SOIL-PLANT NITROGEN DYNAMICS FOLLOWING INCORPORATION OF A MATURE RYE COVER CROP IN A LETTUCE PRODUCTION SYSTEM

Winter non-leguminous cover crops are included in crop rotations to de crease nitrate (NO3-N) leaching and increase soil organic matter. This study examined the effect of incorporating a mature cover crop on sub sequent N transformations. A field trial containing a winter cover cro p of Merced rye and a fallow control was established in December 1991 in Salinas, California. The rye was grown for 16 weeks, so that plants had headed and were senescing, resulting in residue which was difficu lt to incorporate and slow to decompose. Frequent sampling of the surf ace soil (0-15 cm) showed that net mineralizable N (anaerobic incubati on) rapidly increased, then decreased shortly after tillage in both tr eatments, but that sustained increases in net mineralizable N and micr obial biomass N in the cover-cropped soils did not occur until after i rrigation, 20 days after incorporation. Soil NO3-N was significantly r educed compared to winter-fallow soil at that time. A N-15 experiment examined the fate of N fertilizer, applied in cylinders at a rate of 1 2 kg N-15/ha at lettuce planting, and measured in the soil, microbial biomass and lettuce plants after 32 days. In the cover-cropped soil, 5 9% of the N-15 was recovered in the microbial biomass, compared to 21% in the winter-bare soil. The dry weight, total N and N-15 content of the lettuce in the cover-cropped cylinders were significantly lower; 2 8 v. 39% of applied N-15 was recovered in the lettuce in the cover-cro pped and winter-bare soils, respectively. At harvest, the N content of the lettuce in the cover-cropped soil remained lower, and microbial b iomass N was higher than in winter-bare soils. These data indicate tha t delayed cover crop incorporation resulted in net microbial immobiliz ation which extended into the period of high crop demand and reduced N availability to the crop.