Coupling Cover Crops with Alternative Swine Manure Application Strategies: Manure-15N Tracer Studies.

Integration of rye cover crops with alternative liquid swine (Sus scrofa L.)  manure application strategies may enhance retention of manure N in corn (Zea mays L.) - soybean [Glycine max (L.) Merr] cropping systems. The objective of this study was to quantify uptake of manure derived-N by a rye (Secale cereal  L.) cover crop and a subsequent corn crop after fall application of ¹⁵N-labelled liquid swine manure. An additional objective was to quantify retention of manure-derived N within soil N pools. Swine manure was applied using conventional (CONV) and low disturbance (ALT) application strategies to plots with and without a fall-planted rye cover crop (CC).  Deep soil cores (to 100 cm in 20 cm increments) were collected in the fall before manure application,the following spring immediately before the cover crop was killed, and in the fall after corn harvest. Rye biomass was harvested in the spring following manure application immediately before the cover crop was killed. Corn grain and stover biomass were collected in the fall at harvest. Soil, corn leaves, corn stalks, corn grain, and rye biomass were analyzed for TN, NO₃-N, NH₄-N, T¹⁵N, ¹⁵NO₃-N, and ¹⁵NH₄-N. Most of the manure-derived T¹⁵N and ¹⁵N-labelled inorganic-N was in the top 60 and 40 cm of soil, respectively, in the spring 2008. When cover crops were present, ¹⁵N-labelled inorganic N was found at 40-60 cm but was confined to the top 20 cm when CC were absent. There were no significant effects of manure treament or cover crop at any depth for TN, NO₃-N, NH₄-N, ¹⁵NO₃-N,  or ¹⁵NH₄-N except at the 0-20 cm depth, where there was signficantly more NH₄-N for the CONV+CC than for the ALT+CC. Mean inorganic ¹⁵N at% (1.5790, ¹⁵NO₃-N; 1.6031, ¹⁵NH₄-N) was 2-3X higher than mean T¹⁵N at%  (0.6677) at 0- 20 cm and the difference persisted at 20-40 cm (1.4483, ¹⁵NO₃-N; 1.6867, ¹⁵NH₄-N; 0.4231, T¹⁵N). Cover crop biomass N (mean 3.41%N) and ¹⁵N (mean 0.9799 at%) were not significantly different for the two manure treatments. There was no significant effect of manure treatment or cover crop on corn grain N (1.25%N) or ¹⁵N ( 0.6469 at%), corn leaf N (0.92%N) or ¹⁵N (0.6854 at%), and corn stalk N (0.38%N) or ¹⁵N (0.6640 at%). Further work will quantify manure ¹⁵N-enrichment in soil, plant and leached N pools and will  develop partial N budgets for the cropping systems.