Evaluating Inorganic Nitrogen and Rye-Crimson Clover Mixture Fertilization of Spring Broccoli and Lettuce by ¹⁵Nitrogen Tracing and Mass Balance

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^{− 1}N. Lettuce yield and fertilizer N recovery efficiency (by mass balance) was maximized at 140 g·ha^{− 1} N.

Keywords:

• ¹⁵N recovery
• Brassica oleracea var. italica
• broccoli
• covers crops
• crimson clover
• fertilizer N erecovery
• Latuca sativa
• lettuce
• rye
• N uptake
• Secale cereale, spring vegetables
• Trifolium incarnatum

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.