太湖地区稻麦轮作农田改葡萄园对土壤氮转化过程的影响

Effects of Conversion of Paddy Field into Vineyard on Soil Nitrogen Transformation in the Taihu Lake Region of China

采用15N成对标记技术结合数值模型,测定太湖地区两种土地利用方式(稻麦轮作农田和葡萄园)下的土壤氮素初级转化速率,探讨了土地利用方式改变对土壤供氮和保氮能力的影响。结果表明,葡萄园土壤初级矿化速率高于稻麦轮作农田土壤,但是其NH4+-N同化速率几乎可以忽略不计(0.02 mg kg-1 d-1),自养硝化成为培养条件下葡萄园土壤NH4+-N的唯一去向。葡萄园土壤初级自养硝化速率(15.85 mg kg-1 d-1)显著高于稻麦轮作农田土壤(13.65 mg kg-1 d-1),但两者初级异养硝化速率和NO3--N同化速率均接近零值。可见,太湖地区稻麦轮作农田改种为葡萄园后,土壤NH4+-N同化速率显著降低而自养硝化速率增加,由此导致更多的NO3--N在土壤中累积,进而可能增加土壤中N的淋溶和径流损失风险。

In response to the growing demand for fruits,farmers in the Taihu Lake region are rushing to convert paddy fields into fruit orchards in recent years. Changes in land-use and management may affect or alter physico-chemical properties of the soil,and hence cycling of soil N and fate of N fertilizer. Up to date,little has been reported on quantification of effects of changes in land use on soil N gross transformation rate,besides some works that have been mainly focused on effects of the conversion of non-agricultural land into agricultural land,and rarely on the effects of the conversion from one type to another type of agricultural land use. In the Taihu Lake region,paddy fields under rice-wheat crop rotation and orchards coverted from paddy fields are the two typical types of agricultural land-use,which differ sharply in water regimes（periodically waterlogged for paddy fields and water-unsaturated for orchards）and fertilizer management（no input of organic manure for paddy fields and combined application of chemical fertilizer and organic manure for orchards）. Therefore,gross N processes（e.g.,nitrification and denitrification）in the soils under the two types of land use also differ sharply,as affected by their different aeration conditions and fertilizer managements. The paddy field under rice-wheat crop rotation and the vineyard converted from paddy field under study are located in the upper-streams of the Zhushan Bay Catchment in the Taihu Lake Region of China. Gross transformation rates of soil N under the two types of land use were measured using the 15 N tracing technique combined with the Markov Chain Monte Carlo（MCMC）algorithm-based numerical optimization model,and effects of the conversion on soil N supply and N retention capacity were investigated. Results show that the conversion reduced soil p H（from 5.74 in paddy field to 5.14 in vineyard,on average）and contents of soil organic C and total N,though not much. In the soils of the paddy field and vineyard,the inorganic-N pools were dominated with nitrate,with NH4＋/NO3- being 0.26 and 0.06,respectively,and the gross N mineralization rate（mineralization of labile and recalcitrant soil organic matter）was N 3.90 mg kg-1 d-1 and 4.52 mg kg-1 d-1,respectively. Obviously the differences between the two were not very sharp. In the paddy field,the gross NH4＋ assimilation rate was 0.56 mg kg-1 d-1,accounting for only 14% of the total NH4＋ produced,while in the vineyard it was almost negligible. The gross N autotrophic nitrification rate in the vineyard was 15.85 mg kg-1 d-1,significantly higher than that（13.65 mg kg-1 d-1）in the paddy field,while the gross heterotrophic nitrification rate and NO3- assimilation rate were both negligible in both soils. Through fitting with the MCMC algorithm-based numerical optimization model,consumption of NO3- in the soils was found to have two pathways,namely assimilation of NO3- and dissimilatory reduction of NO3- to NH4＋（DNRA）. However,in both of the soils,NO3- assimilation was not detected,turning DNRA into the major pathway of NO3- consumption,moreover,the two soils did not differ much in DNRA rate. The ratio of total nitrification to gross NH4＋ assimilation（N/NA）in the soil was 24 in the paddy field and 793 in the vineyard,indicating that ammonia oxidizing bacteria are stronger than heterotrophic nitrifiers in competition for NH4＋,and hence autotrophic nitrification is the dominant fate of NH4＋,especially in the vineyard. On the whole,the conversion of paddy field into vineyard significantly affects soil autotrophic nitrification,increasing the N autotrophic nitrification rate in the soil,but its influence on NH4＋ assimilation rate was almost negligible,thus making autotrophic nitrification the only fate for NH4＋ in the vineyard. The decreased NH4＋ assimilation rate and the increased autotrophic nitrification rate in the vineyard enhanced NO3- accumulation in the soil,which may in turn increase the risk of N leaching and losing with runoff. It is recommended that nitrification inhibitor and/or organic manure high in C/N ratio should be applied to mitigate the risk.