Fate of Soil Residual Fertilizer-15N as Affected by Different Drip Irrigation Regimes

Soil residual N is a potential factor threatening the environment, but it is also an N fertilizer resource. Few studies have evaluated the fate of soil residual N under agronomic practice. The objective of this study was to investigate the distribution of residual N and its possible influencing factors with different irrigation regimes. Under three N residual situations created by the previous season using the N-15 labeled urea, we employed lettuce as the plant material and three lower limits of drip irrigation including 75% (DR1), 65% (DR2), and 55% (DR3) accounting for the field water capacity as experimental treatments. A furrow irrigation treatment (FI) with the same irrigation regime as DR2 was used as control. Results showed that 2.1-4.8% of the residual N-15 from the previous season was absorbed by the succeeding lettuce, 78.0-84.4% was still remained in the 0-80 cm soil, and 10.9-20.0% was unaccounted for. After harvest of succeeding lettuces, the soil residual N-15 mainly existed in the mineral form. Moreover, the lettuce reuse efficiency for(15)N was positively correlated with the total residual N-15 amount (p < 0.01) and the mineral N-15 amount (p < 0.01). The overall results indicated that an appropriate irrigation regime (DR2) was conducive to promoting absorption of residual N by succeeding crop.