Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 197

intra-aggregate between the two management practices after 7 years of
organic inputs.
The soil microbial biomass is responsible for the organic matter
turnover and nutrient cycling in soils. Microbial biomass C ranged from
150 to 404 kg C ha −1 , equivalent to 0.67-1.2% in the soils investigated
(Table 3.13.2), comparable with the typical range of 1-4% (Gaunt et al .,
1995). Microbial biomass C was greater in ecological fields at all villages,
with the exception of Gabtali. Basal respiration followed a pattern similar
to total C (Table 3.13.2), being greater in ecological than conventional
fields at Daulatpur and Shibganj, but less at Gabtali and Dhamrai;
differences at Shibganj and Dhamrai were significant ( P = 0.05). There was
no significant difference in potential N mineralization at Dhamrai and
Gabtali, whereas both Daulatpur and Shibganj had significantly larger
mineralizable N pools under the ecological management (Table 3.13.2).
Farmers who are engaged in ecological farming also continue with
'conventional' practices on other fields, but the two systems were not
practised in isolation. Nutrient budgets for the same fields (data not shown)
showed, even on conventional fields, that farmers added up to two-thirds of
their fertilizer as organic manure.
Thus it is not surprising that there are not large differences in soil
properties between the two systems. Fields at Dhamrai that have been
under ecological management for a longer time demonstrate a significant
increase in total soil C, C in free and intra-aggregate organic matter
fractions, microbial biomass C and basal respiration.
Conclusion
Given that the organic inputs varied annually for both farming systems, it is
not surprising that differences between systems after 3 years of ecological
management were not entirely consistent. We will re-analyse these fields
after a further 3 years to assess the longer term impact of ecological man-
agement. The data in Table 3.13.2 suggest that a suite of measurements
should be used as indicators of soil fertility rather than an individual
measurement.
If farmers' observations and measurable soil properties can be linked,
then we can develop learning exercises that are related to this biophysical
understanding. We also wish to link our predictive tools and quantitative
indicators. In this way, we hope to use organic matter fractions as
measurable pools in a soil organic matter turnover model (Gaunt et al .,
1999). Linking models and indicator measurements in this way has the
advantage that models can, in principle, be used in a predictive manner
without previous site knowledge (Arah and Gaunt, 2000).