Agriculture Reference
In-Depth Information
and it can increase SOM. With respect to the latter, long-term experiments
in vineyards suggest that the effects are markedly dependent on climate and
soil type, and that very large additions of organic materials may be required
to significantly increase SOM in some situations. For example, in the Loire
Valley, France, under an annual rainfall of 525 mm, changes in SOM were
measured in a clean-cultivated vineyard on sandy soil (86% sand), following
additions of prunings, manure, or compost. The results in table 5.6 show that,
after 28 years of large annual additions (up to 16 to 20 t/ha of fresh materi-
als), the organic C content of the topsoil had increased to only 1.24% and
1.31%, respectively, compared with the unamended soil C content of 0.63%.
As expected, gains in soil C below 30 cm were small. Clearly, in this sandy soil
under low rainfall, much of added organic material decomposed each year.
Nevertheless, the high turnover rate of C in the soil was beneficial for the
microbial biomass, which roughly trebled in size under the manure and com-
post treatments.
Biochar and Similar Materials
Historically, a form of biochar was used by indigenous people in the Amazon
Basin to enrich the poor soils around their settlements. The resultant dark,
organic-enriched soils, called
terra preta do indio
, were rediscovered by Western soil
scientists about 50 years ago and are now variously referred to as Anthropogenic
or Amazonian Dark Earths. Concern about the forcing effect of greenhouse gas
emissions, especially of CO
2
, in warming the earth's climate has stimulated inter-
est in recent years in the use of biochar for storing C in soil.
Table 5.6
Changes in Organic C in a Vineyard Soil (86% sand) in the Loire Valley Region,
France, after 28 Years of Organic Amendments
Topsoil (0-0.3 m)
Subsoil (0.3-0.6 m)
Initial organic
C (%)
Final organic C
(%)
Initial organic
C (%)
Final organic
C (%)
Treatment
Control—no
amendment
0.78
0.63
0.47
0.44
2 t/ha/year
crushed dry
prunings
0.85
0.79
0.45
0.44
20 t/ha/year fresh
cow manure
0.90
1.21
0.42
0.58
16 t/ha/year spent
mushroom
compost
0.84
1.34
0.44
0.64
Note
. C = carbon.
After Morlat and Chaussod (2008).
