长期保护性耕作对稻田土壤团聚体稳定性和碳氮含量的影响

为研究双季稻(Oryza sativa L.)土壤团聚体稳定性及C、N含量对耕作方式的响应,该研究利用已进行12 a的包括翻耕+秸秆不还田(CT),翻耕+秸秆还田(CTS),旋耕+秸秆还田(RTS)和免耕+秸秆还田(NTS)的保护性耕作稻田定位试验,运用湿筛等方法测算了团聚体的构成与稳定性,C、N含量及其对土壤总C、N的贡献。结果表明:长期秸秆还田显著增加0～10 cm土层大团聚体比重,弱化翻耕、旋耕和免耕等不同耕法对表层大团聚体的差异化影响(P0.05),但5～30 cm土层大团聚体随耕作强度的减弱有所提高。总体来看,稻田土壤团聚体以2 mm粒径为主(占35.02%～64.44%),其C、N贡献率分别达52.12%和52.16%;秸秆还田有利于微团聚体向大团聚体的转化,外源C、N更多被大团聚体固持和保护。NTS在0～20 cm的2 mm团聚体对土壤C、N的贡献率显著大于其他处理;土壤C、N含量与团聚体稳定性呈显著正相关关系(P0.05)。相比于CTS与RTS,长期采取NTS显著改善土壤C、N含量,促进大团聚体的形成和稳定,对改善稻田耕层(尤其0～20 cm)土壤团聚体稳定性具有显著的效果(P0.05)。因此,采取保护性耕作措施对南方双季稻田土壤质量及农业生态持续性具有积极的作用。

Effects of long-term conservation tillage on soil aggregate stability and carbon and nitrogen in paddy field

Abstract: Soil structure stability, aggregate-associated C and N play an important role in soil conservation and nutrient supply. Tillage practices can affect the soil aggregate stability and C and N distribution, thus affecting the farmland ecological security. To estimate the effects of tillage practices on soil aggregate stability and its C and N distribution of double paddy field in Southern China, a long-term field experiment initiated from 2005 with four treatments (no-till with residue retention, NTS), rotary tillage with residue retention, RTS), plow tillage with residue retention, CTS), and plow tillage with residue removed, CT)) was conducted in a double rice cropping system in Ningxiang, Hunan. After 12-years of the experiment, the soil water-stable aggregates, stability, and C, N concentration were determined from four soil depths of 0-5, 5-10, 10-20, and 20-30 cm. The results showed that there were significant positive correlations between soil C, N and aggregate stability (P <0.05). The percentage of soil aggregate decreased with the particle size decreases in paddy fields. It mainly composed of macro-aggregate (>0.25 mm), accounting for 66.90%-87.82%, of which >2 mm part accounted for 35.02%-64.44% in 0-30 cm soil layers under different tillage practices. For >2 mm soil aggregate, NTS was significantly higher than RTS (P <0.05) in the 5-30 cm soil layers.NTS was significantly higher than CTS at 5-20 cm (P < 0.05), but NTS, RTS and CTS were not significant in the 0-5 cm soil layer. The <0.25 mm soil aggregate accounted for 12.18%-33.10% in 0-30 cm soil layers under different tillage practices. In terms of aggregate stability, NTS was significantly higher than RTS (10-30 cm) and CTS (5-20 cm), but NTS, RTS, and CTS were not significant in the 0-5 cm soil layer. The contribution rate of macro-aggregate to soil C, N in paddy fields were 76.58%-90.62% and 72.28%-89.76%, respectively, and the contribution rates of >2 mm aggregates to C and N were52.12% and 52.16%, respectively. Compared with straw removal, the contribution rate of >2 mm aggregate treated with straw returning to the soil C, N increased by 8.20 percentage point and 7.35 percentage point, while the contribution of 0.25-2 mm aggregate decreased by 4.96 percentage point and 4.84 percentage point, respectively. Further analysis of the relationship between soil C and N and aggregate stability showed that SOC and GMD (geometric mean diameter), TN and GMD were significantly positively correlated. Thus, straw returning was conducive to the transformation of micro-aggregate to macro-aggregate. Compared with CTS and RTS, NTS significantly increased the C, N content in soil surface and promoted the stable macro-aggregate formation, which had significant effects on improving aggregate stability in paddy fields (especially 0-20 cm) (P <0.05). Therefore, no-till with residue retention is an effective measure to maintain and improve soil performance of the paddy field in Southern China.