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«: AGROCHEMICALS: FATE IN FOOD AND THE ENVIRONMENT PROCEEDINGS OF A SYMPOSIUM, ROME, 7 - 1 1 JUNE 1982 JOINTLY ORGANIZED BY IAEA AND FAO l^J I N T E R ...»

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In plants butonate is metabolized by deacylation via trichlorphone and demethyl trichlorphone with dichlorvos as a minor metabolite and by demethylation via demethyl butonate. Evaporation f r o m plant surfaces is increasing in the order trichlorphone butonate dichlorvos, which is a minor part o f evaporating insecticidal c o m p o u n d s following spraying o f butonate.

Photodegradation following 5 h o f ultraviolet irradiation o n glass plates is more expressed than 2 0 h o f sunlight irradiation and is preferred f o r dichlorvos ( 1 0 0 and 7 5 % ) in relation to butonate ( 3 0 and 2 % ) and trichlorphone (7 and 6%).

In mammals, according t o the preferred pH 7 and the increased activity o f certain enzymes, the formation o f vinylbutonate and trichlorphone with preferred rapid detoxication by demethylation to give the n o n - t o x i c demethylated metabolites is observed. T h e b l o o d level as well as residues in milk are given b y the graduation : trichlorphone vinylbutonate butonate dichlorvos.

The half-life o f butonate and metabolites in water, plants and animals in vitro and in vivo and the c o m p l e t e scheme o f suggested chemical degradation and metabolic pathways are given.

FERTILIZERS AND PESTICIDES

(Session VII) Chairman

–  –  –

FATE OF FERTILIZER NITROGEN IN SOIL-PLANT SYSTEMS WITH EMPHASIS ON THE

TROPICS.

The fate of fertilizer nitrogen in soil-plant systems is considered by analysing the different components of nitrogen balance. Special emphasis is given to research conducted in the tropics, using isotope techniques. Nitrogen fertilization, biological fixation, rainfall additions, nitrogen movement in the soil profile, nitrogen leaching, run-off and gaseous losses, and crop extraction/ export are considered. Data indicate that the fertilizer application rates cover a wide range;

in developed areas they are approximately 6.5 times higher than in developing areas. Nitrogen fixation is 17 to 40% of plant nitrogen and rainfall inputs are very low. A more extended analysis is made of nitrogen leaching losses, and it is concluded that they are not a problem if fertilizer is applied at normal levels, being of the order of 90 kg N/ha. Run-off losses do not seem to be a major nitrogen loss mechanism; gaseous losses are extremely variable and can rise to 30% of fertilizer nitrogen, and crop export is the most variable component of the balance.

–  –  –

* This work was supported by Conselho Nacional de Pesquisas (CNPq) fellowships to K. Reichardt and P.L. Libardi and an IAEA fellowship to S. Caballero Urquiaga.

–  –  –

demands rational use, something that has still not been established for most agrosystems. The need for basic information on the dynamics o f nitrogen under different soil-plant conditions is essential to achieve this goal.

Nitrogen is a nutrient element o f vital importance to all organisms. Although it is the most abundant element in the atmosphere, it is the one that most frequently limits agricultural production in both temperate and tropical regions.

While in most natural ecosystems the mineral nutrient balance is found to be in equilibrium, in agrosystems the highly mobile nutrients are lost, often on a catastrophic scale, as has been observed after deforestation o f rain forests. Soil management practices can cause enormous losses, especially o f nitrate, due to rapid oxidation o f organic matter, which is accelerated by these practices.

Owing to constant losses in agrosystems, use o f fertilizers has increased.

This in turn might lead to progressive losses, mainly to the atmosphere (in the case o f nitrogen) and by leaching. Many studies have been developed to evaluate nitrogen movement in soil profiles and also leaching losses, but it is first necessary to obtain both information and an understanding of the interaction of the different factors affecting the nitrogen balance in a system, as well as to optimize productivity, diminish losses and reduce the risk o f pollution. Isotope techniques are in this contex-t o f extreme importance and in many instances they are the only means o f achieving an answer.

Nitrogen balance in a soil-plant system is the summation o f all inputs ( N i n ) and outputs ( N o u t ) o f this element in a known soil volume (which should include the entire root system) in a given period o f time (t 2 —1[ ), so that AN = ( S N i n - 2 N o u t ) = N 2 - N (1) where AN is the change in nitrogen storage (kg/ha) in a soil layer L ( c m ) and N! and N 2 are the nitrogen storages (kg/ha) at times t, and t 2 (d) in a soil layer L.

Nitrogen inputs can come f r o m mineral fertilizers and/or organic matter additions ( N f ), biological nitrogen fixation ( N b ) and rainfall (N r ). Outputs can occur by leaching (Ng), run-off ( N 0 f ), gaseous losses ( N g ) and crop export ( N c e ), so that (2)

–  –  –

2. NITROGEN INPUTS

2.1. Nitrogen fertilization The amount o f nitrogen applied to crops depends on the mineral nitrogen content that the soil can supply during crop growth, and the crop need.

Malavolta [ 1 ] suggests that nitrogen fertilization should be equal to the difference between crop need and soil supply, multiplied by a coefficient К (equal to or greater than 1), which is a function o f fertilizer type, soil conditions, climate and plant, it being a measure o f the fertilizer use efficiency o f the plant. Regarding soil supply, it is generally assumed that organic matter is the main source o f nitrogen.





According to Bremner [2] and Cheng [3], the nitrogen content o f organic matter is about 5%. Total soil nitrogen, normally more than 85% [4] or 98% [2], is in the organic form and its content is extremely variable.

Organic nitrogen has to be mineralized to the ammonia form (N-NH 4 ) and the nitric form ( N - N 0 3 ) before it can be used by plants. Mineralization is a complex biological process affected by a great number o f factors, but these are not discussed here. The extreme variability o f these factors leads to variable mineralization rates, but these are generally very low. Only 1 to 2% o f the total organic nitrogen is mineralized annually in temperate climate areas. Under humid tropical conditions information is very limited, but there is evidence that its contribution to the plant is low due to the high rainfall rates [5]. In these areas good correlation between organic nitrogen content and crop response is not found, probably due to the intense leaching o f mineral nitrogen by rainfall. In many o f these areas it can be observed that during periods o f low rainfall, in which high soil temperatures promote partial sterilization, there is an accumulation o f products that can be nitrified. These, at the beginning o f the rainy season, are highly subject to leaching at a time when crop roots are not sufficiently developed to use all the nitrates produced [4, 6—10]. In Brazil, in order to estimate the soil nitrogen availability, an average mineralization rate o f organic matter o f 2% is assumed for the period o f 1 year. In the past, the total soil nitrogen content used to be considered [5].

Crop needs are related to crop growth rates which in turn depend on species, variety and yield. They also depend on the climatic factors and the technology level. Malavolta [ 1 ] and Sanchez [8] present tables of the nitrogen needs (extraction and export) o f the main tropical crops in which the influence o f the above factors can be seen, mainly the effect on yield. In Brazil [5], because o f lack o f an efficient critérium to discriminate crop response to nitrogen fertilization, average crop response curves for nitrogen are used with which it is possible to define economic application rates. On a world-wide basis the level o f agricultural development is influencing significantly the quantities o f fertilizer applied per hectare. Cooke [ 1 1 ] shows that greater use o f fertilizer is restricted to developed areas, in which rates are 6.5 times higher than in developing areas.

280 REICHARDT et al.

2.2. Biological nitrogen fixation

Biological nitrogen fixation constitutes one o f the main forms by which nitrogen is added to soil-plant systems, in temperate as well as in tropical regions.

Epstein [ 1 2 ] and Postgate and Hill [13] present a list of the different biological nitrogen fixing systems that occur in nature. Among these, the legame-Rhizobium associations are considered the most efficient [8]. Henzel and Norris [14] report that these associations contribute inputs o f 16 to 500 kg N/ha per year. The wide range depends to a large extent on the methodology and conditions under which the evaluation was made. Sanchez [8] affirms that in most cases the fixed quantities o f nitrogen are below those expected, mainly in tropical areas. This might be due to low concentrations o f phosphorus and calcium and high concentrations o f toxic elements, as Al in the majority o f soils.

In the last decade much evidence o f non-symbiotic associative N 2 -fixation, especially with C-4 tropical grasses such as Digitaria decumbens, Panicum maximum and Cynodon has accumulated. Azospirillum spp. appears to be dactylon, ubiquitous in many o f these systems for C-3 barley, rice, rye and wheat, while has the greatest affinity for maize, sorghum and C-4 grasses [15].

A. lipoferum Other diazotrophic biocoenoses have been found in spring wheat (Bacillus rice ( A c h r o m o b a c t e r ) Spartina (Campylobacter) and sugar-cane polymixa), and Bacillaceae), and recent information is presented in (Enterobacteriaceae Vose and Ruschel [16].

Estimating amounts o f N 2 fixed in associative systems is very difficult.

Rennie [ 1 7 ] presented a consensus that it was at least 30 kg N/ha per year.

Ruschel [16] noted that for sugar-cane there was reasonable evidence that 17 to 30% of plant nitrogen was due to fixation. Boddey and Dôbereiner [15] noted that for Azospirillum inoculation experiments in Israel and Brazil as much as 40% of the plant nitrogen might come from biological fixation.

An especially important symbiotic association is Azolla-Anabaena, which is frequently used in south Asia, mainly in flooded rice paddies. Under field conditions fixation values o f 62 t o 125 kg N/ha per year have been reported [18].

In a general way, according to Sanchez [8], additions o f atmospheric N 2 to the soil-plant system in the tropics can be as low as 4 and not greater than 50 kg N/ha per year in annual crops; however, in tropical forest systems it can vary from 46 to 147 kg N/ha per year.

2.3. Rainfall inputs

–  –  –

those in England and the United States o f America rainfall contains appreciable quantities o f nutrients, some o f which come from the sea as an aerosol and others from fossil fuel combustion, which also contributes nitrogen and sulphur to the atmosphere. Owing to the great variability o f all these processes, composition o f rainfall varies a great deal from place to place and from time to time. Cooke [ 1 1 ] states that in industrial areas nitrogen incorporated into soil by rainfall is o f the order o f 17 kg/ha per year and in undisturbed areas o f the order o f 1.5 kg N/ha per year. Sanchez [8] noted that for five different ecosystems, including agricultural and forest systems, the contribution o f rainfall nitrogen was in the range of 4 to 8 kg N/ha per year. Libardi and Reichardt [ 2 0 ] report that for tropical conditions (during the rainy season) 4.6 kg N/ha were received by the crop in a total o f 661.4 mm o f rain, which fell in 120 d. So it seems that an average contribution o f 8 kg N/ha per year is reasonable; slightly higher values could be expected in areas o f intense atmospheric lightning, where ammonia inputs are significantly higher.

3. NITROGEN LOSSES

3.1. Nitrogen movement in the soil profile and leaching Mineral soil nitrogen in the N - N 0 3 form (and sometimes N-NH4 and N-NO2) is normally not strongly retained by soil colloids, it remaining free in the soil solution and being subject to water movement in the soil profile. In this way it can be lost by drainage (leaching), which decreases soil fertility, thereby increasing the cost o f production through the need for higher fertilization rates and opening the possibility o f groundwater contamination. The quantities o f N - N 0 3 in the soil profile that are susceptible to leaching are extremely variable in space and time, depending on the amount o f nitrogen applied to the soil, mineralization rates, crop removal, soil management practices, type o f crop and volume o f water drained. All these factors are significantly affected b y soil properties and climate.

Sampling and analysis o f the soil below the root zone in areas o f free drainage is a procedure used by many scientists for estimation o f leaching losses [20 — 26]. Other more sophisticated methods involve measurement o f mass-flow and hydrodynamic dispersion [27 — 32] which, coupled with mathematical models, give the actual rates o f movement in soil profiles.

Wild [33], studying N - N 0 3 movement in a well aggregated Alfisol in Nigeria, found that this ion moved downward at a rate o f 0.5 mm per millimetre o f rainfall, which is considered low when compared with the rates found by Terry and McCants [ 3 4 ] o f 1 to 5 mm per millimetre o f rainfall for sandy soils in North Carolina. Pratt et al. [35], working with soils o f different textures, found 282 REICHARDT et al.

that the volume o f drained water, the concentration o f N - N 0 3 and the leaching losses o f these ions were significantly related to irrigation treatments and soil characteristics, mainly saturated hydraulic conductivity. The whole picture becomes more complicated when it is realized that these soil characteristics can present a high spatial variability over large field areas.



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