过量施氮对旱地土壤碳、氮及供氮能力的影响

Effects of excessive nitrogen fertilization on soil organic carbon and nitrogen and nitrogen supply capacity in dryland

【目的】过量施氮会影响土壤有机碳、氮的组成与数量,进而改变土壤供氮能力,但关于西北旱地长期过量施用氮肥后土壤有机碳、氮及土壤供氮能力变化的研究尚缺乏。本文在长期定位试验的基础上,通过分析不同氮肥水平特别是过量施氮条件下土壤硝态氮,有机碳、氮和微生物量碳、氮的变化,探讨长期过量施氮对土壤有机碳、氮及供氮能力的影响。【方法】长期定位试验位于陕西杨凌西北农林科技大学农作一站。在施磷(P2O5)100kg/hm2的基础上,设5个氮水平,施氮量分别为N 0、80、160、240、320 kg/hm2。重复4次,小区面积40 m2,完全随机区组排列。种植冬小麦品种为小堰22。本文选取其中3处理,以不施氮为对照(N0)、施氮量N 160 kg/hm2为正常施氮(N160),施氮量N 320 kg/hm2为过量施氮(N320),分别于2012年6月小麦收获后和10月下季小麦播前采集土壤样品,进行测定分析。【结果】过量施氮导致下季小麦播前0—300 cm各土层硝态氮含量显著增加,平均由对照的2.8 mg/kg增加到15.5 mg/kg;同时,0—60 cm和0—300 cm土层的硝态氮累积量分别由对照的47.2和108.9 kg/hm2增加到76.5和727.7 kg/hm2。过量施氮也增加了夏闲期间0—300 cm土层土壤有机氮矿化量,由对照的72.4 kg/hm2增加到130.7 kg/hm2。但过量施氮未显著增加土壤的有机碳含量,却显著增加了土壤有机氮含量,过量施氮0—20、20—40 cm土层土壤有机碳分别为9.24和5.39 g/kg,有机氮分别为1.05和0.71 g/kg,较对照增加52.2%和54.3%。同样,过量施氮未显著影响0—20、20—40 cm土层土壤微生物量碳含量,其平均含量分别为253和205 mg/kg,却显著提高了0—20、20—40 cm土层土壤微生物量氮含量,由对照的24.1和7.5 mg/kg提高到43.6和16.1 mg/kg。【结论】过量施氮可以显著增加旱地土壤剖面中的硝态氮累积量、夏闲期氮素矿化量、小麦播前土壤氮素供应量和土壤微生物量氮含量,但对土壤有机碳和微生物量碳没有显著性影响,同时过量施氮增加了土壤硝态氮淋溶风险,故在有机质含量低的黄土高原南部旱地冬小麦种植中不宜施用高量氮肥,以减少土壤氮素残留和农业投入,达到保护环境和培肥土壤的目的。

【Objectives】Excessive nitrogen( N) fertilization affects fractions and quantities of soil organic carbon( C) and nitrogen( N) and therefore influences soil nitrogen supply capacity. However,there were few studies on effects of long term excessive nitrogen fertilization on soil organic C and N in dryland of southern Loess Plateau.Therefore,a long-term experiment was carried out to determine changes of nitrate N,organic C and N,andmicrobial biomass C and N under different N treatments and to study effects of excessive N fertilization on soil carbon and nitrogen and soil nitrogen supply capacity. 【Methods】A long-term experiment was established in fall of 2004 with five N levels,0,80,160,240 and 320 kg / ha. The plot size was 40 m2 and all treatments were arranged in a completely randomized block design with four replications,and winter wheat( Triticum aestivum L.)cultivar,Xiaoyan 22,was grown. The N levels of 0,160 and 320 kg / ha were chosen as control( N0,with no N fertilization),normal N( N160) and excessive N( N320) treatments in this study. Soil samples were randomly collected from four sites at two depths( 0-20 and 20-40 cm) using a 2. 5- cm diameter auger in each plot on June15,2012( at wheat harvest) and October 1( at next winter wheat sowing) and then prepared for analysis.【Results】The excessive N fertilization treatment significantly increases soil nitrate N contents in 0-300 cm soil layers before the next winter wheat sown,i. e. from an average of 2. 8 mg / kg( control) to 15. 5 mg / kg which is higher than the normal N treatment by 8. 1 mg / kg. Meanwhile,the nitrate N accumulation amounts in 0- 60 cm and 0-300 cm soil layers are increased from 47. 2 and 108. 9 kg / ha of the control treatment to 76. 5 and 727. 7kg / ha of the excessive N treatment,respectively,which are higher than the normal N treatment by 22% and122%. During the summer fallow season,the excessive N fertilizer use also increases soil N mineralization,i. e.from 72. 4 kg / ha of the control treatment to 130. 7 kg / ha of the excessive N treatment. The excessive N treatment does not significantly increase soil organic C,but it does significantly increase organic N,and soil organic nitrogen C and N contents in the excessive N treatment are 9. 24,5. 39 g / kg and 1. 05,0. 71 g / kg( with increases by52. 2% and 54. 3% than control,respectively) in 0-20 and 20-40 cm soil layers,respectively. In addition,the excessive N fertilization does not significantly affect the microbial biomass C in 0-20,20-40 cm soil layer( with the average value of 253 and 205 mg / kg,respectively),and significantly increases the soil microbial biomass N,i. e. from 24. 1 and 7. 5 mg / kg in the control treatment to 43. 6 and 16. 1 mg / kg in the excessive N treatment.【Conclusions】The excessive N fertilizer application significantly increases the nitrate N accumulation and mineralization in 0-300 cm soil profile,the quantities of soil N supply at the next winter wheat sowing and the soil microbial biomass N. However,the excessive N fertilization does not significantly affect soil total organic C and microbial biomass C,and increases the risk of nitrate N leaching in soil. Therefore,in order to reduce nitrate N leaching,input costs and the risk of environment pollution,high rate of N fertilizer is not suggested to apply to the dryland in southern Loess Plateau where the soil is low in organic matter concentration.