ABSTRACT
Broccoli (Brassica oleraceaL. var. italica) and lettuce (Latuca sativaL.) were grown under greenhouse conditions with nitrogen (N) from a cover crop mixture of rye (Secale cerealeL.) and crimson clover (Trifolium incarnatumL.) and ammonium nitrate (NH4NO3). Individual cover crop species were produced with non-enriched or enriched (5 atom % NH4 15NO3) Hoagland Nutrient Solutions resulting in enriched rye [0.799% atom % 15N, 24:1 carbon (C):N ratio] and enriched clover (0.686% atom % 15N, 19:1 C:N ratio). Cover crops were applied as an equal mixture of rye and clover at 1884, 3768, and 5652 kg·ha− 1 dry weight to supply 26, 52, and 78 kg·ha− 1 N. Enriched materials were only applied at the 3768 kg·ha− 1 rate, either as enriched rye plus non-enriched clover or non-enriched rye plus enriched clover. Additional treatments consisted of an unfertilized control and three NH4NO3 fertilizer rates; 112, 224, and 336 kg·ha− 1 N for broccoli and 70, 140, and 210 kg·ha− 1 N for lettuce. Combination treatments were the standard cover crop rate (3768 kg·ha− 1) plus the lowest N fertilizer rate for each vegetable. Cover crops did not increase yield of either broccoli or lettuce, and contributed only 17% of the N in broccoli and 15% of the N in lettuce. The majority of cover crop 15N remained in the soil: 54.8% and 81.3% of rye and clover N, respectively, after broccoli harvest; and 68.1% and 79.2% of rye and clover N, respectively, after lettuce harvest. Broccoli plant tissue recoveries were 8.0% of the rye and 11.0 % of the clover 15N; while lettuce plant tissue recoveries were 6.3% (rye) and 4.1% (clover). Broccoli yield could not be assessed due to lack of floret development, but dry matter accumulation was maximized at 224 kg·ha− 1N. Lettuce yield and fertilizer N recovery efficiency (by mass balance) was maximized at 140 g·ha− 1 N.
ACKNOWLEDGMENTS
Financial support for this study was provided by the NCA & TSU Agricultural Research Program (Evans-Allen Funds). The authors acknowledge the assistance of Howard Sanford, NCSU for N15 analysis; Dr. W.A.R.N. Fernando for statistical analysis and manuscript review; Dr. V. Shirley, P. Nouri, H. Parks and Pam Britt for sample analysis the samples and maintenance of the crops in the greenhouse; and Pamela E. Puryear of the Tobacco Literature Service for reference identification.
This paper is a portion of a thesis submitted by R.L. Holness. The use of trade names in this publication does not imply endorsement of the products named, or criticism of similar ones not mentioned.
Notes
zUnless otherwise indicated, the cover crops are an equal mixture by weight of non-enriched clover and rye.
yThis treatment was an equal mixture by weight of non-enriched clover plus 15N enriched rye.
xThis treatment was an equal mixture by weight of 15N enriched clover plus non-enriched rye.
v The ANOVA for residual soil inorganic N in broccoli was not significant at P ≤ 0.05.