Asymbiotic biological nitrogen fixation in a temperate grassland as affected by management practices

Estimates of asymbiotic biological N fixation (BNF) in temperate grasslands are few with large variations. In the past six decades, European grasslands have been subjected to intensive management practices and presently it is not known how asymbiotic BNF is influenced by these practices. Our objective was to assess the impact of fertilizer application and mowing frequency on asymbiotic BNF in a Central European grassland. In 2008, we established a three-factorial experiment with two fertilizer treatments (no fertilizer application and combined nitrogen (N), phosphorus (P) and potassium (K) fertilization at 180–30–100 kg ha⁻¹ yr⁻¹), two mowing frequencies (cut once and thrice per year) and three sward compositions through the application of herbicides (control, monocot- and dicot-enhanced swards). Three years after the initial sward manipulation, there was no more difference in functional group composition. Between June 2011 and May 2012, we measured in-situ asymbiotic BNF using the acetylene reduction assay, calibrated with ¹⁵N₂-fixation method. Across treatments, asymbiotic BNF rates in the 0–5-cm soil depth ranged from 1.7 (±0.2 SE) kg ha⁻¹ yr⁻¹ for fertilized plots cut once a year to 5.7 (±2.3 SE) kg ha⁻¹ yr⁻¹ for unfertilized plots cut thrice a year. Fertilization decreased asymbiotic BNF, suggesting that the potential positive effect of increased soil P levels might have been overruled by the negative effect of increased soil mineral N levels. Intensive mowing stimulated asymbiotic BNF, which was probably due to an increase in rhizodeposition. Our calibration of the acetylene reduction assay with the ¹⁵N₂-fixation method resulted in a conversion factor of 0.61, which largely deviates from the theoretical conversion factor of 3. Furthermore, laboratory incubations under increased soil moisture and temperature conditions overestimated BNF rates compared to in-situ measurements. Thus, laboratory measurements with altered soil moisture, temperature or disturbed soil may lead to strong biases in estimates of asymbiotic BNF. Our results suggest that input of N through BNF may be considerable in temperate grasslands. We conclude that BNF studies should be conducted in-situ and that the acetylene reduction assay should be calibrated against ¹⁵N₂-fixation calibration for reliable estimates.     

Added nitrogen interaction as affected by soil nitrogen pool size and fertilization – significance of displacement of fixed ammonium

Displacement of NH₄⁺ fixed in clay minerals by fertilizer ¹⁵NH₄⁺ is seen as one mechanism of apparent added nitrogen interactions (ANI), which may cause errors in ¹⁵N tracer studies. Pot and incubation experiments were carried out for a study of displacement of fixed NH₄⁺ by ¹⁵N‐labeled fertilizer (ammonium sulfate and urea). A typical ANI was observed when ¹⁵N‐labeled urea was applied to wheat grown on soils with different N reserves that resulted from their long‐term fertilization history: Plants took up more soil N when receiving fertilizer. Furthermore, an increased uptake of ¹⁵N‐labeled fertilizer, induced by increasing unlabeled soil nitrogen supply, was found. This ANI‐like effect was in the same order of magnitude as the observed ANI. All causes of apparent or real ANI can be excluded as explanation for this effect. Plant N uptake‐related processes beyond current concepts of ANI may be responsible. NH₄⁺ fixation of fertilizer ¹⁵NH₄⁺ in sterilized or non‐sterile, moist soil was immediate and strongly dependent on the rate of fertilizer added. But for the tested range of 20 to 160 mg ¹⁵NH₄⁺‐N kg^–1, the NH₄⁺ fixation rate was low, accounting for only up to 1.3 % of fertilizer N added. For sterilized soil, no re‐mobilization of fixed ¹⁵NH₄⁺ was observed, while in non‐sterile, biologically active soil, 50 % of the initially fixed ¹⁵NH₄⁺ was released up to day 35. Re‐mobilization of ¹⁵NH₄⁺ from the pool of fixed NH₄⁺ started after complete nitrification of all extractable NH₄⁺. Our results indicate that in most cases, experimental error from apparent ANI caused by displacement of fixed NH₄⁺ in clay is unlikely. In addition to the low percentage of only 1.3 % of applied ¹⁵N, present in the pool of fixed NH₄⁺ after 35 days, there were no indications for a real exchange (displacement) of fixed NH₄⁺ by ¹⁵N.     

Nutrient composition of leaves and fruit juice of grapevine as affected by soil and nitrogen fertilization

Effects of soil type and nitrogen (N) fertilizer–application rates on the nutrient composition of grapevine (Vitis vinifera L. cv. Riesling) leaves during a growing cycle were compared with the composition of the resulting grape juice. Grapevines were planted in 75 L containers that had been installed in a vineyard and filled with three different vineyard soils (loess, shell lime, and Keuper). Four typical levels of N fertilizer (40, 80, 120, and 160 kg N ha^–1) were applied. Elemental composition of mature leaves sampled seven times during the growing cycle as well as of the extracted grape juice was analyzed. The time of sampling affected all measured elements (C, N, Ca, K, P, Mg, S, Fe, Zn, Mn, and B) in the leaves. Nitrogen‐fertilizer rate affected the concentrations of all elements except Ca and Mg, while the soil type had significant effects on elemental composition of the leaves with the exception of N, B, and Ca. Soil type had a significant effect on K, S, Mn, and B in the grape juice. Increasing rates of N fertilizer increased C concentration in the grape juice significantly and affected its elemental composition similar to the effects in leaves. This may be explained with the role of leaves as the source for supplying the grapes during ripening via phloem transport. Cluster analysis for the elemental composition of soils, leaves, and grape juice revealed no consistent relationships indicating that other soil characteristics in addition to the mineral concentration influence the elemental composition of grapevine leaves and grape juice.     

不同施氮水平下土壤的生化性质对干湿交替的响应

以中国科学院封丘农业生态试验站水氮耦合长期试验地为研究平台,采集五个施氮水平(施氮0、150、190、230、270 kg hm-2)下表层0 ～ 20 cm土壤并测定其土壤肥力参数(土壤pH、全氮、全磷、全钾、碱解氮、速效磷、速效钾、有机碳).结果表明:施氮降低了土壤pH、速效磷、全钾,增加了全氮、碱解氮、有机碳,除有机碳随施氮水平的增加而增加外,其他肥力参数并未随施氮水平的增加而发生规律性变化.土壤经过0、3、6、10次干湿交替,培养60 d后测定其生物和化学性质(土壤铵态氮、硝态氮、溶解性有机碳、脲酶活性、脱氢酶活性、微生物生物量碳、土壤基本呼吸).双因素方差分析显示干湿交替次数对铵态氮、硝态氮、无机氮、溶解性有机碳、脱氢酶活性、微生物生物量碳和土壤基本呼吸均有极显著作用,而干湿交替次数和施氮水平对除脱氢酶活性以外的其他土壤性质均无交互作用.五个施氮水平下土壤硝态氮、无机氮、溶解性有机碳、脲酶活性、脱氢酶活性和微生物生物量碳均随干湿交替次数增加而增加,土壤基本呼吸随干湿交替次数增加而降低.高施氮水平(施氮超过190 kg hm-2)下土壤性质的变异系数更小并能更好地发生聚类.研究表明当土壤遭遇干湿交替时,高施氮水平下土壤更能维持其生化性质的稳定.     

冬小麦-夏玉米轮作体系中不同施氮水平对玉米生长及其根际土壤氮的影响

在河北衡水潮土上进行田间试验,以当地习惯高氮用量(小麦季施N 300 kg/hm2,玉米季施N 240 kg/hm2)为对照,研究冬小麦-夏玉米轮作体系中减少氮肥用量对玉米季植株生长、氮素吸收及根际土壤中无机氮与微生物量氮的影响。结果表明,两季作物氮肥施用量减少25%和40%,对玉米产量、生物量及植株体内氮累积量未产生明显影响,氮肥利用率提高。不同氮肥施用量对根际和非根际土壤铵态氮含量的影响不显著;减少氮肥施用量,对玉米根际土壤硝态氮含量也没有明显影响。在玉米苗期、抽雄期和成熟期,习惯高施氮量处理的非根际土壤硝态氮含量较高,其中抽雄期,非根际土壤硝态氮含量较氮肥减施40%用量处理高出近一倍,但非根际土壤微生物量氮水平含量明显降低。氮肥减施未影响根际土壤微生物量碳、氮含量,反而增加了非根际土壤微生物量碳、氮水平。在高肥力的潮土上,冬小麦/夏玉米轮作体系中适当减施氮肥并未影响玉米根际土壤氮素水平,可保证玉米稳产,实现减氮增效。     

Nitrous oxide emission as affected by changes in soil water content and nitrogen fertilization

Nitrous oxide emission was measured in laboratory incubations of an alluvial soil (58% clay, pH 7.4). The soil was amended with 40 mg N kg⁻¹ as NaNO₃ or NH₄Cl, or with NaCl as a control. Each fertilization treatment was adjusted to three different water contents: constant 60% WHC (water-holding capacity), constant 120% WHC, and water content alternating between 60 and 120% WHC. During an 8-day incubation period N₂O emission rates and inorganic nitrogen concentrations in soil (NH₄⁺, NO₂⁻, NO₃⁻) were determined at regular intervals. In the control and after nitrate application small N₂O emission rates occurred with only minor variations over time, and no differences between the water treatments. In contrast, with ammonium application N₂O emission rates were much higher during the first two days of incubation, with peaks in the constant 60% WHC and 120% WHC at day 1 and in the changing-water treatment at day 2, when the first wet period (120% WHC) was completed. This N₂O peak in the changing-water treatment was 4 to 9 times higher than with constant WHC and occurred when both, NH₄⁺ and NO₂⁻ concentrations declined sharply. Thus, this N₂O emission flush can be attributed to nitrifier denitrification. After the second rewetting of the NH₄⁺-amended soil no further N₂O emission peak was observed, being in accordance with small NH₄⁺ and NO₂⁻ concentrations in soil at that time. The unexpectedly small N₂O fluxes in the constant 120% WHC treatment after nitrate application were probably caused by the reduction of N₂O to N₂ under the prevailing conditions. It can be concluded that continuous wetting or flooding of a soil is an effective measure to reduce N₂O emissions immediately after the application of NH₄⁺ fertilizers.     

Grain yield,nitrogen use efficiency and tillage intensity in wheat as affected by precision nutrient management

ABSTRACT

The present studies were conducted to evaluate the effect of different nutrient management practices under two tillage options in wheat. The experiments were laid out in split-plot design with a combination of two varieties (WH 1105 and HD 2967) and two tillage options (Conventional and No tillage) in the main plot and six precision nutrient management practices [absolute control, site-specific nutrient management with Nutrient Expert for wheat (SSNM-NE)(170 kg nitrogen (N)/ha), SSNM NE+GreenSeeker (GS)(153/158 N kg/ha), N₁₂₀ (120 kg N/ha) before irrigation, N₁₂₀ after irrigation and N Rich (180 kg N/ha)] in subplot replicated thrice. The grain yield and quality characters in no tillage (NT) and conventional tillage (CT) were similar but agronomic efficiency was higher in NT. Both the varieties (WH 1105 and HD 2967) gave similar grain yield and quality. Wheat variety WH 1105 recorded significantly higher sodium dodecyl sulfate sedimentation (SDS) and gluten index. The treatment SSNM NE+GS had resulted in 107.1% higher grain yield than no nitrogen control but similar to enriched N plot (180 kg N/ha). The grain protein, SDS and gluten index in need-based nutrient management (SSNM+GS) treatment were found to be similar as recorded in SSNM-NE (170 kgN/ha) and N enriched plot (180 kg N ha^?1). The agronomic efficiency and recovery efficiency in SSNM+GS were also better than SSNM NE.     

Phosphorus uptake by cowpea plants from sparingly available or soluble sources as affected by nitrogen form and arbuscular‐mycorrhiza‐fungal inoculation

In most plant species, nutrient uptake is facilitated upon root association with symbiotic arbuscular mycorrhizal (AM) fungi. The aim of the present experiment was to test how the form in which nitrogen (N) is supplied to the growth medium affects substrate pH, AM development, and contribution of the symbiosis to phosphorus (P) uptake from sparingly available or soluble resources. Cowpea (Vigna unguiculata L. Walp) plants inoculated or noninoculated with AM fungi (Glomus sp.) were grown in pots with a sand substrate supplied with nutrient solution. The nutrient solution was prepared either with a high or a low concentration of soluble P, and NO ‐N : NH ‐N ratios of 9:1 or 5:5. The substrate supplied with low‐P nutrient solution was either or not additionally amended with ground rock phosphate. Despite a high level of root colonization, AM fungi used in the present study did not appear to increase plant availability of rock phosphate. It cannot be excluded that the ability of AM root systems to acquire P from sparingly available resources differs depending on the plant and fungal genotypes or environmental conditions. The absence from the growth substrate of P‐solubilizing microorganisms able to associate with AM mycelia might also have been a reason for this observation in our study. Increased supply of NH relative to NO improved plant P availability from rock phosphate, but also had a negative effect on the extent of AM‐fungal root colonization, irrespective of the plant P‐nutritional status. Whether increasing levels of NH can also negatively affect the functioning of the AM symbiosis in terms of plant element uptake, pathogen protection or soil‐structure stabilization deserves further investigation.     

Soil phosphorus dynamics in a Vertisol as affected by cattle manure and nitrogen fertilization in soybean‐wheat system

Repeated application of phosphorus (P) as superphosphate either alone or in conjunction with cattle manure and fertilizer N may affect the P balance and the forms and distribution of P in soil. During 7 years, we monitored 0.5 M NaHCO₃ extractable P (Olsen‐P) and determined the changes in soil inorganic P (P_i) and organic P (P_o) caused by a yearly dose of 52 kg P ha^—1 as superphosphate and different levels of cattle manure and fertilizer N application in a soybean‐wheat system on Vertisol. In general, the contents of Olsen‐P increased with conjunctive use of cattle manure. However, increasing rate of fertilizer nitrogen (N) reduced the Olsen‐P due to larger P exploitation by crops. The average amount of fertilizer P required to increase Olsen‐P by 1 mg kg^—1 was 10.5 kg ha^—1 without manure and application of 8 t manure reduced it to 8.3 kg ha^—1. Fertilizer P in excess of crop removal accumulated in labile (NaHCO₃‐P_i and P_o) and moderately labile (NaOH‐P_i and P_o) fractions linearly and manure application enhanced accumulation of P_o. The P recovered as sum of different fractions varied from 91.5 to 98.7% of total P (acid digested, P_t). Excess fertilizer P application in presence of manure led to increased levels of Olsen‐P in both topsoil and subsoil. In accordance, the recovery of P_t from the 0—15 cm layer was slightly less than the theoretical P (P added + change in soil P — P removed by crops) confirming that some of the topsoil P may have migrated to the subsoil. The P fractions were significantly correlated with apparent P balance and acted as sink for fertilizer P.     

Biological nitrogen fixation in faba bean (Vicia faba L.) in the Ethiopian highlands as affected by P fertilization and inoculation

 N₂ fixation by leguminous crops is a relatively low-cost alternative to N fertilizer for small-holder farmers in developing countries. N₂ fixation in faba bean (Vicia faba L.) as affected by P fertilization (0 and 20 kg P ha^–1) and inoculation (uninoculated and inoculated) with Rhizobium leguminosarium biovar viciae (strain S-18) was studied using the ¹⁵N isotope dilution method in the southeastern Ethiopian highlands at three sites differing in soil conditions and length of growing period. Nodulation at the late flowering stage was significantly influenced by P and inoculation only at the location exhibiting the lowest soil P and pH levels. The percentage of N derived from the atmosphere ranged from 66 to 74%, 58 to 74% and 62 to 73% with a corresponding total amount of N₂ fixed ranging from 169 to 210 kg N ha^–1, 139 to 184 kg N ha^–1 and 147 to 174 kg N ha^–1 at Bekoji, Kulumsa and Asasa, respectively. The total N₂ fixed was not significantly affected by P fertilizer or inoculation across all locations, and there was no interaction between the factors. However, at all three locations, N₂ fixation was highly positively correlated with the dry matter production and total N yield of faba bean. Soil N balances after faba bean were positive (12–58 kg N ha^–1) relative to the highly negative N balances (–9–44 kg N ha^–1) following wheat (Triticum aestivum L.), highlighting the importance of rotation with faba bean in the cereal-based cropping systems of Ethiopia. Received: 13 January 2000     

Energy balances and SOC and N stocks as affected by organic amendments and inorganic N fertilization in a semi-arid environment (IOSDV-Madrid)

A long-term field experiment (1984–2011), was conducted on a Calcic Haploxeralf from semi-arid central Spain to evaluate the combined effect of three treatments: farmyard manure (FYM), straw and control without organic amendments (WOA) and five increasing rates of mineral N on: (1) some energetic parameters of crop production, and (2) the effect of the different treatments on soil organic carbon (SOC) and total N stocks. Crop rotation included spring barley, wheat and sorghum. The energy balance variables considered were net energy produced (energy output minus energy input), the energy output/input ratio and energy productivity (crop yield per unit energy input). Results showed small differences between treatments. Total energy inputs varied from 9.86 GJ ha^?1 year^?1 (WOA) to 11.14 GJ ha^?1 year^?1 in the FYM system. For the three crops, total energy inputs increased with increasing rates of mineral N. Energy output was slightly lower in the WOA (33.40 GJ ha^?1 year^?1) than in the two organic systems (37.34 and 34.96 GJ ha^?1 year^?1 for FYM and straw respectively). Net energy followed a similar trend. At the end of the 27-year period, the stocks of SOC and total N had increased noticeably in the soil profile (0–30 cm) as a result of application of the two organic amendments. Most important SOC changes occurred in the FYM plots, with mean increases in the 0–10 cm depth, amounting an average of 9.9 Mg C ha^?1 (667 kg C ha^?1 year^?1). Increases in N stocks in the top layer were similar under FYM and straw and ranged from 0.94 to 1.55 Mg N ha^?1. By contrast, simultaneous addition of increasing rates of mineral N showed no significant effect on SOC and total N storage.     

Soil organic carbon and nitrogen fractions and water‐stable aggregation as affected by cropping and grassland reclamation in an arid sub‐alpine soil

This paper investigates effects of cropping abandonment and perennial grass growing on soil organic C and N pools and aggregate stability, by comparing soils under native grassland, crop cultivation, perennial grass growing and cropping abandonment, in degraded cropland at a sub‐alpine site in north‐western China. The pools of total and particulate organic C (115 and 37 Mg ha⁻¹) in the 0–30 cm soil layer of native grassland were reduced by 31 and 54% after 30 years of crop cultivation. After 4 years of conversion from cropland to perennial grass growing total and particulate organic C pools were increased by 29 and 56%, whereas 4 year cropping abandonment increased particulate organic C by 36%. Rapid increases in total and particulate N were also found in perennial grass growing and cropping abandonment soils. The native grassland soil and soils of cropping abandonment and perennial grass growing had higher carbohydrate C concentrations in the 0–10 cm layer than the cropped soil. The rapid recovery of particulate organic fraction and carbohydrates in the re‐vegetated soils were probably due to higher plant biomass inputs and lower organic matter decomposition compared with those in the cropped soil. Aggregate stability of the 0–30 cm soil layer was significantly decreased by crop cultivation but showed a good recovery after 4 year re‐vegetations. This study suggests that reduction of soil organic matter and aggregate stability under crop cultivation may be remedied by cropping abandonment or perennial grass growing. Copyright © 2008 John Wiley & Sons, Ltd.