Depth dynamics of soil N contents and natural abundances of 15N after 43 years of long-term fertilization and liming in sub-tropical Alfisol

The aim of this study was to understand impacts of long-term (43 years) fertilization on soil aggregation, N accumulation rates and δ¹⁵N in surface and deep layers in an Alfisol. Soil samples from seven treatments were analysed for N stocks, aggregate-associated N in 0–30 cm and the changes in δ¹⁵N in 0–90 cm depths. The treatments were: unfertilized control (control); recommended N dose (N); recommended N and phosphorus doses (NP); recommended N, P and potassium doses (NPK); 150% of recommended N, P and K doses (150% NPK); NPK + 10 Mg FYM ha^?1 (NPK + FYM) and NPK + 0.4 Mg lime ha^?1 (NPK + L). Results revealed that plots under NPK + FYM had ~39% higher total N concentrations than NPK + L in 0–30 cm soil layers. In NPK + L, macro-aggregates had 35 and 11% and microaggregates had 20 and 9% lower δ¹⁵N values than NPK + FYM in 0–15 and 15–30 cm soil layers, respectively. However, plots receiving NPK + FYM had ~39% greater deep soil (30–90 cm) N accumulation than NPK + L. These results would help understanding N supplying capacity by long-term fertilization and assist devising N management strategies in sub-tropical acidic Alfisols.     

Wheat Grain Nitrogen Uptake,as Affected by Soil Total and Mineral Nitrogen,for the Determination of Optimum Nitrogen Fertilizer Rates for Wheat Production

Determination of appropriate nitrogen (N) fertilization for wheat (Triticum aestivum L.) production with respect to the available resources can result in the enhanced efficiency of agricultural systems and ecosystem health. Hence, a 3-year field experiment was conducted to determine (1) the effects of soil total N and soil mineral N (including nitrate, NO₃-N, and ammonium, NH₄-N) measured at seeding and postseeding for wet and dry soil samples at 0- to-30 cm and 0- to 60-cm depths on wheat grain N uptake and (2) the regression equations that can best explain the variation in wheat grain N uptake by N fertilizer and soil total and mineral N. Determination of wheat grain N uptake as affected by soil NO₃-N in areas with reasonable amounts of organic matter can also be used as a very useful tool for determination of appropriate N fertilization, which is of great agricultural and environmental implications.     

Tillage and N application effects on crop yield,N uptake and soil properties in a corn-based rotation

The objective of this study was to determine the influence of tillage methods (conventional tillage (CT) and minimum tillage (MT)) and N rates (0, 50, 150, 250 kg N ha^?1) on crop yield, N uptake and soil organic carbon (SOC), bulk density (BD), total N (TN), electrical conductivity (EC), pH and soil nutrient contents on a clay-loam near Hashtgerd, Iran. A successive corn-based rotation (2012–2014) was conducted as a split-plot in a randomized complete block design in which tillage methods were considered as main plots, and N rates as subplots. Tillage had no significant effect on corn 2012 and canola 2012–2013 grain yields. CT and MT systems showed different critical N rates to reach their maximum grain yield in corn (2013) and wheat (2013–2014). MT system required more N application to reach its maximum grain yield. Tillage × N rate effect on none of the soil properties was significant. Tillage had no significant (P ≤ 0.05) effect on soil pH, BD, TN and SOC. However, soil EC of 0–5 cm depth in MT system was higher than CT system by 64%. MT system under higher N application could increase corn grain yield, but on the other hand probably adversely changes soil chemical properties.     

氮肥减施对稻-麦轮作体系作物氮素吸收、利用和土壤氮素平衡的影响

田间试验研究了稻-麦轮作体系中减施氮肥对作物氮素吸收、利用和土壤氮素平衡的影响。结果表明,与当地习惯施肥(小麦:N 225 kg/hm2,基肥与分蘖肥各半;水稻:N 210 kg/hm2,基肥和分蘖肥为3∶2)相比,减氮20%～30%处理产量并没有降低,而氮肥当季利用率、氮素农学利用率以及氮素偏因子生产力则有所增加;而且,氮肥分次追施,能增加子粒产量,并减少氮肥成本。虽然减氮20%～30%处理0—40 cm土层无机氮含量较习惯施肥处理降低,但是并没有降低植株地上部对氮素的吸收。在小麦和水稻收获期,减施氮肥处理0—100 cm土壤无机氮残留量低于习惯施肥处理;且稻-麦轮作系统中氮的表观损失主要发生在水稻季。初步认为,在长江中下游平原稻-麦轮作体系氮素过量施用地区,第一个轮作周期减施氮肥20%～30%不仅不影响产量,而且可提高氮素利用率,有利于保护环境。     

Impact of mineral and organic fertilization on yield,C content in the soil,as well as on C,N and energy balances in a long-term field experiment

Data from a 49-year-long organic–mineral fertilization field experiment with a potato–maize–maize–wheat–wheat crop rotation were used to analyse the impact of different fertilizer variations on yield ability, soil organic carbon content (SOC), N and C balances, as well as on some characteristic energy balance parameters. Among the treatments, the fertilization variant with 87 kg ha^?1 year^?1 N proved to be economically optimal (94% of the maximum). Approximately 40 years after initiation of the experiment, supposed steady-state SOC content has been reached, with a value of 0.81% in the upper soil layer of the unfertilized control plot. Farmyard manure (FYM) treatments resulted in 10% higher SOC content compared with equivalent NPK fertilizer doses. The best C balances were obtained with exclusive mineral fertilization variants (?3.8 and ?3.7 t ha^?1 year^?1, respectively). N uptake in the unfertilized control plot suggested an airborne N input of 48 kg ha^?1 year^?1. The optimum fertilizer variant (70 t ha^?1 FYM-equivalent NPK) proved favourable with a view to energy. The energy gain by exclusive FYM treatments was lower than with sole NPK fertilization. Best energy intensity values were obtained with lower mineral fertilization and FYM variants. The order of energy conversion according to the different crops was maize, wheat and potato.     

Influence of Long-term Tillage,Straw, and N Fertilizer Management on Crop Yield,N Uptake,and N Balance Sheet in Two Contrasting Soil Types

Field experiments (established in autumn 1979, with monoculture barley from 1980 to 1990 and barley/wheat–canola–triticale–pea rotation from 1991 to 2008) were conducted on two contrasting soil types (Gray Luvisol [Typic Haplocryalf] loam soil at Breton; Black Chernozem [Albic Agricryoll] silty clay loam soil at Ellerslie) in north-central Alberta, Canada, to determine the influence of tillage (zero tillage and conventional tillage), straw management (straw removed [S_Rem] and straw retained [S_Ret]), and N fertilizer rate (0, 50 and 100 kg N ha^?1in S_Ret, and only 0 kg N ha^?1in S_Rem plots) on seed yield, straw yield, total N uptake in seed + straw (1991–2008), and N balance sheet (1980–2008). The N fertilizer urea was midrow-banded under both tillage systems in the 1991 to 2008 period. There was a considerable increase in seed yield, straw yield, and total N uptake in seed + straw with increasing N rate up to 100 kg N ha^?1 under both tillage systems. On the average, conventional tillage produced greater seed yield (by 279 kg ha^?1), straw yield (by 252 kg ha^?1), and total N uptake in seed + straw (by 6.0 kg N ha^?1) than zero tillage, but the differences were greater at Breton than Ellerslie. Compared to straw removal treatment, seed yield, straw yield, and total N uptake in seed + straw tended to be greater with straw retained at the zero-N rate used in the study. The amounts of applied N unaccounted for over the 1980 to 2008 period ranged from 1114 to 1846 kg N ha^?1 at Breton and 845 to 1665 kg N ha^?1 at Ellerslie, suggesting a great potential for N loss from the soil-plant system through denitrification, and N immobilization from the soil mineral N pool. In conclusion, crop yield and N uptake were lower under zero tillage than conventional, and long-term retention of straw suggests some gradual improvement in soil productivity.     

Different rates of biochar application change 15N retention in soil and 15N utilization by maize

Biochar application to soil may impact soil nitrogen (N) dynamics, but the effects on N uptake and utilization by crop remain largely unknown, especially the effects of the rate of biochar application. To investigate the effects of biochar on soil ¹⁵N retention rate and ¹⁵N utilization efficiency (¹⁵NUE) by maize, a six-month ¹⁵N isotope tracer technique combined with in situ pot experiment was conducted in Mollisol. The experiment included four treatments: no biochar applied (CK) and biochar applied at the rates of 12 t ha⁻¹ (P12), 24 t ha⁻¹ (P24) and 48 t ha⁻¹ soil (P48). Compared with CK, biochar application reduced soil bulk density and ¹⁵N loss rate, and significantly improved total N and ¹⁵N retention amount in the 0–30 cm soil depth. The P24 treatment had the largest increase in ¹⁵N retention rate throughout the 0–40 cm depth. After biochar application, the ¹⁵N uptake and ¹⁵NUE were significantly increased in the grain and leaf, which promoted grain yields. Contrary to this, the P48 treatment appeared to lower ¹⁵N uptake and ¹⁵NUE compared with P12 and P24. In conclusion, biochar application improves the potential of the soil to retain N and the improvement in ¹⁵N uptake and utilization are more pronounced in maize leaves and grain. Moreover, biochar application promotes ¹⁵N utilization in maize plant and improves maize yield. However, when biochar application rate is high (i.e. P48 treatment), the ¹⁵N retention by the soil and ¹⁵N utilization by the maize are reduced markedly compared with P12 and P24.     

Fate of residual 15N-labeled fertilizer in dryland farming systems on soils of contrasting fertility

Abstract

Up to 50% of nitrogen (N) fertilizer can remain in soil after crop harvest in dryland farming. Understanding the fate of this residual fertilizer N in soil is important for evaluating its overall use efficiency and environmental effect. Nitrogen-15 (¹⁵N)-labeled urea (165 kg N ha^?1) was applied to winter wheat (Triticum aestivum L.) growing in three different fertilized soils (no fertilizer, No-F; inorganic nitrogen, phosphorus and potassium fertilization, NPK; and manure plus inorganic NPK fertilization, MNPK) from a long-term trial (19 years) on the south of the Loess Plateau, China. The fate of residual fertilizer N in soils over summer fallow and the second winter wheat growing season was examined. The amount of the residual fertilizer N was highest in the No-F soil (116 kg ha^?1), and next was NPK soil (60 kg ha^?1), then the MNPK soil (43 kg ha^?1) after the first winter wheat harvest. The residual fertilizer N in the No-F soil was mainly in mineral form (43% of the residual ¹⁵N), and for the NPK and MNPK soils, it was mainly in organic form. The loss rate of residual ¹⁵N in No-F soil over summer fallow was as high as 48%, and significantly (P < 0.05) higher than that in the NPK soil (22%) and MNPK soil (19%). The residual ¹⁵N use efficiency (RNUE) by the second winter wheat was 13% in the No-F soil, 6% in the NPK soil and 8% in the MNPK soil. These were equivalent to 9.0, 2.0 and 2.2% of applied ¹⁵N. The total ¹⁵N recovery (¹⁵N uptake by crops and residual in 0–100 cm soil layer) in the MNPK and NPK soils (84.5% and 86.6%, respectively) were both significantly higher than that in the No-F soil (59%) after two growing seasons. The ¹⁵N uptake by wheat in two growing seasons was higher in the MNPK soil than in NPK soil. Therefore, we conclude that a high proportion of the residual ¹⁵N was lost during the summer fallow under different land management in dryland farming, and that long-term combined application of manure with inorganic fertilizer could increase the fertilizer N uptake and decrease N loss.     

Leaf litter C and N cycling from a deciduous permanent crop

Our understanding of leaf litter carbon (C) and nitrogen (N) cycling and its effects on N management of deciduous permanent crops is limited. In a 30-day laboratory incubation, we compared soil respiration and changes in mineral N [ammonium (NH₄⁺-N) + nitrate (NO₃^–-N)], microbial biomass nitrogen (MBN), total organic carbon (TOC) and total non-extractable organic nitrogen (TON) between a control soil at ¹⁵N natural abundance (δ¹⁵N = 1.08‰) without leaf litter and a treatment with the same soil, but with almond (Prunus dulcis (Mill.) D.A. Webb) leaf litter that was also enriched in ¹⁵N (δ¹⁵N = 213‰). Furthermore, a two-end member isotope mixing model was used to identify the source of N in mineral N, MBN and TON pools as either soil or leaf litter. Over 30 d, control and treatment TOC pools decreased while the TON pool increased for the treatment and decreased for the control. Greater soil respiration and significantly lower (p < 0.05) mineral N from 3 to 15 d and significantly greater MBN from 10 to 30 d were observed for the treatment compared to the control. After 30 d, soil-sourced mineral N was significantly greater for the treatment compared to the control. Combined mineral N and MBN pools derived from leaf litter followed a positive linear trend (R² = 0.75) at a rate of 1.39 μg N g^?1 soil day^?1. These results suggest early-stage decomposition of leaf litter leads to N immobilization followed by greater N mineralization during later stages of decomposition. Direct observations of leaf litter C and N cycling assists with quantifying soil N retention and availability in orchard N budgets.     

Impact of top-pruning time on the fertilizer N use efficiency of flue-cured tobacco as assessed by 15N tracing technique

To evaluate the impact of top-pruning time on fertilizer N use efficiency (NUE) of flue-cured tobacco, we adopted ¹⁵N tracing technique and conducted a 2-year experiment from 2014 to 2015 in eastern China. The experiment included three top-pruning points of time: 5th, 25th and 45th day after flowering (DAF), abbreviated, respectively, as TP5, TP25 and TP45. The amounts of plant N derived from fertilizer (Ndff) and soil (Ndfs) were observed during 0–55th DAF. Results showed that top-pruning slowed down the increase of Ndff in tobacco organs, particularly in the leaf, but accelerated the increase of Ndfs dramatically. The proportion of Ndff (%) accounted for the total N reduced dramatically after top-pruning. This reduction might attribute to the selectivity of plant to different N sources as influenced by top-pruning while had little relationship with soil N supply, according to the analysis on the soil total mineral N and mineral ¹⁵N. The average NUE for the 2 years was 32%, 41% and 47%, respectively, for TP5, TP25 and TP45, showing significant (p < 0.05) differences. We concluded that the tobacco preferred to uptake soil N rather than fertilizer N after top-pruning; thus, optimizing the top-pruning time might be one of the approaches to improve the in-season NUE of flue-cured tobacco.     

Assessment of nitrogen uptake and utilization in drought tolerant and Striga resistant tropical maize varieties

Maize (Zea mays L.) is an important food crop in the Guinea savannas of Nigeria. Despite its high production potential, drought, Striga hermonthica parasitsim, and poor soil fertility particularly nitrogen deficiency limit maize production in the savannas. Breeders at IITA have developed drought- and Striga-tolerant cultivars for testing, dissemination, and deployment in the region. Information on the response of these cultivars to N fertilization is, however, not available. This study evaluated grain yield, total N uptake (TNU), N uptake (NUPE), N utilization (NUTE), and N use efficiency (NUE) of selected maize cultivars along with a widely grown improved maize cultivar at two locations in the Guinea savannas of northern Nigeria. Maize grain yield increased with N application. The average grain yield of the maize cultivars was 76% higher at 30, 156% higher at 60, and 203% higher at 120 kg N ha^?1 than at 0 kg N ha^?1. This suggests that N is a limiting nutrient in the Nigerian savannas. Five drought-tolerant cultivars produced consistently higher yields when N was added at all levels. These cultivars had either high NUPE or NUTE confirming earlier reports that high N uptake or NUTE improves maize grain yield. The study also confirms earlier reports that maize cultivars that are selected for tolerance to drought are also efficient in uptake and use of N fertilizer. This means that these cultivars can be grown with application of less N fertilizer thereby reducing investment on fertilizers and reduction in environmental pollution.     

Effect of Fertilizer Nitrogen (N) on Soil Organic Carbon,Total N,and Soil pH in Long-Term Continuous Winter Wheat (Triticum Aestivum L.)

Carbon sequestration via sound agronomic practices can assist in combating global warming. Three long-term experiments (Experiment 502, Experiment 222, and The Magruder Plots) were used to evaluate the effect of fertilizer nitrogen (N) application on soil organic carbon (SOC), total nitrogen (TN), and pH in continuous winter wheat. Soil samples (0–15 cm) were obtained after harvest in 2014, analyzed, and compared to soil test results from these experiments in 1993. Soil pH decreased with increasing N fertilization, and more so at high rates. Nitrogen application significantly increased TN in Experiment 502 from 1993 to 2014, and TN tended to be high at high N rates. Fertilizer N significantly increased SOC, especially when N rates exceeded 90 kg ha^?1. The highest SOC (13.1 g kg^?1) occurred when 134 kg N ha^?1 was applied annually. Long-term N application at high rates increased TN and SOC in the surface soil.     

Increased Dryland Winter Wheat Yields by Nitrogen Fertilizer Topdressing and its Relationship to Soil Moisture,Available N,P and K in Northwestern China

Winter wheat (Triticum aestivum L.) production in northwestern China as a monoculture is hampered by unfertile soil and drought. With the fast-developing Chinese chemical fertilizer industry, many farmers now use more nitrogen (N) fertilizer as topdressing for winter wheat in early spring, in addition to a basal dose of N fertilizer applied in the previous autumn at seeding time. The objective of this study was to evaluate the increase in grain yield of dryland winter wheat by early spring N fertilizer topdressing, and its relationship to soil moisture, available N, phosphorus (P) and potassium (K). Field experiments with no N fertilizer topdressing (F_b) and N fertilizer topdressing (F_b+t) treatments were carried out over two growing seasons at 54 site-years to assess the relationship between increase in winter wheat grain yield by early spring N fertilizer topdressing and soil moisture, available N, P and K in Changwu county, Shaanxi province, China. Compared to F_b treatment, the F_b+t treatment produced grain yields lower at 10 site-years, and increased by <10% at 21 site-years and by >10% at 23 site-years. The results indicated that topdressing N fertilizer could increase wheat grain yield when soil nitrate-N accumulation in the 0–20, 20–40 and 40–60 cm depths was less than 121.7, 36.4 and 24.1 kg N ha^?1, and soil moisture content in the 40–60, 60–80 and 80–100 cm depths was more than 15.7%, 16.7% and 16.9%, respectively. The findings also suggested that it is not necessary to analyze soil for ammonium-N, available P and K before topdressing N fertilizer. It is necessary to analyze 0–60 cm soil profile for nitrate-N and 40–100 cm depth for soil moisture before topdressing N fertilizer for winter wheat in dryland areas of northwestern China.     

Development of Soil N Testing for Wheat Production using Soil Residual Mineral N

The effects of soil residual nitrate (NO₃-N) and ammonium (NH₄-4) on wheat (Triticum aestivum L.) grain yield and N uptake were evaluated in a three-year experiment from 1991 to 1993, in the province of Quebec, Canada. Soil samples were collected at harvest from the depth of 0-30 and 30–60 cm from 20 wheat sites and analyzed in the laboratory for NO₃-N and NH₄-N. Wheat yield and N-uptake was also determined. Using statistical analysis system (SAS) and Matlab, data were analyzed and the best-fitted regression equations were plotted. Significant amounts of soil NO₃-N was found in the soil ranging from 31.6 to 61.6 kg/ha. The amount of soil NH₄-N varied from 9.2 to 23.3 kg/ha. The statistical effects of model parameters indicated that soil NO₃-N and soil NH₄-N significantly influenced wheat yield and N uptake. It may be possible to investigate wheat behavior at harvest using soil residual NO₃-N and NH₄-N.     

Assessment of urea coated with pomegranate fruit powder as N slow-release fertilizer in maize

It has been hypothesized that plant polyphenol plus lignin, cellulose, and hemicelluloses can immobilize/remineralize inorganic fertilizers, such as urea, better than polyphenol alone. To test this hypothesis, urea was coated with pomegranate (Punica granatum L.) fruit powder (containing polyphenol+lignin+cellulose) at rates of 0%, 30%, 70%, and 100% (w/w) of fertilizer urea. Fertilizer nitrogen was applied at 100 and 200 mg kg^?1 soil in a clay loam soil. This natural coating material, particularly at the rate of 100% (w/w), improved the distribution of mineral nitrogen (N) and available phosphorus (P) in soil during both early and advanced growth stages of maize and significantly increased total crop N and P uptake at both rates of fertilizer nitrogen compared to urea alone. The results suggest that urea coated with the powder of pomegranate fruit could potentially be an N slow-release fertilizer for use in better synchronizing crop N demand with soil N supply.     

Changes in the soil C and N contents,C decomposition and N mineralization potentials in a rice paddy after long-term application of inorganic fertilizers and organic matter

A long-term experiment on combined inorganic fertilizers and organic matter in paddy rice (Oryza sativa L.) cultivation began in May 1982 in Yamagata, northeastern Japan. In 2012, after the 31^st harvest, soil samples were collected from five fertilizer treatments [(1) PK, (2) NPK, (3) NPK + 6 Mg ha^?1 rice straw (RS), (4) NPK + 10 Mg ha^?1 rice straw compost (CM1), and (5) NPK + 30 Mg ha^?1 rice straw compost (CM3)], at five soil depths (0–5, 5–10, 10–15, 15–20 and 20–25 cm), to assess the changes in soil organic carbon (SOC) content and carbon (C) decomposition potential, total nitrogen (TN) content and nitrogen (N) mineralization potential resulting from long-term organic matter addition. The C decomposition potential was assessed based on the methane (CH₄) and carbon dioxide (CO₂) produced, while the N mineralization potential was determined from the potassium chloride (KCl)-extractable ammonium-nitrogen (NH₄⁺-N), after 2, 4, 6 and 8 weeks of anaerobic incubation at 30°C in the laboratory. Compared to NPK treatment, SOC in the total 0–25 cm layer increased by 67.3, 21.0 and10.8%, and TN increased by 64.2, 19.7 and 10.6%, in CM3, RS and CM1, respectively, and SOC and TN showed a slight reduction in the PK treatment by 5.2 and 5.7%, respectively. Applying rice straw compost (10 Mg ha^?1) instead of rice straw (6 Mg ha^?1) to rice paddies reduced methane production by about 19% after the soils were measured under 8 weeks of anaerobic incubation at 30°C. Soil carbon decomposition potential (Co) and nitrogen mineralization potential (No) were highly correlated with the SOC and TN contents. The mean ratio of Co/No was 4.49, lower than the mean ratio of SOC/TN (13.49) for all treatments, which indicated that the easily decomposed organic matter was from soil microbial biomass and soil proteins.     

On‐farm comparison of variable rates of nitrogen with uniform application to maize on canopy reflectance,soil nitrate,and grain yield

Recent development in canopy optical‐sensing technology provides the opportunity to apply fertilizer variably at the field scale according to spatial variation in plant growth. A field experiment was conducted in Ottawa, Canada, for two consecutive years to determine the effect of fertilizer nitrogen (N) input at variable‐ vs. uniform‐application strategies at the V6–V8 growth stage, on soil mineral N, canopy reflectance, and grain yield of maize (Zea mays L.). The variable N rates were calculated using an algorithm derived from readings of average normalized difference vegetation index (NDVI) of about 0.8 m × 4.6 m, and N fertilizer was then applied to individual patches of the same size of NDVI readings (0.8 m × 4.6 m) within a plot (2184 m²). Canopy reflectance, expressed as NDVI, was monitored with a hand‐held spectrometer, twice weekly before tasseling and once a week thereafter until physiological maturity. Soil mineral N (0–30 cm depth) was analyzed at the V6 and VT growth stages. Our data show that both variable and uniform‐application strategies for N side‐dressings based on canopy‐reflectance mapping data required less amount of N fertilizer (with an average rate of 80 kg N ha^–1 as side‐dressing in addition to 30 kg N ha^–1 applied at planting), and produced grain yields similar to and higher nitrogen‐use efficiency (NUE) than the preplant fully fertilized (180 kg N ha^–1) treatment. No difference was observed in either grain yield or NUE between the variable‐ and uniform‐application strategies. Compared to unfertilized or fully fertilized treatments, the enhancements in grain yield and NUE of the variable‐rate strategy originated from the later N input as side‐dressing rather than the variation in N rates. The variable‐rate strategy resulted in less spatial variations in soil mineral N at the VT growth stage and greater spatial variations in grain yield at harvest than the uniform‐rate strategy. Both variable‐ and uniform‐application strategies reduced spatial variations in soil mineral N at the VT stage and grain yield compared to the unfertilized treatment. The variable‐rate strategy resulted in more sampling points with high soil mineral N than the uniform‐rate strategy at the VT stage.     

Effects of Liming and Fertilization (N,PK) on Chemistry and Nitrogen Turnover in Acidic Forest Soils in SW Sweden

SW Sweden has very acidic forest soils because of deposition ofair-borne pollutants. Large-scale liming and fertilization have been proposed as countermeasures against a possible future development of forest decline. To test the effects of suggested treatments, liming (3 or 6 t ha¹) and fertilization with easily soluble PK (25 or 50 kg P, 80 or 160 kg K ha¹) or N(20 kg N ha¹ annually in the form of NH₄ NO₃) were applied in different combinations in four experiments in 30–60 yr-old Picea abies forests in SW Sweden. Four yearsafter the initial application of the fertilizers, samples were taken from the O-horizon and the two uppermost 5 cm thick layersof the mineral soil. Their pH(H₂O) and easily extractable Ca, Mg, K, P and inorganic N contents were analyzed. Samples werealso incubated to estimate net N mineralization and potential nitrification rates. Liming increased the pH by 0.6–1 unit in the O-horizon, and by 0.1 unit in the mineral soil. The Ca + Mg content increased by 15–25 kmol_c ha¹ (4–8 foldincrease) in the O-horizon of the limed plots, while an increaseof 5 kmol_c ha¹ (two-fold increase) was observed in theuppermost 5 cm of the mineral soil. Liming did not affect extractable P, K or inorganic N contents. Net N mineralization and potential nitrification rates in the O-horizon were enhanced 1.5- and 6-fold, respectively, by liming, but it had no apparenteffect in the mineral soil. N fertilization caused a slight increase (1.5 kg ha¹) in the content of inorganic N, buthad no effects on the other variables measured. The amount ofextractable P was raised by 16 kg ha¹ in plots given the high P dose (50 kg ha¹), but no other effects of PK fertilization were detected.     

施氮量对冬小麦氮素吸收、转运及产量的影响

2004至2005年在田间条件下,研究了施氮量0、105、2103、15.kg/hm2对冬小麦氮素吸收、累积、转运、产量及氮肥利用率的影响。结果表明,施用氮肥可显著提高冬小麦的子粒、秸秆产量及成熟期地上部总吸氮量,但过量施用氮肥对子粒和秸秆增产不显著;各施氮处理的氮肥利用率在34.2%～38.3%之间,随施氮量增加而略有降低。植株中氮素含量随生育期的延长而降低,氮素累积量总体呈增加趋势。施氮量对冬小麦氮素吸收有显著影响,同一生育时期,氮素含量和累积量都随着施氮量增加而提高。施氮可显著地促进氮素在子粒中累积,其中69%～87%的氮素是靠营养体的转运而来的。施氮量影响氮素的转运效率,随施氮量增加,转运效率降低。本试验条件下,冬小麦的合理施氮量应控制在105～210.kg/hm2之间。     

Estimation of Plant‐Available Nitrogen in Soils using Rapid Chemical and Biological Methods

The relationships between potential laboratory indices for plant‐available nitrogen (N) and the plant N uptake in a pot experiment with ryegrass were assessed for 13 mineral soils and 2 peat soils. The methods included aerobic soil incubation, soil incubation in a bioreactor, hot potassium chloride (KCl)–extractable mineral N, 0.01 M calcium chloride (CaCl₂)–extractable N, and N loss at heating. The indices for total plant‐available N accounted for 63–93% of the variance in N uptake in a statistical analysis with all soils (n = 15) and 27–89% for the mineral soils (n = 13). Most indices were not a direct quantitative measure of the plant N uptake. The N mineralization indices accounted for 57–86% of the variance in N mineralization for all soils and 5–50% for the mineral soils. Hot KCl‐extractable mineral N and 0.01 M CaCl₂–extractable N were the most promising rapid indices for plant‐available N.