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1.
The potential of modified spring barley crops with improved nitrogen (N) use efficiency to reduce nitrogen (N) leaching and to increase soil organic carbon (SOC) storage was assessed at the regional scale. This was done using simulation model applications designed for reporting according to the Helcom (Helsinki Commission) and Kyoto protocols. Using model simulations based on modified crops N dynamics and SOC were assessed for three agro-ecological regions (latitudes ranging 55°20′–60°40′ N) in Sweden over a 20-year period. The modified N use properties of spring barley were implemented in the SOILNDB model (simulating soil C, N, water and heat, and plant N dynamics) by changing the parameters for root N uptake efficiency and plant N demand within a range given by previous model applications to different crops. A doubling of the daily N uptake efficiency and increased N demand (by ca 30%) reduced N leaching by 24%–31%, increased plant N content by 9%–12%, depending on region. The effects of the modified crop on SOC was simulated with the ICBM model, resulting in an increased SOC content (0–25 cm depth) by 57–79 kg?C?ha?1?y?1. The results suggest that a modified crop might reduce N leaching from spring barley area, in a range similar to the targets of relevant environmental protection directives, a result which held more in the northern than southern regions. The simulated SOC increase on a hectare basis was highest for the central region and least for the most northern region. For the total agricultural area the share of spring barley area was small and more crops would need to be modified to reach the emission reduction targets.  相似文献   

2.
Summary A crop of barley was grown on plots which had previously supported pure stands of lupins, canola, ryegrass, and wheat. The plots were labelled with 15N-enriched fertilizers at the time of sowing of the antecedent crops. The crop of lupins, which derived 79% of its N from symbiotic N2 fixation at physiological maturity, conferred an N benefit to barley of 3.4 g N m-2 when compared to barley following wheat. Lupins used less fertilizer N and less unlabelled soil N compared to the other crops, but the ratios of these sources of N in the plant tops were similar. The apparent sparing of soil+fertilizer N under lupins compared with wheat was 13.6 g N m-2, which was much larger than the measured N benefit. Barley following lupins was less enriched in 15N compared to barley following wheat, and the measured isotope dilution was used to estimate the proportion of barley N derived from biologically fixed N in the lupin residues. This in turn enabled the N benefit to be partitioned between the uptake of spared N and the uptake of fixed N derived from the mineralization of legume residues. Spared N and fixed N contributed in approximately equal proportions to the N benefit measured in barley following lupins compared to barley following wheat.  相似文献   

3.

Background

A high use-efficiency of fertilizer N remains essential to sustain high crop productivity with low environmental impact. However, little is known on the long-term lability of mineral fertilizer N.

Aims

To quantify crop uptake and leaching of 15N-labelled mineral fertilizer that has been retained in an agricultural soil for 25–30 years in crops with variable growing season.

Methods

A field plot received 15N-labelled mineral fertilizers over a period of 5 years and was then kept under arable cropping for 12 years. After relocation to 16 lysimeters, the topsoil grew set-aside grassland for the next 13 years. Then crop uptakes and leaching losses of 15N remaining in soil was tested over a 2-year period by either converting set-aside grass to production grassland, or by replacing it with spring barley (+/− autumn cover crop) or vegetation-free fallow. All treatments received unlabelled mineral N fertilizers.

Results

Crop uptake and leaching of 15N were generally highest in the first test year after termination of the set-aside. The leaching of residual 15N in soil declined in the order: vegetation-free soil (4.7%), spring barley (1.9%), spring barley + cover crop (0.7%) and production grassland (0.2%). Corresponding losses for the second leaching period were 2.7%, 0.9%, 0.4% and 0.06%. There was a fixed relationship between leaching losses of 15N and total N.

Conclusions

After residing in soil for 25–30 years, the lability of labelled mineral N fertilizer residues appeared slightly higher than the lability of bulk soil N. Autumn vegetation was crucial for reducing leaching losses.  相似文献   

4.
In Northern Europe, cover crops are traditionally established before spring crops by undersowing, but some cover crops might also have an effect if preharvest sown before spring crops and even winter crops. The effects of cover crop sowing date, sowing technique and succeeding main crop on biomass production, N uptake, nitrate leaching and soil inorganic N were tested in lysimeters and in the field. Cruciferous cover crops (oil radish, white mustard) were sown preharvest by broadcasting into winter wheat in July and were allowed to grow until a following winter wheat was established in September. Other preharvest cover crops were left in place until late autumn. For comparison, the same cruciferous cover crops were established postharvest after light harrowing. Perennial ryegrass undersown in spring barley was also included. Aboveground N uptake in preharvest cover crops amounted to a maximum of 24 kg N/ha in September before sowing winter wheat. When left until late autumn, preharvest oil radish took up a maximum of 66 kg N/ha, and ryegrass and postharvest cover crops 35 kg N/ha. Preharvest establishment of cruciferous cover crops before a spring‐sown crop thus seems promising. The soil was depleted of inorganic N to the same extent in late autumn irrespective of cover crop type, sowing time and technique within winter wheat or spring barley. However, the reduction in nitrate leaching of preharvest cover crops incorporated after 2 months and followed by winter wheat was only half of that achieved by cover crops left until late autumn or spring.  相似文献   

5.
《Soil Use and Management》2018,34(3):335-342
This study investigates the effect of different crop rotation systems on carbon (C) and nitrogen (N) in root biomass as well as on soil organic carbon (SOC ). Soils under spring barley and spring barley/pea mixture were sampled both in organic and conventional crop rotations. The amounts of root biomass and SOC in fine (250–253 μ m), medium (425–250 μ m) and coarse (>425 μ m) soil particulate organic matter (POM ) were determined. Grain dry matter (DM ) and the amount of N in harvested grain were also quantified. Organic systems with varying use of manure and catch crops had lower spring barley grain DM yield compared to those in conventional systems, whereas barley/pea showed no differences. The largest benefits were observed for grain N yields and grain DM yields for spring barley, where grain N yield was positively correlated with root N. The inclusion of catch crops in organic rotations resulted in higher root N and SOC (g C/m2) in fine POM in soils under barley/pea. Our results suggest that manure application and inclusion of catch crops improve crop N supply and reduce the yield gap between conventional and organic rotations. The observed positive correlation between root N and grain N imply that management practices aimed at increasing grain N could also increase root N and thus enhance N supply for subsequent crops.  相似文献   

6.
In organic farming systems, it has been demonstrated that grain pulses such as peas often do not enhance soil N supply to the following crops. This may be due to large N removals via harvested grains as well as N‐leaching losses during winter. In two field‐trial series, the effects of legume (common vetch, hairy vetch, peas) and nonlegume (oil radish) cover crops (CC), and mixtures of both, sown after peas, on soil nitrate content, N uptake, and yield of following potatoes or winter wheat were studied. The overall objective of these experiments was to obtain detailed information on how to influence N availability after main‐crop peas by adapting cover‐cropping strategies. Cover crops accumulated 56 to 108 kg N ha–1 in aboveground biomass, and legume CC fixed 30–70 kg N ha–1 by N2 fixation, depending on the soil N supply and the length of the growing period of the CC. Nitrogen concentration in the aboveground biomass of legume CC was much higher and the C : N ratio much lower than in the nonlegume oil radish CC. At the time of CC incorporation (wheat series) as well as at the end of the growing season (potato series), soil nitrate content did not differ between the nonlegume CC species and mixtures, whereas pure stands of legume CC showed slightly increased soil nitrate content. When the CC were incorporated in autumn (beginning of October) nitrate leaching increased, especially from leguminous CC. However, most of the N leached only into soil layers down to 1.50 m and was recovered more or less by the following winter wheat. When CC were incorporated in late winter (February) no increase in nitrate leaching was observed. In spring, N availability for winter wheat or potatoes was much greater after legumes and, after mixtures containing legumes, resulting in significantly higher N uptake and yields in both crops. In conclusion, autumn‐incorporated CC mixtures of legumes and nonlegumes accomplished both: reduced nitrate leaching and larger N availability to the succeeding crop. When the CC were incorporated in winter and a spring‐sown main crop followed even pure stands of legume CC were able to achieve both goals.  相似文献   

7.
包膜尿素对芹菜产量、品质及氮素平衡的影响   总被引：9，自引：1，他引：9  
通过盆栽试验研究了包膜尿素(自制)和尿素等肥料在两种质地灰潮土上对芹菜产量、品质、氮素吸收和氮平衡的影响。结果表明,施用包膜尿素较尿素使当季芹菜增产11.5％-15.2％,氮素吸收增加5.9％-9.5％,氨挥发损失N减少14.2％-14.9％,淋失和反硝化氮损失减少25.5％-28.3％,氮素利用率提高19.2％-27.1％,土壤持留氮增加32.0％-37.3％。芹菜植株硝态氮含量降低44.2％-58.9％,维生素C含量显著提高,品质改善;后茬作物生菜增产14.4％-35.2％。  相似文献   

8.
The effects of soil incorporation with cereal straw (nil, 2.5, 5 and 10 t straw ha–1) and direct drilling on the proportion and amount of pea N derived from biological N fixation were investigated in three field experiments. Fixed N was determined by15N dilution using barley as a reference plant. The three sites were on acidic, red clay-loams in the cropping zone of southeastern Australia. Seasonal plant available soil N, as determined by the N accumulated in barley, was 31, 56 and 158 kg N ha–1, for the three sites. Incorporated straw reduced soil nitrate at sowing by 10–50 kg N ha–1 (0–30 cm), and 5 or 10 t straw ha–1 reduced barley uptake of N by 10–38 kg N ha–1. However, reducing plant available soil N was generally ineffective for increasing the N fixed by pea. Fixed N increased only at the site with the least plant-available N, and only one-third of the increase could be attributed to lower soil N uptake by pea. There was no evidence that direct drilling pea increased fixed N by decreasing crop uptake of soil N. It is proposed that a lower requirement for soil N by pea as compared to barley, and availability of mineral N beneath the soil layer treated with straw, minimise the effectiveness of straw incorporation for increasing the N fixed by pea.  相似文献   

9.
The Static Fertilization Experiment Bad Lauchstädt (1902) consists of a crop rotation of sugar beets, spring barley, potatoes and winter wheat. Three farmyard manure (FYM) treatments and six mineral fertilizer treatments are combined orthogonally. Comparing the first and last decades, crop yields nearly doubled. In unfertilized plots, yields and N uptake by crops also increased when comparing first and last decades. On average for the decade 2001–2010, N uptake in unfertilized plots amounted 51.6 kg ha?1. Although soil organic carbon (SOC) levels for unfertilized plots remain almost unchanged, SOC increases slowly in the most highly fertilized treatment, resulting in a gradual widening of differences in SOC between the most extreme treatments to 0.952%. Climate change and increased harvesting and root residues due to rising yields are suggested as an explanation. Except for the plot with the highest application of mineral and organic fertilizer, in all treatments more N was taken up by crops than was applied by fertilizers. Higher FYM input leads to more unfavourable N balances because N release from FYM cannot be controlled. Considering atmospheric N input, only in the exclusively mineral fertilized treatment is N balanced out. Similar results are found for C balances: the exclusively mineral fertilized treatment shows the most favourable C balance.  相似文献   

10.
Cover crop and nitrogen(N) fertilization may maintain soil organic matter under bioenergy perennial grass where removal of aboveground biomass for feedstock to produce cellulosic ethanol can reduce soil quality. We evaluated the effects of cover crops and N fertilization rates on soil organic carbon(C)(SOC), total N(STN), ammonium N(NH_4-N), and nitrate N(NO_3-N) contents at the0–5, 5–15, and 15–30 cm depths under perennial bioenergy grass from 2010 to 2014 in the southeastern USA. Treatments included unbalanced combinations of perennial bioenergy grass, energy cane(Saccharum spontaneum L.) or elephant grass(Pennisetum purpureum Schumach.), cover crop, crimson clover(Trifolium incarnatum L.), and N fertilization rates(0, 100, and 200 kg N ha~(-1)). Cover crop biomass and C and N contents were greater in the treatment of energy cane with cover crop and 100 kg N ha~(-1) than in the treatment of energy cane and elephant grass. The SOC and STN contents at 0–5 and 5–15 cm were 9%–20% greater in the treatments of elephant grass with cover crop and with or without 100 kg N ha~(-1)than in most of the other treatments. The soil NO_3-N content at 0–5 cm was 31%–45% greater in the treatment of energy cane with cover crop and 100 kg N ha~(-1)than in most of the other treatments.The SOC sequestration increased from 0.1 to 1.0 Mg C ha~(-1)year~(-1)and the STN sequestration from 0.03 to 0.11 Mg N ha~(-1)year~(-1)from 2010 to 2014 for various treatments and depths. In contrast, the soil NH_4-N and NO_3-N contents varied among treatments,depths, and years. Soil C and N storages can be enriched and residual NO_3-N content can be reduced by using elephant grass with cover crop and with or without N fertilization at a moderate rate.  相似文献   

11.
Nitrogen acquisition by field pea (Pisum sativum L.) and spring barley (Hordeum vulgare L.) grown on a sandy loam soil and availability of N in three subsequent sequences of a cropping system were studied in an outdoor pot experiment. The effect of crop residues on the N availability was evaluated using 15N-labelled residues. Field pea fixed 75% of its N requirement and the N2 fixation almost balanced the N removed with the seeds. The barley crop recovered 80% of the 15N-labelled fertilizer N supplied and the N in the barley grain corresponded to 80% of the fertilizer N taken up by the crop. The uptake of soil-derived N by a test crop (N catch crop) of white mustard (Sinapis alba L.) grown in the autumn was higher after pea than after barley. The N uptake in the test crop was reduced by 27% and 34% after pea and barley residue incorporation, respectively, probably due to N immobilization. The dry matter production and total N uptake of a spring barley crop following pea or barley, with a period of unplanted soil in the autumn/winter, were significantly higher after pea than after barley. The barley crop following pea and barley recovered 11% of the pea and 8% of the barley residue N. The pea and barley residue N recovered constituted only 2.5% and <1%, respectively, of total N in the N-fertilized barley. The total N uptake in a test crop of mustard grown in the second autumn following pea and barley cultivation was not significantly influenced by pre-precrop and residue treatment. In the short term, the incorporation of crop residues was not important in terms of contributing N to the subsequent crop compared to soil and fertilizer N sources, but residues improved the conservation of soil N in the autumn. In the long-term, crop residues are an important factor in maintaining soil fertility and supplying plant-available N via mineralization.  相似文献   

12.
Some studies suggest that incorporation of catch crop residues leads to increased availability of P to plants. However, little information is available on how this affects P leaching in soils with a high P load. We tested the effect of catch‐crop residue incorporation at the end of winter on the P leaching potential in a soil column experiment under unsaturated conditions using a typical sandy loam soil of NW Europe characterized by a high P load. We sampled the catch crops white mustard (Sinapis alba L.), Italian ryegrass (Lolium multiflorum L.), black oats (Avena strigosa L.) and a perennial ryegrass‐white clover mix (Lolium perenne L.‐Trifolium repens L.) from a field trial on catch crops and soil from the plots where they were grown. Plant biomass was incorporated taking account of the differences in conditions of the plant material at the end of winter and the biomass yield of each catch crop. Incorporation of catch‐crop residues decreased P leaching compared to the fallow treatment probably through immobilization of soil P during catch crop residue decomposition. The exception was black oats, where the leaching of P was the same as for fallow soil. We observed clear differences in C/N, C/P, water soluble and total P concentration, and biodegradability between the tested catch crops, which seemed to affect the P leaching. We conclude that the incorporation of catch crop residues under typical soil and weather conditions and agricultural practices of NW Europe does not increase the potential P leaching losses.  相似文献   

13.
梁斌  赵伟  杨学云  周建斌 《土壤学报》2012,49(4):748-757
以长期不同施肥处理土壤为对象,研究了不同施肥土壤中施用氮肥后土壤氮素含量、微生物固持及释放和作物吸收及利用特性。结果表明,施用氮肥显著增加长期不施肥土壤(NF)矿质氮含量,对长期施用化肥土壤(NPK)和有机无机配施土壤(MNPK)矿质态氮含量无显著影响;施用氮肥对NF中土壤微生物生物量氮(SMBN)含量无显著影响,使拔节期NPK和MNPK中SMBN含量分别增加了4.3倍和0.8倍。从小麦拔节期到开花期,NPK和MNPK中土壤微生物生物量氮含量分别显著降低51%和56%。小麦收获时NPK和MNPK土壤氮肥的利用率分别为36%和45%;而NF土壤所施入的氮素几乎未被小麦吸收利用,但在玉米季有34%被吸收。小麦收获时,NF土壤施入的氮肥有50%以上淋溶至土壤30 cm以下土层,施氮也显著提高了NPK土壤30～50 cm土层硝态氮含量,但施用氮肥对MNPK土壤0～100 cm剖面硝态氮含量无显著影响。说明长期有机无机配施增强了土壤氮素的缓冲能力,协调了土壤氮素固持与作物吸氮间的关系,为提高氮素利用率,减少氮素对环境影响的有效手段。  相似文献   

14.
Catch crops might reduce sulfate leaching and thereby increase the overall sulfur (S)‐use efficiency in crop rotations. At two experimental sites in Denmark (a sandy loam and a coarse sand), S uptake of catch‐crop species was measured. Furthermore, net release of S following incorporation of this material (S contents 0.13%–1.03%, C:S ratios of 40–329, and lignin contents of 1%–10.8%) was investigated in a pot experiment with spring barley in sandy soil. The catch crops showed huge differences in their ability to sequester S. The best catch crops (legumes on sandy loam), sequestered 10–12 kg S ha–1, and the poorest catch crops (ryegrass and sorrel on coarse sand) sequestered less than 3 kg S ha–1. The S‐mineralization rates were highest for crucifers (57%–85% of total S added) and lowest for legumes (up to 46% of total S added). Differences can partly be explained by the C:S ratio, whereas no significant relationship was found with the lignin content of the incorporated catch crops. Catch crops may help to avoid S deficiency and increase synchrony between plant demand and available soil S in a crop rotation. However, the release of S will not fulfil the need of S‐demanding crops and even for cereals, the mineralization will most often only make a contribution. In the case of legume catch crops, it is advisable to use a supplemental S source.  相似文献   

15.
The effects of various measures introduced to increase nitrogen (N)‐use efficiency and reduce N losses to water in a 6‐yr crop rotation (winter wheat, spring barley, green manure, winter wheat, spring barley, spring oilseed rape) were examined with respect to N leaching, soil mineral N (SMN) accumulation and grain yield. An N‐use efficient system (NUE) with delayed tillage until late autumn and spring, direct drilling of winter wheat, earlier sowing of winter and spring crops and use of a catch crop in winter wheat was compared with a conventional system (CON) in a field experiment with six separately tile‐drained plots in south‐western Sweden during the period 1999–2011 (two crop rotation cycles). Total leaching of NO3‐N from the NUE system was significantly 46 and 33% lower than in the CON system during the first and second crop rotation cycle, respectively, with the most pronounced differences apparently related to management strategies for winter wheat. Differences in NO3‐N leaching largely reflected differences in SMN during autumn and winter. There was a tendency for lower yields in the NUE system, probably due to problems with couch grass. Overall, the measures for conserving N, when frequently used within a crop rotation, effectively reduced NO3 concentrations in drainage water and NO3‐N leaching losses, without severely affecting yield.  相似文献   

16.
长期水稻-大麦轮作体系土壤供氮能力与作物需氮量研究   总被引：8，自引：3，他引：8  
通过18年稻麦轮作,7个施肥处理,研究了水稻-大麦轮作系统中土壤生产力、氮素自然供应能力、作物氮素内部利用率及氮肥表观利用率。结果表明,在水旱轮作下,土壤对大麦产量的地力贡献率平均为69%,水稻为75%～81%; 肥料的增产贡献率分别为31%和19%～25%,可维持每年生产大麦2.3 t/hm2、稻谷6～7 t/hm2。土壤氮素自然供给力在大麦上平均为75.9%,比水稻的低3.3%～7.2%。在一年三熟水旱轮作制中,土壤和环境年供氮118～299 kg/hm2; 在一年二熟轮作制中为86～199 kg/hm2。施氮肥条件下,大麦的氮素内部利用率为31.0～56.3 kg/kg; 水稻在23.6～50.2 kg/kg之间变动; 大麦的氮肥利用率变幅在27.5%～41.2%,水稻为14.6%～41.2%。在稻麦轮作系统中,如果想获得作物产量12 t/hm2（4 t大麦和8 t 单季稻）,需要每年施氮肥 N 226～337 kg/hm2。为获得更高的作物产量,在氮肥推荐时不但要考虑作物的目标产量,作物对氮素的需要量,还要充分考虑土壤和环境氮素供应能力。  相似文献   

17.
Abstract. The nitrogen (N) conserving effects of Italian ryegrass ( Lolium multiflorum L.) undersown as a nitrate catch crop in spring barley ( Hordeum vulgare L.) were evaluated over a ten-year period in outdoor lysimeters (1.5 m deep, diam. 1 m) with sandy loam soil. Spring barley grown every year received 11.0 or 16.5 g N m−2 before planting or was kept unfertilized. The N was given either as calcium ammonium nitrate or as ammoniacal N in pig slurry. From 1985 to 1989, ryegrass was undersown in the barley in half of the lysimeters while barley was grown alone in the remaining lysimeters. The grass sward was left uncut after barley harvest and incorporated in late winter/early spring. From 1990 to 1994 all lysimeters were in barley only.
Barley dry matter yields and crop N offtakes were not affected by the presence of undersown ryegrass, although grain yields appeared to be slightly reduced. After termination of ryegrass growing, N offtake in barley (grain+straw) was higher in lysimeters in which catch crops had been grown previously.
The loss of nitrate by leaching increased with N addition rate. Regardless of N dressing, ryegrass catch crops halved the total nitrate loss during 1985–1989, corresponding to a mean annual reduction in nitrate leaching of 2.0–3.5 g N m−2. From 1990 to 1994, lysimeters previously undersown with ryegrass lost more nitrate than lysimeters with no history of ryegrass. The extra loss of nitrate accounted for 30% of the N retained by ryegrass catch crops during 1985–1989.
It is concluded that a substantial proportion of the N saved from leaching by ryegrass catch crops is readily mineralized and available for crop offtake as well as leaching as nitrate. To maximize benefits from ryegrass catch crops, the cropping system must be adjusted to exploit the extra N mineralization derived from the turnover of N incorporated in ryegrass biomass.  相似文献   

18.
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.  相似文献   

19.
The aim of this investigation was to prepare and evaluate organic manures (vermicompost, compost and FYM) and mineral fertilizers on crop productivity and changes in soil organic carbon (SOC) and fertility under a four-year-old maize-wheat cropping system. The results demonstrated that yields and nutrient uptake by crops increased significantly in plots receiving manures and mineral fertilizers either alone or in combination than unfertilized control. Application of manures and fertilizers also enhanced SOC, mineral N, Olsen-P and ammonium acetate-extractable K (NH4OAc-K) after both the crops. Surface soil maintained greater build-up in SOC, mineral N, Olsen-P and NH4OAc-K than sub-surface soil. Plots amended with manures at 5 t ha?1 and 50% recommended dose of fertilizer (RDF) had pronounced impact on improving SOC and fertility after both the crops indicating that integrated use of manures and mineral fertilizers could be followed to improve and maintain soil fertility, increase crop productivity under intensive cropping system.  相似文献   

20.
The effects of soil incorporation with cereal straw (nil, 2.5, 5 and 10 t straw ha?1) and direct drilling on the proportion and amount of pea N derived from biological N fixation were investigated in three field experiments. Fixed N was determined by15N dilution using barley as a reference plant. The three sites were on acidic, red clay-loams in the cropping zone of southeastern Australia. Seasonal plant available soil N, as determined by the N accumulated in barley, was 31, 56 and 158 kg N ha?1, for the three sites. Incorporated straw reduced soil nitrate at sowing by 10–50 kg N ha?1 (0–30 cm), and 5 or 10 t straw ha?1 reduced barley uptake of N by 10–38 kg N ha?1. However, reducing plant available soil N was generally ineffective for increasing the N fixed by pea. Fixed N increased only at the site with the least plant-available N, and only one-third of the increase could be attributed to lower soil N uptake by pea. There was no evidence that direct drilling pea increased fixed N by decreasing crop uptake of soil N. It is proposed that a lower requirement for soil N by pea as compared to barley, and availability of mineral N beneath the soil layer treated with straw, minimise the effectiveness of straw incorporation for increasing the N fixed by pea.  相似文献   

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