 or 300 kg N ha⁻¹ was added, NDVI was significantly lower for PB with straw burned than for other treatments. Among other things, this suggests the utility of rotating maize or wheat with crops whose residues have lower C–N ratios, thus avoiding immobilization of large amounts of N for extended periods. PB with residue burning, however, is an unsustainable practice leading to low crop performance and soil and environmental degradation.     

A long‐term nitrogen fertilizer gradient has little effect on soil organic matter in a high‐intensity maize production system

Global maize production alters an enormous soil organic C (SOC) stock, ultimately affecting greenhouse gas concentrations and the capacity of agroecosystems to buffer climate variability. Inorganic N fertilizer is perhaps the most important factor affecting SOC within maize‐based systems due to its effects on crop residue production and SOC mineralization. Using a continuous maize cropping system with a 13 year N fertilizer gradient (0–269 kg N ha^?1 yr^?1) that created a large range in crop residue inputs (3.60–9.94 Mg dry matter ha^?1 yr^?1), we provide the first agronomic assessment of long‐term N fertilizer effects on SOC with direct reference to N rates that are empirically determined to be insufficient, optimum, and excessive. Across the N fertilizer gradient, SOC in physico‐chemically protected pools was not affected by N fertilizer rate or residue inputs. However, unprotected particulate organic matter (POM) fractions increased with residue inputs. Although N fertilizer was negatively linearly correlated with POM C/N ratios, the slope of this relationship decreased from the least decomposed POM pools (coarse POM) to the most decomposed POM pools (fine intra‐aggregate POM). Moreover, C/N ratios of protected pools did not vary across N rates, suggesting little effect of N fertilizer on soil organic matter (SOM) after decomposition of POM. Comparing a N rate within 4% of agronomic optimum (208 kg N ha^?1 yr^?1) and an excessive N rate (269 kg N ha^?1 yr^?1), there were no differences between SOC amount, SOM C/N ratios, or microbial biomass and composition. These data suggest that excessive N fertilizer had little effect on SOM and they complement agronomic assessments of environmental N losses, that demonstrate N₂O and NO₃ emissions exponentially increase when agronomic optimum N is surpassed.