红壤双季稻田土壤活性碳、氮周年变化及影响因素

Annual variation of active carbon and nitrogen contents and the related factors in red paddy soils of subtropical China

【目的】研究红壤稻田土壤活性碳、氮含量及其周年变化和影响因素,为弄清红壤双季稻田土壤活性碳、氮季节变化规律,提高土壤养分利用效率提供理论依据。【方法】以红壤稻田长期定位试验(1990年开始)为对象,选择不施肥(CK)、单施NPK化肥(NPK)和有机物(紫云英、稻草)+NPK化肥(NPKM)三个施肥处理,在早稻、晚稻、休闲期取0—20 cm土样,动态监测稻田土壤几种重要活性碳、氮成分,包括无机态氮(NH_4^+N、NO_3~–-N),可溶性有机碳、氮(DOC、DON)和微生物生物量碳、氮(MBC、MBN)的周年动态变化。【结果】稻田土壤NH_4^+-N含量在早稻期间呈下降状态,晚稻期间变化较平缓,休闲期呈先上升后下降的趋势。土壤无机氮主要以NH_4^+N形式存在,其含量范围为14.9~31.6 mg/kg,其休闲期含量略低于生育期(P>0.05);周年土壤NO_3~–-N含量(<3 mg/kg)远小于NH_4^+N含量,与NH_4^+N含量呈现此消彼长的趋势(P<0.01)。稻田土壤DOC、DON周年变化趋势相似(P<0.01),在生育期呈下降状态,休闲期呈先上升后下降趋势,休闲期土壤DOC、DON含量略低于生育期(P>0.05)。休闲期土壤MBC、MBN含量分别为463~701 mg/kg、31.1~52.4mg/kg,比生育期分别提高25.4%~36.9%和62.8%~125.9%。总体来看,稻田休闲期土壤活性碳(DOC+MBC)、氮(NH_4^+N+NO_3~–-N+DON+MBN)含量高于生育期含量,较生育期增幅分别为10.8%~19.6%、10.3%~34.8%。不同施肥处理结果表明,仅施用化肥(NPK处理)对活性碳、氮影响较小(P>0.05),有机物还田能显著提高水稻生育期和休闲期的土壤活性碳、氮含量(P<0.05),NPKM与CK相比活性碳、氮在水稻生育期分别提高53.8%和81.2%,而在休闲期分别提高了43.5%和63.2%。相关性分析结果表明,稻田休闲期土壤NH_4^+N、DOC、DON含量不仅与土壤SOC、TN含量密切相关(P<0.05或P<0.01),还受温度、水分含量等环境因子影响,如休闲期土壤NH_4^+N含量与5 cm处土壤温度呈极显著负相关关系(P<0.01),而土壤NO_3~–-N含量与该土层温度呈显著正相关关系(P<0.05),土壤DOC和DON与土壤含水量呈显著正相关关系(P<0.05)。【结论】长期化肥配施紫云英及秸秆还田能显著提高稻田土壤周年活性碳、氮含量,其活性含量与土壤有机碳、全氮含量呈显著正相关关系;休闲期稻田能维持较高的活性碳、氮含量,且比水稻生育期有一定程度增加。休闲期土壤无机氮含量变化主要受土壤温度影响,土壤可溶性碳、氮含量变化主要受土壤含水量的影响。

[Objectives] Active carbon and nitrogen play important roles in soil carbon and nitrogen cycle. This study aimed to determine the difference of active carbon and nitrogen contents in soils affected by fertilization and other factors through the year in the subtropical double-rice region of China.[Methods] A long-term field experiment on reddish paddy soil was conducted. The three treatments were CK （non-fertilizer）, NPK （N, P, and K fertilizer application）, and NPKM （NPK fertilizer, milk vetch and rice straw application）. The characteristics and variation trends of inorganic nitrogen （NH4＋-N and NO3--N）, dissoluble organic carbon and nitrogen （DOC and DON） and microbial biomass carbon and nitrogen （MBC and MBN） in soils were investigated during the rice growing and fallow periods.[Results] The NH4＋-N contents in paddy soils fluctuated in all treatments and showed three general trends：decreased during the early rice stage, stable during late rice period, and increased then decreased during the fallow period. NH4＋-N, the main form of inorganic nitrogen in soils, was in range of 14.9-31.6 mg/kg, and no significant difference was found between the rice growing period and fallow period. The contents of soil NO3--N, less than 3.0 mg/kg, were far lower than those of NH4＋-N contents, and there was a reciprocal relationship between them （P 〈 0.01）. Both of soil DOC and DON had a similar annual variation with a declining trend in the rice growing period and an increasing first and decreasing later trend in the fallow period. The contents of MBC and MBN in soils were 463-701 mg/kg and 31.1-52.4 mg/kg during the non-rice growing period, which were increased by 25.4%-36.9% and 62.8%-125.9%, compared to those in the rice-growing period, respectively. Overall, the total contents of active carbon （DOC ＋ MBC） and nitrogen （NH4＋ -N＋ NO3- -N＋ DON ＋ MBN） in soils in the non-rice growing period were increased by 10.8%-19.6% and 10.3%-34.8%, respectively, compared with those in the rice-growing period. The total contents of active carbon or nitrogen in soils in the NPKM treatment were significantly higher than those in the NPK and CK treatments（P 〈 0.05）. The active carbon and nitrogen contents in the NPKM treatment were increased by 53.8% and 81.2% in the growth season, and by 43.5% and 63.2% in the fallow season, compared with those in the CK treatment. No significant differences were found between the NPK and CK treatments during the two periods. During the fallow period, soil NH4＋-N, DOC, and DON had significant and positive correlations with SOC and TN; Soil NH4＋-N and soil NO3--N were significantly correlated with soil temperature at 5 cm （P 〈 0.01 and P 〈 0.05）; Soil DOC and DON were significantly correlated with soil water contents （P 〈 0.01 and P 〈 0.05）.[Conclusions] Combined application of milk vetch and rice straw and chemical fertilizers could significantly increase the contents of active carbon and nitrogen in soils, which were positively correlated with soil organic carbon and total nitrogen contents. Compared with the rice growing period, the contents of active carbon and nitrogen in soils maintained high levels during the fallow period, which were affected by SOC and TN, and environmental factors, i.e., soil NH4＋-N and soil NO3--N were significantly correlated with soil temperature at 5 cm, and soil DOC and DON were significantly correlated with soil water contents.