Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 409

highlights the need to define the boundaries for compiling budgets in
relation to use of the resulting information.
As it is no longer acceptable to judge land management practices
simply on the basis of their current productivity, we must understand
the long-term implications of current practices. The combination of
archaeological knowledge with modern analytical techniques and modelling
is potentially a very powerful tool. McCann et al . provide a fascinating
insight into the origins of the Terra Preter soils in Amazonia, and the
paper by Glaser et al . explores the scientific evidence underlying farmer
observations that these soils are the most productive in the region. In a
contrasting setting, Adderley et al . draw conclusions on the sustainability of
traditional manuring practices on a remote Scottish island.
Major changes in land use, such as the ploughing out of long-term
pasture, are known to result in major disturbances to the carbon and
nitrogen cycles (Johnston, 1991; O'Sullivan et al .). The question of how
best to maintain soil organic matter levels following a more major land use
change is difficult. Hatley et al . and Mazzoncini et al . both assess the impact
of different management treatments following land use change in the
contrasting environments of East Anglia and the Mediterranean.
Nutrients
One of the key roles played by soil organic matter as discussed by Goulding
et al . is that involving the supply of nutrients to plants and soil organisms.
Soil organic matter contains substantial pools of organic N, P, S and a
number of trace elements; however, the availability and mobility of these
elements in organic compounds is generally very much less that that in the
inorganic state. An understanding of the process of transfer between the
organic and inorganic states (mineralization) and the reverse process of
immobilization is therefore critical to our approach in this area. It is often
assumed that high inputs of inorganic fertilizers can substitute for the
nutrient supply from organic matter pools. Work with isotopically labelled
N and P, however, has shown this not to be the case. Even where high
inputs are supplied, organic matter plays an important controlling role.
Results from field trials in the UK and USA have shown that even when N
fertilizers are added in amounts that are sufficient to satisfy the crop's
demand, the crop recovery of fertilizer-derived N is no more than 60% of
that which was added, with the remainder being made up from N released
from organic matter pools and small amounts from atmospheric inputs. In
other words, if we wish to manage fertilizers efficiently, we must be in a
position to understand the interactions between added fertilizer materials
and the organic matter pools. Short-term immobilization by the microbial
biomass potentially can result in crop nutrient shortages. However, work by
Vinten et al . has shown that crop N recovery can be optimized whilst