15N Application Research Articles

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 com- pared 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 15 N-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, respec- tively. 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 drain- age was negligible ( 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 15 Ni n

Parsimonious modelling of nutrient fluxes for a terrestrial ecosystem on Svalbard

MBL-MEL, a simple model of ecosystem biogeochemistry, is amended and applied to plant and soil C, 14N and 15N data for the summers of 2001–2003 from Brandalpynten, a maritime high Arctic site on Svalbard following the application of 15N (99 atom%) as 15NO3-N at or 15NH4-N at concentrations of 1 or 5 kg N ha−1. Variants of this Parent model are also developed to incorporate: temperature dependence into equations describing nutrient fluxes (Temp model); cryptogams (Cryp model); both features combined (CrypTemp model). Goodness-of-fit (GOF) statistics suggest that the addition of temperature-dependence improves the utility of models with and without cryptogams: the residual weighted sums of squares per data point were 5.69, 3.91, 4.31 and 3.93 for the Parent, Temp, Cryp and CrypTemp models respectively. The application of model selection criteria confirm that the addition of temperature-dependence also improves model generalisability. Across all models, the principal discrepancies between observation and prediction are associated with the inorganic soil 15N pool. We conclude that models in which fluxes are described using simple equations that can be augmented to include additional complexity, are an ideal starting point from which the relationship between GOF and model generalisability can be assessed.

Decomposition and distribution of 15N labelled mustard litter ( Sinapis alba) in physical soil fractions of a cropland with high- and low-yield field areas

High-yield (HY) areas of an agricultural cropland were characterized by different positions on a slope and lower silt and clay contents, compared to low-yield (LY) areas, and this was associated with differences in water regime and C and N turnover. To understand differences in N flows of HY and LY areas, a combination of 15N tracer techniques and physical fractionation procedures was applied. Within 570 d after application of 15N labelled mustard litter to an agricultural cropland, the distribution of 15N was measured in particulate organic matter (POM) fractions and in fine mineral fractions (fine silt- and clay-sized fractions). After 570 d, only 2.5% of the initial 15N amount was found in POM fractions, with higher amounts in POM occluded in aggregates than in free POM. After this period, stabilization of the initial 15N in fine silt- and clay-sized fractions amounts to 10% in HY, but 20% in LY soils. 70% to 85% of the added 15N were lost. Initial decomposition of labelled material was faster in HY than in LY areas during the first year, but the remaining 15N amounts in POM fractions of the different areas were similar after 570 d. 15N amounts and concentrations in mineral-associated fractions increased within 160 d after application. From 160 to 570 d, HY and LY areas showed different 15N dynamics, resulting in a decline of 15N amounts in HY, but constant 15N amounts in LY soils. The results indicate faster decomposition processes in HY than in LY areas, due to different soil conditions, such as soil texture and water regime. The higher silt and clay contents of LY areas seem to promote N stabilization in fine mineral fractions. As a whole, N flows were higher in HY compared to LY areas, thus supporting higher yields and accelerated organic matter degradation due to higher N supply.

Effect of Total Mixed Ration Composition on Fermentation and Efficiency of Ruminal Microbial Crude Protein Synthesis In Vitro

The goal of this study was to identify dietary factors that affect fermentation and efficiency of microbial crude protein (CPM) synthesis in the rumen in vitro. We used 16 total mixed, dairy cow rations with known digestibilities that varied in ingredient composition and nutrient content. Each ration was incubated in a Rusitec (n = 3) for 15 d, and fermentation of different fractions was assessed. Observed extents of fermentation in 24h were 35 to 47% for organic matter, 25 to 60% for crude protein, 3 to 28% for neutral detergent fiber, and 31 to 45% for gross energy. Organic matter fermentation depended on the content of crude protein and neutral detergent fiber in the ration. We studied net synthesis of CPM using an 15N dilution technique and found that 7 d of continuous 15N application are needed to achieve an 15N enrichment plateau in the N of isolated microbes in this type of study. The efficiency of CPM synthesis was 141 to 286 g/kg of fermented organic matter or 4.9 to 11.1 g/MJ of metabolizable energy, and these ranges agree with those found in the literature. Multiple regressions to predict the efficiency of CPM synthesis by diet data showed that crude protein was the only dietary chemical fraction that had a significant effect. Fat content and the inclusion rate of corn silage in the ration also tended to improve efficiency. We suggest that microbial need for preformed amino acids may explain the crude protein effect. A large part of the variation in efficiency of microbial activity still remains unexplained.

The fate of 15N added to high Arctic tundra to mimic increased inputs of atmospheric nitrogen released from a melting snowpack

AbstractIncreases in the long‐range aerial transport of reactive N species from low to high latitudes will lead to increased accumulation in the Arctic snowpack, followed by release during the early summer thaw. We followed the release of simulated snowpack N, and its subsequent fate over three growing seasons, on two contrasting high Arctic tundra types on Spitsbergen (79°N). Applications of 15N (99 atom%) at 0.1 and 0.5 g N m−2 were made immediately after snowmelt in 2001 as either Na15NO3 or 15NH4Cl. These applications are approximately 1 × and 5 × the yearly atmospheric deposition rates. The vegetation at the principal experimental site was dominated by bryophytes and Salix polaris while at the second site, vegetation included bryophytes, graminoids and lichens. Audits of the applied 15N were undertaken, over two or three growing seasons, by determining the amounts of labeled N in the soil (0–3 and 3–10 cm), soil microbial biomass and different vegetation fractions.Initial partitioning of the 15N at the first sampling time showed that ∼60% of the applied 15N was recovered in soil, litter and plants, regardless of N form or application rate, indicating that rapid immobilization into organic forms had occurred at both sites. Substantial incorporation of the 15N was found in the microbial biomass in the humus layer and in the bryophyte and lichen fractions. After initial partitioning there appeared to be little change in the total 15N recovered over the following two or three seasons in each of the sampled fractions, indicating highly conservative N retention. The most obvious transfer of 15N, following assimilation, was from the microbial biomass into stable forms of humus, with an apparent half‐life of just over 1 year. At the principal site the microbial biomass and vascular plants were found to immobilize the greatest proportion of 15N compared with their total N concentration. In the more diverse tundra of the second site, lichen species and graminoids competed effectively for 15NH4‐N and 15NO3‐N, respectively. Results suggest that Arctic tundra habitats have a considerable capacity to immobilize additional inorganic N released from the snow pack. However, with 40% of the applied 15N apparently lost there is potential for N enrichment in the surrounding fjordal systems during the spring thaw.

Quantifying the Impact of Regular Cutting on Vegetative Buffer Efficacy for Nitrogen‐15 Sequestration

This study used the stable 15N isotope to quantitatively examine the effects of cutting on vegetative buffer uptake of NO3(-)-N based on the theory that regular cutting would increase N demand and sequestration by encouraging new plant growth. During the summer of 2002, 10 buffer plots were established within a flood-irrigated pasture. In 2003, 15N-labeled KNO3 was applied to the pasture area at a rate of 5 kg N ha(-1) and 99.7 atom % 15N. One-half of the buffer plots were trimmed monthly. In the buffers, the cutting effect was not significant in the first few weeks following 15N application, with both the cut and uncut buffers sequestering 15N. Over the irrigation season, however, cut buffers sequestered 2.3 times the 15N of uncut buffers, corresponding to an increase in aboveground biomass following cutting. Cutting and removing vegetation allowed the standing biomass to take advantage of soil 15N as it was released by microbial mineralization. In contrast, the uncut buffers showed very little change in 15N sequestration or biomass, suggesting senescence and a corresponding decrease in N demand. Overall, cutting significantly improved 15N attenuation from both surface and subsurface water. However, the effect was temporally related, and only became significant 21 to 42 d after 15N application. The dominant influence on runoff water quality from irrigated pasture remains irrigation rate, as reducing the rate by 75% relative to the typical rate resulted in a 50% decrease in total runoff losses and a sevenfold decrease in 15N concentration.

Fate of Nitrogen‐15 in a Long‐Term Nitrogen Rate Study: II. Nitrogen Uptake Efficiency

Increased fertilizer N uptake efficiency (FNUE) leads to more economical corn (Zea mays L.) production and lower environmental impact. Excessive N application reduces FNUE and may affect subsequent crop response through its influence on NO3–N carryover and the amount of readily mineralizable organic N in the soil. Our objective was to determine how prior fertilizer N application rate affects (i) grain yield and agronomic optimum N rate, (ii) contributions of fertilizer‐ and soil‐derived N to N uptake, and (iii) FNUE. Labeled 15NH415NO3 was applied at 0, 67, 134, 201, or 268 kg N ha−1 to subplots within a continuous corn long‐term N rate study. Estimates of FNUE were higher by the difference method (49–69%) than with the isotope (15N) method (31–37%), and different trends were observed with each method as N application rate increased. The disparity between methods is consistent with a differential effect of long‐term N application rate on mineralization–immobilization. Recovery of labeled N from the plant–soil system ranged from 71% at the 67 kg ha−1 N application rate to 64% at the 201 kg ha−1 application rate. Fertilizer N accounted for an increasing proportion of crop N uptake as the N rate was increased, but soil N uptake was always more extensive, accounting for 54 to 83% of total plant N. Crop uptake of labeled N during the second growing season after 15N application ranged from 2.2 kg ha−1 with the lowest N rate to 7.8 kg ha−1 with the two highest rates.

Timing of urea application affects leaf and root N uptake in young Fuji/M.9 apple trees

SummaryLeaf and root nitrogen (N) uptake was compared at different times of the season in young apple trees. One-year-old potted Fuji/M.9 trees were supplied with 1% 15N-labelled urea either by foliar or soil applications in May, July and September. Trees receiving only water served as controls. The trees were harvested 10 d after 15N application, separated into shoots (leaves and current-year stems), stem (previous-year wood) and roots. Biomass, total N and 15N contents of all tissues were determined. New shoot biomass and total tree biomass increased as the season advanced, while root biomass peaked in July. Leaf N uptake was higher than root uptake in May and September, while root N uptake was higher than leaf uptake in July. Leaf N uptake increased as the season advanced, while root N uptake was highest in July. The lowest 15N recovery (11%) was obtained in May with soil N application, and the highest 15N recovery (48%) was obtained in September with foliar N application. Our results suggest that foliar application of N early in the season, followed by soil N application in mid-season, then foliar application again late in the season is an efficient N management strategy for young trees.

Nitrogen-15 Uptake by Pinus contorta Seedlings in Relation to Phenological Stage and Season

This study measured the amount of uptake of labeled nitrogen (15N) of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm) seedlings, at three different phenological stages, in a growth chamber experiment. Thirty days after 15N application, the amount of 15N recovered in seedlings as a percentage of the total 15N fertilizer applied was 4% in early spring, 43% in summer and 33% in autumn. The total 15N recovered in the plant–pot system ranged from 80 to 96%, and is higher than reported in other studies. Total 15N recovered from the pot compartment alone ranged from 48 to 95%, suggesting that substantial pools of N remain in the soil. Results suggest that low 15N uptake in the spring was associated with limited development of new root as a result of low spring soil temperatures. The lack of unsuberized roots in spring could be a key factor decreasing the effectiveness of early spring fertilization in the boreal forest.

Influence of Water Deficit on Uptake and Distribution of Nitrogen in Soybeans Monitored by Soil Injected 15N

AbstractA pot experiment was carried out in a climatic chamber to study 15N‐uptake, translocation and recovery in soybeans [Glycine max (L.) Merr.] as affected by a 13 day interval of water deficits (wd) starting at flowering. Ten or 40 mg 15N (500 or 2000 p.p.m. 15N) was injected at 0.4 m depth into the soil 3 days after flowering (3 DAF). A dose of 10 mg 15N, resulted in a translocation of 50 and 80 % to the shoot at 7 and 13 DAF, respectively; thus the uptake of 15N into the plant organs was limited by the low remaining 15N in the soil after 7 DAF. Therefore, only results referring to the 40 mg 15N application are presented here. The daily 15N‐uptake of well‐water (ww) plants during the 10 days of investigations was about c. 2.5 mg 15N, it was significantly higher than under wd at 7 DAF and tended to be higher at 13 DAF. The reduction by wd was also evident by a lower recovery of 15N (55 % instead of 75 %) at 13 DAF. In almost all leaves and pods there was a trend to a lowered translocation (mg 15N per organ) under wd when compared with the ww plants at 7 and 13 DAF; lowering due to wd was strongest and significant in upper leaves and pods at 13 DAF, whereas translocation to stems was even slightly increased. Relative accumulation of 15N (mg g−1 dry matter) in leaves, stems and pods increased from the bottom to the top indicating an higher sink activity of younger organs. Water deficits tended to lower this relative 15N‐accumulation in all leaves and pods at 7 and 13 DAF, and caused a significant reduction in upper leaves and pods at 7 DAF. Absolute and relative 15N‐accumulation in stems tended to be affected differently by wd, being unaffected or slightly decreased at 7 DAF, but increased at 13 DAF. These differential effects of wd are discussed.

Recovery of Late-season 15N-labeled Fertilizer Applied to Pecan

The recovery of late-season (September) 15N-labeled fertilizer (N at 55 kg·ha-1) was followed in mature pecan trees [Carya illinoinensis (Wangehn.) K. Koch] and soil (0-270 cm) from 1996 (application year) through 2001 (end of study). Recovery of late-season applied 15N was compared to the recovery of six 15N applications (March through June, N at 221 kg·ha-1) of a previously reported study. By Nov. 1996, both fertilizer schedules exhibited considerable 15N accumulation below the rooting zone and just above the water table (280 cm), with 43.4% and 35.3% 15N recovered from the soil sampling profile of the September and March-June schedules, respectively. 15Nitrogen recoveries from perennial storage tissues (root and wood) were 20.6% and 10.1% under the September and March-June schedules, respectively. The 15N recoveries from annual abscission tissues (leaf, shuck, and nut) were 1.4% and 10.6% under the September and March-June schedules, respectively. By the end of the 2001 growing season, 4% and 9% of the 15N remained in the soil following the September and March-June applications, respectively. Under both fertilizer schedules, >80% of the fertilizer-N was lost to the environment through natural processes and very little was removed during harvest. Nearly 6 years following application, perennial storage of 15N remained greater in the September application (4.3% of the 15N applied) than in the March-June application (2.7% of the 15N applied). Late-season application of fertilizer-N during the kernel filling stage was stored in perennial tissues for use the following year; very little was used for current year growth of annual tissues. Increased accumulation of perennial storage N by late-season application may reduce the depletion of N caused during a heavy-cropping on-year and may moderate the alternate-bearing trend in pecan by providing a greater reservoir of N the following year.

N uptake, soil retention and loss of soil-applied 15NH415NO3 in young Fuji/M.26 apple trees with different N status

SummaryNitrogen (N) uptake, soil retention and loss of soil-applied N were studied in young apple trees with different N backgrounds. Bench-graft potted Fuji/M.26 (Malus domestica Borkh) trees were fertigated with 5, 10 or 20 mM N twice a week from June to August, and the trees were removed from soil and bare-root stored in a 2°C cold room in December of the first season. In April of the second season, the trees were washed and replanted in containers with a N-free medium (perlite:vermiculite=l:l v), and received 500 ml Hoagland’s nutrient solution without N weekly through the experiment. The trees received 15NH415NO3 at 1g per plant or no N on June 21. Four trees from each treatment were harvested at one, two and four weeks after 15N application, and 15N and total N in plant tissues and soil were analysed. N fertigation rates during the first growing season increased tree growth and N reserve levels, and N content in trees in the second year. New shoot and leaf growth in the following season was positively related to reserve N. 15N uptake increased during the four weeks after 15N application while soil 15N retention decreased. There was no significant difference in the total 15N uptake per tree. However, trees with the lowest N contents at the end of the first growing season had the highest rate of 15N uptake per unit root dry weight. Four weeks after application of 15N, tree uptake of N accounted for about 60% of applied 15N, while about 20% of the N still remained in the soil, and another 20% of the N was lost. Our results suggested that trees with lower N status are more efficient in N uptake from soil.

Rapid 15N uptake and metabolism in fine roots of Norway spruce

Short-term N uptake by Norway spruce roots was investigated after tracer application both in the field and in the laboratory. In the forest the influence of wood ash or liquid fertiliser treatments on the uptake was studied. A rapid uptake of the 15NH415NO3 tracer into the fine roots was observed. Within 1 day of application about 50% of the maximum δ15N value observed was measured and within 1 week as much as 70–90%. The 15N enrichment in fine roots was not affected by wood ash and liquid fertiliser treatments applied in the previous year. δ15Ν increased continuously until 2 months after 15N application and decreased to 60% of its maximum value within 1 year. Nine months after the tracer application, an analysis of δ15N distribution throughout the root system was conducted. The δ15N values were highest in roots of the topsoil compared to roots at deeper soil layers, and higher in fine roots than in larger roots. The 15N was not translocated within the root system. The laboratory experiment, using Norway spruce seedlings, aimed at describing short-term 15NO3 uptake and assimilation into free amino acids with 2 mM or 20 mM nitrate application. The 15N was detected within 4 h to 1 day in the amino acids Glu, Gln, Asp of roots, while in Asn, Ala and Ser and in the shoots, the 15N was retrieved at the earliest after 1 day. The enrichment factor in the amino acids increased to a maximum within 3–7 days, depending on the nitrate concentration applied.

Effects of 15N application frequency on nitrogen uptake efficiency in Citrus trees

Two irrigation systems were used to compare nitrogen uptake efficiency in citrus trees and to evaluate the NO3- runoff in "Navelina" orange trees [Citrus sinensis (L.) Osbeck] on Carrizo citrange rootstock (Citrus sinensis x Poncirus trifoliata Raf.). These were fertilized with 125 g N as labelled K15NO3 and grown outdoors in containers filled with a sand-loamy soil. Two groups of 3 trees received this N dose either in five equally split applications by a flooding irrigation system or in 66 applications by drip. Trees were harvested at the end of the vegetative cycle (December) and the isotopic ratios of 15N/14N were measured in the soil-plant system. The N uptake efficiency of the whole tree was higher with drip irrigation (75%) than with flooding system (64%). In the 0-90 cm soil profile, the N immobilized in the organic fraction was similar for both irrigation methods (around 13 %), whereas the N retained as NO3- was 1% of the N applied under drip and 10% under flooding. In the last case, most of NO3- remained under root system and it could be lost to leaching either by heavy rainfalls or excessive water applications. These results showed that a drip irrigation system was more efficient for improving water use and N uptake from fertilizer, in addition to potentially reduced leaching losses.

Estimation of symbiotic N 2 fixation by a low-level, large-scale 15N application technique

A modified approach is presented for the 15N isotope dilution method to estimate the proportion of plant nitrogen derived from the atmosphere (Ndfa). This approach utilizes a low-level, large-scale application of 15N enriched fertilizer. In a field study, pea ( Pisum sativum L.) was cultivated in two long-term (since 1970) tillage systems (conventional: CT, minimum tillage: MT). Nitrogen (26 g NO 3–N ha −1 with 10 at.% 15N) was distributed with a conventional field sprayer to increase the plant available soil 15N level up to the desired range of δ 15N>5‰. A formula for determining the desired amounts of 15N application is presented and a model is introduced comparing the apparent Ndfa calculated by field data to the theoretically calculated true values of Ndfa. The model makes it possible to consider three basic factors: (i) the amount of plant available soil nitrogen, (ii) the distribution of 15N enrichment in different soil layers and (iii) the soil N uptake proportions of the legume and the reference plant when the amount of applied 15N fertilizer is the dependent variable. Errors in estimating Ndfa by the 15N dilution method can be quantitatively determined with different soil N uptake patterns of legumes and reference plants at varying 15N enrichment levels. The model shows that results obtained by using the low-level, large-scale 15N enrichment technique produced a slightly smaller margin of error than did the addition of larger amounts of 15N labelled fertilizer when N uptake patterns of the legume and the reference plant differ. The new technique offers distinct advantages, in that it is very practical and makes the enlargement of plots possible at low cost and with marginal effort. The results of the low-level enrichment field study were evaluated using the model as well as the results of an investigation on pea with moderate 15N enrichment by liquid manure (isotope dilution method).

Losses of fertilizer-derived N from transplanted rice after heading

Microplot field experiments with 15N-labeled (NH4)2SO4 were conducted to investigate the changes in the percentage of fertilizer 15N recovery (%FNR) of transplanted rice (Oryza sativa L.) with basal (4 g m−2) and topdress (3 g m−2) applications for two years. In 1999, %FNR from the basal 15N application decreased by 6% (0.23 g N m−2)from heading + 2 d to maturity, and %FNR from the topdress 15N application decreased by 11% (0.32 g N m−2)from heading − 4 d to maturity. In 2000, %FNR from the topdress 15N application decreased by 9% (0.26 g N m−2) from heading − 1 d to maturity, whereas there was no significant decline in %FNR from the basal 15N application after heading. A decrease in %FNR occurred even when total N of rice plants showed no decrease after heading. Nitrogen harvest index of fertilizer 15N with the topdress 15N application was not significantly different from that with the basal 15N application in 1999, whereas N harvest index of fertilizer 15N with the topdress 15N application was higher than that with the basal 15N application in 2000. N harvest index of fertilizer 15N, that indicated the proportion of the 15N remobilized to panicles after heading, was substantially responsible for changes in %FNR from the basal and topdress 15N applications after heading.

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

Application of 15N to the study of hepatic nitrogen metabolism

Application of 15N to the study of hepatic nitrogen metabolism

Short-term nitrogen-15 recovery vs. long-term total soil N gains in conventional and alternative cropping systems

Cropping systems receiving organic N additions may sequester more N than conventional systems receiving inorganic N inputs. A short-term study using 15N-labeled vetch ( Vicia spp.) and 15N urea was conducted to determine whether 9 years of organic N inputs to a cropping system led to lower short-term 15N losses than a cropping system under conventional management. The three cropping systems were: a conventional, receiving 15N urea, a low-input receiving 15N urea or 15N-labeled vetch residue, and an organic receiving 15N-labeled vetch residue with unlabeled poultry manure. Nitrogen recovery was calculated as the amount of 15N recovered in maize ( Zea mays L.) and in the soil. Total N recovery from 15N sources in the three cropping systems were similar after one growing season and unaffected by the form of 15N application, whether organic or inorganic. Similar results were reported from other studies conducted at Rodale (USA), Rothamsted (UK) and Cadriano (Italy) where short-term N recoveries from organic or inorganic 15N sources were similar in conventional and legume or manure based cropping systems. Therefore, short-term 15N recoveries appear to be unaffected by the form of 15N application and whether organic or inorganic N-based systems are evaluated. However, results from the short-term 15N tracer experiments did not reflect known long-term trends of increased total soil N in the organic N-based cropping systems. The apparent discrepancy between short-term 15N recoveries and long-term gains in total soil N may be due to differences in total N inputs between the various cropping systems or the inability of short-term 15N studies to accurately reflect the long-term N retention of various cropping systems.

Effects of green manures on soil organic matter and wheat yields and N nutrition

A field study was conducted for 5 yr (1993-1997) to evaluate the effects of green manure residues applied to the soil in 1993 and 1995, on wheat (Triticum aestivum L.) yields and N nutrition as subsequent crop in 1994, 1996 and 1997. The effect of green manure application was also evaluated on soil microbial activity (CO2), on C and N contents of whole soil and on labile (LF) and heavy fractions (HF) of organic matter (OM). The experiment was initiated on a Le Bras silt loam (Humic Gleysol). The green manures, as a main factor, were clover (Trifolium pratense L.), buckwheat (Fagapyrum esculentum L.), millet (Echinicloa crus galli L.), mustard (Brassica hirta Moench), and colza (Brassica campestris L.), and there was a control without green manure. The sub-factors consisted of four N fertilizer rates for wheat in the subsequent years at 0, 30, 60 and 90 kg N ha–1. Broadcast application of 15N- labelled NH4NO3 was made in 90 kg N ha–1 fertilizer treatments. Two green manure applications did not influence the C and N contents of densimetric fractions of OM (LF and HF), but significantly increased those of whole soil, and microbial respiration (CO2). Green manures significantly increased wheat yields and N uptake in 1994 and 1996. Levels of N derived from fertilizer (Ndff) were lower in all green manure treatments as compared to the control, which indicates that the proportion of N derived from soil and green manures (% Ndfs) was higher in these treatments. The contribution of N from green manure varied in the following order: buckwheat < clover < mustard < millet < colza. With the exception of the clover treatment (< 100%), the N recoveries from the other green manure N (NRGM) ranged from 23 to 34% and from 19 to 36% for 1994 and 1996, respectively. Green manure application provided 15 to 24 kg N ha–1 in 1994 and from 16 to 36 kg N ha–1 in 1996 and this contribution accounted for 25 to 31% of the total wheat N uptake. Significant green manure effects on wheat yield and N nutrition were primarily due to the improvement of soil properties and to high N recoveries from the green manure. In the cold temperate climate of Québec, green manure incorporation into soil in late summer or early fall of the preceding year allowed N synchronization with wheat N needs in subsequent cropping seasons. Nitrogen fertilizer rates could be reduced after the incorporation of green manures having high yields and N contents in the previous season. Key words: Green manure, wheat yields and N uptake, N recoveries, microbial respiration, labile and heavy fractions of OM, C and N contents