N recovery from legume prunings and priming effects are governed by the residue quality

Nitrogen recovery from N-15-labelled prunings of Gliricidia sepium, Peltoph orum dasyrrachis, Calliandra calothyrsus and Leucaena leucocephala, each of two different chemical qualities, was followed over three cropping cycles in a growth room. Half of the pots of each treatment received a further add ition of unlabelled pruning material, from the same species as that previou sly applied, before the second and third crop cycle. The cumulative maize t otal N accumulation revealed the largest benefit from N rich, low lignin an d polyphenols Gliricidia prunings followed by Leucaena, Calliandra and Pelt ophorum. Cumulative N recovery measured using N-15 over the three crop cycl es ranged from 9% from Calliandra prunings to 44% from Gliricidia prunings. The vast majority of this N was recovered during the first crop cycle whic h agreed well with estimates using the N difference method. Recoveries in t he second and third crops ranged from 0.4-5% (N-15 method) and 6-14% (N dif ference method) of the N initially applied. The protein binding capacity of polyphenols was the best predictor of N recovery at both initial and later crop cycles. Treatments which led to a large N recovery initially, continu ed to provide greater N benefits in subsequent cycles although with increas ing harvest time this trend decreased. Thus, there was no compensation in i nitial N release from low quality prunings at later harvests and the agrono mic implications of this are discussed. Addition of unlabelled Gliricidia p runings before the second and third cycle led to a positive apparent primin g effect on previously applied N-15 labelled prunings. By contrast, repeate d additions of Peltophorum residues, rich in lignin and active polyphenols, resulted in a reduced recovery of initially applied pruning-N-15. However, the maximum positive or negative effects on recovery of pruning N amounted to less than 2% recovery of the initial amount of N added over 14 weeks. T hus the scope for regulation of N release from tree prunings during these l ater stages of decomposition appears to be limited.