Programa e Resumos - I Congresso Ibérico de Ciência do Solo 2004

I Congresso Ibérico da Ciência do Solo – 15 a 18 de Junho de 2004, Bragança, Portugal
Assessment of changes in soil C and N – components, methodologies,
issues and implications
Partap K. Khanna
Institute of Soil Science and Forest Nutrition, Buesgenweg 2, 37077 Goettingen, GERMANY e-
mail address: pkhanna@gwdg.de
Resumo
Comunicação: Oral
In recent years demands by policy makers to assess changes due to land management
practices in soil organic C (SOC) and N (SON) contents have given a fresh impetus to
understand what factors and processes affect such changes and in what way one can
bring about certain desired changes in SOC and SON contents. The primary issues are
related to: changes in greenhouse gases, the productivity levels of ecosystems, adverse
effects on the ground water quality and the maintenance of biological diversity. Most
ecosystems are continually changing in their structure and sometimes in their
functioning, both leading to changes in SOC. This paper will present some of these
issues in terms of their needs and the level of accuracy required for assessing SOC
changes.
SOC and SON occur as multi-component substances and any assessment of change
would need the description of individual components and their degree of change, the
significance of which requires further consideration in relation to the natural variability
in the spatial and temporal scales. Components are arbitrarily grouped into two types –
those which are easily and in short term mobilizeable and those which are resistant or
protected and need long time to mobilize, with major consequences on the assessment
methodology and implications. These are sometimes distinguished by physical (size and
density based fractions), chemical (soluble, extractable or to a varying degree
oxidisable), biological (microbial biomass, respiration potential) and biochemical
(lignin, alkyl, phenol and polysaccharides contents). The paper will summarize some of
the essential methods including the use of stable isotopes in terms of their limitations
and usefulness to assess changes in SOC and SON. Quantitative assessment of SOC
and SON requires a reliable estimate of any changes bulk density (BD). Do we have a
reliable method of assessing BD of soils
Sources of SOC and SON are primarily related to plant growth (specifically root
growth) whereas minor inputs can be those of agricultural residues and waste materials.
Any retention of C and N by soils will primarily depend upon microbial and faunal
decomposition activities and soil factors controlling the accessibility (protection factors
– high clay, high Al and Ca, aggregation), retardation (high N, low pH) and acceleration
(priming effects of dissolved OC, low molecular weight organic acids). Commonly
three continuous phases of litter decomposition are outlined which will be described in
terms of their effects on the assessment of SOC changes. There are strong interactions
between SOC and SON and during the discussion on SOC changes, the role of SON is
not adequately considered. The implications of such interactions will be described in
terms of developing greenhouse gases, and changing productivity, biodiversity and
health of ecosystems.
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