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"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. E-37008 Salamanca, Spain
Phone: +34923219606
Fax:
+34923219609
e-mail: emeterio@gugu.usal.es
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"Short communication"
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Nitrogen availability from a mature urban compost determined by the
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N isotope dilution method
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E. Iglesias-Jiménez
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Instituto de Recursos Naturales y Agrobiología de Salamanca. Consejo Superior de Investigaciones
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Científicas (CSIC). Cordel de Merinas 40-52, E-37008 Salamanca (Spain).
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ABSTRACT
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Land application of city refuse compost (CRC), produced from aerobic-
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thermophilic composting of the organic fraction of municipal solid wastes, is an attractive
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alternative for the disposal of these wastes, currently land-filled or incinerated. Knowledge
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of the availability of N in compost is particularly important, given the current concern
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about groundwater contamination by NO3--N. In this work we assessed the capacity of a
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CRC with a high degree of maturity to supply N to a barley crop over 2 months in a
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controlled-phytotron experiment. The CRC was applied at a rate equivalent to 60 t ha-1,
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after incubation of the material (fraction < 2mm) for 3 months at 24 ºC (40-45 %
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moisture). The soil (Eutric Cambisol) was labelled with
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atom %
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labelled soil in an important way; atom %
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after 2 months was approximately 50% in the compost treatment with respect to the non-
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amended soil (0.625 and 1.201 atom %
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should not be considered as a poor-release N material when it has a high degree of
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maturity, i.e. it is highly biologically stabilised and "humified".
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N as (15NH4)2SO4 with 9.614
N excess. Available-N from CRC dilutes the isotopic
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N/14N ratio of the
N excess in the plant material (aerial part)
N excess, respectively). Accordingly, CRC
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Key words: compost, maturity degree, N-availability, 15N isotope dilution.
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Composting of the organic fraction of municipal solid wastes for agronomic use is
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rapidly increasing in certain developed countries as an alternative to the disposal of these
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wastes, normally land-filled or incinerated. The physico-chemical and microbiological
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properties as well as the effects of city refuse compost (CRC) on the soil-plant system
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have been exhaustively studied (Gallardo-Lara and Nogales, 1987; He et al., 1992). The
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CRC normally improves certain important physical and chemical soil properties as
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porosity (fundamentally macroporosity), the water-holding capacity, the water-stability of
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micro and macroaggregates, and the cation-exchange capacity. Similarly, mature CRC
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releases nutrients gradually and improves the soil biological environment, i.e., normally
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increases the soil microbiota, including rhizospheric microbiota, and above all it increases
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certain important soil enzyme activities (Serra-Wittling et al., 1995). Moreover, mature
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CRC also may increase the sorption of hydrophobic pesticides by the soil, avoiding
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groundwater contamination (Iglesias-Jiménez et al., 1997) and may increase pesticide
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degradation by stimulating microbial activity (Abdelhafid et al., 2000). Nevertheless,
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negative effects have also been reported, normally associated with high concentrations of
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heavy metals and the use of non-mature CRCs, i.e., not biologically stabilised and only
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sparingly "humified" (Iglesias-Jiménez and García, 1989).
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A wide range of results has been obtained in relation to the efficiency of composted
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organic materials as a source of N for plants because N availability is closely related to the
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degree of compost maturity (Bernal et al., 1998). Knowledge of the availability of N in
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highly mature composts is particularly important, given the current concern with
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groundwater contamination by NO3--N (Sims, 1990), and the scarcity of values reported
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for N mineralisation from urban composts.
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The aim of the present work was to evaluate the capacity of an highly mature CRC
to supply N over 2 months to barley plants, using the
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N isotope dilution method.
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The CRC was from the treatment plant of municipal solid wastes at
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"Valdemingómez" (Madrid, Spain) with an approximate bio-oxidative period of 30 days
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and a 60-day complementary maturity process. A sample of the commercial CRC was air-
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dried, passed through 2 mm sieve and analysed. The main chemical characteristics of the
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CRC for the experiment are shown in Table 1. This CRC is only slightly matured (Iglesias-
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Jiménez and García, 1992). To achieve a greater degree of maturity, sieved CRC was
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maintained under conditions of incubation for 90 days with a 45-50% % moisture (w/v) at
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24 ºC. The material was turned over periodically and vigorously mixed during the
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incubation period. The final CRC sample used for the experiment had an intense black
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colour with a 35% moisture, and a strong odour similar to that of "damp forest ground",
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owing to the presence of geosmine, a secondary metabolite excreted by mesophilic
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actinomycetes. These factors point to a high degree of maturity.
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The experiment was conducted over 2 months in a controlled phytotron
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environment (temperature 19/14 ºC day/night, relative humidity 80%) using pots
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containing 3.5 kg of soil (fraction<2 mm, dry-weight basis). The soil (arenic, orthicalcic,
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eutric Cambisol) was collected from the Ap horizon (0-20 cm) of an agricultural field from
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"La Armuña" region (Salamanca, Spain). This soil contain 750 g kg-1 sand, 80 g kg-1 silt
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and 170 g kg-1 clay, with pH (H2O): 7.4, organic C: 3.6 g kg-1, cation-exchange capacity:
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12 cmolc kg-1, C/N: 8.6, labile (Olsen) P: 24 mg kg-1 and NH4OAc-extractable K: 453 mg
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kg-1.
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Two treatments were set up in a a complete-randomized block design: (i) the
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control, unamended soil (0 CRC) and (ii) soil amended with 105 g CRC pot-1 (dry-weight
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basis), equivalent to 60 t ha-1 (60 CRC), assuming 2000 t soil ha-1, with 3 replications.
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The amended and non-amended soil samples were initially brought to a water
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potential corresponding to field capacity (pF 2, 17 % moisture) by addition of distilled
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water. The soil and soil+compost mixture was then maintained for 30 days in the phytotron
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before sowing, to increase organic matter mineralization and humification. After 30 days,
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35 mg N pot-1 (20 kg N ha-1 equivalent-rate) of 9.614 atom % 15N excess as (15NH4)2SO4
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was uniformly sprayed as a 50-mL water solution onto the soil-surface of the pots. The
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final moisture level was the field capacity. Also 17.5 mg P pot-1 (10 kg P ha-1 equivalent
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rate) as KPO4H2 and 38.5 mg K pot-1 as K2SO4 were applied.
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Twelve seeds of barley were planted per pot (Hordeum vulgare L. cv Plaixa).
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Germination was completed in 7 days. After emergence, plants were thinned to eight
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plants. The 6 pots were watered with distilled water to bring the soil moisture to a level
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corresponding to 15% (slighly lower than field capacity). Soil moisture was measured
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every 12 hours.
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After 2 months the aerial phytomass was collected. For dry matter production
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determinations the aerial part was dried at 80 ºC in a forced-draught cabinet oven to
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constant weight. Dried plants were ground to powder in a stainless steel mill and stored for
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chemical analysis. Prior to the analytical process, the ground samples were dried again at
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105 ºC for 4 h. The N content was determined following a semimicro-Kjeldahl digestion
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method. Analyses for 15N were carried out at least in triplicate. The technique employed to
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determine
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CHNS-O (Carlo Erba model 1108 analyser); after trapping the H2O and CO2 produced, a
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sample of the resulting gases (mainly N2 and He carrier) was injected into an isotope ratio
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mass spectrometer (Micromass, CF-Isochrom model) continuous flow system (EA-CF-
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IRMS technique). Atom %
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abundance (0.366 atom %). The analytical deviation (reproducibility) obtained (1 σ)
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was +- 0.004 %.
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N abundance entailed the combustion of the sample in an elemental analyser
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N excess was calculated with reference to the natural
An important dilution in the 15N/14N ratio occurred in the aerial part of barley with
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the 60 CRC treatment (Table 2). Atom %
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approximately 50% in the compost treatment with respect to unamended soil (0.625 and
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1.201 atom % 15N excess, respectively). Thus, presumably the directly-available compost-
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N decreased the
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during the 2 months of the experimental period. N in the plant derived from the labelled
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fertiliser (Ndff) was 12.5 % (control) and 6.5% (60 CRC), according to the expression
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(Barraclough, 1991):
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%Ndff = (atom % 15N excess in plant material / atom % 15N excess in fertiliser) x 100
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N excess after 2 months was decreased by
N/14N ratio of the soil solution as well as the mineralized compost-N
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49.6 % of the N exported by the plant crop during the experimental period derived
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from compost-N (Table 2). Thus, the CRC applied at a rate equivalent to 60 t ha-1
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contributed relatively high amounts of available-N to the soil-plant system. Likewise, the
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CRC significantly increases both the yield and plant-N concentration with respect to the
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unamended soil (Table 2).
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These observations agree with the results of the work published in 1993 (Iglesias-
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Jiménez and Alvarez, 1993) indicating that CRC cannot be considered as a poor-release N
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material when it has a high degree of maturity. NO3--N leaching and pollution of the
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groundwater may occur if high doses of mature CRC are employed (>50 t ha -1) (Mamo et
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al., 1999), but a high N-fertiliser value to crops can be attained with lower doses if a very
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high maturity degree is reached in commercial composting plants. Normally, highly
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matured composts increase crop yield and a net inorganic-N accumulation may occur in
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the soil (Iglesias-Jiménez and Alvarez, 1993; Bernal et al., 1998). In contrast, immature
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composts induce a considerable increase in soil microbiota to descompose the excess of
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labile C compounds, potentially causing a strong immobilisation of native and added
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available N, and consequently, N starvation and depressive effects on crop plants may
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occur (Iglesias-Jiménez and García, 1989).
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The increase in yield with respect to the control was very low, as was the value of
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plant-N concentration (Table 2). In principle, these findings suggest that despite the
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incubation of the compost, the degree of maturity is still low. By contrast, the explanation is
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simpler: the substrate incorporated into the soil is highly matured and may be considered as a
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"postmature compost", a term first proposed by Blanco and Almendros (1997) to explain the
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results of a pot-experiment with straw-based compost using rye-grass, based on a time-series
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design with analysis of available N and plant yield. The term "postmature compost" was
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introduced to justify decreased in plant yields as regards preceding composting stages,
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probably resulting from microbial immobilisation of available N, leading to suboptimum
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yields that paralleled N-availability. Maximum yield was obtained after 20 days of
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composting, after which yield decreased. That is, yield decreases with the rise in the degree of
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maturity. If this should be a general phenomenon, the "favourable" maturity would not be
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claimed as a time-dependent aging process, but as a "climax" stage followed by not
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necessarily improving transformations. The "postmature compost" may be considered as a
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slow-release N material to plants. However, as may be deduced from the results of Iglesias-
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Jiménez and Alvarez (1993), and Bernal et al. (1998), and from the important isotopic
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dilution observed here (Table 2), highly mature compost may not be considered as a poor-
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release N material to the soil sensu stricto. Although both facts seem to be contradictory, there
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is a simple explanation: "The pattern of N availability in highly mature compost is a positive
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net mineralization but a partial biological immobilisation" (Iglesias-Jiménez and Alvarez,
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1993). This behaviour is similar to the pattern of N availability in biologically active soils in
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which there is a continuous immobilisation of inorganic N into organic phases and a
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mineralisation of organic N into inorganic forms. This N-behaviour is therefore extremely
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interesting for the conservation of N in agro-ecosystems and particularly important
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considering the current concern about groundwater contamination by NO3--N. The concept
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"postmature compost" is of great interest from an agronomic point of view and opens a wide
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portal for investigation of the agronomic reuse of composted organic materials. It would be
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interesting to see the agronomic use of this type of compost and investigate the most
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appropriate physico-chemical and biological parameters for their generation at industrial scale
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and to establish the optimum quality indices.
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Acknowledgements
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The author thanks Dr. Ramón Redondo (Universidad Autónoma de Madrid,
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Laboratorio de Isótopos Estables) for the
N analysis and Mr. Nicholas Skinner
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(Universidad de Salamanca) for revising the English version of the manuscript. This
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research was carried out in the framework of "Junta de Castilla-León" Project ECS05.
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REFERENCES
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Abdelhafid, R., Houot, S., Barriuso, E. 2000. Dependence of atrazine degradation on C and
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N availability in adapted and non-adapted soils. Soil Biology and Biochemistry 32, 389-
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401.
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Barraclough, D., 1991. The use of mean pool abundances to interpret
15
experiments. I. Theory. Plant and Soil 131, 89-96.
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N tracer
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Bernal, M.P., Navarro, A.F., Sánchez-Monedero., M.A., Roig, A., Cegarra, J., 1998.
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Influence of sewage sludge compost stability and maturity on carbon and nitrogen
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mineralization in soil. Soil Biology and Biochemistry 30, 305-313.
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Blanco, M.J., Almendros, G., 1997. Chemical transformation, phytotoxicity and nutrient
22
availability in progressive composting stages of wheat straw. Plant and Soil 196, 15-25.
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Gallardo-Lara, F., Nogales, R., 1987. Effect of the application of town refuse compost on
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the soil-plant system: A review. Biological Wastes 19, 35-62.
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He, X-T., Traina, S.J., Logan, T.J., 1992. Chemical properties of municipal solid waste
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composts. Journal of Environmental Quality 21, 318-329.
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Iglesias-Jiménez, E., García, V.P. 1989. Evaluation of city refuse compost maturity: A
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review. Biological Wastes 27, 115-142.
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Iglesias-Jiménez, E., García, V.P., 1992. Determination of maturity indices for city refuse
33
composts. Agriculture, Ecosystems and Environment 38, 331-343.
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Iglesias-Jiménez, E., Alvarez, C.E., 1993. Apparent availability of nitrogen in composted
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municipal refuse. Biology and Fertility of Soils 16, 313-318.
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Iglesias-Jiménez, E., Poveda, E., Sánchez-Martín., M.J., Sánchez-Camazano, M., 1997.
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Effect of the nature of
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Archives of Environmental Contamination and Toxicology 33, 117-124.
exogenous organic matter on pesticide sorption by the soil.
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Mamo, M., Rosen, C.J., Halbach, T.R., 1999. Nitrogen availability and leaching from soil
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amended with municipal solid waste compost. Journal of Environmental Quality 28, 1074-
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1082.
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Sims, J.T., 1990. Nitrogen mineralization and elemental availability in soils amended with
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cocomposted sewage sludge. Journal of Environmental Quality 19, 669-675.
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Serra-Wittling, C., Houot, S., Barriuso, E., 1995. Soil enzymatic response to addition of
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Table 1. Main characteristics of city refuse compost
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pH (H20) 1:5
7.3
-1
Ash (g 100g dry matter)
53.6
Total organic C (g C kg-1 dry matter)
253
Oxidizable C (g C kg-1 dry matter)
218
Alkali-extractable C (g C kg-1 dry matter)
89
-1
Humic acid (HA)-like C (g C kg dry matter)
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Fulvic acid (FA)-like C (g C kg-1 dry matter)
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HA/FA ratio
1.2
C/N ratio
16.4
N content (g kg-1)
15.4
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Table 2. Yield, plant-N concentration and 15N enrichment in the aerial part of barley
after 2 months in the phytotron experiment. The results correspond to each of the
three replications (n=3).
Yield
-----------------------g pot-1 (dry-weight)
-----------------------0 CRC
60 CRC
8.9
11.6
8.5
11.7
8.4
12.2
-----------------------Mean 8.6
11.8
SD
0.3
0.3
plant-N concentration
-----------------------plant-N (mg N g-1)
-----------------------0 CRC
60 CRC
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11.1
15.8
11.0
15.5
10.3
16.2
-----------------------10.8
15.8
0.4
0.4
1.142
0.605
1.205
0.635
1.256
0.636
------------------------1.201
0.625
0.057
0.018
N enrichment
------------------------atom % 15N excess
------------------------0 CRC
60 CRC
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