亚热带几种典型稻田与旱作土壤中外源输入秸秆的分解与转化差异

Decomposition and transformation of input straw in several typical paddy and upland soils in subtropical China

选取亚热带四种典型母质(花岗岩风化物、第四纪红色粘土、板页岩风化物、近代河流沉积物)发育的稻田土壤,以毗邻的旱作土壤为对比,通过室内模拟培养试验研究45%田间持水量(WHC)条件下稻田和旱作土壤中外源输入秸秆矿化和转化的特征与差异。结果表明:在180 d的培养期内,所选4种稻田土壤中外源输入秸秆的累积矿化率(18%—21%)均显著低于对应的旱作土壤(21%—28%),外源秸秆的输入对土壤原有有机碳矿化的激发效应也是以稻田土壤(5%—30%)明显低于对应的旱作土壤(17%—65%)。外源秸秆在土壤中的分解产物主要向颗粒有机碳(POC)和铁铝结合态有机碳(Fe/Al-OC)分配,分配比例分别为9%—21%和12%—24%,其次为腐殖质碳(HMC)(11%—15%),而向微生物生物量碳(MBC)和溶解性有机碳(DOC)分配的比例极小,分别仅为2%—7%和0.1%-0.7%。与旱作土壤相比,稻田土壤中外源秸秆的分解产物向POC、Fe/Al-OC和MBC分配的比例较高,分别为15%—21%、17%—24%和6%—7%,而旱作土壤为9%—17%、13%—18%和2%—4%。此外,外源秸秆分解产物向2000—250μm水稳性粗团聚体分配的比例也以稻田土壤(10%—13%)高于旱作土壤(6%—7%),其它粒径中稻田与对应的旱作土壤之间并无显著差异。本研究结果说明,稻田土壤中外源输入秸秆的矿化率低于旱作土壤的现象在不同母质类型的土壤中可能普遍存在,这可能与稻田土壤中外源秸秆分解产物受水稳性团聚体的物理保护、与氧化铁铝的化学键合以及向有机碳稳定组分的分配作用较强有关,从而贡献于稻田土壤较高的有机碳积累。

Decomposition and transformation of input straw in four types of paddy soils were investigated under incubation at 25℃ and 45% water holding capacity （WHC） for 180 d. The soils were derived from different parent materials （weathered granite, quaternary red clay, weathered shale, and river alluvial） in subtropical China, and the adjacent upland soils were selected as a control. During the 180 d period, the mineralization ratios of input straw in the selected paddy soils （18%-21%） were lower than those in the corresponding upland soils （21%-28%）. The priming effects of straw amendment on native soil organic carbon mineralization were also lower in the paddy （5%-37%） than in the corresponding upland soils （23%-65%）. The decomposed products of input straw were mainly distributed in particulate organic carbon （POC, 9%-21%） and Fe/Al-bound organic carbon （Fe/Al-OC, 12%-24%）, followed by humus carbon （HMC） （11%-15%）, whereas only a small part was distributed as microbial biomass carbon （MBC, 2%-7%） and dissolved organic carbon （DOC, 0.1%-0.7%）. In paddy soils, the conversion ratios of input straw in POC, Fe/Al-OC, and MBC （15%-21%, 17%-24%, and 6%-7%） were higher than those in upland soils （9%-17%, 13%-18%, and 2%-4%）. In addition, the 2,000-250 μm coarse water-stable aggregates in paddy soils tended to receive more decomposed products of input straw than those in upland soils （10%-13% vs. 6%-7%）, whereas no significant difference was observed between paddy and upland soils in other small sizes of aggregates. The results indicated that the mineralization of input straw may be lower in paddy than in upland soils derived from different parent materials, possibly owing to stronger physical protection in soil coarse aggregates, chemical protection by binding with Fe/Al oxyhydrates, and larger transformation to stable fractions of input straw during its decomposition in paddy soils. This fate of input straw decomposition may contribute to a higher organic carbon accumulation in paddy than in upland soils.