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Characteristics of composts and manure

The spread organic waste products (OWP) an OM (organic matter) content of between 34% and 57% DM (BIO < GWS < FYM < MSW). However, it is the stability of the OM that determines the biodegradeability of OWP and their potential ability to restore OM levels in the soil. Indicators allows us to estimate the proportion of OM that is likely to be incorporated into the OM in the soil. This figure varies from 33% to 54% (MSW < FYM < GWS < BIO).

OM content and biochemical fractionation of OWP

The results of the biochemical fractionation of the OM in OWP is shown in Figure 1.

LIC: lignins + cutins, CEL: celluloses, HEM: hemicellulose, SOL: soluble substances, expressed as a % of DM (Van Soest fractionation).

Fraction-Biochim-PRO_reference
Figure 1 : Content and fractionation of OM in distributed OWP (averages in the years 1998, 2000, 2002, 2004, 2006, 2007, 2009 and 2011)

The average OM content of the OWP varies from 33.7% to 56.6% DM, in order BIO<GWS<FYM<MSW. This order is linked with the duration of the composting processes: from none for manure to the longest duration for biowaste composts.

OWP are also characterised by the fraction of OM:

- FYM is set apart by an HEM fraction of 12% of the OM (compared with less than 7% for composts).

- MSW compost has a CEL fraction of 36% of OM (compared with between 20% abd 25% for others OWP).

- GWS and BIO composts have an LIC fraction of nearly 30% of OM (compared with less than 23% for other OWP).

These characteristics match those normally measured in these OWP.

The LIC fraction is a good indicator of the proportion of recalcitrants OM in the OWP. Meanwhile, the SOL fraction is not necessarily a good indicator of the biodegradation of OWP.

Mineralisation of organic C during incubation

The biodegradeability of OWP is measured by monitoring the mineralisation of organic C during incubations under controlled laboratory conditions.This parameter represents the reactivity of OM in OWP in the short term. The more biodegradeable a product is, the more it helps to stimulate biological activity in the soil. This effect also impacts on other parameters such as nitrogen availability and the stability of the soil's structure.

Mineralisation-72dpi

Figure 2 : Mineralisation of organic carbon in distributed OWP (average in the years 1998, 2000, 2002 and 2004)

MSW compost has superior biodegradability to other OWP, with around 40% of the organic C mineralised at the end of the mineralisation. It is followed by manure, BIO compst and GWS compost, for which the proportion of organic C mineralised varies from 10% to 20%. These differences in biodegradability of OWP are linked with the type and maturity of the OM in the OWP.

Indicators of the stability of OM in OWP

Presentation of indicators

The stability of the OM in the OWPs is evaluated based on 3 indicators that estimate the proportion of OM likely to be incorporated into the OM in the soil over the long term and therefore maintain the OM level of the soil:  the BSI (biological stability index), the Rc (residual carbone) and the IROC (indicator of recaclcitrant organic carbon). 
These indicators are calculated based on the biochemical fractionation of the OM (standardised protocol XPU 44-162, AFNOR 2004). The IROC includes a further piece of data supplied by the mineralisation of organic C during incubations under controlled conditions (XPU 44-163, AFNOR pending). 
Their values range from 0 to 1 (or 0% to 100%). They represent the proportion of stabilised OM in OWPs that is likely to be incorporated into the OM in the soil. They are used to estimate the conditioning value of OWPs over the long term.

The formulae for calculating the indicators are as follows:

BSI = 2.112 - 0.02009 SOL - 0.02378 HEM - 0.02216 CEW + 0,00840 LIC

Rc = (0.3221 SOL - 0.7155 HEM + 0.6717 CEL + 1.8919 LIC) x TOM x 0.01 + 0.0271 MM

IROC = 44.5 + 0.5 SOL - 0.2 CEL + 0.7 LIC - 2.3 Cm3

LIC: lignins + cutins, CEL: celluloses, HEM: hemicellulose, SOL: soluble substances, expressed as a % of DM (Van Soest fractionation).
CEW: celluloses extracted from Wend fractionation, TOM: total organic matter (%DM), MM: mineral matter (% DM), Cm3: percentage of organic C mineralised after 3 months (incubation under controlled laboratory conditions)

Results for OWP

The values for the BSI, Rc and IROC of the OWP are shown in Figure 3.

ISB-ISMO-et-Tr_1998-2011_72dpi

Figure 3 : Average values of the indicators of the conditioning value of distributed OWPs (averages in the years 1998, 2000, 2002, 2004, 2006, 2007, 2009 and 2011)

These indicators allow us to evaluate the proportion of OM that is resistant to degradation, which is likely to enrich the OM content soil over the long term. The indicators do not provide identical results in terms of the value and classification of OWP. The IROC appears to give higher values compared with the BSI. The Rc does not offer much distinction between OWP. Meanwhile, the BSI and the IROC tend to single out MSW compost as having the highest biodegradeability.