Nitrogen Forms in Humic Substances

In this paper,the nitrogen forms in newly-formed humic substances,including humic acid (HA),fulvic acid (FA) and humic acid in humin (HAI),were studied by using the ^15N CP-MAS NMR technique in combination with chemical approaches.Results show that the majority of nitrogen in HA,FA and HAI was in the amide form with some presented as aliphatic and/ or aromatic amines and some as pyrrole type nitrogen,although the contents of nonhydrolyzable nitrogen in them differed greatly from each other (15-55%).     

Direct Estimation of Nitrogen Gases Emitted from Flooded Soils During Denitrification of Applied Nitrogen

Denitrification losses measured by direct method (measuring the evolution of (N2 N2O)-^15N) were compared with the apparent denitrification losses (calculated from the difference between the total N loss and ammonia loss), for fertilizers applied to flooded soils.The direct measured denitrification losses from potassium nitrate were 23.0%,40.0%,and 63.1-79.7% of applied N in rice field,and in incubations of 7 cm deep layer of soil and 2 cm deep layer of soil,respectively;while the corresponding apparent denitrification losses were 96.0%,98.4%,and 97.7-97.9%,respectively.In field experiments with urea,the direct measured denitrification losses ranged from 0.1-1.8%,which were much less than the apparent denitrification losses (41.3-45.7%).Such discrepancies were primarily due to the entrapment of the gaseous products of denitrification in the soil as revealed by the facts:(1) stirring the floodwater and the surface soil markedly increased the fluxes of (N2_N2O)-^15N from urea or potassium nitrate applied to the flooded rice field,and (2) reducing the pressure in the headspace of the incubation bottle with the 7 cm soil layer during gas sampling decreased the discrepance between the direct measured and apparent denitrifecation losses from 58.4% to 21.2%.The advantage of reducing the pressure in the headspace is that there is minimal disturbance of the soil.Further testing of this technique in rice field is needed to determine its effectiveness in releasing the entrapped gaseous products of denitrification so that denitrification losses can be quantified directly.     

Effects of Fertilizer Nitrogen on Short‐Term Nitrogen Loss in Bypass Flow in a Vertisol

Bypass flow, the vertical flow of free water along the walls of macropores or preferential flow paths in the soil, can lead to movement of fertilizer nutrients beyond the reach of plants. Fertilizer type and the rate of application, as well as the amount, frequency, and intensity of rainfall, can influence the amount of fertilizer nitrogen (N) loss in leaching or bypass flow. The effect of fertilizer N form and rate of application on N recovery in bypass flow in a Kenyan Vertisol was determined. Calcium nitrate and ammonium sulfate, used to supply nitrate (NO₃ ^?)‐N and ammonium (NH₄ ⁺)‐N, respectively, were surface‐broadcast to 40‐cm‐long undisturbed soil columns at equivalent rates of 50, 100, and 200 kg N ha^?1. Using a rainfall simulator, two rainfall events (30 mm of water applied in 1 h) were applied to the soil columns, one before and the other after fertilizer application. Total N, NO₃ ^?‐N, and NH₄ ⁺‐N concentrations in the bypass flow were determined after the second rainfall event. The application of NH₄ ⁺‐N, regardless of the rate, had no effect on N recovery in the bypass flow. When nitrate N was applied, the amount of fertilizer N recovered in the bypass flow significantly increased with the rate of NO₃ ^?‐N application. Of the total N in the bypass flow, 24 to 48% was derived from the soil, the bulk of which was organic N. It is concluded that following the application of NO₃ ^?‐N, bypass flow is an important avenue of loss of both fertilizer and soil N from Vertisols.     

Efficient Management of Nitrogen Fertilizers for Flooded Rice in Relation to Nitrogen Transformations in Flooded Soils

Recent progresses in efficient management of nitrogen fertilizers for flooded rice in relation to nitrogen transformations in flooded soil were reviewed.Considerable progress has been achieved in the investigation on the mechanism of ammonia loss and the factors affecting it .However,little progress has been obtained in the investigations on nitrification-denitrification loss owing to the lack of method for estimating the fluxes of gaseous N products.Thus,so far the management practices developed or under investigation primarily for reducing ammonia loss are feasible or promising,while those for reducing nitrification-denitrification loss seem obscure,except the point deep placement. In addition,it was emphasized that the prediction of soil N supply and the recommendation of the optimal rate of N application based on it are only semi-quantitative.The priorities in research for improving the prediction are indicated.     

Nitrogen fertigation of greenhouse‐grown tomato

Abstract

This greenhouse study was conducted to determine the response of trickle‐irrigated tomato (Lycopersicon esculentum cv. Dombo) to 6.4, 12.8, or 19.2 mmol N/L applied via the irrigation stream. The plants were grown in pots filled with 12 kg of soil. The amount of N applied in a total of 438 L of water per plant was 39.4, 78.8, or 118.2 g for the three N levels, respectively. The residual NO₃‐N concentration in the root volume was negligible with the 6.4 mmol N/L treatment, whereas, with the highest N level increased sharply for the first 16 weeks before reaching a value around 32 mmol N/L, which continued for the remainder of the experiment. With the highest N level there was also increase of soil solution EC, and NO₃‐N concentration in laminae and petioles was in excess. With the lowest N treatment, NO₃‐N concentration in laminae and petioles was at deficient levels. With 12.8 mmol N/L, NO₃‐N in petioles and laminae was at the sufficient level and yet no substantial increase of soil solution EC or NO₃‐N concentration occurred, suggesting efficient use of N by crop. The highest yield (12.6 kg marketable fresh fruit per plant) was obtained with 12.8 mmol N/L due to increased number of fresh weight of fruits. It was concluded that 12.8 mmol N/L applied via the irrigation stream is adequate for high tomato yield without unduly raising soil salinity or wasting fertilizer N.     

Nitrogen–Potassium Concentrated Complex Fertilizers versus Nitrogen–Potassium Blends: Effects on Potassium Uptake by Perennial Ryegrass

Abstract

It was hypothesized that supplying potassium (K) in concentrated complex fertilizer (CCF) form with nitrogen (N) (NK CCF) to all fertilizer microsites, rather than in NK‐blended fertilizer form to a fraction of the total fertilizer microsites, should enhance the rate of K uptake by perennial ryegrass. Two complementary pot experiments were conducted to test this hypothesis. The results demonstrated that plants fertilized with an NK CCF absorbed K at faster rates than those fertilized with an NK blend and that use of K₂SO₄ in place of KCl as the K source lowered the rate of K uptake by plants regardless of fertilizer form. Form of fertilizer (i.e., CCF or blend), however, had no effect on NH₄ ⁺ or NO₃ ^? uptake. Unfortunately, the positive effects of the CCF on K absorption were only manifest during the second 2 weeks of regrowth and did not result in significant improvements in dry matter production by the end of the 5‐week regrowth periods.     

Mitrification-Denitrification Loss of Added Nitrogen in Flooded RiceRhizosphere

Nitrification-denitrification losses of 15N-labelled nitrate and ammonium applied to the rhizos phere and nonrhizosphere of flooded rice were evaluated in 2 greenhouse rhizobox experiments.The loss of added N via denitrification was estimated directly by measuring the total fluxes of (N2O N2)12N,It was found that 67% and 51%-56% of 15N-nitrate added to rice rhizosphere were lost as (N2O N2)-15N in the 2 experiments,respectively,which were comparable to that added to norhizosphere soil(70%and 47%,respectively),implying that the denitrifying activity in rice rhizosphere was as high as that in nonrhizosphere soil.However,only trace amounts (0-0\3% of added N)were recovered as (N2O N2)-15N when 15N-ammonium was applied to either rhizosphere or nonrhizosphere,which seems to indicate that the nitrifying activity in the either rhizosphere of nonrhizosphere soils was quite low.The apparent denitrification calculated from 15N balance studies was 10%-47% higher than the total flux of (N2O N2)-15N.Reasons for the large differences can not be explained satisfactorily.Though the denitrifying activity in rhizosphere was high and comparable to that in nonrhizosphere soil.presumably due to the low nitrifying activity and /or the strong competition of N uptake against denitrification.the nitrification-denitrification taking place in rhizosphere could not be an important mechanism of loss of ammonium N in flooded rice-soil system.     

Nitrogen‐Supplying Capacity of Soils for Rice and Wheat and Nitrogen Availability Indices in Soils of Northwest India

Abstract

Quantitative assessment of soil nitrogen (N) that will become available is important for determining fertilizer needs of crops. Nitrogen‐supplying capacity of soil to rice and wheat was quantified by establishing zero‐N plots at on‐farm locations to which all nutrients except N were adequately supplied. Nitrogen uptake in zero‐N plots ranged from 41.4 to 110.3 kg N ha^?1 for rice and 33.7 to 123.4 kg N ha^?1 for wheat. Availability of soil N was also studied using oxidative, hydrolytic, and autoclaving indices, salt‐extraction indices, light‐absorption indices, and aerobic and anaerobic incubation indices. These were correlated with yield and N uptake by rice and wheat in zero‐N plots. Nitrogen extracted by alkaline KMnO₄ and phosphate borate buffer and nitrogen mineralized under aerobic incubation were satisfactory indices of soil N supply. For rice, 2 M KCl and alkaline KMnO₄ were the best N‐availability indices. Thus, alkaline KMnO₄ should prove a quick and reliable indicator of indigenous soil N supply in soils under a rice–wheat cropping system.     

Nitrogen‐uptake by plants following soil incubation

Abstract

Many methods of evaluating organic soil nitrogen (N) mineralization and N availability indexes have been proposed. Chemical methods are more rapid but they do not measure the soil microorganisms and plant root activities. Incubation‐leaching procedure may remove some of the readily mineralizable soil organic N compounds. Continuous‐incubation procedure may sometimes increase soil acidity or cause toxins accumulation. The objective of this study was to determine, in a greenhouse experiment, the relative capabilities of 10 soils with organic matter (O.M.) content ranging from 2.38 to 8.63% to supply plant‐available N by combining two procedures, i.e., soil incubation and plant N‐uptake. In method one (M₁), N‐uptake by 3 successive oat crops of 8 weeks each, without soil preincubation was studied. Method two (M₂) involved a soil preincubation of 8 weeks, and the subsequent determination of N‐uptake by two successive crops of oats (Avena sativa L.) of 8 weeks each. No soil‐leaching was used. The results show that there was a large difference in plant N‐uptake according to soil organic matter. The highest correlation between soil O.M. and plant N‐uptake (r = 0.91**) was given by the first crop following incubation. The N‐uptake by the first crop in M₁ (without soil incubation) was much less correlated with soil O.M. (r = 0.74*) and was significantly influenced by soil initial NO₃ and NH₄‐N. The results of this study show that the preincubation of soil samples minimized the influence of soil initial mineral N and that a preincubation was necessary before the plant N‐uptake measurement, even on a 8‐week cropped soil period.     

Nitrogen mineralization from ‘AU Golden' sunn hemp residue

The tropical legume sunn hemp (Crotalaria juncea L.) cultivar ‘AU Golden’ has the potential to provide substantial nitrogen (N) to subsequent crops to reduce recommended application rates of synthetic N fertilizers. A mineralization field trial was conducted to measure mass decomposition and N and carbon (C) amounts remaining from sunn hemp residue following three planting dates (May, June, and July) during the 2013 growing season at the Tennessee Valley (TVS) and Coastal Plain (WGS) locations of AL. Residue from June and July plantings contained 50.0% and 61.1% N at WGS and 41.5% and 66.5% N at TVS by the end of their respective incubation periods compared to residue from the May planting, which contained 21.1% N at WGS and 47.8% at TVS. In order to create a more synchronous relationship between ‘AU Golden’ residue N mineralization and crop demand, termination must be delayed until approximate planting of the following crop.     

Nitrogen and mineral composition of autumn‐grazed pasture

Abstract

Grazing management in autumn can influence the botanical composition and productivity of a sward. Cycling of nutrients as a result of grazing livestock activity and variable canopy growth rates may influence mineral nutrient supply and demand in a dynamic canopy. An experiment was conducted to determine the influence of autumn grazing practices on the growth and composition, including minerals in terms of ruminant requirements, of a grass/legume sward. Paddocks were established and three replicates grazed by growing lambs for 30‐, 60‐, or 90‐d intervals beginning in late summer. Herbage samples were collected at the beginning of the grazing interval and at the end of each interval (closing date). Herbage mass, and nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca), and sulfur (S), as well as copper (Cu) and zinc (Zn) were examined in terms of the influence of sampling date, closing date, year, and the interaction of these factors from stockpiled and grazed canopies. Soil mineral composition was determined as well. Concentrations of all minerals declined with increasing soil depth and P, Na, Mg, and Ca increased in soil over the course of the experiment. Soil N concentration was reflected in the pattern of herbage growth in autumn. In general, closing date had no influence on herbage mineral composition and concentrations were within the recommended levels for a range of livestock. Phosphorus was the exception and concentrations in herbage were low in terms of requirements for high producing livestock such as lactating dairy cattle. Uptake or mineral reallocation within the plant remained constant during the autumn growth interval, since mineral yields were stable as growth rates declined in 1991 and increased when growth rates were stable in 1992. Mineral related nutritional problems in grazed mixed‐species pasture, would most likely be a function of mineral bioavailability or interactions, rather than low concentrations in the herbage.     

Soil Total Nitrogen and Natural 15Nitrogen in Response to Long-Term Fertilizer Management of a Maize–Wheat Cropping System in Northern China

The Fengqiu long-term field experiment was established to examine effects of organic manure and mineral fertilizers on soil total nitrogen (N) and natural ¹⁵N abundance. Fertilizer regimes include organic manure (OM), one-half N from organic manure plus one-half N from mineral N fertilizer (1/2OMN), mineral fertilizers [N–phosphorus (P)–potassium (K), NP, NK, PK], and a control. Organic manure (OM and 1/2OMN) significantly increased soil total N and δ¹⁵N, which was expected as a great amount of the N applied remained in soils. Mineral NPK fertilizer and mineral NP fertilizer significantly increased total N and slightly increaed δ¹⁵N. Phosphorus-deficient fertilization (NK) and N-deficient fertilization (PK) had no effect on soil total N. Significantly greater δ¹⁵N was observed in the NK treatment as compared to the control, suggesting that considerable N was lost by ammonia (NH₃) voltalization and denitrification in this P-deficiency fertilization regime.     

Nitrogen accumulation and partitioning in hail‐damaged soybeans 1

Hail damage to an experiment that was being used to investigate nitrogen (N) nutrition of soybeans [Glycine max (L.) Merr.] with ¹⁵N methodology provided a unique opportunity to study the effects of hail damage at the R3 stage of development on N uptake and partitioning through stage R5.8. Field plots were established on a silt loam soil (Typic Hapludol 1). Severely damaged (mean 72% leaf loss) and slightly damaged (mean 26% leaf loss) soybeans were compared for total reduced N and for ¹⁵N concentration in leaflets, petioles, stems, roots, pod walls, and seeds during the 28 days following the hailstrom. The concentration of total N and of ¹⁵N in all organs in both damage treatments declined significantly after the storm, but less in green leaflets (total N), and in green leaflets, green petioles, and pod walls (¹⁵N) of severely than of slightly damaged plants. Measurements on senesced leaflets and petioles showed that the concentration of ¹⁵N also decreased to a greater extent than that of the total N in these organs. This differential loss of ¹⁵N compared with total N suggests that the ¹⁵N was in a form that was less refractory than was the bulk tissue N, and provides evidence of separate mobile pools of N in the plant. Nitrogen budgets were calculated to compare the loss of N and ¹⁵N from abscising leaflets and petioles to the N accumulation of the damaged plants during podfill. These showed that loss from the leaflets and petioles contributed only 7% of the total N accumulated by the plants between R3 and R5.8. This study has exemplified the usefulness of ¹⁵N methodology in investigations of the nutrition and physiology of soybeans suffering leaf damage by hail.     

Nitrogen Dynamics and Losses in Direct‐Drilled Maize Systems

Abstract

The knowledge of nitrogen (N) losses in direct‐drilling agrosystems is essential to develop strategies to increase fertilizer efficiency and to minimize environmental damage. The objectives were i) to quantify the magnitude of N volatilization and leaching simultaneously as affected by different urea fertilization rates and ii) to evaluate the capacity of these specific plant–soil systems to act as a buffer to prevent nitrate leaching. Two experiments were conducted during 2001/02 and 2002/03 growing seasons in Alberti, Argentina. The crop was direct‐drilled maize and the soil a Typic Argiudoll. Ammonia losses, N uptake by crop at flowering and harvest, grain yield, N in previous crop residues, and soil nitrate content up to 2‐m depths were determined. Nitrogen availability, soil nitrate (NO₃)‐N up to 1 m plus fertilizer N, was linearly and highly associated with crop N uptake at flowering (R²=0.93, P<0.01) and at harvest (R²=0.852, P<0.01). Around 17.5% of fertilizer N was lost by volatilization in 10 days. The obtained values of residual nitrate N up to the 150‐cm depth were associated (R²=0.960, P<0.001) with those predicted by the nitrate leaching and economic analysis package (NLEAP) model. Maize in the direct‐drilling system was able to cycle N from the previous crop residues, N from soil organic matter, and N from fertilizers with few losses.     

Dry Matter and Nitrogen Accumulation and Root Morphological Characteristics of Two Clonal Selections of ‘Russet Norkotah’ Potato as Affected by Nitrogen Fertilization

New clonal selections with increased vine vigor and stress resistance have been identified for the potato cultivar ‘Russet Norkotah’. However, the importance of clonal variation in nitrogen (N) uptake and root morphological properties is not well known. The objective of this study was to determine the effect of N fertilization on dry matter and N accumulation and root morphological parameters of two clonal selections of ‘Russet Norkotah’. A field experiment was conducted in 2002 using the standard ‘Russet Norkotah’ clone (SRC) and Texas selection 112 (TX112) of ‘Russet Norkotah’, grown at 0 and 150 kg N ha^{? 1}. Whole plants were excavated at 54, 76, and 96 days after planting; partitioned into tubers, vines, roots, stolons, and fruits; and their dry matter and N accumulation were determined. Soil cores were obtained from 10 spatial locations relative to the plant, and used for determination of root length (RL), root length density (RLD), root average diameter (RAD), and root dry weight (RDW). Soil inorganic N content was also measured. Nitrogen fertilization increased tuber yield and dry matter and N accumulation. Fertilizer N application did not affect RL, RLD, or RDW, but resulted in a larger proportion of roots close to the top of the potato hill. Tuber yield and dry matter and N accumulation were similar for the two clonal selections. The TX112 clone, however, partitioned more dry matter and N to vines and less dry matter and N to tubers compared with the SRC clone. Soil nitrate concentration was significantly higher for SCR than for the TX112 clone in the fertilized treatment at 54 DAP, and was low and similar between clones thereafter. Root length and RLD were significantly higher for the TX112 clone compared with SRC, and both clones had a similar spatial distribution of roots. Under the conditions of this study where moisture and disease stress were limited and under a short growing season, the larger root system and increased vine vigor of the TX112 clone did not provide any advantage in terms of plant production as either dry-matter accumulation or tuber yield.     

Leaf Color Chart and Chlorophyll‐Meter‐Based Leaf Nitrogen Estimation and Their Threshold Values for Real‐Time Nitrogen Management in Cassava

The critical leaf and the threshold values of leaf color chart (LCC) and chlorophyll meter (SPAD‐502) for cassava have been evaluated. The nitrogen (N) rates and cultivars had a significant effect on LCC score, SPAD values, and leaf N concentration of leaf 1 in most cases. Among the three leaf positions studied, the youngest fully expanded leaf (YFEL) blade (leaf 1) had significant, positive correlation of tuber yield with LCC score, SPAD value, and leaf N concentration. The regression between LCC score and leaf N concentration of leaf 1 was LCC = 0.358 (Leaf N) + 0.78 (r² = 0.81) and that between LCC score and SPAD value was SPAD = 10.981 (LCC) – 3.51 (r² = 0.82). A threshold LCC score of 2.65 and threshold SPAD value of 25 were suitable to determine the optimal timing of N top‐dressing for cassava.     

Single‐Hole Soil Sampling for Nitrogen in the Potato Hill

Abstract

The nitrate distribution in the soil profile varies with fertilization and tillage practices in potato (Solanum tuberosum L.) production. Band‐applied fertilizers localized near the seed at planting must diffuse through the bulk soil during the growing season. The hilling operation transforms soil surface into an undulating field landscape and redistributes the split‐applied nitrogen fertilizers between the hill and the interrow. The soil sampling procedure during the growing season thus becomes extremely tedious when searching to quantify nitrate accumulation in the entire soil volume. The objective of this study was to assess seasonal nitrate accumulation in a soil volume from a single boring in the potato hill. An intensive sampling was conducted at four places in the 0‐ to 50‐cm profile in potato fields receiving three rates of split-applied nitrogen (N) before hilling. Treatment and time effects provided a large range of nitrate concentrations throughout the soil profile. Nitrate content increased with N fertilization and organic‐matter mineralization and decreased as a result of plant uptake and nitrate leaching. Averaged across the season, nitrate accumulation in the 0‐ to 50‐cm profile represented 78% of that accumulated in the center of the hill on a per ha basis (r²=0.90). A single boring in the center of the hill considerably reduced sampling time and cost and provided a fair estimate of seasonal nitrate accumulation in the 0‐ to 50‐cm soil profile.     

Nitrogen transfer from warm‐season annual legumes to pearl millet

A glasshouse study was conducted to determine and quantify direct transfer of nitrogen (N) between 3 selected warm‐season annual legumes and a warm‐season annual grass during the growing season, ‘Tifleaf’ pearl millet [Pennisetum americanum (L.) Leeke] was grown in pots as a monoculture with and without N applied as inorganic fertilizer, or with either ‘Iron and Clay’ cowpeas [Vigna unguiculata (L.) Walp], common alyceclover [Alysicarpus vaginalis (L.) DC.], or ‘Comanche’ partridge pea (Cassia fasciculata Michx.). Sixty‐three percent of the N contained in pearl millet grown with alyceclover was derived directly from alyceclover as determined by the ¹⁵N dilution technique. Partridge peas and cowpeas transferred 34% and 32%, respectively, of the N contained in companion pearl millet plants. Pearl millet grown with partridge peas produced dry matter yields similar to pearl millet that received the equivalent of 112 kg N/ha. Pearl millet grown with legumes contained lower levels of neutral detergent fiber than did pearl millet that received inorganic fertilizer. Nitrogen content of pearl millet grown with legumes was not as great as pearl millet that received N‐fertilizer.     

Nitrogen mineralization in the ruderal sub-alpine communities in Mount Uludağ, Turkey

Nitrogen mineralization and nitrification in the soil of sub-alpine ruderal community of Mount Uludağ, Bursa, Turkey was measured for 1 year, under field conditions with Verbascum olympicum and Rumex olympicus being the dominant pioneer species under dry and wet sites, respectively. Seasonal fluctuations were observed in N mineralization and nitrification. The net N mineralization and nitrification were high in early summer and winter, due to high moisture. The annual net N mineralization rate (for the 0–15 cm soil layer) was higher under R. olympicus (188 kg N ha⁻¹ yr⁻¹) than under V. olympicum (96 kg N ha⁻¹ yr⁻¹). A significant positive correlation between net N mineralization and soil organic C (r² = 0.166), total N (r² = 0.141) and water content (r² = 0.211) was found. Our results indicate that N mineralization rate is high in soils of ruderal communities on disturbed sites and varies with dominant species and, a difference in net N mineralization rate can be attributed to organic C, total N and moisture content of soils.