The performance of the Danish simulation model DAISY in prediction of Nmin at spring

In Denmark the Danish Agricultural Advisory Centre has for some years used the soil content of mineral nitrogen in spring in fertilizer recommendations. Since 1986 these recommendations have been based on soil samples carried out at all intersections of a nationwide 7 km square grid in Denmark. It was hoped that it may be possible to replace soil measurements with values of soil mineral-N calculated with a model. The Danish simulation model DAISY, which among other things simulates changes in the inorganic N content of the soil, was evaluated with respect to the N_min content in the early spring under bare soil and under winter cereal. For both situations the precrop was cereals. The performance of the model was evaluated in farming systems receiving mineral fertilizer and in some instances organic manures. The results were analysed according to type of subsoil: sandy or loamy. Predictions were 11 kg N ha^–1 less than the measured values as a mean and the differences between simulated and measured values were high for fields receiving organic manures. Predictions were less than ± 10 kg N ha^–1 of measured values in 25–58% of cases for the different types of crop cover at the time of soil sampling, type of subsoils and fertilizer strategies, respectively. Predictions were less than ± 20 kg N ha^–1 of measured values in 48–89% of cases for the different situations. The best predictions were obtained for sandy subsoils covered by winter cereal and supplied with mineral fertilizer only. It is concluded that the quality of the data used as input in the model has to be increased and that further developments of parts of the DAISY model are needed before modelling can be a useful tool in N fertilizer recommendations.     

Impact of different forms of N fertilizer on N2O emissions from intensive grassland

Nitrous oxide (N₂O) emissions were measured over two years from an intensively managed grassland site in the UK. Emissions from ammonium nitrate (AN) and urea (UR) were compared to those from urea modified by various inhibitors (a nitrification inhibitor, UR(N), a urease inhibitor, UR(U), and both inhibitors together, SU), as well as a controlled release urea (CR). N₂O fluxes varied through time and between treatments. The differences between the treatments were not consistent throughout the year. After the spring and early summer fertilizer applications, fluxes from AN plots were greater than fluxes from UR plots, e.g. the cumulative fluxes for one month after N application in June 1999 were 5.2 ± 1.1 kg N₂O-N ha^–1 from the AN plots, compared to 1.4 ± 1.0 kg N₂O-N ha^–1 from the UR plots. However, after the late summer application, there was no difference between the two treatments, e.g. cumulative fluxes for the month following N application in August 2000 were 3.3 ± 0.7 kg N₂O-N ha^–1 from the AN plots and 2.9 ± 1.1 kg N₂O-N ha^–1 from the UR plots. After all N applications, fluxes from the UR(N) plots were much less than those from either the AN or the UR plots, e.g. 0.2 ± 0.1 kg N₂O-N ha^–1 in June 1999 and 1.1 ± 0.3 kg N₂O-N ha^–1 in August 2000. Combining the results of this experiment with earlier work showed that there was a greater N₂O emission response to rainfall around the time of fertilizer application in the AN plots than in the UR plots. It was concluded that there is scope for reducing N₂O emissions from N-fertilized grassland by applying UR instead of AN to wet soils in cool conditions, e.g. when grass growth begins in spring. Applying UR with a nitrification inhibitor could cut emissions further.     

Nitrogen mineralization in paddy soils of the Chinese Taihu Region under aerobic conditions

In order to make more efficient use of mineral nitrogen fertilizers, the mineralization of organic N has to be fully understood and taken into account when meeting the nitrogen demand of crops. Aerobic long-term incubation experiments (147 days) based on the method by Stanford and Smith (1972, Soil Sci Soc Am Proc 36: 465), modified by Nordmeyer and Richter (1985, Plant Soil 39: 433), were carried out with paddy soils (Typic Epiaquepts) from two different locations in the Taihu Region in Eastern China, each in three depth increments (0–30, 30–60, 60–90 cm). The two soils had pH (H₂O) values between 6 and 7, silty clay loam texture, C _org contents (0–30 cm) of 8.9 and 16.6 g kg^–1 and N _tot contents of 1030 and 1650 mg kg^–1, respectively. Mineralization parameters were estimated from the cumulative amounts of leached nitrate-N via non-linear regression, using a two-pool approach based on first-order kinetics reactions. The results were compared to those from incubation experiments carried out with topsoils from the southern edge of the Chinese Loess Plateau (Roelcke et al., 1996, Trans. 9th Nitrogen Workshop, Braunschweig, p, 313). The optimized parameters were used to simulate the N regime in paddy soils with a deterministic model (Han et al., 2001, Proc. Int. Conf. China, p. 411). A winter wheat growing period (November 1995 to June 1996) in Wuxi County was simulated. A total of 220 kg ha^–1 mineral N fertilizer were applied as NPK and urea, split into four doses. Simulated results were compared to the measured mineral nitrogen (N _min) contents over the growing season. Cumulative aerobic N mineralization (0–1.0 m profile) yielded 84 kg N ha^–1. The modelled total cumulative N losses amounted to 76 kg N ha^–1 or 35% of the N applied.     

The N-cycle as determined by intensive agriculture – examples from central Europe and China

Using a scientific assessment concept of sustainability in crop-production based on the entropy production minimization principle of thermodynamics, formation and non-use of soluble and volatile (by-)products of the nutrient cycles within the system are interpreted as indicators or measures of the low efficiency/sustainability of recent forms of intensive agriculture. The simultaneous high energy input in modern crop production systems further shows the difference between these and quasi-stationary natural systems with maximum bioproduction having minimum energy dissipation and entropy production. Using balance sheets and dynamic approaches, the practical implications regarding the nitrogen cycle in central Europe (FR Germany) and China are exemplified and discussed. The average N balance of arable systems in Germany shows surplus N amounts of 110–130 kg N ha^-1 yr^-1. A high N immobilization in accordance with deepened top soil layers has governed N balances in Germany since about 1960. In China Nbalance surpluses in intensive agricultural (double-cropping) systems on the southern edge of the Loess Plateau now reach 125–230 kg N ha^-1 yr^-1. In field experiments, mineral N contents in the profiles (0–1.2 m depth) were 72–342 and 78–108 kg ha^-1 at harvest of summer maize and winter wheat, respectively. In the Taihu region in eastern China, surpluses in the N balance (rice-wheat double cropping) amount to 217–335 kg N ha^-1 yr^-1. N_min contents in the 0–0.9 m profiles of between 50 and 100 kg N ha^-1 were frequently found after winter wheat harvest. In two separate investigations of ground and well water samples in China, nitrate contents exceeded the critical WHO value for drinking water in 38–50% of the locations investigated.     

A farm-level evaluation of nitrogen and phosphorus fertilizer use and planting density for pearl millet production in Niger

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers' fields. Farmers in this region plant at low density (average 5,000 pockets ha^–1, 3 plants pocket^–1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha^–1 and 30 kg P₂O₅ ha^–1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets ha^–1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets ha^–1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets ha^–1 had average yields of 317 kg grain ha^–1 while those planting at densities higher than 6,500 pockets ha^–1 showed average yields of 977 grain ha^–1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets ha^–1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets ha^–1) or no economic benefit from fertilizer use will be realized.     

On-farm estimation of indigenous nitrogen supply for site-specific nitrogen management in the North China plain

Estimating indigenous nitrogen supply (INS) by measurement of crop N uptake in N omission plots for site-specific N management is not feasible on a routine basis because it involves destructive plant sampling and plant tissue analysis, which is time-consuming and expensive. The objective of this study was to determine the amount of INS and develop a method to estimate it using soil testing in the North China plain (NCP). On-farm experiments at 229 sites were conducted from 2003 to 2005 in seven key winter wheat (Triticum aestivum L.)/summer maize (Zea mays L.) production regions of the NCP. The mean INS during the wheat-growing season was129 kg N ha⁻¹ with a range from 62 to 212 kg N ha⁻¹, and it varied from 69 to 202 kg N ha⁻¹ with a mean of 142 kg N ha⁻¹ during the maize-growing season. Considering all sites, the variability of INS was not simulated by initial soil N _min or apparent N mineralization (N _organic) alone, while together they could explain about 38% and 60% of INS during the wheat and maize-growing seasons, respectively. During the wheat-growing season, mean N _organic was 63 kg N ha⁻¹, and 59% and 33% of its variation could be explained by SOM in high-yielding regions (mean yield, 7.6 t ha⁻¹) and low-yielding regions (mean yield, 5.3 t ha⁻¹), respectively. Mean N _organic during the maize-growing season was 109 kg N ha⁻¹, 22% of which could be explained by SOM across all sites. An average of 40% and 42% of INS variation could be explained by both SOM and initial soil N _min content during the wheat and maize-growing seasons, respectively. We conclude that the accuracy of estimating crop N requirement for site-specific N management will be increased by using initial soil N _min and SOM.     

Effect of nitrogen,phosphorus and molybdenum application on growth and symbiotic N2-fixation of groundnut in an acid sandy soil in Niger

Two field experiments were conducted in 1988 and 1989 on an acid sandy soil in Niger, West Africa, to assess the effect of phosphorus (P), nitrogen (N) and micronutrient (MN) application on growth and symbiotic N₂-fixation of groundnut (Arachis hypogaea L.). Phosphorus fertilizer (16 kg P ha^–1) did not affect pod yields. Addition of MN fertilizer (100 kg Fetrilon Combi 1 ha^–1; P + MN) containing 0.1% molybdenum (Mo) increased pod yield by 37–86%. Nitrogen concentration in shoots at mid pod filling (72 days after planting) were higher in P + MN than in P – MN fertilizer treatment. Total N uptake increased from 53 (only P) to 108 kg N ha^–1 by additional MN application. Seed pelleting (P + MoSP) with 100 g Mo ha^–1 (MoO₃) increased nitrogenase activity (NA) by a factor of 2–4 compared to P treatment only. The increase in NA was mainly due to increase in nodule dry weight and to a lesser extent to increase in specific nitrogenase activity (SNA) per unit nodule dry weight. The higher NA of the P + MoSP treatment was associated with a higher total N uptake (55%) and pod yield (24%). Compared to P + MoSP or P + MN treatments application of N by mineral fertilizer (60 kg N ha^–1) or farmyard manure (130 kg N ha^–1) increased only yield of shoot dry matter but not pod dry matter. Plants supplied with N decreased soil water content more and were less drought tolerant than plants supplied with Mo. The data suggest that on the acid sandy soils in Niger N deficiency was a major constraint for groundnut production, and Mo availability in soils was insufficient to meet the Mo requirement for symbiotic N₂-fixation of groundnut.     

Influence of long-term changes in nitrogen flows on the environment: A case study of a city in Hokkaido, Japan

There is an urgent need to establish sustainable nutrient cycling. Changes in amounts of N flow and separation of production and consumption sectors are becoming a serious environmental problem. In this study, the yearly N in- and outflow of a city in northern Japan from 1912 to 2002 was investigated based on the statistics and inventory data. Based on the characteristics of the N flow, the period was divided into manure-based period (MBP, 1912–1950), transition period from manure- to chemical fertilizer-based period (TP, 1950–1970), and chemical fertilizer-based period (CBP, 1970–2002). The highest amount of N inflow (up to 350 Mg N y^–1) was observed at the end of the MBP, and the second peak (about 300 Mg N y^–1) at the beginning of the CBP. The N application rate on farmland increased from 68 kg N ha^–1 in 1912 to above 250 kg N ha^–1 in the 1950s, then decreased to 168 kg N ha^–1 in 2002. The farmland productivity increased from 30 kg N ha^–1 at the end of the 1950s to 90 kg N ha^–1 in 2002, due to improvement in crop varieties and management methods. In MBP surplus N in farmland and NH₃ volatilization accounted for 90% of the N outflow from the city, then in CBP, disposal N and surplus N in farmland became the main N outflow. All these outputs are considered to increase the N concentration in rivers and/or underground water. In the case of surplus N in farmland, it exceeded the amount of optimum N management (<50 kg N ha^–1; , Agricult. Ecosyst. Environ. 72: 35–52) during 1935–1970 and 1981–1997. In order to prevent degradation of the environment through artificially altered nutrient flow, we need to be aware of the environmental impact of the N flow and establish proper N management practices.     

Rate,source and time of N application for meadow bromegrass in central Alberta,Canada

Field experiments were conducted from 1988 to 1991 or 1992 at two sites (Lacombe-Black Chernozem and Eckville-Gray Luvisol) in central Alberta, Canada to determine the effect of rate (0 to 300 kg N ha^–1), source [urea and ammonium nitrate (AN)] and time (early fall, late fall, early winter, early spring and late spring) of N application on dry matter yield (DMY), protein yield (PY), protein concentration (PC), N-use efficiency (NUE), % N recovery (% NR) and nitrate-N (NO₃–N) concentration in meadow bromegrass (Bromus bibersteinii Roem and Shult. cv. Regar). The DMY, PY and PC increased with increasing applied N, but the NUE and % NR decreased at high N rates. The increases in PY from fertilizer N were proportionately greater than DMY due to increase in PC at high N rates. Potentially toxic NO₃–N levels (>2.3 g kg^–1) were not found in the forage. Urea generally produced lower DMY, PY, PC, NUE and % NR than AN, regardless of time of application and cut. Early spring application had the highest and early winter application had the lowest DMY and PY. In conclusion, urea was less effective than AN as a forage fertilizer and early spring application was most effective.     

Estimates of the interannual variations of N2O emissions from agricultural soils in Canada

The DNDC model was used to estimate direct N₂O emissions from agricultural soils in Canada from 1970 to 1999. Simulations were carried out for three soil textures in seven soil groups, with two to four crop rotations within each soil group. Over the 30-year period, the average annual N₂O emission from agricultural soils in Canada was found to be 39.9 Gg N₂O–N, with a range from 20.0 to 77.0 Gg N₂O–N, and a general trend towards increasing N₂O emissions over time. The larger emissions are attributed to an increase in N-fertilizer application and perhaps to a trend in higher daily minimum temperatures. Annual estimates of N₂O emissions were variable, depending on timing of rainfall events and timing and duration of spring thaw events. We estimate, using DNDC, that emissions of N₂O in eastern Canada (Atlantic Provinces, Quebec, Ontario) were approximately 36% of the total emissions in Canada, though the area cropped represents 19% of the total. Over the 30-year period, the eastern Gleysolic soils had the largest average annual emissions of 2.47 kg N₂O–N ha^–1 y^–1 and soils of the dryer western Brown Chernozem had the smallest average emission of 0.54 kg N₂O–N ha^–1 y^–1. On average, for the seven soil groups, N₂O emissions during spring thaw were approximately 30% of total annual emissions. The average N₂O emissions estimates from 1990 to 1999 compared well with estimates for 1996 using the IPCC methodology, but unlike the IPCC methodology our modeling approach provides annual variations in N₂O emissions based on climatic differences.     

Nitrogen requirements of corn (Zea mays L.) as affected by monocropping and intercropping with Alfalfa (Medicago sativa)

Intercropping perennials with corn has the potential to improve utilization of the growing season over monocropping corn in regions where a substantial portion of the growing season is too cool for corn growth. The biomass potential and fertilizer N requirements of monocropped corn (Zea mays) grown using conventional tillage were compared with those of corn intercropped with alfalfa (Medicago sativa) in 1987 and 1988. The intercropped alfalfa was harvested once prior to planting the corn each spring. Rotation effects on and N fertilizer requirements for monocropped corn following these treatments and also following monocropped alfalfa, were evaluated in 1989 and 1990. During the two years of intercropping for which data is presented, the critical intercropped corn biomass (13.05 Mg ha^–1) estimated using a quadratic-plus plateau model, was close to the monocropped corn biomass (13.01 Mg ha^–1), but an estimated 83 kg ha^–1 more N was required for intercropped corn to reach the critical biomass. Total biomass (intercropped corn and alfalfa) was 25% greater than that of the monocropped corn, and the total N uptake was 55% greater than that by monocropped corn over the two- year period. After rotation to monocropped corn using conventional tillage in 1989, corn biomass averaged over N rates following intercropping or monocropped corn was lower (P=0.01) than following monocropped alfalfa. Critical corn biomass estimated was highest following alfalfa and lowest following monocropped corn, and more N fertilizer was required to attain the critical biomass under continuous monocropped corn in 1989. Corn yields and N uptake values in 1990 were not significantly different among the cropping systems. The N fertilizer replacement values due to intercropping decreased from above 90 kg N ha^–1 in the first year of rotation to less than 40 kg N ha^–1 in the second year of rotation. Considering the higher potential for total biomass production and rotation benefit, intercropping is a viable alternative to conventional corn monoculture for forage production.     

Nitrogen uptake by autumn sown sugar beet

The influence of N fertilizer rate on uptake and distribution of N in the plant,¹⁵N labelled fertilizer uptake and sugar yield were studied for 3 years on autumn sown sugar beet (Beta vulgaris L.) under Mediterranean (Southern Spain) rain-fed and irrigated conditions. Available average soil N prior to sowing was 69 kg N ha^–1, and net mineralisation in the soil during the growth period was 130 kg N ha^–1. Maximum N uptake occurred in the spring and increased with increasing fertilizer rates in the irrigated crop. There was no increase in N uptake in the sugar beet cropped under rain-fed conditions because of water shortage. Maximum average N uptake both by roots and tops was between 200 and 250 kg N ha^–1. When N fertilizer was not applied, average uptake from the soil was between 130 and 140 kg N ha^–1. At the end of the growth period there was a marked translocation of N from the leaves to the root which increased with the N fertilizer rate. The N ratio top/roots at harvest was 0.45–0.5 and 0.8- - 1 in the irrigated and rain-fed sugar beet, respectively. Maximum¹⁵N labelled fertilizer uptake took place in May-June, being larger in irrigated sugar beet or when spring rainfall was more abundant. Fertilizer use efficiency varied between 30% and 68%. Sugar yield response to N fertilizer rates depended on the N available in the soil and on the total water input to the crop, particularly in spring. The response was more constant in the irrigated crop, where optimum yield was obtained with a fertilizer rate of 160 kg N ha^–1. In the rain-fed crop, the optimum dose proved more erratic, with an estimated mean of 100 kg N ha^–1. The amount of N required to produce 1 t of root and of sugar ranged between 1.5 and 3.8 kg N and between 11.1 and 22.4 kg N respectively, and varied according to the N fertilizer rates applied.     

Effect of climate change on greenhouse gas emissions from arable crop rotations

The DAISY soil–plant–atmosphere model was used to simulate crop production and soil carbon (C) and nitrogen (N) turnover for three arable crop rotations on a loamy sand in Denmark under varying temperature, rainfall, atmospheric CO₂ concentration and N fertilization. The crop rotations varied in proportion of spring sown crops and use of N catch crops (ryegrass). The effects on CO₂ emissions were estimated from simulated changes in soil C. The effects on N₂O emissions were estimated using the IPCC methodology from simulated amounts of N in crop residues and N leaching. Simulations were carried out using the original and a revised parameterization of the soil C turnover. The use of the revised model parameterization increased the soil C and N turnover in the topsoil under baseline conditions, resulting in an increase in crop N uptake of 11 kg N ha^–1 y^–1 in a crop rotation with winter cereals and a reduction of 16 kg N ha^–1 y^–1 in a crop rotation with spring cereals and catch crops. The effect of increased temperature, rainfall and CO₂ concentration on N flows was of the same magnitude for both model parameterizations. Higher temperature and rainfall increased N leaching in all crop rotations, whereas effects on N in crop residues depended on use of catch crops. The total greenhouse gas (GHG) emission increased with increasing temperature. The increase in total GHG emission was 66–234 kg CO₂-eq ha^–1 y^–1 for a temperature increase of 4°C. Higher rainfall increased total GHG emissions most in the winter cereal dominated rotation. An increase in rainfall of 20% increased total GHG emissions by 11–53 kg CO₂-eq ha^–1 y^–1, and a 50% increase in atmospheric CO₂ concentration decreased emissions by 180–269 kg CO₂-eq ha^–1 y^–1. The total GHG emissions increased considerably with increasing N fertilizer rate for a crop rotation with winter cereals, but remained unchanged for a crop rotation with spring cereals and catch crops. The simulated increase in GHG emissions with global warming can be effectively mitigated by including more spring cereals and catch crops in the rotation.     

Response to N-fertilizer of Italian ryegrass grown alone and in mixture with berseem clover under continental irrigated mediterranean conditions

In a field experiment over three growing seasons, the potential benefits of planting berseem clover (Trifolium alexandrinum L) with Westerwold Italian ryegrass (Lolium multiflorum Lam.) were examined under irrigated continental Mediterranean conditions.Similar N rates (0, 30, 60, 90 and 120 kg N ha^–1 cut^–1) were applied to both pure Italian ryegrass stands and mixtures, each given three successive cuts. One previously unfertilized cut was performed in late winter. Species in the mixture were established at 50:50 seed ratio but the mean proportion of berseem clover was 14%. Mean winter survival of berseem was 87% but 88% of the plants had leaves damaged by the frost. Forage production varied with both N rate and cutting sequence in both the pure stand and the mixture but differences between the two types of swards were significant only at low levels of fertilizer N. Total DM production over the four cuts in plots with N applications of 0,90, 180, 270 and 360 kg N ha^–1 a^–1 were 7.14, 9.51, 11.66, 13.91 and 14.36 t DM ha^–1 a^–1 in pure stand, respectively. Corresponding values for the mixture were 8.80, 10.94, 12.90, 14.05 and 13.64 t DM ha^–1 a^–1. The mean response of Italian ryegrass in the range of 0–360 kg N ha^–1 a^–1 was 20 kg DM per kg N applied. The corresponding value for the mixture was 13 kg DM per kg N applied. At the berseem clover proportions reached in this work, N equivalence showed values of about 80 kg N ha^–1 a^–1. As rates of N increased from 0 to 120 kg N ha^–1 cut^–1, nitrogen concentration increased by 78%. In the applied range of N fertilizers, N0₃-N was not affected.

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Résumé Dans un essai réalisé au cours de trois saisons agricoles, on a étudié le potentiel de 1'association du bersim avec le raygrass italien. Les cultures ont été emenées et irrigué dans des conditions continentales méditerranéennes. On a appliqué, aussi bien pour la culture pure du raygrass que pour l'association, une fertilisation azotée avec les doses suivantes (0, 30, 60, 90 et 120 kg N/ha/coupe) après chacune des trois coupes successives. Une autre coupe avant fertilisation a été faite à la fin de 1'hiver. Les proportions du bersim et du raygrass dans le mélange de graines étaient de 50:50. Cependant, dans la culture en association, les plantes du bersim n'étaient préentés qu'avec un 14 pourcent. 87% des plantes du bersim ont pu survivre en hiver, dont 88% avaient des feuilles endommagées par les gelées. La production d'herbe a été proportionnelle aux doses de fertilisation pour la culture pure et l'association. Néanmoins, différence entre les rendements de chacune de ces dernières était d'autant plus nette que les doses d'azote incorporées dans le sol étaient faibles. La production de la MS pour les quatre coupes dans les parcelles avec les applications de 0, 90, 180, 270 et 360 kg N ha^–1 a^–1 étaient de 7.14, 9.51, 11.66, 13.51, 14.36 tMS ha^–1 a^–1. Le rendement moyen du raygrass italien dans un intervalle de 0-360 kg N ha^–1 a^–1 a été de 20 kg MS par kg de N de fertilisation. Concernant le bersim, les valeurs équivalentes de N étaient de 1'ordre de 80 kg N ha^–1 a^–1. Au fur et à mesure que les doses de fertilisation azotée augmente de 0 à 120 kg ha^–1 coupe, la concentration en azote augmente de 78%. Dans l'intervalle de la fertilisation azotée appliqué NO₃ -N n'a pas été affectée.

     

Ammonia volatilization from grassland receiving nitrogen fertilizer and rotationally grazed by dairy cattle

The micrometeorological mass balance method was used to measure ammonia (NH₃) volatilization from rotationally grazed swards throughout the 1987 and 1988 growing seasons. In both years the swards were dressed with calcium ammonium nitrate (CAN) split over 7 dressings. In 1987 the sward received a total of 550 kg N ha^–1, in 1988 a total of 550 or 250 kg N ha^–1. For the 550 kg N ha^–1 treatments there were 8 and 9 grazing cycles, respectively, in 1987 and 1988 and 7 for the 250 kg N ha^–1 treatment. Losses from the 550 N sward were 42.2 and 39.2 kg N ha^–1 in 1987 and 1988, respectively; this was equivalent to 8.5 and 7.7% of the N returned to the sward in the excreta of the grazing cattle. The NH₃ loss from the 250N sward was 8.1 kg N ha^–1 in 1988, which was equivalent to 3.1% of the N returned to the sward in excreta during the growing season. There was a wide variation in NH₃ volatilization between the individual grazing periods. This indicates the necessity of continued measurements throughout the growing season to obtain reliable data on NH₃ volatilization. Soil humidity is suggested to be a key factor, because emissions were high from wet soil, and low from drier soil. Results of a Monte Carlo simulation study showed that the measured NH₃ loss from the 250 and 550 N swards had a standard deviation of 13 and 5% of the mean, respectively.     

Uptake of fertilizer and soil nitrogen by ryegrass swards during spring and mid-season

Double-labelled¹⁵N ammonium nitrate was used to determine the uptake of fertilizer and soil N by ryegrass swards during spring and mid-season. The effects of water stress (40% of mean rainfall v 25 mm irrigation per 25 mm soil water deficit) and the rate of application of N in the spring (40 v 130 kg ha^–1) on the recovery of 130 kg N ha^–1 applied in mid-season were also evaluated. Apparent recovery of fertilizer N (uptake of N in the fertilized plot minus that in the control expressed as a percentage of the N applied) was 95 and 79% for fertilizer N applied in the spring at rates of 40 and 130 kg ha^–1, respectively. Actual recovery of the fertilizer N assessed from the uptake of¹⁵N was only 31 and 48%, respectively. The uptake of soil N by the fertilized swards was substantially greater than that by the control. However, the increased uptake of soil N was always less than the amount of fertilizer N retained in or lost from the soil. Broadly similar patterns for the uptake of fertilizer and soil N were observed during mid-season. Uptake of N in mid-season was highest for swards which received 40 kg N ha^–1 in the spring and suffered minimal water stress during this period. Application of 130 kg N ha^–1 in spring reduced the uptake of N in mid-season to an extent similar to that arising from water stress. Only 1.8 to 4.2 kg ha^–1 (3 to 10%) of the N residual from fertilizer applied in the spring was recovered during mid-season. Laboratory incubation studies suggested that only a small part of the increased uptake of soil N by fertilized swards could be attributed to increased mineralisation of soil N induced by addition of fertilizer. It is considered that the increased uptake of soil N is partly real but mostly apparent, the latter arising from microbially mediated exchange of inorganic¹⁵N in the soil.     

Emissions of N2O from Scottish agricultural soils, as a function of fertilizer N

Potato fields and cut (ungrazed) grassland in SE Scotland gave greater annual N₂O emissions per ha (1.0–3.2 kg N₂O–N ha^-1) than spring barley or winter wheat fields (0.3–0.8 kg N₂O–N ha^-1), but in terms of emission per unit of N applied the order was potatoes > barley > grass > wheat. On the arable land, especially the potato fields, a large part of the emissions occurred after harvest.When the grassland data were combined with those for 2 years' earlier work at the same site, the mean emission over 3 years, for fertilization with ammonium nitrate, was 2.24 kg N₂O–N ha^-1 (0.62% of the N applied). Also, a very strong relationship between N₂O emission and soil nitrate content was found for the grassland, provided the water-filled pore space was > 70%. Significant relationships were also found between the emissions from potato fields and the soil mineral N content, with the added feature that the emission per unit of soil mineral N was an order of magnitude larger after harvest than before, possibly due to the effect of labile organic residues on denitrification.Generally the emissions measured were lower, as a function of the N applied, than those used as the basis for the current value adopted by IPCC, possibly because spring/early summer temperatures in SE Scotland are lower than those where the other data were obtained. The role of other factors contributing to emissions, e.g. winter freeze–thaw events and green manure inputs, are discussed, together with the possible implications of future increases in nitrogen fertilizer use in the tropics.     

Effect of irrigation and nitrogen fertilizer on yield,oil content,nitrogen accumulation and water use of canola (Brassica napus L.)

Effects of N application and water supply on yield, oil content and N accumulation by canola, cultivar Marnoo, grown on a heavy clay soil in the Goulburn Murray Irrigation Region were investigated. Treatments were rainfed (R_f) or watered at a deficit of 50 mm (40–60 mm, I₅₀) beginning in the spring. N treatments were 0, 50, 100 or 200 kg N ha^–1 at sowing or as split applications of 20/80, and 50/50 kg N ha^–1 at sowing and rosette, respectively.Yield (Y_g) ranged from 170 to 520 g m^–2. Irrigation and N increased yield in both years. Grain yields were increased by N application on the irrigated treatments when 100 or 200 kg N ha^–1 was applied. Oil concentrations ranged from a maximum of 46.4% in treatment N₀ to a minimum of 40.6% in treatment N₂₀₀ and was inversely related to seed N concentration. Although fertilizer N decreased oil concentration, it increased the yield of oil.Nitrogen accumulation (N_b) limited yield of all treatments and was described by the equation, Y_g = 806[1-EXP(–0.039*N_b)]. This implied a decrease in yield per unit of N_b at the higher rates of fertilizer addition with consequent increases in grain N concentration.The efficiency of water use in the production of grain (WUE_g) and biomass (WUE_b) were 7.5 and 23 kg ha^–1 mm^–1 respectively. Nitrogen additions increased WUE_g and WUE_b in both seasons. Maximum values of 8.9 (WUE_g 1986) and 26.8 (WUE_b 1987) were measured from treatment N₂₀₀. These data suggest that the crops made efficient use of the applied water.     

Effects of deep application of urea on NO and N2O emissions from an Andisol

A modeling study revealed that the depth of nitric oxide (NO) production in soil is crucial for its flux, while that of nitrous oxide (N₂O) is not. To verify this result, laboratory experiments with soil columns classified as Andisol (Hydric Hapludand) were conducted, with changing the depth of urea application, at 0–0.1 or 0.1–0.2 m. All the NO concentration profiles in soil exhibited a sharp peak at each fertilized layer within 5 days of fertilizer application. NO concentration in soil decreased abruptly as the distance from the fertilized layer increased. These findings imply that NO is produced mainly within the fertilized layer, but does not diffuse widely in the soil columns, because of rapid NO uptake within the soil. As a result, the NO flux from soil columns fertilized at 0.1–0.2 m depth over the 48-day study period was reduced to almost the same rate as that of the unfertilized one. The total NO emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m depth were 0.02, 1.39 (± 0.05) and 0.05 (± 0.03) kg N ha^–1, respectively, suggesting that NO emission derived from N fertilizer could be reduced to 2% by shifting the depth of fertilizer application by 0.1 m. On the other hand, soil N₂O concentration profiles exhibited a gentler peak, because of the lower uptake by soil. N₂O fluxes were affected more by the soil conditions, e.g. soil water content, than the distance between fertilized depth and soil surface. The total N₂O emissions from soil columns unfertilized and fertilized at 0–0.1 and 0.1–0.2 m were 0.02, 0.16 (± 0.03) and 0.25 (± 0.04) kg N ha^–1, respectively.     

Effects of type and amount of applied nitrogen fertilizer on nitrous oxide fluxes from intensively managed grassland

Five field experiments and one greenhouse experiment were carried out to assess the effects of nitrogen (N) fertilizer type and the amount of applied N fertilizer on nitrous oxide (N₂O) emission from grassland. During cold and dry conditions in early spring, emission of N₂O from both ammonium (NH ₄ ⁺ ) and nitrate (NO ₃ ^– ) containing fertilizers applied to a clay soil were relatively small, i.e. less than 0.1% of the N applied. Emission of N₂O and total denitrification losses from NO ₃ ^– containing fertilizers were large after application to a poorly drained sand soil during a wet spring. A total of 5–12% and 8–14% of the applied N was lost as N₂O and via denitrification, respectively. Emissions of N₂O and total denitrification losses from NH ₄ ⁺ fertilizers and cattle slurry were less than 2% of the N applied. Addition of the nitrification inhibitor dicyandiamide (DCD) reduced N₂O fluxes from ammonium sulphate (AS). However, the effect of DCD to reduce total N₂O emission from AS was much smaller than the effect of using NH ₄ ⁺ fertilizer instead of NO ₃ ^– fertilizer, during wet conditions. The greenhouse study showed that a high groundwater level favors production of N₂O from NO ₃ ^– fertilizers but not from NH ₄ ⁺ fertilizers. Inereasing calcium ammonium nitrate (CAN) application increased the emitted N₂O on grassland from 0.6% of the fertilizer application rate for a dressing of 50 kg N ha^–1 to 3.1% for a dressing of 300 kg N ha^–1. In another experiment, N₂O emission increased proportionally with increasing N rate. The results indicate that there is scope for reducing N₂O emission from grasslands by choosing the N fertilizer type depending on the soil moisture status. Avoiding excessive N application rates may also minimize N₂O emission from intensively managed grasslands.