| This study attempts to quantify the losses of C, N, and P from three prairie soils of different textures during cultivation. For this purpose cultivated and adjacent uncultivated soils were sampled and their C, N, and P contents as well as their bulk densities and horizon depths were compared.;Losses of C, N, and P from soil particle-size fractions during cultivation were examined. Four years of cultivation of a silt loam resulted in an extensive depletion of organic matter associated with particles > 50 (mu)m (43% of the initial C lost). With continued cultivation this accumulated material was slowly transformed within the system, accompanied by net losses of 34% C and 29% N from the total soil over 60 years. Fine clay (> 0.2 (mu)m) associated organic matter was rapidly depleted during the first 60 years but showed little change thereafter. Losses of fine silt (5 to 2 (mu)m) and coarse clay (2 to 0.2 (mu)m) associated organic materials were substantially less, and the proportion of total soil organic matter in these forms increased with time of cultivation.;A sequential chemical extraction was used to remove several forms of inorganic phosphorus (Pi) and organic phosphorus (Po). In the uncultivated coarse-textured soil significant amounts (7%) of secondary (NaOH extractable) Pi forms were associated with high levels of labile (bicarbonate and resin extractable) Pi. These secondary Pi forms, which were concentrated in the finer particle size fractions ( > 2 (mu)m), contributed to the P loss during cultivation, whereas all P loss in the silt-loam soils was due to Po losses alone. Labile P fractions were greatly reduced during cultivation, indicating a significant reduction in available P and P fertility of cultivated soils which was closely tied to soil organic matter losses.;Reductions of about 35% in the C concentration were observed in clay and silt loam soils after 60 to 70 years of cultivation. At the same time reductions in N concentrations were greatly influenced by the presence or absence of legume crops grown in the fields and losses varied between 18 and 34%. Phosphorus concentrations were reduced by 12% and all P losses were accounted for by the organic fraction. During a similar period of cultivation a lighter textured sandy loam had experienced greater reductions in C, N, and P concentrations of 46, 46, and 29%, respectively. Prolonged cultivation of 90 years did not result in a decrease in the rates of losses of C, N, and P on the silt loam soil. Conversion of concentration data to area based total C, N, and P budgets resulted in a decrease in the differences seen between cultivated and uncultivated soils. |
