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1.
Fate of 15N after one single application of labelled N fertilizer in the long-term experiment Eternal Rye Cropping Halle To elucidate the fate of N in the long-term field experiment with continuous rye cropping in Halle, Germany (Haplic Phaeozem derived from sandy loess; mean annual precipitation: 466 mm) micro plots (9 m2) were established within the NPK main plot in 1993 and 1994 and the routine N dose (60 kg N ha—1 a—1) was once applied as 15N labelled fertilizer. In the subsequent four years each micro plot was analyzed for the 15N withdrawn with grain and straw and that remaining in the soil after harvest. Parallel to this, micro plots on the unfertilized main plot received a single dose of 5 kg ha—1 of high-label 15N in order to achieve labelling of the annual N input from the air (˜40 kg ha—1 a—1) and to follow its fate. — In the NPK treatment about one third of the labelled fertilizer N applied (= *N) was taken up by the rye in the first year, while nearly half of it remained in the soil (0—100 cm), and 10—15% was not recovered (most likely lost via leaching). During the following three years the amount of *N remaining in the soil decreased from 32.7 kg ha—1 to 21.5 kg ha—1. However, less than half of the remobilized *N had been taken up by the crop. On the unfertilized plot the uptake of *N was lower at the end of the experiment compared to the NPK plot, while the loss exceeded the comparative figure correspondingly.  相似文献   

2.
Nitrate leaching from intensively and extensively grazed grassland measured with suction cup samplers and sampling of soil mineral‐N I Influence of pasture management Leaching of nitrate (NO3—) from two differently managed cattle pastures was determined over four winters between 1993 and 1997 using ceramic suction cup samplers (with min. 34 cups ha—1); additionally, vertical soil mineral‐N content in 0—0.9 m (Nmin) was measured at the beginning and end of two winters (with min. 70 different sample cores ha—1). The experimental site in the highlands north‐east of Cologne, Germany, is characterized by high annual precipitation (av. 1,362 mm between 1993 and 1996). An intensive continuous grazing management (1.3 ha, fertilized with 250 kg N ha—1 yr—1, average stocking density 4.9 LU ha—1, = [I]) was tested against an extensive continuous grazing system (2.2 ha, av. 2.9 LU ha—1; no N‐fertilizer but an estimated proportion of Trifolium repens up to 15 % of total dry matter in the final year, = [E]). The results can be summarized as follows: (1) Mean leaching losses of NO3‐N, estimated from suction cup sampling and balance of drainage volume, were 85 kg NO3‐N ha—1 [I] and 15 kg NO3‐N ha—1 [E] during three wet winters with drainage volumes between 399 and 890 mm; in a dry winter with 105 mm calculated percolation, nitrate leaching decreased by a factor of 5 for both grazing treatments. (2) Although the amount of mineral N in soil (Nmin) sampled in late autumn showed differences between intensive and extensive grazing, the Nmin method permits no certain indication of the risk of NO3 leaching. For example, during the winter period 1994/95 a reduction of mineral N in the soil (0—0.9 m) in both grazing treatments was found (—33 [I] / —8 [E] kg NO3‐N ha—1 and —26 [I] / —21 [E] kg NH4‐N ha—1) whereas during the winter 1996/97 an increase in almost all mean mineral N values occurred (+10 [I] / +2 [E] kg NO3‐N ha—1 and +10 [I] / —10 [E] kg NH4‐N ha—1). (3) In spite of the differences between both methods, the experiment shows that NO3‐N leaching under extensive grazing could be reduced almost to levels close to those under mown grassland.  相似文献   

3.
In 1991, field experiments on loess (with winter wheat) and sandy soils (with summer barley) were conducted to study N dynamics in the microbial biomass and non-exchangeable NH inf4 sup+ . The measurements showed a mass change in microbial N, with a maximum increase of 100 kg N ha-1 30 cm-1 from March to July in the loess soil, and a change for only 1 month (May) in the sandy soil. Plots treated with conventional levels of N fertilizer (213 kg N ha-1 on a loess soil to winter wheat and 130 kg ha-1 on the sandy soil to summer barley), reduced levels of N (83% and 62% of the conventional N application), or no N showed no consistent fertilizer N effect on microbial biomass N. From March to July, non-exchangeable NH inf4 sup+ in loess soils under winter wheat decreased by 110 kg N ha-1 30 cm-1 in conventionally fertilized plots and by 200 kg N ha-1 30 cm-1 in a plot with no N fertilizer. After harvest, the pool of non-exchangeable NH inf4 sup+ increased due to increasing mineral N concentrations in the soil.  相似文献   

4.
In a two‐year field trial at the sites Ruthe (Germany, loess soil, Orthic Luvisol) and Schermer (The Netherlands, marine clay soil, Eutric Fluvisol) the cauliflower F1‐hybrids Marine, Lindurian and Linford were compared in their efficiency of N use from limiting and optimum supplies of N. Limiting N was Nmin at planting. Optimum N was 250 kg ha—1 as the sum of inorganic N content of the soil (Nmin) at planting and fertilizer‐N. Marine was the most efficient variety, producing the highest shoot dry‐matter and quality (% class 1 curds) at both limiting and optimum N supplies. The N supply did not affect the horizontal and vertical distribution of root length density per soil volume (RLD, cm cm—3) irrespective of variety. The RLD decreased exponentially with increasing soil depth. Varietal differences in RLD were not found at Ruthe, whereas at Schermer Marine had the highest RLD in all soil layers investigated (0 to 60 cm). No correlations were found between RLD and residual Nmin at harvest, except at limiting N supply in Schermer where a strong negative correlation was found between RLD in the 45 to 60 cm layer and Nmin at harvest. Thus, varietal differences in N efficiency are speculated to be rather due to different internal N‐use efficiency than to differences in N‐uptake efficiency.  相似文献   

5.
Natural and mutant strains of A. chroococcum were isolated from Indian soils. Their ability to dissolve phosphate and their phytohormone production were tested under in vitro conditions. In addition the effect of bacterial inoculation of Azotobacter on N, P, K uptake by three P responsive wheat genotypes (Triticum aestivum L.) under greenhouse conditions at five nutrient levels (Control, 90 kg N ha—1, 90 kg N + 26 kg P ha—1, 120 kg N ha—1 and 120 kg N + 26 kg P ha—1) was studied. In vitro phosphate solubilization and growth hormone production by mutant strains was more than by the soil isolates. Inoculation of wheat varieties with the soil isolates and mutant strains of A. chroococcum showed greater NPK uptakes as compared with parent soil isolates. Mutant strains M15 and M37 were proved to be the most effective for all three wheat varieties with regard to NPK uptake as well as root biomass production under greenhouse conditions.  相似文献   

6.
Field estimation of the microbial biomass of soils derived from natural and anthropogenic parent materials Whereas the estimation and evaluation of physical and chemical soil properties is possible with the use of widely accepted methods, there is still no procedure with respect to microbiological parameters. Therefore, our objective was to develop a field procedure for estimating the integral parameter ”︁microbial biomass”. For this purpose, microbial biomass data (Cmic in dry matter) of soils developed in natural parent materials from 116 sites in Germany and abroad were analyzed. Additionally, Cmic in 33 German soils developed in anthropogenic parent materials was determined. In the soils under consideration, Cmic varied between 100 and 4000 kg ha—1 in the upper 30 cm. For soils of natural substrates statistical relations between Cmic and the humus and clay content as well as pH were derived. From these parameters which are combined as the diagnostic characteristics of humus a simple procedure for estimating Cmic of arable soils was developed. For soils developed in anthropogenic parent materials, however, an estimation procedure was developed that uses the nature of the parent material and the degree of soil development. To evaluate the estimated amount of Cmic, a frame consisting of 6 classes is proposed: < 200 kg ha—1 (= very low), 200—400 kg ha—1 (= low), 400—800 kg ha—1 (= moderate), 800—1600 kg ha—1 (= medium), 1600—3200 kg ha—1 (= high), and > 3200 kg ha—1 (= very high).  相似文献   

7.
N20 emissions and denitrification N-losses. precipitation, air temperature, soil moisture, bulk density and content of mineral N were monitored in 9 different agricultural soils in 6 European countries throughout the vegetation period (April to September) 1992 and 1993. N2O emissions and denitrification N-losses were log-normal distributed, reflecting high temporal changes. While small flux rates (< 2 g N ha?1 d?1) were detectable every day, high rates (> 10 g N ha?1 d?1) were measured after fertilization. An attempt to relate the emission variables to climate and soil variables was made through the use of correlation analysis. The mean N20 emissions from soil were significantly correlated with the soil properties clay, organic C and mineral N content and the amount of applied mineral N fertilizer. The best prediction of the N2O emission rates (r2 = 0.734) was achieved by multiple linear regression using the soil parameter clay and mineral N. Only 50% of the observed variation could be explained by the factors Corg and mineral N, which describe the substrate availability for microbial processes. No successful statistical model was found for the prediction of denitrification N-losses.  相似文献   

8.
Significance of microbial biomass and non-exchangeable ammonium with respect to the nitrogen transformations in loess soils of Niedersachsen during the growing season of winter wheat. I. Change of pool sizes Nitrogen transformations in loess soils have been examined by laboratory and field experiments. After straw application (· 8 t · ha?1), N in microbial biomass (Nmic) increased by about 20 mg · kg?1 soil (· 90 kg N · ha?1 · 30 cm?1) after 9 days of incubation (20 °C). Another laboratory experiment yielded an increase of about 400 mg of NH4+-N · kg?1 fixed by minerals within 1 h after addition of 1 M NH4+-acetate. Defixation of the recently fixed NH4+ after addition of 1 M KCl amounted to only 60 mg · kg?1 within 50 days. In a field experiment with winter wheat 1991, an increase in Nmic of about 80 kg N · ha?1 · 30 cm?1 was observed from March to June. After July, growth of the microbes was limited by decreased soluble carbon concentrations in the rhizosphere. Different levels of mineral N-fertilizer (0, 177 and 213 kg N · ha?1) did not affect significantly the microbial biomass. The same field experiment yielded a decrease of non-exchangeable ammonium on the “zero”-fertilized plot in spring by 200 kg N · ha?1 · 30 cm?1. The pool of fixed ammonium increased significantly after harvest. After conventional mineral N-fertilizer application (213 kg N · ha?1). NH4+-defixation was only about 120 kg N · ha?1 · 30 cm?1 until July.  相似文献   

9.
Denitrification losses from a horticultural soil as affected by incorporation of fresh plant residues Denitrification in the Ap-horizon of a Luvisol under horticulture as affected by fresh cauliflower residues (Brassica oleracea L. var. botrytis) was measured using the acetylene inhibition technique. The residues were chopped and ploughed in 25 cm deep. Denitrification rates, soil temperature, moisture, nitrate and watersoluble carbon were determined from the end of July until the end of September. One day after incorporation of plant residues the denitrification rate showed a rapid increase to the highest value (5.15 kg N · ha?1 · day?1) of the experimental period. During the following 57 days the rates declined to the level of the untreated control. The highest denitrification rates occurred in the experimental sites which received plant residues. The estimated N-loss for the whole experimental period (57 days) totalled 43.6 kg N · ha?1 in the treatment with plant residues and 2 kg N · ha?1 in the treatment without plant residues. Coefficients of variation of denitrification rates were high (29–206%). Within the treatment ‘with plant residues’ denitrification rate and watersoluble carbon were positively correlated.  相似文献   

10.
Dissolved organic nitrogen (DON) substantially contributes to N leaching from forest ecosystems. However, little is known about the role of DON for N leaching from agricultural soils. Therefore, the aim of our study was to quantify the contribution of DON to total N leaching from four agricultural soils. Concentrations and fluxes of DON and mineral N were monitored at two cropped sites (Plaggic Anthrosols) and two fallow plots (Plaggic Anthrosol and Gleyic Podzol) from November 1999 till May 2001 by means of glass suction plates. The experimental sites were located near the city of Münster, NW Germany. Median DON concentrations in 90 cm depth were 2.3 mg l—1 and 2.0 mg l—1 at the cropped sites and 1.6 mg l—1 and 1.3 mg l—1 at the fallow sites. There was only a slight (Anthrosols) or no (Gleyic Podzol) decrease in median DON concentrations with increasing depth. Total N seepage was between 19 kg N ha—1 yr—1 and 46 kg N ha—1 yr—1 at the fallow sites and 16—159 kg N ha—1 yr—1 at the cropped sites. For the fallow plots, DON seepage contributed 10—21 % to the total N flux (4—5 kg DON ha—1 yr—1), at the cropped sites DON seepage was 6—21 % of the total N flux (6—10 kg DON ha—1 yr—1). Thus, even in highly fertilized agricultural soils, DON is a considerable N carrier in seepage that should be considered in detailed soil N budgets.  相似文献   

11.
Influence of iron content in sewage sludges on parameters of phosphate availability in arable soils The use of iron salts for the P elimination in sewage plants is widely used. But it is not clear whether the P availability in arable soils is negatively influenced by iron compounds or not. The aim of the investigations was, therefore, to study the influence of two sewage sludges with a high and a low Fe content respectively on P sorption and phosphate concentration (Pi) in the soil solution after application of CaHPO4 or sewage sludge to 5 loamy and 4 sandy soils (pot experiments and 1 silty loam (field experiment)). Soils were analyzed 1, 6, and 13 months after P application. Sludge Gö contained 12 kg P and 65 kg Fe (t DM)—1 (P : Fe = 1 : 5.4) and sludge Sh 25 kg P and 39 kg Fe (t DM)—1 (P : Fe = 1 : 1.5). The basic P application was 60 kg P ha—1 (= 30 mg P (kg soil)—1 in the pot experiment, as sludge or as CaHPO4). P uptake by maize was determined in a separate pot experiment with a loamy soil and the same P application rate. The P sorption capacity remained similar in all soils after application of sludge Sh (P : Fe = 1:1.5) compared with soils without sludge, however, after application of sludge Gö the P sorption increased by 16% (0—59%). After application of sludge Sh the mean Pi concentration increased in loamy soils by 34% and in sandy soils by 15%. On the other hand the Pi concentration decreased after applying sludge Gö by 13% and 36% as compared to the controls of the respective soils. In the field experiment the Pi concentration of plots with a high P level (50 mg lactate soluble P (kg soil)—1) was also significantly decreased after application of 10 t sludge Gö (126 kg P ha—1) in comparison with triple phosphate. One month after the application of increasing amounts of sludge Gö (5, 10, 15 t DM ha—1) both the concentration of oxalate‐soluble Fe in the soil and the P sorption were increased. The elevated relationship between these two parameters was highly significant (r2 = 0.6 — 0.97). Plant uptake of P was less after application of sludge Gö than after application of sludge Sh and much less than P uptake from CaHPO4. Sewage sludges with a P : Fe ratio of 1 : 5 should not be recommended for agricultural use, as the P availability is significantly reduced. Iron salts should not be used for conditioning of sludges.  相似文献   

12.
Significance of microbial biomass and mineral fixed ammonium with respect to the nitrogen transformations in loess soils of Niedersachsen during the growing season of winter wheat. II. 15N-turnover Field experiments 1988/89 on a fallow plot of the southern Niedersachsen loess area with straw application (δ 10 t · ha?1, homogeneously incorporated by hand) yielded an increase in microbial biomass-N (Nmic) by 60 kg N · ha?1 · 30 cm?1 until March 1989 and further 40 kg N · ha?1, till May which was almost completely remobilized until harvest. For a cropped plot (with winter wheat and 10 t · ha?1 straw incorporation), N immobilization was of similar magnitude. Up to 18% of the applied 15N-fertilizer (185 kgN · ha?1) were microbially immobilized. In contrast to 1988/89, no significant mass change of Nmic occurred in 1991 due to straw application (δ 10t · ha?1). Variations in the amount of Nmic were nearly independent on the treatment (crop, with 140 kg fertilizer-N · ha?1 or without N-fertilizer, respectively; fallow plot without fertilizer-N) within a range of 225-400 kg N · ha?1 · 30 cm?1. Directly after N-application (each 70 kg N · ha?1 in March and in May), up to 100% of the fertilizer-N were assimilated by the microbes. Subsequently, remobilization of the immobilized nitrogen occurred within 2 (in March) or 6 weeks (in May), respectively. Simultaneously, organic soil-N was mineralized after each N-application and minerally fixed for us biggest part. Between March and June, the fixed NH4+ decreased by about 112 kg · ha?1 · 30 cm?1.  相似文献   

13.
Intensification of homegardens in the Nuba Mountains may lead to increases in C and nutrient losses from these small‐scale land‐use systems and potentially threaten their sustainability. This study, therefore, aimed at determining gaseous C and N fluxes from homegarden soils of different soil moisture, temperature, and C and N status. Emissions of CO2, NH3, and N2O from soils of two traditional and two intensified homegardens and an uncultivated control were recorded bi‐weekly during the rainy season in 2010. Flux rates were determined with a portable dynamic closed chamber system consisting of a photo‐acoustic multi‐gas field monitor connected to a PTFE coated chamber. Topsoil moisture and temperature were recorded simultaneously to the gas measurements. Across all homegardens emissions averaged 4,527 kg CO2‐C ha?1, 22 kg NH3‐N ha?1, and 11 kg N2O‐N ha?1 for the observation period from June to December. Flux rates were largely positively correlated with soil moisture and predominantly negatively with soil temperature. Significant positive, but weak (rs < 0.34) correlations between increasing management intensity and emissions were noted for CO2‐C. Similarly, morning emissions of NH3 and increasing management intensity were weakly correlated (rs = 0.17). The relatively high gaseous C and N losses in the studied homegardens call for effective management practices to secure the soil organic C status of these traditional land‐use systems.  相似文献   

14.
A long‐term fertilizer experiment, over 27 years, studied the effect of mineral fertilizers and organic manures on potassium (K) balances and K release properties in maize‐wheat‐cowpea (fodder) cropping system on a Typic Ustochrept. The treatments consisted of control, 100% nitrogen (100% N), 100% nitrogen and phosphorus (100% NP), 50% nitrogen, phosphorus, and potassium (50% NPK), 100% nitrogen, phosphorus, and potassium (100% NPK), 150% nitrogen, phosphorus, and potassium (150% NPK), and 100% NPK+farmyard manure (100% NPK+FYM). Nutrients N, P, and K in 100% NPK treatment were applied at N: 120 kg ha—1, P: 26 kg ha—1, and K: 33 kg ha—1 each to maize and wheat crops and N: 20 kg ha—1, P: 17 kg ha—1, and K: 17 kg ha—1 to cowpea (fodder). In all the fertilizer and manure treatments removal of K in the crop exceeded K additions and the total soil K balance was negative. The neutral 1 N ammonium acetate‐extractable K in the surface soil (0—15 cm) ranged from 0.19 to 0.39 cmol kg—1 in various treatments after 27 crop cycles. The highest and lowest values were obtained in 100% NPK+FYM and 100% NP treatments, respectively. Non‐exchangeable K was also depleted more in the treatments without K fertilization (control, 100% N, and 100% NP). Parabolic diffusion equation could describe the reaction rates in CaCl2 solutions. Release rate constants (b) of non‐exchangeable K for different depth of soil profile showed the variations among the treatments indicating that long‐term cropping with different rates of fertilizers and manures influenced the rate of K release from non‐exchangeable fraction of soil. The b values were lowest in 100% NP and highest in 100% NPK+FYM treatment in the surface soil. In the sub‐surface soil layers (15—30 and 30—45 cm) also the higher release rates were obtained in the treatments supplied with K than without K fertilization indicating that the sub‐soils were also stressed for K in these treatments.  相似文献   

15.
N mineralization in sandy soils of the ‘Fuhrberg well field’ (Hannover) during winter Net N mineralization was measured under field conditions during winter and spring 1991/92 in sandy arable soils (Gleyic Podzols, Mollic Gleysols, Gleyic Arenosols) of the ‘Fuhrberg well field’, a drinking water catchment north-east of Hannover. The aim was to assess leaching losses of nitrate from mineralization processes during the winter on soils formerly used as grassland. Two field procedures were used: the incubation of soil material in polyethylene bags at its original location and rain sheltered fallow plots. Between 6 and 40 (100) kg N ha?1 were mineralized during 73 days from Dec., 17th to March, 2nd. Mineralisation rates were closely correlated to the organic N and C contents of the soils (r2 ± 0.9). In the uncovered soils, the NO3 was completely leached out. On five out of seven fields the process ‘N-mineralization during winter’ alone was sufficient to exceed the official limit for drinking water (50 mg 1?1 NO3? ) in the uppermost groundwater. It is concluded that even 15 years after converting grassland into arable land the Norg and Corg levels in the soils had not reached a new equilibrium.  相似文献   

16.
Drainage and cultivation of organic soils often result in large nitrous oxide (N2O) emissions. The objective of this study was to assess the impacts of nitrogen (N) fertilizer on N2O emissions from a cultivated organic soil located south of Montréal, QC, Canada, drained in 1930 and used since then for vegetable production. Fluxes of N2O were measured weekly from May 2004 to November 2005 when snow cover was absent in irrigated and non‐irrigated plots receiving 0, 100 or 150 kg N ha−1 as NH4NO3. Soil mineral N content, gas concentrations, temperature, water table height and water content were also measured to help explain variations in N2O emissions. Annual emissions during the experiment were large, ranging from 3.6 to 40.2 kg N2O‐N ha−1 year−1. The N2O emissions were decreased by N fertilizer addition in the non‐irrigated site but not in the irrigated site. The absence of a positive influence of soil mineral N content on N2O emissions was probably in part because up to 571 kg N ha−1 were mineralized during the snow‐free season. Emissions of N2O were positively correlated to soil CO2 emissions and to variables associated with the extent of soil aeration such as soil oxygen concentration, precipitation and soil water table height, thereby indicating that soil moisture/aeration and carbon bioavailability were the main controls of N2O emission. The large N2O emissions observed in this study indicate that drained cultivated organic soils in eastern Canada have a potential for N2O‐N losses similar to, or greater than, organic soils located in northern Europe.  相似文献   

17.
Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.  相似文献   

18.
During the last three decades, large amounts of soil organic matter (SOM) and associated nutrients have been accumulated in arable soils of Western Germany (former FRG) due to deepening of the plough layers (from < 25 to > 35 cm) and to fertilizer application rates which have exceeded the amounts of nutrients removed in harvested crops. Organic carbon and total nitrogen balances (1970—1998) on 120 plots from 16 farms in southern Lower Saxony yielded a cumulative increase of up to 16 t C ha−1 and 1 t N ha−1 in loess soils used for cash crop production and up to 26 t C ha−1 and 2.4 t N ha−1 in sandy soils under livestock production. The buffering capacity for reactive compounds, particularly of C, N, S and P and of other (organic or inorganic) pollutants will reach its limits in the near future, after organic matter ”︁equilibria” have been re‐established. An immediate adaptation of the current fertilizer application rates to the nutrient export by field crops is therefore urgently needed.  相似文献   

19.
The rice‐wheat annual double cropping system occupies some 0.5 million ha in the Himalayan foothills of Nepal. Alternating soil drying and wetting cycles characterize the 6–10 weeks long dry‐to‐wet season transition period (DWT) after wheat harvesting and before wetland rice transplanting. Mineral fertilizer use in the predominant smallholder agriculture is low and crops rely largely on native soil N for their nutrition. Changes in soil aeration status during DWT are likely to stimulate soil N losses. The effect of management options that avoid the nitrate build‐up in soils during DWT by N immobilization in plant or microbial biomass was studied under controlled conditions in a greenhouse (2001/2002) and validated under field conditions in Nepal in 2002. In potted soil in the greenhouse, the gradual increase in soil moisture resulted in a nitrate N peak of 20 mg (kg soil)–1 that rapidly declined as soil moisture levels exceeded 40 % water‐filled pore space (equiv. 75 % field capacity). Similarly, the maximum soil nitrate build‐up of 40 kg N ha–1 under field conditions was followed by its near complete disappearance with soil moisture levels exceeding 46 % water‐filled pore space at the onset of the monsoon rains. Incorporation of wheat straw and/or N uptake by green manure crops reduced nitrate accumulation in the soil to < 5 mg N kg–1 in pots and < 30 kg N ha–1 in the field (temporary N immobilization), thus reducing the risk for N losses to occur. This “saved” N benefited the subsequent crop of lowland rice with increases in N accumulation from 130 mg pot–1 (bare soil) to 185 mg pot–1 (green manure plus wheat straw) and corresponding grain yield increases from 1.7 Mg ha–1 to 3.6 Mg ha–1 in the field. While benefits from improved soil N management on lowland rice are obvious, possible carry‐over effects on wheat and the feasibility of proposed options at the farm level require further studies.  相似文献   

20.
Consistent use of mulches over several years can provide significant N to avocado. Study of a 3-year-old Ventura, California, avocado orchard mulched annually for 3 years with 12–14 Mg ha–1 chipped eucalyptus showed that total N in the mulched soil was double that of the control. Mulched intact soil cores released 53 kg ha–1 more N annually than control treatments. A litterbag study showed that net mineralization of the applied mulch commenced approximately 8.5 months following application. Mulched soils tended to be warmer and moister than control soils and temperatures varied less. Laboratory incubations of mulch and soil layers showed that net mineralization rates (mg kg–1 day–1) were greatest in the lowest decomposed mulch layer, but that more N mineralized overall (g m2) in the soil due to its greater density.  相似文献   

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