Effect of microbial nitrogen immobilization during the growth period on the availability of nitrogen fertilizer for winter cereals

 Pot and field experiments were conducted to determine microbial immobilization of N fertilizer during growth periods of winter wheat and winter barley. In a pot experiment with winter wheat, Ca(¹⁵NO₃)₂ was applied at tillering [Zadok's growth stage (GS) 25)], stem elongation (GS 31) and ear emergence (GS 49). Rates of 100 mg N pot^–1, 200 mg N pot^–1 or 300 mg N pot^–1 were applied at each N application date. At crop maturity, ¹⁵N-labelled fertilizer N immobilization was highest at the highest N rate (3×300 mg N pot^–1). For each N-rate treatment about 50% of the total immobilized fertilizer N was immobilized from the first N dressing, and 30% and 20% of the total ¹⁵N immobilized was derived from the second and third applications, respectively. In field trials with winter wheat (three sites) and winter barley (one site) N was applied at the same growth stages as for the pot trial. N was also applied to fallow plots, but only at GS 25. N which was not recovered (neither in crops nor in soil mineral N pools) was considered to represent net immobilized N. A clear effect of N rate (51–255 kg N ha^–1) on net N immobilization was not found. The highest net N immobilization was found for the period between GS 25 (March) and GS 31 (late April) which amounted to 54–97% of the total net N immobilized at harvest (July/August). At GS 31, non-recovered N was found to be of similar magnitude for cropped and fallow plots, indicating that C from roots did not affect net N immobilization. Microbial biomass N (N_mic) was determined for cropped plots at GS 31. Although N_mic tended to be higher in fertilized than in unfertilized plots, fertilizer-induced increases in N_mic and net N immobilization were poorly correlated. It can be concluded that microbial immobilization of fertilizer N is particularly high after the first N application when crop growth and N uptake are low. Received: 6 July 1999     

Legumes as dry season fallow in upland rice-based systems of West Africa

Declining fallow length in traditional upland rice-based cropping systems in West Africa results in a significant yield reduction due mainly to increased weed pressure and declining soil fertility. Promising cropping system alternatives include the use of weed-suppressing legumes as short duration fallows. N accumulation, N derived from the atmosphere (Ndfa), weed suppression, and the effects on rice yield were evaluated in 50 legumes, grown at four sites in Côte d'Ivoire with contrasting climate, soils, and rice production systems. The sites were located in the derived and the Guinea savanna and in the bimodal and the monomodal rainfall forest zones. Legume and weed biomass during the fallow were determined at bimonthly intervals. Percent Ndfa by biological N fixation was determined by ¹⁵N natural abundance. Fallow vegetation was cleared and rice seeded according to the practice of local farmers and the cropping calendar. Weed biomass and species composition were monitored at monthly intervals. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Biomass was in most instances significantly greater in the legume fallow than in the "weedy" fallow control, and several legume species suppressed weed growth. N accumulation by legumes varied between 1–270?kg N ha^–1 with 30–90%?Ndfa. Across sites, Mucuna spp., Canavalia spp., and Stylosanthes guianensis showed consistently high N accumulation. Grain yields of rice which had been preceded by a legume fallow were on average 0.2?Mg ha^–1 or about 30% greater than that preceded by a natural weedy fallow control. At the savanna sites where fallow vegetation was incorporated, Mucuna spp. and Canavalia ensiformis significantly increased rice yield. In the bimodal forest zone, the highest rice yield and lowest weed biomass were obtained with Crotalaria anagyroides. In general, the effects of legume fallows on rice yield were most significant in environments with favourable soil and hydrological conditions.     

Field evaluation of lentil cultivars inoculated with Rhizobium leguminosarum bv. viciae strains for nitrogen fixation using nitrogen-15 isotope dilution

 A ¹⁵N isotope dilution technique was applied to quantify the extent of N₂ fixation in lentil (Lens culinaris Medik.) cultivars as influenced by Rhizobium leguminosarum bv. viciae strains in a field experiment in Pakistan. The experiment was conducted on a soil with a very small indigenous rhizobial population and where N was a limiting factor for crop production. Significant variations in number of nodules, dry weight of nodules, biomass yield, grain yield, total N yield, proportion of plant N derived from N₂ fixation (P_fix) and amount of N derived from the atmosphere (N_dfa) were observed among combined treatments of four rhizobial strains and six lentil varieties. In a field previously labelled with ¹⁵N, to which a basal dose of 75 kg P₂O₅ ha^–1 was applied as single super phosphate, N_dfa ranged from 15 to 24 kg N ha^–1 when calculated according to rhizobial strain and from 4 to 38 kg N ha^–1 when calculated according to lentil variety. Lc 26 was the most effective strain and fixed 243% more N than the indigenous population in the uninoculated control. In treatments with the lentil variety PL-406, N_dfa was 38 kg N ha^–1, which was 850% higher than with the lentil variety Precoz/F6-20-1×M-85. Generally, the varieties with greater P_fix produced a higher dry matter yield. Received: 26 May 1999     

Influence of the presence of nitrite and nitrate in soil on maize biomass production, nitrogen immobilization and nitrogen recovery

 When comparing nitrite (NO₂ ^–) and nitrate (NO₃ ^–) toxicity to maize (Zea mays L.) growth, it is important to know the fate of applied nitrogen (N). A pot experiment, using potassium nitrite (K¹⁵NO₂) and potassium nitrate (K¹⁵NO₃) was conducted to determine the fate of N (0, 75, 150, and 225 mg N kg^–1 soil) applied to a sandy loam soil collected from Gistel (Belgium). The total dry weight of the plants treated with NO₂ ^– was lower than that of the plants treated with NO₃ ^– at 15 and 26 days after N application (harvest 1 and harvest 2, respectively). Shoot and root biomass reduction started at a relatively low NO₂ ^– application rate (75 mg NO₂ ^–-N kg^–1). Biomass reduction increased, at both harvests with increasing amounts of NO₂ ^– to more than 55% at the highest application rate (225 mg NO₃ ^–-N kg^–1). In the NO₃ ^– treatment, a reduction of 16% in total plant dry biomass was recorded only at the highest application rate (225 mg NO₂ ^–-N kg^–1), at both harvest times. The ¹⁵N plant uptake (shoots plus roots) at harvest 1 decreased with increasing N application rates of both N forms (KNO₂ and KNO₃). Twenty-six days after the N application, the total ¹⁵N taken up by the plant increased in all treatments in comparison with 15 days after the N application. However, only at higher rates of N application (150 and 225 mg N kg^–1) was the ¹⁵N uptake by the NO₂ ^– fed plants significantly lower than by the NO₃ ^– fed plants. The percentage of immobilized N from the applied N was low (0–17.7%) at both harvests, irrespective of the N source. However, with relatively low N application rates (75 mg N kg^–1), the immobilized N in the soil decreased with time. This may be due to the re-mineralization of the applied N. The percentage of inorganic ¹⁵N in the soil in NO₂ ^– treatments was slightly lower than in equivalent doses of NO₃ ^–. This might be due to higher losses of N as N-oxides. Unaccounted for N from the applied N ranged from 21% to 52% for the NO₂ ^– treatments and from 3% to 38% for the NO₃ ^– treatments. Received: 17 July 1997     

Carbon additions increase nitrogen availability in northern hardwood forest soils

 The effects of acetate additions to northern hardwood forest soils on microbial biomass carbon (C) and nitrogen (N) content, soil inorganic N levels, respirable C and potential net N mineralization and nitrification were evaluated. The experiment was relevant to a potential watershed-scale calcium (Ca) addition that aims to replace Ca depleted by long-term exposure to acid rain. One option for this addition is to use calcium-magnesium (Mg) acetate, a compound that is inexpensive and much more readily soluble than the Ca carbonate that is generally used for large-scale liming. Field plots were treated with sodium (NA) acetate, Na bicarbonate or water (control) and were sampled (forest floor – Oe and Oa combined) 2, 10 and 58 days following application. It was expected that the addition of C would lead to an increase in biomass C and N and a decrease in inorganic N. Instead, we observed no effect on biomass C, a decline in biomass N and an increase in N availability. One possible explanation for our surprising results is that the C addition stimulated microbial activity but not growth. A second, and more likely, explanation for our results is that the C addition did stimulate microbial growth and activity, but there was no increase in microbial biomass due to predation of the new biomass by soil fauna. The results confirm the emerging realization that the effects of increases in the flow of C to soils, either by deliberate addition or from changes in atmospheric CO₂, are more complex than would be expected from a simple C : N ratio analysis. Evaluations of large-scale manipulations of forest soils to ameliorate effects of atmospheric deposition or to dispose of wastes should consider microbial and faunal dynamics in considerable detail. Received: 13 March 1998     

Genetic variability in dinitrogen fixation between cowpea [Vigna unguiculata (L.) Walp] cultivars determined using the nitrogen-15 isotope dilution technique

 N fixed in 16 cultivars of cowpea [Vigna unguiculata (L.) Walp] inoculated with effective Bradyrhizobium strains collected from the West African MIRCEN culture collection was measured by ¹⁵N isotope dilution technique. In all plant parts, significant differences in the percentage of N derived from the atmosphere (%Ndfa) and the amount of Ndfa occurred between the cultivars. Ndoute variety exhibited the highest %Ndfa (74.33% in shoots; 60.90% in roots) and accumulated more fixed N (960 mg N plant^–1 and 38 mg N plant^–1 in shoots and roots, respectively). Therefore this cultivar should be selected as the highest N-fixing cowpea cultivar. It also should be used in a breeding programme to contribute to the development of cultivars that could stimulate an intensive use of cowpea in many different cropping systems in Africa with a view to maintaining soil fertility. Received: 14 June 1999     

Long-term winter cover cropping effects on corn (Zea mays L.) production and soil nitrogen availability

 This study was conducted to determine effects of long-term winter cover cropping with hairy vetch, cereal rye and annual ryegrass on soil N availability and corn productivity. From 1987 to 1995, with the exception of the first year of the study, the cover crops were seeded each year in late September or early October after the corn harvest and incorporated into the soil in late April or early May. Corn was seeded 10 days to 2 weeks after the cover crop residues had been incorporated, and N fertilizer was applied as a side-dressing at rates of 0, 67, 134, or 201 kg N ha^–1 each year. While the average annual total N input from the above-ground biomass of the cover crops was highest for hairy vetch (72.4 kg N ha^–1), the average annual total C input was highest for cereal rye (1043 kg C ha^–1) compared with the other cover crops. Hairy vetch was the only cover crop that significantly increased pre-side-dressed NO₃ ^–-N (N_i) corn biomass and N uptake at 0 N. At an N fertilizer rate of 134 kg N ha^–1 or higher, the cover crops had a minimal effect on corn biomass. This indicated that even after 9 years of winter cover cropping, the effect of the cover crops on corn growth resulted primarily from their influence on soil N availability. The amount of available N estimated from the cover crops (N_ac) was significantly correlated with relative corn biomass production (r ²=0.707, P<0.001). The total amount of available N, comprising N_ac and N added from fertilizer (N_f), was strongly correlated (r ²=0.820, P<0.001)) with relative corn biomass production. The correlation was also high for the available N comprising N_i and N_f (r ²=0.775, P<0.001). Although cereal rye and annual ryegrass did not improve corn biomass production in the short term, they benefited soil organic N accumulation and gradually improved corn biomass production compared with the control over the long term. Received: 10 August 1999     

The fate of tree root and pruning nitrogen in a temperate climate alley cropping system determined by tree-injected 15N

 Nitrogen (N) fluxes through the major plant pools of an alder (Alnus sinuta)-sweet corn (Zea mays) alley cropping system were determined over the course of two cropping seasons. Alder trees were injected with ¹⁵NO₃–N to directly follow the flow of N between alder and corn. The contribution of the above- and below-ground tree N to corn was determined by exchanging the labeled above-ground prunings (green manure) with those from unlabeled plots. During the first growing season after coppicing of the injected alders, 18% of the alder ¹⁵N was taken up by the corn with 12% coming from the above-ground prunings. Of the ¹⁵N remaining in the tree/stump following coppicing, the majority was recovered by corn plants within the rows next to the labeled trees during the first growing season. Earlier recovery of ¹⁵N by corn in the labeled root plots compared to the labeled pruning plots indicated the importance of root turnover in supplying N to corn, especially following coppicing. By the end of the first and second growing seasons, 34% and 38% of the ¹⁵N initially present in prunings was recovered in corn plants, respectively. Approximately 80% of the total injected ¹⁵N was found in the soil during the second growing season; however, the turnover of above- and below-ground alder components supplied only 3–4% of the N required by corn during the year of green manure application. Thus, most of the corn N demand was met by mineralization of residual soil N within the 2 years of coppicing and green manure additions. Continued internal cycling of tree N and movement of soil N into more labile pools would presumably allow more alder N to become available over time. The synchronization between N mineralization from the hedgerow green manure components and nutrient uptake of the alley crop remains a major challenge in alley cropping and other green manure systems. Received: 9 April 1999     

Effects of nitrogen input and grazing on methane fluxes of extensively and intensively managed grasslands in the Netherlands

 Generally, grasslands are considered as sinks for atmospheric CH₄, and N input as a factor which reduces CH₄ uptake by soils. We aimed to assess the short- and long-term effects of a wide range of N inputs, and of grazing versus mowing, on net CH₄ emissions of grasslands in the Netherlands. These grasslands are mostly intensively managed with a total N input via fertilisation and atmospheric deposition in the range of 300–500 kg N ha^–1 year^–1. Net CH₄ emissions were measured with vented, closed flux chambers at four contrasting sites, which were chosen to represent a range of N inputs. There were no significant effects of grazing versus mowing, stocking density, and withholding N fertilisation for 3–9 years, on net CH₄ emissions. When the ground-water level was close to the soil surface, the injection of cattle slurry resulted in a significant net CH₄ production. The highest atmospheric CH₄ uptake was found at the site with the lowest N input and the lowest ground-water level, with an annual CH₄ uptake of 1.1 kg CH₄ ha^–1 year^–1. This is assumed to be the upper limit of CH₄ uptake by grasslands in the Netherlands. We conclude that grasslands in the Netherlands are a net sink of CH₄, with an estimated CH₄ uptake of 0.5 Gg CH₄ year^–1. At the current rates of total N input, the overall effect of N fertilisation on net CH₄ emissions from grasslands is thought to be small or negligible. Received: 27 January 1998     

Combining field incubation with nitrogen-15 labelling to examine nitrogen transformations in low to high intensity grassland management systems

 The ¹⁵N isotope dilution method was combined with a field incubation technique to provide simultaneous measurements of gross and net rates of N turnover in three long-term swards: unfertilized (Z) or receiving N either from N fixation as clover (C), or as 200 kg fertilizer N ha^–1 year^–1 (F). Uniform N enrichment of soil microplots was achieved with a multi-point soil injector to measure mineralization/immobilization turnover and nitrification over a 4-day incubation. Net rates of mineralization ranged between 0.6 and 2.9 μg N g^–1 day^–1 and in all three treatments were approximately half the gross rates. Nitrification rates (gross) were between 1.0 and 1.6 μg N g^–1 day^–1. In the F treatment, the turnover of NH₄ ⁺-N and NO₃ ^–-N pools was on a 2- and 4-day cycle, respectively, whereas in the N-limited treatments (C and Z) turnover rates were faster, with the NO₃ ^–-N pools turning over twice as fast as the NH₄ ⁺-N pools. Therefore, available N was recycled more efficiently in the C and Z treatments, whereas in the F treatment a higher N pool size was maintained which would be more vulnerable to leakage. A large proportion of the added ¹⁵N was recovered in the soil microbial biomass (SMB), which represented a 4–5 times larger sink for N than the plant biomass. Although the C treatment had a significantly lower SMB than the grass-only treatments, there were no differences in microbial activity. Gross rates of nitrification increased along the gradient of N input intensity (i.e. Z<C<F), and the addition of a nitrification inhibitor (C₂H₂) tended to increase microbial immobilization, but did not influence plant N uptake. In this study, the value of combining different techniques to verify net rates was demonstrated and the improved methodology for ¹⁵N labelling of soil enabled measurements to be obtained from relatively undisturbed soil under natural field conditions. Received: 25 May 1999     

Uptake of 15N by wetland rice in response to application of 15N-labelled Sesbania rostrata and urea

 Pot experiments were carried out to evaluate the response of rice to Sesbania rostrata green manure N as compared to urea fertilizer N under flooded conditions. After growing S. rostrata for 21 days with a ¹⁵N-labelled N source, the labelled Sesbania was applied to wetland rice as a green manure and the uptake of ¹⁵N from this substrate was compared to that from labelled urea. Rice was cultivated twice in the same pots. The rice was grown for a period of 49 days in each case, separated by a period of 21 days when the soil was allowed to dry. The ¹⁵N content of the soil and shoots and roots of rice was determined and ¹⁵N balances established. The total N content of the shoots and roots of rice was determined by a non-tracer method. The percentage recovery of ¹⁵N from shoot material which was derived from urea N was more than twice that from S. rostrata. The recovery of ¹⁵N from the pots receiving both green manure and urea was low, and not significantly different from that recovered from the green manure treatment. As much as 64.5–73.5% and 40.1–41% of the ¹⁵N remained in the soil which had received green manure or urea, respectively. The overall recoveries of ¹⁵N varied between 86.5% and 94.4%. At the second harvest, the oven-dry weight of shoots was significantly (P<0.05) higher in green-manure treated pots, but the total N content did not differ significantly. Labelled N remaining in the soil after amendment with the green manure was much more available to the rice crop than that remaining after the addition of urea-N. The total recovery of labelled N (shoots plus roots) amounted to 65.5% and 74%, respectively of the residual labelled N in the two S. rostrata treatments (i.e. 19.55 mg ¹⁵N pot^–1 and 39.10 mg ¹⁵N pot^–1) and 23.2% and 23.2% of the residual labelled N in the two urea treatments (i.e. 19.55 mg ¹⁵N pot and 39.10 mg ¹⁵N pot^–1), respectively. Received: 8 December 1997     

The use of nitrogen-15 natural abundance in white clover (Trifolium repens L.) to determine nitrogen fixation under different management practices

 The ¹⁵N natural abundance (δ¹⁵N) of white clover (Trifolium repens L.) grown in pasture under different management practices was determined. Plants were split into leaflets, petioles and stolons and the ¹⁵N signature of each tissue was measured. The δ¹⁵N of leaflet tissue from plants of two non-N₂-fixing species (Lolium perenne L. and Ranunculus repens L.), growing in close proximity to the sampled T. repens, was also measured. By using T. repens plants grown in the absence of mineral N to provide reference material, the proportion of N derived from N₂ fixation (%Ndfa) in pasture plants was calculated. Within a plot, variation was present in the δ¹⁵N between the tissues of T. repens. Variation was also present between the same tissues under different management practices. The %Ndfa in the leaf material of T. repens varied from 34% to 100% between the plots. The use of different reference species did not affect the estimate of %Ndfa. Received: 14 December 1998     

Dynamics of soil microbial biomass and nitrogen availability in a flooded rice soil amended with different C and N sources

 A greenhouse experiment was conducted to compare effects of different C and N sources applied to a flooded soil on soil microbial biomass (SMB) C and N, extractable soil organic N (N_ORG), and NH₄ ⁺-N in relation to plant N accumulation of rice (Oryza sativa L.). In addition to a control without inputs (CON), four treatments were imposed receiving: prilled urea (PU), rice straw (RS), RS and PU (RS+PU), or Sesbania rostrata as green manure (SES). Treatments were arranged according to a completely randomized design with four replicates and further consisted of pots with and without transplanted rice. While plant effects on the SMB were relatively small, the application of organic N sources resulted in a rapid increase in SMB until 10 days after transplanting (DAT) followed by a gradual decline until 73 DAT. Plant N accumulation data in these treatments clearly indicated that the SMB underwent a transition from a sink to a source of plant-available soil N during the period of crop growth. Seasonal variation of the SMB was small in treatments without amendment of organic material (CON, PU) presumably due to a lack of available C as energy source. Extractable N_ORG was significantly affected by soil planting status and organic N source amendment, but represented only a small N pool with little temporal variation despite an assumed rapid turnover. Among the three treatments receiving the same amount of N from different sources, the recovery efficiency of applied N was 58% for PU and 28% for both RS+PU and SES treatments at 73 DAT. The N uptake of rice, however, was not driven by N availability alone, as most evident in the RS+PU treatment. We assume that root physiological functions were impeded after application of organic N sources. Received: 1 June 1999     

Within-field variability of mineral nitrogen in grassland

 The within-field variability of soil mineral nitrogen (N_min) in a grazed grassland of 8000 m² was examined. NO₃ ^–-N concentrations were characterized by a high spatial variability. This can be explained by the uneven deposition of animal excreta. All NH₄ ⁺-N as well as NO₃ ^–-N values were lognormally distributed, before and after the grazing season. At the end of the grazing season the largest part of the variability of NO₃ ^–-N was found for NO₃ ^–-N concentrations measured within a distance of a few metres. A high variability for NO₃ ^–-N over very short distances was also indicated by a large nugget variance. During the grazing season, observed mean N_min values increased from 22 to 132 kg N ha^–1. Regions with clearly higher NO₃ ^–-N concentrations could be identified. These zones matched with the drinking place and the entrance of the pasture, places which were more frequently visited than others. High residual N levels in autumn led to relatively high losses of N, mostly by leaching, during the subsequent drainage period. Knowing the variability of N_min, the number of samples needed to estimate the average N_min in a field could be calculated for different probabilities and various degrees of precision. From the spatial distribution of the N_min concentrations and the restrictions imposed by the new European decree, adapted fertilizer strategies can be proposed at least for places where systematically higher N_min concentrations can be expected. Received: 14 December 1999     

Quantification of nitrogen excretion rates for three lumbricid earthworms using 15N

 Nitrogen excretion rates of ¹⁵N-labeled earthworms and contributions of ¹⁵N excretion products to organic (dissolved organic N) and inorganic (NH₄-N, NO₃-N) soil N pools were determined at 10  °C and 18  °C under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clitellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rubellus (Hoffmeister) were labeled with ¹⁵N by providing earthworms with ¹⁵N-labeled organic substrates for 5–6 weeks. The quantity of ¹⁵N excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were calculated. N excretion rates ranged from 274.4 to 744 μg N g^–1 earthworm fresh weight day^–1, with a daily turnover of 0.3–0.9% of earthworm tissue N. The N excretion rates of juvenile L. terrestris were significantly lower than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, particularly NH₄-N, were greater in soils incubated with earthworms for 48 h than soils incubated without earthworms. Between 13 and 40% of excreted ¹⁵N was found in the ¹⁵N-mineral N (NH₄-N+NO₃-N) pool, and 13–23% was in the ¹⁵N-DON pool. Other fates of excreted ¹⁵N may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N forms, or gaseous N losses. Earthworm excretion rates were combined with earthworm biomass measurements to estimate N flux from earthworm populations through excretion. Annual earthworm excretion was estimated at 41.5 kg N ha^–1 in an inorganically-fertilized corn agroecosystem, and was equivalent to 22% of crop N uptake. Our results suggest that the earthworms could contribute significantly to N cycling in corn agroecosystems through excretion processes. Received: 12 April 1999     

Carbon, nitrogen and phosphorus mineralization of tree leaves and manure

 Farmers in developing countries cannot afford inorganic fertilizers. Multipurpose tree leaves or livestock manure are major sources of nutrients for soil fertility replenishment. Nutrient release from these organic inputs depends on their chemical composition and on soil properties. This study determined the chemical composition of leaves of four African browse species and manure from goats fed leaves as protein supplements, and their mineralization of C, N and P. Cumulative evolved CO₂ was significantly correlated with the initial N content of the organic inputs (r 0.83, P<0.05) and the C : N ratio (r 0.80, P<0.05), and was negatively correlated with the lignin : N ratio (r–0.71, P<0.05). Cumulative P released was negatively correlated with the C : P ratio (r 0.76, P<0.05) and positively correlated with initial P content of the organic amendments (r 0.76, P<0.05). Cumulative N mineralized was not significantly correlated with initial N, lignin or P concentrations of the organic inputs. Leaves from Acacia karro and Acacia nilotica had high concentrations of polyphenols, which may have caused immobilization of N in both leaves and manure. Gliricidia sepium leaves had low amounts of soluble polyphenols, a high N content and a high rate of N mineralization, but the manure from goats fed Gliricidia leaves immobilized N. The leaves of all browse species immobilized P, but the manure released P. The results suggested that some browse leaves cannot meet the N and P requirements of crops due to their low P content and prolonged N and P immobilization. However, the manures had higher P contents and rates of P mineralization, which suggested that manure is a good source of P for crops. The implications of these results for nutrient cycling in mixed farming systems is discussed. Received: 28 October 1998     

秸秆还田下减氮增密对双季稻田土壤氮素库容及氮素利用率的影响

为推动传统稻作技术向资源节约型与环境友好型方向转型,本研究在稻草还田基础上设置不施氮常密（T1）、常氮常密（T2）、常氮增密（T3）、减氮常密（T4）、减氮增密（T5）5种双季稻栽培模式,研究稻草还田下减氮增密对双季稻田土壤氮素库容及氮素利用率的影响。结果表明：稻草还田下,减氮增密（基肥减施总氮量的20%,增加密度27.3%）的0~10 cm、10~20 cm的土壤全氮含量及库容量与常氮常密无显著差异,但碱解氮含量分别显著降低15.6%、8.8%,碱解氮总库容量显著降低10.7%。与常氮常密相比,减氮增密可显著降低双季稻田的土壤氮素表观盈亏量,氮素损失量及损失率分别显著降低51.7%及15.5个百分点;早、晚稻的氮素农学利用率分别显著增加33.6%、23.0%,吸收利用率分别显著增加5.9个百分点、6.3个百分点,生理利用率分别显著增加16.3%、3.7%。表明稻草还田下短期内的减氮增密不会显著降低土壤的全氮库容,但会显著降低土壤的碱解氮库容,可显著降低氮素损失,提高氮素利用率。     

氮肥用量及其分施比例对棉花氮利用和土壤氮平衡的影响

The Yellow River valley is one of the three largest cotton production areas in China.An experiment was performed in cotton fields of Anyang,China from 2013 to 2014 to investigate the effects of nitrogen(N) application rate and the ratio between basal and topdressing N fertilizer on N balance in a soil-plant system,N use efficiency,and cotton yield.Five N application rates as treatments were applied with the same split application ratio.Half of the N(50% basal fertilizer) was applied at pre-planting and the other half(50% topdressing fertilizer) at the initial flowering stage.These treatments were:zero N(N0,control),90 kg N ha~(-1)(N90(5/5)),180 kg N ha~(-1)(N180(5/5)),270 kg N ha~(-1)(N270(5/5),a reduced N rate),and 360 kg N ha~(-1)(N360(5/5),a conventional N rate).Additional 2 split application ratios as treatments were applied with the same N rate of 270 kg N ha~(-1).The split application ratios between basal N and topdressing N were 30%:70%(N270(3/7)) and 70%:30%(N270(7/3)).Results demonstrated that soil NH_4-N content in the 0–60 cm layer and NO3-N content in the 0–20 cm layer increased with increased N rate at the squaring and boll-opening stages and then decreased to lower levels at the initial flowering and harvest stages.Soil NO_3-N content in the 20–60 cm layer after the initial flowering stage increased with the increase of topdressing N rate.Soil apparent N surplus varied at different growth stages,while the soil apparent N surplus over the entire growth period exhibited a positive relationship at N rates over 180 kg ha~(-1).Seed cotton yield of N270(3/7) was the highest of all treatments.Plant N uptake,N agronomic efficiency,and apparent N recovery efficiency of N270(3/7) were significantly higher than those of N270(5/5) and N270(7/3) in both growing seasons.These suggest both economic and ecological benefits in cotton production in the Yellow River valley could be created,by appropriately reducing total N application rate and increasing the ratio of topdressing to basal N fertilizer at the initial flowering stage.     

Short-term effects of nitrogen on methane oxidation in soils

 The short-term effects of N addition on CH₄ oxidation were studied in two soils. Both sites are unfertilized, one has been under long-term arable rotation, the other is a grassland that has been cut for hay for the past 125 years. The sites showed clear differences in their capacity to oxidise CH₄, the arable soil oxidised CH₄ at a rate of 0.013 μg CH₄ kg^–1 h^–1 and the grassland soil approximately an order of magnitude quicker. In both sites the addition of (NH₄)₂SO₄ caused an immediate reduction in the rate of atmospheric CH₄ oxidation approximately in inverse proportion to the amount of NH₄ ⁺ added. The addition of KNO₃ caused no change in the rate of CH₄ oxidation in the arable soil, but in the grassland soil after 9 days the rate of CH₄ oxidation had decreased from 0.22 μg CH₄ kg^–1 h^–1 to 0.13 μg CH₄ kg^–1 h^–1 in soil treated with the equivalent of 192 kg N ha^–1. A ¹⁵N isotopic dilution technique was used to investigate the role of nitrifiers in regulating CH₄ oxidation. The arable soil showed a low rate of gross N mineralisation (0.67 mg N kg^–1 day^–1), but a relatively high proportion of the mineralised N was nitrified. The grassland soil had a high rate of gross N mineralisation (18.28 mg N kg^–1 day^–1), but negligible nitrification activity. It is hypothesised that since there was virtually no nitrification in the grassland soil then CH₄ oxidation at this site must be methanotroph mediated. Received: 31 October 1997     

Differential effects of soil properties on leaf nitrogen release

 Identifying the determinants of the N dynamics of plant prunings or litter is important for the efficient management of agroecosystems in order to improve their productivity. The plant materials in these ecosystems are managed as soil surface mulches or are incorporated into the soil. Numerous studies have been conducted to investigate which plant chemical parameter best governs N release. In these studies, different plant materials have been incorporated into a soil with a set of known characteristics. The objective of the present study was to examine the effects of different soil properties on N release from plant leaves, when they were incorporated into soils under non-leaching conditions. A laboratory incubation experiment (for 8 weeks) was carried out with dried and ground leaves of six leguminous plants and wild sunflower, which were mixed with three soils (alfisol; ultisol, udult; ultisol, humult). Leaf cellulose was the major chemical parameter that determined leaf N release in the alfisol and ultisol, udult. In the ultisol, humult, the C/N ratio and hemicellulose concentration were better related to N release. Cellulose was not a good indicator of N release in the ultisol, humult, possibly due to a low soil pH which did not favour the activity of the cellulose-degrading enzymes of microbes active in decomposition. Soil pH determined the specific C source that was used to generate energy for microbial action and N mineralization/immobilization. It also had an effect on the nitrification of the mineralized N. The levels of labile soil C fractions governed the mode or nature of N release (i.e. mineralization or immobilization). The levels of labile leaf C fractions incorporated into the soils governed the extent of N release. The soil N concentration in the decomposable organic matter pool, as compared to the leaf N concentration, determined whether leaf N limited its own release. It is recommended from this study that, in grouping different leaf materials as sources of N, the properties of soils into which they are incorporated should also be considered, in addition to leaf quality in terms of its chemical composition. In future studies, the relationships identified under laboratory conditions in this experiment should be verified under field conditions. Received: 3 December 1997