S.J. González-Prieto

PHYSICAL AND CHEMICAL CHARACTERIZATION OF FOUR COMPOSTED URBAN REFUSES

Abstract The physical and chemical characteristics of four composted urban refuses (one of them amended with CaCO 3 during the composting process) from Spanish industrial composting plants were studied from the point of view of their use as organic fertilizers. The four composts were very fine in texture with low bulk density and high salinity. The pH was close to neutrality; the organic matter content ranged from 42 to 60% and the C/N ratio from 16 to 22 (7 for the amended urban refuse). Most of the total N was in organic forms; NH 4 + -N largely predominatedNO 3 − -N in the composts, except for the amended composted refuse where the predominant inorganic form was NO 3 − -N. Calcium was the most abundantnutrient followed by K, Na, Mg and P. Most of the Ca and Na were in available forms; available K and Mg were lower and available P very small. Total Al and Fe were extremely abundant followed by Zn, Mn, Pb, Cu, Cr, Ni and Cd. The percentage of available Mn was very high, followed by available Cu and Pb in two of the composts, and available Zn and Cd. Available Al, Fe, Ni and Cr were very low or negligible. Most of the total Zn, important percentages of total Pb, Mn, Al and Fe, but very low proportions of total Cr and Ni and only traces of Cd, were complexed with organic matter; these compounds seemed to be soluble organo-metallic complexes, except part of those formed by Al and Cd that could be stable complexes. Although the four composted refuses were unbalanced with regard to the main nutrients they all had potential agronomic value. Total C contents and C/N ratios in the three non-amended composts were in the range for stabilized composts; however, the NH 4 + -N content seemed to point to the presence of non-stabilized substances.

Effect of soil characteristics on N mineralization capacity in 112 native and agricultural soils from the northwest of Spain

N mineralization capacity and its main controlling factors were studied in a large variety (n=112) of native (forest, bush) and agricultural (pasture, cultivated) soils from several climatic zones in Spain. The available inorganic N content, net N mineralization, and net N mineralization rate were determined after 6 weeks of aerobic incubation. NHinf4sup+−N largely predominated over NOinf3sup--N (ratio near 10:1) except in some agricultural soils. Net N mineralization predominated (83% of soils) over net N immobilization, which was more frequent in agricultural soils (25%) than in native soils (9%). In forest soils, both net N mineralization and the net N mineralization rate were significantly higher than in the other soil groups. The net N mineralization rate of pasture and cultivated soils was similar to that of bush soils, but available inorganic N was lower. The net N mineralization rate decreased in the order: soils over acid rocks>soils over sediments>soils over basic rocks or limestone; moreover, the highest net N mineralization and available inorganic N were found in soils over acid rocks. The highest N mineralization was found in soils with low C and N contents, particularly in the native soils, in which N mineralization increased as the C:N ratio increased. N mineralization was higher in soils with a low pH and base saturation than in soils with high pH and base saturation values, which sometimes favoured N immobilization. Soils with an Al gel content of >1% showed lower net N mineralization rates than soils with Al gel contents of <1%, although net N mineralization and available inorganic N did not differ between these groups. The net N mineralization rate in silty soils was significantly lower than in sandy and clayey soils, although soil texture only explained a low proportion of the differences in N mineralization between soils.

Carbon and nitrogen mineralization in an acid soil fertilized with composted urban refuses

Abstract The C- and N-mineralization kinetics of a Cambisol over granite fertilized with four composted urban refuses (one of them amended with CaCO 3 in the composting process) as well as the degradation kinetics of the wastes themselves were studied. The C-mineralization was determined by incubation of the samples for 42 days at 28°C in a thermostat bath and measurement of the CO 2 evolved from the samples. The N-mineralization was performed by aerobic incubation in an incubator at the same temperature and for the same time as in the case of C. Addition of the composts to the soil significantly increased the C-mineralization rate. About 30% of the organic C of the composts (only 16% in the amended composts) was mineralized after 6 weeks incubation. Most C mineralization took place in the first 3 weeks for only about 3% of the organic C (0·9% in the amended composts) was mineralized between weeks 3 and 6. The amended compost was the only one that stimulated the soil N-mineralization rate. The other composts provoked N-immobilization (or denitrification) during the incubation and although an increase of the net inorganic-N production was detected after week 2, the net N-mineralization rate was almost nil at the end of the incubation. Net nitrification predominated over net ammonification in the soil with or without addition of composts. The non-amended compost behaviour seem to show that they had not undergone enough stabilization; as for the amended composted refuse its singular behaviour could be due to its high carbonate content.

Nitrogen mineralization and its controlling factors in various kinds of temperate humid-zone soils

The N mineralization capacity of 41 temperate humid-zone soils of NW Spain was measured by aerobic incubation for 15 days at 28°C and 75% of field capacity. The main soil factors affecting organic N dynamics were identified by principal components analysis. Ammonification predominated over nitrification in almost all soils. The mean net N mineralization rate was 1.63% of the organic N content, and varied according to soil parent materials as follows: soils on basic and ultrabasic rocks < soils over acid metamorphic rocks < soils developed over sediments < soils over acid igneous rocks < soils on limestone. The N mineralization capacity was lower in natural soils than in cropped soils or pastures. The accumulation of organic matter (C and N) seems to be due to poor mineralization which was caused, in decreasing order of importance, by high exchangeable H-ion levels, high Al and Fe gel contents and, to a lesser extent (though more markedly in cropped soils), by silty clay texture and exchangeable Al ions.

Evaluation of three organic wastes for reclaiming burnt soils: Improvement in the recovery of vegetation cover and soil fertility in pot experiments

Abstract The efficacy of three abundant organic wastes: poultry manure (PM), cattle slurry (CS) and sewage sludge (SS) for the reclamation of burnt soils was evaluated. A forest soil, previously furnace-heated in order to simulate exposure to a high-intensity wildfire, was labelled with nitrogen-15 (15N) to evaluate the contribution of N derived from the organic waste to the burnt soil and vegetation. Four treatments were performed with the heated 15N-labelled soil: an unamended control soil (S) and three waste amended soils (S+PM, S+CS and S+SS) at a dose waste of 167mg total N kg–1 soil. Lolium perenne was grown in all the pots for 3 months. In each treatment the phytomass produced and its N content decreased significantly in the following order of treatments: S+PM S+CS > S+SS S. The percentage of plant N derived from the waste was similar in the S+PM (22.8%) and S+CS (24.0%) treatments, but significantly lower in the S+SS treatment (16.5%). At the end of the 3 month experimental period, the available N reserves (phytomass N+soil inorganic N) in the control soil accounted for 51.5–71.5% of those in the S+PM, S+CS and S+SS treatments, whereas the yield of the plants was only 13.4–29.8% of that in the manured soils. These results demonstrated the importance of the addition of organic wastes, particularly PM, for the recovery of the vegetation cover and for the stabilization of the soil ash layer. They also showed that the level of N was not the main controlling factor of plant growth in the control soil, which, moreover, did not show evidence of a shortage of macronutrients, i.e. phosphorus, potassium, calcium or magnesium. It is hypothesized that, as occurs in heat-sterilized soils, phytomass production in the control-heated soil could have been inhibited by the heat-induced production of phytotoxic compounds, their negative effects being microbially or chemically suppressed by the addition of organic wastes.

Successional dynamics of soil characteristics in a long fallow agricultural system of the high tropical Andes

To detect soil changes related to vegetation and fertility restoration in a long fallow agricultural system of the Venezuelan Andes, 32 soil (A horizon, 0 ‐ 15 cm depth) and litter characteristics were studied in plots at different stages of the fallow-cultivation cycle. Four sectors of the valley were sampled, each one including seven plots: recently ploughed after a long fallow period; 1 and 2 years under potato crop; 1, 4 ‐ 5 and 8 years in fallow and natural vegetation. Each sector had similar topography, parent material and exposure to reduce the spatial heterogeneity that can hinder the synchronic analysis of the succession. Data of each sector were standardized before the statistics comparisons. Although all soils were acidic, those involved in the cropping cycle, or with only 1 year of fallow, had a significantly lower pH than the others, indicating that soil cultivation triggered off acidifying processes intense enough to overcome the strong buffering indices of the soils. These acidifying processes, facilitated by the acidity generating ions that widely dominate the desaturated CEC, are surely due to the stimulation of litter and soil organic matter mineralization after ploughing the soil and also to the N fertilization of the cultivated soils. The high soil contents of exchangeable Al 3þ and free Al oxides suggest that Al plays an important role in SOM stabilization, lowering its mineralization. No successional increase of any main plant nutrient was found in soil or litter. Moreover, soil available P and litter-P contents are higher during the cultivation phase and at the beginning of the succession, probably as a consequence of fertilization. Water holding capacity was similar for all soils, indicating that this long fallow agriculture system does not change the water storage capability of the soil, an

Short- and medium-term effects of fire and fire-fighting chemicals on soil micronutrient availability.

The impact of fire and three fire-fighting chemicals (FFC) on soil micronutrient availability was evaluated 1, 90 and 365 days after a prescribed fire. Five treatments were considered: unburnt soil (US) and burnt soil with 2 l m(-2) of water (BS) or water with foaming agent Auxquímica RFC-88 at 1% (BS+Fo), Firesorb at 1.5% (BS+Fi) and FR-Cross ammonium polyphosphate at 20% (BS+Ap). Pre-fire contents of available micronutrient were homogeneous among plots and high (Fe, Zn) or insufficient (Co, Cu, Mn) for plant nutrition. At t=1 day, Fe availability decreased greatly in burnt treatments, with significant differences in BS+Fi (-50%) and BS+Ap (-75%), contrasting with Fe richness of the ammonium polyphosphate. The fire induced a significant increase (9-16x) of available Mn in burnt treatments that lasted for at least three months; the FFC effect on soil available Mn was imperceptible, despite the noticeable amounts of Mn they supplied (especially Firesorb and ammonium polyphosphate). In burnt soils, the Fe/Mn ratio also decreased strongly (92-99%) and significantly till t=90 days. A high increase was also found, at t=1 day, for the available Zn in all burnt treatments and, although the ammonium polyphosphate provided more Zn than the Firesorb, the increment was only significant in BS+Fi plots (+100%). Neither fire nor FFC effects on soil Cu availability were found. The slight increase of Co availability in BS, BS+Fo and BS+Ap at t=1 day was followed by a transient decrease in all burnt treatments at t=90 days. Except the Mn and the Fe/Mn ratio in BS+Ap, which remained significantly higher and lower, respectively, the indices of available micronutrients at t=365 days in all burnt soils were similar to the pre-fire levels.

Changes in the soil organic N fractions of a tropical Alfisol fertilized with 15N-urea and cropped to maize or pasture

Qualitative and quantitative changes in soil and fertilizer-derived organic N fractions were assessed during a cropping season in an intertropical Alfisol, under maize and pasture, fertilized with15 N-urea. Before the sowing, after fertilizing and after the harvest, the organic N of top soil samples was fractionated by a two-step acid hydrolysis under reflux (H1 = 1 M HCl for 3 h; H2 = 3 M HCl for 3 h). The total hydrolysable N (HN) from H1 decreased significantly during the cropping season in both maize and pasture soils. Contrastingly, the content of HN from H2 and that of non-hydrolysable N did not vary significantly during the cropping season. The easily hydrolysable fractions, especially amino acid N, amino sugar N and amide N, were the most active N pools and the major source of N potentially available for plants. The urea-derived N that remained in the soil was mainly in organic forms at both 7 and 108 d after fertilizing (70–82% and 93–98%, respectively), higher figures being found in pasture than in maize soil. The total amount of urea-derived HN decreased significantly during the crop period in both maize and pasture soils. This decrease was largely due to the decline in HN from H1. The amount of non-hydrolysable urea-derived N was significantly higher in pasture than in maize soil and it decreases in the former and increases in the latter, during the cropping season. During the crop period, the decrease of urea-derived organic N was 4.6 to 9.1 times higher than that of native organic N. Shortly after fertilizing, the proportion of urea-derived N in the easily hydrolysable (H1) organic fractions was higher than that of soil N, whereas the reverse was true for the slowly hydrolysable (H2) or insoluble fractions. These differences were less marked, but still significant, at the end of cropping. The easily hydrolysable organic N fractions were more sensitive than total N to the impact of land use intensification and are, therefore, a more useful index for early detection of soil biological degradation.

Carbon- and nitrogen-containing compounds in composted urban refuses

Abstract The composition of the organic matter of four composted urban refuses (one of them amended with CaCO 3 in the composting process) from Spanish industrial composting plants was studied. Fundamental components and C-bearing compounds (humic substances) were assessed by classical fractionation methods; N-bearing compounds were determined by acid step-wise hydrolyses. Cellulose and hemicelluloses largely predominated over lignin, which was the second important component, followed by water-soluble compounds, the content of lipids being very low. Water-soluble compounds represented 3–14% of the organic C. Alkali-soluble compounds were quite low since humic compounds (humic and fulvic acids) only represented about 17% of the organic C (24% in the amended compost); humic acids predominated over fulvic acids, the most polymerized compounds being higher than the less polymerized ones. The predominant fraction (about 70% of the organic C) was the insoluble one which comprised unhumified compounds, insolubilizable humin, microbial humin (1·4–9·0% of the organic C) and residual humin (20–40% of the organic C). The extraction percentage was very low whereas the degree of humification was close to 50%. About 85% of the organic N was hydrolysable. Hydrolysable unidentified-N was the predominant organic N form, followed by α-aminoacid-N. NH 4 + -N from organic compounds and hexosamine-N were very small and amide-N was not detectable. According to the percentage of humification the four composted refuses were stabilized materials.

Organic nitrogen mineralization in temperate humid-zone soils after two and six weeks of aerobic incubation

The N mineralization rate in 11 soils was studied by aerobic incubation at 28°C and at a moisture content of 75% of field capacity for 2 weeks (short term) and 6 weeks (medium term). Relationships between the N mineralization indices for each period were evaluated. Ammonification largely predominated during the first 2 weeks of incubation, whereas nitrification was the predominant process between weeks 2 and 6. The net N mineralized in the different soils varied from 0 to 2.85% of the organic N after 2 weeks of incubation and from 0.32 to 3.36% of the organic N after 6 weeks of incubation, the mean values for each period being 0.82 and 1.51% of the organic N, respectively. The quantities of NHinf4sup+-N, NOinf3sup--N, and total inorganic N produced and the percentage of organic N mineralized after 2 weeks of incubation were highly and positively correlated with the coresponding values after 6 weeks of incubation. These results showed that either length of incubation could be used to determine the potential N mineralization capacity of the soils. Information obtained from two incubation periods was largely supplementary for the kinetic study of N mineralization, ammonification, and nitrification; therefore a medium-term incubation with intermediate measurements of N mineralization over a short term may be more useful than a single measurement using either of the two incubation periods.