Contributions of Shoot and Root Nitrogen-15 Labeled Legume Nitrogen Sources to a Sequence of Three Cereal Crops

Legume mulches are important sources of N for cereal crop production, particularly for organic and resource-poor producers. A field study was conducted using a direct method to determine if the amount of N in cereal crops derived from either the shoots or roots of preceding tropical legume cover crops was affected by their chemical composition and mineralization potential. Desmodium ovalifolium Guill. & Perr. [= D adscendens (Sw.) DC. and Pueraria phaseoloides (Roxb.) Benth.], were grown in 6.0-m² microplots and foliar-labeled with 99 atom % ¹⁵N urea. A cereal sequence of maize (Zea mays L.)–rice (Oryza sativa L.)–maize followed the legumes. Cereal accumulation of legume N from either the shoot (shoot + leaf litter) or the root-soil sources was evaluated by spatially separating the legume N sources. This was achieved by interchanging surface applications of nonlabeled and ¹⁵N-labeled legume shoots with in situ ¹⁵N-labeled and nonlabeled legume roots. Initially the Desmodium shoot N source contained 316 kg N ha⁻¹ and roots contained 12.5 kg N ha⁻¹ Pueraria shoots and root N sources initially contained 262 and 14.8 kg N ha⁻¹, respectively. About 90 g kg⁻¹ of the initial N of each legume shoot was recovered in the total aboveground tissues from the three cereal crops, while 490 g kg⁻¹ of Desmodium and 280 g kg⁻¹ of Pueraria root-soil N sources were recovered. Of the 181 kg N ha⁻¹ accumulated aboveground by the cereal sequence, the contribution of shoot plus root-soil N sources was 200 g kg⁻¹ from Desmodium and 150 g kg⁻¹ from Pueraria Cereal N was derived primarily from mineralization of soil organic matter present before the legumes and possibly from N deposition (precipitation and dry) occurring during the cereal crop sequence. After harvest of the last cereal crop, 13 and 180 g kg⁻¹ of the initial legume N was present as inorganic and organic N fractions, respectively, in the top 75 cm of soil. Even though Pueraria shoots had a lower C:N ratio and concentration of polyphenols than Desmodium shoots, the relative contributions of the shoot N source were similar for both legumes. Decomposition of legume residues, particularly legume shoots, make a meaningful contribution to the N economy of cereal crops grown in the tropics. The legume cover crops (root + shoot) contributed nearly 280 g kg⁻¹ of the aboveground N in the first cereal crop and as much as 110 g kg⁻¹ of the N in the third crop during the 15-mo sequence of cereals.