P. E. Rasmussen

Because soil is a limited resource, agricultural production is dependent on improving soil quality. Improved soil quality also has an impact on water use, as high quality soil more effectively collects and stores water, reducing the need for irrigation. Intensive use of soil throughout history has led to depletion in soil quality, leading in turn to reduced yields because of the consequent reduced organic matter. Recognizing the lessons of history, scientists at research stations such as Rothamstead in England; Pendleton, Oregon; Champaign, Illinois; and Columbia, Missouri, began long-term studies on the effects of crop rotation, crop fertilization, manure additions, and residue management on the productivity and organic matter of cropped soils. In general, it was found that soil cultivation caused a decline in organic carbon content (which constitutes about half of the organic matter), or at best stabilized organic matter, even with heavy manure treatment, as long as conventional tillage continued. In the 1960s and 197Os, many investigators noted that tillage made soils more erodible, and that crop residues left on the surface were highly effective in reducing erosion. The introduction of more and …

Long-term agroecosystem experiments can be defined as large-scale field experiments more than 20 years old that study crop production, nutrient cycling, and environmental impacts of agriculture. They provide a resource for evaluating biological, biogeochemical, and environmental dimensions of agricultural sustainability; for predicting future global changes; and for validating model competence and performance. A systematic assessment is needed to determine the merits of all known experiments and to identify any that may exist in tropical and subtropical environments. The establishment of an international network to coordinate data collection and link sites would facilitate more precise prediction of agroecosystem sustainability and future global change.

Abstract Understanding microbial dynamics is important in the development of new management strategies to reverse declining organic‐matter content and fertility of agricultural soils. To determine the effects of crop rotation, crop residue management, and N fertilization, we measured changes in microbial biomass C and N and populations of several soil microbial groups in long‐term (58‐yr) plots under different winter wheat ( Triticum aestivum L.) crop rotations. Wheat‐fallow treatments included: wheat straw incorporated (5 Mg ha −1 ), no N fertilization; wheat straw incorporated, 90 kg N ha −1 ; wheat straw fall burned, no N fertilization; and wheat straw incorporated, 11 Mg barnyard manure ha −1 . Annual‐crop treatments were: continuous wheat, straw incorporated, 90 kg N ha −1 ; wheat‐pea ( Pisum sativum L.) rotation (25 yr), wheat and pea straw incorporated, 90 kg N ha −1 applied to wheat; and continuous grass pasture. Total soil and microbial biomass C and N contents were significantly greater in annual‐crop than wheat‐fallow rotations, except when manure was applied. Microbial biomass C in annual‐crop and wheat‐fallow rotations averaged 50 and 25%, respectively, of that in grass pasture. Residue management significantly influenced the level of microbial biomass C; for example, burning residues reduced microbial biomass to 57% of that in plots receiving barnyard manure. Microbial C represented 4.3, 2.8, and 2.2% and microbial N 5.3, 4.9, and 3.3% of total soil C and N under grass pasture, annual cropping, and wheat‐fallow, respectively. Both microbial counts and microbial biomass were higher in early spring than other seasons. Annual cropping significantly reduced declines in soil organic matter and soil microbial biomass.