CHALK, P. M.

INACIO, C. de T.

MAGALHÃES, A. M. T.

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  IAEA-CN-191-235  FROM FERTILIZER TO FOOD: TRACING N DYNAMICS IN CONVENTIONAL AND ORGANIC CROPPING SYSTEMS USING 15N C.T. Inácio1, P.M. Chalk1,2, A.M.T. Magalhães1 1EMBRAPA-Soils, Rua Jardim Botânico 1024, Rio de Janeiro, Brazil 2International Atomic Energy Agency, Vienna, Austria E-mail address of main author:  caio@cnps.embrapa.br 1. Gaseous N losses and 15N signatures during storage or composting of animal wastes Ammonia volatilization of stored manure is most significant during early storage (e.g. 3 days) and 90% of the emitted ammonia-N comes from the urinary N as a consequence of rapid urea hydrolysis [1]. The ammonia-N lost is highly depleted in 15N which results in an increase of 15N of the total N remaining in the stored manure. The relationship between cumulative ammonia losses and 15N of manure was highly significant (p<0.001; r² = 0.76) (Fig. 1) [2].  The intramolecular distribution of 15N in N2O was used to identify denitrification as the main source of N2O evolved during composting with a minor contribution from nitrification in the surface of the compost pile [3].  FIG.1. Relationship between cumulative losses of ammonia N from manure and 15N of manure [2] 2. 15N signatures of synthetic vs. organic fertilizers Soluble N fertilizers (e.g. urea, ammonium salts) are often slightly depleted or slightly enriched in 15N as they are derived from atmospheric N2 ( 15N = 0 . Organic fertilizers such as manures and composts are usually enriched in 15N compared with feedstocks (Table 1). Ammonia volatilization and nitrification in the early stages (10 days) are the primary reasons for the increase in 15N in the composted material whereas denitrification in the latter stage (after 60 days) leads to 15N enrichment of residual nitrate in the compost [4, 5]. TABLE 1. RELATIVE 15N ABUNDANCE IN AGRICULTURAL WASTES AND DERIVED COMPOSTS Residue  15  Reference Cattle manure + 7.6 [4] Cattle manure + rice hulla compost + 11.0 Cattle manure + 11.4 Cattle manure + sawdustb compost + 15.6 Corn silage + 0.3 ± 1.3 [5] Corn silage compost + 8.2 ± 0.4  a 15N of rice hull = +  b 15N of sawdust = +    3. N fertilizer value of manure and derived compost Animal excreta (urine, faeces) and composts derived from the excreta have been artificially labelled with 15N by feeding 15N enriched roughage or grain to sheep or poultry. Alternatively, 15N labelled compost can be produced from unlabelled feedstock by co-composting with 15N-labelled fertilizer. Many studies have shown that the N fertilizer value of compost is low compared with the feedstock [6, 7] (Table 2). However, compost can play an important role in reducing N fertilizer losses by conserving N through promoting biological N immobilization. However, compost and fertilizer should not be applied simultaneously due to the danger of enhanced compost-induced urease activity which can promote NH3 volatilization, particularly from alkaline-hydrolysing N fertilizers such as urea [7]. TABLE 2. RECOVERY OF 15N LABELLED POULTRY MANURE AND DERIVED COMPOST BY RYEGRASS [6] Material C: N ratio Recovery (%) Poultry manure 30.0 25.7 Poultry manure + straw compost 14.9   3.8  4. Conventional vs. organic plant products Because organic and synthetic fertilizer sources differ markedly in 15N composition, it would appear to be a promising marker to distinguish organically- and conventionally-fertilized plant products (Table 3). The greater the difference between organic and synthetic fertilizer the more robust will be the differentiation.  Nevertheless, many production or external factors may confound product designation. e.g. (i) legume products or the use of legume cover crops (BNF) on organic farms (ii) crop species with a low N requirement (iii) annual vs. perennial growth habit (iv) use of organic fertilizers by conventional farmers (v) marketing of organic products as conventional products [8]. TABLE 3. 15N RANGE OF CONVENTIONAL AND ORGANIC PLANT PRODUCTS [8] Production system 15N ( ) Conventional a 2.5 to +8.7 Organic b +0.3 to +14.6 aSynthetic N-fertilizer alone or with organic fertilizer bSynthetic N-fertilizer excluded References [1] LEE, C., et al., Nitrogen isotope fractionation and origin of ammonia nitrogen volatilized from cattle manure in simulated storage, Atmosphere 2 (2011) 256 270. [2] HRISTOV, A.N., et al., Nitrogen losses from dairy manure estimated through nitrogen mass balance and chemical markers, J. Environ. Qual. 38 (2009) 2438 2448. [3] MAEDA, K., et al., Source of nitrous oxide emissions during the cow manure composting process as revealed by isotopomer analysis of and amoA abundance in betaproteobacterial ammonia-oxidizing bacteria, Appl. Environ. Microbiol. 76 (2010) 1555 1562. [4] KIM, Y-J., et al., Changes in nitrogen isotopic compositions during composting of cattle feedlot manure: Effects of bedding material type, Bioresource Technol. 99 (2008) 5452 5458. [5] LYNCH, D.H., VORONEY, R.P., WARMAN, P.R., Use of 13C and 15N natural abundance techniques to characterize carbon and nitrogen dynamics in composting and in compost-amended soils, Soil Biol. Biochem. 38 (2006) 103 114. [6] KIRCHMANN, H., Nitrogen interactions and crop uptake from fresh and composted 15N-labelled poultry manure, Eur. J. Soil Sci. 41 (1990) 379 385. [7] CHALK, P.M., MAGALHÃES, A.M.T., INÁCIO C.T., Towards an understanding of the dynamics of compost N in the soil-plant-atmosphere system using 15N tracer. Plant Soil (in press). [8] INÁCIO, C.T., CHALK, P.M., MAGALHÃES, A.M.T., Principles and limitations of stable isotopes in differentiating organic and conventional foodstuffs: 1. Plant products, Crit. Rev. Food Sci. Nutr. (in press). 

From fertilizer to food: tracing N dynamics in conventional and organic cropping systems using 15N.

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