Effects Of Tillage Practices On Water-Stable Aggregation And Aggregate-Associated Organic Carbon In Lou Soil

A field experiment was conducted on the Guanzhong plain to study the effects of tillageon the distribution and property of water-stable aggregates and aggregate-associated organicC. Four tillage practices were compared: sub-soiling, rotary tillage, no-tillage, andconventional tillage. The soil type was a Lou soil and the cropping system was a winterwheat-summer maize rotation. Meanwhile, a culture experiment was conducted in laboratoryto study the effects of drying-wetting cycling and maize straw mix on the distribution ofwater-stable aggregates and aggregate-associated organic C. Four treatments were compared:constant wetting without maize straw mixed (CW), constant wetting with maize straw mixed(CW+S), drying-wetting cycling without maize straw mixed (DW), and drying-wettingcycling with maize straw mixed (DW+S). The results showed that:1. Effects of tillage practices on the distribution of water-stable aggregates andaggregate-associated organic CThe mass of macro-aggregates (i.e.,>2mm and0.25~2mm size classes), thecontribution of macro-aggregate-associated organic C to total soil organic C (SOC), and themean weight diameter (MWD), total SOC and the aggregate-associated organic C of all sizeclasses increased as soil depth increased. In contrast, the mass of the micro-aggregate (i.e.,0.053~0.25mm) and mineral (i.e.,<0.053mm) fractions and the contribution of these sizefractions to total SOC decreased as soil depth increased. The content of organic carbondecreased as aggregate size class decreased. Compared to conventional tillage, nine years ofsub-soiling, rotary tillage, or no-tillage increased the mass of macro-aggregates, thecontribution of macro-aggregate-associated organic C to total SOC, and the MWD in the0~40cm depth. In comparison, the mass of the micro-aggregate and mineral fractions and thecontribution of these size fractions to total SOC were greater in the conventional tillagetreatment. Sub-soiling, rotary tillage, and no-tillage also increased total SOC at all depths andthe aggregate-associated organic C of all size classes at the0~10cm depth. The return ofmaize straw to the soil increased the total SOC content as well as the aggregate-associatedorganic C content of all aggregate size classes at the0~40cm depth. The return of straw alsofacilitated the formation and stabilization of macro-aggregates at all depths. Sub-soiling every other year had better effects than sub-soiling every year. Specifically, sub-soiling every otheryear increased total SOC as well as aggregate-associated organic C in each size class at alldepths. Compared to annual sub-soiling, sub-soiling every other year also increased theformation and stability of macro-aggregates at the0~40cm depth. Also, Sub-soiling hadbetter effects than surface tillage.2. Effects of tillage practices on the property of aggregate-associated organic CThe content of aggregate-associate readily oxidizable organic C and aggregate-associatedhighly labile organic C decreased as aggregate size class decreased or as soil depth increased.In comparison, oxidation stability of aggregate-associate readily oxidizable organic C (Kos)increased as aggregate size class decreased or as soil depth increased. Compared toconventional tillage, nine years of sub-soiling, rotary tillage, or no-tillage increased thecontent of aggregate-associate readily oxidizable organic C and aggregate-associated highlylabile organic C, decreased the Kos in the0~40cm depth. Compared to no straw return,sub-soiling every year, and surface tillage, maize straw return, sub-soiling every other year,and sub-soiling had such positive effects respectively.3. Effects of drying-wetting cycling and maize straw mix on the distribution of water-stableaggregates and aggregate-associated organic CAs the culture experiment processed, DW treatment decreased the mass of>0.053mmaggregate size classes at the0~40cm depth continually and increased continually the mass ofmineral fractions. In comparison, CW+S treatment increased continually the mass ofmacro-aggregates and decreased the mass of micro-aggregate and mineral fractionscontinually. DW+S treatment and CW treatment had the tendency of facilitating the formationof macro-aggregates. Differences of MWD were significant between straw mix treatmentsand no straw mix treatments. Treatments without drying-wetting cycling increased MWDcontinually. However, in treatments with drying-wetting cycling, DW decreased MWDcontinually, while DW+S firstly decreased MWD and then increased it. In treatments withoutstraw mix, no matter drying-wetting cycling or not, total SOC and aggregate-associatedorganic C of all size classes had the tendency of continual decrease. In contrast, in treatmentswith straw mix, no matter drying-wetting cycling or not, total SOC and aggregate-associatedorganic C of all size classes had the tendency of continual increase. DW treatment enrichedthe percentage of organic C in mineral fractions, while CW+S treatment enriched thepercentage of organic C in macro-aggregates. For treatments DW+S and CW, there was thetendency of percentage enrichment of organic C in macro-aggregates.