长期施肥对红壤性水稻土团聚体稳定性及固碳特征的影响

施用有机肥是提高土壤有机碳(SOC)含量、促进土壤团聚体形成和改善土壤结构的重要措施。本研究旨在探讨长期作物残留和投入有机物料对水稻土团聚体分布及稳定性的影响,分析不同粒级团聚体的固碳特征及其与团聚体形成的相关性,以及土壤和不同粒级团聚体对累积碳投入的响应。长期定位施肥试验始于1986年,设不施肥(CK)、单施化肥(CF)、秸秆化肥混施(RS)、低量粪肥配施化肥(M1)和高量粪肥配施化肥(M2)5个处理。2009年采集0~10 cm土壤样品,测定总土以及大团聚体(LM,2 mm)、较大团聚体(SM,0.25~2 mm)、微团聚体(MA,0.25~0.053 mm)和黏粉粒(SC,0.053 mm)的质量比例及其SOC浓度,并分析闭蓄于SM内部的颗粒有机物(POM)、微团聚体(MA-SM)和黏粉粒(SC-SM)的质量含量和SOC浓度。结果表明,与CK和CF比较,有机肥混施化肥处理(RS、M1和M2)均显著提高了LM和SM的质量比例和平均当量直径(MWD),降低了SC质量含量;两个粪肥配施化肥处理(M1和M2)的效果优于秸秆化肥混施(RS),但是M1和M2间差异不显著;单施化肥则降低了稳定性团聚体的比例。团聚体的SOC浓度没有随粒级增大而增加,各处理均为LM和SM结合的SOC浓度最高,其次为SC,最小为MA。与CK比较,有机肥混施化肥处理均显著提高了各粒级团聚体的SOC浓度。总土SOC的增加主要取决于SM的SOC含量,而MA-SM组分决定了SM固持SOC的能力。总土、LM和SM的SOC含量以及从SM分离出的POM、MA-SM和SC-SM的SOC含量均与累积碳投入量呈显著正相关,但总土分离出的MA和SC的SOC含量对累积碳投入量反应不敏感,表现出碳饱和迹象。因此,尽管长期大量施用有机物料促进了红壤性水稻土大团聚体的形成和团聚体稳定性,增加了其SOC的固持,但有机质可能不是该土壤水稳性团聚体形成的最主要黏结剂。

Influences of long-term organic and chemical fertilization on soil aggregation and associated organic carbon fractions in a red paddy soil

Organic fertilizer application is an essential measure for improving soil organic carbon (SOC) content, promoting soil aggregate formation and improving soil structure. The objectives of this study were to investigate the influences of long-term organic and chemical fertilizer applications on the distribution and stability (in terms of mean weight diameter, MWD) of soil aggregates, aggregate formation in relation to SOC concentration and responses of SOC content of whole soils and different aggregates to cumulative C input. The experiment was initiated in 1986 and included five treatments - no fertilizer (CK), only chemical fertilizer (CF), rice straw plus chemical fertilizer (RS), low manure rate plus chemical fertilizer (M1), and high manure rate plus chemical fertilizer (M2). Soil samples (in the 0-10 cm soil layer) were collected in 2009 to determine aggregate size distribution and SOC content in bulk soil and different aggregate fractions - they are, large macro-aggregates (LM, >2 mm), small macro-aggregates (SM, 0.25-2 mm), micro-aggregates (MA, 0.25-0.053 mm) and silt plus clay (S&C, <0.053 mm) fractions. The ratios and SOC content of particulate organic matter (POM), micro-aggregates (MA-SM) and silt plus clay (S&C-SM) fractions within SM fraction were also analyzed. Compared with CK and CF treatments, combined application of rice straw or manure with chemical fertilizer significantly improved the proportions of LM and SM, and also MWD of the soil aggregates, but reduced S&C content. Changes were more evident in manure treatments (M1 and M2) than in RS treatment, but no significant difference was noted between M1 and M2. Sole chemical fertilization reduced the stability of soil aggregates. In term of SOC concentration within aggregate fractions, it was highest in LM and SM fractions, followed by S&C and then lowest in MA fraction. SOC concentration of aggregates did not increase linearly with aggregates size. Comparing the four aggregate sizes, SM fraction contributed the most to SOC sequestration in bulk soils. Within SM, MA fraction formed the key site for SOC storage. SOC contents of bulk soils, LM, SM and fractions within SM (i.e., POM, MA-SM and S&C-SM) responded positively to cumulative C input, but no apparent changes in SOC were noted for MA and S&C fractions, indicating C saturation in the two fractions. It was concluded that organic amendment (of either rice-residue or manure) promoted the formation of macro-aggregates and improved aggregate stability. However, soil organic matter was not likely the major binding agent driving soil aggregation in red paddy soils.