Soil Teeming with Life: New Frontiers for Soil Science - Sustainable Management of Soil Organic Matter - page 408

recently, it was widely accepted that equilibrium levels of carbon and
nitrogen in soil were controlled largely by net input, and that qualitative
aspects were relatively unimportant. Drinkwater et al . (1998), however,
found that quantitative differences in inputs alone could not explain
observed changes in soil carbon and nitrogen, and that plant species
composition and litter quality influenced soil organic matter turnover. They
suggested that managing the quality of inputs could help to increase carbon
sequestration and reduce CO 2 emissions, in accordance with the Kyoto
protocol. Cadisch and Giller move this idea forward by suggesting that soil
organic matter management should begin with the decision as to whether
we are managing organic matter for carbon sequestration or for crop
nitrogen supply. The key residue characteristics that govern the outcome
are carbon to nitrogen ratio, lignin and polyphenol content. They also
highlight the importance of understanding differences in decomposition of
root material compared with above-ground material. Recent research has
suggested that root turnover is relatively short for many temperate species,
for example ~30% of grass and clover roots survive for < 3 weeks under UK
field conditions (Watson et al ., 2000). Although there are now reliable
estimates of root turnover for many tree and agricultural species (Black
et al ., 1998; Watson et al ., 2000), there is still a lack of quantitative data on
soil organic matter inputs from this source.
In addition to the influence of quantity and quality of residues on
potential nutrient release and soil organic matter accumulation, physical
management of residues is also a key issue. Baggs et al . and Vinten et al .
both address the question of particle size of residues. The use of crop
residues and off-farm organic wastes within cropping systems may require
alterations to normal fertilizer and cultivation practices in order to
maximize crop uptake and minimize nutrient losses (Shepherd et al .,
Robertson and Thorburn, and Vinten et al .).
As stated in the Introduction, nutrient budgets are used increasingly
as international indicators of sustainability. Nutrient budgets can be used
for a number of purposes; they can identify the long-term sustainability of
a system and may be able to suggest management options that can improve
nutrient retention. They can also be used to identify gaps in our knowledge
of nutrient fluxes by using simple models to calculate fluxes that would
otherwise be difficult to measure experimentally. Finally, they can be useful
as a tool for policy makers in order to allow the synthesis of data at the scale
of a catchment or region. Fortune et al . point out some of the difficulties
in interpreting budgets, due in part to the different methodologies used
in their compilation, and also the need to understand the reliability and
limitations of the data available. Pilbeam et al ., working in Nepal, use
nutrient budgets to illustrate how sustainability at one level, in this case the
household, may jeopardize the sustainability of the system at a higher level,
when the origin of imports to the household is taken into account. This