Submitted to: Journal of Plant Nutrition and Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/1996
Publication Date: N/A
Interpretive Summary: Nitrogen fertilizer is one of the most expensive energy and monetary inputs to cropping systems. Application of only the amount of fertilizer needed to achieve optimum yield helps to reduce costs and avoid environmental risk to water quality. Seasonally changing environmental conditions must be considered when predicting nitrogen mineralized from soil organic matter. Potential soil nitrogen mineralization after eight years of no tillage was greater than under conventional tillage, suggesting conservation of nutrients with no tillage systems. Seasonal increases in nitrogen mineralization occurred several months after input of low-nitrogen wheat and sorghum residues, but more immediately after input of high-nitrogen soybean residues. Competition for nitrogen between crops and soil microorganisms can limit crop growth, but also reduce nitrogen losses from the rooting zone. Our results indicate that soil organic matter can be managed to release or conserve soil nitrogen to increase short-term productivity or increase long-term conservation. Seemingly opposite goals of productivity and conservation could be optimized with high-intensity cropping systems using no tillage with moderate nitrogen fertilization during active crop growth.
Technical Abstract: Active fractions of soil carbon (C) and nitrogen (N) vary seasonally due to environmental and cultural factors, thereby influencing plant N availability and soil organic matter (SOM) conservation. Our objective was to determine the effect of tillage (conventional and none) on seasonal dynamics of potential C and N mineralization, soil microbial biomass C (SMBC), specific respiratory activity of SMBC (SRAC), and inorganic soil N in a sorghum [Sorghum bicolor (L.) Moench]-wheat (Triticum aestivum L.)/soybean [Glycine max (L.) Merr.] rotation and in a wheat/soybean double crop. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas was sampled to 200 mm depth 57 times during a 2-yr period. Potential C mineralization was lowest midway during the sorghum and soybean growing seasons and highest at the end of the wheat growing season and following harvest of all crops. Seasonal variation of inorganic soil N was 19 to 27%, of potential C and N mineralization and SRAC was 8 to 23%, and of SMBC was 7 to 10%. Soil under conventional tillage experienced greater seasonal variation in potential C and N mineralization, SRAC, bulk density, and water-filled pore space than under no tillage.