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Land conversion and cultivation effects on belowground biomass and C were assessed at a fescue (Festuca altaica Trin. ssp. hallii) prairie site and an adjacent cultivated field seeded to spring wheat (Triticum aestivum L.). Belowground biomass and soil organic C (SOC) were measured in three landform element groups (upper level and convex shoulder elements [UL], low-catchment-area footslope elements [FS], and low-elevation level and high-catchment-area footslope elements [LL]) at both sites. Upper level, FS, and LL landscape positions in the prairie had, respectively, eight, 12, and 13 times more belowground biomass in the top 60 cm of soil than their cultivated field counterparts. In the upper 1.8 m of soil, belowground biomass in the prairie grassland increased downslope (UL = 1849 [+ or -] 306, FS = 2533 [+ or -] 899, and LL = 3663 [+ or -] 1248 g [m.sup.-1]). In the cultivated field, LL landscape positions had higher levels of belowground biomass than either UL or FS positions (UL = 228 [+ or -] 54.5, FS = 200 [+ or -] 47.0, and LL = 294 [+ or -] 76.8 g [m.sup.-2]). Losses of C from belowground biomass accounted for 17.6% of total SOC losses in UL landscape positions, and 71.7% of losses in FS positions after cultivation. Lower level positions had the greatest loss of belowground plant C (-15.6 Mg [ha.sup.-1]) but overall the SOC had increased by 9.0 Mg [ha.sup.-1]. In these position the loss of belowground C was offset by significantly higher SOC additions to the surface through soil redistribution. On a whole landscape basis, the C loss because of changes in belowground biomass is a large part of the C change that occurs during the transition from native prairie to arable agriculture.

The effect of topographical position on the yield response of canola (Brassica rapa var. Maverick) to N fertilizer was evaluated in a 3-yr field study in Saskatchewan, Canada. The hummocky, glacial till research site was stratified into three topographically defined landform complexes (convex, linear, and concave). Five N treatments (0, 0.5, 1.0, 1.5, and 2 times the recommended soil test rates; treatments will be referred to as 0xN, 0.5xN, 1xN, 1.5xN, and 2xN, respectively) were randomized in replicated blocks, and each block spanned the three landform complexes. Substantial yield differences occurred among the study years and were associated with year-to-year differences in spring soil moisture. Canola seed yields (averaged across years and treatments) increased from 0.53 Mg [ha.sup.-1] in convex complexes to 0.95 Mg [ha.sup.-1] in linear, and 1.42 Mg [ha.sup.-1] in concave landform complexes. The greatest yield responses to N occurred in the concave landform units in years where spring soil moisture was high. The N fertilizer rate required to achieve the economically optimum yield was significantly correlated to spring available moisture (required N rate [kilogram per hectare] = 40.9 + 14.87 [spring available water [in centimeters] water to 60-cm depth], [R.sup.2] = 0.73, sig. = 0.003) but was not significantly related to spring (i.e., preseeding) available soil N. The results confirm earlier studies indicating the importance of spring available water for crop production in the northern Great Plains, and suggest that intensive spring sampling for soil moisture conditions may be the most useful diagnostic tool for the implementation of a variable rate N program in this region.

The 137Cs technique has greatly expanded our knowledge of the topography–soil redistribution relationship. For the technique to be useful in upscaling of process models and regional-scale conservation planning, we must be able to show that a consistent relationship exists between 137Cs-derived soil redistribution rates and terrain attributes in a given region. In this paper, the association between 137Cs-derived soil redistribution rates and quantitatively defined landform elements was examined at nine hummocky terrain sites in southern Saskatchewan, Canada. Shoulder (SH) elements with convex plan curvatures had the highest mean soil loss rates of 33 t ha−1 yr−1, followed closely by other SH and backslope (BS) elements. The erosional behavior of level elements (i.e. those with gradients less than 3°) was highly dependent on the specific dispersal area (SDA) of the element—elements with high dispersal areas were dominantly erosional (mean soil loss of 14 t ha−1 yr−1), whereas level elements with low dispersal areas were depositional (mean soil gain of 15 t ha−1 yr−1). Doubly concave footslope (FS) elements had mean soil gain of 10 t ha−1 yr−1. The dispersion of values across the nine sites was much greater for the depositional units than the erosional units, indicating a complex relationship between deposition and terrain attributes in the depositional units. The results clearly indicate that regional-scale patterns of soil redistribution can be developed using the 137Cs technique. [Copyright &y& Elsevier]

Examines the effects of land conversion and cultivation on belowground biomass and organic carbon. Measurement of belowground biomass and soil organic carbon (SOC) content; Estimation of SOC loss; Relationship between root growth and SOC dynamics.