| Knowledge about the changes in soil nitrogen pools and the recovery of fertilizer nitrogen under different tillage management is necessary to assess the feasibility of adoption of conservation practices for sustaining productivity and protecting the environment in dryland farming in northern China. To evaluate the recovery of fertilizer nitrogen under no-tillage practice can provide theoretical basis for the determination of reasonable fertilizer nitrogen rate under conservation tillage. We investigated the long-term(24 years) effects of no-till with residue retention(NT) on soil N and fertilizer N in a dry-land winter wheat(Triticum aestivum L.) cropping system in northern China.(1) Effects of the long-term no-tillage practice on soil total nitrogen and its fractionsCompared with conventional tillage without residue retention(CT), significantly higher soil total N(STN) concentrations were observed in the surface soil layer(0-10 cm) under NT. Meanwhile, more soil N accumulated for the whole soil profile(0 to 60 cm) in the NT(3.38 Mg ha-1) treatment relative to the CT(3.17 Mg ha-1) treatment. The particulate organic matter N(PON), microbial biomass N(MBN), and water extractable organic N(WEON) levels in the NT treatments were 52.3%, 116%, and 69.4% greater at a depth of 0-5 cm and 41.6%, 108%, and 44.9% greater at a depth of 5-10 cm, respectively, compared with the CT treatment. However, no differences were observed below the 10 cm layer. At a depth of 0-60 cm, the soil NH4-N content under CT was higher than that under NT. However, the soil NO3-N contents in the NT treatments were significantly greater at a depth of 0-10 cm and were not significantly different at a depth of 10-60 cm, relative to the CT treatment. Significantly positive correlations were observed between the STN and the labile organic N fractions. Overall, the results show that no-till coupled with residue retention is an effective management method for improving soil N stocks and increasing soil fertility. Nonetheless, other benefits associated with NT and residue retention present greater challenges regarding their popularization and application in the dryland farming areas in northern China.(2) Effects of the long-term no-tillage practice on the distribution of residual fertilizer N in soil N poolsTillage and residue management affected the fate of residual fertilizer N in soils significantly. In the soil organic N pools, the residual fertilizer N mainly exists in the form of heavy fraction organic nitrogen(HFOMN). More fertilizer N accumulated for the whole soil profile in the CT(32.94 kg/hm2) treatment relative to the NT(27.72 kg/hm2) treatment, which accounted for 16.9% and 14.2% of the application N rate respectively. While the residual fertilizer N which exists in the form of light fraction organic nitrogen(LFOMN) is less. The fertilizer content in the LFOMN accounted for 2.5% and 2.0% in the NT and CT treatment, respectively. In the soil total nitrogen pool, the residual fertilizer content in CT(72.3 kg/hm2) was 6.4% higher than that in NT(67.9 kg/hm2) which accounted for 36.5% and 34.8% of the application N rate respectively. About 90%-93% of the residual fertilizer N was accumulated at 0-20 cm depth. And the residual fertilizer content in CT was 73.6% greater at a depth of 0-5 cm and 21.3% greater at a depth of 5-10 cm, respectively, compared with the CT treatment.(3) Effects of the long-term conservation tillage on the loss of fertilizer N through N2 O and NH3No-tillage with residue retention had significantly improved the emission flux of N2 O and the NH3 volatilization rate. Higher N2 O emission fluxes were found in NT treatment than that in CT treatment during the whole year, especially in fallow and late growing period(P< 0.05). The average N2 O emission rate in NT(19.6 μg N m-2 h-1) was 44.1% higher than CT(13.6 μg N m-2 h-1). In the growth period, the fertilizer loss dynamics in the form of N2 O was similar to the soil N2 O emission dynamics. The gross N2 O fluxes in NT(0.96 kg N hm-2 yr-1) were 32% than CT(0.73 kg N hm-2 yr-1), of which 32% and 30% were from fertilizer N. Fertilizer N loss amount in the form of N2 O in NT accounted for 0.16% of fertilizer application rate, which was 22.9% higher than CT(0.11%). In wheat growth period, the whole ammonia volatilization rate in NT was higher than CT. Compared to CT, the gross NH3 volatilization fluxes in NT(54.0 kg N hm-2) were 22.9% greater than CT(43.9 kg N hm-2). The fertilizer loss rate in the form of N2 O and NH3 was 38.3% under NT relative to CT(35.5%).(4) Effects of the long-term conservation tillage on the uptake and utilization dynamics of fertilizer N by cropsIn rainfed agricultural region, NT can significantly improve the yield of winter wheat than CT by 22.6%-31.6%. The results of two years experiments indicate that the fertilizer recovery efficiency under CT and NT in 2014 and 2015 was 33% and 31%, 28% and 26%, respectively. No-tillage with residue retention reduced the fertilizer recovery efficiency in the two years. The total uptake N by winter wheat in NT in 2014 and 2015 was 151.4 kg N hm-2 and 132.5 kg N hm-2, respectively, which was 16.7% and 6.4% higher than CT. While the fertilizer N absorbed by winter wheat in NT in 2014 and 2015 was 61.4 kg N hm-2 and 52.5 kg N hm-2, which was 3.9% and 2.1% lower than CT. It implies that the crops in NT treatment may absorb and utilize more soil N than fertilizer N. |
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