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Humic balance of spreadings

The theoretical change in C content in soils is calculated according to the humic balance proposed by Hénin-Dupuis (1945) and compared with results measured in the field. The theoretical results based on the BSI or OMSI are very satisfactory and correlate with the field measurements.

Principle of the humic balance

Every year, a K2 proportion of the organic matter (OM) in soils is mineralised. These losses through mineralisation are offset by the incorporation of crop residues and the spreading of organic waste products (OWP). Only a K1 proportion of the organic C entering into the soil via the residues and OWP  helps to offset the mineralisation of OM in the soil.
The annual balance of inputs and outputs can be used to calculate the theoretical change in OM levels in the soil surface horizon:

C (t+1) = C(t) - K2 . C(t) + K1PRO . CPRO(t) + K1RES . CRES(t)

C(t): the level of organic C over time t,
C(t+1): the level of organic C over time t+1,
COWP(t): the flow of organic C through OWP inputs,
CRES(t): the flow of organic C through the incorporation of crop residues,
K1ROP and K1RES: the proportions of organic C from OWP and crop residues that enrich the level of organic C in the soil,
K2: the proportion of organic C in the soil that is mineralised each year:

K2 = [ 0,03 . (1 + 0,2 . (TMA - 10) ] / [ (1 + 0,005 . Arg) . (1 + 0,0015 . CaCO3) ]

AAT: the annual average temperature,
Clay: the clay content of the soil,
CaCO3: the carbonate content of the soil.

Calculating the humic balance in the QualiAgro field experiment

The values taken into account for the QualiAgro field experiment are as follows:

K2: 0.0206 (AAT: 11°C, Clay: 150g/kg soil, CaCO3: 0)
K1RES: 0.13
K1OWP is estimated based on the BSI, the OMSI or the RL

The change in the organic C content in the soil C(t) is compared with the values measured in the soil.
The results of the calculation with the BSI or the IROC produced satisfactory results that are well correlated with measurements in the field (r2: respectively 0.90 and 0.92). The RL produces the poorest results (r2 of 0.73).
The results obtained are slightly underestimated with the BSI and overestimated with the IROC. As such, the change in organic C content in the soil can be calculated based on the BSI and the IROC, which provide the low and high points of the actual change.


Figure 1-a.  ISB comparaison


Figure 1-b.  ISMO comparaison

Figure 1. Comparison of C content measured in the soils of the various treatments from 1998 to 2006, with C content calculated based on the humic balance, taking into account the annual mineralisation of organic C in the soil and inputs of C through crop residues and supplies of OWP: the BSI and OMSI indicators are compared with the K1ROP coefficient of the humic balance.