W. Roder et al., DYNAMICS OF SOIL AND VEGETATION DURING CROP AND FALLOW PERIOD IN SLASH-AND-BURN FIELDS OF NORTHERN LAOS, Geoderma, 76(1-2), 1997, pp. 131-144
Slash-and-burn rice production systems in northern Laos are undergoing
dramatic changes. Increased population pressure and regulations limit
ing access to land have resulted in shorter fallow periods. Limited in
formation is available on nutrient dynamics in slash-and-burn systems
of Southeast Asia in general and particularly on effects of reduced fa
llow length. Crop and fallow effects on soil parameters and fallow veg
etation were quantified in slash-and-burn fields in Luang Prabang, nor
thern Laos from 1991 to 1994. Over the cropping season from May to Oct
ober declines of 8, 7, and 3% organic C and 33, 40, and 53% extractabl
e P, were observed for the depth intervals of 0-3, 3-10 and 10-25 cm;
respectively. Over the same period extractable K declined by 34% in th
e 0-3 cm interval and increased by 15 and 17% in the 3-10 and 10-25 cm
intervals. The declining trend continued over the 3 year crop-fallow
cycle with losses (depth 0-100 cm) of 29 +/- 7.6 t organic C ha(-1), 2
.0 +/- 1.1 t total N ha(-1), and 0.7 +/- 0.8 t extractable K ha(-1). A
t the end of the fallow period the above ground biomass contained 100
kg N ha(-1), 5 kg P ha(-1), and 140 kg K ha(-1). The fallow vegetation
was dominated by Chromolaena odorata with a gradual succession toward
s tree and bamboo species. The nutrients in the above ground fallow ve
getation represent only a small fraction of the N and C lost due to mi
neralization and leaching. With the present no-till system, mineraliza
tion losses are far more serious than losses due to soil erosion. Shor
t fallows will result in a fast decline and low equilibrium of soil or
ganic C levels, reducing the potential for rice yields and limiting fa
rmers choice for other land use options which may become available wit
h better market access.