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A field micro-plot experiment for winter wheat was conducted in an irrigated winter wheat (Triticum aestivum)-summer maize (Zea mays L.) rotation system in Mazhuang, Xinji of Hebei province in the North China Plain, using the 15 N isotope method to determine the effects of N application (rates and timing), and irrigation frequency on urea-15 N fate, residual-N and N recovery efficiency (NRE) of wheat. The experiment was conducted under two irrigation treatments (I2 and I3, representing for two and three irrigations, respectively), at three N rates (150, 210, and 270, kg ha -1 ), divided between two 15 N-labeled applications of basal-15 N (90 kg ha -1 ) and topdress-15 N (60, 120, and 180, kg N ha -1 , respectively). The total N uptake by wheat (ranging from 186 to 238 kg ha -1 ) and the fertilizer-derived N (Ndff, about 34-55%) were measured. The Ndff from labeled basal-15 N and from labeled topdress-15 N were about 15-22% and 16-40%, respectively. The NRE (measured either as recovery in grain or as the total N recovery in the plant) was higher with I3 (39-41 or 47-49%) than with I2 (35-40 or 42-47%), showing maximum NRE in grain of about 40% both at N210 with I2 and at N150 with I3 treatment. The NRE by the first wheat crop (in grain or the total N recovery in plant) was higher with labeled topdress-15 N (39-48 or 45-56%) as compared to that with labeled basal-15 N (30-37 or 36-45%), while the unaccounted N losses were lower with labeled basal-15 N (14-22%) relative to labeled topdress-15 N (14-35%). Higher residual N in soils was found with labeled basal-15 N (41-51%), as compared to labeled topdress-15 N (18-35%). Residual N in the 0-to 150-cm soil depth ranged from 26 to 44% while the unaccounted N losses ranged from 14 to 30%. Recovery of residual N by the 2nd and 3rd crops in the rotation was 5-10% in the maize crop and a further 1.7-3.5% in the subsequent wheat crop. The accumulated N recovery and the unaccounted N losses in continuous wheat-maize-wheat rotations derived from labeled topdress-15 N were 54-64% and 16-37%, respectively while they were 47-53% and 16-28%, respectively from labeled basal-15 N. This study also suggested that an N rate of 210 kg ha -1

To ascertain the effects of long-term conservation tillage and residue retention on soil organic carbon (SOC) content and aggregate distribution in a deep soil (>20-cm depth) in a dryland environment, this paper analyzed the SOC and aggregate distribution in soil, and the aggregate-associated organic carbon (OC) and SOC physical fractions. Conservation tillage (reduced tillage with residue incorporated (RT) and no-tillage with residue mulch (NT)) significantly increased SOC sequestration and soil aggregation in deep soil compared with conventional tillage with residue removal (CT). Compared with CT, RT significantly increased the proportion of small macroaggregates by 23%-81% in the 10-80 cm layer, and the OC content in small macroaggregates by 1%-58% in the 0-80 cm layer. RT significantly increased (by 24%-90%) the OC content in mineral-SOC within small macroaggregates in the 0-60 cm layer, while there was a 23%-80% increase in the 0-40 cm layer with NT. These results indicated that: (1) conservation tillage treatments are beneficial for soil aggregation and SOC sequestration in a deep soil in a dryland environment; and (2) the SOC in mineral-associated OC plays important roles in soil aggregation and SOC sequestration. In conclusion, RT with NT is recommended as an agricultural management tool in dryland soils because of its role in improving soil aggregation and SOC sequestration.

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