"Short communication"
Nitrogen availability from a mature urban compost determined by the 15N isotope dilution method
E. Iglesias-Jiménez
Instituto de Recursos Naturales y Agrobiología de Salamanca. Consejo Superior de Investigaciones Científicas (CSIC). Cordel de Merinas 40-52, 37008 Salamanca (Spain).
ABSTRACT
City refuse compost (CRC) produced from thermophilic-aerobic composting of the organic fraction of municipal solid wastes should not be considered as a poor and slow-release N material when has a high degree of maturity, i.e. highly biologically stabilized and "humified". Under these conditions, is expected that high quantities of mineral-N appear, after a suitable maturity period. In this work se evaluó the capacity of a CRC with a high degree of maturity to supply N to a barley crop during 2 months in a controlled-phytotron experiment. CRC was applied at a rate equivalent to 60 t ha-1, after incubation of the material (fraction < 2mm) during 3 months at 24 ºC (40-45 % moisture). The soil (arenic, orthicalcic, eutric Cambisol -FAO 1999-) was labelled with 15N as (15NH4)2SO4 with 9.6137 atom % 15N excess. Mature CRC increases significantly yield and plant-N concentration with respect to the non-amended soil. These results have been described before. But the most important result (here described for the first time) is that mineral-N from CRC dilutes in an important way the isotopic 15N/14N ratio of the labelled soil. Atom % 15N excess in the plant material (aerial part) after 2 months was approximately 50% in the compost treatment with respect to non-amended soil (0.6250 and 1.2007 atom % 15N excess, respectively). Thus, these results indicate that highly matured CRC contain appreciable amounts of available N, exchangeable N-NH4+ but sobre todo N-NO3- because the nitrification is enhanced during a long period of maturation after the bio-oxidative phase of composting process.
Key works: compost, maturity degree, N-availability, 15N isotope dilution.
INTRODUCTION
The composting of the organic fraction of municipal solid wastes for agronomic utilization is rapidly increasing in some developped countries, as Spain, as an alternative to the disposal of these wastes, normally landfilled or incinerated. Physico-chemical and microbiological properties as well as the effects of soil organic matter, city refuse compost (CRC) and related products on the soil-plant system have been broadly studied in Spain in the last 15 years (Acea and Carballas, 1996, 1999; Almendros et al., 1996, 1998; Bernal et al. 1998a, 1998b; Díaz-Burgos et al., 1993; Gallardo-Lancho et al., 1995; Gallardo-Lara and Nogales, 1987; García et al., 1992; González-Vila and Martín, 1985; González-Vila et al. 1999; Iglesias-Jiménez, 1990, 1996; Iglesias-Jiménez and García, 1989, 1992; Iglesias-Jiménez and Alvarez, 1993; Iglesias-Jiménez et al., 1993, 1997; Roig et al. 1988; Sánchez-Monedero et al., 1999, among others). CRC, with a high degree of maturity ("humification"), increases some important physical and chemical soil properties as porosity (fundamentally macroporosity), water-holding capacity, water-stability of micro and macroaggregates, cation-exchange capacity. Similarly, mature CRC releases nutrients gradually and improves the soil biological environment, i.e., normally increases soil microbiota (total microbial biomass) and sobre todo increases some important soil enzyme activities (Serra-Wittling et al., 1995). Moreover, mature CRC also increases sorption of hidrophobic pesticides by the soil, avoiding ground waters contamination (Iglesias Jiménez et al., 1997) and may increase pesticide degradation by stimulating microbial activity (Abdelhafid et al. 2000). Nevertheless, negative effects have also been reported, normally associated to high concentrations of xenobiotic heavy metals and the use of inmature CRCs, i.e. no biologically estabilized and scarcely "humified" (Iglesias-Jiménez and García, 1989).
The objective of the work presented here, was to evaluate the capacity of an highly mature CRC to supply N during two months to barley plants, mediante el 15N isotope dilution method.
MATERIALS AND METHODS
The results are in the framework of a pot-experiment designed to evaluate the effects of a CRC in the dinitrogen fixation of a lentil crop (Lens culinaris Medik.) in a phytotron-controlled environment, using a soil-plant system with a complete-randomized block design. Barley (Hordeum vulgare L. cv Plaixa) as been used only as a reference plant to the calculations made to quantify the amount of atmospheric N2 fixed by lentil during all the vegetive-growth period, by the 15N isotope dilution method.
The CRC was from the MSW-treatment plant at "Valdemingómez" (Madrid) with a approximate biooxidative period of 30 days and a 60-day complementary maturity process. A sample of the commercial CRC was air-dried, passed by 2 mm sieve and analyzed.. The main chemical characteristics of the CRC for the experiment are shown in Table 1. This compost is slighly matured (Iglesias-Jiménez and García, 1992). To achieve a highly degree of maturity the sieved CRC was maintained in incubation during 90 days with a 45-50% % moisture (w/v) at 24 ºC. The material was turned-off periodically and strongly mixed during the incubation period. The final CRC sample for the experiment had an intense black colour with a 35% moisture, and a strong odour similar to that of "damp forest ground" produced by the presence of geosmine, a secondary metabolite excreted by mesophilic actynomicetes. These facts indicate a high degree of maturity.
The experiment was conducted over 2 months under a controlled phytotron environment (temperature 19/14 ºC day/night, relative humidity 80%) using pots (20 cm diameter) containing 3.5 kg of soil (fraction<2 mm, dry-weight basis). The soil (Eutric Cambisol) was collected from the Ap horizon (0-20 cm) of an agricultural field of the "La Armuña" region (Salamanca, Spain). The soil sample was passed, with its natural moisture, through a 2-mm screen to remove rocks, roots, and other large particles. This soil contain 75% sand, 8% silt and 17 % clay (montmorillonite>illite>caolinite), with pH (H20): 7.4, organic C: 3.6 g kg-1, cation-exchange capacity: 12 cmolc kg-1, C/N: 8.6, labile (Olsen) P: 24 mg kg-1 and NH4Oac-extractable K: 453 mg kg-1.
The results shown in this communication correspond only to the 15N isotope dilution observed between two treatments corresponding to the reference crop. Treatments were (i) the control, unamended soil (0 CRC) and (ii) soil amended with 105 g CRC pot-1 (dry-weight basis), equivalent to 60 t ha-1 (60 CRC), assuming 2000 t soil ha-1, with 3 repetitions and 8 plants per pot. The results shown in Table 2 correspond to each of the three repetitions.
The amended and non amended soil samples were initially brought to a water potential corresponding to field capacity (pF 2, 17 % moisture) by addition of distilled water. The soil and soil+compost was then incubated at 24 º C during 30 day incubation period in the phytotron, to increase organic matter mineralization and humification. After 30 days of incubation 35 mg N pot-1 (20 kg N ha-1 equivalent-rate) of 9.6137 atom % 15N excess as (15NH4)2SO4 was sprayed as a 50-mL water solution uniformly onto the soil-surface of the pots. The final moisture was the field capacity. Also 17.5 mg P pot-1 (10 kg P ha-1 equivalent rate) as PO4H2K and 38.5 mg K pot-1 as K2SO4 were applied to the six pots.
Twelve seeds were planted per pot. After emergence plants were thinned to eight plants. Germination was completed in 7 days. The 6 pots were watered with distilled water to bring the soil moisture to a level corresponding to 15% (slighly lower than field capacity). Soil moisture was measured every 12 hours.
After 2 months the phytomass was collected and weighted. For dry matter production determinations the aerial part was dried at 80 ºC in a forced-draught cabinet oven to constant weight. Dried plants were ground in a stainless steel mill to powder (Culatti mill, 0.5 mm) and stored for chemical analysis. Prior to the analytical process, the ground samples were dried again at 105 ºC for 4 h. N content was determined following a semimicro-Kjeldahl digestion method. Analysis for 15N were carried out at least in triplicate. The technique employed to determine 15N abundance in the plant material was EA-CF-IRMS. Basically it consists in the combustion of the sample in an elemental analyzer CHNS-O (Carlo-Erba model 1108 microanalyzer); after trapping H2O and CO2 produced, a sample of the resulting gases (mostly N2 and He carrier) se inyecta en un isotope ratio mass spectrometer (Micromass, Isochrom model) continuous flow system. Atom % 15N excess was calculated with reference to the natural abundance (0.3663 atom %). The analytical deviation (reproductivity) obtained (1 sigma) was +- 0.004 %.
RESULTS AND DISCUSSION
In Table 2 an important isotope dilution is observed in the aerial part of barley with the treatment 60 CRC. Atom % 15N excess after 2 months was approximately 50% in the compost treatment with respect to non-amended soil (0.6250 and 1.2007 atom % 15N excess, respectively). Thus, the compost applied at a rate equivalent to 60 t ha-1 aporta relatively high amounts of available-N (mineral-N) to the soil-plant system. The directly available compost-N dilutes the rapport 15N/14N of the soil solution as well as the mineralized compost-N during the 2 months of the experimental period. These facts are in accordance with the results of the work published in 1993 (Iglesias-Jiménez and Alvarez, 1993) indicating that CRC may not be considered as a poor and slow-release N material when (tiene) has a high degree of maturity, i.e. highly biologically stabilized and highly "humified", with highly poly-condensed and polymerized "humic substances"..
Nitrate-N leaching and contamination of the ground waters may occur if high doses of mature CRC are employed (>50 t ha-1), but a high N-fertilizer value to crops can be obtained with lower doses if a very high maturity degree is reached in the commercial composting plants (Iglesias-Jiménez and Alvarez, 1993).
In conclusion, highly mature CRC contains important amounts of available N, principally exchangeable N-NH4+, but also N-NO3- because the nitrification is enhanced during a long period of maturation after the bio-oxidative phase of composting process durante la cual a intensa mineralization of organic containing-N compounds is observed (Iglesias-Jiménez, 1990).
Acknowledgements
The author wish to thank Dr. Ramón Redondo (Universidad Autónoma de Madrid, Laboratorio de Isótopos Estables) for 15N analysis. Research was carried out in the framework of "Junta de Castilla-León" Project ECS05.
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Analytical
pH was measured in a 1:5 ratio (w/v) of solid:water suspension; Total Organic Matter by calcination at 600 º C (4 hours); Total Organic Carbon (TOC) was deduced from Total Organic Matter applying the factor 1.84 deduced by Iglesias-Jiménez and García, 1992b; Oxidizable Carbon by the Walkley-Black's dichromatometric method. Total K, Ca, Mg, Fe, Cu, Mn, Zn, Cr, Pb, Ni and Cd concentrations were determined by ASS after reflux in "aqua regia", the xenobiotic heavy metals with graphite furnace. N was determined titrimetrically following a semimicro-Kjeldahl digestion method and P according to the molybdivanadophosphoric acid procedure after mineralization of the sample in a muffle furnace at 480 ºC and redissolution of the ash with 6 N HCl. Cation-exchange capacity was determined by the method of Harada and Inoko (1980).