Rasmussen et al (1998) give an overview of the need for long-term studies on the sustainability of agricultural systems as an element of understanding global change. Some long-term measurements of soil characteristics date back to the mid 1800s in England. Soil organic matter, as measured by soil organic carbon content, is one of the key indicators of soil quality. Annual changes in soil carbon are small, so at least 20 years of continuous data are needed to detect changes in soil carbon. Below-ground input of carbon from roots is sufficient to maintain soil carbon in cool humid climates but not tropical or arid temperate climates. Therefore, removing surface crop residue tends to reduce soil carbon content in some but not all soils. Long-term experiments also show that heavy reliance on ammonium or urea-based nitrogen fertilizer increases soil acidity and negative effects on soil quality. Long-term agroecosystem experiments are needed to evaluate biological, biogeochemical, and environmental dimensions of agricultural sustainability.
Reference
Rasmussen, P. E., K. W. T. Goulding, J. R. Brown, P. R. Grace, H. H. Janzen, and M. Korschens, 1998: Long-term agroecosystem experiments: Assessing agricultural sustainability and global change. Science, 282, 893-896.