Impact of farming intensity reduction in the Šumava foothills region on changes in soil organic matter and surface water quality

Samples of soils and surface waters were analysed in the Šumava foothills region in nine watersheds with an area 78.5 km2 in 1986-1987 and in 2001. Lower intensity of fertilization and liming and other changes in Czech agriculture were found to result in a decrease in the content of cold and hot water soluble carbon (Cw and Chws) in the soil; the content of biologically decomposable matter expressed as BOD5 and BODt of water extract of soil also decreased. The values of rate constants k of BOD kinetics of soil suspension indicate that the most valuable fast decomposable organic matter in the soils of this region have practically been mineralized to HCO3-, which are gradually eluted from soils and their increased amounts are present in surface waters. On the contrary, the transport of N-NO3- decreased to 66.3% of the value in 1986, that of N-NH4+ to 7.1% while the ratios C:N and N:P increased as a result of lower P elution in relation to lower N elution. CODMn showing the proportion of readily decomposable matter increased. A decrease in conductivity, i.e. in the content of dissolved salts in waters, was highly significant. These results warn against an optimistic interpretation of surface water analyses that indicate positive impacts of farming intensity reduction on the landscape. The results in the Šumava foothills region demonstrate that a major part of the most valuable components of soil productivity, decomposable organic matter, has already been decomposed, so their content in the soil has decreased. Natural sources (roots, root exudates, plant residues) are not able to ensure an equilibrium balance of these valuable matters at a lower farming intensity.