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Abstract
Soil analyses (pH, N status, organic matter (OM), bulk density (BD), penetrometer resistance (PR), microbial biomass (MB), acetylene reduction activity (ARA), N mineralization potential), leaf nitrogen (LN) and grain yield comparisons were made on an 8-year cropping system experiment consisting of the following cropping patterns: sorghum (Sorghum bicolor (L.) Moench)-sorghum (SR(,c)), soybean (Glycine max (L.) Merr.)-soybean (SB(,c)), sorghum-soybean (SR(,sb)) or soybean-sorghum (SB(,sr)). Fertilizer (NH(,4)NO(,3)) was applied each crop year on sorghum at the rate of 0 (ON), 56 (56N), 112 (112N), and 168 (168N) kg N ha('-1). The aim is to determine the effects of long-term sorghum-soybean rotation and N-fertilizer on the yield of sorghum and on some physical and biochemical properties of the soil.
SR(,sb) produced higher yields than SR(,c) at or below 56N but above 56N, yield of SR(,c) was greater. SR(,sb) receiving ON yielding similar to 56N.
Values of OM, pH, and total N varied with cropping pattern, N rate and depth. Higher LN was obtained from SR(,sb) than from SR(,c). SR(,c) and SR(,sb) had contrasting soil ARA's. Mixed rotations had lower BD than the monocrops.
The rotations had significantly higher amounts of potentially mineralizable N (N(,0)) and longer N(,0) half-lives (t(, 1/2)) than the monocrops. Fertilizer N had no effect on N(,0)'s under both years (1981 and 1983). Across cropping patterns, half-lives increased linearly with increasing fertilizer rates up to 112 kg N ha('-1), but tapered off at higher rates.
