Experimental results for model initialization

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Chapitre 3 : Etude du devenir des Mafor dans les sols – Minéralisation et effet sur la stabilité

3.2. Influence des caractéristiques biochimiques des Mafor sur la minéralisation du

3.2.3. Results

3.2.3.1. Experimental results for model initialization

3.2.3.1.1.Composition of organic amendments

The five studied organic amendments covered a wide range of chemical and biochemical characteristics (Tableau 23). The manure (MAN) had the highest levels of TC, TN and water-soluble fraction (SOLH2O) with the lowest level of the LIC fraction. The digestates from the two different sources presented a similar amount of C but higher than that of the composts, and with different TN contents, resulting in different C/N ratios. The two composts were similar in both TC and TN contents and differed in their biochemical characteristics.

Depending on their origin, digestates differed in their biochemical fractions, specially the SOLH2O and LIC fractions. The two composts had low amounts of HEM+CEL as compared to the manure and the digestates, as this fraction is highly degraded during composting. The

170 composts had higher level of LIC and SOLnd as compared to their respective digestates. The comparison of MAN and ADMAN, suggests that anaerobic digestion induced a small reduction of the SOLH2O fraction and a concentration of the LIC fraction.

The raw waste influenced the initial characteristics, observable in the digestates and composts, as the EOM stemming from the manure had a higher TN and water-soluble C than their biowaste counterpart. The distribution of TN in the biochemical fractions was also different amongst EOM: for the manure, a high percentage of TN was found in the SOLH2O fractions of the EOM, whereas this part was slightly lower in the EOM for the biowaste.

As the added EOM corresponded to 4 gC.kg-1 dry soil, the quantity of inorganic nitrogen added to the soil depended both on the TC and the EOM inorganic content. Composts bring relatively small amounts of inorganic nitrogen (2mgN.kg-1 dry soil for CBIO and 6mgN.kg-1 dry soil for CMAN). Between 70 (CMAN) and 80 % (CBIO) of the added inorganic nitrogen was in the form of nitrates, the other part being ammonium. Thus, NH4+

is expected to represent only a small amount of inorganic nitrogen, as ammonia volatilization and nitrification during compositing drops the NH4+

/NO3

ratio. Digestates brought respectively 11mgN.kg-1 dry soil (ADMAN) and 5mgN.kg-1 dry soil (ADBIO), including 100% and 72%

of the inorganic nitrogen added as NH4+

, respectively, for ADMAN and ADBIO. De la Fuente et al. (2013) found that phase separation lead to almost all inorganic nitrogen being in the liquid phase. Nevertheless, consistent quantities of inorganic nitrogen were found to persist in the solid phase of anaerobic digestates. The cattle manure MAN brought 11 mg.kg-1 of dry soil, and 37% of this added inorganic nitrogen was made up of nitrates.

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Tableau 23 : Initial characteristics of the five incubated exogenous organic matters

MAN: Cattle manure; ADMAN: Solid phase of the anaerobic digestate of MAN ; CMAN: Compost of MAN ; ADBIO: Solid phase of the anaerobic digestate of biowaste ; CBIO: compost of biowaste. TC: Total carbon; TN:

Total nitrogen; SOLH2O: Soluble in hot water; SOLnd: Soluble in neutral detergent; HEM: Hemicellulose-like;

CEL: Cellulose-like; LIC: Lignin and cutin-like; NA: No nitrogen was quantified in the fraction.

3.2.3.1.2.Initial microbial biomass in the mixtures

The initial total microbial biomass C content in the soil without EOM was 161 mgC.kg-1 dry soil, thus representing about 1.4 % of the soil total organic C content. This value is slightly higher than the 0.6-1.1% range proposed by Fierer et al. (2009) but is within the typical range reported for cultivated soils with low clay and C contents (Dalal, 1998). Initial total microbial biomass C content was 190, 194, 174,170, and 207 mgC.kg-1dry soil for the MAN, ADMAN, CMAN, ADBIO and CBIO respectively. Higher levels of the microbial biomass were expected in the soil + EOM mixtures compared to the controls, because of the EOM microbial biomass content. The EOM microbial biomass addition was between 0.2 and 1.2% of the total C added for all five EOM. These values are in agreement with Gomez-Brandon et al. (2008), who found that microbial biomass represented about 1 % of the TC in composted manure but less than 0.4 % in mature compost.

MAN ADMAN CMAN ADBIO CBIO

172 3.2.3.1.3.Experimental results for model calibration

The soil without EOM mineralized a cumulated amount of 559 mg C-CO2.kg-1 dry soil at the end of the incubation period of 120 days (Figure 39), which represented 4.7 % of the soil organic C content. Some 55.5 mg N-NO3

-.kg-1 dry soil was measured at the end of the incubation period (Figure 40), thus representing a net mineralization of 42.6 mgN-NO3-.kg-1 dry soil which represented about 3.6 % of the soil TN.

Figure 39 : Experimental (dots) and simulated (lines) cumulated C mineralization of the soil or soil+EOM mixtures. Vertical bars represent the standard errors of the means. MAN: Cattle manure; ADMAN: Solid phase of the anaerobic digestate of MAN; CMAN: Compost of MAN; ADBIO: Solid phase of the anaerobic digestate of biowaste; CBIO: compost of biowaste.

0

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Figure 40 : Experimental (dots) and simulated (lines) nitrate contents of the soil or soil+EOM mixtures.

Vertical bars represent the standard errors of the means. MAN: Cattle manure; ADMAN: Solid phase of the anaerobic digestate of MAN; CMAN: Compost of MAN; ADBIO: Solid phase of the anaerobic digestate of biowaste; CBIO: compost of biowaste.

At the end of the 120 days incubation period, the C mineralized in MAN and ADMAN were similar (Figure 39; difference not significant as attested by the LSD test) and equal to 42 % of the added C. However, these two measures were significantly different for CMAN with 13 % of the added C. The ADBIO mineralized an intermediate amount of C, namely 32 % of that added C. The CBIO mineralized the lowest amount of C, at 8 % of that added, which was significantly lower than the C mineralized by all other EOM.

Only few studies compared C and N mineralization patterns for digestates, their composts and their raw counterparts in soils (de la Fuente et al., 2013). Bernal et al. (1998b) found that

174 between 9.0 and 37.9 % of the TOC of various mature composts were mineralized after 70 days of soil incubation period at 28°C. The present results fell within the lowest values reported by Bernal et al. (1998b) but are within the range measured by Annabi et al. (2007) after 112 days of soil incubation at 28°C, for three mature composts, representing approximately 10 to 15 % of added C.

The kinetics of nitrogen mineralization differed among EOM (Figure 40). MAN exerted a slight net N immobilization during the first days of incubation followed by an important net mineralization. ADMAN and CMAN showed an important and almost constant net mineralization all along the incubation period. The amounts of inorganic nitrogen in the mixtures were similar at the end of the incubation period for MAN and ADMAN (difference not significant). The CBIO also exert an almost constant net mineralization all along the incubation period, but at a more limited extent. Thus, at the end of the incubation period, the amount of inorganic N in the CBIO mixture was significantly smaller than that of CMAN.

The ADBIO exerted a slight net immobilization of N during the first month thus leading to nitrates in the mixture at day 120 equivalent to that of the soil without EOM. At the end of the incubation period, the mineral nitrogen provided by the two composts represented respectively 8 and 4 % of the TN added by CMAN and CBIO, respectively. These values are in the typical range proposed by Amlinger et al. (2003) who found that between –2.5% to +35% of TN added was generally nitrified during medium term incubation periods for various types of composts.

To conclude, the amount of C mineralized was highly dependent on the treatment process, the composts having a significantly lower C mineralization than that of the other EOM. On the other hand, the amount of mineral nitrogen seemed to be better related to the origin of the EOM, as CBIO and ADBIO provided significantly lower amounts of mineral nitrogen at day 120, than CMAN and ADMAN. As previously noticed, net N mineralization was not explained by the total C/N ratio of the organic input (Lashermes et al., 2010), since the highest immobilization during the first experimental days was found in the MAN treatment with a low C/N ratio.

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