Effect of compost application rate on carbon degradation and retention in soils

We investigated the effect of a single compost application at two rates (50 and 85 Mg ha-1) on carbon (C) degradation and retention in an agricultural soil cropped with maize after 150 d. We used both C mass balance and soil respiration data to trace the fate of compost C. Our results indicated that compost C accumulated in the soil after 150 d was 4.24 Mg ha-1 and 6.82 Mg C ha-1 for 50 and 85 Mg ha-1 compost rate, respectively. Compost C was sequestered at the rate of 623 and 617 g C kg-1 compost TOC for 50 and 85 Mg ha-1 compost dose, respectively. These results point to a linear response between dose of application and both C degradation and retention. The amount of C sequestered was similar to the total recalcitrant C content of compost, which was 586 g C kg-1 compost TOC, indicating that, probably, during the short experiment, the labile C pool of compost (414 g C kg-1 of compost TOC) was completely degraded. Soil respiration measured at different times during the crop growth cycle was stable for soils amended with compost (CO2 flux of 0.96 ± 0.11 g CO2 m-2 h-1 and 1.07 ± 0.10 g CO2 m-2 h-1, respectively, for 50 and 85 Mg ha-1), whereas it increased in the control. The CO2 flux due to compost degradation only, though not statistically significant, was always greatest for the highest compost doses applied (0.22 ± 0.40 g CO2 m-2 h-1 and 0.33 ± 0.25 g CO2 m-2 h-1 for the 50 and 85 Mg ha-1 compost dose, respectively). This seems to confirm the highest C degradation for the 85 Mg ha-1 compost dose as a consequence of the presence of more labile C. Unlike other studies, the results show a slight increase in the fraction of carbon retained with the increase in compost application rate. This could be due to the highly stable state of the compost prior to application, although it could also be due to sampling uncertainty. Further investigations are needed to better explain how the compost application rate affects carbon sequestration, and how characterization into labile and recalcitrant C can predict the amount of C sequestered in the soil

WHAT IS THE DIGESTATE?

As anaerobic digestion (AD) is quickly being harnessed in Italy and in other European countries, there is a need for a more in-depth description of the main by-product of the process, the digestate. Little information on digestate characteristics and composition is available and unclear legislation causes problems in biogas plant management. In this work, the organic matter (OM) of this matrix was described through chemical, biological, spectroscopic, and statistical approaches. It was shown that AD results in a strong reduction of the easily degradable fraction of the OM and an accumulation of recalcitrant molecules (possible humus precursors). This contributes to a relatively high biological stability of the residual OM content in the digestate and may lead to good amendment properties. Besides, the observed relative accumulation and the high mineralisation of nitrogen and phosphorus may point to the digestate as a readily available liquid fertiliser for agronomic use. Moreover, xenobiotics and pathogens respected limits for both biosolids and compost in Italian and European legislation

Bio-based surfactant-like molecules from organic wastes : the effect of waste composition and composting process on surfactant properties and on the ability to solubilize tetrachloroethene (PCE)

In this work, four surfactant-like humic acids (HAS) obtained from garden lignocellulose wastes and kitchen food wastes mixed with garden-lignocellulose wastes, both before and after composting, were tested for surfactant properties and the ability to solubilize tetra chloroethene (PCE). The waste-derived HAS showed good surfactant properties, lowering the water surface tension from 74 mN m(-1) to 45.4 +/- 4.4 mN m(-1), with a critical micelle concentration (CIVIC) of 1.54 +/- 1.68 g L-1, which is lower than many synthetic ionic surfactants. CMC was affected by both waste origin and composting processes. The addition of food waste and composting reduced CIVIC by adding alkyl-C (measured by CP MAS C-13 NMR) and N- and S-HA contents (amide molecules), so that a multistep regression was found [CIVIC = 24.6 - 0.189 alkyl C - 2.64 (N + S); R-2 = 0,77, P < 0.10, n = 61. The four HAS solubilized PCE at the rate of 0.18-0.47 g PCE/g aqueous biosurfactant. These results were much higher than those reported in the literature for a commercial HA (0.026 g/g), but they were in line with those measured in this work for nonionic surfactants such as Tween-80 (0.69 g/g) and Triton X-100 (1.08 g/g)

Qualitative modifications of humic acid-like and core-humic acid-like during high-rate composting of pig faces amended with wheat straw

Humic acid-like (RA-like) and core-humic acid-like (core-RA-like) were characterized during the high-rate composting process by CP-MAS C-13 NMR, pyrolysis-gas chromatography (GC) / mass spectrometry (MS), and elemental analysis. Results obtained indicated that humification proceeded through a relative concentration of aromatic fractions due to the faster degradation of the O-alkyl and alkyl fractions. Core-RA-like, after purification of the parent material, showed a large reduction of the O-alkyl fraction in terms of HA-like. We concluded that HA-like consisted of refractory organic molecules, such as lignin and biopolymers, which formed a stable structure (core-HA-like) coated with degradable material associated with the core by weak physical association, ether or ester bounds

The determination of biological stability of composts using the Dynamic Respiration Index : the results of experience after two years

Biological stability was ascertained by using the Dynamic Respiration Index (DRI) on 144 samples of compost during the years 2003 and 2004, as a routine service for private subjects. Data obtained were collected and are critically discussed in this paper by using other parameters registered during tests, i.e., biomass temperature (T), specific airflow rate (Q(s)) and biomass analytical data (pH). Good linear correlations were obtained for DRI vs. T, DRI vs. Q(s) and DRI vs. pH, confirming expected results based on the theoretical discussion. Consequently, using the analytical method proposed in this paper means both T and Q can be used as additional parameters for measurement of the biological stability of compost. As a result, T values of 25.8 and 30.5(degrees)C, and specific airflow rate of 8.6 and 13.4 m(3) Mg(-1) VS h(-1) were found corresponding to 500 and 1000 mg O(2) kgVS(-1) h(-1), which, respectively, indicate a high and a medium degree of biological stability. (copyright) 2005 Elsevier Ltd. All rights reserved