Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 195

7 years of ecological management; all other fields had been under ecological
management for between 3 and 5 years. Soils did vary across sites and a
composite soil sample was taken from the plough layer (i.e. to ~12-15 cm).
Concurrently, samples were collected for soil moisture content and bulk
density determination using undisturbed 5-cm cores taken from just below
the soil surface.
Soils were dried and ground to pass through a 2-mm sieve before
analysis. Soil properties measured were bulk density, total C, anaerobic N
mineralization (Anderson and Ingram, 1996) as an index of potential N
mineralization, and soil organic matter fractions that are postulated as
being sensitive to changes in management. The organic matter fraction-
ation procedure (Sohi et al ., 1998) uses sequential density separations
to separate free organic matter (with no dispersion other than manual
swirling) and intra-aggregate organic matter (after ultrasonic disruption
of aggregates). Carbon and N in the fractions were determined by
combustion, using a combined C and N analyser and mass spectrometer
(Europa Integra CN). Sub-samples were also re-wetted and incubated for
11 days at 30 C until they came to microbial equilibrium. Soil microbial

biomass C and basal respiration were then determined (Witt et al ., 2000).

°

Results and Discussion

It is known that organic matter influences soil physical, chemical and
biological properties upon which soil fertility depends (Gaunt et al ., 1995).
One of the main benefits from using regular organic inputs on the ecologi-
cal fields was a significant difference in the bulk densities (Table 3.13.1).
Ecological management led to lower average bulk densities in three villages,
with Shibganj being the exception. Shibganj lies on the Barind Tract and
has higher clay content than the other sites, the parent material consisting
of Madhupur clay. These results appear to confirm farmers' perception of
the ecological practice that the fields were easier to cultivate, whilst the
conventional fields were found to be 'hard'.

Table 3.13.1. Bulk density of soils from all four villages.

Bulk density (g cm − 3 )

Farming system Dhamrai Daulatpur Gabtali Shibganj

Conventional 1.07 1.29 1.29 1.28

Ecological * 0.94 * * 1.23 * * 1.22 * * 1.31 *

* Significant difference at the 1% level.